SYNTHESIS of NEW NANOCRYSTAL MATERIALS by Yasser Hassan Abd El-Fattah Hassan a Thesis Submitted in Conformity with the Requireme

Home , Nanocrystal

SYNTHESIS OF NEW NANOCRYSTAL MATERIALS

Yasser Hassan Abd El-Fattah Hassan

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Department of Chemical Engineering and Applied Chemistry University of Toronto

Synthesis of New Nanocrystal Materials

Yasser Hassan

Doctor of Philosophy

Department of Chemical Engineering and Applied Chemistry University of Toronto

2016

Abstract

Colloidal semiconductor nanocrystals (NCs) have sparked great excitement in the scientific community in last two decades. NCs are useful for both fundamental research and technical applications in various fields owing to their size and shape-dependent properties and their potentially inexpensive and excellent chemical processability. These NCs are versatile fluorescence probes with unique optical properties, including tunable luminescence, high extinction coefficient, broad absorption with narrow photoluminescence, and photobleaching resistance. In the past few years, a lot of attention has been given to nanotechnology based on using these materials as building blocks to design light harvesting assemblies. For instant, the pioneering applications of NCs are light-emitting diodes, lasers, and photovoltaic devices.

Synthesis of the colloidal stable semiconductor NCs using the wet method of the pyrolysis of organometallic and chalcogenide precursors, known as hot-injection approach, is the chart-topping preparation method in term of high quality and monodisperse sized NCs. The advancement in the synthesis of these artificial materials is the core step toward their applications in a broad range of technologies.

This dissertation focuses on exploring various innovative and novel synthetic methods of different types of colloidal nanocrystals, both inorganic semiconductors NCs, also known as quantum dots (QDs), and organic-inorganic metal halide-perovskite materials, known as perovskites. The work presented in this thesis focuses on pursuing fundamental understanding of the synthesis, material properties, photophysics, and spectroscopy of these nanostructured semiconductor materials.

This thesis contains 6 chapters and conclusions. Chapters 13 focus on introducing theories and background of the materials being synthesized in the thesis. Chapter 4 demonstrates our synthesis of colloidal linker–free TiO2/CdSe NRs heterostructures with CdSe QDs grown in the presence of

TiO2 NRs using seeded–growth type colloidal injection approach. Chapter 5 explores a novel approach of directly synthesized CdSe NCs with electroactive ligands. The last Chapter focuses on a new class of perovskites. I describe my discovery of a (bottom-up) simple method to synthesize colloidally stable methyl ammonium lead halide perovskite nanocrystals seeded from high quality PbX2 NCs with a pre-targeted size. This chapter reports advances in preparation of both these materials (PbX2, and lead halide perovskite NCs).

iii

Dedication To My Parents and My Stepmother To My Wife Marwa H. Ibrahim To My Brothers and Sisters To My Beautiful Kids To My Teachers To My Family

To Every knowledge seeker:

Al-Khatib al-Baghdadi stated in his book “The History of the City of Peace: The History of Baghdad” that:

Imam Al-Juwayni (died 1085) wrote to one of his students said

Imam Al-Shafi‘i wrote a rhyme of poetry:

أخي لن تنال العلم إال بستة سأنبئك عن تفصيلها ببيان

ذكاء وحرص وافتقار وغربة وتلقين أستاذ وطول زمان

“Oh my brother you will never become a scholar until you

complete the six pillars of THE REQUISITES OF

KNOWLEDGE:

A QUICK MIND, ZEAL FOR LEARNING, POVERTY,

FOREIGN LAND, A PROFESSOR'S INSPIRATION, AND OF

LIFE A LONG SPAN.”

End of Quote!

Acknowledgments

I would like to thank a number of individuals, without their unique insights and wisdom, I would have struggled to find the motivation, enthusiasm, inspiration, support, and guidance to complete this work.

First of all, I wish to express my deepest gratitude to my advisors, Professor Gregory D. Scholes and Professor Mitch Winnik for their inspiration, continued encouragement, priceless suggestions, and support while working in Toronto. Their mentorship has helped me become a better scientific writer, strategic planner and critical thinker about science. I would like to express my profound appreciation to my advisor Professor Scholes who graciously welcomed my visit to his lab

20092010 and then provided me with the incredible opportunity to complete my Ph.D. at the

University of Toronto. There is no doubt without his continued encouragement, invaluable suggestions, invaluable mentorship, exceptional support and keen insights, patient guidance, I would not have made it to this stage of success. Greg became more of a mentor and friend than a professor. Moreover, the work environment in Professor Scholes’ lab was a fantastic experience.

Additionally, I would like to thank Professor Tim Bender and Professor Dwight Seferos for being members in my committee and for all of their help and kind suggestions related to my research. I am indebted to Professor Bender to be a friend and mentor. Thanks Tim! This is highly appreciated.

I would like to thank Professor Zheng-Hong Lu and Professor Peng Zhang for their willingness to spend time reading this thesis and be part of my departmental examination.

I would also like to thank and to express deep appreciation to my coworkers who assisted me on these research projects. I am so thankful to my colleagues and friends at the university, especially the ones in the Scholes and Winnik groups. They were all exceptional people. All of their support

vi and encouragements helped me significantly toward the completion of my studies at the University of Toronto. The following is a list of all my former and current coworkers and colleagues who helped me carry out the different research projects at the University of Toronto. I was fortunate to work within this environment. In particular I would like to thank my friend and colleague Ahmed

Abdelrahman for working on the documents, renting an apartment, and his countless support during my the past seven years. Thanks Ahmed. I feel you are one part of the family. I would to thank Dr. Anna Lee and Dr. Gerald Guerin for their advice and support. I thank my current and former fellow labmates from Scholes group. I would like to thank Dr. Haizheng Zhong and Yaser

Khan for all their insightful comments and help provided in the beginning of my work in Scholes’ lab. I would like to thank my friend Dr. Mahmoud Dawood (University of Zagazig) for helping in reproducing Figures 2.12, 3.1 and 3.2.

An acknowledgement with a very special thanks goes out and deeply appreciation to Dr. Ryan

Pensack, Dr. Yoichi Kobayashi, Dr. Yin Song, and Dr. Elsa Cassette. Without whose encouragement and technical support, I would not have finished up my Chapters 5 and 6 of this thesis.

Special thanks for Dr. Ryan Pensack (the Boss) for being a fantastic mentor, always helping and being the greatest friend. Boss, I am so indebted for having you during my PhD!!

I must also acknowledge my collaborators: (a) Professor Clemens Burda and his student Chi-Hung

Chuang for their insightful comments and help provided for transient absorption measurements done for the colloid linker-free TiO2/CdSe Nanorods synthesis project which presented in Chapter

4. (b) Professor Datong Song and his student Trevor Janes for their insightful comments and involvement in the “Direct Synthesis of CdSe Nanocrystals with Electro-active Ligands” project

vii found in Chapter 5 of this thesis. (c) Professor Axel Guenther and his students Milad Abolhasani,

Parnian Saberi for their collaborations.

I gratefully acknowledge the funding sources that funded my Ph.D. work. First I thank the Egyptian

Ministry of Higher Education for funding my visit to U of T in 20092010 and the Natural Sciences and Engineering Research Council of Canada (NSERC) for funding my PhD work. This financial support has enabled me to focus on my professional and scientific development. It is my hope that our efforts in this thesis can add to a better quality of life throughout the world.

I would like to thank my entire family and long-time friends for their love, support and encouragement. I would like to express profound gratitude and deeply appreciation to my parents and all siblings from whom I got unconditional love and inspiration that shaped my actions and morals.

Last and not least, I am greatly indebted to of my beautiful wife, Marwa Hamza Ibrahim. None of this could have happened without her patience and sacrifices. I am thankful for her endless love and encouragement. Marwa, you are always there cheering me up, supporting me spiritually throughout my good times and bad times of our life. I thank my lovely kids. You are a blessing in my life. Thanks God!!

To you all, without you this thesis would not have been possible. I thank you.

viii

Table of Contents

Abstract…………………………………………………………………………………………..ii

Acknowledgments ...... vi

Table of Contents ...... ix

List of Tables ...... xiv

List of Figures ...... xv

List of Abbreviations ...... xxvi

List of Mathematical Terms ...... xxviii

List of Materials ...... xxix

Chapter 1: Introduction ...... 1

1.1 Motivation and Background: The Global Energy Demand ...... 1

1.2 Fossil Fuels and Climate Change ...... 3

1.3 Renewable Energy Resources ...... 4

1.4 Development of Material Architectures for Solar Energy ...... 6

1.5 Thesis Goal ...... 8

1.6 References ...... 9

Chapter 2: Semiconductors: Review and Theory...... 10

2.1 Semiconductor Conductivity ...... 10

2.2 The interaction of Light with Semiconductors ...... 11

2.3 Semiconductor Crystal Structures ...... 12

2.3.1 The Bravais Lattice ...... 13

2.3.2 The Atom Positions and Crystal Structure ...... 15

2.3.3 Coordination Number ...... 16

2.3.4 Miller Indices ...... 17

2.3.5 Interplanar Spacing ...... 19

2.4 Popular Types of Semiconductor Crystal Structures ...... 20

2.4.1 Wurtzite Crystal Structure ...... 20

2.4.2 Zinc Blende Crystal Structure ...... 22

2.4.3 Perovskite Crystal Structure ...... 23

2.5 The Crystal Structure and Physical Properties of the Material ...... 25

2.6 Crystal Structure and Origin of the Band Gap ...... 26

2.6.1 Drude-Lorentz model-Free Electron Theory ...... 26

2.6.2 Braggs’ Diffraction ...... 28

2.6.3 Kronig-Penney Model (Periodically Repeated Potential Well) ...... 30

2.7 Exciton in Semiconductors ...... 34

2.7.1 Frenkel Exciton ...... 34

2.7.2 Wannier-Mott exciton ...... 36

2.8 Different types of Perovskite materials and Their Applications ...... 40

2.8.1 Inorganic Perovskite Materials ...... 40

2.8.2 Organic-Inorganic Metal Halide-Perovskite Materials ...... 41

2.9 References ...... 45

Chapter 3 Semiconductor Nanocrystals: An Overview ...... 48

3.1 Quantum Confinement: Definition ...... 48

3.2 Major Milestones toward Nanocrystals ...... 49

3.2.1 The Idea of Small Scale ...... 49

3.2.2 Nanometer-Thick Foils Called Quantum Wells (2D Structures) ...... 50

3.2.3 From 2D QW Heterostructures to 1D and to 0D Structures ...... 51

3.2.4 Colloidal Quantum Dots-Wet Chemistry ...... 53

3.3 Discovery of Organometallic Synthesis of QDs: ...... 54

The Hot-injection Approach ...... 54

3.3.1 Progress in Nanocrystals Synthesis Using Hot-injection Approach ...... 56 x

3.4 Synthesis of Semiconductors in the Nanocrystals Regime: The Mechanisms ...... 57

3.4.1 Theory of Homogeneous Nucleation and Growth ...... 58

3.4.2 Heterogeneous Nucleation-Growth: Seeded Growth Method ...... 64

3.5 Drawback of the Conventional Surfactant-Assisted Synthesis ...... 65

3.6 References ...... 66

Chapter 4: Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods ...... 70

4.1 Abstract: ...... 71

4.2 Introduction ...... 72

4.2.1 Titanium Dioxide as Light Harvester ...... 72

4.2.2 Semiconducting Nanocrystals as Sensitizers ...... 73

4.2.3 Advantages of Quantum Dot Sensitized Solar Cells (QDSSCs) ...... 74

4.2.4 QDSSCs Efficiency Challenges ...... 75

4.2.5 Common Methods for Assembling QDs into Mesoporous TiO2 Thin Films ...... 77

4.2.6 Objectives and Approach of Our Work in This Chapter ...... 79

4.3 Experimental Section ...... 80

4.3.1 Materials...... 80

4.3.2 Synthesis of TiO2 nanorod seeds ...... 82

4.3.3 TiO2 Short Rods (30–50 nm): ...... 82

4.3.4 TiO2 Long Rods (80–100 nm): ...... 83

4.3.5 Preparation of TiO2 Seeds Bundled Sample: ...... 83

4.3.6 Synthesis of TiO2/CdSe Nanohybrids...... 84

4.3.7 Synthetic Method for TiO2/CdSe Samples 1 to 4: ...... 84

4.3.8 Synthesis of TiO2/CdSe NHs sample (5) ...... 86

4.3.9 CdSe QDs Control Experiment ...... 86

4.4 Characterization...... 88

4.5 Results and Discussion ...... 89 xi

4.5.1 Morphology Characterization...... 90

4.5.2 Chemical Analyses ...... 95

4.5.3 STEM–EELS Analysis ...... 96

4.5.4 Examination of CdSe Alone as a Control Experiment ...... 99

4.5.5 Optical Properties...... 101

4.5.6 Transient Absorption Analysis ...... 104

4.6 Conclusions ...... 120

4.7 References ...... 121

Chapter 5: Direct Synthesis of CdSe Nanocrystals with Electro-active Ligands ...... 137

5.1 Abstract...... 138

5.2 Introduction ...... 139

5.3 Experimental Methods ...... 141

5.3.1 Materials...... 141

5.3.2 Synthesis of the Electroactive Ligands...... 142

5.3.3 Direct Synthesis of CdSe NCs in Ferrocene Derivatives...... 144

5.3.4 Characterization...... 147

5.4 Results and Discussion ...... 150

5.4.1 Our Approach in This Chapter ...... 150

5.4.2 Synthesis and Characterization of the Electro-active Ligands ...... 151

t 5.4.3 Testing the Stability of FcP(O) Bu2 Ligand at High Temperature ...... 155

5.4.4 Characterization of Precursor Complexes ...... 157

5.4.5 CdSe NCs Directly Synthesized with Electroactive Ligands...... 158

5.4.6 Shape Characterization...... 159

5.4.7 Electroactive Ligand Attached to the Surface of the NCs...... 163

5.5 Conclusions ...... 180

5.6 References ...... 182 xii

Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals ...... 188

6.1 Abstract...... 189

6.2 Introduction ...... 190

6.3 Experimental Section ...... 191

6.3.1 Synthesis of Lead Iodide (PbI2) NCs...... 193

6.3.2 Synthesis of Methyl Ammonium Iodide...... 194

6.3.3 Synthesis of Lead Halide Perovskite...... 195

6.3.4 XRD Samples Preparation ...... 196

6.3.5 Characterization ...... 198

6.4 Results and Discussion ...... 200

6.4.1 Lead Halide NC Synthesis...... 200

6.4.2 Lead Halide Perovskite NC Synthesis...... 204

6.4.3 Perovskite NCs with Different 2D Structures...... 207

6.4.4 Ligand and Solvent Effects on the Optical Properties of Perovskite NCs ...... 209

6.4.5 Powder X-ray Diffraction (XRD) Measurements ...... 209

6.4.6 X-ray Photoelectron Spectroscopy Analysis ...... 213

6.4.7 Transient Absorption Spectroscopy Ananlysis ...... 215

6.4.8 Time-Correlated Single Photon Counting Measurements for 2D Perovskite (n=3) ...... 217

6.5 Conclusion ...... 218

6.6 References ...... 219

Conclusions and Future Directions ...... 225

7.1 Thesis Summary ...... 225

7.2 Future Directions and Closing Remarks ...... 229

7.2.1 TiO2/CdSe Heterostructure NRs ...... 229

7.2.2 Direct Synthesis of the QDs with Electro-active Ligands ...... 230

7.2.3 Perovskite nanocrystallites ...... 233 xiii

List of Tables

Table 4.1. Experimental quantities for Cd- precursors used in each sample……………………87

Table 4.2 Time constants for transient bleach recovery measured for CdSe NRs and TiO2/CdSe hybrid NRs using femtosecond pump-probe experiments………………………………………110

Table 6.3: Amplitudes and Time Constants Obtained from Biexponential Fit to TCSPC Measurements on 2D Perovskite (n=3) Nanocrystal Suspension…….…………………………217

xiv

List of Figures

Figure 1.1. The total world primary energy supply by fuel (Mtoe) in the year 2014. Mtoe is the “tone of oil equivalent”...... 2

Figure 2.1: Energy band diagram for a semiconductor showing the formation of an exciton due to the interaction with light or high-energy electrons. In an exciton, the electron and hole are bound by the electron-hole Coulomb interaction...... 11

Figure 2.2: Elementary unit cells forming the crystal lattice. Drawing of a crystal structure diagram normally represented by replacing each unit cell by point (each with identical environments)...... 12

Figure 2.3: Unit cell with primitive lattice vectors; lengths a,b and c & angles ,  and  ...... 13

Figure 2.4: The four different types of Bravais lattices...... 14

Figure 2.5: Schematic representation of the arrangement possibilities of atoms in a crystal (each atom is represented by equal-sized spheres: (a) a square close packing and (b) close packing structure...... 16

Figure 2.6: Miller indices. (a) Single unit cell with a set of coordinate axes (X, Y, Z) that describe the edges of the unit cell. (b) Low index surfaces of a cubic crystal system (100), (010) and (001)...... 19

Figure 2.7: Wurtzite (ZnS or CdSe) lattice (one unit cell structure). Zinc (or cadmium) atoms are shown in blue, sulfur (or selenium) atoms are shown in yellow, with the lines representing the unit cell of the lattice...... 21

Figure 2.8: Zinc blende (ZnS) lattice (one unit cell structure). Zinc atoms are shown in violet, sulfur atoms are shown in golden yellow, with the lines representing the unit cell of the lattice. 22

2+ + Figure 2.9: Perovskite lattice (one unit cell structure). Larger cation (e.g. Ca or RNH4 ) atoms are shown in golden yellow, smaller cation (e.g. Ti4+ or Pb4+) atoms are shown in red, the oxygen atoms are shown in blue, and the solid red line denotes the cubic structure...... 23

Figure 2.10: Diagram of Bragg’s crystal diffraction where electron waves reflected from atomic planes with a spacing dhkl...... 29

Figure 2.11: Illustration of the Kronig-Penney model. One-dimensional periodic hard-wall potential simplified as a series of potential barriers with identical barrier width and period. The electrons have zero potential at the positive ion site and possess maximum value at the intermediate lattice points...... 32

Figure 2.12: Energy dispersion curve “E-k diagram” of an electron in the Kronig-Penney model. In this E-k diagram, “a” is the lattice constant and the electron reflection occurs at ± π/a. (b) The allowed and forbidden bands are plotted in the resulting energy dispersion E as a function of the superlattice wave-vector k...... 33

Figure 2.13: Two types of exciton; (a) Frenkel excitons; comprising of a strong electron–hole attraction, the electron and the hole are tightly bound to each other within the same or nearest- neighbor unit cells within the crystal. (b) Wannier-Mott excitons; where the Coulomb interaction between the electron and hole is strongly screened by the valence electrons via the large dielectric constant that results in electrons and holes that are only weakly bound...... 35

Figure 2.14: Optical absorption transitions of the Wannier-Mott exciton. The Wannier-Mott exciton levels lie just below the conduction band. The energy states of a Wannier exciton demonstrate both its bound states n = 1 to 3 and the continuum states. Eop is the optical band gap while Eg is the electronic band gap...... 39

Figure 2.15: Schematic structures of the 2D layered organo-lead halide perovskites, where n=1, 2 and 3...... 43

Figure 3.1: Illustration of the mechanisms of monodisperse colloid formation in the framework of LaMer’s theory. The stages of nucleation and growth of monodisperse NCs are represented by the change of the atomic concentration/degree of supersaturation of growth species in solution as a function of time. Cs is the solubility concentration, Cmin is the minimum concentration for nucleation, and Cmax is the critical limiting concentration for nucleation...... 60

Figure 3.2: Change in the Gibbs free energy (G, solid line) as a function of the nucleus radius (r) as a sum of the volume and surface free energies...... 62

xvi

Figure 4.1: Illustration cartoon with TEM images of linker-free TiO2/CdSe nanorods prepared in this chapter...... 71

Figure 4.2: Principle of the quantum dot sensitized solar cells. e denoted to electrons, h is the hole or vacant orbital, VB is the valence band, CB is the conduction band, arrows refer to the direction of electrons in the cell...... 75

Figure 4.3: (a) cartoon to illustrate the nature of hydrophilic nature of TiO2 vs the hydrophobic nature of as-prepared QDs. (b) Illustration of the bifunctional linker technique used to apply QD- sensitizers on the surface of TiO2...... 78

Figure 4.4: Non-hydrolytic Synthesis scheme of TiO2 Nanorods. The three neck flask and reaction system cartoon (top left) drawn by Dr. Tihana Mirkovic...... 84

Figure 4.5: Transmission electron microscope (TEM) images (annular–dark field imaging mode) of isolated (a) short TiO2 NRs (used as a seed for sample 1) and (b) long TiO2 NRs (used as a seed for sample 2, 3 and 4). (c) TEM image of bundled TiO2 (d) HRTEM images of long TiO2 NRs...... 91

Figure 4.6: Annular dark-field STEM images of TiO2/CdSe NRs (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4, respectively...... 92

Figure 4.7: X-ray diffraction patterns of (a) isolated TiO2 (short nanorods, 30-50 nm in length),

(b) bundled TiO2, (c) isolated TiO2/CdSe, and (d) bundled TiO2/CdSe NRs. TiO2 seeds show the anatase structure while the bundled one has lower crystallinity. Both isolated and bundled

TiO2/CdSe samples have the wurtzite structure after the CdSe deposition...... 93

Figure 4.8: (a and b) HRTEM images of isolated (sample 1) and bundled (sample 5) TiO2/CdSe NRs, respectively...... 94

Figure 4.9: (a) Energy-dispersive X-ray spectrum (EDX) of TiO2/CdSe heterostructured NRs

(sample #5). A STEM image of TiO2/CdSe NRs: The rectangular box is for EDX analysis corresponding to the spectrum. (b) EDS of TiO2/CdSe NRs (sample #2): The inset shows an area selected for EDX analysis...... 95

xvii

Figure 4.10: The STEM and EELS spectra from TiO2 NRs sample which used as a seed for the hybrid. The spectrum shows the titanium’s L edge and the oxygen’s K edge in TiO2...... 97

Figure 4.11: The STEM (left) and EELS spectrum (right) from TiO2/CdSe NRs sample 3 hybrid.

The spectrum shows the titanium L edge and the oxygen K edge in TiO2 plus the M edge in case of Cd of CdSe part...... 97

Figure 4.12: Region of analysis for a TiO2/CdSe hybrid rod: a) STEM with regions identified as 1, 2 and 3 in the field of analysis. b) EELS spectra demonstrating the presence of Ti and oxygen in region 1, Cd in region 2 and Ti and oxygen within region 3...... 98

Figure 4.13: Annular–dark–field STEM image of (a) TiO2 NRs as prepared, (b) TiO2 NRs mixture with Cd-precursor after heating at 270 °C and before adding the Se–precursor directly, (c) TiO2/

CdSe sample 2 formed after adding the Se-precursor to TiO2 shown in (b) and (d) CdSe sample prepared under the same conditions used for sample 2, but without adding TiO2 NR seeds. .... 100

Figure 4.14: (a and b) the brightness of the CdSe QDs against the heterostructure in day light and under UV illumination, respectively. (c) represents sketch of TiO2 and CdSe band offset the estimated mechanism of the charge separation inside the heterostructure sample...... 101

Figure 4.15: Solution phase (a) UV–vis absorption spectra of TiO2 (red) and TiO2/CdSe NRs (blue). (b) PL spectra of CdSe QDs prepared alone and compared to the mixture of this CdSe with titanium isopropoxide, and a previously prepared TiO2 NRs. (c) and (d) UV–vis absorption (blue) and PL (red) spectra of TiO2/CdSe NRs of samples 1 and 2, respectively...... 102

Figure 4.16: (a and b) Transient absorption spectra of TiO2/CdSe NRs for samples 2 and 1, respectively, recorded by a white light probe pulse after femtosecond excitation at 620 nm and 650 nm, respectively. (c and d) Decay of the bleach of TiO2/CdSe NRs (samples 2 and 1) in toluene following pulsed excitation...... 106

Figure 4.17: (a and b) Transient absorption spectra of TiO2/CdSe NRs for samples 3 and 4, respectively, recorded by a white light probe pulse after femtosecond excitation at 620 nm. (c and d) Decay of the bleach of TiO2/CdSe NRs (samples 3 and 4) assemblies in toluene following pulsed excitation...... 108

xviii

Figure 4.18: (a and b) Transient absorption spectra of TiO2/CdSe NRs for (sample 5) and CdSe NRs recorded by a white light probe pulse after femtosecond excitation at 620 nm and 585 nm, respectively. (c and d) Decay of the bleach of TiO2/CdSe NRs (sample 5) and CdSe QDs assemblies in toluene following pulsed excitation...... 109

Figure 4.19: Fitting results of transient absorption spectra of CdSe/TiO2 NRs sample #2 with tri- Gaussian functions at different time delay. (a, d, g) shows the amplitude, (b, e, h) shows peak energy and (c, f, i) shows the width of tree Gaussian functions, respectively...... 112

Figure 4.20: Fitting results of transient absorption spectra of CdSe/TiO2 #1 with tri-Gaussian functions at different time delay. (a, d, g) shows the amplitude, (b, e, h) shows peak energy and (c, f, i) shows the width of tree Gaussian functions, respectively...... 113

Figure 4.21: Fitting results of transient absorption spectra of CdSe/TiO2 #3 with tri-Gaussian functions at different time delay. (a, d, g) shows the amplitude, (b, e, h) shows peak energy and (c, f, i) shows the width of tree Gaussian functions, respectively...... 114

Figure 4.22: Fitting results of transient absorption spectra of CdSe/TiO2 #4 with tri-Gaussian functions at different time delay. (a, d, g) shows the amplitude, (b, e, h) shows peak energy and (c, f, i) shows the width of tree Gaussian functions, respectively...... 115

Figure 4.23: Fitting results of transient absorption spectra of CdSe/TiO2 #5 with tri-Gaussian functions at different time delay. (a, d, g) shows the amplitude, (b, e, h) shows peak energy and (c, f, i) shows the width of tree Gaussian functions, respectively...... 116

Figure 4.24: Fitting results of transient absorption spectra of CdSe NRs with tri-Gaussian functions at different time delay. (a, d, g) shows the amplitude, (b, e, h) shows peak energy and (c, f, i) shows the width of tree Gaussian functions, respectively...... 117

Figure 4.25: Relaxation models of excited carriers observed at (a) X0 bleach and (b) X2 bleach for

TiO2/CdSe NRs...... 118

Figure 5.1: CdSe nanocrystals are prepared directly with their surfaces passivated by electro- active ligands like ferrocene phosphine and phosphinoxide derivatives...... 138

t Figure 5.2: Photograph of unbound FcP(O) Bu2 ligand prepared in this work...... 144 xix

Figure 5.3: Schematic approach of our synthesis of CdSe capped with electro-active ligands. Schlenck flask is drawn by Dr. Tihana Mirkovic...... 146

1 31 t Figure 5.4: HNMRs and PNMR of di-tert-butylphosphinylferrocene (FcP(O) Bu2) in CDCl3 ...... 152

1 31 t Figure 5.5: HNMRs (a) and PNMR (b) of di-tert-butylphosphinoferrocene (FcP Bu2)...... 153

t Figure 5.6: Mass spectrum of unbound free FcP(O) Bu2 ligand, Molecular weight of 346.12 gram/mole...... 154

t Figure 5.7: Single crystal XRD of FcP(O) Bu2 crystals...... 154

t Figure 5.8: TLC of unbound FcP(O) Bu2 ligand dissolved in 1-octadecene at different temperatures (right spot) versus the same mixture at room temperature (left spot)...... 155

Figure 5.9: Thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) t measurement of the FcP(O) Bu2 ligand...... 156

Figure 5.10: 1HNMRs (a) and 31PNMR (b) of Se-precursor, the di-tert-butylphosphinoferrocene t complex with Se (FcP(Se) Bu2)...... 157

t Figure 5.11: Mass spectrum of Se-precursor, the Se-FcP Bu2 complex. Peak at 411.0441 m/z t indicate the formation of the FcP(Se) Bu2 complex...... 158

Figure 5.12: Cartoon illustration of the inorganic core of the new QDs and the nature of their surface...... 159

Figure 5.13: Low-magnification transmission electron micrograph (TEM) analysis for CdSe t t capped with capped with ferrocene derivatives (FcP(O) Bu2 and FcP Bu2); directly synthesized in

FcPO(tBu2) at 250 °C with the size of (a) 3.5 nm (b) 6–7 nm. Inset: High-resolution electron microscopy (HRTEM), (c) SAED of as-prepared CdSe together with the simulated electron diffraction patterns pattern of the CdSe hexagonal structure and (d) EDX analysis of CdSe nanocrystals (size: 6–7 nm)...... 160

t Figure 5.14: X-ray diffraction patterns of unbound FcP(O) Bu2 ligand...... 162

t Figure 5.15: X-ray diffraction patterns of CdSe-Fc(O) Bu2 NCs...... 162

t Figure 5.16. UV-vis absorption spectra of FcP(O) Bu2 in toluene (yellow), CdSe NC (synthesized t in FcP(O) Bu2) in toluene (red). The ferrocene absorption peak associated with the CdSe NCs is red shifted from the original pure ligand by 0.155 eV...... 164

t Figure 5.17. FTIR transmittance spectra: FcPO( Bu2) (blue line) and CdSe NCs dispersed in KBr (green line). The characteristic sym. and anti-sym. absorption bands of P=O stretching of t FcP(O) Bu2 were shifted to lower wavenumber and broadened indicating complexation between the P=O moiety and the CdSe surface...... 166

Figure 5.18. Representative X-ray photoelectron spectroscopy (XPS) analysis of CdSe NCs as prepared with ferrocene derivative ligands. (a) Typical XPS survey spectra of CdSe nanocrystals bound to Si/SiO2 wafer using a monochromatic Al K-Alpha X-ray source. (b) Enlargement of the low-energy region of the CdSe NCs survey spectrum (binding energy range 0 - 250 eV) showing the overlap between the selenium peaks with the iron and phosphorous peaks...... 167

Figure 5.19. XPS data for Cd 3d, Se 3d core levels, respectively, for the CdSe with 7 and 3 nm highlighting their transitions...... 169

Figure 5.20. The high resolution XPS data for P 2p core level comparison between CdSe-capped t t with FcP Bu2 with sizes of 7 and 3 nm and the free FcP Bu2 ligand, highlighting their transitions...... 170

Figure 5.21. The high resolution XPS data for Fe 2p core level comparison between CdSe-capped t t with FcP Bu2 with sizes of 7 and 3 nm and the free FcP Bu2 ligand, highlighting their transitions...... 171

Figure 5.22. 1H NMR spectra of free and bound ligands with chemical shifts indicated in ppm. (a) 1 t 1 t HNMR of free FcPO( Bu2) ligand in CDCl3. (b) ) HNMR of bound FcPO( Bu2) ligand in d- toluene; the peaks of 4.77, 4.82 and 5.48 ppm correspond to the protons of the ferrocene cyclopentadienyl rings...... 173

t Figure 5.23: Mass spectrum of CdSe-FcP(O) Bu2 NCs samples washed by acetonitrile/heptane mixture...... 174

xxi

t t Figure 5.24. Cyclic voltammogram: CV response of FcP(O) Bu2 and FcP Bu2 free ligands. ... 176

t Figure 5.25. Cyclic voltammogram: CV response of CdSe- FcP(O) Bu2 and CdSe-TOPO NCs...... 177

t Figure 5.26. The energy alignment diagram between CdSe NCs and the FcP(O) Bu2 molecule which determined by the cyclic voltammetry...... 178

t Figure 5.27. Transient absorption kinetics of the excitonic bleach of CdSe–FcP(O) Bu2 (orange), CdSe–TOPO (black), and CdSe–OA (red) NCs. All kinetics are nonmonoexponential. The CdSe– TOPO and CdSe–OA NCs exhibit a weighted-average decay of ca. 1 and >> 2 ns, respectively, t whereas the CdSe–FcP(O) Bu2 NCs exhibit an initial rapid decay of ca. 2 ps...... 179

Figure 6.1: Colloidally stable suspensions of lead halide perovskite NCs are prepared from high quality PbX2 NCs seeds...... 189

Figure 6.2: Photograph of a colloidal lead iodide NC suspension and the corresponding 2D organo-lead iodide perovskite NCs. a, Photograph of a colloidal lead iodide NC suspension in toluene and the corresponding 2D organo-lead iodide perovskite (n= 3) NCs in mixture of toluene: chloroform (2:1) under exposure to ambient and UV light. (b) Photograph of a colloidal 2D perovskite (n=3) suspension in a mixture of chloroform and toluene...... 196

Figure 6.3: Structural, optical, and chemical characterization of lead iodide nanocrystals. a-c, o TEM images of PbI2 NCs prepared at 200 C at a growth interval time of (a) 5, (b) 15 and (c) 30 minutes exhibiting a diameter of 3.4- 4.2 nm, 4.7-5.8 nm and 9.5-10 nm, respectively. d, High- resolution electron microscopy (HRTEM) of the PbI2 NCs prepared in this work, that shows a lattice parameter of 0.34 nm. e, Absorption spectra of colloidal PbI2 NCs in toluene solution where the nanocrystal size has been varied from 2 to 6 nm diameter and clearly exhibiting a quantum confinement effect. f, EDX spectra obtained from 6 nm sized nanocrystals of PbI2. The Cu signals arise from the TEM grid...... 201

Figure 6.4: Transmission Electron Microscopy of PbI2 NCs. a, TEM images of PbI2 NCs prepared o at 160 C. b,c, High-resolution electron microscopy (HRTEM) of the PbI2 NCs. d, Fast Fourier transform (FFT) of the HRTEM image showing the (101), (102), (111) and (004) lattice fringes...... 202

xxii

Figure 6.5: Experimental selected area electron diffraction (SAED) patterns of PbI2 NCs. SAED image of PbI2 NCs of size 10 nm prepared at 200 oC, 30 minutes growth...... 203

Figure 6.6: Experimental and simulated selected area electron diffraction (SAED) patterns of PbI2

NCs. a, Comparison of the SAED of PbI2 NCs and the simulated XRD patterns of the PbI2 hexagonal structure (PDF card no. 43-1484). b, Integrated SAED and simulated XRD patterns of

PbI2 NCs. The simulation of the SAED was performed using open-source software Diffraction Ring Profiler...... 204

Figure 6.7. Structural and optical properties of 2D organo-lead iodide perovskite nanocrystals. a- c, TEM images of the 2D organo-lead iodide perovskite (n=3) NCs prepared from PbI2 NCs that exhibit a diameter of (a) 3.5, (b) 6 and (c) 10 nm, respectively. d, Normalized absorption and PL spectra of 2D lead iodide perovskite (n=3) nanocrystals in a toluene/chloroform mixture as prepared from PbI2 NCs of three different diameters; 3 (green line), 5 (blue line) and 10 nm (orange line)...... 206

Figure 6.8: 2D layered perovskite nanocrystals. a, Schematic structures of the colloidal 2D organo-lead iodide perovskites NCs (C18H35NH3)2(CH3NH3)n-1[PbnI3n+1] synthesized in the present study, where n=1, 2 and 3. b, Photograph of colloidal 2D organo-lead iodide perovskite NC solutions of (from left to right) n= 1, 2 and 3 respectively, under ambient light. c, Absorption spectrum of colloidal PbI2 NCs in toluene with a diameter of 4–5 nm compared to the absorption and emission spectra of the resultant 2D lead iodide perovskite (n=3) colloidal nanocrystal solution in toluene/chloroform mixture. All spectra have been normalized to the lowest-energy optical transition. The 2D lead iodide perovskite (n=3) nanocrystal absorption exhibits an onset at 593 nm. Energy bandgap estimated from the intersection point of the normalized curve was 2.09 eV. The steady-state photoluminescence spectrum (dotted red) consists of a single peak slightly redshifted from the excitonic peak with a maximum at 607 nm. The photoluminescence was measured with an excitation wavelength of 480 nm. Absorption spectra of the three 2D organo- lead iodide perovskite samples (blue, green and red for n= 1, 2 and 3, respectively), and steady- state photoluminescence that peaks at 520, 580 and 610 nm (dotted blue, green and red lines for n= 1, 2 and 3, respectively). The photoluminescence was measured with an excitation wavelength of 480 nm...... 208

xxiii

Figure 6.9: Absorption spectra of perovskite NCs synthesized with different ligands and solvent mixture ratios. a, Absorption spectra of perovskite NCs synthesized with butyl-, octyl-, and olelylammonium ligands. b, Absorption spectra of perovskite NCs synthesized with olelylammonium ligand with different solvent mixture ratios, i.e., toluene (T) to chloroform (CF) = 2:1, 1:1 and 1:1.5 v/v ratio...... 209

Figure 6.10: Powder X-ray diffraction (XRD) measurements of PbI2 and the corresponding 3D organo-lead iodide perovskite NCs CH3NH3PbI3. a, X–ray diffractogram of PbI2 after washing by methanol (red) and after the transformation to lead halide perovskite (blue). The blue line indicates the XRD results for perovskite powder, which was deposited on a silicon substrate at room temperature from a concentrated colloidal NCs solution without any further treatment, while PbI2 NCs were precipitated using a methanol/heptane mixture before centrifugation and collection for measurements. Perovskite reflection peaks are assigned to the tetragonal CH3NH3PbI3 perovskite crystal lattice except the peaks at 19 and 29 degrees coming from the unreacted precursors. * denote diffraction peaks corresponding to unreacted precursors and not from the perovskite. The plot displays the X–ray intensity as a function of twice the diffraction angle (2). b, X–ray diffractogram of PbI2 films deposited by spin coating without washing (black line) and the calculated XRD spectra of PbI2 hexagonal phase (red line)...... 210

Figure 6.11: Powder X-ray diffraction (XRD) measurements of 2D organo-lead iodide perovskite NCs (n=1, 2 and 3). a, X–ray diffractogram of the 2D lead halide perovskite (n=1 green line, n=2 orange line, and n=3 red line). The XRD results for 2D perovskite (n=1 and 2) powder, which was deposited on a silicon substrate at room temperature from a concentrated colloidal NCs solution without any further treatment, while 2D perovskite (n=3) was spin coated on a glass. Perovskite reflection peaks are assigned to the orthorhombic perovskite crystal lattice which in agreement with ref. (37).The plot displays the X–ray intensity as a function of twice the diffraction angle (2)...... 212

Figure 6.12. Representative X–ray photoelectron spectroscopy (XPS) analysis of a, Pb 4f core level and b, I 3d core level of 2 D perovskite film deposited from evaporated NCs solution. XPS showing the Pb(II) 4f7/2 and 4f5/2 transitions with a binding energy of 137.8 eV and 142.7 eV respectively with no metallic Pb component observed, and I 3d5/2 and 3d3/2 transitions with a binding energy of 619.28 and 630.78 eV, respectively. c and d, Gaussian-Lorentzian peak shape

xxiv fitting along with a ‘Smart’ background of Pb 4f core level and I 3d core level of 2 D perovskite NCs...... 214

Figure 6.13. X–ray photoelectron spectroscopy (XPS) analysis of PbI2 NCs. a, Gaussian- Lorentzian peak shape fitting along with a ‘Smart’ background of a, Pb 4f core level and b, I 3d core level of PbI2 NCs film deposited from evaporated NCs solution. XPS showing the Pb(II) 4f7/2 and 4f5/2 transitions with a binding energy of 138.5 eV and 143.42 eV respectively with no metallic

Pb component observed, and I 3d5/2 and 3d3/2 transitions with a binding energy of 619.18 and 630.68 eV, respectively...... 215

Figure 6.14. Transient absorption dynamics of perovskite nanocrystals. a, Transient absorption spectra of perovskite nanocrystals in toluene:chloroform mixture (2:1). The corresponding pump- probe time delays are indicated in the legend. b, Decay-associated spectra obtained from a global analysis of the transient absorption data. The spectra have been normalized to the most intense bleach feature. The inset displays the decay-associated spectra without normalization. c, Dynamics of GSB and PIA at 599 and 530 nm, respectively. The square’s and x’s represent the experimental data while the lines represent the fits obtained from the global analysis...... 216

Figure 6.15. Time-Correlated Single Photon Counting (TCSPC) Measurements for 2D Perovskite (n=3). TCSPC measurements of the 2D perovskite (n=3) nanocrystal suspension. The excitation wavelength was 374 nm and the emission wavelength was 646 nm. Overlaying the data (black) is a biexponential fit (red) with time constants of 14 and 46 ns...... 217

xxv

List of Abbreviations

NCs Colloidal semiconductor nanocrystals

QDs Quantum dots: a term refer to colloidal semiconductor nanocrystals

NRs nanorods

IEA International Energy Agency

EIA the U.S. Energy Information Administration

VB the valence band

CB the conduction band

(hkl) Miller indices: notation vectors in the 3-dimensional lattice through the, where h, k, and l are reciprocals of the point at which the vector exits the unit cell nm nanometer: a unit refer to the small size regime

WLED white light emitting diodes

QW quantum well structure

TEM transmission electron microscopy

HRTEM high–resolution transmission electron microscopy

STEM scanning transmission electron microscopy

EELS electron energy loss spectroscopy

TA femtosecond transient absorption

PXRD powder X-ray diffraction

xxvi

EDX energy–dispersive X-ray

SILAR successive ionic layer adsorption and reaction

UV-vis UV-vis absorption spectroscopy

PL photoluminescence

XPS X-ray photoelectron spectroscopy

CV cyclic Voltammetry

NMR nuclear magnetic resonance spectroscopy

1HNMR proton NMR

31P NMR phosphorous NMR

TGA thermogravimetric analysis

DSC differential scanning calorimetry

SAED the selected area electron diffraction pattern

SEM-EDX scanning electron microscopy energy-dispersive X-ray spectroscopy

FTIR Fourier transform infrared spectroscopy

TCSPC the time-correlated single photon counting

DSSC dye-sensitized solar cell

QDSSCs Quantum Dot Sensitized Solar Cells

xxvii

List of Mathematical Terms

Å angstrom

 wavelength

aB The Exciton Bohr radius

휀 dielectric constant

휀표 electromagnetic permittivity constant of free space

휀∞ is dielectric constant of the material

ħ the reduced Planck's constant (Planck’s constant divided by 2π)

푒 the electromagnetic elementary charge (electric charge carried by electron/hole; equal to 1.602×10−19 coulombs)

∗ 푚푒 the effective mass of electron

∗ 푚ℎ the effective mass of holes

xxviii

List of Materials

TiO2 titanium dioxide

CdSe cadmium selenide

TTIP titanium isopropoxide

CdO cadmium oxide

Se selenium (powder)

OA oleic acid

LA lauric acid

OLA oleylamine

ODE 1–octadecene

TOPO trioctylphosphine oxide

TOP trioctylphosphine

TDPA tetradecyl phosphonic acid

HPA n–hexyl phosphonic acid

OPA octyl phosphonic acid

t FcP Bu2 di-tert-butylphosphinoferrocene

t FcP(O) Bu2 di-tert-butylphosphinylferrocene

PbI2 lead iodide

xxix

PbBr2 lead bromide

Pb(II)O lead oxide

MAI methyl ammonium iodide

TBAI tetrabutylammonium iodide

TBAB tetrabutylammomium bromide

xxx Introduction Motivation and Background

Chapter 1: Introduction

1.1 Motivation and Background: The Global Energy Demand

All modern necessities and technological improvements are aimed at higher levels of convenience—and all depend on one form of energy or another. As developed and developing nations all continue to strive for a better quality of life, consumer demand for energy across the globe increases. Energy is the dynamic element driving economic growth and prosperity. All routes towards improving the quality of life for people rely on energy. This becomes clearer if one imagines in our daily lives the food we eat, the medical system keeping us healthy, the means of transportation we use to reach our work, the industrial system that provides a better quality of life, the electricity lighting our houses and all electronic devices that keep us connected and entertained; all of these systems require energy.

There are several causes that fire up the worldwide energy demand. These sources include industrialization, especially in emerging economies/markets (e.g. China and India), globalization and transportation. Use of petroleum-based fuels in transportation represent the largest energy consumption (~60%).

According to calculations reported by Nathan Lewis and Daniel Nocera (2006)1-2, the global demand for energy at the current usage rate of 14 terawatt/year (TW/yr) is projected to more than double by 2050. In addition, the World Energy Outlook, published in 2014 by the International

Energy Agency (IEA), estimated that the “global energy demand” will grow up to 37% by 2040, with an average annual rate of growth of 1.1%. Similarly, increases in energy demand rates close to ~40% by 2035, compared to the 2013 levels, have been estimated by the U.S. Energy

Information Administration (EIA).3 More than two-thirds of the increase in world energy

1 Introduction Motivation and Background consumption will come from developing countries because their economic and population growth are the highest.

Figure 1.1 shows a pie chart of the total world primary energy supply in 2014—both renewable and nonrenewable sources—reported in “2014 Key World Energy Statistics” by the International

Energy Agency (IEA).4 From the chart, one can see that the major energy sources (~80%) coming from the non-renewable (fossil fuels—oil, coal, and natural gas) sources will deplete with time and use. Both the facts that the rising global energy demand and the main resources being non- renewable present formidable challenges to the world. In addition, the non-renewable sources are limited and are depleting at a faster rate, adding more challenges to the difficulties in affordability, non-sustainability and supply security.

Figure 1.1. The total world primary energy supply by fuel (Mtoe) in the year 2014. Mtoe is the “tone of oil equivalent”.

2 Introduction Motivation and Background

1.2 Fossil Fuels and Climate Change

Environmentally, the biggest challenge is the greenhouse gases (mainly carbon dioxide, CO2)

4 emitted from fossil fuels and therefore the threat of “Climate Change”. Typically, CO2 is the side product from total combustion of the hydrocarbon fossil fuels, which is emitted to the atmosphere.

Here, CO2 is accountable as an important heat-trapping (greenhouse) gas. Briefly, as the sun strikes the earth with sunlight irradiation warming its surface, the earth re-emits this radiation as heat.

There are certain gases in the atmosphere including CO2— known as natural greenhouse gases, their job is to block this sun heat from escaping.

The US National Aeronautics and Space Administration (NASA) provides the definition of greenhouse gases as:

“Life on Earth depends on energy coming from the sun. About half the light reaching Earth's atmosphere passes through the air and clouds to the surface, where it is absorbed and then radiated upward in the form of infrared heat. About 90 percent of this heat is then absorbed by the greenhouse gases and radiated back toward the surface, which is warmed to a life-supporting average of 59 degrees Fahrenheit (15 degrees Celsius)”.5

As reported by NASA, since the advent of the “Industrial Revolution”, human activities have increased the atmospheric CO2 concentration by a third. Increasing the content of CO2 in the atmosphere will eventually make the earth much warmer which is known as the greenhouse effect.5

Practically, a stronger greenhouse effect will heat up the oceans and partially melt glacial masses and finally lead to increasing sea levels—i.e. “Global Warming Effect”. Accordingly, if the human continue to depend on the dwindling non-renewable fossil fuel, the environmental deterioration will have progressed too far to be reversible.

3 Introduction Motivation and Background

Taking into account all previously mentioned facts and factors (especially the Global Warming

Effect against the up growing energy demand), the world needs to make every effort in pursuing advancements in the ongoing search for alternative energy resources that would adequately cover energy demands. Typically, in order to meet our aspirations for a better quality of life for all, as well as the growing economic needs, global societies must strive to create solutions that extend the global energy supply networks and achieve environmental objectives by taking practical and cost-effective actions that do not contribute to the environmental problems. This means we must focus on searching for sustainable, cheap and clean energy conversion, as well as high-density energy storage systems.

1.3 Renewable Energy Resources

There are seven different renewable energy sources; solar energy, wind energy, hydropower, biomass energy, hydrogen, geothermal energy and ocean energy. These resources originate from the sun either directly or indirectly. All of these resources are considered as sources that emit no

CO2.

Renewable energy resources (solar, wind, and geothermal) have multiple advantages such as: (1) they are clean sources of energy that have a much lower environmental impact than conventional energy resources—i.e. no air pollution or hazardous waste, (2) they are sustainable and secure resources that can be constantly replenished, i.e., will not run out, (3) as part of new and growing technological environments, they support job creation and strengthening the economy, and (4) these renewable resources are free and abundant.

4 Introduction Motivation and Background

In the white paper6 reported in 2006 by Jeff Tsao (previously from the U.S. Department of Energy,

Office of Basic Energy Science and currently located at Sandia National Laboratories), Nate Lewis

(California Institute of Technology) and George Crabtree (Argonne National Laboratory), a series of questions were discussed and answers were provided regarding the potential of the sun as a supplier of energy to the world. In this initial short report and the subsequent detailed report, these researchers provide calculations for the global energy need and the amount of demand every renewable energy resource can fulfill in future. According to these two reports, there are four renewable sources with theoretical potentials exceeding 15 TW: solar, wind, geothermal, and ocean waves, where the two extractable potentials exceeding 15 TW are solar and wind. Wind power, however, has lower technical potential mostly due to most of its power resides over the relatively inaccessible deep oceans.

Practically speaking, sunlight by far provides the largest source of clean and abundant energy of all the carbon-neutral energy sources. More energy from sunlight strikes the Earth in one hour (4.3

× 1020 J) than all the energy consumed on the planet in a year (4.1 × 1020 J).6 This value was calculated after taking into account the surface area of the earth intercepting the sun’s flux and deducting the scattered and absorbed flux in its path to earth. This means the theoretical potential of the sun is 89 TW/yr. Therefore, solar energy could be used to generate 15 TW of C-neutral

power from 10%-efficient solar-conversion systems covering only 0.17% of the earth’s surface

2 area (~858,792 km ).

Solar energy is a form of electromagnetic radiation. Solar energy can be harvested directly in three different ways:

(i) Solar electricity, where the sun photoenergy is converted into electricity using a method

called “photovoltaic”, where electricity is produced by semiconductor solar cells.

5 Introduction Motivation and Background

(ii) Solar fuels, where solar energy is converted into chemical fuel. Solar fuels include

biomass (known as non-fossilized solid or liquid organic material derived from

photosynthetic processes of plants) and hydrogen.

(iii) Solar thermal, where solar energy is converted into heat. Examples include commercial

and residential heating use (e.g. providing hot air to low-temperature spaces as well as

utilization in water heating) and thermal-power generated electricity.

1.4 Development of Material Architectures for Solar Energy

Utilizing solar energy in any of the three aforementioned processes combines the three functional steps of capture, conversion, and storage. In order for one of these three processes to provide an effective commercial potential that meets the global clean energy demand, the process should be able to harvest solar energy with high efficiency and have affordable manufacturing costs comparable with available fossil fuel energy resources.7-8

Photovoltaics (solar cells) are one of the main areas where the solar industry and research are concentrating their efforts and focus today. Single-crystal and polycrystalline silicon-based solar cells dominate worldwide photovoltaic technology markets. Although these solar cells are efficient compared to other modules, the manufacturing processing is still expensive. Similarly, in the field of the solar fuels, researchers still cannot split water into oxygen and hydrogen without an expensive platinum catalyst.

Therefore, the development of novel materials for solar energy conversion represents an outstanding challenge towards the development of alternative energy sources.

6 Introduction Motivation and Background

To achieve competitive cost-to-efficiency of alternative energy sources requires breakthroughs in the development of materials for solar energy conversion and/or storage, which is the most significant contribution that can be achieved by researchers. To do so, researchers must continue working towards design and building a new generation of materials that strive to achieve high conversion efficiencies and scale up manufacturing to the global level at low cost.

Materials can be classified into three different types depending on their electrical conductivity; metals, semiconductors, and insulators. Among these three types, only semiconductors are capable of harnessing solar energy. Obtaining cheap semiconductor material having an appropriate band gap to absorb the solar spectrum effectively with the ability to be used in junction formation suitable for efficient energy conversion is highly sought.7-8

It has been proven in the last three decades that advancements in nanometer-scaled semiconductors has provided new directions in science, including: (1) by synthesizing smaller structures, we lower material consumption needed for the reaction and therefore reduce cost, (2) by having control on the nanomaterials using bottom-up approaches, we can control the self-assembling molecules and materials to build efficient structures at high-volume with low-cost production, (3) by reducing the size to nanoscale, we can control the optical properties of materials and the light being absorbed by these materials, and (4) at nanoscale, we can design new materials with hybrid architectures capable of setting electrons into motion efficiently from their surface at conversion efficiencies that exceed those available in conventional bulk materials.

7 Introduction Motivation and Background

1.5 Thesis Goal

The work in this thesis focuses on introducing various innovative and novel synthetic methods of different types of colloidal nanocrystals (NCs). There are two types of NCs presented in this thesis:

(1) Inorganic semiconductors, also known as quantum dots (QDs), in which the NCs are composed of an inorganic semiconductor core that exhibits nanoscale sizes and stabilizers of organic shell ligands. (2) Organic-inorganic metal halide-perovskite materials. These two classes of materials are unique light-harvesting materials. This thesis pursues a fundamental understanding of the synthesis, material properties, and photophysics and spectroscopy of these nanostructured semiconductor materials.

Semiconductor NCs are versatile fluorescence probes with unique optical properties, including tunable luminescence, high extinction coefficient, broad absorption with narrow photoluminescence, and photobleaching resistance. Using these materials as building blocks to design light-harvesting assemblies has drawn a lot of attention in past years.

In this work I aimed morphologically-controlled synthesis and characterization of semiconductor nanomaterials, as well as investigating the relationship between their structures/surfaces and charge transfer properties to outer media. An outstanding challenge towards alternative energy sources is the ability to develop new materials for solar energy conversion that harvest solar energy with high efficiency and have affordable manufacturing costs comparable with available fossil fuel energy resources. To achieve competitive cost-to-efficiency of alternative energy sources requires breakthroughs in the development of materials for solar energy conversion and/or storage, which is the most significant contribution that can be achieved by researchers.

8 Introduction Motivation and Background

1.6 References

1. Lewis, N. S.; Nocera, D. G., Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2007, 104 (50), 20142-20142.

2. Lewis, N. S.; Nocera, D. G., Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103 (43), 15729-15735.

3. http://www.eia.gov/forecasts/aeo/pdf/0383%282015%29.pdf.

4. International Energy Agency, Key World Energy Statistics. 2014, p 6.

5. http://climate.nasa.gov/causes/.

6. http://www.sandia.gov/~jytsao/Solar%20FAQs.pdf.

7. Pensack, R. D. Measuring the Barrier to Charge Transfer State Dissociation in Organic Photovoltaic Materials with Ultrafast Vibrational Spectroscopy. The Pennsylvania State University, 2012.

8. Milliron, D. J. New Materials for Nanocrystal Solar Cells. University of California, Berkeley, 2004.

9 Introduction Semiconductors: Review and Theory

Chapter 2: Semiconductors: Review and Theory

2.1 Semiconductor Conductivity

Semiconductor materials are a class of materials that have the main characteristics (beside others) of moderate electricity conductivity higher than insulators and lower than that of metals. Typically, the metallic conductivity is between 106 and 104 (cm)-1 and the insulators have conductivities of less than 10-10 (cm)-1.1 Semiconductors have conductivity between 104 and 10-10 (cm)-1.1

Semiconductors are either an element such as silicon (Si) and germanium (Ge) or compound semiconductors as gallium arsenide (GaAs), indium phosphide (InP), the cadmium based family of CdE (where E=S, Se, Te), titanium dioxide (TiO2), …etc. Each material takes its characteristics from the structure of its band energies, especially the valence band (the highest occupied band with electrons), the conduction band (the first unoccupied band) and the energy band gap between them.1 Semiconductors have drawn a lot of attention after the discovery of transistors in 1947 due to their tunable conductivity properties. In particular, their conductivity can be changed (mostly raised) by different factors. For example, material state (chemical purity), whereby the conductivity depends on the concentration of the impurities in the material and doping.

Conductivity of the semiconductors rises exponentially with temperature. Last but not least, conductivity can also be raised with irradiation by light or high-energy electron interactions.1

These parameters affect the conductivity since they are able to excite electrons from the valence band (VB) across the band gap to the conduction band (CB). Thus, the conductivity of semiconductors can be tuned by adjusting the density of electrons in the conduction band.

10 Introduction Semiconductors: Review and Theory

2.2 The interaction of Light with Semiconductors

Interaction of light (which consists of electromagnetic radiation with certain energy) with matter is a much-studied area in science that has led to many photochemical and photophysical processes incorporated in our daily lives. When the light with energy higher than the absorption threshold of the semiconductor pass into the semiconductor or interact with it, is strongly absorbed by the semiconductor initiating excitation process of the VB’s electron into the CB leaving a positive hole

(empty orbital) in the VB.2 The photoinduced hole and excited electrons have a columbic interaction binding them together. This bound electron–hole pairing is termed as “exciton” (Error!

Reference source not found.).3 Therefore, the material now exhibits electronic excited states whereby the atomic structure of the molecule remains unchanged, but one (or more) electrons are excited to higher energy orbital, the conduction band (CB). These excited states are the basis of many photochemical, photophysical and biological processes in our lives.2

11 Introduction Semiconductors: Review and Theory

2.3 Semiconductor Crystal Structures

When atoms assemble to form solid material, they take relatively simple atomic arrangements in an infinite periodic replication of identical building blocks in three-dimensional (3D) internal orders. The basic building block of a crystal is called unit cell or basis group —where the unit cell is the smallest group of atoms, ions or molecules. For example, in the CdSe crystal, each two atoms of CdSe represents a unit cell. However, in the case of elemental semiconductors, the unit cell (or the basic group) is a single atom. The repetition of the unit-cell in 3D within the crystal will produce what is called a “crystal lattice”. Error! Reference source not found. represents a diagram of a crystal lattice. Since a crystal is built up of an extremely large number of unit cells, the determination of the spatial arrangement of the atoms within a single unit cell, the size and shape of each unit cell and the spatial arrangement of atoms in 3D with respect to neighboring atoms and the number of surrounding atoms as well as the distance between neighboring atoms and unit cells are all linked to the crystal structure. In theory, repetition of the unit-cell in 3D within the crystal is infinite. However, in practice, their periodicity extends over a distance between ~103 to ~1020 atomic or molecular dimensions, where normally crystal defects take place.4

Figure 2.2: Elementary unit cells forming the crystal lattice. Drawing of a crystal structure diagram normally represented by replacing each unit cell by point (each with identical environments).

12 Introduction Semiconductors: Review and Theory

2.3.1 The Bravais Lattice

In 1848, a French physicist and mathematician Auguste Bravais discovered what is called the

“Bravais lattice”, which is the basic building block that all crystals can be constructed from.

Bravais first proposed the unit cell is the basic building block of the crystal then he studied the crystal structure as a topological problem by replacing each unit cell by point (i.e. each unit cell with identical environments). As a crystal is built of extremely large number of repeated unit cells, he solved the arrangement of unit cells by considering each unit cell as a mathematical point (a unique location or position in Euclidean space). Each point would be surrounded by an identical set of points that would be indistinguishable from each other. He also discovered that there were

14 different collections of the arranged groups of points; these points became known as the Bravais lattice (3D lattice). These 14 lattices are arranged in 3D to form 7 ways in which rotational

(translation) symmetry can lead to infinitely repeatable unit cells. These are the 7 crystal systems

(shapes)—cubic, tetragonal, orthorhombic, trigonal/rhombohedral, hexagonal, monoclinic, and triclinic. The transformation of these 7 groups are based on the distance between points in the unit cell (a, b and c)—dimensions along the three edges—as well as the relationship between the angles

(,  and ) between the sides of the unit cell (Error! Reference source not found.). Hence, the 7

Bravais lattice groups are differentiated based on these six parameters.4

Figure 2.3: Unit cell with primitive lattice vectors; lengths a,b and c & angles ,  and 

13 Introduction Semiconductors: Review and Theory

According to Bravais’ theory, the Bravais lattices contain 7 crystal systems and 4 lattice centering types (Figure 2.4); (a) simple unit cell or primitive unit cell, (b) face-centered cubic lattice, (c) end-centered, and (d) body-centered lattices.

(a) Primitive unit cell is a unit cell having lattice points only at the corners of the unit cell. A crystal lattice having primitive unit cells is called a simple crystal lattice.

(b) End centered lattice is a unit cell having lattice points at the center of only one set of faces as well as at the corners.

(d) Body-centered lattice is a unit cell in which a lattice point located at the center of the body as well as at each corner.

These 14 lattice types are distributed among metals, non-metals (insulators) and semiconductors.

In the case of semiconductors, the face centered cubic system is the most important.

Figure 2.4: The four different types of Bravais lattices.

14 Introduction Semiconductors: Review and Theory

2.3.2 The Atom Positions and Crystal Structure

The method that atoms take to accumulate in the crystal as well as the spatial position of the repeated stacking sequence of layers determines the structure of the crystal being formed. The way in which the atoms (ions) are arranged to build the crystal structure are normally described in terms of packing of equal sized spheres as shown in Figure 2. 5.

Atoms can take two possible arrangements for forming a layer (2D arrangement): (A) a square array of spheres (Figure 2.5a) or (B) a closed packed layer of spheres after squeezing the square layer to achieve less space (Figure 2.5b). In order to obtain maximum density, the close packing approach is mostly utilized. With the aim of forming a close packed structure in the crystal 3D- growth process, a second layer is stacked on top of the first layer, following which layer B is added in a way that each sphere will assemble on the vacancy between the three spheres of layer A to achieve maximum density—i.e. only in contact with the three spheres from layer A— (see Figure

2.5b). The process of adding layer after layer will continue in repetition to form an infinite sheet, until the end of the crystal growth.

There are two ways in which the close packed structure in three dimensional packing can be completed. In this case, the resulting structures can be either hexagonal or cubic close packed structures, depending on the way the third layer is added.

2.3.2.1 Hexagonal close packed structure:

During the generation of hexagonal packing, the third layer will rest on top of layer B in a manner where it is aligned vertically in exactly the same position as layer A, making a sequence of

…..ABABABAB….., resulting in hexagonal close packing (hcp). In this structure type, the

15 Introduction Semiconductors: Review and Theory vacancies been created by deposition of layer B (marked with a red dot in Figure 2.5b) are never occupied by spheres, leaving very small channels through the layers.

2.3.2.2 Cubic close packed structure:

On the other hand, in the case of a second type of close packing structure, the third layer’s spheres may fill the vacant areas—i.e. the thus far unfilled positions—(red spot in Error! Reference source not found.b), forming layer C and resulting in a sequence of …..ABCABCABC……. In this manner, layer C does not exactly cover layer A. This is known as cubic close packing (ccp) or face-centered cubic (fcc), where the unit cell of a cubic close packed structure is in fact that of a face-centered cubic (fcc).

2.3.3 Coordination Number

Since the atoms packed inside the crystal form different crystal structures, the number of the adjacent atoms that in contact with a particular atom will influence the properties of that atom. The

16 Introduction Semiconductors: Review and Theory number of the neighboring atoms/ions around a given atom/ion is termed as the coordination number of that atom.

Both the face-centered cubic (fcc) and hexagonally close packed (hcp) structures have the coordination number of 12; where 6 spheres surround the sphere of interest in one layer as well as

3 from the top layer and 3 from the bottom layer. Therefore, the coordination number of both (fcc) and (hcp) is 12 while the coordination polyhedron is cubic for the former and hexagonal for the latter.

2.3.4 Miller Indices

The process of crystal growth or crystal formation leads to a tendency towards growth in a certain direction versus another. In this energetically favored direction, the crystal forms planes or faces containing a high density of atoms.

In material chemistry, it is important to understand multiple phenomena—such as the mechanisms of crystal formation (nucleation and growth), explaining the shapes of a single crystal, understanding X-ray diffraction patterns, and the mechanical properties of a certain material.

Moreover, it was discovered that some properties of materials (e.g. electrical conductivity and thermal conductivity) can vary in a crystal in relation to orientation. Thus, it is important and useful to be able to identify certain directions and planes (faces) of crystals.

In 1839, William Hallowes Miller (Professor of Mineralogy at Cambridge University) introduced

(for a first time) a description method for the orientation of a plane or set of planes within a lattice in relation to the unit cell. This method is known later as the “Law of Rational Indices”. Here,

Miller states that each face of a crystal (plane) can be described by three rational numbers; these

17 Introduction Semiconductors: Review and Theory three integer numbers are written enclosed in parentheses or square brackets as (hkl) and is known as the crystal direction notation.5

In his method, Miller chose an arbitrary point called point O or (original) at the edges of a selected unit cell in the crystal. This start point of all vectors is assumed as (000). Since three points in 3D can uniquely define a geometric plane, he defined three vectors in the space coordinates (X, Y and

Z directions) in order to define a crystallographic plane (crystal face)— i.e. these vectors are OX,

OY and OZ, see Figure 2.6a. The crystallographic plane intersects each axis of OX, OY and OZ making intercepts a/h, b/k, and c/l with the edges of the unit cell in terms of unit cell dimensions

(lengths a, b, and c)—i.e., the plane has unit intercepts on each coordinate axis. The length of a, b and c is defined as the distance of N number of lattice parameters from the origin (point O). Lattice parameters are the length of the edges of a unit cell in a crystal. It has been found that a consistent designation results if the Miller indices, h, k, and l, are identified as reciprocals of the x, y, and z axis intercepts instead of the intercepts themselves.5-6 Therefore, Miller annotated the vectors in the three dimensional lattice through the notation of (hkl), where h, k, and l are reciprocals of the point at which the vector exits the unit cell. Figure 2.6b shows some examples of the various directions in the cubic unit cell.

In summary, calculating the Miller Indices for a plane requires two steps:

1. Determine the intercepts (a/h, b/k, and c/l) of the face/plane along the crystallographic axes

(OX, OY and OZ) in terms of unit cell dimensions (a, b, and c).

2. Take the reciprocals and reduce them to the lowest integrates—i.e. their greatest common

divisor should be 1. i.e. Miller indices are made up of the lowest combination of integers and represent unit distances rather than actual distances. For instance, the (222) plane is identical to (111). If a crystal face is

18 Introduction Semiconductors: Review and Theory parallel to a crystal axis, its intercept on that axis is at infinity (∞), such that the corresponding

“Miller index” is zero (1/∞). For instance, the intercepts = a, ∞, ∞ and then the reciprocals (hkl)

= (100). Using this scheme, one can identify different planes in a crystal unit cell.

2.3.5 Interplanar Spacing

An important notice one can be taken at this point is that in some crystal structures, different directions can be equivalent (in length and types of atoms encountered), e.g. cubic crystals. The directions are all equivalent by symmetry. Similarly, given any plane in a lattice, there is an infinite set of parallel planes that are equally spaced from each other; all collectively called a family of planes.

Any set of equivalent planes in a crystal is denoted with curly brackets as {hkl}, where they have the same indices regardless of order or sign. In case of cubic crystal, for instance, the faces of the

19 Introduction Semiconductors: Review and Theory cubic unit cell are {001}, the planes orthogonal to the area diagonals are {110}, and the planes orthogonal to the body diagonals are {111}.5

 [hkl] represents a direction.

 represents a family of directions.

 (hkl) represents a plane.

 {hkl} represents a family of planes.

The perpendicular spacing distance between successive parallel lattice planes in a family of planes

(distance between each adjacent pair of planes) is known as the interplanar spacing (dhkl). This distance is measured in angstroms or nanometers. Each material has a set of distinctive dhkl spacing, where they are directly related to lattice parameters and atom location in the crystal.

2.4 Popular Types of Semiconductor Crystal Structures

As the focus of this thesis is on semiconductor nanocrystals, it is important to mention the crystal structures that are regularly represented in semiconductors.

There are 5 types of crystal structures that occur frequently in elemental and compound semiconductors. These are the rocksalt, diamond, zincblende, wurtzite and perovskite structures.

The latter three are present in compound semiconductors.

2.4.1 Wurtzite Crystal Structure

The wurtzite crystal is named after the French chemist Charles-Adolphe Wurtz (1861), when he described the crystal structure of the wurtzite mineral, a form of zinc iron sulfide mineral

((Zn,Fe)S). Certain compound semiconductors are able to crystalize into this type of crystal. The wurtzite structure is a hexagonal crystal with dimensions of a = b 3.82 Å and c = 6.26 Å originally

20 Introduction Semiconductors: Review and Theory composed of a hexagonal closest-packed (hcp) array structure of sulfide ions with alternate tetrahedral holes occupied by zinc ions. This crystal shape (Figure 2.7) has a tetrahedral (four- fold) coordination, i.e. each metal atom (e.g. Zn) is surrounded by 4 non-metal atoms (e.g. S) at the corners of a tetrahedron, and vice versa—having a stoichiometric composition.7-8

Examples of compound semiconductors that crystallize into the wurtzite structure are:

 The II–VI compound semiconductors such as: ZnS, and CdSe.

21 Introduction Semiconductors: Review and Theory

2.4.2 Zinc Blende Crystal Structure

In 1930, Coster, Knol, and Prins identified the configuration of the crystal structure of the ZnS semiconductor giving it the name of zinc blende crystal, also known as a (sphalerite).9-10 The zinc blende structure is a cubic crystal with dimensions of a = b = c = 5.42 Å where the sulfide ions were originally implemented in a cubic closely-packed structure during the growth stages.

Briefly, the zinc blende structure is a face centered cubic (fcc) lattice with two atoms in the base; atom A (metal) at (0, 0, 0) and atom B (nonmetal) at (1/4, 1/4, 1/4)a ,Figure 2.8. Moreover, it could be described as a zinc blende lattice as it consists of two interpenetrating face centered cubic

(fcc) lattices; one lattice consists of (A) type of atoms (e.g. Cd in CdSe), with the other lattice comprising of (B) type of atoms (e.g. Se in CdSe). In the zinc blende structure, both types of atoms

(metal and non-metal) have tetrahedral (four-fold) coordination, i.e. each atom is bonded to 4 opposite ions—having a stoichiometric composition.

Examples of compound semiconductors that crystallize into the zincblende structure are:

 The II–VI compound semiconductors such as: ZnS, ZnSe, HgTe, CdSe and CdTe.

 The III-V compound semiconductors such as: GaAs, InAs, InP, and GaP.

Figure 2.8: Zinc blende (ZnS) lattice (one unit cell structure). Zinc atoms are shown in violet, sulfur atoms are shown in golden yellow, with the lines representing the unit cell of the lattice.

22 Introduction Semiconductors: Review and Theory

2.4.3 Perovskite Crystal Structure

Following the discovery of calcium titanate (CaTiO3) in the Ural mountains of Russia by Gustav

Rose in 1839, this mineral was entitled as perovskite after the Russian mineralogist, L. A.

11 Perovski. Any compound having the same crystal structure as CaTiO3 is known as a perovskite compound. The basic chemical formula of cubic perovskite composites is ABX3, where A and B are cations of different sizes; A cation is larger than cation B while X is usually oxygen but may also be fluorine, chlorine, nitrogen, sulphur or carbon. Ideally, the structure of perovskite compounds comprise a cubic structure with the larger A cation (Ca, Ba or Sr); (charge state 2+), on the corner positions of the cube (0, 0, 0) and the smaller cation B, e.g. (Ti4+), in the centre of the body (1/2, 1/2, 1/2) while the O ions (2−) are located in the centre of each cubic face (1/2, 1/2, 0).

Therefore, A-cation coordinated by 12 oxygen anions while the B-cation in the octahedral interstices,12 Figure 2.9Error! Reference source not found.. This BX6 octahedra are corner- connected in order to form a three-dimensional framework.

This ideal crystal structure of perovskite normally refers to a face centred cubic crystal of an arrangement. In the process of perfect close packing process of this ideal structure, the A cation

23 Introduction Semiconductors: Review and Theory must fit into a space surrounded by the four adjacent octahedral which were connected by the shared corners. For such a limit space if the A cation size is too large, it cannot be fit into the hole.

Therefore, the perfectly packed structure depends on the cations ionic radii. In early 1920s, when

Victor Goldschmidt studied the geochemistry of crystal structure of elements, he proposed a tolerance factor (t) that present the geometrical relationship between these three components of

13 ABX3 structure. Goldschmidt’s tolerance factor (t), defined by the equation (1):

r + r 푡 = 퐴 푋 ≈ 1 (1) √2 (푟퐵+r푋)

where t is the tolerance factor, r퐴, 푟퐵, and r푋 are the ionic radii of the elements A, B and X, respectively.

According to the Goldschmidt, the stability of the perovskite structure depend on this ratio of the distance (A−X) to (B−X). For most cubic perovskite, t is in the range of approximately 0.78 < t <

1.05. The t value of an ideal perovskite should be 1.0; however, experimentally, the t value for most cubic perovskites is in the range 0.8–0.9. The crystal structure of perovskites are distorted of the ideal structure for different reasons.

There are five different reasons that make distortion from the ideal cubic structure14: (i) distortion of the MX6 octahedron by the Jahn–Teller effect, (ii) off-center displacement of the M cations in the MX6 octahedron, (iii) tilting of the octahedron framework, because of a too small A cation at the cuboctahedral site, (iv) ordering of more than one kind of cations A or M, and (v) ordering of more than one kind of anions X, or vacancies. The physical properties such as the electronic, magnetic and dielectric properties of perovskites depend significantly on the details of these distortions.

24 Introduction Semiconductors: Review and Theory

Although the tolerance factor is essential for perovskite formability, it is proven that tolerance factor not sufficient condition for the perovskite structure stability. It is found that the relationship between the small B cation ionic radii to the X anion ionic radii is another an important parameter, known as the octahedral factor. This is due to the octahedron [BX6] is the repeated unit cell in the crystal formation. This octahedral factor (μ) defined as the ratio of radii of cation B to the radii of

X anion.

푟 (휇) = 퐵 ≈ 0.425 − 0.6785 (2) 푟푋

Perovskite materials can occur also in the orthorhombic and tetragonal phases as non-cubic variants. More details about perovskite different types are present in section 2.8 of this thesis.

2.5 The Crystal Structure and Physical Properties of the Material

To understand the effect of the crystal structure on the net physical properties of a certain type of matter, one needs to recall the following concepts:

1. As mentioned earlier, the atoms or molecules are stacked together to form the crystal structure during the crystal growth process. In this process of packing, the stacking faults or the defects

(imperfection) which result from disorders in the stacking sequence lead to deviation from the ideally perfect crystal.

2. Solid crystals are classified according to the symmetry of regularity of the atomic building blocks arrangement (lattice), whereby they are symmetric or asymmetric. Most types of semiconductors are highly symmetric crystals.

25 Introduction Semiconductors: Review and Theory

3. The nature of the electronic properties on any matter (atoms or molecules) depends on the electronic states—i.e. electron density distribution, motion and the electron potentials.

4. Since crystals are states of matter self-assembled from the same kind of atom/molecule that are systematically repeated within a spatial space, a solid crystal is, of course, a much larger and extended collection of atoms. Therefore, the electronic structure of a crystal depends on the nature of the electronic structure of atoms as well as the formation of bonds between pairs of atoms and molecules in the material. Consequently, the crystal structure has a relation to the nature of the optical and electrical properties of the atoms and molecules in the solid material, whereby the energy band structure and energy band gap are integrally interrelated to the crystal structure of the material. As a result, the electron density in the crystal is an important factor affecting the physical properties of the crystal.

5. There are several different types of bonding are possible inside the crystalline structures.

These bonds include ionic, covalent, or metallic bonding. The structure of a specific crystal is determined by the bonding type, accompanied by the sizes of the atoms involved in the bond formation.

6. The atomic properties together with the real crystal structure describe several physical properties of crystalline materials.

2.6 Crystal Structure and Origin of the Band Gap

2.6.1 Drude-Lorentz model-Free Electron Theory

Questions behind thermal conductivity, heat capacity, and electrical conductivity have been raised in the physics community as early as 1900. That year, Paul Drude proposed the Drude model of

26 Introduction Semiconductors: Review and Theory free electron that described the transport of electrons in materials to explain the electrical conductivity (DC and AC conductivity) of metals. In 1905, Nobel laureate Hendrik Lorentz

(1902 Nobel Prize in Physics) extended this model to what became called the Drude-Lorentz model of free electron theory (classical theory).15 However, this theory fails to differentiate between insulator, semiconductor and conductor (metal) materials.

In this model, the following assumptions are considered15: since the crystal is an assembly of stacked atoms, it is imagined that each atom’s nucleus (positive charge) is in a fixed (non-motion) state shielded by the core electrons leaving the valence electrons, i.e. those coming from the outer shells of the atoms in a free motion process throughout the crystal. Ignoring the nuclei and the core electrons leads to a simple quantum mechanical model of the valence electron motion. Describing only one electron and imagining that it is in a one dimensional lattice would be much simpler to explain. In 1900, Drude used the classical physics of kinetic theory to describe the behavior of electrons in solids. He suggested that the valence electrons are totally free electrons as they are free particles in gas. Hence, electrons obey Maxwell-Boltzmann statistics. As a result, the electrical and thermal conductivity of materials is due to the presence of free electrons transporting freely in the material.

According to this model, the crystalline solid specimen is treated as if it contains two forms of matter; one being the fixed positive atoms and the other the free valence electrons. The latter bounce and re-bounce between atoms in the solid while, according to this model, not suffering from any effect except at the boundary of the solid specimen. Therefore, this model neglects the interaction between the electrons and positive atoms or between electrons. Accordingly, the electron total energy is due to the kinetic energy with a neglected potential energy of almost zero.

27 Introduction Semiconductors: Review and Theory

This model then took the term of “particle in a box” where the conductive material is a box filled with electrons.15

2.6.2 Braggs’ Diffraction

In 1895, German physicist Wilhelm Conrad Roentgen (1845–1923) discovered X-rays and was subsequently awarded the first ever Nobel Prize in Physics in 1901. Thereafter, the question was raised on whether X-rays were particles or waves. By 1912, Max von Laue discovered the diffraction of X-rays by crystals of copper sulfate.4 W.H. and W. L. Bragg (father and son) realized that this diffraction took place because the crystal was formed by 3D arrangement of atoms in 1913. They discovered that there were very sharp peaks observable at certain angles in the reflection of X-rays from crystals—i.e. at specific orientations of the crystal with respect to the X- rays’ source and detector. These orientations became later known as the atom planes within the crystal. The Braggs determined the structure of the NaCl crystal first, following which they

6 determined many other structures including those of KCl, ZnS, CaF2, CaCO3, and diamond. This work won the Braggs the Nobel Prize in 1915.

The Braggs noted that there was a relationship between the diffraction angle, wavelength of the

X-ray, and interplanar spacing. If one imagines that the crystal planes are parallel to one another and equally spaced, they will then behave as separate-scattering objects. When the X-ray waves pass through the crystal, each plane reflects a wave like a simple plane mirror. The Braggs also noticed that the X-ray reflected beams were observed with a reasonable peak intensity only when the conditions for constructive interference were satisfied.6 The constructive interference term refers to “the sum of the reflections from a large number of parallel mirrors all separated by the same distance, d, which must reinforce and arrive in phase with one another”. Braggs noticed that the constructive interference takes place only when two factors satisfied: (a) the difference in path

28 Introduction Semiconductors: Review and Theory lengths of the interfering beams differ by an integral number of wavelengths (n) for certain angles of scattering, and (b) the angle of the reflection radiations is equal to the angle of the incidence radiations.6

In Figure 2.10, the difference in path length of beam 1 and beam 2 is equal to the sum of distance

“AB” + distance “BC”. If AB=BC=푑ℎ푘푙푠𝑖푛휃, then the difference in path length = 2푑ℎ푘푙푠𝑖푛휃

Using these assumptions, the Braggs derived their famous equation or Braggs’ law, (Error!

Reference source not found.):

푛휆 = 2푑ℎ푘푙푠𝑖푛휃 (3)

From this model, W.H. and W. L. Bragg established that the X-ray crystal diffraction can be used as a means of determining the crystal structure.

Figure 2.10: Diagram of Bragg’s crystal diffraction where electron waves reflected from atomic planes with a spacing dhkl.

29 Introduction Semiconductors: Review and Theory

2.6.3 Kronig-Penney Model (Periodically Repeated Potential Well)

After the discovery of X-rays and the work of the Braggs that proved that the crystalline solid is a regular array of atoms, the nature of materials’ electricity conductivity still remained a mystery. In

1931, the German physicist Ralph Kronig, and the British mathematician William Penney jointly proposed the Kronig-Penney model, known as One-dimensional model of a Crystal or Periodically

Repeated Potential Well model.16 The Kronig-Penney model explains how the electrons in a crystal are dispersed into allowed and forbidden bands by scattering from the extended linear array of atoms where an electron is in a periodic potential of lattices. This model explains clearly the origin of band gaps, and consequently the difference between insulator, semiconductor and conductor materials.

In order to differentiate between insulators and conductors, the Kronig-Penney model extended the free electron model by taking into consideration the concept of lattice existence in the solid specimen. This led them to the concept of “energy band gap” presence in materials.

By considering that the crystal lattice is formed of periodic and infinite arrangement of atoms, the electron interacts with this array of positive fixed atoms. Kronig and Penney also represented this interaction by a potential that manipulated an electron in the crystal—they called it the crystal potential. In their model, they took into account only the valence electrons where the core electrons are then firmly tied to the nucleus. Hence, the crystal potential implicitly centered on the atomic position in the crystal and took into account the Coulomb repulsion between electrons—i.e. the valence electrons are independent electrons, yet they are not totally free.16

Energy gaps are caused when electrons’ undergo Bragg’s reflection. In practice, the ionic lattice

(or the ion cores of the crystal) introduces an attractive periodic potential, which scatters the

30 Introduction Semiconductors: Review and Theory conduction electron waves as they travel through the crystal, thereby resulting in the band gaps that are forbidden energy regions—i.e. regions in energy for which no wavelike electron orbitals exist.16-17 This scattering process is known as Braggs’ reflection of electron waves.

In order to simplify their calculations, Kronig and Penney took the solid as a one-dimensional array of atoms rather than three-dimensional, in which the atoms are regularly spaced by a distance a, the lattice parameter. They assumed each atom as a fixed point (x) that are separated by (n) number of distances a. As the electron travels from atom to atom, it will undergo segments of superposition of potential barriers, vx separated by width of (a), (Figure 2.11). The crystal potential, i.e. one- dimensional potential V(x) for instance, is the sum of these atomic potentials (Equation 4).

∞ 푉푥 = ∑ 푣(푥 − 푛푎) (4) 푛=−∞

In the Kroning-Penney model, electrons experience a periodic square wave potential V(x) (Figure

2.11)—i.e. experience an infinite series of rectangular barriers separated by the well width of a and barrier width b, and thus the period d = a + b. According to this model, the electrons have zero potential at the positive atoms (ions)—i.e. the potential is zero in the well—and reach the maximum potential value at the intermediate lattice points or in the barrier.8, 17 This periodicity of the potential is the central property that produces electronic band structure.16

31 Introduction Semiconductors: Review and Theory

Energy band gaps play a critical role in ascertaining a solid material as an insulator or a conductor.

Kroning and Penney found that the size of these gaps is proportional to the strength of the crystal potential.8 Typically, (a) If electrons are tightly bound to ion cores of a crystal (i.e. strong potential barrier), the energy bands are spaced far apart making big bandgap and accordingly yield insulators. (b) On the other side, if a potential barrier is weak, energy bands spaced close to each other creating metals. (c) However, in case of semiconductors, the band gap is small enough for a significant number of electrons to be excited across the energy gap by any source of excitation energy like irradiation.

Although the Kroning-Penney model is oversimplified and not realistic, it is still able to demonstrate the band structure and calculate the allowed and forbidden energies by solving the corresponding Schrödinger equation for the motion of any one electron in a single direction in a periodic potential (Figure 2.12). As the Schrödinger equation depicts the “wave” like nature of

32 Introduction Semiconductors: Review and Theory electron, the resulting energy dispersion (E) of the allowed and forbidden energies of the electron can be presented as a function of the superlattice wave-vector k.

Figure 2.12: Energy dispersion curve “E-k diagram” of an electron in the Kronig-Penney model. In this E- k diagram, “a” is the lattice constant and the electron reflection occurs at ± π/a. (b) The allowed and forbidden bands are plotted in the resulting energy dispersion E as a function of the superlattice wave- vector k.

33 Introduction Semiconductors: Review and Theory

2.7 Exciton in Semiconductors Referring to section 2.1 of this chapter, when semiconductor materials interacting with energy (or light) that is sufficient to promote the electron to the conduction band cross the band gap, leaving a hole in the valence band—i.e. form an exciton through a photoexcitation process. If the photoinduced hole and excited electron have a Columbic interaction that produces a bound state between them, then the pair is known as an “exciton” or electrically neutral quasiparticle, Error!

Reference source not found.. The Columbic attraction forces between the electron and the hole in the exciton causes their motion to be correlated—where they both bind together and propagate through the crystal. Thus, the spatial extent of an electronic excited state is spread out through coherent sharing of the excitation among atoms and lattice of the material.3 This depends on the nature of the electronic coupling among the lattices of the material. On the other hand, if the attraction forces between the photo-excited electron/hole pair is negligible—where each wave function is spread over an expanse of atoms—then the “electron/hole” pair are free carriers and not termed an exciton.

The distance of separation between the excited electron and hole is known as the effective Bohr radius of an exciton in bulk crystal—briefly, exciton Bohr radius.

There are two types of excitons in semiconductors; Frenkel excitons and Wannier-Mott excitons

(Figure 2.13):

2.7.1 Frenkel Exciton

In the late 1920s, the low-temperature spectroscopy of organic molecular crystals performed by two independent research groups Pringsheim and Kronenberger, and Obreimov and de Haas respectively showed narrow photoemission lines that contradicted the Bloch theory available at

34 Introduction Semiconductors: Review and Theory that time.18 In 1931, Soviet physicist Yakov Frenkel introduced for the first time the concept of

“exciton” in order to explain these data.19 It was not until 1936 when Frenkel first used the term

“exciton”18. In his work on neutral excitation of atoms in a lattice of ionic crystals (insulators) by light, he showed that the electron and hole are strongly bound to each other within the same or nearest-neighbor unit cells wherein they can move together inside the crystal lattice without the net transfer of charge in a particle-like motion. He identified the bound electron and hole as a quasi-particle—the exciton.15, 19

A distinguishable feature of a Frenkel exciton is that it has a small Bohr radius (< 5 Å) and the electron is bound strongly, basically confined within a single lattice constant.

Figure 2.13: Two types of exciton; (a) Frenkel excitons; comprising of a strong electron–hole attraction, the electron and the hole are tightly bound to each other within the same or nearest-neighbor unit cells within the crystal. (b) Wannier-Mott excitons; where the Coulomb interaction between the electron and hole is strongly screened by the valence electrons via the large dielectric constant that results in electrons and holes that are only weakly bound.

Since the electronic coupling between ions (or molecules) in these materials is strong, the excitation energy (exciton) can essentially be transported from one ion to another through the crystal without the ions themselves having to change places—the exciton is delocalized in the

35 Introduction Semiconductors: Review and Theory crystal. In other words, the electronic structure of a crystal containing an exciton can be described as a quantum-mechanical superposition of states, in which it is equally probable that the excitation is associated with any ion in the crystal.15

Frenkel excitons mostly occur in ionic crystals like alkali halides (e.g. NaCl), and organic crystals and aggregates (e.g. molecular crystals and polymers).18 Recently, Frenkel excitons have attracted attention as their energy transport properties were shown to be involved in biomolecular and light- harvesting antennas in photosynthesis.3 Accordingly, Frenkel excitons can be one of two types; one with a small radius that occurs in molecular crystals wherein the spatial extension of the excitation is approximately restricted to a single unit cell. These excitons are, to a large extent, localized at a specific atom or molecule, and their movement through the crystal is limited to a hopping mechanism. The second type is a charge transfer exciton that occurs mainly in ionic crystals. One can imagine its concept as the following: An electron is transferred from a lattice’s anion to the adjacent cation, thereby maximizing the electron charge density in the region. The radius of the charge transfer exciton is therefore to some extent larger than that of the Frenkel exciton.3, 18

2.7.2 Wannier-Mott exciton

Research on low temperature optical properties in pure semiconductor crystals in 1937 by the

Swiss physicist Gregory Hugh Wannier and in 1938 by the English theoretical physicist Sir

Nevill Francis Mott (1977 Nobel laureate in physics) led to the development of new exciton concepts in semiconductor crystals. In semiconductor materials with a large dielectric constant, as opposed to the strongly bound Frenkel excitons, the electric field screening of the crystal reduces the Columbic attraction between the electron and hole. Consequently, excitons with weak binding are predominant. In this kind of exciton, the exciton Bohr radius is much larger than the lattice

36 Introduction Semiconductors: Review and Theory parameter—typically a length in order of magnitude larger than tens of lattice parameters (40–100

Å). In such a case, the exciton wavefunction is strongly delocalized over the crystal unites, i.e. the crystal lattices. Subsequently, the exciton can move freely inside the crystal where the electron and hole typically are a quasi-particle— also called free exciton. These excitons are known as Wannier-

Mott excitons or simply “Wannier excitons”. Since the Columbic attraction force between the electron and hole is weak, we shall consider the electron and hole as two particles moving within the approximation of the effective masses.

Since the exciton is a bound electron–hole in which the hole and the electron circulate around each other, it resembles a hydrogen atom (more similar to positronium) in which the hole is taking the role of the proton.

In the Bohr model of a hydrogen atom, the attractive force between the electron and proton results from the Coulomb potential defined as

푒2 푈푟 = − (5) 4휋휀표 휀 푟 where r is the electron–hole distance and 휀 is the dielectric constant of the substance.

The Exciton Bohr radius (aB) in a crystal with a dielectric constant of 휀 will be different from the hydrogen atom in two aspects20:

(a) The hole mass is not very heavy as in the case of protons. Instead, either of the electron/hole

pair components are light quasi-particles with approximately comparable masses (me, mh). As

a result, the exciton has lower stability in comparison with the hydrogen atom, and thus a

larger electron orbit radius.

37 Introduction Semiconductors: Review and Theory

(b) In the presence of the electron and hole in a crystalline semiconductor, the dielectric constant

provides shielding and consequently reduces the attractive force between them. i.e. an increase

in the exciton radius.

Hence, the exciton is more analogous to positronium than the hydrogen atom.

In this case, the exciton radius can be defined as:

휀 푎퐵 = 푚∗ 푎표 (6) ⁄푚표

푚 푚 where 푚∗ is the reduced mass of the exciton (which equal to 푒 ℎ ), m is the rest mass of the 푚푒+푚ℎ free particle (electron), and 푎표 is the Bohr radius of the hydrogen atom. i.e. a low value of the

∗ reduced mass (푚 /푚표 ) along with the high dielectric constant reduces the stability of the exciton.

Hence, it can be written as in Equation 7

2 4휋휀표휀∞ ħ 1 1 푎퐵 = 2 ( ∗ + ∗ ) (7) 푚표 푒 푚푒 푚ℎ where 휀표 is the electromagnetic permittivity constant of free space, 휀∞ is dielectric constant of the material, ħ is the reduced Planck's constant (Planck’s constant divided by 2π), 푒 is the electromagnetic elementary charge (electric charge carried by electron/hole; equal to 1.602×10−19

∗ ∗ coulombs), and 푚푒 and 푚ℎ are the effective mass of electron and holes respectively.

The effective mass of the electron and hole are nearly the same. The exciton can be formed only when the electron and hole velocities are the same—to be able to move together in the bound- state.21 In order for this condition to be satisfied, in an ideal pure crystal, all the spin and orbital electron momenta in the valence bands are compensated by the ground state of wave vector k=

|푶⟩. At this point, the exciton is formed through a direct transition of an electron to the conduction band, Figure 2.14. At the point k= |푶⟩, the gradients of the conduction and valence bands are the same at the point of the Brillouin zone where the transition occurs.21 The direct transition of the

38 Introduction Semiconductors: Review and Theory electron takes place through k= |푶⟩, and forms free electron-hole pairs—the exciton has zero kinetic energy.

Figure 2.14 shows the optical properties of the Wannier-Mott exciton20, 22, where the energy “E” of the electrons in the different bands is plotted against the electron wave vector k. Experimental studies by Philip Baumeister in 1961 examining the low temperature optical properties of the

Cu2O (direct band gap semiconductor) showed that the optical absorption spectra exhibit lines that resemble the Rydberg transitions in hydrogen.23 These lines appear in the optical absorption spectra at energies just below the conduction band—i.e. below the fundamental band gap.21

The optical absorption of a photon by a direct-bandgap semiconductor to form the Wannier exciton is equal to

퐸 ℎ휈 = 퐸 푥 (8) 𝑔− 푛2 where Ex is the exciton binding energy, Eg is the band gap, and n=1, 2, 3, ….∞, is the exciton levels.

39 Introduction Semiconductors: Review and Theory

2.8 Different types of Perovskite materials and Their Applications

Since pioneering work in 2009 by Tsutomu Miyasaka and coworkers24, incredible advances have been made in the development of organic-inorganic metal halide-perovskite materials (known as perovskites) for thin film solar photovoltaic cells (PV) which resulted in devices with efficiencies exceeding 20%. These perovskites have a unique combination of inorganic and organic properties including panchromatic absorption profile (400-800 nm), intense and narrow-band luminescence, excellent ambipolar charge carrier mobilities in addition to long exciton lifetimes/diffusion length of charge carriers, and very low exciton binding energy. Due to low nonradiative losses, perovskites have also found applications in white light emitting diodes (WLED), low threshold lasers, and electroluminescence (EL) devices.

As previously mentioned in F, perovskites are a class of materials that has crystal structure similar to CaTiO3 or (ABX3 structure). Perovskites are exist naturally in the earth in form of oxides as

CaTiO3 and magnesium silicate perovskite (MgSiO3) which the latter is believed as the most abundant mineral in the mantle of the earth.11, 14 In addition to these inorganic oxide, there are other class of alternate stacking sheets of organic and inorganic components with the same ABX3 crystal structure of perovskite known as organic-inorganic perovskite hybrids. In the late 1980s and early 1990s perovskite materials attracted greatly attention due to their unique physical properties such as high superconductivity and ferroelectricity as well as their unique crystal structures.

2.8.1 Inorganic Perovskite Materials

There are many examples of inorganic perovskite all share the general ABX3 crystal structure, but classified into different families due to the structure distortion. For instance, (i) undistorted cubic perovskite structures include SrTiO3, and CsSnBr3, (ii) structure is distorted because of cation

40 Introduction Semiconductors: Review and Theory

displacements, as in BaTiO3, and (iii) structure is distorted as a result of tilting of the octahedra,

2- - - - - as in CaTiO3. There are too many other alternatives where X anion can be (O , Cl , B , F , I , or in a few instances S2-), while B cation can be (Na1+, K1+, Ca2+, Sr2+, Ba2+).14

2.8.2 Organic-Inorganic Metal Halide-Perovskite Materials

Hybrid organic–inorganic perovskite compounds adopt the ABX3 perovskite structure in which the organic moiety with positive charge, most commonly methylammonium (MA), replace the large cation A. In the last three decades these class of materials have attracted a much attention due to combination of inorganic and organic properties. Typically, the inorganic covalent and ionic interactions offer the properties of high electrical mobility, interesting magnetic properties, high dielectric properties and thermal stability. On the other side, the organic part of this family of organic–inorganic hybrid perovskite offers possibility of structural diversity, highly efficient luminescence, and their film processability. This excellent combination of optical and electronic properties enables the use of this family of materials in many applications.

In particular, attention has been focused on organic–inorganic lead halide perovskites. In the early

1990s, David Mitzi, and co-workers at IBM undertook an extensive investigation of the optoelectronic properties of these lead halide perovskites due to their strong excitonic features.

Lead halide perovskites has two different classes of structure: (i) either self-organization of

4− inorganic [MX6] octahedra to form an extended three dimensional (3D) network type taking the general formula of CH3NH3PbX3. Alternatively, (ii) low-dimensional structures known to be self- assembly forming an ideal two-dimensional layered structures.

41 Introduction Semiconductors: Review and Theory

2.8.2.1 2D Lead halide perovskites

In early 1990s, David Mitizi’s group discovered the optical and electronic significance of low- dimensional organometal halide materials.25 They could easily grow these crystals from solution and by deposited them by spin-coating on a substrate. These materials have general formula

(R2(CH3NH3)n-1PbnX3n+1). In this generic structural formula; n is an integer refers to the number of layers (n=1, 2, 3, …), R is long primary aliphatic or aromatic alkylammonium cation, M is divalent metal (IV group in the periodic table; e.g. Sn, Pb), and X is a halide anion. Typically, the

4− 2D layered perovskite-type compounds composed of inorganic layers of corner-sharing [MX6] octahedra confined between bilayers of bulky alkylammonium cations in method known as intercalation.25

This unique hybrid combination between the organic and inorganic layer create perfect quantum well (QW) structure, in which structures in which the semiconducting inorganic layers act as

+ “wells” and the insulating organic layers act as “barriers”. Typically, organic (CnH2n+1NH3 ) and

4- inorganic (PbX2 ) layers are alternatively stacked with each other. This enables excitons in these compounds with binding energy as large as a few hundreds meV due to the quantum and dielectric confinement effects.26-27 Therefore, these low-dimensional organometal halide materials has a potential applications in the nonlinear optical devices as well as the electroluminescent device.

Figure 2.15 represent Schematic structures of this family of the 2D layered organo-lead halide perovskites.

42 Introduction Semiconductors: Review and Theory

Figure 2.15: Schematic structures of the 2D layered organo-lead halide perovskites, where n=1, 2 and 3.

2.8.2.2 3D Lead halide perovskites

The 3D organometal halide takes the generic structural formula of CH3NH3PbX3 built up of all corner sharing octahedra (MX6). In order to form the 3D network only the small organic cations can be integrated.

Following the discovery of the 2D organometal halide materials by Mitzi’s group in the 1990s, an extensive focus went on magnetic, optical, and electric properties of these low-dimension materials. On the other side, few studies have been investigated about the 3D network perovskite materials. It was until 2009, when a Japanese research group of Tsutomu Miyasaka and coworkers when they tested the photovoltaic properties of methylammonium lead triiodide

43 Introduction Semiconductors: Review and Theory

24 perovskite (CH3NH3PbI3). In their work, Miyasaka et.al pioneered the first perovskite solar cells based on mesoporous TiO2 photoanodes implemented the CH3NH3PbI3 as the light sensitizer and shown efficiency of 3.8 %. Due to the instability (e.g. corrosion) of these materials in the

– 3– electrolyte redox couple (usually iodide/tri-iodide (I /I )), no spotlight on these CH3NH3PbI3- based solar cells was received until the concept of all-solid-state introduced in 2012. In 2012, a major breakthrough by Mercouri Kanatzidis and his coworkers28 when they fabricated all-solid- state dye-sensitized solar cells with high efficiency (10.2 %) based on using adopted alkali metal halide perovskite of CsSnI3. In the same year, 2012, Michael Grätzel, and Nam-Gyu Park and their coworkers29 collaborated on fabricating solid-state mesoscopic heterojunction solar cells employing nanoparticles (NPs) of methyl ammonium lead iodide (CH3NH3)PbI3 as light harvesters exceeding 9% efficiency. Since these two breakthroughs, this class of material rapidly attracted tremendous attention in the research community. The reader is referred to excellent recent reviews.14, 30-31

44 Introduction Semiconductors: Review and Theory

2.9 References

Seeger, K., Semiconductor Physics: An Introduction. Springer Berlin Heidelberg: 2004.

2. Suppan, P., Chemistry and Light. The Royal Society of Chemistry Thomas Graham House, The Science Park, Cambridge CB4 4WF 1994; p - 295.

3. Scholes, G. D.; Rumbles, G., Excitons in nanoscale systems. Nat Mater 2006, 5 (9), 683- 696.

4. Pecharsky, V. K.; Zavalij, P. Y., Fundamentals of powder diffraction and structural characterization of materials. Springer: 2009.

5. Miller, W. H., A Treatise on Crystallography. For J. & J. J. Deighton: 1839.

6. A.Moore, L. E. S. a. E., Solid State Chemistry: An Introduction. CRC Press May 29, 2012; p 494.

7. Download for free at: http://cnx.org/contents/f46e8679-ee00-4073-9f5e- [email protected].

8. Grundmann, M., The Physics of Semiconductors: An Introduction Including Devices and Nanophysics. Springer: 2006.

9. Coster, D.; Knol, K. S.; Prins, J. A., Unterschiede in der Intensität der Röntgenstrahlen- reflexion an den beiden 111-Flächen der Zinkblende. Z. Physik 1930, 63 (5-6), 345-369.

10. Deschamps, J. R., X-ray Crystallography of Chemical Compounds. Life Sciences 2010, 86 (15–16), 585-589.

11. Bhalla, A. S.; Guo, R.; Roy, R., The perovskite structure – a review of its role in ceramic science and technology. Materials Research Innovations 2000, 4 (1), 3-26.

12. Hassan, Y.; Song, Y.; Pensack, R. D.; Abdelrahman, A. I.; Kobayashi, Y.; Winnik, M. A.; Scholes, G. D., Structure-Tuned Lead Halide Perovskite Nanocrystals. Advanced Materials 2015, n/a-n/a.

13. Bhalla, S. A.; Guo, R.; Roy, R., The perovskite structure – a review of its role in ceramic science and technology. Material Research Innovations 4 (1), 3-26.

14. Cheng, Z.; Lin, J., Layered organic-inorganic hybrid perovskites: structure, optical properties, film preparation, patterning and templating engineering. CrystEngComm 2010, 12 (10), 2646-2662.

15. Mermin, N. W. A. a. N. D., Solid state physics. Harcourt. Inc.: USA, 1976; Vol. Chapter 30 Defects In Crystals, p 848.

45 Introduction Semiconductors: Review and Theory

16. de L. Kronig, R.; Penney, W. G., Quantum Mechanics of Electrons in Crystal Lattices. Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 1931, 130 (814), 499-513.

17. Kittel, C., Introduction to Solid State Physics. Wiley: 2004.

18. Knoester, J.; Agranovich, V. M., Frenkel and Charge-Transfer Excitons in Organic Solids. In Thin Films and Nanostructures, Academic Press: 2003; Vol. Volume 31, pp 1-96.

19. Frenkel, J., On the Transformation of light into Heat in Solids. I. Physical Review 1931, 37 (1), 17-44.

20. Valenta, I. P. a. J., Luminescence Spectroscopy of Semiconductors. Oxford University Press United States, 2012; Vol. Luminescence of excitons.

21. Fox, M., Optical Properties of Solids. Oxford University Press: OUP UK, 2010; Vol. Chapter 4.

22. Pope, M. a. S., Charles E, Electronic Processes in Organic Crystals and Polymers Oxford University Press: United States, 1999; p 1351.

23. Baumeister, P. W., Optical Absorption of Cuprous Oxide. Physical Review 1961, 121 (2), 359-362.

24. Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. Journal of the American Chemical Society 2009, 131 (17), 6050-6051.

25. Mitzi, D. B., Synthesis, Crystal Structure, and Optical and Thermal Properties of (C4H9NH3)2MI4 (M = Ge, Sn, Pb). Chemistry of Materials 1996, 8 (3), 791-800.

26. Tabuchi, Y.; Asai, K.; Rikukawa, M.; Sanui, K.; Ishigure, K., Preparation and characterization of natural lower dimensional layered perovskite-type compounds. Journal of Physics and Chemistry of Solids 2000, 61 (6), 837-845.

27. Kitazawa, N.; Watanabe, Y., Optical properties of natural quantum-well compounds (C6H5-CnH2n-NH3)2PbBr4 (n=1–4). Journal of Physics and Chemistry of Solids 2010, 71 (5), 797- 802.

28. Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G., All-solid-state dye-sensitized solar cells with high efficiency. Nature 2012, 485 (7399), 486-489.

29. Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G., Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci Rep 2012, 2, 591.

46 Introduction Semiconductors: Review and Theory

30. Park, N.-G., Organometal Perovskite Light Absorbers Toward a 20% Efficiency Low-Cost Solid-State Mesoscopic Solar Cell. The Journal of Physical Chemistry Letters 2013, 4 (15), 2423- 2429.

31. Gao, P.; Gratzel, M.; Nazeeruddin, M. K., Organohalide lead perovskites for photovoltaic applications. Energy & Environmental Science 2014, 7 (8), 2448-2463.

47 Introduction Semiconductor Nanocrystals: An Overview

Chapter 3 Semiconductor Nanocrystals: An Overview

Materials having a particle size between 1 and 100 nanometers (nm) are normally considered as nanomaterials. Semiconductor nanocrystals are semiconductors of nanomaterial sizes. As mentioned earlier, semiconductors have an energy gap between the valence band and conduction band known as the bandgap. Within the macroscopic size, the width of the band gap is a physical constant which is known for each material. However, in the case of nanoscale semiconductor particles with sizes smaller than ~10 nm (typically less than the Bohr radius of the material), this bandgap is no longer constant—i.e. its value changes.

3.1 Quantum Confinement: Definition

Advances in synthetic chemistry and material science has led to control over the architectural shape and size of the semiconductor crystals. This latter class of semiconductor crystallizes range in the nanometer size regime, known as semiconductor nanocrystals (NCs) and also quantum dots (QDs), whereas they exhibit a new mesoscopic phenomena due to their size. More specifically, if the diameter of the synthesized semiconductor nanocrystal, d, is comparable to or smaller than the exciton Bohr radius (aB), then the exciton in these nanomaterials is subject to confinement by the boundaries of the material. Quantum confinement leads to a blue-shift of the optical absorption and emission spectra of a material. This phenomenon is known as the quantum-size effect— whereby the material exhibits unique size-dependent physical (mainly the electronic and optical) properties. More interestingly, the material demonstrates atomic-like optical behavior in addition to its bulk bands becoming quantized—a characteristic of this phenomenon. As their infinite

48 Introduction Semiconductor Nanocrystals: An Overview

dimensions that affect adjustment of the energy bands is due to quantum confinement, the nanocrystal is an intermediate state of matter, between bulk and molecular systems. This is a fundamental property of nanomaterials which is not present in bulk materials.

Colloidal inorganic semiconductor NCs, or the quantum dots (QDs), have attracted great attention in the last two decades for both their fundamental research and technical applications in various fields owing to their size and shape-dependent properties and their excellent chemical processability. For instance, the pioneering applications of QDs are light-emitting diodes, lasers, and photovoltaic devices.

3.2 Major Milestones toward Nanocrystals

In this section, I present the historical background of the semiconductor NCs and some important milestones in the development of these promising type of materials.

3.2.1 The Idea of Small Scale

The idea of nanotechnology is generally attributed to the American theoretical physicist Richard

Phillips Feynman (1965 Nobel Laureate) in his historical and legendary lecture under the title

“There is Plenty of Room at the Bottom: Invitation to Enter a New Field of Physics”1 that he delivered on 29 December 1959 at the annual meeting of the American Physical Society. In this lecture, he addressed the audience with his vision of a new field of physics that should emerge— the problem of manipulating and controlling things on a small scale. By this legendary lecture,

Feynman opened the field of nanotechnology. Here, he asked questions such as: Why cannot we write the entire 24 volumes of the Encyclopedia Britannica on the head of a pin? He also

49 Introduction Semiconductor Nanocrystals: An Overview

encouraged the community to think about making the electron microscope a hundred times better, develop smaller robots, and learn from molecular biology to make efficient miniaturized computers. The most significant prediction by Feynman in this lecture was “when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have”.

3.2.2 Nanometer-Thick Foils Called Quantum Wells (2D Structures)

The early history of the synthesis of QDs began in the 1970s with one-dimensional periodic potential superlattices called quantum wells.2

As early as the 1950s and 1960s, interest in forming ultra-thin layers for studies of size quantization effects was profound. The discoveries during these two decades was only theoretical, yet led to greater awareness. Although the focus was on thin films of semi-metals (e.g. Bi) on mica substrates obtained by vacuum deposition, in 1963 L. V. Iogansen theoretically studied the possibility of the resonance-tunneling effect (resonant tunneling of electrons) in semiconductor structures with dielectric barriers—double-barrier structures—for obtaining negative differential resistance.3

By the end of 1970, experimental fabrication of semiconductor QWs became possible due to the development of molecular beam epitaxial growth techniques for semiconductor heterostructures.

The pioneering experimental quantum engineering of microelectronic semiconductor materials— one-dimensional periodic potential superlattices known as quantum well (QW) —was achieved by the Japanese physicist L. Esaki and coworkers at IBM. This work also led to Esaki’s discovery of tunneling in semiconductors in 1970.4 Later in 1973, L. Esaki was awarded a Nobel Prize for this work.

50 Introduction Semiconductor Nanocrystals: An Overview

Most significantly, the mesoscopic variation of the material composition at that time enabled the variation of the band gap to be achieved affording desired electronic potentials, tunneling probabilities and optical absorption. In 1971, Esaki and coworkers showed a clear demonstration of size quantization effects in their new lattice-matched GaAs/GaAlAs QW structures—double- barrier structures having an ultra-thin GaAs semiconductor sandwiched between two GaAlAs barriers using molecular beam epitaxy electronic spectroscopy.5

In the same year (1974), R. Dingle, W. Wiegmann and C. H. Henry at Bell labs observed for the first time the energy quantization in the optical absorption of a QW. In their optical spectroscopy work, they observed that the photon absorption of a very thin, molecular beam-grown, AlxGa1-xAs-

GaAs-AlxGa1-x heterostructure exhibits a staircase of discrete exciton resonances (up to eight resolved exciton transitions), whereas in the photon absorption of a bulk semiconductor only one exciton peak and the associated continuum is found.6

3.2.3 From 2D QW Heterostructures to 1D and to 0D Structures

Following the work of Esaki and the interesting findings by Dingle et al., an exponential growth in the field turned up during the 1970s. During this and the following decade, the foremost properties of superlattices and QWs were understood, and the researchers’ interests focused in the direction of smaller sized structures with further reduced dimensionality—1D and 0D structures.

Further advances in electron-beam lithography—microfabrication technology—allowed W.J.

Skocpol and coworkers (1983) at Bell labs in New Jersey to fabricate a Si with quantum confinement to one dimension—quantum wires with a width less than 100 nm in order to be used as metal–oxide–semiconductor field-effect transistors (MOSFETs). 7

51 Introduction Semiconductor Nanocrystals: An Overview

Research progression in epitaxial two-dimensional semiconductor devices and quantum wires eventually reached zero-dimensional nanostructures in 1981. The development of transmission electron microscopy (TEM), scanning tunneling microscopy (STM) and atomic force microscopy

(AFM) enabled researchers to gain more information of the nanostructures at the atomic-level.

Between 1981–1984, two Russian solid state physicists, A. Ekimov and A. Onushenko, discovered zero-dimensional size semiconductor nanocrystals (later known as QDs) while working at the Vavilov State Optical Institute. In this work, they observed experimental validation of quantum confinement and quantum size effect in the optical absorption spectra of CuCl (in 1981)8 and CdS (in 1984)9 nanocrystals dispersed within a transparent insulating silicate glassy matrix.

The optical spectra were shown to depend on a crystallite size when the later approaches the Bohr exciton radius of the semiconductor.

However, it was not until late 1988 when Mark Reed and coworkers (then at Texas Instruments, now at Yale University) coined the term “quantum dots” for the first time to describe their GaAs nanostructures.10

The aforementioned milestones to reach the scale of the QDs were all solid state physics work that were based on creating quantum dots with epitaxial growth techniques and/or nanoscale patterning.

Generally, there are two families of quantum dots according to their preparation: physical approach or chemical approach. Physicists, as mentioned previously in section (3.2.1, 3.2.2 and 3.2.3), work with epitaxial quantum dots through the combination of high-resolution electron beam lithography and subsequent etching. On the other hand, chemists use the pyrolysis of organometallic and chalcogenide precursors to produce relatively large volumes of colloidal quantum dots (wet method), which are typically passivated by organic molecules on their surface. The latter is easily controlled by rapid nucleation followed by slower and steady growth that gives an average

52 Introduction Semiconductor Nanocrystals: An Overview

distribution size that varies within 5–10%, while the former is more painstaking and involves many disadvantages including defect formation, size polydispersity and poor interface quality.

3.2.4 Colloidal Quantum Dots-Wet Chemistry

Although metal nanoparticles date back to antiquity (as old as the ancient Egyptians), the origins of colloidal semiconductor quantum dots (QDs) can be traced to the early 1980s.

In the late 1970s, Michael Grätzel and coworkers were involved in studying the conversion of light into electricity or chemical fuels in photoelectrochemical devices equipped with semiconductor electrodes. In order to overcome the photocorrosion of the semiconductors, Grätzel and his coworkers (at Institut de Chimie Physique, Ecole Polytechnique de Lausanne, Switzerland) reported for the first the synthesis of colloidal CdS in the maleic anhydride/styrene copolymer.11

In the early 1980s, Louis E. Brus (then at Bell laboratories, now S. L. Mitchell Professor of

Chemistry at Columbia University) began working on complex chemistry and organic oxidation and reduction (redox) reactions taking place on the surfaces of photoexcited semiconductors.12

Using the same synthetic method utilized by Michael Grätzel for aqueous colloidal semiconductors in order to obtain a higher surface area for his reactions, L. Brus accidently observed that the band gap of his colloidal semiconductors decreased over several days. Due to their higher surface area, he explained that the small particles were less stable and thus grew spontaneously.13 This accidental observation led to the discovery of colloidal semiconductor nanocrystals known nowadays as quantum dots. In 1984, L. Brus accomplished “theoretical modeling of quantum confinement in three dimensions”, concluding that “the band gap of the semiconductor was experimentally a function of particle size”—the quantum confinement effect.14 In his theoretical model, Brus applied a particle in a sphere model approximation to the wave function for bulk

53 Introduction Semiconductor Nanocrystals: An Overview

semiconductors which explained the relation between size and bandgap for semiconductor nanoparticles.

Independently, Michael Steigerwald (then a researcher at Bell labs, now a researcher in the Brus lab at Columbia University) is the pioneer of the evolution of many of the early chemical synthetic routes of inorganic precursor chemistry, which are now routinely used in quantum dot synthesis.

Steigerwald was inspired by early work on the deposition of semiconductors by metal organic chemical vapor deposition (MOCVD) which is based on organometallic precursors designed for gaseous-phase reactions. He modified these precursors toward organometallic phosphine chalcogenides that opened up quantum dots synthesis in solution chemistry—the “wet method” that is widely used today.15-17

3.3 Discovery of Organometallic Synthesis of QDs:

The Hot-injection Approach

A new advancement in QD research started with the productive and successful collaboration between physicist L. Brus and his research group with organometallic synthetic chemist M.

Steigerwald in order to make smaller particles and to understand the evolution from molecules to bulk semiconductors.18-19 This led to the successful synthesis of colloidal—the first hot injection route for the production of high-quality CdE (E = S, Se, Te) semiconductor NCs or QDs with size- tunable band-edge absorption and emissions by Murray et al. in 1993.20 Since this first report by

Chris Murray and coworkers on the hot-injection synthesis approach20, extensive research has taken place that has led to the advanced progress in the field of QDs seen today.

54 Introduction Semiconductor Nanocrystals: An Overview

The hot injection method developed by Murray et al is based on the injection of a combination of metal-organic precursors into a hot solution containing coordinating phosphine chalcogenides ligands (trioctylphosphine oxide; also known as TOPO) at high temperatures (reaching above 300 oC). In this method, rapid burst nucleation at a high temperature immediately begins following the injection, and is then followed by slower and steady growth at a relatively lower temperature that facilitates the formation of nanoparticles with monodisperse sized particles—i.e. narrow size distributions.21 The presence of these surfactants helps in rectifying the internal defects and yields a uniform surface passivation, allowing well-defined physical properties.

However, the main disadvantage of this method is the cadmium chalcogenides nanocrystals prepared using dimethyl cadmium (Cd(CH3)2) as the cadmium precursor. Dimethyl cadmium beside it is expensive, it has proven to be extremely toxic, pyrophoric, unstable at room temperature, and explosive at elevated temperatures when releasing large amounts of gas.22

This method, with some modifications of the surfactants utilized to allow for more green chemistry, is nowadays the most widely employed chemical synthetic technique for NC preparation.

55 Introduction Semiconductor Nanocrystals: An Overview

3.3.1 Progress in Nanocrystals Synthesis Using Hot-injection Approach

Since the discovery of the hot-injection technique by Murray et al in 1993, the past two decades have resulted in tremendous efforts and progress in optimizing the high-temperature solution- phase chemical synthesis of QDs.

In 2007, Haizheng Zhong et al.21 listed “the major milestones of progress in nanocrystal synthesis” since Murray et al.’s work as follows:

 In 1996, Hines and Guyot-Sionnest demonstrated the synthesis of the type-I heterostructure of the CdSe/ZnS core–shell with high photoluminescence yields up to 50%.23

 In 2000, Paul Alivisatos’s group reported the shape control and evolution of CdSe NCs.24-

25 In their work, they reported a synthesis method that yields soluble and monodisperse particles of CdSe NCs that are quantum-confined in two of their dimensions. This method enables variation in the shapes of the resulting particles from nearly spherical particles to soluble and processable rod-, arrow-, teardrop-, and branched tetrapod-shaped CdSe nanocrystals.

 In 2001, Norris and coworkers prepared high quality and highly fluorescent ZnSe NCs doped with paramagnetic manganese (Mn2+) impurities using a high-temperature reaction.26

 Between 2001–2002, Xiaogang Peng’s group developed green synthesis by using air- stable, less toxic and low-cost chemicals. They used cadmium oxide (CdO) and cadmium acetate

(Cd(Ac)2) instead of the environmentally toxic dimethyl cadmium (Cd(CH3)2). Moreover, they introduced oleic acid as an alternative ligand of TOPO, and 1-octadecene (ODE) as a noncoordinating solvent.22, 27-28

 In 2003, Moungi Bawendi and coworkers reported the preparation of heterostructure type-

II CdTe/CdSe and CdSe/ZnTe core–shell NCs.29

 In 2001, Chris Murray and coworkers reported the high-temperature solution-phase

56 Introduction Semiconductor Nanocrystals: An Overview

synthesis of PbSe NCs.30 In 2003, Scholes and Hines adapted this approach to synthesize colloidal size-tunable PbS NCs with a monodispersed size that emit in the near infrared region.31

 Between 2004–2005, Charles Cao and coworkers developed the one-pot synthesis of CdS,

CdSe and CdTe semiconductor NCs without precursor injections—i.e. the non-injection route.

This route enables scaling up the synthesis and making nanocrystals in large quantities.32-33

 In 2007, Dmitri V. Talapin and coworkers designed heterostructures of CdSe/CdS with complex shapes and morphologies ranging between nanorods and nanotetrapods using a seeded growth technique. By tailoring the crystalline phase of the seeds, Talapin et al. were able to engineer the shape of the NCs towards multicomponent nanostructures with precisely designed physical and chemical properties.34

3.4 Synthesis of Semiconductors in the Nanocrystals Regime: The Mechanisms

Referring to the section (3.3) in this chapter, the quest to develop reproducible and in the same time of reliable methods for producing relatively large volumes of monodisperse sized NCs has attracted material chemists over the last three decades. The term of “monodispersity” of NCs refers to “all nanostructures produced from a reaction mixture must have narrow size and shape distribution—i.e. particles should be as similar as possible in sizes and shapes that they are indistinguishable”.28

Colloid scientists often explain the properties of suspensions as the sum of contributions of individual particles. Obviously, to obtain well-defined physical properties of such suspensions, one needs to create monodisperse particles—all particles in the suspension have the same size and shape.

57 Introduction Semiconductor Nanocrystals: An Overview

Colloidal synthesis of the semiconductor NCs using the wet method of the pyrolysis of organometallic and chalcogenide precursors, or the hot-injection approach, is considered the most successful preparation method in terms of high quality and resulting monodisperse sized NCs.

These NCs are typically passivated by organic molecules on their surface forming suspensions. In this approach, pyrolysis of metal-organic precursors in hot coordinating solvents causes the precipitation of a NC or the solid phase from solution. A good understanding of the parameters and mechanisms controlling this precipitation process helps to improve the engineering of nanoparticles with the desired size and shape as well as may further provide some insight for understanding of the size-dependent phenomena.

3.4.1 Theory of Homogeneous Nucleation and Growth

The mechanism of the formation of monodispersed inorganic NCs can be understood in terms of the model of the “nucleation and growth” theory of colloidal particles proposed by Victor K.

LaMer and Robert H. Dinegar in 1950.35 According to LaMer and Dinegar, the controlled synthesis of colloidal hydrosol particles from a homogeneous solution includes two main phases: nucleation and growth.

In this section, we will present a simple description of LaMer et. al’s theory35 in order to understand how different factors influence nucleation and growth and can be employed to obtain monodisperse sized nanomaterials.

In the early 1940s, LaMer and his coworkers pursued extensive research on the preparation of uniform colloidal particles of various oil aerosols and sulfur hydrosols. They proposed that small monodispersed colloid formation results when growth from a homogeneous solution experiences

58 Introduction Semiconductor Nanocrystals: An Overview

kinetically controlled sequences in three stages: (I) supersaturation (or pre-nucleation stage), (II) nucleation, and (III) growth and Ostwald ripening.

Error! Reference source not found. shows a schematic illustration diagram of the LaMer and

Dinegar monodispersity formation mechanism.21, 35-37

(I) Pre-nucleation stage:

Typically, for any precipitation to be formed from a solution, the concentration of the monomer

(solute) exceeds its solubility in the mixing solvent to reach supersaturation. In a homogenous solution, the concentrations of the solute (monomeric species) rises above the saturation concentration (Cs) and builds up in Stage I until the concentration reaches a critical supersaturation level (Cmin), whereby the nucleation may start to be observed.

(II) Nucleation stage:

There are different kinds of nucleation of particles from solution. Depending on the presence of solid surfaces or interface in the solution, there can be homogenous nucleation or heterogeneous nucleation. Typically, homogeneous nucleation occurs in the absence of any solid interface/surface while the heterogeneous nucleation occurs at nucleation sites on surfaces that work as seeds.

LaMer’s theory focuses on the mechanism of homogeneous nucleation that takes place due to the thermodynamic driving force of supersaturated solution energy non-stability.

In this model, they proposed that a critical condition of “burst nucleation” should take place in order to have control on the size distribution and consequently yields monodispersed particles.

Burst nucleation, whereby many nuclei are generated at the same time and subsequently grow simultaneously while no additional nucleation occurs during growth. This means that the nucleation of all particles should occur temporally and discretely followed by slower and

59 Introduction Semiconductor Nanocrystals: An Overview

controlled growth on all existing ensemble of nuclei. Alternatively, if the nucleation process occurred concurrent to the growth, the growth histories of the particles become different from one another and eventually yield polydispersity.

During the nucleation process, the atoms assemble themselves to their crystalline structure, which is a thermodynamically stable state, starting with infinitesimal clusters—denoted as nuclei. These clusters will eventually become a site for the subsequent packing of further atoms during growth.36

In the context of colloidally stable nanocrystals synthesis using the “wet method” of organometallic chalcogenides introduced by Steigerwald15-19 and modified by Murry et al.,20 the concentrations of the monomeric solute are built up by chemical decomposition of these

60 Introduction Semiconductor Nanocrystals: An Overview

chalcogenides. During the decomposition of these chalcogenide complexes, the concentration rises first above the solubility concentration Cs. Due to the concentration building up during stage I, a critical concentration (Cmin) and the nucleation threshold is reached whereby a discrete homogeneous nucleation event takes place generating small clusters, seen as metastable stage II in

Figure 3.1.

The thermal decomposition reaction that leads to the critical supersaturation point and then initiation of “burst nucleation” occurs either by: (a) swift injection of the solute precursors at a higher temperature into coordinating solvents and then cooling to low temperatures—denoted as hot injection method, or (b) by directly dissolving these solutes at a higher temperature—known as the one-pot synthesis method.

(III) Growth Stage:

Due to the sudden decrease in the concentration of monomers due to the particle formations, the growth stage starts when no more new nuclei are formed. In order to obtain monodispersed size particles, the concentration of the monomers should be rapidly depleted below the critical concentration (Cmin) to prevent further nucleation events to occur. This condition takes place if the rate of monomer consumption by nucleation and the rate of the growth process exceeds the rate of monomer supply.

During this stage, stage (III) in Figure 3.1, the particles grow by uniform diffusion-controlled growth as long as the solute concentration is supersaturated which guarantees a continuous supply of solute (atoms) by the continuing decomposition reaction.35 The diffusional growth mechanism can be understood in the sense of the classical nucleation theory, where the supersaturated solution and, as a result, “the new born nuclei” or “embryo”, are thermodynamically unstable in energy.

61 Introduction Semiconductor Nanocrystals: An Overview

Therefore, the reduction of the overall Gibbs free energy takes place during the generation of a new solid phase, the embryo, and is the driving force for the nucleation.

In the supersaturated solution, thermal fluctuations of the freely dispersed atoms cause the formation of the “embryo” through assembling themselves by a diffusion process from the solution phase to the solid phase. Here, nanocrystals pass the barrier of activation energy in order to form the thermodynamically stable nanocrystal (see Figure 3.2).21, 38

Nucleation is driven by the difference in free energy between two phases: the solution phase and solid phase.38 Therefore, the overall free energy change, ∆푮, of the nucleation (i.e. the formation of the “embryo”) is a combination of two criteria: the change in Gibbs free energy per unit volume (∆푮푽), i.e. the energy due to the formation of a new volume, and the surface energy per unit area (γ) due to the formation of new surface. See Equation 9.

Figure 3.2: Change in the Gibbs free energy (G, solid line) as a function of the nucleus radius (r) as a sum of the volume and surface free energies.

62 Introduction Semiconductor Nanocrystals: An Overview

The change in Gibbs free energy per unit volume of the solid phase (∆퐺푉) is dependent on the concentration of the monomer,

−푘퐵푇 퐶 ∆퐺푉 = ln (9) Ω 퐶표 where, 푘퐵 is the Boltzmann constant, T is the temperature, Ω is the atomic volume, C is the

퐶 concentration of the solute (monomer), C0 is the equilibrium concentration, and refers to the 퐶표 saturation ratio.

For C > C0 or S > 1; ∆퐺푉 will be negative and nucleation is spontaneously favored due to the reduction of energy. This reduction in energy is caused by the loss in entropy due to the gain in the chemical potential when non-interacting atoms form a new bond.

Considering a formation of a spherical particle or nucleus with effective radius (r), the total overall change in Gibbs free energy (∆퐺) which is required for nucleating an “embryo” will then be equal to:

4 ∆퐺 = π푟3 ∆퐺 + 4휋푟2훾 3 푉 (10)

volume term surface term

In Equation (10), the surface energy per unit area (γ) is always positive and disfavors the growth of the “embryo” into a bigger nanocrystal. Thus, the overall free energy change, ∆푮, of the nucleation is increased in the beginning of the process due to the surface cost until the cluster reaches critical size at r =r*, Figure 3.2. Henceforth at the point of r>r* the thermal fluctuations of the “embryo” become more costly in relation to energy that the atoms become bound into a dimensional structure of proper crystal shape. At this stage, the concentration of the monomeric solute species reaches a critical supersaturation point resulting in a single short burst of nucleation creating the particle seeds or the “embryo” followed by uniform diffusional growth. Conversely,

63 Introduction Semiconductor Nanocrystals: An Overview

before this critical point, i.e. r

In some systems during growth, stage (III) in Figure 3.1, the continuous addition of new atoms from the solution leads to an increase in the surface energy cost due to the presence of dangling bonds at the surface of the growing particles from the “embryo”—i.e. incomplete saturation of the surface bonds. Accordingly, the small droplets solubilize back into atoms/molecules by dissolution before they are redeposited onto the larger NCs by condensation and agglomeration as a second growth phase. This means that the larger NCs will grow in a coarsening process known as Ostwald ripening. As a result, the average NC size increases over time with a compensating decrease in NC number.

3.4.2 Heterogeneous Nucleation-Growth: Seeded Growth Method

In another scenario, whereby the formation of the new solid phase or material is taking place in the presence of another material in the media (denoted as seed material). In this case, the nucleation of the new solid phase takes place on the surface of the second material rather than followed the

LaMer mechanism—i.e. there is no embryo formation. Heterogeneous nucleation possess lower

ΔG*(Equation 10), and consequently are more thermodynamically favored relative to homogeneous nucleation.39

This method used for synthesizing heterostructure core-shell or hybrid materials as well as making shape-controlled materials like rods, tetrapods, …etc.

64 Introduction Semiconductor Nanocrystals: An Overview

3.5 Drawback of the Conventional Surfactant-Assisted Synthesis

In the conventional surfactant-assisted synthesis of NCs, surfactants need to be anchored to the

NCs’ surface during their growth.20, 40-41 Long chain organic surface capping ligands are important to control the NC growth, to prevent aggregation, to maintain the desired size, and to passivate surface electronic trapping states in the NCs.20, 28, 40-43 Moreover, the solubility of the semiconductor nanoparticles is dictated by these capping ligands.

A challenge, however, is that this conventional surfactant-assisted synthesis yields NCs with surfaces functionalized with insulating organic ligands. These insulating ligands act as a barrier for charge transport between NCs.44-46 It has been proven by Kuno et al. that complete removal of surface ligands is difficult and can create surface dangling bonds and charge-trapping centers that open up pathways for nonradiative relaxation.47 However, an important feature of these surfactants is that they can be easily exchanged on the surface of the nanocrystals, i.e. process of ligand exchange.

The surface of NCs in solution is usually capped by organic surfactants, such as trioctyl phosphine oxide-functionalized surfaces typical of NCs (CdSe-TOPO NCs). This capping is necessary to prevent NC aggregation as well as passivating the surface states.47-51 However, since the performance of photovoltaic devices depends to a large extent on the mobility of charge carriers through the NC film, organic surfactants negatively affect the performance by acting as a barrier to charge transport between the NCs. The electronic coupling between NCs—and consequently the charge carrier mobility—can be enhanced by removing the organic surfactants from the NC surface by a method to facilitate direct contact between the absorber and the metal-oxide surface.

65 Introduction Semiconductor Nanocrystals: An Overview

3.6 References

Feynman, R. P., There's Plenty of Room at the Bottom. Engineering and Science 1960, 23 (5).

2. Tulkki, F. B. J., Quantum Dots: Phenomenology, Photonic and Electronic Properties, Modeling and Technology. In The Handbook of Nanotechnology. Nanometer Structures: Theory, Modeling, and Simulation, Lakhtakia, A., Ed. 2004; pp 107-143.

3. Iogansen, L. V., The Possibility of Resonance Transmission of Electrons in Crystals through a System of Barriers. Soviet Physics Jetp-Ussr 1964, 18 (1), 146-150.

4. Esaki, L.; Tsu, R., Superlattice and Negative Differential Conductivity in Semiconductors. IBM J. Res. Dev. 1970, 14 (1), 61-65.

5. Chang, L. L.; Esaki, L.; Tsu, R., Resonant tunneling in semiconductor double barriers. Applied Physics Letters 1974, 24 (12), 593-595.

6. Dingle, R.; Wiegmann, W.; Henry, C. H., Quantum States of Confined Carriers in Very Thin AlxGa1−xAs-GaAs-AlxGa1−x As Heterostructures. Physical Review Letters 1974, 33 (14), 827-830.

7. Skocpol, W. J.; Jackel, L. D.; Howard, R. E.; Hu, E. L.; Fetter, L. A., Nonmetallic localization and interaction in one-dimensional (0.1 μm) Si MOSFETs. Physica B+C 1983, 117– 118, Part 2, 667-669.

8. Ekimov, A.; Onushchenko, A., Quantum size effect in three-dimensional microscopic semiconductor crystals JETP lett 1981, 34 (6), 345-349.

9. Ekimov, A. I.; Onushchenko, A. A., Size quantization of the electron energy spectrum in a microscopic semiconductor crystal. Jetp Letters 1984, 40 (8), 1136-1139.

10. Reed, M. A.; Randall, J. N.; Aggarwal, R. J.; Matyi, R. J.; Moore, T. M.; Wetsel, A. E., Observation of discrete electronic states in a zero-dimensional semiconductor nanostructure. Physical Review Letters 1988, 60 (6), 535-537.

11. Kalyanasundaram, K.; Borgarello, E.; Duonghong, D.; Grätzel, M., Cleavage of Water by Visible-Light Irradiation of Colloidal CdS Solutions; Inhibition of Photocorrosion by RuO2. Angewandte Chemie International Edition in English 1981, 20 (11), 987-988.

12. Davis, T., Biography of Louis E. Brus. Proceedings of the National Academy of Sciences of the United States of America 2005, 102 (5), 1277-1279.

13. Rossetti, R.; Brus, L., Electron-hole recombination emission as a probe of surface chemistry in aqueous cadmium sulfide colloids. The Journal of Physical Chemistry 1982, 86 (23), 4470-4472.

14. Brus, L. E., Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. The Journal of Chemical Physics 1984, 80 (9), 4403-4409.

66 Introduction Semiconductor Nanocrystals: An Overview

15. Stuczynski, S. M.; Kwon, Y. U.; Steigerwald, M. L., The use of phosphine chalcogenides in the preparation of cobalt chalcogenides. Journal of Organometallic Chemistry 1993, 449 (1), 167-172.

16. Steigerwald, M. L., Clusters as small solids. Polyhedron 1994, 13 (8), 1245-1252.

17. Green, M., Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis. the Royal Society of Chemistry: 2014; p 277.

18. Steigerwald, M. L.; Brus, L. E., Synthesis, Stabilization, And Electronic-Structure Of Quantum Semiconductor Nanoclusters. Annual Review of Materials Science 1989, 19, 471-495.

19. Alivisatos, A. P.; Harris, A. L.; Levinos, N. J.; Steigerwald, M. L.; Brus, L. E., Electronic states of semiconductor clusters: Homogeneous and inhomogeneous broadening of the optical spectrum. The Journal of Chemical Physics 1988, 89 (7), 4001-4011.

20. Murray, C. B.; Norris, D. J.; Bawendi, M. G., Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites. Journal of the American Chemical Society 1993, 115 (19), 8706-8715.

21. Zhong, H.; Mirkovic, T.; Scholes, G. D., 5.06 - Nanocrystal Synthesis. In Comprehensive Nanoscience and Technology, Wiederrecht, D. L. A. D. S. P., Ed. Academic Press: Amsterdam, 2011; pp 153-201.

22. Peng, Z. A.; Peng, X., Formation of High-Quality CdTe, CdSe, and CdS Nanocrystals Using CdO as Precursor. Journal of the American Chemical Society 2001, 123 (1), 183-184.

23. Hines, M. A.; Guyot-Sionnest, P., Synthesis and Characterization of Strongly Luminescing ZnS-Capped CdSe Nanocrystals. The Journal of Physical Chemistry 1996, 100 (2), 468-471.

24. Manna, L.; Scher, E. C.; Alivisatos, A. P., Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, and Tetrapod-Shaped CdSe Nanocrystals. Journal of the American Chemical Society 2000, 122 (51), 12700-12706.

25. Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P., Shape control of CdSe nanocrystals. Nature 2000, 404 (6773), 59-61.

26. Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A., High-Quality Manganese-Doped ZnSe Nanocrystals. Nano Letters 2001, 1 (1), 3-7.

27. Qu, L.; Peng, Z. A.; Peng, X., Alternative Routes toward High Quality CdSe Nanocrystals. Nano Letters 2001, 1 (6), 333-337.

28. Peng, Z. A.; Peng, X., Nearly Monodisperse and Shape-Controlled CdSe Nanocrystals via Alternative Routes: Nucleation and Growth. Journal of the American Chemical Society 2002, 124 (13), 3343-3353.

29. Kim, S.; Fisher, B.; Eisler, H.-J.; Bawendi, M., Type-II Quantum Dots: CdTe/CdSe(Core/Shell) and CdSe/ZnTe(Core/Shell) Heterostructures. Journal of the American Chemical Society 2003, 125 (38), 11466-11467.

67 Introduction Semiconductor Nanocrystals: An Overview

30. Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; et al., Colloidal synthesis of nanocrystals and nanocrystal superlattices. IBM Journal of Research and Development 2001, 45 (1), 47-56.

31. Hines, M. A.; Scholes, G. D., Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Advanced Materials 2003, 15 (21), 1844-1849.

32. Cao, Y. C.; Wang, J., One-Pot Synthesis of High-Quality Zinc-Blende CdS Nanocrystals. Journal of the American Chemical Society 2004, 126 (44), 14336-14337.

33. Yang, Y. A.; Wu, H.; Williams, K. R.; Cao, Y. C., Synthesis of CdSe and CdTe Nanocrystals without Precursor Injection. Angewandte Chemie International Edition 2005, 44 (41), 6712-6715.

34. Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisatos, A. P., Seeded Growth of Highly Luminescent CdSe/CdS Nanoheterostructures with Rod and Tetrapod Morphologies. Nano Letters 2007, 7 (10), 2951-2959.

35. LaMer, V. K.; Dinegar, R. H., Theory, Production and Mechanism of Formation of Monodispersed Hydrosols. Journal of the American Chemical Society 1950, 72 (11), 4847-4854.

36. Camargo, P. H. C.; Rodrigues, T. S.; da Silva, A. G. M.; Wang, J., Controlled synthesis: Nucleation and growth in solution. In Metallic Nanostructures: From Controlled Synthesis to Applications, Springer International Publishing: 2015; pp 49-74.

37. Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E., Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angewandte Chemie International Edition 2009, 48 (1), 60-103.

38. Kudera, S.; Carbone, L.; Manna, L.; Parak, W. J., Growth mechanism, shape and composition control of semiconductor nanocrystals. Springer: 2008.

39. Xiong, Y.; Lu, X., Metallic Nanostructures: From Controlled Synthesis to Applications. Springer International Publishing: 2014; pp 49-74.

40. Yu, W. W.; Wang, Y. A.; Peng, X., Formation and Stability of Size-, Shape-, and Structure- Controlled CdTe Nanocrystals: Ligand Effects on Monomers and Nanocrystals. Chemistry of Materials 2003, 15 (22), 4300-4308.

41. Yu, W. W.; Peng, X., Formation of High-Quality CdS and Other II–VI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity of Monomers. Angewandte Chemie International Edition 2002, 41 (13), 2368-2371.

42. Peng, X.; Wickham, J.; Alivisatos, A. P., Kinetics of II-VI and III-V Colloidal Semiconductor Nanocrystal Growth: “Focusing” of Size Distributions. Journal of the American Chemical Society 1998, 120 (21), 5343-5344.

43. Pradhan, N.; Reifsnyder, D.; Xie, R.; Aldana, J.; Peng, X., Surface Ligand Dynamics in Growth of Nanocrystals. Journal of the American Chemical Society 2007, 129 (30), 9500-9509.

68 Introduction Semiconductor Nanocrystals: An Overview

44. Hassan, Y.; Chuang, C. H.; Kobayashi, Y.; Coombs, N.; Gorantla, S.; Botton, G. A.; Winnik, M. A.; Burda, C.; Scholes, G. D., Synthesis and optical properties of linker-free TiO2/CdSe nanorods. Journal of Physical Chemistry C 2014, 118 (6), 3347-3358.

45. Kovalenko, M. V.; Scheele, M.; Talapin, D. V., Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands. Science 2009, 324 (5933), 1417-1420.

46. Draguta, S.; McDaniel, H.; Klimov, V. I., Tuning Carrier Mobilities and Polarity of Charge Transport in Films of CuInSexS2–x Quantum Dots. Advanced Materials 2015, 27 (10), 1701-1705.

47. Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G., The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state. Journal of Chemical Physics 1997, 106 (23), 9869-9882.

48. Micic, O. I.; Sprague, J.; Lu, Z.; Nozik, A. J., Highly efficient band-edge emission from InP quantum dots. Applied Physics Letters 1996, 68 (22), 3150-3152.

49. Mattoussi, H.; Cumming, A. W.; Murray, C. B.; Bawendi, M. G.; Ober, R., Characterization of CdSe nanocrystallite dispersions by small angle x-ray scattering. Journal of Chemical Physics 1996, 105 (22), 9890-9896.

50. Majetich, S. A.; Carter, A. C., Surface effects on the optical properties of cadmium selenide quantum dots. The Journal of Physical Chemistry 1993, 97 (34), 8727-8731.

51. Majetich, S. A.; Carter, A. C.; Belot, J.; McCullough, R. D., 1H NMR Characterization of the CdSe Nanocrystallite Surface. The Journal of Physical Chemistry 1994, 98 (51), 13705-13710.

69 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

Chapter 4:

Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

This chapter is reproduced in part with permission from the following publication:

“Yasser Hassan, Chi-Hung Chuang, Yoichi Kobayashi, Neil Coombs, Sandeep Gorantla, Gianluigi A. Botton, Mitchell A. Winnik, Clemens Burda, and Gregory D. Scholes, J. Phys. Chem. C 2014, 118, 3347−3358”.

Contribution of Authors in this Chapter:  YH had the idea for, conceived and directed the project, designed the experiments,

carried out the synthesis of materials, and performed data analysis.

 CC performed the transient absorption measurements. YK performed the transient

absorption data analysis. CB supervised this experiment results and discussion.

 YH and NC carried out the electron microscope measurements. YH did the morphology

analysis of the materials.

 SG carried out the electron energy loss spectroscopy measurements. GB supervised this

experiment’s results and its discussion.

 YH, and GDS wrote the manuscript for publication in JPCC.

 All authors discussed the results and commented on the manuscript.

 GDS, MAW and CB supervised the whole project.

70 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.1 Abstract:

Linker–free TiO2/CdSe hybrid nanorods (NRs) were prepared by growing CdSe QDs in the presence of TiO2 NR seeds using seeded–growth type colloidal injection approach. TEM studies revealed the abundant formation of anatase TiO2 NRs with different lengths. We found that the as- prepared TiO2 seeds determined the morphology of TiO2/CdSe NRs. The photophysics of these materials were studied by photoluminescence (PL) and femtosecond transient absorption spectroscopy. While we observed the efficient PL quenching in TiO2/CdSe NRs, the bleach dynamics of TiO2/CdSe NRs is similar to that of CdSe NRs. It suggests that while surface traps that arise from the lattice mismatch between CdSe and TiO2 are mainly observed in transient absorption measurements, the ultrafast exciton dissociation over the experimental time resolution occurs in TiO2/CdSe NRs.

Figure 4.1: Illustration cartoon with TEM images of linker-free TiO2/CdSe nanorods prepared in this chapter.

71 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.2 Introduction

4.2.1 Titanium Dioxide as Light Harvester

Conversion of solar energy to electrical energy can be initiated by injecting electrons into a metal oxide from a photo-excited sensitizer. Using inorganic semiconductor materials as light harvesting sensitizers has drawn a lot of attention in the past years.1-15 Titanium dioxide is concerned a great consideration since the phenomenon of photocatalytic splitting of water on TiO2 electrodes which discovered by Fujishima and Honda in 1972.16 It is considered as one of the most important wide gap semiconductor and is widely explored for exploiting in hydrogen production17, photocatalysis18, environmental purification19, gas sensing20 and heterojunction solar cells.21,22

On their own, TiO2 films are inadequate absorbs of visible light due to their large band gap (3.2 eV).23,24 O’Regan and Grätzel in 1991 invented a new design of solar cells known as dye-sensitized solar cell (DSSC) which has attracted much attention all over the world in both academic and industrial field as a promising alternative to silicon-based solar cells.25,26 DSSC is based on photosensitization of photoanode of mesoporous wide-bandgap semiconductors oxide materials, such as TiO2 (or alternatively SnO2 or ZnO) films, using sensitizers to harvest sun light in visible region. Due to the wide band gap, 3.2 eV, of TiO2 which prevents the TiO2 from efficient absorption of sunlight in the visible region23,24, the performance of a these types of cells is intimately controlled by the sensitizer adsorbed on the TiO2 surface. Sensitizers of organic ruthenium complexes and narrow band gap semiconductors, which absorb light in the visible, can serve as sensitizers since they are able to transfer electrons into large band gap material.

72 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.2.2 Semiconducting Nanocrystals as Sensitizers

Semiconducting nanocrystals (NCs), also known as quantum dots (QDs) are versatile fluorescence probes with interesting physical, electronic properties27-28, and unique optical properties, including tunable luminescence, high extinction coefficient, broad absorption with narrow photoluminescence, and photobleaching resistance.27-31 Using these materials as building blocks to design light harvesting assemblies, also known as quantum dot sensitized solar cells (QDSSCs) has drawn a lot of attention in the past years.1-14, 32

Although natural organic dyes have achieved an energy conversion efficiency approximately 11%

33,34-35, they have many limitations, for instance its higher cost, shorter lifetime and poorer stability weighed against inorganic sensitizers. Over the past two decades, low band gap QDs have been shown to be effective as an alternative to molecular dyes for sensitizing TiO2 films and thereby generating photocurrents—i.e. they are promising solution to the dyes’ limitations. QDs can be easily adsorbed to the surfaces of wide-bandgap semiconductors oxide materials, such as SnO2,

ZnO, and TiO2.

In a semiconductor sensitized solar cell, nanocomposites of QDs with TiO2 or SnO2 have been widely used to separate exciton into individual electron onto TiO2 and hole onto QDs.

Among the colloidal semiconductor, QDs, examples include CdS36-58, CdSe43, 50, 59-97, PbS14, 45, 98-

100 100 68, 101-103 104 105 87, 106 , Bi2S3 , CdTe , InAs , InP and PbSe have continuously been a focus of concern in sensitized solar cells. Moreover, recently combination such as CdS/CdSe91, 107-108 and

CdS/CdSe/ZnS64, 109 systems have been suggested as they are advantageous over single QDs- sensitization, relating to the extension of spectral response in the visible light region and charge injection from QDs to TiO2.

73 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.2.3 Advantages of Quantum Dot Sensitized Solar Cells (QDSSCs)

The potential Advantages of Quantum Dot Sensitized Solar Cells (QDSSCs) include:

(1) Quantum confinement effect110, size quantization effect, the most important feature of

QDs, allows one to tune the band energy and the energy of the electronic levels that achieved by manipulating and changing the particle size and composition of the quantum dots the size of semiconductor nanocrystals. As a result, improves the visible response by covering a large fraction of the solar spectrum over a wide range of visible-IR light.111-112 This effect furthermore tunes the energetics at the interfaces of the QDs with the surrounding media (hole and electron transporting materials).

(2) QDs have the properties of high absorption cross-section, which increase light absorption process, due to their extremely high molar extinction coefficients of ~1 × 105 cm−1M −1 at the first excitonic peak in comparison with ~1.3 × 104 cm−1 M−1 for a typical Ru-based dyes113. This nature helps to reduce the dark current and increase the overall efficiency of the solar cells; additionally these two advantages can improve the short circuit current density of these photovoltaics. The former and later advantages together can improve the short circuit current density of photovoltaic devices.

(3) The inorganic nature of QDs offers the advantages of enhanced-stability compared to conventional dyes.

(4) Preparation of well-defined colloidal QDs via wet chemical synthesis, 114 is simple and cheap process by which different compositions can be obtained with a high luminescence quantum yield and a narrow size distribution.

74 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

(5) Their quantum confinement effect makes it possible to exploit hot electrons in QDs to generate multiple electron-hole pairs, or excitons, per photon through the impact ionization effect (an inverse Auger process).115,116

4.2.4 QDSSCs Efficiency Challenges

Conversion of solar energy to electrical power requires sequential steps to be involved, See

cartoon in Figure 4.2. These steps include: (1) Absorption of photons and forming excitons, (2)

Separation of exciton into individual electron and “vacant orbital or hole” pair at lattice boundary

interfaces, 3) Transportation of carriers to respective electrodes, and (4) Collection of carriers at

the electrodes. The power conversion efficiency is the product of the efficiency of each step.

Despite such potential advantages over organic dyes sensitizers of DSSCs, quantum dot sensitized solar cells (QDSSCs) have not achieved efficiencies or stabilities that are satisfactory competitive

75 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods with conventional dye sensitized solar cells. In other words the photovoltaic performance of the device is still much lower than that of the DSSCs.

These limitations are attributed to:

(1) The difficulties of assembling and filtration of QDs into the pores of the mesoporous TiO2 matrix to obtain a well-covering QD as monolayer on the TiO2 crystalline surface whilst preserving control over particle size, spectral properties, and the interconnectivity of QD substrate.

(2) Deposition of QDs may block the entrance of TiO2’s pores, and therefore the electrolyte poorly contact the QDs inside the pores which leads to dropping the open-circuit voltages (Voc) for

QDSSCs. 109

(3) The serious electron loss due to the charge recombination at TiO2/CdSe interface where the hole removal is slow that enhances the recombination efficiency and make the QDs undergo serious degradation as they oxidized by iodine-based electrolytes, presumably due to CdI2 formation117, and lead to instability of the nanocrystals.

(4) In addition, the spectral properties of the applicable sensitizers require the use of more than one absorber in order to harvest the solar spectrum.

(5) Finally, a biggest challenge is that QDs have surfaces functionalized with organic ligands, which are essential as they prevent QD aggregation and passivate surface states118-122. Similarly,

TiO2 surfaces are hydroxylated (Figure 4.3a). Because the performance of photovoltaic devices depends to a large extent on the mobility of charge carriers through the QD film, unfortunately, these insulating organic surfactants act as a barrier for charge transport between the QDs and TiO2.

76 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.2.5 Common Methods for Assembling QDs into Mesoporous TiO2 Thin Films

The electronic coupling between QDs, and consequently the charge carrier mobility, can be enhanced by removing the organic surfactants from the QD surface to facilitate direct contact between the absorber and the metal-oxide surface.1, 123-125

In the literature, three techniques were generally used to assemble QDs into a mesoporous thin film of TiO2 (large band gap material), with the ultimate goal of trying to enhance the charge carrier mobility.

The first technique involves assembling pre-prepared QDs using bifunctional linkers to achieve the attachment of the QDs to the oxide, Figure 4.3b.50, 126-127 It was found that the chemical nature of the linker plays a decisive role in determining the efficiency of electron injection into the matrix.

Moreover, linker molecules used to bridge nanocomposites have substantially hindered the electron transfer yielding the low efficiency.

The second technique is physisorption of QDs to the metal oxide substrate as reported by Arthur

J. Nozik and his coworkers.104, 128-129

The third technique involves avoiding the use of separately synthesized colloidal quantum dots and as an alternative; utilized chemical bath deposition61, 130-131 or successive ionic layer adsorption reaction (SILAR) to grow semiconductor QDs directly on the oxide architecture.132-135 In addition, some people reported a certain combination of these techniques, efficiency as high as 1.84–4.22% which can be obtained for the coupling-layered CdS/CdSe or CdSe/ZnS electrodes.75, 136-138

Although these techniques guarantee high optical densities in the visible region, and a direct contact between the oxide and the QDs is achieved in some cases, they have disadvantages. For

77 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods instance, there are two main disadvantages affecting their solar cell efficiency. First, linker molecules used to bridge nanocomposites hinder the electron injection into TiO2 and reduce carrier mobility, as in the case of using bifunctional linkers.127, 139 Second, there may be difficulties preparing stable anions such Se2- and Ti2- and losing the control over the QD size and size distributions can be a problem in case of applying chemical bath deposition or SILAR.140

78 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.2.6 Objectives and Approach of Our Work in This Chapter

As previously mentioned the big challenge, however, is that trioctylphosphine oxide functionalized surfaces typical of QDs (CdSe-TOPO QDs) and hydroxylated TiO2 surfaces inhibit interfacial charge transfer processes. The electronic coupling between QDs, and consequently the charge carrier mobility, can be enhanced by removing the organic surfactants from the QD surface by one method to facilitate direct contact between the absorber and the metal-oxide surface.

Here in this chapter, as a contribution to alleviate this problem, we investigate the direct contact between the nanocrystalline absorber and the metal-oxide surface. We present a colloidal route for the preparation and characterization of linker-free TiO2/CdSe type II heterostructure—hybrid— nanorods (NRs) that prepared by growing CdSe QDs in the presence of TiO2 NR seeds using seeded-growth type colloidal injection approach.141 Using this method, we were able to produce a new composite of linker-free isolated and bundled TiO2/CdSe NRs. This method based on growing the semiconductor QDs directly on the oxide architecture132-135 and avoiding the use of separately synthesized colloidal quantum dots.

In this chapter, we report characterization results of the linker-free TiO2/CdSe with different morphological structure and their photophysics with a focus on characterizing the morphology- dependent exciton dynamics and electron transfer (ET) efficiency. Our synthetic technique of

TiO2/CdSe NRs accomplished in two steps;

(a) First, the synthesis of TiO2 nanocrystals (seeds) with nanorod shapes were accomplished by an aminolysis reaction of titanium isopropoxide by oleylamine (OLA) at high temperature using a modification of previous methods of nonhydrolytic synthetic approach.142-145 This method yields good anatase titania nanocrystals146 caped with the appropriate ligands which used normally in the

QDs preparation.

79 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

(b) In a second step, TiO2/CdSe NRs, prepared by incorporating CdSe NCs grown in the presence of TiO2 NRs, which were synthesized by the seeded-type colloidal injection growth technique used previously for CdTe/CdSe type II heterostructures.141

This method enabled us to control and tuning the average QDs sensitizer’s diameter during synthesis and consequently adjustment the electronic levels which is an essential parameter to tune the band offset between the donor and acceptor conduction band edges. Moreover, our synthetic technique lends a hand of the formation of a near-epitaxial interface between the two composite and enables a rapid and a direct injection of photoinduced carriers from CdSe into the oxide

147 TiO2.

4.3 Experimental Section

General Methods. All procedures were carried out using standard Schlenk line techniques under oxygen-free conditions and nitrogen flow.

4.3.1 Materials.

All chemicals were used as received without further purification.

Titanium isopropoxide (TTIP) (99 %), cadmium oxide (CdO) (99.99 %), selenium (Se) (100 mesh,

99.5 %,), oleic acid (OA) (90 %), oleylamine (OLA) (70 %), 1–octadecene (90 %), lauric acid

(LA) (99%), trioctylphosphine oxide (TOPO) (99 %) and trioctylphosphine (TOP) (90 %) were purchased from Sigma-Aldrich and used without any purification.

Phosphonic acids like tetradecyl phosphonic acid (TDPA), n–hexyl phosphonic acid (HPA) and octyl phosphonic acid (OPA) were purchased from PCI Synthesis Inc., and used without any purification.

80 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

All the solvents such as chloroform, methanol, toluene and isopropanol are anhydrous.

81 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.3.2 Synthesis of TiO2 nanorod seeds

The synthesis of TiO2 nanocrystals with nanorod shapes were accomplished by an aminolysis reaction of titanium isopropoxide by oleylamine (OLA) at high temperature using a modification of previous methods.142-145

Typically, TiO2 nanocrystals were synthesized by the following procedure:

4.3.3 TiO2 Short Rods (30–50 nm):

1. Lauric acid (LA) and oleic acid (OA), each 16 mmol in 5 ml 1–ODE were dried and degassed at 110 °C in a 50 mL three–neck flask for 1 h to remove all the traces of water and oxygen.

2. After that, TTIP (8 mmol) was injected into the solution and hold under vacuum for 30 minutes. The solution gradually turned from colorless to a pale yellow, indicating the formation of titanium carboxylate complexes.143

3. Then the system was exchanged to nitrogen flow and the temperature was raised to 270 °C.

The solution turned to green yellowish then become lighter when reaching 270 °C.

4. The aminolysis reaction was initiated by the rapid injection of previously degassed OLA

(3 ml) under vigorous stirring and the reaction kept at that point for 1.5 hours at 250 °C. A white precipitate (suspension) was formed 30 minutes after OLA injection.

82 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.3.4 TiO2 Long Rods (80–100 nm):

In order to elongate the rods length, we increased the reaction time, after OLA injection, to be 3 hours instead of 1.5 hours. In this case we used oleic acid as source of carboxylate complexes143 without the lauric acid.

Once the system cooled down to room temperature, excess anhydrous toluene was added to the

TiO2 nanorods, which were then precipitated by adding anhydrous methanol and TiO2 was separated by centrifugation. The sample was washed several times by repeated methanol precipitation and toluene redispersion (volume ratio of methanol: toluene = 3:1). The resulting white powder could be redispersed easily into non–polar solvents such as toluene or chloroform for further characterization.

4.3.5 Preparation of TiO2 Seeds Bundled Sample:

Typically, bundled TiO2 nanocrystals were synthesized by the following procedure:

1. Lauric acid (LA) + oleic acid (OA) each 8 mmol in 5 ml 1-ODE was dried and degassed at

110 °C in a 50-mL three-neck flask for 1 h to remove all the traces of water and oxygen.

2. After that a TTIP (8 mmol) was injected into the solution and continue under vacuum for more 30 minutes. The solution gradually turned from colorless to a pale yellow, indicating the formation of titanium carboxylate complexes.

3. Then the system was exchanged to nitrogen flow and the temperature was raised to 270 °C.

The aminolysis reaction was initiated by the rapid injection of previously degassed OLA (3 ml) under vigorous stirring and the reaction kept at that point for 30 minutes. The solution was turned to remarkable viscous solution with orange yellowish which decreased with cooling down to room temperature.

83 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4. Once the system cooled down to room temperature, excess anhydrous ethanol was added to precipitate TiO2 nanorods, which were then separated by centrifugation.

Figure 4.4: Non-hydrolytic Synthesis scheme of TiO2 Nanorods. The three neck flask and reaction system cartoon (top left) drawn by Dr. Tihana Mirkovic.

4.3.6 Synthesis of TiO2/CdSe Nanohybrids.

We prepared five batches of TiO2/CdSe heterostructure nanohybrids (NH). Four samples were prepared by using the two previously mentioned short and long TiO2 NR seeds. For the fifth sample we used bundled shape TiO2.

4.3.7 Synthetic Method for TiO2/CdSe Samples 1 to 4:

1. In a typical synthesis of TiO2/CdSe short rods heterostructure, calculated amount of

(TOPO, TDPA, HPA/or OPA) and CdO were mixed in a 50 mL three–necked flask. Details are presented in Table 4.1.

84 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

2. After the reaction flask was pumped under vacuum for ~1 h at 120°C, the solution was heated to 320~340°C under argon to yield a colorless Cd complex solution.

3. The solution was then cooled to 120°C and held under vacuum for 2 h to remove water.

4. A fixed amount (in mg) of well–washed TiO2 short (30–50 nm in length), used for sample

1 or long (80–100 nm in length) NRs, used for samples 2, 3 and 4, was dissolved in a mixture of

2mL ODE and 2mL TOP, degassed well by nitrogen. This solution was then added to the Cd complex solution.

5. The mixture was held under vacuum for a further 1 h before the flask was allowed to return to 270°C under nitrogen.

6. At that point the Se–precursor was added drop–wise into the reaction at a rate of 0.4 mL/min. The Se (0.1 mmol/mL) solution, prepared by dissolving 0.039 g (0.5 mmol) Se powder in 2 mL TOP and 3 mL ODE. This Se–precursor degassed and stored under nitrogen before it used.

7. The reaction temperature was adjusted to 250 °C, and the reaction was stopped after 30 minutes by the removal of the heating mantle and the injection of anhydrous toluene.

8. The nanorods were isolated and cleaned by a few precipitation and re–dissolution cycles using toluene or chloroform as solvent and methanol and isopropanol mixture as the non–solvent

(1:3 solvent to non–solvent ratio). Precipitation was achieved by centrifugation for 20–30 min under 6000 rpm. It is worth to mention here that these heterostructures are easily soluble in organic nonpolar solvent (toluene) and precipitated in organic polar solvent like methanol.

9. The growth of TiO2/CdSe NRs was monitored by taking aliquots from the reaction and measuring absorption and photoluminescence spectra.

85 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

These samples are stable colloidal solutions for more than one year if they are firmly sealed.

4.3.8 Synthesis of TiO2/CdSe NHs sample (5)

Sample 5 of TiO2/CdSe (bundled NRs) has been prepared using these bundled TiO2 nanoparticles.

In a typical synthesis of TiO2/CdSe NHs sample (5):

1. 3.4 g TOPO, 0.3 g TDPA, 0.3 g HPA and 0.128 g CdO were mixed and pumped under vacuum for ~1 h at 120°C, the resulting solution was heated to 320~340°C under argon to yield a colorless Cd complex solution.

2. The solution was then cooled to 120°C and put under vacuum for 2 h for removal of water.

3. A fixed amount (in mg) of well-washed and stored TiO2 NCs is dissolved in mixture of

(2mL) ODE and (2mL) TOP, was then added to the preformed Cd complex solution.

4. Then the mixture allowed continuing under vacuum for further 1 h and the flask was allowed to return to 270°C under nitrogen.

5. At that point the as prepared Se-precursor was added drop-wise into the reaction at a rate of 0.4 mL/min. The Se (0.1 mmol/mL) solution, prepared by dissolving 0.039 g (0.5 mmol) Se- powder in 2 mL TOP and 3 mL ODE and degassed very well.

6. The reaction temperature was adjusted to 250 °C, and the reaction was stopped after 30 min by the removal of the heating mantle and the injection of excess of anhydrous toluene.

4.3.9 CdSe QDs Control Experiment

For comparison, CdSe nanorods were also synthesized following the similar procedures without adding any TiO2 seeds.

1. 3.4 g TOPO, 0.3 g TDPA, 0.3 g HPA and 0.128 g CdO were mixed in a 50 mL three necked flask. After the flask was pumped under vacuum for ~1 h at 120°C, the resulting solution was heated to 320~340°C under argon to yield a colorless Cd complex solution.

86 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

2. The solution was then cooled to 120°C and put under vacuum for 2 h for removal of water.

3. Then the flask was allowed to return to 270°C under nitrogen. At that point the as prepared

Se-precursor was added drop-wise into the reaction at a rate of 0.4 mL/min. The Se (0.1 mmol/mL) solution, prepared by dissolving 0.039 g (0.5 mmol) Se-powder in 2 mL TOP and 3 mL ODE and degassed very well.

4. The reaction temperature was adjusted to 250 °C, and the reaction was stopped after 30

min by the removal of the heating mantle and the injection of excess of anhydrous toluene.

Table 4.1. Experimental quantities for Cd- precursors used in each sample.

CdO TOPO TDPA HPA OPA HAD TiO2-seeds (g) (g) (g) (g) (g) (g)

Sample 1 0.128 3.4 0.3 0.3 -- -- Short rods 30-50 nm

Sample 2 0.0512 5 0.7 -- -- 0.3 Long rods 80-100 nm Sample 3 0.0512 5 0.3 -- 1.0 -- Long rods 80-100 nm Sample 4 0.0512 5 0.3 -- -- 0.7 Long rods 80-100 nm Sample 5 0.128 3.4 0.3 0.3 -- -- Bundled

(bundled) TiO2

CdSe 0.128 3.4 0.3 0.3 -- -- No TiO2 NRs alone used

*Short TiO2 NRs (30-50 nm) represented in Figure 4.5a & Figure 4.5c.

*Long TiO2 NRs (80-100 nm) represented in Figure 4.5b & Figure 4.5d.

87 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.4 Characterization.

1. Size distributions and energy–dispersive X–ray spectra of the prepared TiO2/CdSe NRs were determined by transmission electron microscopy (TEM) with a Libra 200EF (Zeiss) at 200 kV.

2. UV–vis absorption and PL spectra were recorded using a Varian Cary 50 and Varian

Eclipse fluorescence spectrometer, respectively. UV–vis characterization took place in 1 cm cuvette.

3. High–resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM) images and electron energy loss spectroscopy (EELS) data were recorded using a FEI Titan 300 keV aberration–corrected microscope (CCEM facility at McMaster

University).

4. Femtosecond transient absorption (TA) measurements were conducted using a Clark MXR

2001 femtosecond laser system producing 780 nm, 150 fs pulses from a regenerative amplifier.148-

149 The laser pulse train was split to generate a white light continuum (450–800 nm) probe pulse in a sapphire crystal and a tunable (460–1100 nm) pump pulse. The TiO2/CdSe samples were excited at 620 or 650 nm wavelength light generated by using an optical parametric amplifier OPA

(TOPAS, Light conversion). All femtosecond laser experiments were carried out in a 2 mm path length quartz cuvette at room temperature. The instrument time resolution was determined to be

~150 fs via a pump–probe cross–correlation analysis. The pump power was carefully controlled to avoid multi–exciton production in the NRs. The excitation power in TA (~150 J/cm2 fluence per pulse) measurements was considered carefully to ensure that the average number of excitations per rod was less than one.

88 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.5 Results and Discussion

The TiO2/CdSe nanohybrids investigated in our work were prepared according to a colloidal route.

142- TiO2 seeds were first synthesized using a slightly modified procedure from the previous work,

145 by an ester aminolysis non-hydrolytic synthetic approach to form anatase titania nanocrystals146 capped with lauric acid and/or oleic acid ligands. TiO2/CdSe nanorods (NRs), incorporating CdSe

NCs grown in the presence of TiO2 NRs were synthesized by the seeded-type colloidal injection growth technique used previously for CdTe/CdSe type II heterostructures.141

It was found that the time of the aminolysis reaction affected the structure of the TiO2 product. In the first experiment, the reaction took place for only 30 minutes after oleylamine (OLA) injection and resulting TiO2 bundled structure (consisting of a mixture of TiO2 nanoparticles and nanorods), shown in Figure 4.5c. Accordingly, we modified the recipe by increasing both the concentration of ligand and the time of the reaction. The time of the reaction in this experiment considered after injection of oleylamine to the oleic/lauric acid solution mixture with titanium tetraisopropoxide.

In other experiments, the time of the reaction was extended to 1.5 and 3 hours. This adaptation led to well-formed and isolated TiO2 short (30–50 nm) and long NRs (80–100nm) with anatase lattice structure, Figure 4.5.

In this reaction, TiO2 is formed by ester aminolysis reaction that involves the nucleophilic attack of an amine group of oleylamine on the carbonyl carbon atom of titanium carboxylate (titanium oleate) complexes at high temperature to produce good nanorods of TiO2 in the presence of oleic acid and lauric acid as the capping ligands. We found experimentally this method provides a TiO2 without byproducts, such as HCl in other ordinary methods, which affect the CdSe formation in

142 the next step as in other TiO2 preparation methods. In the second step, the heterostructure of

89 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

TiO2/CdSe is then prepared by introduce the first prepared TiO2 as a seed for the further reaction synthesis of CdSe.

We have prepared two different kinds of TiO2/CdSe heterostructure morphologies using different structures of TiO2 which presented in Figure 4.5; TiO2/CdSe (bundled NRs) using the mixed TiO2 nanoparticles (Figure 4.5a) and TiO2/CdSe (isolated NRs) using TiO2 nanorods (Figure 4.5b).

The synthesis of the TiO2/CdSe NRs in this work demonstrated good reproducibility.

TiO2 NRs (30–100nm) length capped with lauric and/or oleic acids were used as a seed for the preparation of TiO2/CdSe nanohybrids. By changing the synthetic recipe, different shapes of TiO2

NRs, either “short or long” isolated nanorods or bundled nanorods, were obtained. The morphologies of TiO2 seeds (“short” rods, 50 nm in length and “long” rods of 100 nm in length) and TiO2/CdSe NRs were characterized by TEM, shown in Figure 4.5 and Figure 4.6, respectively.

4.5.1 Morphology Characterization.

The morphology of TiO2 NR seeds after preparation was characterized by electron microscopy and powder X-ray diffraction. Short TiO2 (30–50 nm) NRs and long TiO2 (80–100nm) NRs are shown in (Figure 4.5a and Figure 4.5b), respectively. High resolution TEM (HRTEM) of the isolated long TiO2 NRs sample showed a crystalline structure with 0.35 nm spacing consistent with the (101) plane of anatase TiO2, Figure 4.5d.

90 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

2, 3 and 4). (c) TEM image of bundled TiO2 (d) HRTEM images of long TiO2 NRs.

We found that the nature of the as–prepared TiO2 seeds dramatically determined the morphology of TiO2/CdSe NRs. Figure 4.6a shows TiO2/CdSe sample 1, “short” NRs with 50 nm in length, similar to their TiO2 seeds. The isolated “long” TiO2/CdSe NRs samples 2, 3 and 4 in Figure 4.6b– d have average dimensions of ~100 nm in length and ~7 nm in diameter, estimated by examining more than 200 NRs. It is clear from these TEM images that the particles are significantly different only in their thickness from the TiO2 rods used to seed the reaction.

91 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

Figure 4.6: Annular dark-field STEM images of TiO2/CdSe NRs (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4, respectively.

Figure 4.7 shows the powder X-ray diffraction (PXRD) spectra which we carried on to examine the structure and crystalline nature of two TiO2/CdSe samples heterostructures and their corresponding TiO2 seeds.

We found that our aminolysis reaction produces anatase titanium dioxide lattice structures for the bundled and isolated NRs TiO2, see Figure 4.7a and Figure 4.7b. In Figure 4.7b, the peaks of

TiO2 crystals are not well-formed which due to the lack of crystallinity in this sample. However, for the other modified sample of TiO2, short NRs, Figure 4.7a, it shows a perfect anatase TiO2 features. It is possible that a longer reaction time at high temperature (270 °C) improves the crystallinity of TiO2 by annealing of the defects and due to adding more atoms of TiO2 on the c- axes (001 peak) which is preferred in presence of oleic acid.

92 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

Figure 4.7: X-ray diffraction patterns of (a) isolated TiO2 (short nanorods, 30-50 nm in length), (b) bundled

TiO2, (c) isolated TiO2/CdSe, and (d) bundled TiO2/CdSe NRs. TiO2 seeds show the anatase structure while the bundled one has lower crystallinity. Both isolated and bundled TiO2/CdSe samples have the wurtzite structure after the CdSe deposition.

The PXRD pattern of TiO2/CdSe nanohybrids, sample 2 and 5, synthesized at 270 °C, shows only the peaks of the CdSe phase with the wurtzite structure, and one cannot see any clear indication of

TiO2. This is because the first three diffraction peaks of the cubic CdSe phase overlaps with the

150 (101) peak of the anatase TiO2 phase. In Figure 4.7c and Figure 4.7d, the TiO2/CdSe heterostructure samples confirms a formation of CdSe hexagonal wurtzite structure with the characteristic triplet of (110), (103) and (112) peaks, upon the surface of anatase TiO2 crystals where these peaks are indicative of elongation in a preferred direction.

It is also clear that the (002) reflection is significantly narrower and longer than both its neighbors

(100) and (101). This is evidence for elongation of the domains along the c-direction, the (001) direction by 50 nm.151-153 The (110) peak is associated with the diameter, which we find to be 7 nm

93 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods for this particular sample. Both unusually strong and well-defined peaks of PXRD and the TEM images indicate a narrow particle size distribution of the as-prepared heterostructures.

High-resolution TEM (HRTEM) HRTEM (taken for one single nanorod sample of the composite

(isolated sample)), reveal an apparent contrast between the rods and the dark spot on its surface

(Figure 4.8a). This might indicate the existence of CdSe nanoparticles on the surface of TiO2 nanorods. On the other hand, Figure 4.8b, under HRTEM we obtained three kinds of lattice planes in a single nanorod for the bundled structure. The crystalline structure with 0.35 nm spacing could

154 be from (101) plane of anatase TiO2 while the d-spacing of 0.33 and 0.31 nm lattice planes is close to the interplane distance of the (101) planes in hexagonal CdSe, indicating the shell was

155 hexagonal CdSe. This result indicates that CdSe NRs are growing directly off the TiO2 seeds.

Since the lattice d-spacing of TiO2 and CdSe is very similar, so energy- dispersive X-ray (EDX) measurements (Figure 4.9) and confirmed by XRD (Figure 4.7) have been carried in order to figure out the constitution of original TiO2 NRs and CdSe deposition.

Figure 4.8: (a and b) HRTEM images of isolated (sample 1) and bundled (sample 5) TiO2/CdSe NRs, respectively.

94 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.5.2 Chemical Analyses

To check the chemical composition of the original TiO2 NRs compared to the TiO2/CdSe composite samples several measurements were carried out such as energy–dispersive X-ray

(EDX), TEM, HRTEM and scanning transmission electron microscopy–electron energy loss spectroscopy (STEM–EELS).

4.5.2.1 EDX Analysis

EDX results, shown in Figure 4.8, indicate the traces of Cd, Se, Ti, and O elements for TiO2/CdSe composite sample, respectively. However, EDX measurements of our hybrids did not exhibit a strong peak for TiO2. Therefore, STEM–EELS measurements were performed on one patch of these rods to check whether CdSe did deposit on the TiO2 or not.

Figure 4.9: (a) Energy-dispersive X-ray spectrum (EDX) of TiO2/CdSe heterostructured NRs (sample #5).

A STEM image of TiO2/CdSe NRs: The rectangular box is for EDX analysis corresponding to the spectrum.

(b) EDS of TiO2/CdSe NRs (sample #2): The inset shows an area selected for EDX analysis.

95 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.5.3 STEM–EELS Analysis

The STEM-EELS technique is a transmission scattering experiment in which a monochromatic beam of electrons is incident on the sample with inelastic scattering and passing through the sample

(which is one nanorod in our case here).156-157 Once focused, an X-ray beam is made incident on the sample, whereby a non-valence electron of the sample absorbs the beam and makes a transition to a higher energy unoccupied state. Analysis of the spectrum reveals structural information

(including chemical composition) of the material under study. In other words, the corresponding core edge (L edge in case of Ti and M edge in case of Cd) in the spectrum carries information about the electronic structure and symmetry of the corresponding excited atom157 while the amount of energy lost is dependent on the atomic number of the specific atom being imaged.158

To collect EELS spectrum image data of the area of interest, we used a reference area—a spot on the grid with no rods—for tracking spatial drift occurring during acquisition. For instance, the

EELS spectrum in Figure 4.10b was integrated over the rectangle marked “spectrum image” in the STEM image, red box shown in Figure 4.10a. Similarly, we did in Figure 4.11 and Figure

4.12.

In our system here, we firstly measured the EELS spectra for the TiO2 NRs seeds alone as a reference for the hybrid spectra in the next step, shown in Figure 4.10. EELS spectra of the TiO2

NR seeds, Figure 4.10, shows three significant signals for Ti–L2 and L3 edges, and O–K edge, in particular, the titanium L edge and the oxygen K edge in TiO2. So this demonstrates that we can easily detect Ti and O in the TiO2 NRs of 2–3 nm in diameter using the EELS technique. The

EELS spectra for both Ti and O in Figure 4.10 are in the agreement with previously reported

159-162 literature values for the anatase TiO2.

96 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

In a second step, the EELS spectrum was acquired for the TiO2/CdSe hybrid NRs sample 2 (shown in Figure 4.11 and Figure 4.12). EELS spectra for the TiO2/CdSe hybrid sample 2 shows,

Figure 4.11b, three edges including Ti–L2,3 edges (at ~461 eV), oxygen–K edge (at ~530 eV) overlapping with the Cd–M4,5 edge at ~437 eV which indicates the coexistence of both TiO2 and

CdSe in the sample.

Figure 4.10: The STEM and EELS spectra from TiO2 NRs sample which used as a seed for the hybrid. The spectrum shows the titanium’s L edge and the oxygen’s K edge in TiO2.

Spectrum image

Figure 4.11: The STEM (left) and EELS spectrum (right) from TiO2/CdSe NRs sample 3 hybrid. The spectrum shows the titanium L edge and the oxygen K edge in TiO2 plus the M edge in case of Cd of CdSe part.

97 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

Figure 4.12 shows the EELS spectra recorded from one TiO2/CdSe hybrid rod for the purpose of determining the chemical distribution at different points along the nanorod. Accordingly, EELS spectra were extracted at three points around the selected nanorod for the measurement; on the periphery of the rod and its core. Interestingly, it is pertinent to note that we could detect significant titanium signal at the edges of most of the NRs in sample 2 (shown in Figure 4.12a with spectra extracted from region 1 and 3) that we investigated. However, in the core of each nanorod, the Cd peak is predominant, Figure 4.12b middle line. We studied many single nanorods at different locations to test the reproducibility of the results.

According to these results, we believe that in the case of the TiO2/CdSe hybrid NRs formed, the TiO2 NRs totally covered by a thick shell of heavy metals that absorbs the beams before it reaches to the core. Moreover, TiO2 small particles may be predominantly adsorbed to the surface of CdSe NRs after the shelling process. Further studies, at this point, is essentially needed to study this phenomenon in order to improve the hybrid structure. We think that controlling couple of

98 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

factors such as: the Cd and Se concentration to have thinner CdSe shell, the thickness of TiO2 NRs seeds and the reaction time may improve the structure of the resultant hybrid.

4.5.4 Examination of CdSe Alone as a Control Experiment

In order to investigate the role of TiO2 in the hybrid formation, we ran a control experiment in which we compared the STEM images of TiO2 NR seeds used in sample 2 before and after adding them to the Cd-precursor. It was found that the TiO2 NRs maintained their shape even after heating at 270 °C with Cd-precursor, Figure 4.13a and Figure 4.13b. Similarly, for comparison, we prepared CdSe QDs following the same recipe used for preparing the TiO2/CdSe nanocomposite sample 2, but without adding TiO2 NR seeds to the reaction flask. In this experiment we added Se- precursor drop-wise similar to the preparation of TiO2/CdSe nanocomposite experiment. It was noticed for this CdSe TEM image that CdSe are in rice shape (short rods), Figure 4.13d, and do not form elongated nanorods that resemble TiO2/CdSe sample 2, shown in Figure 4.13c. This control experiment for CdSe formation in the absence of TiO2 but under the same conditions as heterostructure formation indicates that the presence of TiO2 NRs is responsible for the TiO2/CdSe

NRs elongation (100 nm) in the case of sample 2.

99 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

Figure 4.13: Annular–dark–field STEM image of (a) TiO2 NRs as prepared, (b) TiO2 NRs mixture with

Cd-precursor after heating at 270 °C and before adding the Se–precursor directly, (c) TiO2/ CdSe sample 2 formed after adding the Se-precursor to TiO2 shown in (b) and (d) CdSe sample prepared under the same conditions used for sample 2, but without adding TiO2 NR seeds.

In sum, we have presented strong evidence that we have made a hybrid material wherein CdSe is precipitated on TiO2 NRs with irregular coverage. First, we did not find any sign of TiO2 in the

EELS experiment in which the predominant spectral features arose from CdSe, Figure 4.12.

However, we did find through TEM that the thin nanorods of TiO2 ended up much thicker after adding CdSe to them, Figure 2.13a–c, highly suggestive that CdSe is in fact growing along the

TiO2 nanorods. This interpretation is further supported by the control experiment where we show that in the absence of TiO2 NR seeds, the CdSe nanorods did not grow long (rice shape),

Figure 4.13d compared to the CdSe NRs formed in the presence of TiO2 NR seeds, Figure 4.13c.

100 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.5.5 Optical Properties.

4.5.5.1 UV-vis spectroscopy and Photoluminescence Measurements:

We studied the optical properties of our hybrid NRs using UV-vis spectroscopy and PL measurements. The photoinduced charge separation at the interface of TiO2/CdSe has been investigated by the emission spectrum of the heterostructure. It was found that presence of TiO2 is quenching the PL of CdSe which leads to a weak PL emission quantum yield of the new composite.

Figure 4.14: (a and b) the brightness of the CdSe QDs against the heterostructure in day light and under

UV illumination, respectively. (c) represents sketch of TiO2 and CdSe band offset the estimated mechanism of the charge separation inside the heterostructure sample.

Figure 4.15 displays the absorption and emission spectra of TiO2/CdSe nanohybrids. Since TiO2 has a wide band gap of ~3.2 eV, the visible absorption from 500 to 700 nm (shown in Figure 4.15a; blue) are assigned to CdSe exciton transitions. Figure 4.15a shows the absorption of a dilute dispersion of TiO2/CdSe hybrid NRs, in chloroform (the same behavior was found in toluene) and the absorption peak of TiO2 NR seeds capped with the organic surfactants, oleic acid and oleylamine, dissolved in toluene. The short TiO2/CdSe NRs (sample1) have photoluminescence quantum yields on the order of ~ 1% (Figure 4.15c), in which a fraction of TiO2 is uncovered by

CdSe, while the other three samples (samples 2–5) have no detectable emission (Figure 4.15d).

101 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

To aid the subsequent discussion of the transient absorption bleach features, we first discuss assignment of the bands observed in the CdSe absorption spectrum. Since CdSe covers TiO2 NRs and the mean rod radius is few times smaller than the CdSe exciton Bohr radius the CdSe exciton experiences strong–quantum confinement mainly in the direction of the thickness of the shell diameter.163 In this case, one dimensional (1D) or two–dimensional (2D) exciton model is suitable to explain the electronic structure. Experimentally, the striking difference between 1–dimensional or 2D CdSe nanostructures is the oscillator strength of the second absorption peak (X1 in our transient absorption studies, for example look at Figure 4.16). While the second absorption of

102 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

CdSe NRs (1S(e)–2S(h)) is weaker than the lowest absorption, the second absorption of CdSe nanopletelets (light hole) is comparable to the lowest absorption peak.164 When we see the bleaches of transient absorption spectra (shown later) the bleach of the second peak (X1) is much smaller than the lowest peak (X0). On the basis of this discussion, we use the 1D exciton model to explain the electronic structures of CdSe shells.165 The lowest peak and a shoulder at higher energy (~540 nm) of the TiO2/CdSe absorption spectra are therefore assigned to the 1e–13/2 and the

166 superposition of 1e–1 1/2 and 1e–13/2, respectively.

Two control experiments were performed in order to examine the effect of TiO2 on the PL quenching in the TiO2/CdSe nanohybrids. CdSe QDs with high photoluminescence quantum yield

(~ 30 %) were first prepared, and then mixed separately in two different vials with: (1) titanium tetraisopropoxide as a Ti–ion source and (2) TiO2 NRs (the same material used as a seed for the

TiO2/CdSe nanohybrids) added at room temperature. These mixtures were stirred at room temperature for few days before PL measurements were carried out. It was found that neither Ti– ions nor the TiO2 NRs affect the PL of the CdSe Figure 4.15b, meaning no quenching occurs if the CdSe is not bound to the TiO2 surface. These results clearly show that CdSe PL is quenched only when TiO2 NRs are used as a seed for CdSe growth.

The PL quenching in TiO2/CdSe NRs is rather due to the exciton dissociation by electron transfer than due to the surface defects at the interface of the CdSe and the TiO2 because the bleach decay due to the surface trapping does not explain the efficient PL quenching (discuss later).50, 60, 69, 126,

147, 153, 167-182 The charge separation process, in our system, is probably proceeding via tunneling transfer of electrons from the CdSe to the TiO2 NCs according to the type II alignment in a close–

50, 69, 172, 177, 179, 183 packed TiO2/CdSe heterostructure. Kamat and his coworkers have reported that

50, 69, 172, 177, 179 51 the TiO2 colloids are able to interact with excited CdSe and CdS particles. As the

103 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods visible light excites the CdSe QDs nanocrystallites, electrons are excited to the conduction band

(CB) with holes left in the valence band (VB) forming the excitons, then the excitons diffuse to the interface between the two materials of the heterostructure followed by the charge separation into the two different materials (see carton in Figure 4.2 and Figure 4.14c).167

4.5.6 Transient Absorption Analysis

4.5.6.1 Analysis of Pump–Probe Spectra:

The relaxation dynamics of the photoinduced charge separation at the interface of two components is investigated the by the ultrafast pump–probe spectroscopy which is been employed to study the dynamics of charge carrier in nanoparticles and to probe the electron transfer between the two components of any hybrid. In this technique, the excitation-induced changes in the absorption spectra (ΔA) are measured while information on the relative population of excited states in the system is provided. Carrier transfer process can be seen as transient spectral bleaching due to the difference in electron occupation numbers in the particular conduction bands169 and those bleaching considered as a measure of charge separation. This phenomenon is attributed to the production of electron–hole pairs, exciton, can completely bleach the excitonic absorption of a semiconductor particle. As a result, the transfer of electrons to another compound disrupts the exciton and then elongates the rate of the bleaching recovery which can be monitored by kinetic analysis.

To examine the charge carrier relaxation pathways in TiO2/CdSe NRs following photoexcitation, we measured the exciton dynamics in TiO2/CdSe NRs by femtosecond (fs) transient absorption

(TA) spectroscopy.184

104 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

We tuned the pump wavelength to 650 nm to resonantly excite the CdSe exciton state, by an optical parametric amplifier (OPA) for the case of TiO2/CdSe NRs sample 1 and sample 5. The pump wavelength was set to 620 nm for samples (2, 3 and 4) and for the CdSe QDs. With the 150 fs white light continuum as the probe pulse, we obtained the TA spectra shown in Figure 4.16–

Figure 4.18. The pump power (less than 20 W) was carefully controlled to avoid multi–exciton production in the NCs.

Figure 4.16a and Figure 4.16b show the TA spectra of TiO2/CdSe NRs samples 2 and 1, respectively. In the TA spectra of TiO2/CdSe sample 2, Figure 4.16a, two bleaches are mainly observed at 1.82 eV and 2.2 eV. However, since the peak at 1.82 eV has a tail at higher energy there is an additional peak around 1.98 eV. We determined the spectral peaks by fitting the spectrum with three Gaussian functions. We observed similar spectral features for the samples 3 and 4, but the TA spectra of samples 1 and 5 do not have the intermediate peak. We refer to these peaks as X0, X1 and X2 from low to high energy as indicated in Figure 4.16–Figure 4.18.

We assign X0 and X1 to the bleach of 1e–1 1/2 and 1e–13/2, respectively. Although some reports

163, 166 assign X2 to 1e–11/2, we assign X2 to the bleach of 1e–11/2 for the following reasons.

First, the X2 bleach appears in spite of the excitation energy being lower than X2, which indicates that the electron or hole state is the same as X0 or X2, i.e. they share a common ground or excited state. Second, bleach of the 1.98 eV feature lasts much longer than the intraband transition of electron and hole from higher states. For example, we still see the bleach even on a time scale of nanoseconds, which means the 1.98 eV features is not the bleach of a higher energy electronic transition. At short times, such as sub–picoseconds, after photo-excitation, the peak should involve a bleach corresponding to the 1e–11/2 transition, but after several picoseconds; the 1e–11/2 transition relaxes to the lowest excited sigma-related transition. As a consequences, the reason we

105 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

see the bleach at the X2 even on a nanosecond time scale is because the 1e–11/2 exists near 1e–

11/2 transition and X0 and X2 populate the same electron state, most probably 1(e). These results are in agreement with the report by Norris et al.166 They measured photoluminescence excitation line-narrowed spectra of CdSe nanocrystals and assigned the third peak, i.e. X2, to 1Se–1S1/2 and not 1Pe–1P1/2. Although, the notation, fine structures and polarity of nanocrystals and nanorods systems are different, the order of states is basically the same, even when shapes change.

We also investigated the relaxation dynamics of CdSe NRs sample that prepared without TiO2 seeds, which is shown in Figure 4.13d, as a reference to investigate the surface traps accumulated with CdSe, Figure 4.18d. As we can see from the Figures (4.16a and 4.16b), (4.17a and 4.17b), and (4.18a and b), the bleach peaks shift with time. This time-dependent shift means that the decay extracted at only one wavelength does not give correct time constants for recovery of the bleach.

106 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

We fitted a TA spectrum at each time delay with three Gaussian functions and determined the time evolution of the amplitude, peak and width of each Gaussian component. Dynamics of all parameters obtained by fitting (amplitude, peak energy and band width) are shown in Figures

4.20–4.26.

In Figures (4.16c and 4.16d), (4.17c and 4.17d), and (4.18c) we display the amplitude dynamics of X0 and X2 bleaches of the five samples of TiO2/CdSe NRs, while Figure 4.16c and Figure

4.16d display the amplitude dynamics of X0 and X2 bleaches of CdSe NRs.

The relaxation dynamics in TiO2/CdSe NRs sample 5, like the other samples, were investigated by this technique, but we tuned the pump wavelength to (650 nm) using an OPA. Femtosecond transient absorption spectra show the time evolution of photoexcited states in the QDs where this transient spectra were recorded at different delay times following the laser pulse excitation.

Pumping the sample leads to the development of two distinctly different excited states by comparing spectra-bleach positions as well as the absorption spectrum of hetero-NRs. This two spectrum can be defined, for instance in case of sample 5 as : one at (666 nm) refers to that one electron has been propped up from the valence band edge of CdSe to the conduction band edge of

169 CdSe , CdSe1S-TiO2 , and a second one at (548nm ) which attributed to the electron that has

+ − been injected from the conduction band of CdSe to the conduction band of TiO2, CdSe –TiO2 .

107 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

108 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

109 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

Table 4.2 Time constants for transient bleach recovery measured for CdSe NRs and TiO2/CdSe hybrid NRs using femtosecond pump-probe experiments.

X0 bleach X2 bleach Red Shift τ1 (ps) τ2 (ps) τ3 (ns) τ1 (ps) τ2 (ps) τ3 (ns) (meV)

CdSe NRs 11±2 800±200 28±12 36±9 700±170 24±10 33 (25%) (33%) (42%) (35%) (27%) (38%)

TiO2/CdSe #1 190±4( 1200±0.5 8.0±0.9 16±3 310±20 5.0±0.2 23 23%) (23%) (54%) (14%) (27%) (59%)

TiO2/CdSe #2 -- 480±2 4.3±0.2 18±5 350±70 3.5±0.6 46 (54%) (46%) (24%) (31%) (47%)

TiO2/CdSe #3 11±1 170±20 2.1±0.1 11±2 170±20 2.6±0.3 116 (23%) (51%) (27%) (32%) (42%) (26%)

TiO2/CdSe #4 -- 270±390 3.2±0.2 14±1 270±60 3.1±0.1 79 (14%) (86%) (23%) (33%) (44%)

TiO2/CdSe #5 49±8 390±3 0.0039±0.1 17±1 280±30 0.0035±0 25 (28%) (13%) (31%) .1 (55%) (34%) (66%)

110 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

It is obvious that the transient absorption bleaches shift with time in both CdSe NRs and TiO2/CdSe

NRs samples. The red-shift of the bleach band of each sample is tabulated in table 4.2. In literature, the significant red-shift bleaching in TA measurements have not been observed in spherical nanoparticles.185-186 However, a red-shift of the bleach feature has been reported in studies of anisotropic heterostructures.12-13, 174 Trapping on the surface of CdSe layers weakens the quantum confinement and causes a dynamical shift of the lowest bleach.187 In this case, the analysis of the higher bleach, such as X2, helps us to characterize the dynamics.

For instance, the bleach of the 1S(e)–1S3/2(h) transition of CdSe nanocrystals contains predominantly the state filling of the excited electron state for the main contribution and the decrease of the ground state population, which has been revealed by electrochemical and time- resolved spectroscopic studies.188-190 This is explained by the fewer density of states of conduction band states due to the lighter effective mass of electron (0.13 m0 for electron and 0.45m0 for hole

188-190 respectively; m0 is the mass of free electron). On the other hand, the bleaches of 1S(e)–

2S3/2(h) and 1S(e)–1S1/2(h) excited at lower energy than these bleaches contain only the effect of the state filling of the electron because the hole cannot be populated to the higher state than the

191 excited energy. Since we excited our samples at X1 in this experiment, the X2 bleach contains electron dynamics and the X0 bleach contain the electron and hole dynamics. We analyzed X0 and

X2 dynamics to get more reliable information. It is worth mentioning that this behavior is valid only for strong confinement regime, in which the electron and hole move independently.

111 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

Figure 4.19: Fitting results of transient absorption spectra of CdSe/TiO2 NRs sample #2 with tri-Gaussian functions at different time delay. (a, d, g) shows the amplitude, (b, e, h) shows peak energy and (c, f, i) shows the width of tree Gaussian functions, respectively.

112 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

113 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

114 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

115 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

116 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

117 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.5.6.2 Analysis of X0 and X2 decays.

We observe slightly different dynamics between X0 and X2, which may be a result of hole relaxation process in the X0 dynamics, Figure 4.25. The decay of TiO2/CdSe 1, 3 and 5 are fitted with tri–exponential functions, but 2 and 4 can be fitted with bi–exponential functions. Each decay time constant for bleach recovery kinetics is tabulated in Table 4.2. The fast (τ1) and middle (τ2) decay time constants become about two times faster than those of CdSe NRs, while the long decay

(τ3) time constants become more than 5 times faster than those of CdSe NRs. However, these time constants do not explain the huge decrease of the emission quantum yield (from 30 % for CdSe

NRs to <1% for TiO2/CdSe nanohybrids).

Figure 4.25: Relaxation models of excited carriers observed at (a) X0 bleach and (b) X2 bleach for

TiO2/CdSe NRs.

The complete quenching of CdSe photoluminescence for the TiO2/CdSe nanohybrids indicates that the decay constants extracted from analysis of the bleach recovery are not related to the electron

118 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

transfer from CdSe to TiO2. The fast (τ1) and the middle (τ2) decay components are probably due to trapping processes.

The reason why we observed somewhat faster decays in all TiO2/CdSe nanohybrids is likely due to the large lattice mismatch between CdSe and TiO2 and the some interface acts as additional carrier–trap sites (a = 0.430 nm, c = 0.702 nm for wurtzite CdSe while a = 0.374 nm, c = 0.939 nm for anatase TiO2). Because of this large mismatch, the interface strain accumulates dramatically with increasing CdSe shell thickness, and eventually leads to the formation of misfit dislocations.

These lattice defects can act as trapping sites for the photogenerated charge carriers and degrading the fluorescence properties of the NCs. Another effect of the lattice mismatch is the formation of particles with irregular shape and a broad size distribution upon increasing CdSe-shell thickness.

192 It has been previously reported by Cheng et al that CdSe can grow upon the surface of TiO2 by the by successive ionic layer adsorption and reaction (SILAR) method, but with via interface lattice distortion area. They found that lattice distortion area is created between the {101} planes of CdSe and to the {004} planes of TiO2. Moreover, Cheng et al, compared their (heterojunction) of

CdSe/TiO2 with the simple adsorption interface and they found the growth interfaces

(heterojunction) of QDs/TiO2 result in more charge transfer channel and more effective coverage, which resulted in enhancement in the electronic injection between the two materials.192

The long decay (τ3) component is the nonradiative return of the exciton to the ground state, likely

191 via recombination of the electron trapped in TiO2. The absence of a decay component in the bleach recovery corresponding to electron transfer from photoexcited CdSe to TiO2 is not unreasonable because the bleach should not recover after the electron is lost from the conduction band of CdSe. Bearing in mind that the electron transfer from dye to TiO2 in dye–sensitized solar

193-197 cells occurs within 100 fs, it is conceivable that the electron transfer from CdSe to TiO2

119 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods occurs very rapidly, but we cannot detect it in these experiments. For example, such ultrafast

198 electron injection has been reported in PbS NCs/TiO2 systems.

4.6 Conclusions

We synthesized colloidal linker–free TiO2/CdSe NRs heterostructures with TiO2 as a seed and

CdSe QDs grown in the presence of TiO2 NRs using seeded–growth type colloidal injection approach. We prepared these type–II heterostructures without any ligand exchange process for the

QDs or using bifunctional linker compound to bind the two semiconductor nanocrystals (TiO2 and

CdSe) together. Electron microscopy showed that the aminolysis reaction of titanium oleate can lead to the formation of anatase TiO2 NRs with different lengths. We found that the as–prepared

TiO2 seeds determined the morphology of TiO2/CdSe NRs. By changing the synthetic recipe, the different shape of NRs, either isolated (long or short rods) or entangled was obtained. The TiO2 bundle structure followed by the CdSe deposition results in the entanglement of TiO2/CdSe NRs.

On the other hand, the isolated TiO2 seed rods lead to the formation of isolated TiO2/CdSe NRs with the average dimension of ~100 nm in length and ~7 nm in diameter.

Comparing these five samples of TiO2/CdSe NRs, the initial excited state of the sensitizer CdSe

QDs is efficiently quenched according to PL measurements. Femtosecond transient absorption revealed that exciton states decays ps-to-ns time scale most probably due to the surface trapping at the interface of the heterostructures and decay kinetics of the heterostructures is similar to that of CdSe QDs. The efficient PL quenching suggests that the ultrafast exciton dissociation is over our experimental time resolution occurs in TiO2/CdSe NRs.

120 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

4.7 References

Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P., Hybrid Nanorod-Polymer Solar Cells. Science 2002, 295 (5564), 2425-2427.

2. Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A., Chemistry and Properties of Nanocrystals of Different Shapes. Chemical Reviews 2005, 105 (4), 1025-1102.

3. Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V., Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films. Journal of the American Chemical Society 2006, 128 (7), 2385-2393.

4. Robel, I.; Kuno, M.; Kamat, P. V., Size-Dependent Electron Injection from Excited CdSe Quantum Dots into TiO2 Nanoparticles. Journal of the American Chemical Society 2007, 129 (14), 4136-4137.

5. Lee, H.; Wang, M.; Chen, P.; Gamelin, D. R.; Zakeeruddin, S. M.; Grätzel, M.; Nazeeruddin, M. K., Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process. Nano Letters 2009, 9 (12), 4221-4227.

6. Loef, R.; Houtepen, A. J.; Talgorn, E.; Schoonman, J.; Goossens, A., Study of Electronic Defects in CdSe Quantum Dots and Their Involvement in Quantum Dot Solar Cells. Nano Letters 2009, 9 (2), 856-859.

7. Guijarro, N. s.; Lana-Villarreal, T.; Shen, Q.; Toyoda, T.; Gómez, R., Sensitization of Titanium Dioxide Photoanodes with Cadmium Selenide Quantum Dots Prepared by SILAR: Photoelectrochemical and Carrier Dynamics Studies. The Journal of Physical Chemistry C 2010, 114 (50), 21928-21937.

8. Gao, X. F.; Sun, W. T.; Ai, G.; Peng, L. M., Photoelectric performance of TiO2 nanotube array photoelectrodes cosensitized with CdS/CdSe quantum dots. Applied Physics Letters 96 (15).

9. Shin, K.; Seok, S. i.; Im, S. H.; Park, J. H., CdS or CdSe decorated TiO2 nanotube arrays from spray pyrolysis deposition: use in photoelectrochemical cells. Chemical Communications 46 (14), 2385-2387.

10. Yang, L.; Luo, S.; Liu, R.; Cai, Q.; Xiao, Y.; Liu, S.; Su, F.; Wen, L., Fabrication of CdSe Nanoparticles Sensitized Long TiO2 Nanotube Arrays for Photocatalytic Degradation of Anthracene-9-carbonxylic Acid under Green Monochromatic Light. The Journal of Physical Chemistry C 114 (11), 4783-4789.

11. Chakrapani, V.; Tvrdy, K.; Kamat, P. V., Modulation of Electron Injection in CdSe-TiO2 System through Medium Alkalinity. Journal of the American Chemical Society 132 (4), 1228- 1229.

12. Bang, J. H.; Kamat, P. V., Solar Cells by Design: Photoelectrochemistry of TiO2 Nanorod Arrays Decorated with CdSe. Advanced Functional Materials 20 (12), 1970-1976.

121 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

13. Hyun, B.-R.; Bartnik, A. C.; Sun, L.; Hanrath, T.; Wise, F. W., Control of Electron Transfer from Lead-Salt Nanocrystals to TiO2. Nano Letters 11 (5), 2126-2132.

14. Hyun, B.-R.; Zhong, Y.-W.; Bartnik, A. C.; Sun, L.; Abruña, H. D.; Wise, F. W.; Goodreau, J. D.; Matthews, J. R.; Leslie, T. M.; Borrelli, N. F., Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles. ACS Nano 2008, 2 (11), 2206-2212.

15. Acharya, K. P.; Hewa-Kasakarage, N. N.; Alabi, T. R.; Nemitz, I.; Khon, E.; Ullrich, B.; Anzenbacher, P.; Zamkov, M., Synthesis of PbS/TiO2 Colloidal Heterostructures for Photovoltaic Applications. The Journal of Physical Chemistry C 2010, 114 (29), 12496-12504.

16. Fujishima, A.; Honda, K., Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238 (5358), 37-38.

17. Hartig, K. J.; Getoff, N., Production and testing methods of different TiO2 photoanodes. International Journal of Hydrogen Energy 1986, 11 (12), 773-781.

18. Hassan, Y., Treatment of Some Industrial Pollutants by Photocatalytic Techniques. Master Thesis, Zagazig University 2006.

19. Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W., Environmental Applications of Semiconductor Photocatalysis. Chemical Reviews 1995, 95 (1), 69-96.

20. Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A., A review on highly ordered, vertically oriented TiO2 nanotube arrays: Fabrication, material properties, and solar energy applications. Solar Energy Materials and Solar Cells 2006, 90 (14), 2011-2075.

21. Grätzel, M., Corrigendum to "Conversion of sunlight to electric power by nanocrystalline dye-sensitized solar cells" [J. Photochem. Photobiol. A: Chem. 164 (2004) 3-14]. Journal of Photochemistry and Photobiology A: Chemistry 2004, 168 (3), 235-235.

22. Grätzel, M., The advent of mesoscopic injection solar cells. Progress in Photovoltaics: Research and Applications 2006, 14 (5), 429-442.

23. Koo, H.-J.; Kim, Y. J.; Lee, Y. H.; Lee, W. I.; Kim, K.; Park, N.-G., Nano-embossed Hollow Spherical TiO2 as Bifunctional Material for High-Efficiency Dye-Sensitized Solar Cells. Advanced Materials 2008, 20 (1), 195-199.

24. Barnes, P. R. F.; Anderson, A. Y.; Koops, S. E.; Durrant, J. R.; O’Regan, B. C., Electron Injection Efficiency and Diffusion Length in Dye-Sensitized Solar Cells Derived from Incident Photon Conversion Efficiency Measurements. The Journal of Physical Chemistry C 2008, 113 (3), 1126-1136.

25. O'Regan, B.; Grätzel, M., A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353 (6346), 737-740.

26. Hagfeldt, A.; Grätzel, M., Molecular Photovoltaics. Accounts of Chemical Research 2000, 33 (5), 269-277.

122 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

27. Scholes, G. D.; Rumbles, G., Excitons in nanoscale systems. Nat Mater 2006, 5 (9), 683- 696.

28. Scholes, G. D., Controlling the Optical Properties of Inorganic Nanoparticles. Advanced Functional Materials 2008, 18 (8), 1157-1172.

29. Alivisatos, A. P.; Gu, W.; Larabell, C., Quantum Dots As Cellular Probes. Annual Review of Biomedical Engineering 2005, 7 (1), 55-76.

30. Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H., Quantum dot bioconjugates for imaging, labelling and sensing. Nat Mater 2005, 4 (6), 435-446.

31. Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S., Quantum Dots for Live Cells, in Vivo Imaging, and Diagnostics. Science 2005, 307 (5709), 538-544.

32. Acharya, K. P.; Hewa-Kasakarage, N. N.; Alabi, T. R.; Nemitz, I.; Khon, E.; Ullrich, B.; Anzenbacher, P.; Zamkov, M., Synthesis of PbS/TiO2 Colloidal Heterostructures for Photovoltaic Applications. The Journal of Physical Chemistry C 114 (29), 12496-12504.

33. Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Grätzel, M., A stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nature Materials 2003, 2 (6), 402-407.

34. Kuang, D.; Klein, C.; Ito, S.; Moser, J.-E.; Humphry-Baker, R.; Evans, N.; Duriaux, F.; Grätzel, C.; Zakeeruddin, S. M.; Grätzel, M., High-Efficiency and Stable Mesoscopic Dye- Sensitized Solar Cells Based on a High Molar Extinction Coefficient Ruthenium Sensitizer and Nonvolatile Electrolyte. Advanced Materials 2007, 19 (8), 1133-1137.

35. Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Grätzel, M., Combined Experimental and DFT-TDDFT Computational Study of Photoelectrochemical Cell Ruthenium Sensitizers. Journal of the American Chemical Society 2005, 127 (48), 16835-16847.

36. Baker, D. R.; Kamat, P. V., Photosensitization of TiO2 nanostructures with CdS quantum dots: Particulate versus tubular support architectures. Advanced Functional Materials 2009, 19 (5), 805-811.

37. Banerjee, S.; Mohapatra, S. K.; Das, P. P.; Misra, M., Synthesis of Coupled Semiconductor by Filling 1D TiO2 Nanotubes with CdS. Chemistry of Materials 2008, 20 (21), 6784-6791.

38. Breeze, A. J.; Schlesinger, Z.; Carter, S. A.; Tillmann, H.; Hörhold, H.-H., Improving power efficiencies in polymer - Polymer blend photovoltaics. Solar Energy Materials and Solar Cells 2004, 83 (2-3), 263-271.

39. Chen, S.; Paulose, M.; Ruan, C.; Mor, G. K.; Varghese, O. K.; Kouzoudis, D.; Grimes, C. A., Electrochemically synthesized CdS nanoparticle-modified TiO2 nanotube-array photoelectrodes: Preparation, characterization, and application to photoelectrochemical cells. Journal of Photochemistry and Photobiology A: Chemistry 2006, 177 (2-3), 177-184.

123 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

40. Gerischer, H.; Lübke, M., A particle size effect in the sensitization of TiO2 electrodes by a CdS deposit. Journal of Electroanalytical Chemistry 1986, 204 (1-2), 225-227.

41. Larramona, G.; Choné, C.; Jacob, A.; Sakakura, D.; Delatouche, B.; Péré, D.; Cieren, X.; Nagino, M.; Bayón, R., Nanostructured photovoltaic cell of the type titanium dioxide, cadmium sulfide thin coating, and copper thiocyanate showing high quantum efficiency. Chemistry of Materials 2006, 18 (6), 1688-1696.

42. Lee, W.; Lee, J.; Lee, S.; Yi, W.; Han, S. H.; Cho, B. W., Enhanced charge collection and reduced recombination of CdS-TiO2 quantum dots sensitized solar cells in the presence of single- walled carbon nanotubes. Applied Physics Letters 2008, 92 (15).

43. Lin, S. C.; Lee, Y. L.; Chang, C. H.; Shen, Y. J.; Yang, Y. M., Quantum-dot-sensitized solar cells: Assembly of CdS-quantum-dots coupling techniques of self-assembled monolayer and chemical bath deposition. Applied Physics Letters 2007, 90 (14).

44. Peter, L. M.; Riley, D. J.; Tull, E. J.; Wijayantha, K. G. U., Photosensitization of nanocrystalline TiO2 by self-assembled layers of CdS quantum dots. Chemical Communications 2002, (10), 1030-1031.

45. Plass, R.; Pelet, S.; Krueger, J.; Gratzel, M.; Bach, U., Quantum Dot Sensitization of Organicâˆ’Inorganic Hybrid Solar Cells. The Journal of Physical Chemistry B 2002, 106 (31), 7578-7580.

46. Prabakar, K.; Seo, H.; Son, M.; Kim, H., CdS quantum dots sensitized TiO2 photoelectrodes. Materials Chemistry and Physics 2009, 117 (1), 26-28.

47. Prabakar, K.; Takahashi, T.; Nakashima, T.; Kubota, Y.; Fujishima, A. Optimization and deposition of CdS thin films as applicable to TiO2/CdS composite catalysis, Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 2006; 24 (4) 1613-1617.

48. Prabakar, K.; Takahashi, T.; Nezuka, T.; Takahashi, K.; Nakashima, T.; Kubota, Y.; Fujishima, A., Preparation and photocatalytic activity of TiOxNy/CdS heterojunctions. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 2007, 25 (4), 1188-1192.

49. Qian, X.; Qin, D.; Bai, Y.; Li, T.; Tang, X.; Wang, E.; Dong, S., Photosensitization of TiO2 nanoparticulate thin film electrodes by CdS nanoparticles. Journal of Solid State Electrochemistry 2001, 5 (7-8), 562-567.

50. Robel, I. n.; Subramanian, V.; Kuno, M.; Kamat, P. V., Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films. Journal of the American Chemical Society 2006, 128 (7), 2385-2393.

51. Sant, P. A.; Kamat, P. V., Interparticle electron transfer between size-quantized CdS and TiO2 semiconductor nanoclusters. Physical Chemistry Chemical Physics 2002, 4 (2), 198-203.

52. Shen, Y. J.; Lee, Y. L., Assembly of CdS quantum dots onto mesoscopic TiO2 films for quantum dot-sensitized solar cell applications. Nanotechnology 2008, 19 (4).

124 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

53. Sun, W.-T.; Yu, Y.; Pan, H.-Y.; Gao, X.-F.; Chen, Q.; Peng, L.-M., CdS Quantum Dots Sensitized TiO2 Nanotube-Array Photoelectrodes. Journal of the American Chemical Society 2008, 130 (4), 1124-1125.

54. Tachibana, Y.; Umekita, K.; Otsuka, Y.; Kuwabata, S., Performance improvement of CdS quantum dots sensitized TiO2 solar cells by introducing a dense TiO2 blocking layer. Journal of Physics D: Applied Physics 2008, 41 (10).

55. Toyoda, T.; Sato, J.; Shen, Q., Effect of sensitization by quantum-sized CdS on photoacoustic and photoelectrochemical current spectra of porous TiO2 electrodes. Review of Scientific Instruments 2003, 74 (1 II), 297-299.

56. Vogel, R.; Hoyer, P.; Weller, H., Quantum-Sized PbS, CdS, Ag2S, Sb2S3, And Bi2S3 Particles As Sensitizers For Various Nanoporous Wide-Bandgap Semiconductors. J. Phys. Chem. 1994, 98 (12), 3183-3188.

57. Vogel, R.; Pohl, K.; Weller, H., Sensitization of highly porous, polycrystalline TiO2 electrodes by quantum sized CdS. Chemical Physics Letters 1990, 174 (3-4), 241-246.

58. Wijayantha, K. G. U.; Peter, L. M.; Otley, L. C., Fabrication of CdS quantum dot sensitized solar cells via a pressing route. Solar Energy Materials and Solar Cells 2004, 83 (4), 363-369.

59. Ahn, J. H.; Mane, R. S.; Todkar, V. V.; Han, S. H., Invasion of CdSe nanoparticles for photosensitization of porous TiO2. International Journal of Electrochemical Science 2007, 2 (7), 517-522.

60. Bang, J. H.; Kamat, P. V., Quantum Dot Sensitized Solar Cells. A Tale of Two Semiconductor Nanocrystals: CdSe and CdTe. ACS Nano 2009, 3 (6), 1467-1476.

61. Chang, C. H.; Lee, Y. L., Chemical bath deposition of CdS quantum dots onto mesoscopic TiO2 films for application in quantum-dot-sensitized solar cells. Applied Physics Letters 2007, 91 (5).

62. Chen, J.; Song, J. L.; Sun, X. W.; Deng, W. Q.; Jiang, C. Y.; Lei, W.; Huang, J. H.; Liu, R. S., An oleic acid-capped CdSe quantum-dot sensitized solar cell. Applied Physics Letters 2009, 94 (15).

63. Diguna, L. J.; Murakami, M.; Sato, A.; Kumagai, Y.; Ishihara, T.; Kobayashi, N.; Shen, Q.; Toyoda, T., Photoacoustic and photoelectrochemical characterization of inverse opal TiO2 sensitized with CdSe quantum dots. Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers 2006, 45 (6 B), 5563-5568.

64. Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T., High efficiency of CdSe quantum-dot- sensitized Ti O2 inverse opal solar cells. Applied Physics Letters 2007, 91 (2).

65. Diguna, L. J.; Shen, Q.; Sato, A.; Katayama, K.; Sawada, T.; Toyoda, T., Optical absorption and ultrafast carrier dynamics characterization of CdSe quantum dots deposited on different morphologies of nanostructured TiO2 films. Materials Science and Engineering: C 2007, 27 (5- 8), 1514-1520.

125 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

66. Fang, J.; Wu, J.; Lu, X.; Shen, Y.; Lu, Z., Sensitization of nanocrystalline TiO2 electrode with quantum sized CdSe and ZnTCPc molecules. Chemical Physics Letters 1997, 270 (1-2), 145- 151.

67. Guijarro, N. s.; Lana-Villarreal, T.; Mora-Seró, I. n.; Bisquert, J.; Gómez, R., CdSe Quantum Dot-Sensitized TiO2 Electrodes: Effect of Quantum Dot Coverage and Mode of Attachment. The Journal of Physical Chemistry C 2009, 113 (10), 4208-4214.

68. Hyo, J. L.; Kim, D. Y.; Yoo, J. S.; Bang, J.; Kim, S.; Park, S. M., Anchoring cadmium chalcogenide quantum dots (QDs) onto stable oxide semiconductors for QD sensitized solar cells. Bulletin of the Korean Chemical Society 2007, 28 (6), 953-958.

69. Kamat, P. V., Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. The Journal of Physical Chemistry C 2008, 112 (48), 18737-18753.

70. Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V., Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdS-TiO2 Architecture. Journal of the American Chemical Society 2008, 130 (12), 4007-4015.

71. Lee, H. J.; Yum, J.-H.; Leventis, H. C.; Zakeeruddin, S. M.; Haque, S. A.; Chen, P.; Seok, S. I.; Grätzel, M.; Nazeeruddin, M. K., CdSe Quantum Dot-Sensitized Solar Cells Exceeding Efficiency 1% at Full-Sun Intensity. The Journal of Physical Chemistry C 2008, 112 (30), 11600- 11608.

72. Lee, W.; Kang, S. H.; Min, S. K.; Sung, Y.-E.; Han, S.-H., Co-sensitization of vertically aligned TiO2 nanotubes with two different sizes of CdSe quantum dots for broad spectrum. Electrochemistry Communications 2008, 10 (10), 1579-1582.

73. Lee, W.; Kwak, W.-C.; Min, S. K.; Lee, J.-C.; Chae, W.-S.; Sung, Y.-M.; Han, S.-H., Spectral broadening in quantum dots-sensitized photoelectrochemical solar cells based on CdSe and Mg-doped CdSe nanocrystals. Electrochemistry Communications 2008, 10 (11), 1699-1702.

74. Lee, Y.-L.; Chang, C.-H., Efficient polysulfide electrolyte for CdS quantum dot-sensitized solar cells. Journal of Power Sources 2008, 185 (1), 584-588.

75. Lee, Y.-L.; Huang, B.-M.; Chien, H.-T., Highly Efficient CdSe-Sensitized TiO2 Photoelectrode for Quantum-Dot-Sensitized Solar Cell Applications. Chemistry of Materials 2008, 20 (22), 6903-6905.

76. Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S., Photosensitization of ZnO Nanowires with CdSe Quantum Dots for Photovoltaic Devices. Nano Letters 2007, 7 (6), 1793-1798.

77. Lévy-Clément, C.; Tena-Zaera, R.; Ryan, M. A.; Katty, A.; Hodes, G., CdSe-Sensitized p- CuSCN/Nanowire n-ZnO Heterojunctions. Advanced Materials 2005, 17 (12), 1512-1515.

78. Liu, D.; Kamat, P. V., Electrochemical rectification in CdSe+TiO2 coupled semiconductor films. Journal of Electroanalytical Chemistry 1993, 347 (1-2), 451-456.

126 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

79. Liu, D.; Kamat, P. V., Photoelectrochemical behavior of thin cadmium selenide and coupled titania/cadmium selenide semiconductor films. The Journal of Physical Chemistry 1993, 97 (41), 10769-10773.

80. Lopez-Luke, T.; Wolcott, A.; Xu, L.-p.; Chen, S.; Wen, Z.; Li, J.; De La Rosa, E.; Zhang, J. Z., Nitrogen-Doped and CdSe Quantum-Dot-Sensitized Nanocrystalline TiO2 Films for Solar Energy Conversion Applications. The Journal of Physical Chemistry C 2008, 112 (4), 1282-1292.

81. Mora-Seró, I.; Bisquert, J.; Dittrich, T.; Belaidi, A.; Susha, A. S.; Rogach, A. L., Photosensitization of TiO2 Layers with CdSe Quantum Dots: Correlation between Light Absorption and Photoinjection. The Journal of Physical Chemistry C 2007, 111 (40), 14889- 14892.

82. Mora-Seró, I.; Giménez, S.; Moehl, T.; Fabregat-Santiago, F.; Lana-Villareal, T.; GÃ³mez, R.; Bisquert, J., Factors determining the photovoltaic performance of a CdSe quantum dot sensitized solar cell: The role of the linker molecule and of the counter electrode. Nanotechnology 2008, 19 (42).

83. Mora-Seró, I.; Dittrich, T.; Susha, A. S.; Rogach, A. L.; Bisquert, J., Large improvement of electron extraction from CdSe quantum dots into a TiO2 thin layer by N3 dye coabsorption. Thin Solid Films 2008, 516 (20), 6994-6998.

84. Nasr, C.; Kamat, P. V.; Hotchandani, S., Photoelectrochemical behavior of coupled SnO2CdSe nanocrystalline semiconductor films. Journal of Electroanalytical Chemistry 1997, 420 (1-2), 201-207.

85. Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Rühle, S.; Cahen, D.; Hodes, G., Chemical bath deposited CdS/CdSe-sensitized porous TiO2 solar cells. Journal of Photochemistry and Photobiology A: Chemistry 2006, 181 (2-3), 306-313.

86. Nozik, A. J., Exciton Multiplication and Relaxation Dynamics in Quantum Dots: Applications to Ultrahigh-Efficiency Solar Photon Conversion. Inorganic Chemistry 2005, 44 (20), 6893-6899.

87. Schaller, R. D.; Klimov, V. I., High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion. Physical Review Letters 2004, 92 (18), 186601.

88. Shen, Q.; Arae, D.; Toyoda, T., Photosensitization of nanostructured TiO2 with CdSe quantum dots: effects of microstructure and electron transport in TiO2 substrates. Journal of Photochemistry and Photobiology A: Chemistry 2004, 164 (1-3), 75-80.

89. Shen, Q.; Katayama, K.; Sawada, T.; Yamaguchi, M.; Toyoda, T., Optical absorption, photoelectrochemical, and ultrafast carrier dynamic investigations of TiO2 electrodes composed of nanotubes and nanowires sensitized with CdSe quantum dots. Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers 2006, 45 (6 B), 5569-5574.

127 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

90. Shen, Q.; Katayama, K.; Yamaguchi, M.; Sawada, T.; Toyoda, T., Study of ultrafast carrier dynamics of nanostructured TiO2 films with and without CdSe quantum dot deposition using lens- free heterodyne detection transient grating technique. Thin Solid Films 2005, 486 (1-2), 15-19.

91. Shen, Q.; Kobayashi, J.; Diguna, L. J.; Toyoda, T., Effect of ZnS coating on the photovoltaic properties of CdSe quantum dot-sensitized solar cells. Journal of Applied Physics 2008, 103 (8), 084304.

92. Shen, Q.; Sato, T.; Hashimoto, M.; Chen, C.; Toyoda, T., Photoacoustic and photoelectrochemical characterization of CdSe-sensitized TiO2 electrodes composed of nanotubes and nanowires. Thin Solid Films 2006, 499 (1-2), 299-305.

93. Shen, Q.; Yanai, M.; Katayama, K.; Sawada, T.; Toyoda, T., Optical absorption, photosensitization, and ultrafast carrier dynamic investigations of CdSe quantum dots grafted onto nanostructured SnO2 electrode and fluorine-doped tin oxide (FTO) glass. Chemical Physics Letters 2007, 442 (1-3), 89-96.

94. Shockley, W.; Queisser, H. J., Detailed balance limit of efficiency of p-n junction solar cells. Journal of Applied Physics 1961, 32 (3), 510-519.

95. Si, H.-Y.; Sun, Z.-H.; Zhang, H.-L., Photoelectrochemical response from CdSe-sensitized anodic oxidation TiO2 nanotubes. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2008, 313-314, 604-607.

96. Tena-Zaera, R.; Katty, A.; Bastide, S.; Lévy-Clément, C., Annealing Effects on the Physical Properties of Electrodeposited ZnO/CdSe Core/Shell Nanowire Arrays. Chemistry of Materials 2007, 19 (7), 1626-1632.

97. Tvrdy, K.; Kamat, P. V., Substrate Driven Photochemistry of CdSe Quantum Dot Films: Charge Injection and Irreversible Transformations on Oxide Surfaces. The Journal of Physical Chemistry A 2009, 113 (16), 3765-3772.

98. Hong, J. S.; Choi, D. S.; Kang, M. G.; Kim, D.; Kim, K.-J., Photocurrent instability of PbS- 2- 2- sensitized TiO2 electrodes in S and SO3 solution. Journal of Photochemistry and Photobiology A: Chemistry 2001, 143 (1), 87-92.

99. Hoyer, P.; Könenkamp, R., Photoconduction in porous TiO2 sensitized by PbS quantum dots. Applied Physics Letters 1995, 349.

100. Vogel, R.; Hoyer, P.; Weller, H., Quantum-Sized PbS, CdS, Ag2S, Sb2S3, and Bi2S3 Particles as Sensitizers for Various Nanoporous Wide-Bandgap Semiconductors. The Journal of Physical Chemistry 1994, 98 (12), 3183-3188.

101. Ernst, K.; Engelhardt, R.; Ellmer, K.; Kelch, C.; Muffler, H. J.; Lux-Steiner, M. C.; Könenkamp, R., Contacts to a solar cell with extremely thin CdTe absorber. Thin Solid Films 2001, 387 (1-2), 26-28.

128 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

102. Gao, X.-F.; Li, H.-B.; Sun, W.-T.; Chen, Q.; Tang, F.-Q.; Peng, L.-M., CdTe Quantum Dots-Sensitized TiO2 Nanotube Array Photoelectrodes. The Journal of Physical Chemistry C 2009, 113 (18), 7531-7535.

103. Seabold, J. A.; Shankar, K.; Wilke, R. H. T.; Paulose, M.; Varghese, O. K.; Grimes, C. A.; Choi, K.-S., Photoelectrochemical Properties of Heterojunction CdTe/TiO2 Electrodes Constructed Using Highly Ordered TiO2 Nanotube Arrays. Chemistry of Materials 2008, 20 (16), 5266-5273.

104. Yu, P.; Zhu, K.; Norman, A. G.; Ferrere, S.; Frank, A. J.; Nozik, A. J., Nanocrystalline TiO2 Solar Cells Sensitized with InAs Quantum Dots. The Journal of Physical Chemistry B 2006, 110 (50), 25451-25454.

105. Zaban, A.; Mićić, O. I.; Gregg, B. A.; Nozik, A. J., Photosensitization of Nanoporous TiO2 Electrodes with InP Quantum Dots. Langmuir 1998, 14 (12), 3153-3156.

106. Schaller, R. D.; Sykora, M.; Pietryga, J. M.; Klimov, V. I., Seven Excitons at a Cost of One: Redefining the Limits for Conversion Efficiency of Photons into Charge Carriers. Nano Letters 2006, 6 (3), 424-429.

107. Fan, S. Q.; Kim, D.; Kim, J. J.; Jung, D. W.; Kang, S. O.; Ko, J., Highly efficient CdSe quantum-dot-sensitized TiO2 photoelectrodes for solar cell applications. Electrochemistry Communications 2009, 11 (6), 1337-1339.

108. Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Rühle, S.; Cahen, D.; Hodes, G., Chemical bath deposited CdS/CdSe-sensitized porous TiO2 solar cells. Journal of Photochemistry and Photobiology A: Chemistry 2006, 181 (2-3), 306-313.

109. Lee, Y. L.; Lo, Y. S., Highly efficient quantum-dot-sensitized solar cell based on co- sensitization of CdS/CdSe. Advanced Functional Materials 2009, 19 (4), 604-609.

110. Norris, D. J.; Efros, A. L.; Rosen, M.; Bawendi, M. G., Size dependence of exciton fine structure in CdSe quantum dots. Physical Review B 1996, 53 (24), 16347.

111. Sargent, E. H., Infrared photovoltaics made by solution processing. Nat Photon 2009, 3 (6), 325-331.

112. Evans, C. M.; Cass, L. C.; Knowles, K. E.; Tice, D. B.; Chang, R. P. H.; Weiss, E. A., Review of the synthesis and properties of colloidal quantum dots: the evolving role of coordinating surface ligands. Journal of Coordination Chemistry 2012, 65 (13), 2391-2414.

113. Sambur, J. B.; Riha, S. C.; Choi, D.; Parkinson, B. A., Influence of Surface Chemistry on the Binding and Electronic Coupling of CdSe Quantum Dots to Single Crystal TiO2 Surfaces. Langmuir 26 (7), 4839-4847.

114. Peng, Z. A.; Peng, X., Formation of High-Quality CdTe, CdSe, and CdS Nanocrystals Using CdO as Precursor. Journal of the American Chemical Society 2000, 123 (1), 183-184.

129 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

115. Klimov, V. I.; McBranch, D. W., Femtosecond 1P-to- 1S Electron Relaxation in Strongly Confined Semiconductor Nanocrystals. Physical Review Letters 1998, 80 (18), 4028.

116. Ross, R. T.; Nozik, A. J., Efficiency of hot-carrier solar energy converters. Journal of Applied Physics 1982, 53 (5), 3813-3818.

117. Sambur, J. B.; Parkinson, B. A., CdSe/ZnS Core/Shell Quantum Dot Sensitization of Low Index TiO2 Single Crystal Surfaces. Journal of the American Chemical Society 132 (7), 2130- 2131.

118. Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G., The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state. Journal of Chemical Physics 1997, 106 (23), 9869-9882.

119. Mićić, O. I.; Sprague, J.; Lu, Z.; Nozik, A. J., Highly efficient band-edge emission from InP quantum dots. Applied Physics Letters 1996, 68 (22), 3150-3152.

120. Mattoussi, H.; Cumming, A. W.; Murray, C. B.; Bawendi, M. G.; Ober, R., Characterization of CdSe nanocrystallite dispersions by small angle x-ray scattering. Journal of Chemical Physics 1996, 105 (22), 9890-9896.

121. Majetich, S. A.; Carter, A. C., Surface effects on the optical properties of cadmium selenide quantum dots. The Journal of Physical Chemistry 1993, 97 (34), 8727-8731.

122. Majetich, S. A.; Carter, A. C.; Belot, J.; McCullough, R. D., 1H NMR Characterization of the CdSe Nanocrystallite Surface. The Journal of Physical Chemistry 1994, 98 (51), 13705-13710.

123. Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.; Nozik, A. J., Schottky Solar Cells Based on Colloidal Nanocrystal Films. Nano Letters 2008, 8 (10), 3488- 3492.

124. Knowles, K. E.; Tice, D. B.; McArthur, E. A.; Solomon, G. C.; Weiss, E. A., Chemical Control of the Photoluminescence of CdSe Quantum Dot-Organic Complexes with a Series of Para-Substituted Aniline Ligands. Journal of the American Chemical Society 2010, 132 (3), 1041- 1050.

125. Frederick, M. T.; Weiss, E. A., Relaxation of Exciton Confinement in CdSe Quantum Dots by Modification with a Conjugated Dithiocarbamate Ligand. ACS Nano 2010, 4 (6), 3195-3200.

126. Robel, I. n.; Kuno, M.; Kamat, P. V., Size-Dependent Electron Injection from Excited CdSe Quantum Dots into TiO2 Nanoparticles. Journal of the American Chemical Society 2007, 129 (14), 4136-4137.

127. Dibbell, R. S.; Watson, D. F., Distance-dependent electron transfer in tethered assemblies of cds quantum dots and TiO2 nanoparticles. Journal of Physical Chemistry C 2009, 113 (8), 3139- 3149.

128. Zaban, A.; Mićić, O. I.; Gregg, B. A.; Nozik, A. J., Photosensitization of Nanoporous TiO2 Electrodes with InP Quantum Dots. Langmuir 1998, 14 (12), 3153-3156.

130 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

129. Blackburn, J. L.; Selmarten, D. C.; Ellingson, R. J.; Jones, M.; Micic, O.; Nozik, A. J., Electron and Hole Transfer from Indium Phosphide Quantum Dots. The Journal of Physical Chemistry B 2005, 109 (7), 2625-2631.

130. Mane, R. S.; Lokhande, C. D., Chemical deposition method for metal chalcogenide thin films. Materials Chemistry and Physics 2000, 65 (1), 1-31.

131. Prabakar, K.; Seo, H.; Son, M.; Kim, H., CdS quantum dots sensitized TiO2 photoelectrodes. Materials Chemistry and Physics 2009, 117 (1), 26-28.

132. Lee, H.; Leventis, H. C.; Moon, S.-J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nüesch, F.; Geiger, T.; Zakeeruddin, S. M.; Grätzel, M.; Nazeeruddin, M. K., PbS and CdS Quantum Dot- Sensitized Solid-State Solar Cells: ldquoOld Concepts, New Resultsrdquo. Advanced Functional Materials 2009, 19 (17), 2735-2742.

133. Metin, H.; Erat, S.; Ari, M.; Bozoklu, M., Characterization of CdSe films prepared by chemical bath deposition method. Optoelectronics and Advanced Materials, Rapid Communications 2008, 2 (2), 92-98.

134. Lee, H. J.; Chen, P.; Moon, S.-J.; Sauvage, F. d. r.; Sivula, K.; Bessho, T.; Gamelin, D. R.; Comte, P.; Zakeeruddin, S. M.; Seok, S. I.; Grätzel, M.; Nazeeruddin, M. K., Regenerative PbS and CdS Quantum Dot Sensitized Solar Cells with a Cobalt Complex as Hole Mediator. Langmuir 2009, 25 (13), 7602-7608.

135. Li, J. J.; Wang, Y. A.; Guo, W.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X., Large-Scale Synthesis of Nearly Monodisperse CdSe/CdS Core/Shell Nanocrystals Using Air- Stable Reagents via Successive Ion Layer Adsorption and Reaction. Journal of the American Chemical Society 2003, 125 (41), 12567-12575.

136. Chang, C. H.; Lee, Y. L., Chemical bath deposition of CdS quantum dots onto mesoscopic TiO2 films for application in quantum-dot-sensitized solar cells. Applied Physics Letters 2007, 91 (5).

137. Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T., High efficiency of CdSe quantum-dot- sensitized TiO2 inverse opal solar cells. Applied Physics Letters 2007, 91 (2).

138. Fan, S. Q.; Kim, D.; Kim, J. J.; Jung, D. W.; Kang, S. O.; Ko, J., Highly efficient CdSe quantum-dot-sensitized TiO2 photoelectrodes for solar cell applications. Electrochemistry Communications 2009, 11 (6), 1337-1339.

139. Watson, D. F., Linker-Assisted Assembly and Interfacial Electron-Transfer Reactivity of Quantum Dot−Substrate Architectures. The Journal of Physical Chemistry Letters 2010, 1 (15), 2299-2309.

;.tzel, MˆجLee, H.; Wang, M.; Chen, P.; Gamelin, D. R.; Zakeeruddin, S. M.; Gra .140 Nazeeruddin, M. K., Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process. Nano Letters 2009, 9 (12), 4221-4227.

131 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

141. Zhong, H.; Scholes, G. D., Shape Tuning of Type II CdTe-CdSe Colloidal Nanocrystal Heterostructures through Seeded Growth. Journal of the American Chemical Society 2009, 131 (26), 9170-9171.

142. Jun, Y.-w.; Casula, M. F.; Sim, J.-H.; Kim, S. Y.; Cheon, J.; Alivisatos, A. P., Surfactant- Assisted Elimination of a High Energy Facet as a Means of Controlling the Shapes of TiO2 Nanocrystals. Journal of the American Chemical Society 2003, 125 (51), 15981-15985.

143. Zhang, Z.; Zhong, X.; Liu, S.; Li, D.; Han, M., Aminolysis route to monodisperse titania nanorods with tunable aspect ratio. Angewandte Chemie - International Edition 2005, 44 (22), 3466-3470.

144. Zhang, T.; Ge, J.; Hu, Y.; Yin, Y., A general approach for transferring hydrophobic nanocrystals into water. Nano Letters 2007, 7 (10), 3203-3207.

145. Cozzoli, P. D.; Kornowski, A.; Weller, H., Low-Temperature Synthesis of Soluble and Processable Organic-Capped Anatase TiO2 Nanorods. Journal of the American Chemical Society 2003, 125 (47), 14539-14548.

146. Zhang, Z.; Zhong, X.; Liu, S.; Li, D.; Han, M., Aminolysis Route to Monodisperse Titania Nanorods with Tunable Aspect Ratio. Angewandte Chemie 2005, 117 (22), 3532-3536.

147. Acharya, K. P.; Alabi, T. R.; Schmall, N.; Hewa-Kasakarage, N. N.; Kirsanova, M.; Nemchinov, A.; Khon, E.; Zamkov, M., Linker-Free Modification of TiO2 Nanorods with PbSe Nanocrystals. The Journal of Physical Chemistry C 2009, 113 (45), 19531-19535.

148. Chuang, C.-H.; Doane, T. L.; Lo, S. S.; Scholes, G. D.; Burda, C., Measuring Electron and Hole Transfer in Core/Shell Nanoheterostructures. ACS Nano 5 (7), 6016-6024.

149. Chuang, C.-H.; Lo, S. S.; Scholes, G. D.; Burda, C., Charge Separation and Recombination in CdTe/CdSe Core/Shell Nanocrystals as a Function of Shell Coverage: Probing the Onset of the Quasi Type-II Regime. The Journal of Physical Chemistry Letters 1 (17), 2530-2535.

150. Kim, J.; Choi, S.; Noh, J.; Yoon, S.; Lee, S.; Noh, T.; Frank, A. J.; Hong, K., Synthesis of CdSe-TiO2 Nanocomposites and Their Applications to TiO2 Sensitized Solar Cells. Langmuir 2009, 25 (9), 5348-5351.

151. Fritz, K. P.; Perovic, A.; Sreekumari Nair, P.; Petrov, S.; Perovic, D. D.; Scholes, G. D., Structural characterization of CdSe nanorods. Journal of Crystal Growth 2006, 293 (1), 203-208.

152. Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisatos, A. P., Seeded Growth of Highly Luminescent CdSe/CdS Nanoheterostructures with Rod and Tetrapod Morphologies. Nano Letters 2007, 7 (10), 2951-2959.

153. Kumar, S.; Jones, M.; Lo, Shun S.; Scholes, Gregory D., Nanorod Heterostructures Showing Photoinduced Charge Separation. Small 2007, 3 (9), 1633-1639.

132 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

154. Pawar, S. G.; Patil, S. L.; Chougule, M. A.; Jundale, D. M.; Patil, V. B., Synthesis and characterization of nanocrystalline TiO2 thin films. Journal of Materials Science: Materials in Electronics, 1-5.

155. Tian, Z.-Q.; Zhang, Z.-L.; Gao, J.; Huang, B.-H.; Xie, H.-Y.; Xie, M.; Abruna, H. D.; Pang, D.-W., Color-tunable fluorescent-magnetic core/shell multifunctional nanocrystals. Chemical Communications 2009, (27), 4025-4027.

156. Morales, F.; de Groot, F. M. F.; Gijzeman, O. L. J.; Mens, A.; Stephan, O.; Weckhuysen, B. M., Mn promotion effects in Co/TiO2 Fischer-Tropsch catalysts as investigated by XPS and STEM-EELS. Journal of Catalysis 2005, 230 (2), 301-308.

157. Lazar, S.; Botton, G. A.; Wu, M. Y.; Tichelaar, F. D.; Zandbergen, H. W., Materials science applications of HREELS in near edge structure analysis and low-energy loss spectroscopy. Ultramicroscopy 2003, 96 (3-4), 535-546.

158. Yu, Z.; Guo, L.; Du, H.; Krauss, T.; Silcox, J., Shell Distribution on Colloidal CdSe/ZnS Quantum Dots. Nano Letters 2005, 5 (4), 565-570.

159. Stoyanov, E.; Langenhorst, F.; Steinle-Neumann, G., The effect of valence state and site geometry on Ti-L3,2 and O-K electron energy-loss spectra of TixOy phases. American Mineralogist 2007, 92 (4), 577-586.

160. Brydson, R.; Sauer, H.; Engel, W.; Thomass, J. M.; Zeitler, E.; Kosugi, N.; Kuroda, H., Electron energy loss and X-ray absorption spectroscopy of rutile and anatase: a test of structural sensitivity. Journal of Physics: Condensed Matter 1989, 1 (4), 797.

161. Wang, W.; Varghese, O. K.; Paulose, M.; Grimes, C. A.; Wang, Q.; Dickey, E. C., A study on the growth and structure of titania nanotubes. Journal of Materials Research 2004, 19 (02), 417-422.

162. Zarazúa, I.; De la Rosa, E.; López-Luke, T.; Reyes-Gomez, J.; Ruiz, S.; Ángeles Chavez, C.; Zhang, J. Z., Photovoltaic Conversion Enhancement of CdSe Quantum Dot-Sensitized TiO2 Decorated with Au Nanoparticles and P3OT. The Journal of Physical Chemistry C 2011, 115 (46), 23209-23220.

163. Ekimov, A. I.; Hache, F.; Schanne-Klein, M. C.; Ricard, D.; Flytzanis, C.; Kudryavtsev, I. A.; Yazeva, T. V.; Rodina, A. V.; Efros, A. L., Absorption and intensity-dependent photoluminescence measurements on CdSe quantum dots: assignment of the first electronic transitions. Journal of the Optical Society of America B 1993, 10 (1), 100-107.

164. Ithurria, S.; Tessier, M. D.; Mahler, B.; Lobo, R. P. S. M.; Dubertret, B.; Efros, A. L., Colloidal nanoplatelets with two-dimensional electronic structure. Nat Mater 2011, 10 (12), 936- 941.

165. Shabaev, A.; Efros, A. L., 1D Exciton Spectroscopy of Semiconductor Nanorods. Nano Letters 2004, 4 (10), 1821-1825.

133 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

166. Norris, D. J.; Sacra, A.; Murray, C. B.; Bawendi, M. G., Measurement of the size dependent hole spectrum in CdSe quantum dots. Physical Review Letters 1994, 72 (16), 2612-2615.

167. Burda, C.; Green, T. C.; Link, S.; El-Sayed, M. A., Electron Shuttling Across the Interface of CdSe Nanoparticles Monitored by Femtosecond Laser Spectroscopy. The Journal of Physical Chemistry B 1999, 103 (11), 1783-1788.

168. Dibbell, R. S.; Youker, D. G.; Watson, D. F., Excited-State Electron Transfer from CdS Quantum Dots to TiO2 Nanoparticles via Molecular Linkers with Phenylene Bridges. The Journal of Physical Chemistry C 2009, 113 (43), 18643-18651.

169. Dooley, C. J.; Dimitrov, S. D.; Fiebig, T., Ultrafast Electron Transfer Dynamics in CdSe/CdTe Donor-Acceptor Nanorods. The Journal of Physical Chemistry C 2008, 112 (32), 12074-12076.

170. Greenham, N. C.; Peng, X.; Alivisatos, A. P., Charge separation and transport in conjugated-polymer/semiconductor-nanocrystal composites studied by photoluminescence quenching and photoconductivity. Physical Review B 1996, 54 (24), 17628.

171. Gross, D.; Mora-Seró, I.; Dittrich, T.; Belaidi, A.; Mauser, C.; Houtepen, A. J.; Como, E. D.; Rogach, A. L.; Feldmann, J., Charge separation in type II tunneling multilayered structures of CdTe and CdSe nanocrystals directly proven by surface photovoltage spectroscopy. Journal of the American Chemical Society 132 (17), 5981-5983.

172. Harris, C.; Kamat, P. V., Photocatalysis with CdSe Nanoparticles in Confined Media: Mapping Charge Transfer Events in the Subpicosecond to Second Timescales. ACS Nano 2009, 3 (3), 682-690.

173. Hewa-Kasakarage, N. N.; El-Khoury, P. Z.; Schmall, N.; Kirsanova, M.; Nemchinov, A.; Tarnovsky, A. N.; Bezryadin, A.; Zamkov, M., The effect of dielectric friction on the rate of charge separation in type II ZnSe/CdS semiconductor nanorods. Applied Physics Letters 2009, 94 (13).

174. Hewa-Kasakarage, N. N.; Kirsanova, M.; Nemchinov, A.; Schmall, N.; El-Khoury, P. Z.; Tarnovsky, A. N.; Zamkov, M., Radiative Recombination of Spatially Extended Excitons in (ZnSe/CdS)/CdS Heterostructured Nanorods. Journal of the American Chemical Society 2009, 131 (3), 1328-1334.

175. Jin, S.; Lian, T., Electron Transfer Dynamics from Single CdSe/ZnS Quantum Dots to TiO2 Nanoparticles. Nano Letters 2009, 9 (6), 2448-2454.

176. Klimov, V. I., Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Nanocrystals. The Journal of Physical Chemistry B 2000, 104 (26), 6112-6123.

177. Kongkanand, A.; Tvrdy, K.; Takechi, K.; Kuno, M.; Kamat, P. V., Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe-TiO2 Architecture. Journal of the American Chemical Society 2008, 130 (12), 4007-4015.

178. Logunov, S.; Green, T.; Marguet, S.; El-Sayed, M. A., Interfacial Carriers Dynamics of CdS Nanoparticles. The Journal of Physical Chemistry A 1998, 102 (28), 5652-5658.

134 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

179. Tvrdy, K.; Kamat, P. V., Substrate Driven Photochemistry of CdSe Quantum Dot Films: Charge Injection and Irreversible Transformations on Oxide Surfaces. The Journal of Physical Chemistry A 2009, 113 (16), 3765-3772.

180. Underwood, D. F.; Kippeny, T.; Rosenthal, S. J., Charge carrier dynamics in CdSe nanocrystals: Implications for the use of quantum dots in novel photovoltaics. European Physical Journal D 2000, 8 (3), 241-244.

181. Wang, D.; Zhao, H.; Wu, N.; El Khakani, M. A.; Ma, D., Tuning the Charge-Transfer Property of PbS-Quantum Dot/TiO2-Nanobelt Nanohybrids via Quantum Confinement. The Journal of Physical Chemistry Letters 1 (7), 1030-1035.

182. Zhang, J. Z., Interfacial Charge Carrier Dynamics of Colloidal Semiconductor Nanoparticles. The Journal of Physical Chemistry B 2000, 104 (31), 7239-7253.

183. Zhu, H.; Song, N.; Lian, T., Controlling Charge Separation and Recombination Rates in CdSe/ZnS Type I Core-Shell Quantum Dots by Shell Thicknesses. Journal of the American Chemical Society 132 (42), 15038-15045.

184. Lou, Y.; Chen, X.; Samia, A. C.; Burda, C., Femtosecond Spectroscopic Investigation of the Carrier Lifetimes in Digenite Quantum Dots and Discrimination of the Electron and Hole Dynamics via Ultrafast Interfacial Electron Transfer The Journal of Physical Chemistry B 2003, 107 (45), 12431-12437.

185. Lo, S. S.; Mirkovic, T.; Chuang, C.-H.; Burda, C.; Scholes, G. D., Emergent Properties Resulting from Type-II Band Alignment in Semiconductor Nanoheterostructures. Advanced Materials 23 (2), 180-197.

186. Burda, C.; Link, S.; Green, T. C.; El-Sayed, M. A., New Transient Absorption Observed in the Spectrum of Colloidal CdSe Nanoparticles Pumped with High-Power Femtosecond Pulses. The Journal of Physical Chemistry B 1999, 103 (49), 10775-10780.

187. Burda, C.; El-Sayed, M. A., High-density femtosecond transient absorption spectroscopy of semiconductor nanoparticles. A tool to investigate surface quality. Pure and Applied Chemistry 2000, 72 (1-2), 165-177.

188. Wang, C.; Shim, M.; Guyot-Sionnest, P., Electrochromic Nanocrystal Quantum Dots. Science 2001, 291 (5512), 2390-2392.

189. Wang, C.; Shim, M.; Guyot-Sionnest, P., Electrochromic semiconductor nanocrystal films. Applied Physics Letters 2002, 80 (1), 4-6.

190. Klimov, V. I., Nanocrystal Quantum Dots. 2010 ed.; CRC Press: 2010; p 485.

191. Knowles, K. E.; Peterson, M. D.; McPhail, M. R.; Weiss, E. A., Exciton Dissociation within Quantum Dot: Organic Complexes: Mechanisms, Use as a Probe of Interfacial Structure, and Applications. The Journal of Physical Chemistry C 2013, 117 (20), 10229-10243.

135 Chapter 4 Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods

192. Cheng, S.; Fu, W.; Yang, H.; Zhang, L.; Ma, J.; Zhao, H.; Sun, M.; Yang, L., Photoelectrochemical Performance of Multiple Semiconductors (CdS/CdSe/ZnS) Cosensitized TiO2 Photoelectrodes. The Journal of Physical Chemistry C 2012, 116 (3), 2615-2621.

193. Benkö, G.; Kallioinen, J.; Korppi-Tommola, J. E. I.; Yartsev, A. P.; Sundström, V., Photoinduced Ultrafast Dye-to-Semiconductor Electron Injection from Nonthermalized and Thermalized Donor States. Journal of the American Chemical Society 2001, 124 (3), 489-493.

194. Asbury, J. B.; Ellingson, R. J.; Ghosh, H. N.; Ferrere, S.; Nozik, A. J.; Lian, T., Femtosecond IR Study of Excited-State Relaxation and Electron-Injection Dynamics of Ru(dcbpy)2(NCS)2 in Solution and on Nanocrystalline TiO2 and Al2O3 Thin Films. The Journal of Physical Chemistry B 1999, 103 (16), 3110-3119.

195. Duncan, W. R.; Stier, W. M.; Prezhdo, O. V., Ab Initio Nonadiabatic Molecular Dynamics of the Ultrafast Electron Injection across the Alizarin-TiO2 Interface. Journal of the American Chemical Society 2005, 127 (21), 7941-7951.

196. Bräm, O.; Cannizzo, A.; Chergui, M., Ultrafast fluorescence studies of dye sensitized solar cells. Physical Chemistry Chemical Physics 14 (22), 7934-7937.

197. Wenger, B.; Grätzel, M.; Moser, J.-E., Rationale for Kinetic Heterogeneity of Ultrafast Light-Induced Electron Transfer from Ru(II) Complex Sensitizers to Nanocrystalline TiO2. Journal of the American Chemical Society 2005, 127 (35), 12150-12151.

198. Yang, Y.; Rodríguez-Córdoba , W.; Xiang, X.; Lian, T., Strong Electronic Coupling and Ultrafast Electron Transfer between PbS Quantum Dots and TiO2 Nanocrystalline Films. Nano Letters 12 (1), 303-309.

136 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

Chapter 5: Direct Synthesis of CdSe Nanocrystals with Electro-active Ligands

This chapter is reproduced in part with permission from this following publication:

“Yasser Hassan, Trevor Janes, Ryan D. Pensack, Shahnawaz Rafiq, Peter M. Brodersen, Datong

Song, Mitchell A. Winnik, and Gregory D. Scholes, submitted to Journal of Chemistry of

Materials”.

Contribution of Authors in this Chapter:  YH and DS had the idea for.

 YH conceived and directed the project, designed the experiments, carried out the

synthesis of materials, and performed data analysis.

 TJ synthesized the ferrocene ligands throughout the project and tested their NMR.

Discussed the project progress with YH. My work described in this Chapters would not

have been possible without the generous synthesis donation of ferrocene samples by

Trevor!

 RP and SR performed the transient absorption measurements. RP performed the transient

absorption data analysis.

 YH carried out the electron microscope measurements. YH did the morphology analysis

of the materials. PB carried out the XPS measurements and supervised this experiment

results and discussion in the manuscript.

 YH, RP and GDS wrote the manuscript for publication.

 All authors discussed the results and commented on the manuscript.

 GDS, MAW and DS supervised the whole project.

137 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5.1 Abstract.

We report the synthesis and characterization of cadmium selenide nanocrystals with electro-active ligands directly attached to the surface. The conventional surfactant-assisted synthesis yields nanocrystals with surfaces functionalized with insulating organic ligands. These insulating ligands act as a barrier for charge transport between nanocrystals. Electro-active (reducing/oxidizing) ligands like ferrocene and cobaltocene have potential for applications as photo-excited hole conductors and photoredox systems. Although ferrocene ligands anchored to the nanocrystal surface through insulating long-chain hydrocarbon spacers have previously been reported, this approach is limited because the charge transfer between nanocrystal and ferrocene is highly sensitive to their separation. We report here ferrocene directly bound to the inorganic core of the nanocrystal and as a result the distance between the nanocrystals and the electro-active moiety is minimized.

Figure 5.1: CdSe nanocrystals are prepared directly with their surfaces passivated by electro-active ligands like ferrocene phosphine and phosphinoxide derivatives.

138 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5.2 Introduction

Owing to their tunable physical properties that come from the quantum confinement effect, colloidal inorganic nanocrystals (NCs) or quantum dots have been examined for a diverse range of applications that include optoelectronic devices,1-2 hydrogen reduction photocatalytic processes3-4 and biosensors.5-7 Facile transport of charge carriers between individual NCs is an essential requirement in optoelectronic devices based on these materials.8 In addition, the removal of photo-generated holes from the surface of the NCs, i.e., post photo-oxidation, was found to be essential to obtain efficient devices.9

In the conventional surfactant-assisted synthesis of NCs, surfactants need to be anchored to the

NCs’ surface during their growth.10-12 These long chain organic surface capping ligands are important to control the NC growth, to prevent aggregation, to maintain the desired size, and to passivate surface electronic trapping states in the NCs.10-15 Moreover, the solubility of the semiconductor NCs is dictated by these capping ligands. A challenge, however, is that this conventional surfactant-assisted synthesis yields NCs with surfaces functionalized with insulating organic ligands. These insulating ligands act as a barrier for charge transport between NCs.8, 16-17

It has been proven by Kuno et al. that complete removal of surface ligands is difficult and can create surface dangling bonds and charge-trapping centers that open up pathways for nonradiative relaxation18. However, an important feature of these surfactants is that they are easily exchanged on the surface of the NCs.

In addition, it has been reported that in situ synthesis of NCs in a polymer matrix improves the polymer-nanoparticle interface, and consequently facilitates efficient electronic interaction between the polymer and NC.19-23 As reported previously,10-13 controlling the nucleation and growth process during NC synthesis is essential to obtaining monodisperse materials and this

139 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands parameter is dramatically affected by the bonding strength and steric hindrance effects of the ligands as well as the presence of non-coordinating media.12 This hypothesis is found to be applicable in the case of the in situ synthesis of nanocrystals in polymers.19-20, 22

In deposited films, electronic coupling between NCs, and consequently the charge carrier mobility, can be enhanced by removing the organic surfactants from the NC surface.24-26 The introduction of other competing ligands, through surfactant exchange, makes it possible to access a wide range of chemical functionalities.27 Ligand exchange has been used to adapt the surface of NCs for specific applications.8-9, 25, 28 It was found that the chemical nature of the linker or the new ligand plays a decisive role in determining the efficiency of charge transfer from the NC surface to the surrounding media. An ideal ligand then, should be highly conducting, provide colloidal stabilization, and facilitate stable and facile electronic interactions between the NCs.8 Kovalenko et.al. introduced highly promising metal chalcogenide surface ligands in a ligand exchange process that increased the conductivity of NC solids () by ~11 orders of magnitude compared to the ordinary organic ligands, approaching  values of ( ~200 S cm−1).8

Electroactive ligands (reducing/oxidizing species) like ferrocene9, 26, 29-33 and cobaltocene34 have been studied for applications that include photo-excited hole conductors and photoredox systems.

In previous reports, ferrocene was anchored to the surface of the NCs through hydrocarbon spacers with alkyl chains length of (3–12 carbon units).9, 30 It was suggested by theory and simulations,35-

36 and experimentally31, 33 that the hole transfer rate from the photoexcited NCs surface (donor) to the ferrocene moiety (acceptor) depends on the hydrocarbon spacer length. The donor-acceptor charge transfer rate constant decreases exponentially with increasing donor-acceptor bridge distance28, 33, 37. In addition, complete removal of organic ligands through ligand exchange processes is difficult.

140 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

The structure of the ligand can dramatically affect the shape and properties of the semiconductor

NCs. The optimum ligands should meet the conditions of (i) highly conducting, (ii) adhere chemically to the NCs surface, thereby providing colloidal stabilization, (iii) providing stable and facile electronic communication between the NCs as the distance between the donor and acceptor is short.8

In this study, we report the direct synthesis and characterization of CdSe NCs with electroactive ligands of ferrocene phosphine and phosphine oxide derivatives without an intervening alkyl chain spacer. These metallocenes are directly bound to the inorganic core of the NC and as a consequence the distance between the NCs and the electroactive moiety is minimized. Ferrocene phosphinoxide derivatives, in particular, were found to be promising candidates toward the goal of a direct synthesis of CdSe NCs with electro-active ligands meanwhile the alkyl side chain, tert-butyl, necessary for colloidal stability is attached.

5.3 Experimental Methods

General Methods. All procedures were carried out using standard Schlenk line techniques under oxygen-free conditions and nitrogen flow.

5.3.1 Materials.

All chemicals were used as received without further purification. Cadmium acetate (98.0 %), cadmium oxide (99.99 %), selenium (100 mesh, 99.5 %,), 1-octadecene (90 %), were purchased from Sigma-Aldrich and used without further purification. All solvents used including chloroform, methanol, toluene and isopropanol were anhydrous.

141 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5.3.2 Synthesis of the Electroactive Ligands.

t t Di-tert-butylphosphinoferrocene (FcP Bu2) and Di-tert-butylphosphinylferrocene (FcP(O) Bu2)

(Scheme 5.1) were synthesized according to the method of Xiao et al38 and Hartwig et al39, sections 5.3.2.1 and 5.3.2.2.

Scheme 5.1 Chemical structure of ligand used in this study (a) Di-tert-butylphosphinylferrocene

t t (FcP(O) Bu2), (b) di-tert-butylphosphinoferrocene (FcP Bu2).

Briefly,

t 5.3.2.1 Di-tert-butylphosphinoferrocene (FcP Bu2)

1. To an oven-dried 100-mL three-nick flask, a (9.36 g, 50.3 mmol) of ferrocene was dissolved

in THF (25 mL).

2. 1.7 M tertbutyllithium (tBuLi) in pentane (30.0 mL, 50.3 mmol) was added dropwise over

5 min to the ferrocene solution at 0 oC.

3. The reaction was allowed to stir at 0 oC stirred for 25 minutes while the temperature

maintained and then the solvent was evaporated under vacuum.

4. The residue was then redissolved in a solvent mixture of pentane (100 mL) and THF (5

t mL), then (5.30 mL, 27.7 mmol) of di-tert-butylchlorophosphane (ClP( Bu)2) were added

at room temperature and leave the mixture to stir for 3 hours.

142 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5. A 6 mL of degassed methanol was added to the flask at that point and was allowed to stir

for 0.5 h before the mixture solvents were totally evaporated under vacuum. 6 mL of

MeOH.

6. The byproduct contained a small amount of chloro-di-tert-butylphosphine and unreacted

ferrocene. These byproduct were eluted with pentane through a plug of silica under N2,

followed by collecting the crude product by elution with ethanol (Et2O).

7. The product extract in Et2O was collected separately and concentrated under vacuum to

give an orange/red precipitate of di-tertbutylphosphinoferrocene.

This material synthesized and collected and secured under N2 all the time because it is so sensitive to oxygen.

t 5.3.2.2 B. Di-tert-butylphosphinylferrocene (FcP(O) Bu2)

1. (9.15 g, 27.7 mmol) of di-tert-butylphosphinoferrocene prepared in the previous step was

dissolved in 100 mL ethyl acetate EtOAc, and then a couple of drops of H2O2 was added at 0

oC, and stirred.

2. The reaction monitored by thin layer chromatography (TLC) using EtOAc until the oxidation

is completed.

3. Once oxidation was complete, the solution was treated with aqueous sodium thiosulfate

(Na2S2O3) and extracted twice with ethanol (2 × 50 mL).

4. The organic extracts were dried over MgSO4 and filtered.

5. Then the solution concentrated via evaporation under vacuum and purified by flash

t chromatography (EtOAc/MeOH 100:1) to give the FcP(O) Bu2 compound as an orange

precipitate.

This compound is stable in air and can be prepared in grams.

143 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t Figure 5.2: Photograph of unbound FcP(O) Bu2 ligand prepared in this work.

5.3.3 Direct Synthesis of CdSe NCs in Ferrocene Derivatives.

We prepared two batches of CdSe quantum dots with different sizes.

5.3.3.1 CdSe NCs 3.5 nm size

In a typical direct synthesis of CdSe in ferrocene ligand (Scheme 5.1):

t 1. 0.3 g of FcP(O) Bu2 and 0.1 mmol of Cd-acetate were mixed with 5 mL of ODE in a 50 mL

three-necked flask.

2. After the reaction flask was pumped under vacuum for ~1 h at 120°C, the solution was heated

to 220~250°C under argon or N2 flow.

3. At that point, the Se-precursor was added swiftly into the reaction. The Se solution, prepared

t by dissolving 0.012 g Se powder and 0.3 g of FcP Bu2 were mixed with 3 mL ODE at 70°C

under N2. This Se-precursor degassed and stored under nitrogen at 70°C before it used.

4. The reaction temperature was adjusted to 10 degrees lower than the injection temperature, and

the reaction was stopped after several minutes by the removal of the heating mantle and the

injection of anhydrous toluene.

5. The growth CdSe NCs was monitored by taking aliquots from the reaction with interval of

144 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

time (2, 5 and 10 minutes) and measuring absorption and photoluminescence spectra.

6. The reaction solution color changed, almost after the first two minutes, from bright orange to

dark brown indicating the formation of CdSe nanoparticles.

5.3.3.2 CdSe NCs 7 nm size.

In order to increase the size of the particle, we increased the concentration of the both cadmium and selenium precursors and added the selenium precursor drop-wise in two additions.

Typically,

t 1. Cadmium precursor was prepared by mixing 0.55 g of FcP(O) Bu2 and was dissolved in 7 mL

of ODE and was mixed with 0.045 g of Cd-acetate in a 50 mL three-necked flask.

2. The reaction flask was pumped under vacuum for ~1 h at 120°C and the solution was heated

to 220~250°C under N2 flow.

3. The mixture was further held under vacuum for 10 minutes before the flask was allowed to

return to 220°C under nitrogen.

4. At that point, the Se-precursor (3 mL) was added drop-wise into the reaction at a rate of 0.4

mL/min. After 10 minutes from the first injection, the rest of the Se-precursor (4 mL) was

added drop-wise into the reaction. The Se solution, prepared by dissolving 0.036 g Se powder

t and 0.256 g of FcP Bu2, was mixed with 7 mL ODE at 70°C and degassed under N2 before it

used.

5. The reaction temperature was adjusted to 220–250 °C, and the reaction was stopped after

several minutes by the removal of the heating mantle and the injection of anhydrous toluene.

6. Nanoparticles were allowed to grow at 220 °C. The growth of CdSe NCs was monitored by

taking aliquots from the reaction with interval of time (10, 15 and 30 minutes) from the second

injection.

145 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

7. The NCs were isolated and cleaned by several precipitation and re-dissolution cycles using

toluene or chloroform as solvent and acetonitrile as the non-solvent (1:3 solvent to non-solvent

ratio). Precipitation was achieved by centrifugation for 10–20 min under 6000 rpm.

It is worth mentioning that the “inverse-injection technique” works well for this system. Typically, the Cd-precursor, once prepared at 220–250 °C was cooled to 50 oC before it was injected to a hot solution of Se-precursors at 220–250 °C. We found that the Se-precursor was more stable at high temperature than the Cd-precursor.

As a comparison, we also prepared CdSe according to Cao and coworkers method40 using cadmium acetate and selenium powder in ODE at 240°C to reach the desired size (3nm particles).

t Post-synthesis, we stabilized the NCs in FcP(O) Bu2 ligands by performing ligand exchange.

Figure 5.3: Schematic approach of our synthesis of CdSe capped with electro-active ligands. Schlenck flask is drawn by Dr. Tihana Mirkovic.

146 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5.3.4 Characterization.

TEM, HRTEM and EDX: Size distributions of the prepared CdSe NCs were determined by transmission electron microscopy

(TEM) using a JEOL JEM–2010 TEM with a LaB6 filament operating at 200 kV for both low- magnification (TEM) and high-resolution (HRTEM) images. The energy-dispersive X-ray spectra of the prepared CdSe NCs were determined using Hitachi S-5200; high resolution 'in lens' scanning electron microscope with resolution of 0.4nm at 30kV and 1.8nm at 1kV with Oxford Inca EDX.

SAED: Selective Area Diffraction pattern (SAED) for collection of CdSe NC particles deposited on carbon grid were characterized using transmission electron microscopy (TEM; JEOL 2010 at 200 kV). SAED were simulated using the open-source software: (GDIS41 and Diffraction Ring

Profiler42), with the input of known space group information of the CdSe hexagonal phase

(Wurtzite)43-44. The diffraction ring profiler integrates the SAED ring pattern intensities to accurately calculate the center point of each ring.

UV-vis Absorption and PL: UV-vis Absorption and PL spectra were recorded using a Varian Cary 100 and Varian Eclipse fluorescence spectrometer, respectively, in a 1 cm cuvette. The PL recorded with excitation wavelength of 450 nm.

Powder X-ray diffraction (PXRD): PXRD measurements were carried out on a SiemensD-5000 diffractometer using a high-power Cu

Kα source operating at 50 kV and 35mA with a Kevex solid-state detector. A step scan mode was used for data collection with a step size of 0.028 and time of 2.0 s per step.

147 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

X-ray Crystallography: t Single crystals of FcP(O) Bu2 were grown from a cold ethyl acetate solution. The X-ray diffraction data were collected on a Bruker Kappa Apex II diffractometer with graphite-monochromated Mo

Kα radiation (λ = 0.71073 Å) at 150 K controlled by an Oxford Cryostream 700 series low- temperature system and processed with the Bruker Apex 2 software package (Apex 2 Software

Package; Bruker AXS Inc., Madison, WI, 2013). The structures were solved by direct methods and refined using SHELXL-2013.45 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were calculated using the riding model.

X-ray photoelectron spectroscopy (XPS):

XPS data was acquired on a ThermoFisher Scientific K-Alpha spectrometer with a monochromatic

Al Kα X-ray radiation source in an ultrahigh–vacuum chamber with base pressure of 10−9 Torr.

The generated X-ray photons is 1486.7 eV in energy. XPS analysis were performed on samples drop-casted onto Si(100) substrates whereby both survey and regional spectra were acquired from all samples measured. All data analyses were carried out using the Avantage software fitting program (provided from the vendor of the instrument) to confirm the incorporation of Cd, Se, P and Fe in the CdSe nanocrystals. In order to investigate the chemical state(s) present in each sample; the elemental composition was calculated and the experimental data was mathematically modelled to deconvolute the peak shapes. The peak shapes were deconvoluted utilizing

Lorentzian-Gaussian peak shapes and smart background function to approximate the experimental backgrounds. Surface elemental compositions were calculated from background-subtracted peak areas derived from transmission function corrected regional spectra. Scofield Al K-alpha sensitivity factors were used to calculate the relative atomic percentages. The binding energies were referenced to the NIST-XPS database.

148 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

Cyclic Voltammetry:

CV scans were run using a BASi Epsilon Electrochemical Workstation in a nitrogen-filled glovebox. A 3 mm diameter glassy carbon disk, a platinum wire, and a silver wire were used as working, counter, and pseudo-reference electrodes, respectively. Experiments were conducted in dry, deoxygenated dichloromethane; 0.1 M NBu4PF6 (recrystallized from EtOH) was used as supporting electrolyte. CV scans were obtained without compensation for internal resistance.

1,2,3,4,5-pentamethylferrocene (Me5Fc) was synthesized according to a previously published procedure.46 Potentials were calibrated using ferrocene as internal standard unless otherwise stated and are reported vs. Fc/Fc+.

NMR:

1H and 31P NMR spectra were recorded at ambient temperature on VARIAN Mercury 300 MHz and AGILENT DD2 600 spectrometer (600 MHz for 1H and MHz for the 31P) with 5 mm OneNMR

H/F{X} probe. Chemical shifts were referenced to deuterated solvent peaks).

Mass Spectroscopy:

The dual electrospray ionization (ESI) mass spectra of the electroactive ligands before and after capping the CdSe NCs were acquired in positive-ion mode using a 6538 UHD model quadrupole time-of-flight mass spectrometer equipped with a 1200 Infinity Series HPLC (Agilent

Technologies, Santa Clara, CA).

Transient absorption (TA) spectroscopy:

The experimental setup has been described in detail previously.47 Briefly, a Ti:sapphire-based laser amplifier (Spectra-Physics, Spitfire Pro) that outputs about 3.5 W of 150 fs, 800 nm pulses of light at 5 kHz was used as a laser light source. The amplifier output was directed into a custom-built noncollinear optical parametric amplifier (NOPA) to convert the wavelengths to encompass a

149 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands spectral range from 520 to 620 nm. Pulse compression was achieved with a combination of a grating and prism compressor. The pulse was compressed to about 20 fs (as estimated from a polarization-gated frequency-resolved optical gating measurement48). The compressed NOPA light was split into three beams; pump, probe and a reference beam. The time delay between pump and probe beams was controlled by a motorized translation stage in the pump path. The pump beam was chopped at 625 Hz. The relative polarization of pump and probe beams was set to magic angle.

5.4 Results and Discussion

5.4.1 Our Approach in This Chapter

In this work we demonstrate a direct synthesis approach yielding high-quality CdSe NCs with electro-active ligands. This method includes synthesis of precursor materials in mild and simple reaction conditions. In a typical synthesis, the electro-active ligands include both di-tert-

t t 38 butylphosphinylferrocene (FcP(O) Bu2) and di-tert-butylphosphinoferrocene (FcP Bu2) complexing ligands (Scheme 1) for cadmium and selenium precursors, respectively.

We found that the direct synthesis with FcPO is important over post-synthesis ligand exchange.

Because the electron transfer rates decrease exponentially with increasing donor-acceptor

28, 33, 37 t t distance, we selected these two ligands, FcP(O) Bu2 and FcP Bu2 to ensure the short NC’s surface-to-ligand distance. X-type ligands49 like ferrocene monothiol, 1, 1′-ferrocene dithiol, ferrocene monocarboxylic acid, 1, 1′-ferrocene dicarboxylic acid can be used as short chain conductive ligands and provide better stability. For example, we carried out ligand exchange experiments for CdSe NCs post synthesis in trioctyl phosphinoxide system (TOPO, 90 %) using

150 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands other short chain ferrocene derivatives like ferrocene monothiol, 1, 1′-ferrocene dithiol, ferrocene monocarboxylic acid, 1, 1′-ferrocene dicarboxylic acid. These experimental tests showed that ligand exchange resulted in the aggregation of the NCs and their precipitation out of solution. On

49 t t the other hand the direct synthesis with L-type ligands of FcP(O) Bu2 and FcP Bu2 is important for several reasons. Though our L-type ligands might not provide adequate stability due to weaker interaction of P-atom with the NCs surface, the presence of an alkyl side chain provides some steric hindrance that controls the growth process and prevents aggregation in solutions.

5.4.2 Synthesis and Characterization of the Electro-active Ligands

t t 38 We synthesized the FcP Bu2 and FcP(O) Bu2 ligands according to the method of Xiao et al, then we carried out the post-synthesis characterization for these ligands, e.g., using NMR, see

Figure 5.5 and Figure 5.5, and FTIR to confirm the purity of the products. Moreover, we tested

t the mass spectra of the FcP(O) Bu2 using the dual electrospray ionization (ESI) mass spectra,

Figure 5.6, that shows agreement with the expected molecular mass of this component, i.e.

t molecular mass equal to 346.12 g/mol. Figure 5.7 shows the single crystals of FcP(O) Bu2 that we had grown from a cold ethyl acetate solution. All abovementioned characterization confirm the the

t purity of the FcP(O) Bu2 product we synthesized in this study.

151 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

1 31 t 1) HNMRs and PNMR of di-tert-butylphosphinylferrocene (FcP(O) Bu2):

1 31 t Both HNMRs and PNMR results confirm the purity of the FcP(O) Bu2 product.

1 31 t Figure 5.4: HNMRs and PNMR of di-tert-butylphosphinylferrocene (FcP(O) Bu2) in CDCl3

152 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

1 31 t 2) HNMRs and PNMR of di-tert-butylphosphinoferrocene (FcP Bu2):

1 31 t Both HNMRs and PNMR results confirm the purity of the FcP Bu2 product.

1 31 t Figure 5.5: HNMRs (a) and PNMR (b) of di-tert-butylphosphinoferrocene (FcP Bu2).

153 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t 3) Mass Spectroscopy of the FcP(O) Bu2 ligand

t Figure 5.6: Mass spectrum of unbound free FcP(O) Bu2 ligand, Molecular weight of 346.12 gram/mole.

t Figure 5.7: Single crystal XRD of FcP(O) Bu2 crystals.

154 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t t The results of these characterizations of ferrocene derivatives (FcP(O) Bu2/FcP Bu2) ligands we synthesized in this work are in good agreement with the results obtained by Xiao et al38 and

Hartwig et al39.

t 5.4.3 Testing the Stability of FcP(O) Bu2 Ligand at High Temperature

t In order to examine the effect of the reaction temperature on FcP(O) Bu2 stability, a blank reaction

t of FcP(O) Bu2 in ODE without any added Cd or Se salts was carried out. Based on absorption spectroscopy, thin layer chromatography, thermogravimetric analysis (TGA), and differential

t scanning calorimetry (DSC) measurements, no degradation of FcP(O) Bu2 was observed until 280

°C, see Figure 5.8 and Figure 5.9. Therefore, we carried the semiconductor NC synthesis at temperatures between 200 and 250 °C. Therefore, we carried the semiconductor NC synthesis in the following sections at temperatures between 200 and 250 °C.

t 4) Thin Layer Chromatography Analysis (TLC) of FcP(O) Bu2 ligand:

t Figure 5.8: TLC of unbound FcP(O) Bu2 ligand dissolved in 1-octadecene at different temperatures (right spot) versus the same mixture at room temperature (left spot).

155 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5) Thermogravimetric analysis (TGA:

Figure 5.9: Thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) measurement

t of the FcP(O) Bu2 ligand.

156 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5.4.4 Characterization of Precursor Complexes

The starting precursor complexes of our reaction here were characterized separately by NMR and mass spectroscopy.

1) 1H and 31PNMR Characterization of Se-precursor

Figure 5.10: 1HNMRs (a) and 31PNMR (b) of Se-precursor, the di-tert-butylphosphinoferrocene complex

t with Se (FcP(Se) Bu2).

2) Mass spectrum of Se-precursor

157 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t Figure 5.11: Mass spectrum of Se-precursor, the Se-FcP Bu2 complex. Peak at 411.0441 m/z indicate the

t formation of the FcP(Se) Bu2 complex.

5.4.5 CdSe NCs Directly Synthesized with Electroactive Ligands.

In a typical synthesis, we used ODE as a high-boiling point non-coordinating solvent with

t traditional ligands. The Se-precursor of selenium powder coordinated by FcP Bu2 was introduced in a stoichiometric amount with respect to the Cd-precursor of cadmium acetate coordinated by

t o FcP(O) Bu2 in ODE under inert condition at 200–250 C. A quantitative analysis of the TEM images reveals that the resulting NCs are nearly monodisperse without any size separation.

In order to understand the role of the ferrocene ligands during synthesis and the nature of the

t nanocrystal-ligand interaction, we used various methods to study CdSe capped with FcP(O) Bu2

NCs. These characterization methods include infrared spectroscopy, X-ray photoelectron spectroscopy, solution nuclear magnetic resonance spectroscopy (NMR). To investigate the charge

158 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands transfer between the NCs and the ligands, we used photoluminescence spectroscopy, transient absorption spectroscopy and cyclic voltammetry.

Figure 5.12: Cartoon illustration of the inorganic core of the new QDs and the nature of their surface.

5.4.6 Shape Characterization.

t To check the morphology and chemical composition of the CdSe capped with FcP(O) Bu2 several measurements were carried out such as powder X-ray diffraction (PXRD), energy–dispersive X- ray (EDX), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and the selected area electron diffraction pattern (SAED). TEM images of the synthesized NCs show crystalline spherical particles, Figure 5.13a and Figure 5.13b. HRTEM analysis of CdSe nanocrystals synthesized in ferrocene ligands shows well-developed lattice fringes revealing the

(101) planes with a spacing of 0.35 nm (inset Figure 5.13b). HRTEM together with the selected area electron diffraction pattern (SAED) confirm the high crystallinity of the CdSe NCs sample.

159 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

Figure 5.13: Low-magnification transmission electron micrograph (TEM) analysis for CdSe capped with

t t capped with ferrocene derivatives (FcP(O) Bu2 and FcP Bu2); directly synthesized in FcPO(tBu2) at 250 °C with the size of (a) 3.5 nm (b) 6–7 nm. Inset: High-resolution electron microscopy (HRTEM), (c) SAED of as-prepared CdSe together with the simulated electron diffraction patterns pattern of the CdSe hexagonal structure and (d) EDX analysis of CdSe nanocrystals (size: 6–7 nm).

TEM diffraction patterns (SAED) were analyzed with Diffraction Ring Profiler software, which was developed for phase identification in complex microstructures42. The SAED pattern of a collection of CdSe NC particles (in Figure 5.13c) exhibits broad diffuse rings that are typical of nano-sized particles and show crystalline particles with the patterns index of (100), (002), (101),

(102), (110), (103), and (112) reflections of the hexagonal structure of CdSe43-44. Elemental analysis was performed to confirm the composition of the synthesized NCs using scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDX). EDX results, shown in

160 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t Figure 5.13d, indicate traces of Cd, Se, Fe, and P elements in the CdSe capped with FcP(O) Bu2 composite sample. TEM studies revealed the abundant formation of nearly homogeneously dispersed spherical CdSe nanoparticles.

t The powder X-ray diffraction (PXRD) patterns of the free FcP(O) Bu2 ligand and the CdSe–

t FcP(O) Bu2 nanocrystals are presented in Figure 5.14 and Figure 5.15, respectively. CdSe NCs have been purified by iteratively precipitating in acetonitrile several cycles and subsequently re- dissolving in toluene in order to remove excess and weakly bound surface ligands. Strong reflection peaks at 23.89 o, 25.39 o, 27.09 o, 41.99 o and 49.73 o, corresponding to the (100), (002),

(101), (110) and (112) planes, respectively, indicate high crystallinity of a typical diffraction pattern of hexagonal Wurtzite CdSe phase (space group P63mc)43-44. The main reflection peaks of

o o o o t CdSe at 25.39 , 35 , 41.99 and 50 are overlapped with peaks from FcP(O) Bu2 ligand crystalline

t phase, Figure 5.14 for comparison. The CdSe–FcP(O) Bu2 PXRD spectrum is in agreement with the SAED results (Figure 5.13c) for the same sample.

161 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t Figure 5.14: X-ray diffraction patterns of unbound FcP(O) Bu2 ligand.

t Figure 5.15: X-ray diffraction patterns of CdSe-Fc(O) Bu2 NCs.

162 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

The results of these characterizations of CdSe NCs directly synthesized in ferrocene derivatives

t t (FcP(O) Bu2/FcP Bu2), Figure 5.13 and Figure 5.15, are in good agreement with previous studies of CdSe NCs prepared with the conventional surfactant-assisted synthesis method. This confirms that we have pure CdSe NCs cores without doping.

5.4.7 Electroactive Ligand Attached to the Surface of the NCs.

5.4.7.1 Optical Properties.

We studied the optical properties of our NCs using UV–vis spectroscopy and PL measurements.

Figure 5.16 shows the absorption spectra of the electro-active ligands di-tert-butylphosphinyl

t t t ferrocene (FcP(O) Bu2) and CdSe–(FcP(O) Bu2) dissolved in toluene. The spectrum of FcP(O) Bu2 shows a broad peak in the range 400–520 nm with a maximum at 451 nm that is characteristic of

t the ferrocene moiety. For the solution of the CdSe–(FcP(O) Bu2) composite, the absorption spectrum exhibits a peak at 555 nm that we attribute to the first CdSe exciton peak, i.e. the s-like

(1S3/2 –1Se) transition of the CdSe NCs, as well as significant absorption at wavelengths shorter than 380 nm. 28 A tail extending out to longer wavelengths than 555 nm is also apparent and we attribute this to NC aggregation in the solution that occurs over time. These features, absent in the

t case of FcP(O) Bu2 alone, indicate the presence of CdSe nanocrystals. The p–like (1P3/2–1Pe) states of CdSe NCs manifest in the peak at 488 nm is obscured by the absorption of the ferrocene moieties on the surface of the NCs. It is worth mentioning here that the ferrocene absorption is redshifted in comparison with the neat ligand by 0.155 eV, indicating strong complexation between the nanocrystal and the electro-active ligand.

The absorption peak at 488 nm degraded with strong washing by acetonitrile indicating removal of ferrocene from surface of the NCs. We observed no photoluminescence, indicating that the

163 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands excited state of the CdSe NCs is efficiently quenched. This is attributed to the rapid non-radiative hole transfer to ferrocene as well as an increase in surface trap states due to the steric-hindrance of the cyclopentadienyl rings of ferrocene on the surface of the NCs.

t Figure 5.16. UV-vis absorption spectra of FcP(O) Bu2 in toluene (yellow), CdSe NC (synthesized in

t FcP(O) Bu2) in toluene (red). The ferrocene absorption peak associated with the CdSe NCs is red shifted from the original pure ligand by 0.155 eV.

164 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5.4.7.2 Surface Characterization.

5.4.7.2.1 Fourier transform infrared spectroscopy

t t FTIR spectra of the FcP(O) Bu2 ligand and FcP(O) Bu2 capped CdSe quantum dots are presented

t 2 in Figure 5.17. FTIR of the free unattached FcP(O) Bu2 ligand, blue line, exhibit (C–H)Fc sp stretching, aromatic (C–C)Fc stretching, (C–H)Fc in plane bending and (C–H)Fc out of plane bending vibrations at 3089 cm-1, 1476 cm-1, 1022 cm-1 and 815 cm-1, respectively, which are associated with the aromatic rings of ferrocene moiety. 50-51 The vibrations at 2950 cm-1 and 2867 cm-1 are assigned to asym (C–H) and sym (C–H) of the tert-butyl side chains, on the other hand, the

-1 -1 -1 vibrations at 1369 cm , 1144 cm and 1108 cm are assigned to the P–CH3 bending and P=O symmetric and anti-symmetric stretching absorption bands of the phosphinoxide moiety,

t respectively. FTIR spectra of the CdSe NCs capped with FcP(O) Bu2 ligand (green line in Fig. 3),

t show that the P=O stretching of the FcP(O) Bu2 ligand is broadened, split and downshifted in the range of 1000–1110 cm-1 relative to the free ligand which is attributed to complexation with the

CdSe surface.52-53 This is in agreement with previous IR measurements performed on P=O stretching frequencies of complex-formed by phosphine oxides with salts. 54

165 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t Figure 5.17. FTIR transmittance spectra: FcPO( Bu2) (blue line) and CdSe NCs dispersed in KBr (green

t line). The characteristic sym. and anti-sym. absorption bands of P=O stretching of FcP(O) Bu2 were shifted to lower wavenumber and broadened indicating complexation between the P=O moiety and the CdSe surface.

5.4.7.2.2 X-ray photoelectron spectroscopy

t Multiple samples of CdSe quantum dots capped with FcP(O) Bu2 were examined by X-ray photoelectron spectroscopy (XPS) in order to study the change in composition and chemical bonding at the surfaces of CdSe nanocrystals and the capping ligand. Figure 5.18 shows a typical

t low resolution XPS full survey scan of CdSe NCs (6–7 nm) capped with FcP(O) Bu2. The presence of Si is from the substrate, while the quantitatively significant peaks for Cd, Se, P and Fe are from

166 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands the nanocrystals and their surfaces. The presence of C and O might be from the CdSe NCs surface and/or from atmospheric contamination.

Figure 5.19–5.21 show high-resolution XPS spectra taken of the Cd, Se, P and Fe regions of CdSe

NCs deposited from methanol solution. Several features can be assigned by analyzing the binding

167 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands energy measured from these regions. Typically, one can define the chemical environment of each species on the surface of the nanocrystals and their ratios. XPS indicated a binding energy of core level Se 3d peak at 55.08 eV (Figure 5.19a) and Cd 3d5/2 peak at 405.62 eV (Figure 5.19b) and the difference between their peak positions is about 350.6 eV that is in agreement with the binding energy of bulk CdSe (Ref.52, Katari et. al. (1994) and references therein).52-53

Moreover, we have investigated the XPS spectra of both Cd 3d5/2 and Se 3d for the two samples we prepared in this study of CdSe with size of 3 and 7 nm using identical experimental conditions,

Figure 5.19. The results indicate three main characters in both Cd 3d5/2 and Se 3d: (a) broadening increased with decreasing particle size, (b) decreasing the CdSe NCs size from 7 to 3 nm results in shifting to higher binding energy side, for instance the Cd 3d5/2 core level shifted from 404.98 eV for size of 7nm to 405.08 eV for size of 3nm, and Se 3d shifted from 55.08 eV for size of 3nm to 54.68 eV for size of 7nm as well as (c) an increase in the cumulative intensity of both the Cd

3d5/2 and Se 3d core levels lines with increasing the size of the nanoparticles from 3 to 7 nm.

It has been reported previously that the size dependent broadening of the XPS spectra is due to the surface reconstructions or “atomic displacements” at the surface in the nanocrystals that cause structural disorder within the nanocrystals.55 However, the red-shifts of the core photoemission peak (e.g. Cd 3d5/2 and Se 3d in our case here) toward higher binding energy with decreasing the size of the nanoparticles have been proposed to be caused by quantum confinement effects and increasing the number of lattice strain in small clusters.56-61

Data for different samples show the reproducibility of the results and show no shake-up peaks in the region of Cd and Se peaks. In order to determine the stoichiometry and chemical environments present in these NCs, their peaks were fitted according to a Gaussian-Lorentzian peak shape along

168 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands with a Smart background. We also determined the area under the peaks,62 calculated the ratio of

Cd:Se, and found the ratio to be 1:1 in all samples measured.

Figure 5.19. XPS data for Cd 3d, Se 3d core levels, respectively, for the CdSe with 7 and 3 nm highlighting their transitions.

On the other hand, high-resolution scans of the P and Fe regions of the nanocrystals that provide information about the surface of the nanocrystals, were carried out to confirm the presence of ferrocene molecules attached to the surface of CdSe NCs. Moreover, the surface bonding interactions can be examined by XPS analysis. Figure 5.20 and Figure 5.21 shows the XPS spectrum for the P 2p and Fe 2p regions, respectively, of CdSe NC samples and of the free ligands, for the reason of comparison. The XPS results of the P 2p peak position of the CdSe NC different samples ranged from 132.29 to 132.94 eV, with a normal distribution centered at 132.63 eV. This

t value is shifted by  = + 0.6 eV higher than the P-peak position of the FcP(O) Bu2 free ligand with

132.6 eV. This small shift (0.6 eV) suggests weaker surface chemical interactions between the phosphinoxide (P=O) group attaching ferrocene to CdSe NCs. We attribute this to the electron withdrawing nature of ferrocene that results in decreasing the electron density at the P=O group making it less polarized.

169 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

Figure 5.20. The high resolution XPS data for P 2p core level comparison between CdSe-capped with

t t FcP Bu2 with sizes of 7 and 3 nm and the free FcP Bu2 ligand, highlighting their transitions.

Comparing the shape of the P 2p peak in both free ligand and the CdSe NCs surface, the P 2p peak in the free ligand gives multiple head peaks while the P 2p peak acquired from CdSe samples surface have one smoothed peak. This is due to free P=O bonds in case of free ligand while on the other hand the P=O moiety is bound to the surface of CdSe NC. Because we use two ligands in

t t our synthetic technique, FcP(O) Bu2 and FcP Bu2, we expect two types of P compounds that can

t appear on the surface of our nanocrystals, P=O–Cd as in case of FcP(O) Bu2 ligand and/or P=Se

t t from FcP( Bu2) ligand. However, no other P=O–Cd (from FcP(O) Bu2 ligand) chemical state was

t found in the XPS spectrum. One possibility is that the FcP Bu2 is oxidized during or after the

t 52, 62 synthesis to FcP(O) Bu2 as in the case of the TOPO/TOP system.

170 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

Figure 5.21. The high resolution XPS data for Fe 2p core level comparison between CdSe-capped with

t t FcP Bu2 with sizes of 7 and 3 nm and the free FcP Bu2 ligand, highlighting their transitions.

Figure 5.1921 shows the XPS examination spectrum for the Fe 2p3/2 and 2p1/2 regions for the free

t t ferrocene ligands, both FcP(O) Bu2 and FcP Bu2, and ferrocene on the surface of CdSe NC samples. In the case of the free ligands, Figure 5.1921, the XPS spectra give two peaks, located at nearly 708.69 eV, and 721.5 eV assigned for the ferrocene Fe 2p3/2 and 2p1/2 transitions respectively.63-64 On other hand, in case of CdSe NCs samples, alongside these two peaks an additional predominant two broad peaks appeared at 711.4 and 725 eV. According to previous reports, these two peaks are characteristic of Fe(III) oxidation state derived from ferrocene ligands redox to ferrocenium molecules.63, 65-66 The formation of ferrocenium molecules may due to the photoexcitation of CdSe followed by a charge transfer between the ligand and the core nanoparticles. As shown in Figure 5.21, while the free ligands do not show the FeIII oxidation

t III state, the CdSe–FcP(O) Bu2 NCs sample exhibits the Fe peak. Moreover, washing the CdSe–

t FcP(O) Bu2 NCs sample from excess unreacted ferrocene ligands show increase in the intensity of

171 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

III t the Fe peak which support our hypothesis of hole transfer from CdSe surface to FcP(O) Bu2

t t ligand. These results confirm that the FcP(O) Bu2/FcP Bu2 ligands were successfully capped on the surfaces of CdSe quantum dots in addition to the existence of charge transfer between the ligands and CdSe.

5.4.7.2.3 Nuclear Magnetic Resonance Analysis

1H and 31P-NMR measurements were used for further investigation of the nature of the inorganic-

t t organic interface in the CdSe capped with FcP(O) Bu2 /FcP Bu2 ligands system. We examined the

1H and 31P-NMR spectra of solution state of CdSe NCs and the free ligands in deuterated solvents

1 t for comparison. Figure 5.22a shows the H NMR of the free unbounded FcP(O) Bu2 ligand; (300

MHz, CDCl3) δ 4.45 (m, 2H), 4.45 (m, 2H), 4.28 (s, 5H), 1.28 (d, J = 13.5 Hz, 18H). The two strong peaks at ∼4.45 ppm corresponding to Fc aromatic protons is of interest to examine in case

1 t of bound ligand (Figure 5.4 and Figure 5.5 for more details). H NMR of CdSe–FcPO( Bu2); (600

MHz, d-toluene) in Figure 5.22b shows that sharp resonances in 1H NMR spectra of free ligands are broadened due to the strong bond to the surfaces of NCs which is in agreement with previous reports.9, 62, 67-68

172 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

Figure 5.22. 1H NMR spectra of free and bound ligands with chemical shifts indicated in ppm. (a) 1HNMR

t 1 t of free FcPO( Bu2) ligand in CDCl3. (b) ) HNMR of bound FcPO( Bu2) ligand in d-toluene; the peaks of 4.77, 4.82 and 5.48 ppm correspond to the protons of the ferrocene cyclopentadienyl rings.

Due to the absorption of 1-octadecene’s double bond in the same region of ferrocene moiety, we identified the peaks of 1-octadecene solvent and suppress them. We attribute the broadness of the

1HNMR features in Figure 5.22b to inhomogeneous distribution of environment found around ferrocene nuclei. The rotation about the centroid-metal-centroid axis in ferrocene is more restricted in the case of bound ligands. Moreover, further broadening of the ferrocene peak in Figure 5.22b

173 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

could be a result of increasing the spin-spin relaxation, i.e., correlation time (T2), of ferrocene due to the interaction of ferrocene with neighboring CdSe NCs.

t In addition to the NMR analysis, we obtained the mass spectra of the CdSe–FcP(O) Bu2 washed sample by acetonitrile/heptane mixture, in Figure 5.23. By comparing the mass spectra of free

t t FcP(O) Bu2 ligand, and washed CdSe–FcP(O) Bu2, Figure 5.6 and Figure 5.23, two main aspects

t can be concluded; first, the bound FcP(O) Bu2 ligand did not degrade during the synthesis under the experimental conditions and second, the ligand bound to the surface of CdSe NCs is

t FcP(O) Bu2.

t Figure 5.23: Mass spectrum of CdSe-FcP(O) Bu2 NCs samples washed by acetonitrile/heptane mixture.

174 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5.4.7.2.4 Cyclic Voltammetry

Cyclic voltammetry (CV) is an electrochemical technique in which the current-potential (i vs E) curves at well-defined scan rates are recorded. By measuring the CV for the nanocrystals, two main pieces of information can be gained; the ionization potential Ip and the electron affinity which is correlated directly to the oxidation and reduction potentials, respectively, represented in the cyclic voltammogram.69

t t t CFcP(O) Bu2: A cyclic voltammogram of a 0.7 mM solution of FcP(O) Bu2 and FcP(O) Bu2 recorded at a scan rate of 20 mV/s is shown in (Figure 5.24). Ferrocene was added as an internal

t t standard in the case of FcP(O) Bu2, for FcP Bu2, Me5Fc was used as an internal reference to

t minimize overlap of the peak potentials with ferrocene. The voltammogram of FcP(O) Bu2, solid line Figure 5.24, displays a quasi-reversible oxidation, with peak separation, ΔEp, = 115 mV. The formal reduction potential E1/2 was taken at the average of the oxidation and reduction peak

t potentials; E1/2 = 183 mV vs Fc. On the other hand, the dashed line in Figure 5.24, for the FcP Bu2

70 in dichloromethane, E1/2 of Fc = 532 mV vs Me10Fc and E1/2 of Me5Fc = 273 mV vs Me10Fc.

Using this relationship (E1/2 of Me5Fc = -259 mV vs Fc) peak potentials are reported vs ferrocene.

The first, quasi-reversible (ΔEp = 142 mV) oxidation occurs at E1/2 = 50 mV vs Fc. At more positive potentials there are additional oxidative (Eo = 693 vs Fc) and reductive (Er = 469 mV vs Fc) features likely due to redox processes involving the nonbonding electron pair on phosphorus.71

175 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t t Figure 5.24. Cyclic voltammogram: CV response of FcP(O) Bu2 and FcP Bu2 free ligands.

CdSe capped with ferrocene. In order for hole transfer between the NCs and the anchored ligands, the band energy alignment between the ligands and the NCs has to be favorable9. Thus, we

t examined the voltammogram of CdSe–FcP(O) Bu2 NCs synthesized according to the method

t mentioned previously. Accordingly, a sample of CdSe–FcP(O) Bu2 NCs is washed from excess unreacted ferrocene ligands using 40,000 rpm centrifugation, and then transferred to a glovebox under inert conditions to exclude oxygen. The NCs were dispersed in toluene prior the measurements. We recorded a cyclic voltammogram of a filtered 1.5 mg/mL solution of the CdSe–

t FcP(O) Bu2 NCs at a scan rate of 10 mV/s. Ferrocene was added after the experiment as a reference.

t Figure 5.25 shows a lone oxidative peak of CdSe–FcP(O) Bu2 NCs—i.e., the ionization potential—at approximately 368 mV versus Fc; there is no corresponding reductive feature. This irreversible redox reaction could be presumably due to the consumption of electrons in a chemical

t 30 reactions between the FcP(O) Bu2 ligands and the NCs whereby a hole transfer takes place. On

176 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands the other hand, the CdSe capped with TOPO with the same size measured in our lab gives an oxidation peak at 881 mV versus Fc.

t Figure 5.25. Cyclic voltammogram: CV response of CdSe- FcP(O) Bu2 and CdSe-TOPO NCs.

177 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t Figure 5.26 shows the energy alignment diagram between CdSe NCs and the FcP(O) Bu2 molecule which is experimentally determined by the cyclic voltammetry. Using typical methods,9, 70, 72 we calculated the energy difference between the valence band of the CdSe NCs and the HOMO level

t + of the FcP(O) Bu2 ligand to be 0.717 V vs the Fc/Fc . This is nearly similar to the results reported by Tarafder and coworkers9 of 0.85 V for the energy alignment difference between the CdSe NCs and ferrocene hexylthiol.

t Figure 5.26. The energy alignment diagram between CdSe NCs and the FcP(O) Bu2 molecule which determined by the cyclic voltammetry.

5.4.7.2.5 Femtosecond Transient Absorption Measurements

t As previously mentioned, the CdSe–FcP(O) Bu2 NCs exhibit no detectable photoluminescence

t (PL) emission. The PL quenching in CdSe–FcP(O) Bu2 NCs can be due to either the dissociation of photo-generated excitons by hole transfer from CdSe to the Fc ligand or to the population of defects at the CdSe NCs surface.16

t To examine the nonradiative decay pathways in CdSe–FcP(O) Bu2 NCs following photoexcitation, we monitored the exciton dynamics using femtosecond (fs) transient absorption (TA)

178 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands spectroscopy. We also investigated the exciton dynamics of CdSe NCs capped with either oleic acid (OA) or TOPO for comparison. The samples were dispersed in toluene for the measurements.

t Figure 5.27 displays the kinetics of the excitonic bleach feature of the CdSe–FcP(O) Bu2, CdSe–

OA, and CdSe–TOPO NCs. The kinetic traces of the excitonic bleach of the three samples, shown in Figure 5.2727, were fitted with a sum of multiple exponential functions. The weighted-average exciton lifetimes as obtained from fitting of the transient absorption signal for the OA and TOPO capped NC systems are >> 2 and 1 ns, respectively. The decay of the bleach signal of the

t FcP(O) Bu2 NCs is more complex and shows an initial time constant of ca. 2 ps in addition to several longer time components. The shortest time component contributes most significantly to the overall decay of the bleach feature. The longer time components may be attributed to residual aggregated NCs present in the sample as noted above.

t Figure 5.27. Transient absorption kinetics of the excitonic bleach of CdSe–FcP(O) Bu2 (orange), CdSe– TOPO (black), and CdSe–OA (red) NCs. All kinetics are nonmonoexponential. The CdSe–TOPO and CdSe–OA NCs exhibit a weighted-average decay of ca. 1 and >> 2 ns, respectively, whereas the CdSe–

t FcP(O) Bu2 NCs exhibit an initial rapid decay of ca. 2 ps.

179 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

t We take the two orders of magnitude reduction in the lifetime of CdSe–FcP(O) Bu2 NCs compared

t to OA and TOPO capped NCs to report on the nonradiative decay in the CdSe–FcP(O) Bu2 NCs.

The nonradiative decay is likely due to hole transfer from NCs to the ferrocene ligand as well as the presence of surface traps on the NCs. In the literature, it has been reported that the surface defects can act as trapping sites for photogenerated charge carriers and degrade the fluorescence properties of the NCs.73 However, the bleach decay due to the surface trapping does not explain the efficient PL quenching. Thus we assume that the rapid decay of the excitonic bleach feature in

t the CdSe–FcP(O) Bu2 NCs is not due to surface trapping, but rather due to the transfer of a hole from the NC to the ferrocene ligand.

5.5 Conclusions

In this chapter we reported the synthesis and characterization of cadmium selenide nanocrystals

(NCs) with electro-active ligands directly attached to the surface.

Facile transport of charge carriers between individual NCs is an essential requirement in optoelectronic devices based on these materials. The conventional surfactant-assisted synthesis yields NCs with surfaces functionalized with insulating organic ligands. These insulating ligands act as a barrier for charge transport between NCs. Electro-active (reducing/oxidizing) ligands like ferrocene and cobaltocene, however, due to their potential for extraordinarily large conductance are particularly appealing for applications as photo-excited hole conductors and photoredox systems. Although ferrocene ligands anchored to the NC surface through insulating long-chain hydrocarbon spacers have previously been reported, this approach is not ideal because the rate of charge transfer is known to be highly sensitive to the distance between donor and acceptor. We

180 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands report here ferrocene directly bound to the inorganic core of the NC and as a result the distance between the NCs and the electro-active moiety is minimized.

In order to ensure a short surface-to-ligand distance, we use ferrocene-based ligands containing phosphine and phosphinoxide functional groups without an intervening alkyl chain spacer. We characterize the morphology of the NCs using transmission electron microscopy and X-ray diffraction and find that the NCs are highly monodisperse and have high crystalline quality of

Wurtzite structure. Optical, nuclear magnetic resonance, mass and X-ray photoelectron spectroscopies confirm that the novel electro-active functional ligands are directly attached to the

NC surface. In order to gauge their potential as a photo-excited hole conductor and photoredox system, we perform cyclic voltammetry on the ferrocene-capped nanocrystals and find that the electronic structure is appropriate for hole transfer from the nanocrystalline core to the electro- active ligand. Complete fluorescence quenching and accelerated exciton decay dynamics suggest a high rate of hole transfer from the NC to electro-active ligand. These results suggest that the ferrocene-capped NCs represent promising photo-excited hole conductor and photoredox systems.

In sum, we have presented strong evidence that we have directly synthesized CdSe NCs with electroactive ligands wherein the electroactive ligands chemically adhere to the NC surface through the favored phosphinoxide functional group that maintains a surface-to-ligand short distance. Ferrocene phosphinoxide derivatives are interesting for electro-active and photoredox systems. The preparation, characterization, and photophysical and redox properties of these materials have been presented.

181 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

5.6 References

Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P., Hybrid Nanorod-Polymer Solar Cells. Science 2002, 295 (5564), 2425-2427.

2. Kamat, P. V., Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. The Journal of Physical Chemistry C 2008, 112 (48), 18737-18753.

3. Han, Z.; Qiu, F.; Eisenberg, R.; Holland, P. L.; Krauss, T. D., Robust Photogeneration of H2 in Water Using Semiconductor Nanocrystals and a Nickel Catalyst. Science 2012, 338 (6112), 1321-1324.

4. Dong, Y.; Choi, J.; Jeong, H.-K.; Son, D. H., Hot Electrons Generated from Doped Quantum Dots via Upconversion of Excitons to Hot Charge Carriers for Enhanced Photocatalysis. Journal of the American Chemical Society 2015, 137 (16), 5549-5554.

5. Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J., A DNA-based method for rationally assembling nanoparticles into macroscopic materials. Nature 1996, 382 (6592), 607- 609.

6. Alivisatos, P., The use of nanocrystals in biological detection. Nat Biotech 2004, 22 (1), 47-52.

7. Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H., Quantum dot bioconjugates for imaging, labelling and sensing. Nat Mater 2005, 4 (6), 435-446.

8. Kovalenko, M. V.; Scheele, M.; Talapin, D. V., Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands. Science 2009, 324 (5933), 1417-1420.

9. Tarafder, K.; Surendranath, Y.; Olshansky, J. H.; Alivisatos, A. P.; Wang, L.-W., Hole Transfer Dynamics from a CdSe/CdS Quantum Rod to a Tethered Ferrocene Derivative. Journal of the American Chemical Society 2014, 136 (13), 5121-5131.

10. Murray, C. B.; Norris, D. J.; Bawendi, M. G., Synthesis and characterization of nearly monodisperse CdE (E = sulfur, selenium, tellurium) semiconductor nanocrystallites. Journal of the American Chemical Society 1993, 115 (19), 8706-8715.

11. Yu, W. W.; Wang, Y. A.; Peng, X., Formation and Stability of Size-, Shape-, and Structure- Controlled CdTe Nanocrystals: Ligand Effects on Monomers and Nanocrystals. Chemistry of Materials 2003, 15 (22), 4300-4308.

12. Yu, W. W.; Peng, X., Formation of High-Quality CdS and Other II–VI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity of Monomers. Angewandte Chemie International Edition 2002, 41 (13), 2368-2371.

13. Peng, X.; Wickham, J.; Alivisatos, A. P., Kinetics of II-VI and III-V Colloidal Semiconductor Nanocrystal Growth: “Focusing” of Size Distributions. Journal of the American Chemical Society 1998, 120 (21), 5343-5344.

182 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

14. Pradhan, N.; Reifsnyder, D.; Xie, R.; Aldana, J.; Peng, X., Surface Ligand Dynamics in Growth of Nanocrystals. Journal of the American Chemical Society 2007, 129 (30), 9500-9509.

15. Peng, Z. A.; Peng, X., Nearly Monodisperse and Shape-Controlled CdSe Nanocrystals via Alternative Routes: Nucleation and Growth. Journal of the American Chemical Society 2002, 124 (13), 3343-3353.

16. Hassan, Y.; Chuang, C. H.; Kobayashi, Y.; Coombs, N.; Gorantla, S.; Botton, G. A.; Winnik, M. A.; Burda, C.; Scholes, G. D., Synthesis and optical properties of linker-free TiO2/CdSe nanorods. Journal of Physical Chemistry C 2014, 118 (6), 3347-3358.

17. Draguta, S.; McDaniel, H.; Klimov, V. I., Tuning Carrier Mobilities and Polarity of Charge Transport in Films of CuInSexS2–x Quantum Dots. Advanced Materials 2015, 27 (10), 1701-1705.

18. Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G., The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state. Journal of Chemical Physics 1997, 106 (23), 9869-9882.

19. Skaff, H.; Sill, K.; Emrick, T., Quantum Dots Tailored with Poly(para-phenylene vinylene). Journal of the American Chemical Society 2004, 126 (36), 11322-11325.

20. Liao, H.-C.; Chen, S.-Y.; Liu, D.-M., In-Situ Growing CdS Single-Crystal Nanorods via P3HT Polymer as a Soft Template for Enhancing Photovoltaic Performance. Macromolecules 2009, 42 (17), 6558-6563.

21. Stavrinadis, A.; Beal, R.; Smith, J. M.; Assender, H. E.; Watt, A. A. R., Direct Formation of PbS Nanorods in a Conjugated Polymer. Advanced Materials 2008, 20 (16), 3105-3109.

22. Dayal, S.; Kopidakis, N.; Olson, D. C.; Ginley, D. S.; Rumbles, G., Direct Synthesis of CdSe Nanoparticles in Poly(3-hexylthiophene). Journal of the American Chemical Society 2009, 131 (49), 17726-17727.

23. Taukeer Khan, M.; Kaur, A.; Dhawan, S. K.; Chand, S., In-Situ growth of cadmium telluride nanocrystals in poly(3-hexylthiophene) matrix for photovoltaic application. Journal of Applied Physics 2011, 110 (4), 044509.

24. Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D.; Chou, K. W.; Fischer, A.; Amassian, A.; Asbury, J. B.; Sargent, E. H., Colloidal-quantum-dot photovoltaics using atomic-ligand passivation. Nat Mater 10 (10), 765-771.

25. Yu, D.; Wang, C.; Guyot-Sionnest, P., n-Type Conducting CdSe Nanocrystal Solids. Science 2003, 300 (5623), 1277-1280.

26. Engel, J. H.; Surendranath, Y.; Alivisatos, A. P., Controlled Chemical Doping of Semiconductor Nanocrystals Using Redox Buffers. Journal of the American Chemical Society 2012, 134 (32), 13200-13203.

183 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

27. Yin, Y.; Alivisatos, A. P., Colloidal nanocrystal synthesis and the organic-inorganic interface. Nature 2005, 437 (7059), 664-670.

28. Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V., Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films. Journal of the American Chemical Society 2006, 128 (7), 2385-2393.

29. Cyr, P. W.; Tzolov, M.; Hines, M. A.; Manners, I.; Sargent, E. H.; Scholes, G. D., Quantum dots in a metallopolymer host: studies of composites of polyferrocenes and CdSe nanocrystals. Journal of Materials Chemistry 2003, 13 (9), 2213-2219.

30. Denis, D.; Nikodem, T.; David, N. R.; Aldrik, H. V.; Vancso, G. J., Ferrocene-coated CdSe/ZnS quantum dots as electroactive nanoparticles hybrids. Nanotechnology 2010, 21 (28), 285703.

31. Dorokhin, D.; Tomczak, N.; Velders, A. H.; Reinhoudt, D. N.; Vancso, G. J., Photoluminescence Quenching of CdSe/ZnS Quantum Dots by Molecular Ferrocene and Ferrocenyl Thiol Ligands. The Journal of Physical Chemistry C 2009, 113 (43), 18676-18680.

32. Malicki, M.; Knowles, K. E.; Weiss, E. A., Gating of hole transfer from photoexcited PbS quantum dots to aminoferrocene by the ligand shell of the dots. Chemical Communications 2013, 49 (39), 4400-4402.

33. Ding, T. X.; Olshansky, J. H.; Leone, S. R.; Alivisatos, A. P., Efficiency of Hole Transfer from Photoexcited Quantum Dots to Covalently Linked Molecular Species. Journal of the American Chemical Society 2015, 137 (5), 2021-2029.

34. Koh, W.-k.; Koposov, A. Y.; Stewart, J. T.; Pal, B. N.; Robel, I.; Pietryga, J. M.; Klimov, V. I., Heavily doped n-type PbSe and PbS nanocrystals using ground-state charge transfer from cobaltocene. Sci. Rep. 2013, 3.

35. Bredow, T.; Tegenkamp, C.; Pfnür, H.; Meyer, J.; Maslyuk, V. V.; Mertig, I., Ferrocene- 1,1′-dithiol as molecular wire between Ag electrodes: The role of surface defects. The Journal of Chemical Physics 2008, 128 (6), 064704.

36. Zhang, G.; Li, D.; Shang, Y.; Zhang, H.; Sun, M.; Liu, B.; Li, Z., Transport Properties of Double Quantum Dots Formed by Ferrocene Units. The Journal of Physical Chemistry C 2011, 115 (13), 5257-5264.

37. Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X., Charge Transfer on the Nanoscale: Current Status. The Journal of Physical Chemistry B 2003, 107 (28), 6668-6697.

38. Baillie, C.; Zhang, L.; Xiao, J., Ferrocenyl Monophosphine Ligands: Synthesis and Applications in the Suzuki−Miyaura Coupling of Aryl Chlorides. The Journal of Organic Chemistry 2004, 69 (22), 7779-7782.

184 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

39. Mann, G.; Incarvito, C.; Rheingold, A. L.; Hartwig, J. F., Palladium-Catalyzed C−O Coupling Involving Unactivated Aryl Halides. Sterically Induced Reductive Elimination To Form the C−O Bond in Diaryl Ethers. Journal of the American Chemical Society 1999, 121 (13), 3224- 3225.

40. Yang, Y. A.; Wu, H.; Williams, K. R.; Cao, Y. C., Synthesis of CdSe and CdTe Nanocrystals without Precursor Injection. Angewandte Chemie International Edition 2005, 44 (41), 6712-6715.

41. Fleming, S.; Rohl, A., GDIS: A visualization program for molecular and periodic systems. Zeitschrift fur Kristallographie 2005, 220 (5-6), 580-584.

42. Zhang, L.; Holt, C. M. B.; Luber, E. J.; Olsen, B. C.; Wang, H.; Danaie, M.; Cui, X.; Tan, X.; W. Lui, V.; Kalisvaart, W. P.; Mitlin, D., High Rate Electrochemical Capacitors from Three- Dimensional Arrays of Vanadium Nitride Functionalized Carbon Nanotubes. The Journal of Physical Chemistry C 2011, 115 (49), 24381-24393.

43. Stevenson, A. W.; Barnea, Z., Anharmonic thermal vibrations and the position parameter in wurtzite structures. II. Cadmium selenide. Acta Crystallographica Section B 1984, 40 (6), 530- 537.

44. Kale, R. B.; Lokhande, C. D., Influence of air annealing on the structural, optical and electrical properties of chemically deposited CdSe nano-crystallites. Applied Surface Science 2004, 223 (4), 343-351.

45. Sheldrick, G. M., Crystal structure refinement with SHELXL. Acta Crystallographica. Section C, Structural Chemistry 2015, 71 (Pt 1), 3-8.

46. Bildstein, B.; Hradsky, A.; Kopacka, H.; Malleier, R.; Ongania, K.-H., Functionalized pentamethylferrocenes: Synthesis, structure, and electrochemistry. Journal of Organometallic Chemistry 1997, 540 (1–2), 127-145.

47. McClure, S. D.; Turner, D. B.; Arpin, P. C.; Mirkovic, T.; Scholes, G. D., Coherent Oscillations in the PC577 Cryptophyte Antenna Occur in the Excited Electronic State. The Journal of Physical Chemistry B 2014, 118 (5), 1296-1308.

48. Trebino, R.; DeLong, K. W.; Fittinghoff, D. N.; Sweetser, J. N.; Krumbügel, M. A.; Richman, B. A.; Kane, D. J., Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating. Review of Scientific Instruments 1997, 68 (9), 3277-3295.

49. Anderson, N. C.; Hendricks, M. P.; Choi, J. J.; Owen, J. S., Ligand Exchange and the Stoichiometry of Metal Chalcogenide Nanocrystals: Spectroscopic Observation of Facile Metal- Carboxylate Displacement and Binding. Journal of the American Chemical Society 2013, 135 (49), 18536-18548.

50. Bodenheimer, J. S.; Low, W., A vibrational study of ferrocene and ruthenocene. Spectrochimica Acta Part A: Molecular Spectroscopy 1973, 29 (9), 1733-1743.

185 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

51. Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D., Infrared spectroelectrochemical characterization of ferrocene-terminated alkanethiolate monolayers at gold. Langmuir 1992, 8 (10), 2521-2530.

52. Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P., X-ray Photoelectron Spectroscopy of CdSe Nanocrystals with Applications to Studies of the Nanocrystal Surface. The Journal of Physical Chemistry 1994, 98 (15), 4109-4117.

53. Liu, I. S.; Lo, H.-H.; Chien, C.-T.; Lin, Y.-Y.; Chen, C.-W.; Chen, Y.-F.; Su, W.-F.; Liou, S.-C., Enhancing photoluminescence quenching and photoelectric properties of CdSe quantum dots with hole accepting ligands. Journal of Materials Chemistry 2008, 18 (6), 675-682.

54. Cotton, F. A.; Barnes, R. D.; Bannister, E., The effect of complex-formation by phosphine oxides on their P-O stretching frequencies. Journal of the Chemical Society (Resumed) 1960, (0), 2199-2203.

55. Hamad, K. S.; Roth, R.; Rockenberger, J.; van Buuren, T.; Alivisatos, A. P., Structural Disorder in Colloidal InAs and CdSe Nanocrystals Observed by X-Ray Absorption Near-Edge Spectroscopy. Physical Review Letters 1999, 83 (17), 3474-3477.

56. Mason, M. G., Electronic structure of supported small metal clusters. Physical Review B 1983, 27 (2), 748-762.

57. Richter, B.; Kuhlenbeck, H.; Freund, H. J.; Bagus, P. S., Cluster Core-Level Binding- Energy Shifts: The Role of Lattice Strain. Physical Review Letters 2004, 93 (2), 026805.

58. Kaden, W. E.; Wu, T.; Kunkel, W. A.; Anderson, S. L., Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO2 Surfaces. Science 2009, 326 (5954), 826-829.

59. Bukhtiyarov, A. V.; Kvon, R. I.; Nartova, A. V.; Bukhtiyarov, V. I., An XPS and STM study of the size effect in NO adsorption on gold nanoparticles. Russian Chemical Bulletin 2012, 60 (10), 1977-1984.

60. Aruna, I.; Mehta, B. R.; Malhotra, L. K.; Shivaprasad, S. M., Size dependence of core and valence binding energies in Pd nanoparticles: Interplay of quantum confinement and coordination reduction. Journal of Applied Physics 2008, 104 (6), 064308.

61. MacDonald, M. A.; Zhang, P.; Qian, H.; Jin, R., Site-Specific and Size-Dependent Bonding of Compositionally Precise Gold−Thiolate Nanoparticles from X-ray Spectroscopy. The Journal of Physical Chemistry Letters 2010, 1 (12), 1821-1825.

62. Morris-Cohen, A. J.; Donakowski, M. D.; Knowles, K. E.; Weiss, E. A., The Effect of a Common Purification Procedure on the Chemical Composition of the Surfaces of CdSe Quantum Dots Synthesized with Trioctylphosphine Oxide. The Journal of Physical Chemistry C 2010, 114 (2), 897-906.

186 Chapter 5 Direct Synthesis of CdSe with Electro-active Ligands

63. Fischer, A. B.; Wrighton, M. S.; Umana, M.; Murray, R. W., An x-ray photoelectron spectroscopic study of multilayers of an electroactive ferrocene derivative attached to platinum and gold electrodes. Journal of the American Chemical Society 1979, 101 (13), 3442-3446.

64. Woodbridge, C. M.; Pugmire, D. L.; Johnson, R. C.; Boag, N. M.; Langell, M. A., HREELS and XPS Studies of Ferrocene on Ag(100). The Journal of Physical Chemistry B 2000, 104 (14), 3085-3093.

65. Dalchiele, E. A.; Aurora, A.; Bernardini, G.; Cattaruzza, F.; Flamini, A.; Pallavicini, P.; Zanoni, R.; Decker, F., XPS and electrochemical studies of ferrocene derivatives anchored on n- and p-Si(100) by Si–O or Si–C bonds. Journal of Electroanalytical Chemistry 2005, 579 (1), 133- 142.

66. Ruther, R. E.; Cui, Q.; Hamers, R. J., Conformational Disorder Enhances Electron Transfer Through Alkyl Monolayers: Ferrocene on Conductive Diamond. Journal of the American Chemical Society 2013, 135 (15), 5751-5761.

67. Kalyuzhny, G.; Murray, R. W., Ligand Effects on Optical Properties of CdSe Nanocrystals. The Journal of Physical Chemistry B 2005, 109 (15), 7012-7021.

68. Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.; Bawendi, M. G., The band edge luminescence of surface modified CdSe nanocrystallites: Probing the luminescing state. The Journal of Chemical Physics 1997, 106 (23), 9869-9882.

69. Kucur, E.; Riegler, J.; Urban, G. A.; Nann, T., Determination of quantum confinement in CdSe nanocrystals by cyclic voltammetry. The Journal of Chemical Physics 2003, 119 (4), 2333- 2337.

70. Noviandri, I.; Brown, K. N.; Fleming, D. S.; Gulyas, P. T.; Lay, P. A.; Masters, A. F.; Phillips, L., The Decamethylferrocenium/Decamethylferrocene Redox Couple: A Superior Redox Standard to the Ferrocenium/Ferrocene Redox Couple for Studying Solvent Effects on the Thermodynamics of Electron Transfer. The Journal of Physical Chemistry B 1999, 103 (32), 6713- 6722.

71. Barrière, F.; Kirss, R. U.; Geiger, W. E., Anodic Electrochemistry of Multiferrocenyl Phosphine and Phosphine Chalcogenide Complexes in Weakly Nucleophilic Electrolytes. Organometallics 2005, 24 (1), 48-52.

72. Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C., Electrochemical Considerations for Determining Absolute Frontier Orbital Energy Levels of Conjugated Polymers for Solar Cell Applications. Advanced Materials 2011, 23 (20), 2367-2371.

73. Hassan, Y.; Chuang, C.-H.; Kobayashi, Y.; Coombs, N.; Gorantla, S.; Botton, G. A.; Winnik, M. A.; Burda, C.; Scholes, G. D., Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods. The Journal of Physical Chemistry C 2014, 118 (6), 3347-3358.

187 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

This chapter is reproduced in part with permission from the following publication:

“Yasser Hassan, Yin Song, Ryan D. Pensack, Ahmed I. Abdelrahman, Yoichi Kobayashi, Mitchell A. Winnik and Gregory D. Scholes, Advanced Materials, 2015, DOI: 10.1002/adma.201503461”.

Contribution of Authors in this Chapter:

 YH had the idea for, conceived and directed the project, designed the experiments,

carried out the synthesis of materials, and performed data analysis.

 YS and RDP performed the transient absorption measurements and data analysis.

 AIA and YH carried out the electron microscope measurements.

 YK contributed to the morphology analysis of the materials.

 YH, RDP, YS, and GDS wrote the manuscript.

 All authors discussed the results and commented on the manuscript for the Journal of

“Advanced Materials”.

 GDS and MAW supervised the project.

188 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.1 Abstract.

In the recent years, incredible advances have been made in the development of organic-inorganic metal halide-perovskite materials for thin film solar cells. However, the uncontrolled size and precipitation methods of the perovskite from solvents produce large variation in photovoltaic devices performance engineered from these materials. The quest to prepare perovskite nanocrystals and control their size becomes a high demand. Here, we report the preparation and characterization of a colloidally stable suspension of methyl ammonium lead halide perovskite nanocrystals. The perovskite nanocrystals are seeded from high quality PbX2 nanocrystals (X = I or Br) with a pre- targeted size. Subsequent reaction with methyl ammonium iodide (MAI) yields perovskite nanocrystals. According to ligand choice and ratio, these nanocrystals are prepared with different layered multiple quantum well like nanocrystal structures, denoted R2([CH3NH3]n-1PbnI3n+1) n = 1,

2, 3 where R is the ligand (butyl, octyl, or oleylammonium). The excitonic gap is tuned stepwise; for example, the photoluminescence peak for 5.5 nm diameter colloids is 505, 565, and 600 nm respectively. The sharp excitonic features make these nanocrystals well suited for systematic investigations of the intrinsic photophysics and spectroscopy of organic-inorganic metal halide- perovskites.

Figure 6.1: Colloidally stable suspensions of lead halide perovskite NCs are prepared from high quality

PbX2 NCs seeds.

189 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.2 Introduction

Since pioneering work in 20091 incredible advances have been made in the development of organic-inorganic metal halide-perovskite materials for thin film solar cells; devices have been

2-9 reported with efficiencies exceeding 20%. These CH3NH3PbX3 (X = Br, Cl and I) materials have a broad absorption profile across the visible spectral region and they exhibit intense and narrow- band luminescence. The excitonic properties and long diffusion length of charge carriers10-13 explain the interplay of efficient energy and charge transport underpinning the efficient photovoltaic properties. However, the nature and dynamics of the excited states that ultimately determine the performance of optoelectronic devices are still being elucidated.

Here, we report the preparation and characterization of a colloidally stable suspension of methyl ammonium lead halide perovskite nanocrystals. The perovskite nanocrystals are seeded from high quality PbX2 nanocrystals (X = I or Br) with a pre-targeted size. Subsequent reaction with methyl ammonium iodide (MAI) yields perovskite nanocrystals. According to ligand choice and ratio, these nanocrystals are prepared with different layered multiple quantum well like nanocrystal structures, denoted R2(CH3NH3)n-1PbnI3n+1; n = 1, 2, 3 where R is the ligand. The excitonic gap is tuned stepwise; for example, the photoluminescence peak for 5.5 nm diameter colloids is 505, 565, and 600 nm respectively. The sharp excitonic features make these nanocrystals well suited for systematic investigations of the intrinsic photophysics and spectroscopy of organic-inorganic metal halide-perovskites.

To date, limited syntheses of CH3NH3PbX3 perovskite materials on the nanoscale have been reported.14-17 Burschka et al. used a sequential deposition method to infiltrate the perovskite pigment within the pores of porous metal oxide film.15 This method enabled control over perovskite morphologies with particle sizes that ranged from 50–200 nm and led to reproducible,

190 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals efficient photovoltaic devices.15 Another recently reported approach utilized a long chain ligand- assisted re-precipitation technique to produce colloidal perovskite material.16 The quest to prepare perovskite nanocrystals and control their size motivated us to investigate a synthetic approach inspired by colloidal semiconductor quantum dot synthesis.18-19

The basis of our synthesis is the preparation of colloidal PbI2 nanocrystals. Since the original report

20 of Sandroff et al. on the quantization effects in PbI2 small clusters in films and solution, these nanocrystals have been prepared by various methods that include Langmuir-Blodgett techniques21, zeolite cages22, silica films23, copolymer films24, colloidal solutions25-26, vapor deposition27-28, reverse micelles29, or surfactant-assisted hydrothermal routes.30-31 Control of the size of colloidally stable PbI2 NCs remains a challenge. In this work we address this issue.

The colloidal perovskite nanocrystals reported in this work were synthesized in a two-step process.

In the first step we synthesize nearly monodisperse PbI2 (or PbBr2) nanocrystals using a method

18, 32-33 inspired by lead chalcogenide semiconductor nanocrystals. Subsequently we react the PbI2 nanocrystals with an alkyl ammonium iodide to produce the corresponding perovskite nanocrystals.

6.3 Experimental Section

All procedures were carried on a Schlenk line under oxygen-free conditions and nitrogen flow.

Materials. All chemicals were used as received without further purification. Lead (II) oxide

(99.99 %), tetrabutyl ammomium iodide (98.0 %), tetrabutyl ammomium bromide (98.0 %,), 1- octadecene (90 %), oleic acid (99.0 %), oleylamine (70 %), HBr (48% in water), HI (57% in water)

191 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals and methylamine solution (33% in absolute ethanol) were purchased from Sigma-Aldrich and used without further purification. All solvents such as chloroform and toluene were anhydrous and were purchased from Sigma-Aldrich.

192 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.3.1 Synthesis of Lead Iodide (PbI2) NCs.

PbI2 NCs 3–5 nm size; were synthesized by mixing the halide and lead precursors in a three-neck round-bottom flask with magnetic stirrer attached to a condenser and connected to a Schlenk line.

For temperature control and measurement, a heating mantle and thermocouple were used.

6.3.1.1 Halide precursor preparation.

Tetrabutylammonium iodide (TBAI) or tetrabutylammomium bromide (TBAB) was dissolved in a mixture of oleylamine (OLA) and 1-octadecene (ODE) in a three-neck round-bottom flask (Flask

I) connected to Schlenk line.

Typically for the iodide precursor: 2 mmol (0.735 g) of TBAI was mixed with a mixture of 5 (or

7 mL) of OLA and 5 (or 3 mL) of octadecene ODE. The total volume of OLA and ODE was 10 mL. The mixture was put under vacuum (pumping) for 1.5 h at 150o C to remove any traces of

o water and oxygen. The solution was subsequently heated at 200 C for 1 h under N2 to reach a colorless or (pale yellow) solution. The heating mantle was removed and the solution then kept at

50o C to avoid solidification. A similar recipe was used for the preparation of bromide precursor solutions. As reported by Zhijun Ning et al.34, heating the halide salts in oleylamine solution under

N2 atmosphere leads to the formation of N-butyloctadec-9-en-1-aminium iodide and tributylamine and it is essential for dissolving the iodide salts in oleylamine solution.

193 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.3.1.2 Lead precursor preparation:

Lead oleate precursor was synthesized according the method used to synthesize Pb-based quantum dots.32-34 In another flask (Flask II), 0.225 g of PbO (1.00 mmol), 1 mL of oleic acid (3.00 mmol), and 15 mL of 1-octadecene (ODE) were mixed. The mixture was held under vacuum at 100o C for

1.5 h before the flask was allowed to return to 80 °C under nitrogen gas flow for another 1 h. At that point, the solution turned colorless indicating the complete formation of lead oleate complex.

6.3.1.3 Lead iodide (PbI2) NCs Synthesis:

“Flask (II)” of lead oleate solution was then heated to around 160–200 °C before the content of

“Flask (I)” was swiftly injected into this hot solution; the temperature of the reaction mixture temperature was adjusted to 120–200 °C for the subsequent growth of the PbI2 semiconductor nanocrystals. The reaction solution color changed, after the first few seconds, to a yellow-gold color indicating the formation of PbI2 nanocrystals. The reaction was stopped after several minutes

(1–30 minutes) by removing the heating mantle and injecting anhydrous toluene or chloroform.

6.3.2 Synthesis of Methyl Ammonium Iodide.

The precursors CH3NH3I (MAI) and CH3NH3Br (MABr) were synthesized from HI (45% in water) and HBr (40% in water), respectively, by reaction with methylamine solution (33% in absolute ethanol) according to previously reported procedures.5-6, 35-36 Typically hydroiodic acid (30 mL,

0.227 mol, 57 wt % in water) was added to methylamine (24 mL, 33% in absolute ethanol) in a

250 mL round-bottom flask. 50 mL of absolute ethanol was added to the mixture and stirred while

194 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals the solution was maintained at 0o C in an ice bath for 2 h under nitrogen atmosphere (CAUTION: this is an exothermic reaction). The resulting solution was evaporated at 40o C via rotary evaporation to half of the volume quantity. A white precipitate of MAI crystals was obtained by washing the solution three times with diethyl ether. The MAI was dissolved in ethanol, recrystallized from diethyl ether, and finally dried at 60 °C for 24 h and used without any further purification. The same method used for preparing alkyl (olyel, octyl or butyl) ammonium iodide from the corresponding amine with HI (45% in water), but at room temperature. MABr were prepared following the same procedure except using HBr acid instead of HI acid in a quantity of

44 mL (HBr, 48% in water).

6.3.3 Synthesis of Lead Halide Perovskite.

A solution of lead bromide or lead iodide-based perovskite NCs was prepared by mixing the PbBr2 or PbI2 NCs and the synthesized powder of MABr or MAI, respectively, in toluene:chloroform mixture at 30–50o C. Stirring this mixture immediately forms the perovskites. The solution of perovskite nanocrystals was filtered twice using a PTFF syringe filter (Whatman, 0.2 µm) and then diluted in toluene, toluene:chloroform or toluene:chlorobenzene mixture for optical measurements.

195 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.3.4 XRD Samples Preparation

X-ray patterns for phase purity were recorded on a Rigaku MiniFlex Benchtop X-ray diffractometer (equipped with Cu Kα X-ray tube) operating at 600 watts (tube voltage 40 kV and

196 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

15 mA) with a time per step of 3 s at range of = 0–60 degree. Samples were rotated during data collection.

In order to study the effect of PbI2 sample preparation method on the PXRD, we prepared films by two different methods: (i) methanol treatment method and (ii) spin coating without methanol treatment.

(i) Methanol treatment method

PbI2 NCs were purified by iteratively precipitating in methanol and subsequently re-dissolving in toluene in order to remove excess and weakly bound surface ligands (Figure 6.10a).

(ii) Spin coating

In this method the PbI2 NC solution in hexane, toluene or chloroform were spin coated on a silicon substrate. The thickness of the deposited films was 10-20 nm. The film was dried by flushing with nitrogen gas prior to the XRD measurements (Figure 6.10b).

Perovskite NCs did not undergo the removal of excessive ligands; i.e. the diffractogram of the perovskite NCs was obtained on a powder of nanocrystals that had been prepared by evaporating the toluene:chloroform solution after deposition on a silicon substrate at room temperature from a concentrated NCs colloidal solution. The diffractogram of the 2D perovskite NCs (in case of n=1 and 2) was obtained on a powder of nanocrystals while the diffractogram of the 2D perovskite NCs

(n=3) was performed on film prepared by spin coating.

197 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.3.5 CHARACTERIZATION

TEM, HRTEM and EDX: Size distributions and energy-dispersive X-ray spectra of the PbI2 and lead halide perovskite NCs were determined by transmission electron microscopy using a JEOL

JEM–2010 TEM with a LaB6 filament operating at 200 kV for both low-magnification (TEM) and high-resolution (HRTEM) images.

UV-vis Absorption and PL: UV-vis Absorption and PL spectra were recorded using a Varian Cary

100 and Varian Eclipse fluorescence spectrometer, respectively, in a 1 cm cuvette.

Powder X-ray diffraction: Powder X-ray diffraction (PXRD) patterns for phase purity were recorded on a Rigaku MiniFlex Benchtop X-ray diffractometer (equipped with Cu Kα X-ray tube) operating at 600 watts (tube voltage 40 kV and 15 mA) with a time per step of 3 s. Samples were rotated during data collection. All the samples were prepared by drying nanoparticle solutions.

X-ray photoelectron spectroscopy (XPS): XPS data was acquired on a ThermoFisher Scientific

K-Alpha spectrometer with a monochromatic Al Kα X-ray radiation source generating X-ray photons of 1486.7 eV in energy. This was performed in an ultrahigh–vacuum chamber with base pressure of 10−9 Torr. The samples for XPS analysis were prepared by drop-casting the samples onto Si(100). Both survey and regional spectra were acquired from the samples. All data analyses were carried out using the Avantage software (provided from the vendor of the instrument) fitting program to confirm the incorporation of Pb and I in the PbI2 and lead halide perovskite nanocrystals. We calculated the elemental composition and mathematically modelled the experimental data to deconvolute the peak shapes and investigate the chemical state(s) present. In both cases, a smart background function was used to approximate the experimental backgrounds.

Surface elemental compositions were calculated from background-subtracted peak areas derived

198 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals from transmission function corrected regional spectra. Scofield Al K-alpha sensitivity factors were used to calculate the relative atomic percentages. The peak shapes were deconvoluted utilizing

Lorentzian-Gaussian peak shapes. The binding energies were referenced to the NIST-XPS database.

Transient absorption (TA) Experiment: The experimental setup has been described in detail previously37. Briefly, 800 nm, 100 fs laser light generated at a repetition rate of 5 kHz by a Spectra-

Physics Spitfire Pro laser amplifier was used as the light source in the experiment. The output was directed into a home-built NOPA to convert the light to the visible with a spectral range from 525 to 720 nm for pump–probe measurements. Pulse compression was achieved with a combination of a grating compressor and a prism compressor. The pulse was compressed to less than 20 fs

(estimated by polarization-gated frequency-resolved optical gating)38. The compressed broadband pulse was split into two beams—pump and probe beam—and the time delay between them was controlled by a motorized translation stage. The pump pulse was chopped at a frequency of 625

Hz. Thus the pump–probe signal captured by the camera was averaged over the signals generated by four continuous pulses. The pump and probe beams were crossed at the sample position to generate the signal. The polarizations of the pump and the probe beams were at the magic angle.

The pump–probe signal was detected by a CCD Newton camera (Andor DU971N-FI Newton).

The signal was balanced by a photodiode recording the intensity fluctuations of the probe to compensate for laser intensity fluctuations. Global target analysis was performed through

Glotaran39.

199 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.4 Results and Discussion

6.4.1 Lead Halide NC Synthesis.

A lead oleate precursor reacts with an iodide-organic complex34 in a noncoordinating solvent such as octadecene (ODE). For instance, to prepare PbI2 NCs, tetrabutylammonium iodide (TBAI, 4 mmol) was dissolved in oleylamine (7 mL) at 200 oC under an inert atmosphere to form the iodide- amine complex. The resulting solution was cooled to 50 oC before it was injected into a hot solution of the lead-oleate complex (2 mmol) in ODE. The solution changed to golden yellow color after a few seconds, indicating the formation of PbI2 nanocrystals. In this ionic metathesis reaction, nucleation occurs and growth begins over the course of a few seconds.40 The temperature of the

ODE solution (160–200 oC) and the reaction time can be used to control particle size. The reaction was halted after several minutes by removing the heat source and injecting anhydrous toluene into the reaction mixture. This synthetic procedure produces homogeneous and crystalline lead halide

(PbX2) nanocrystals with mean sizes in the range of 2–11 nm.

Figure 6.3 presents TEM, HRTEM and EDX measurements of a typical sample of PbI2 NCs

o prepared at 200 C. The TEM images of this PbI2 NC sample, without any size-selection treatment, reveals an abundance of uniform spherical nanocrystals. By controlling the reaction temperature

(120–200 oC) and growth time, particles with a narrow size distribution can be obtained with diameters (d) in the range of 3–10 nm. A quantitative analysis of the TEM images reveals that PbI2

NCs prepared at 200 oC at time intervals of 5, 15 and 30 minutes are characterized by a diameter range of 3.4–4.2 nm, 4.7–5.8 nm and 9.5–10 nm, respectively, (Figure 6.3a–c). Figure 6.4 shows

o TEM, and HRTEM images of PbI2 NCs prepared at 160 C, which show high crystallinity of PbI2

NCs prepared at lower temperatures.

200 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

Figure 6.3: Structural, optical, and chemical characterization of lead iodide nanocrystals. a-c, TEM images

o of PbI2 NCs prepared at 200 C at a growth interval time of (a) 5, (b) 15 and (c) 30 minutes exhibiting a diameter of 3.4- 4.2 nm, 4.7-5.8 nm and 9.5-10 nm, respectively. d, High-resolution electron microscopy

(HRTEM) of the PbI2 NCs prepared in this work, that shows a lattice parameter of 0.34 nm. e, Absorption spectra of colloidal PbI2 NCs in toluene solution where the nanocrystal size has been varied from 2 to 6 nm diameter and clearly exhibiting a quantum confinement effect. f, EDX spectra obtained from 6 nm sized nanocrystals of PbI2. The Cu signals arise from the TEM grid.

HRTEM images (Figure 6.3d) exhibit lattice fringes throughout the entire particle. The lattice fringes of the PbI2 NCs are spaced by 0.344 nm, corresponding to the (101) lattice. HRTEM

(Figure 6.3d) and with the selected area electron diffraction pattern (SAED), Figure 6.5, show the

41 high crystallinity of the PbI2 NCs, assigned to the hexagonal PbI2 phase.

In order to investigate the effect of particle diameter on the optical properties of the PbI2 NCs and the corresponding perovskites, we prepared two different sizes of PbI2 NCs (d = 2 and 6 nm,

Figure 6.3e). Figure 6.3e shows absorption spectra recorded at room temperature for aliquots of

201 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

o PbI2 NCs dispersed in toluene, removed from the reaction mixture at 200 C after reaction for 5 and 15 minutes of growth. We found that with decreasing the size of the PbI2 NCs from 6 to 2 nm, the exciton energy blue shifts by 0.22 eV (max from 413 nm to 385 nm). This quantum confinement effect (the bulk PbI2 bulk semiconductor has a band gap of 2.57 eV) is thought to be mainly a two-dimensional exciton quantum confinement along the crystal c axis of the layered hexagonal structure.25, 27

Energy-dispersive X-ray spectroscopy (SEM-EDX), was utilized to determine the elemental composition of our PbI2 NCs. An example is presented in Figure 6.3f, showing clear evidence for the presence of both Pb and I. The elemental analysis from EDX is in agreement with the atomic ratio calculated from XPS measurements (Figure S7).

Figure 6.4: Transmission Electron Microscopy of PbI2 NCs. a, TEM images of PbI2 NCs prepared at 160 o C. b,c, High-resolution electron microscopy (HRTEM) of the PbI2 NCs. d, Fast Fourier transform (FFT) of the HRTEM image showing the (101), (102), (111) and (004) lattice fringes.

202 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

Selective Area Electron Diffraction (SAED) patterns for a collection of PbI2 nanocrystals deposited on a carbon grid (Figure 6.5) were obtained using transmission electron microscopy

(TEM; JEOL 2010 at 200 kV).

Figure 6.5: Experimental selected area electron diffraction (SAED) patterns of PbI2 NCs. SAED image of PbI2 NCs of size 10 nm prepared at 200 oC, 30 minutes growth.

TEM diffraction patterns (SAED) were simulated and analyzed with open-source software with

GDIS42 and Diffraction Ring Profiler43, which was developed for phase identification in complex microstructures.43 The latter software package integrates the SAED ring pattern intensities to accurately calculate the center point of each ring. The SAED pattern of the selected area of collection of PbI2 nanocrystal particles exhibits diffuse rings that are typical of nanometer-sized particles. The patterns index to the (101), (100), (102), (003), (111), (004) and (104) reflections

41 of the hexagonal structure of PbI2 (PDF card no 43-1484) . More details about simulated SAED are provided in Figure 6.6.

203 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

Figure 6.6: Experimental and simulated selected area electron diffraction (SAED) patterns of PbI2 NCs. a,

Comparison of the SAED of PbI2 NCs and the simulated XRD patterns of the PbI2 hexagonal structure (PDF card no. 43-1484). b, Integrated SAED and simulated XRD patterns of PbI2 NCs. The simulation of the SAED was performed using open-source software Diffraction Ring Profiler.

6.4.2 Lead Halide Perovskite NC Synthesis.

In order to prepare the lead iodide perovskite NCs, the PbI2 nanocrystals suspended in toluene were diluted with chloroform to a volume ratio of 2:1 (toluene: chloroform). An alkyl ammonium halide, methyl ammonium halide (MAI) alone, or a MAI mixture with a long chain alkyl ammonium

o iodide was added at 50 C over 30 seconds. It is known that the PbI2 single crystal consists of strongly bonded I–Pb–I layers, and the interlayer bonding is through van der Waals forces.44 The conversion of lead halide NCs to the corresponding perovskite takes place by intercalation of the

45 organic MAI moiety between the Pb–I–Pb layers of the crystalline PbI2 host forming two-

46- dimensional (2D) organo-lead halide perovskites with the structure of R2(CH3NH3)n-1MnX3n+1

47, where (R = long chain amine, M = Pb, X = halide ion and n = 1, 2, 3).

204 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

46, 48 To obtain lamellar 2D lead iodide perovskites R2(CH3NH3)n-1MnX3n+1 with different n values, we tuned the molar ratio between the long chain organic ammonium iodide (e.g. oleylammonium iodide) and MAI. By adding either oleylammonium ioidide alone, or MAI and oleylammonium iodide mixtures with a molar ratio of (2:1), or with a molar ratio of (1:1), we found the solution immediately changed color to green, orange or red respectively, indicating the formation of 2D organo-lead halide perovskites with number of layers n=1, 2 or 3. It was found that a small quantity of chloroform added to toluene enhances the reaction between the MAI and the lead halides (I or

Br) while the addition of any amount of methanol, acetonitrile or acetone degrades the perovskite

(as evidenced by diminished perovskite absorption). Figure 6.7a–c show the TEM images of the colloidal 2D organo-lead iodide perovskite (n=3), (C18H35NH3)2(CH3NH3)2Pb3I10, prepared from three different sample of PbI2 NCs seeds with particles size of 3.5, 6 and 10 nm, respectively.

On the molecular level, the 2D perovskite naturally self-assembles into a stack of lamellae consisting of alternate layers of organic and inorganic components. The inorganic layers comprise

4- semiconducting corner sharing octahedral PbX6 sheets (wells) stacked in the direction of the c- axis, sandwiched between bi-layers of low dielectric constant organic barriers consisting of alkyl ammonium chains.46-47 These self-assembled structures are similar to inorganic multiple quantum wells, characterized by very stable exciton transport in the inorganic layers with enhanced exciton binding energy that can reach 4 times larger than in the case of 3D perovskite.47, 49

205 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

Figure 6.7. Structural and optical properties of 2D organo-lead iodide perovskite nanocrystals. a-c, TEM images of the 2D organo-lead iodide perovskite (n=3) NCs prepared from PbI2 NCs that exhibit a diameter of (a) 3.5, (b) 6 and (c) 10 nm, respectively. d, Normalized absorption and PL spectra of 2D lead iodide perovskite (n=3) nanocrystals in a toluene/chloroform mixture as prepared from PbI2 NCs of three different diameters; 3 (green line), 5 (blue line) and 10 nm (orange line).

In Figure 6.7d we show the room temperature absorption and photoluminescence (PL) spectra corresponding to the three 2D, n = 3 samples presented in Figure 6.7a–c. We found that the colloidal suspension of (C18H35NH3)2(CH3NH3)2Pb3I10 nanocrystals exhibit a quantum confinement due to the particle size compared to the film absorption value reported by Wu et al.46,

50 In the case of our NC suspension, the first exciton max is 585, 592 and 599 nm for the sizes of

3.5, 5.5 and 10 nm, respectively. This compares to 608 nm in the case of the film.46, 50 As shown

206 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals in Figure 3d, the change in particle size has marginal quantum confinement effect as compared to the effect of the 2D confinement. Perovskite material presented in this study ranged between 3.5–

11 nm which is bigger than the Bohr radius of perovskite, i.e. 2.1 nm.47

6.4.3 Perovskite NCs with Different 2D Structures.

Figure 6.8 shows the versatility of our method to prepare perovskite NCs with different 2D structures, where n = 1, 2 or 3. To prepare 2D perovskite with one layer (n = 1), we used either butylammonium iodide, octylammonium iodide or oleylammonium iodide, while these long chain amines mixed with MAI in a molar ratio of OLI:MAI = 2:1 and 1:1 yielded n = 2 and 3, respectively. For these 5.5 nm diameter particles, the absorption peaks are 505, 565, and 593–

600 nm respectively for n = 1, 2 and 3. The room temperature absorption and photoluminescence

(PL) spectra of 2D lead iodide perovskite (n = 3), (C18H35NH3)2(CH3NH3)2Pb3I10, (Figure 6.7d and Figure 6.8c) exhibit a strong “band-edge exciton” absorption and well-resolved higher- energy band-to-band transitions that is indicative of a narrow particle size distribution. The photoluminescence spectra are symmetric and narrow (full-width at half maximum of 0.12 eV) and the quantum yield of the perovskite nanocrystals in toluene was measured to be ~20%, estimated by comparison with the dye cresyl violet.51

We found no evidence that these materials degraded significantly in the presence of light. This enabled us to carry out transient absorption measurements that will be discussed in more detail in section 6.4.6. We measured the absorption spectra before and after these measurements and found very minor changes.

207 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

1, 2 and 3 respectively, under ambient light. c, Absorption spectrum of colloidal PbI2 NCs in toluene with a diameter of 4–5 nm compared to the absorption and emission spectra of the resultant 2D lead iodide perovskite (n=3) colloidal nanocrystal solution in toluene/chloroform mixture. All spectra have been normalized to the lowest-energy optical transition. The 2D lead iodide perovskite (n=3) nanocrystal absorption exhibits an onset at 593 nm. Energy bandgap estimated from the intersection point of the normalized curve was 2.09 eV. The steady-state photoluminescence spectrum (dotted red) consists of a single peak slightly redshifted from the excitonic peak with a maximum at 607 nm. The photoluminescence was measured with an excitation wavelength of 480 nm. Absorption spectra of the three 2D organo-lead iodide perovskite samples (blue, green and red for n= 1, 2 and 3, respectively), and steady-state photoluminescence that peaks at 520, 580 and 610 nm (dotted blue, green and red lines for n= 1, 2 and 3, respectively). The photoluminescence was measured with an excitation wavelength of 480 nm.

208 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.4.4 Ligand and Solvent Effects on the Optical Properties of Perovskite NCs

In the case of n=1 nanocrystals, we find that increasing the alkyl chain length from R= butyl to oleylammonium has an effect on the first exciton absorption peak energy. In addition, increasing the volume fraction of chloroform in the sample changes the peak position, which might indicate a modification of the dielectric confinement effect47, (Figure 6.9).

Figure 6.9: Absorption spectra of perovskite NCs synthesized with different ligands and solvent mixture ratios. a, Absorption spectra of perovskite NCs synthesized with butyl-, octyl-, and olelyl-ammonium ligands. b, Absorption spectra of perovskite NCs synthesized with olelylammonium ligand with different solvent mixture ratios, i.e., toluene (T) to chloroform (CF) = 2:1, 1:1 and 1:1.5 v/v ratio.

6.4.5 Powder X-ray Diffraction (XRD) Measurements

The conversion of lead halide NCs to the corresponding perovskite NCs was confirmed by investigating their optical properties along with a structural analysis by electron microscopy, X- ray diffraction (XRD) and X-ray photoelectron spectroscopy. For instance, treating the PbI2 NCs with MAI fully converts the PbI2 NCs to perovskites, as is clear from the disappearance of the PbI2 absorption peak at 413 nm (see the dashed line for comparison Figure 6.8a).

209 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

Figure 6.10: Powder X-ray diffraction (XRD) measurements of PbI2 and the corresponding 3D organo- lead iodide perovskite NCs CH3NH3PbI3. a, X–ray diffractogram of PbI2 after washing by methanol (red) and after the transformation to lead halide perovskite (blue). The blue line indicates the XRD results for perovskite powder, which was deposited on a silicon substrate at room temperature from a concentrated colloidal NCs solution without any further treatment, while PbI2 NCs were precipitated using a methanol/heptane mixture before centrifugation and collection for measurements. Perovskite reflection peaks are assigned to the tetragonal CH3NH3PbI3 perovskite crystal lattice except the peaks at 19 and 29 degrees coming from the unreacted precursors. * denote diffraction peaks corresponding to unreacted precursors and not from the perovskite. The plot displays the X–ray intensity as a function of twice the diffraction angle (2). b, X–ray diffractogram of PbI2 films deposited by spin coating without washing

(black line) and the calculated XRD spectra of PbI2 hexagonal phase (red line).

The powder X-ray diffraction (XRD) pattern of the lead iodide nanocrystals and corresponding 3D perovskite nanocrystals (CH3NH3PbI3) are presented in Figure 6.10. The 3D organo-lead halide perovskite NCs (CH3NH3PbI3) sample is prepared by mixing the PbI2 NCs with excess of MAI and spin coated on a glass substrate. This substrate treated by heating at 70-100 oC for 5 minutes before undergoes measurements. The XRD show the phase purity (tetragonal) of organo-lead iodide perovskite NCs with the exception of a small amount of residual MAI ligands, which is in

210 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals very good agreement with the calculated XRD powder pattern for the tetragonal phase of lead halide perovskite.13

Figure 6.10a (blue line) shows strong reflection peaks at 14.03o, 28.13o, 28.3o and 31.67o, corresponding to the (002), (004), (200) and (222) planes, respectively, indicate high crystallinity of a typical diffraction pattern of a tetragonal CH3NH3PbI3 perovskite phase (space group

I4/mcm).1-2, 13, 15, 52 The FWHM of the XRD peaks indicate that the NCs aggregated upon evaporation to form a larger particle size (several tens nanometer). The strong diffraction peaks at

2 =19.55 and 29.57 o (starred) can be attributed to unreacted excess MAI precursor.3 The absence of the PbI2 peak in the perovskite XRD spectrum suggests complete conversion of PbI2 NCs to perovskite which is in agreement with absorption spectrum of perovskite in Figure 6.8c.

The powder X-ray diffraction (XRD) pattern of the 2D organo-lead halide perovskite NCs

(R2(CH3NH3)n-1PbnI3n+1 with number of layers n=1, 2 or 3 (and where we used

R=butylammonium as the long chain ligand) are shown in Figure 4. The diffractogram of the 2D perovskite NCs in the case of n=1 and 2 was obtained on a powder of nanocrystals while the diffractogram of the 2D perovskite NCs with n=3 was performed on film of the material prepared by spin coating. More details about the film preparation for the XRD measurements are presented in the Supporting Information. The XRD data show all expected diffraction peaks of the corresponding orthorhombic structure for the three samples suggesting perfect layer periodicity along the out-of-plane direction (002), c-axis. The (002) peak shifts towards the low diffraction 2q with increasing the number of layers. Using Bragg’s law, one can calculate the interplanar distance i.e., d-spacing, at the (002) diffraction.53 The d-spacing value was measured to be 13.84, 19.68, and 24.79 Å for our 2D perovskites n=1,2 and 3, in a good agreement with literature values of the

211 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals corresponding bulk phase of the same materials.54-56 Then lattice constant of the c-axis was evaluated to be 27.86, 39.37, and 49.60 Å, respectively, where c=2*d002.

Positions of Bragg peaks are established from the Braggs’ law as a function of the wavelength and the interplanar distances, that is, d-spacing. The latter can be easily calculated from the known unit cell dimensions

ퟏ − 흀 풉ퟐ 풌ퟐ 풍ퟐ ퟐ 2hkl=ퟐ풂풓풄푺풊풏 ( ) , where 풅풉풌풍 = ( ퟐ + ퟐ + ퟐ) (9) ퟐ풅풉풌풍 풂 풃 풄

212 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.4.6 X-ray Photoelectron Spectroscopy Analysis

Figure 6.12 shows high resolution XPS spectra taken of the Pb and I regions of perovskite nanocrystal samples prepared by depositing a film through evaporating solvent from the NP solution. Several features can be assigned by analysis of the binding energy measured in these regions. Typically, one can distinguish the chemical environment of each species as well as determine their stoichiometric ratios.

XPS analysis shows a binding energy of symmetric core peak shapes of Pb 4f7/2 level at a binding energy of 138.48 eV; assigned for Pb(II), and I 3d5/2 peak at 619.12 eV (Figure 6.12), which is in

57-58 agreement with values of perovskite in literature. Pb 4f7/2 binding energy of perovskite has a binding energy lower shift of 0.4 eV from the Pb 4f7/2 binding energy of PbI2 NCs (Figure 6.13).

Also, the absence of metallic Pb atoms in the measurements is obvious.

The spin-orbit splitting between the Pb 4f7/2 and Pb 4f5/2 core levels and between I 3d5/2 and I 3d3/2 core levels are 4.86 and 11.5 eV, respectively; which are similar to values previously reported57, 59 for lead halide perovskites. In the case of iodide peaks of the perovskite NCs (n=3), it was found when we used the Gaussian fitting the presence of a small peak at 617 eV (4 % of the total iodide in the sample) which corresponds to the iodide bond in the excess of unreacted methyl ammonium iodide.59

In order to determine the stoichiometry and chemical environments present in the NCs, the peaks were fitted according to a Gaussian-Lorentzian peak shape along with a ‘Smart’ background.

Quantifying the values of the area under the peaks, we calculated the ratio of Pb:I and found it to be 3:10 corresponding to 2D (n= 3) OL2(CH3NH3)2[Pb3I10] perovskite (where OL = oleylamine)

213 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

and molar ratio 2:7 in case of 2D (n=2) OL2CH3NH3[Pb2I7] and these results are consistent in all samples measured.

4f7/2 and 4f5/2 transitions with a binding energy of 137.8 eV and 142.7 eV respectively with no metallic Pb component observed, and I 3d5/2 and 3d3/2 transitions with a binding energy of 619.28 and 630.78 eV, respectively. c and d, Gaussian-Lorentzian peak shape fitting along with a ‘Smart’ background of Pb 4f core level and I 3d core level of 2 D perovskite NCs.

214 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

Figure 6.13. X–ray photoelectron spectroscopy (XPS) analysis of PbI2 NCs. a, Gaussian-Lorentzian peak shape fitting along with a ‘Smart’ background of a, Pb 4f core level and b, I 3d core level of PbI2 NCs film deposited from evaporated NCs solution. XPS showing the Pb(II) 4f7/2 and 4f5/2 transitions with a binding energy of 138.5 eV and 143.42 eV respectively with no metallic Pb component observed, and I 3d5/2 and

3d3/2 transitions with a binding energy of 619.18 and 630.68 eV, respectively.

6.4.7 Transient Absorption Spectroscopy Ananlysis

To characterize the photophysical properties and exciton dynamics of the perovskite nanocrystals

(diameter 5.5 nm, n = 3), we employed transient absorption spectroscopy. We used an excitation and probe pulse with the same spectrum spanning from 525 to 720 nm and a temporal full-width at half maximum of ~20 fs. Figure 6.14a displays transient absorption spectra of the sample recorded at different population times. We observe two main features: a strong, positive feature peaked at ~599 nm and a broad, negative feature extending toward shorter wavelengths. The peak position of the positive feature is consistent with the excitonic peak of 2D organo-lead iodide perovskites as shown in prior reports and is therefore assigned as the ground-state bleach of the excitonic peak.46-47, 50 On the basis of previous studies, the broad, negative feature is assigned to the photoinduced absorption of excitons/charge carriers.10, 60-63

215 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

a b c

Figure 6.14. Transient absorption dynamics of perovskite nanocrystals. a, Transient absorption spectra of perovskite nanocrystals in toluene:chloroform mixture (2:1). The corresponding pump-probe time delays are indicated in the legend. b, Decay-associated spectra obtained from a global analysis of the transient absorption data. The spectra have been normalized to the most intense bleach feature. The inset displays the decay-associated spectra without normalization. c, Dynamics of GSB and PIA at 599 and 530 nm, respectively. The square’s and x’s represent the experimental data while the lines represent the fits obtained from the global analysis.

To examine the time evolution of the photogenerated populations, we performed a global analysis of the transient data, Figure 6.14b and Figure 6.14c. The global analysis (Figure 6.14b) reveals three decay-associated spectra with time constants (amplitudes) of 250 fs (22%), 69 ps (12%), and

5.63 ns (67%). The short- and long-lived species are the dominant species. The spectral profile of the short- and long-lived species match those observed in previous reports60, 62, 64 on perovskite films. The short-lived species exhibits a derivative feature slightly lower in energy than the excitonic peak position (i.e. 599 nm). It is likely to arise from electroabsorption and/or a charge- induced ‘Stark-effect’ 60, 62, 65. The long-lived species decays with a time constant of 5.63 ns, which is generally consistent with the short decay component recovered from time-correlated single photon counting measurements on our nanocrystals (Figure S9) as well as with luminescence lifetime measurements on perovskite films64. We attribute the decay of this species to exciton recombination.

216 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.4.8 Time-Correlated Single Photon Counting Measurements for 2D Perovskite (n=3)

We measured the time-correlated single photon counting (TCSPC) for 2D Perovskite (n=3) NCs suspension. We used an excitation wavelength of 374 nm and the emission wavelength at 646 nm.

Figure 6.15 shows the the TCSPC data (black) which is biexponential fitted (red) with time constants of 14 and 46 ns. The amplitudes and time constants obtained from biexponential fit to

TCSPC measurements on 2D perovskite (n=3) nanocrystal suspension, reported in Table 6.1.

Table 6.1: Amplitudes and Time Constants Obtained from Biexponential Fit to TCSPC Measurements on 2D Perovskite (n=3) Nanocrystal Suspension

Experiment fit

0 50 100 150 Time/ns Figure 6.15. Time-Correlated Single Photon Counting (TCSPC) Measurements for 2D Perovskite (n=3). TCSPC measurements of the 2D perovskite (n=3) nanocrystal suspension. The excitation wavelength was 374 nm and the emission wavelength was 646 nm. Overlaying the data (black) is a biexponential fit (red) with time constants of 14 and 46 ns.

217 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.5 Conclusion

In summary, in this chapter I reported my discovery of a simple method to synthesize colloidally stable methyl ammonium lead halide perovskite nanocrystals seeded from high quality PbX2 NCs

(X = I or Br) with a pre-targeted size. We report the preparation and characterization of methyl ammonium lead halide perovskite as well as high quality PbX2 nanocrystals (X = I or Br).

According to ligand choice and ratio, these nanocrystals were prepared in different layered crystal structures denoted R2(CH3NH)3n-1PbnI3n+1; n = 1, 2, 3 where R is the ligand (butyl, octyl, or oleylammonium) and the excitonic gap was thus tuned stepwise. This work reports advances in preparation of both these materials (PbX2 nanocrystals and 2D and 3D lead halide perovskite nanocrystals). The excitonic gap was thus tuned stepwise. Spectacular excitonic features and the reproducibility of our method with the pre-targeted size make these nanocrystals well suited for systematic investigations of the intrinsic photophysics and spectroscopy of organic-inorganic metal halide-perovskites. Our femtosecond transient absorption measurements indicate that the photophysical properties and excited-state dynamics of the perovskite nanocrystals are very similar to those observed in perovskite films.60, 62 Sharp excitonic features make these nanocrystals well suited for systematic investigations of the intrinsic photophysics and spectroscopy of organic- inorganic metal halide-perovskites. This method produces perovskite NCs with controlled size and represent a potential platform that will improve device performance stability.

218 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

6.6 References

Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as Visible- Light Sensitizers for Photovoltaic Cells. Journal of the American Chemical Society 2009, 131 (17), 6050-6051.

2. Im, J.-H.; Lee, C.-R.; Lee, J.-W.; Park, S.-W.; Park, N.-G., 6.5% efficient perovskite quantum-dot-sensitized solar cell. Nanoscale 3 (10), 4088-4093.

3. Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I., Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nat Mater 2014, 13 (9), 897-903.

4. Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H.-j.; Sarkar, A.; NazeeruddinMd, K.; Grätzel, M.; Seok, S. I., Efficient inorganic-organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors. Nat Photon 2013, 7 (6), 486-491.

5. Kim, H.-S.; Lee, J.-W.; Yantara, N.; Boix, P. P.; Kulkarni, S. A.; Mhaisalkar, S.; Grätzel, M.; Park, N.-G., High Efficiency Solid-State Sensitized Solar Cell-Based on Submicrometer Rutile TiO2 Nanorod and CH3NH3PbI3 Perovskite Sensitizer. Nano Letters 2013, 13 (6), 2412-2417.

6. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338 (6107), 643-647.

7. Green, M. A.; Ho-Baillie, A.; Snaith, H. J., The emergence of perovskite solar cells. Nat Photon 2014, 8 (7), 506-514.

8. Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P. H.; Kanatzidis, M. G., Lead-free solid- state organic-inorganic halide perovskite solar cells. Nat Photon 2014, 8 (6), 489-494.

9. Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G., All-solid-state dye-sensitized solar cells with high efficiency. Nature 2012, 485 (7399), 486-489.

10. Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342 (6156), 341-344.

11. Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C., Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342 (6156), 344-347.

12. Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J., Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347 (6225), 967-970.

219 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

13. Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O. M., Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 2015, 347 (6221), 519-522.

14. Schmidt, L. C.; Pertegás, A.; González-Carrero, S.; Malinkiewicz, O.; Agouram, S.; Mínguez Espallargas, G.; Bolink, H. J.; Galian, R. E.; Pérez-Prieto, J., Nontemplate Synthesis of CH3NH3PbBr3 Perovskite Nanoparticles. Journal of the American Chemical Society 2014, 136 (3), 850-853.

15. Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M., Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499 (7458), 316-319.

16. Zhang, F.; Zhong, H.; Chen, C.; Wu, X.-g.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y., Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015.

17. Tyagi, P.; Arveson, S. M.; Tisdale, W. A., Colloidal Organohalide Perovskite Nanoplatelets Exhibiting Quantum Confinement. The Journal of Physical Chemistry Letters 2015, 6 (10), 1911-1916.

18. Hines, M. A.; Scholes, G. D., Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Advanced Materials 2003, 15 (21), 1844-1849.

19. Hassan, Y.; Chuang, C.-H.; Kobayashi, Y.; Coombs, N.; Gorantla, S.; Botton, G. A.; Winnik, M. A.; Burda, C.; Scholes, G. D., Synthesis and Optical Properties of Linker-Free TiO2/CdSe Nanorods. The Journal of Physical Chemistry C 2014, 118 (6), 3347-3358.

20. Sandroff, C. J.; Hwang, D. M.; Chung, W. M., Carrier confinement and special crystallite dimensions in layered semiconductor colloids. Physical Review B 1986, 33 (8), 5953-5955.

21. Yamaki, T.; Asai, K.; Ishigure, K.; Ema, K.; Yaguchi, H., Quantum Size Effect of Lead Iodide Nanoparticles Formed by a Langmuir-Blodgett Technique. Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals 1999, 337 (1), 225-228.

22. Nozue, Y.; Tang, Z. K.; Goto, T., Excitons in PbI2 clusters incorporated into zeolite cages. Solid State Communications 1990, 73 (8), 531-534.

23. Lifshitz, E.; Yassen, M.; Bykov, L.; Dag, I., Continuous photoluminescence, time resolved photoluminescence and optically detected magnetic resonance measurements of PbI2 nanometer- sized particles, embedded in SiO2 films. Journal of Luminescence 1996, 70 (1–6), 421-434.

24. Goto, T.; Saito, S.; Tanaka, M., Size quantization of excitons in PbI2 microcrystallites. Solid State Communications 1991, 80 (5), 331-334.

220 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

25. Mu, R.; Tung, Y. S.; Ueda, A.; Henderson, D. O., Chemical and Size Characterization of Layered Lead Iodide Quantum Dots via Optical Spectroscopy and Atomic Force Microscopy. The Journal of Physical Chemistry 1996, 100 (51), 19927-19932.

26. Chakrabarty, N.; Mukherjee, A.; Sinha, S.; Basu, S.; Meikap, A. K., Observation of correlated barrier hopping in blue luminescent PbI2 nanoparticles. Physica E: Low-dimensional Systems and Nanostructures 2014, 64 (0), 134-140.

27. Gulia, V.; Vedeshwar, A. G.; Mehra, N. C., Quantum dot-like behavior of ultrathin PbI2 films. Acta Materialia 2006, 54 (15), 3899-3905.

28. Schieber, M.; Zamoshchik, N.; Khakhan, O.; Zuck, A., Structural changes during vapor- phase deposition of polycrystalline-PbI2 films. Journal of Crystal Growth 2008, 310 (13), 3168- 3173.

29. Gopi, K. K.; Norman, R. D.; Temer, S. A., Fabrication and characterization of solid PbI2 nanocrystals. Journal of Physics D: Applied Physics 2007, 40 (6), 1778.

30. Ma, D.; Zhang, W.; Zhang, R.; Zhang, M.; Xi, G.; Qian, Y., A Facile Hydrothermal Synthesis Route to Single-Crystalline Lead Iodide Nanobelts and Nanobelt Bundles. Journal of Nanoscience and Nanotechnology 2005, 5 (5), 810-813.

31. Zhu, G.; Liu, P.; Hojamberdiev, M.; Zhou, J.-p.; Huang, X.; Feng, B.; Yang, R., Controllable synthesis of PbI2 nanocrystals via a surfactant-assisted hydrothermal route. Applied Physics A 2010, 98 (2), 299-304.

32. Weidman, M. C.; Beck, M. E.; Hoffman, R. S.; Prins, F.; Tisdale, W. A., Monodisperse, Air-Stable PbS Nanocrystals via Precursor Stoichiometry Control. ACS Nano 2014, 8 (6), 6363- 6371.

33. Yu, W. W.; Falkner, J. C.; Shih, B. S.; Colvin, V. L., Preparation and Characterization of Monodisperse PbSe Semiconductor Nanocrystals in a Noncoordinating Solvent. Chemistry of Materials 2004, 16 (17), 3318-3322.

34. Ning, Z.; Voznyy, O.; Pan, J.; Hoogland, S.; Adinolfi, V.; Xu, J.; Li, M.; Kirmani, A. R.; Sun, J.-P.; Minor, J.; Kemp, K. W.; Dong, H.; Rollny, L.; Labelle, A.; Carey, G.; Sutherland, B.; Hill, I.; Amassian, A.; Liu, H.; Tang, J.; Bakr, O. M.; Sargent, E. H., Air-stable n-type colloidal quantum dot solids. Nat Mater 2014, 13 (8), 822-828.

35. Etgar, L.; Gao, P.; Xue, Z.; Peng, Q.; Chandiran, A. K.; Liu, B.; Nazeeruddin, M. K.; Grätzel, M., Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells. Journal of the American Chemical Society 2012, 134 (42), 17396-17399.

36. Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I., Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells. Nano Letters 2013, 13 (4), 1764-1769.

221 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

37. McClure, S. D.; Turner, D. B.; Arpin, P. C.; Mirkovic, T.; Scholes, G. D., Coherent Oscillations in the PC577 Cryptophyte Antenna Occur in the Excited Electronic State. The Journal of Physical Chemistry B 2014, 118 (5), 1296-1308.

38. Trebino, R.; DeLong, K. W.; Fittinghoff, D. N.; Sweetser, J. N.; Krumbügel, M. A.; Richman, B. A.; Kane, D. J., Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating. Review of Scientific Instruments 1997, 68 (9), 3277-3295.

39. Joris J. Snellenburg, S. L., Ralf Seger, Katharine M. Mullen, Ivo H. M. van Stokkum, Glotaran: A Java-Based Graphical User Interface for the R Package TIMP. Journal of Statistical Software 2012, 49 (3), 1-22.

40. Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V., Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Letters 2015.

41. Schaeffer, R. W.; Ardelean, M., Powder X-ray diffraction of oriented and intercalated lead iodide. Powder Diffraction 2001, 16 (01), 16-19.

42. Fleming, S.; Rohl, A., GDIS: A visualization program for molecular and periodic systems. Zeitschrift fur Kristallographie 2005, 220 (5-6), 580-584.

43. Zhang, L.; Holt, C. M. B.; Luber, E. J.; Olsen, B. C.; Wang, H.; Danaie, M.; Cui, X.; Tan, X.; W. Lui, V.; Kalisvaart, W. P.; Mitlin, D., High Rate Electrochemical Capacitors from Three- Dimensional Arrays of Vanadium Nitride Functionalized Carbon Nanotubes. The Journal of Physical Chemistry C 2011, 115 (49), 24381-24393.

44. Zheng, Z.; Liu, A.; Wang, S.; Wang, Y.; Li, Z.; Lau, W. M.; Zhang, L., In situ growth of epitaxial lead iodide films composed of hexagonal single crystals. Journal of Materials Chemistry 2005, 15 (42), 4555-4559.

45. Ahmad, S.; Kanaujia, P. K.; Niu, W.; Baumberg, J. J.; Vijaya Prakash, G., In Situ Intercalation Dynamics in Inorganic–Organic Layered Perovskite Thin Films. ACS Applied Materials & Interfaces 2014, 6 (13), 10238-10247.

46. Wu, X.; Trinh, M. T.; Zhu, X. Y., Excitonic Many-Body Interactions in Two-Dimensional Lead Iodide Perovskite Quantum Wells. The Journal of Physical Chemistry C 2015.

47. Ishihara, T., Optical properties of PbI-based perovskite structures. Journal of Luminescence 1994, 60–61 (0), 269-274.

48. Era, M.; Hattori, T.; Taira, T.; Tsutsui, T., Self-Organized Growth of PbI-Based Layered Perovskite Quantum Well by Dual-Source Vapor Deposition. Chemistry of Materials 1997, 9 (1), 8-10.

49. Ishihara, T.; Takahashi, J.; Goto, T., Optical properties due to electronic transitions in two- dimensional semiconductors (CnH2n+1NH3)2PbI4. Physical Review B 1990, 42 (17), 11099-11107.

222 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

50. Wu, X.; Trinh, M. T.; Niesner, D.; Zhu, H.; Norman, Z.; Owen, J. S.; Yaffe, O.; Kudisch, B. J.; Zhu, X. Y., Trap States in Lead Iodide Perovskites. Journal of the American Chemical Society 2015, 137 (5), 2089-2096.

51. Magde, D.; Brannon, J. H.; Cremers, T. L.; Olmsted, J., Absolute luminescence yield of cresyl violet. A standard for the red. The Journal of Physical Chemistry 1979, 83 (6), 696-699.

52. Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Graetzel, M.; White, T. J., Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications. Journal of Materials Chemistry A 2013, 1 (18), 5628-5641.

53. Pecharsky, V. K.; Zavalij, P. Y., Fundamentals of powder diffraction and structural characterization of materials. Springer: 2009.

54. Liang, K.; Mitzi, D. B.; Prikas, M. T., Synthesis and Characterization of Organic−Inorganic Perovskite Thin Films Prepared Using a Versatile Two-Step Dipping Technique. Chemistry of Materials 1998, 10 (1), 403-411.

55. Ishihara, T.; Takahashi, J.; Goto, T., Optical properties due to electronic transitions in two- dimensional semiconductors (CnH2n+1NH3)2PbI4. Physical Review B 1990, 42 (17), 11099-11107.

56. Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G., 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications. Journal of the American Chemical Society 2015, 137 (24), 7843-7850.

57. Lindblad, R.; Bi, D.; Park, B.-w.; Oscarsson, J.; Gorgoi, M.; Siegbahn, H.; Odelius, M.; Johansson, E. M. J.; Rensmo, H., Electronic Structure of TiO2/CH3NH3PbI3 Perovskite Solar Cell Interfaces. The Journal of Physical Chemistry Letters 2014, 5 (4), 648-653.

58. Bhachu, D. S.; Scanlon, D. O.; Saban, E. J.; Bronstein, H.; Parkin, I. P.; Carmalt, C. J.; Palgrave, R. G., Scalable route to CH3NH3PbI3 perovskite thin films by aerosol assisted chemical vapour deposition. Journal of Materials Chemistry A 2015.

59. Ng, T.-W.; Chan, C.-Y.; Lo, M.-F.; Guan, Z. Q.; Lee, C.-S., Formation chemistry of perovskites with mixed iodide/chloride content and the implications on charge transport properties. Journal of Materials Chemistry A 2015, 3 (17), 9081-9085.

60. Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D.-D.; Higler, R.; Hüttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; Atatüre, M.; Phillips, R. T.; Friend, R. H., High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. The Journal of Physical Chemistry Letters 2014, 5 (8), 1421-1426.

61. Manser, J. S.; Kamat, P. V., Band filling with free charge carriers in organometal halide perovskites. Nat Photon 2014, 8 (9), 737-743.

62. Chen, K.; Barker, A. J.; Morgan, F. L. C.; Halpert, J. E.; Hodgkiss, J. M., Effect of Carrier Thermalization Dynamics on Light Emission and Amplification in Organometal Halide Perovskites. The Journal of Physical Chemistry Letters 2015, 6 (1), 153-158.

223 Chapter 6 Structure-Tuned Lead Halide Perovskite Nanocrystals

63. Stamplecoskie, K. G.; Manser, J. S.; Kamat, P. V., Dual nature of the excited state in organic-inorganic lead halide perovskites. Energy & Environmental Science 2015, 8 (1), 208-215.

64. Xing, G.; Wu, B.; Chen, S.; Chua, J.; Yantara, N.; Mhaisalkar, S.; Mathews, N.; Sum, T. C., Interfacial Electron Transfer Barrier at Compact TiO2/CH3NH3PbI3 Heterojunction. Small 2015, n/a-n/a.

65. Shank, C. V.; Fork, R. L.; Leheny, R. F.; Shah, J., Dynamics of Photoexcited GaAs Band-Edge Absorption with Subpicosecond Resolution. Physical Review Letters 1979, 42 (2), 112-115.

224 Chapter 7 Summery and Conclusions

Conclusions and Future Directions

7.1 Thesis Summary

The work presented in this thesis has explored the preparation of a wide variety of colloidal nanocrystals (NCs) both inorganic semiconductors, also known as quantum dots (QDs), and organic-inorganic metal halide-perovskite materials, known as perovskites. In this thesis, we pursued synthesis as well as detailed materials characterization that included photophysics and spectroscopy of these nanostructured semiconductor materials.

These materials represent significant classes of nanocrystals that attracted great attention in both fundamental research and technical applications in various fields owing to their size and shape- dependent properties and their excellent chemical processability. They have the desirable chemical synthetic characteristics making them unique light harvesting materials. For instant, the pioneering applications of QDs are light-emitting diodes, lasers, and photovoltaic devices.

This thesis contains 6 chapters and conclusions. The contributions from each chapter are outlined in the next paragraphs.

The first part of this thesis (Chapters 23) focused on introducing theories and background of the materials being synthesized in the thesis. While chapter (1) raises the awareness toward the world energy demands and challenges facing our current resources of energy as well as introduce the motivation toward the investment in renewable energy research—why this thesis work focuses on synthesis novel light harvesting materials.

225 Chapter 7 Summery and Conclusions

Chapter (4) demonstrated our synthesis of colloidal linker–free TiO2/CdSe NR heterostructures

with TiO2 as a seed and CdSe QDs grown in the presence of TiO2 NRs using seeded–growth type colloidal injection approach. In this chapter, we presented our preparation of these type–II heterostructures without any ligand exchange process for the QDs or using bifunctional linker compound to bind the two semiconductor nanocrystals (TiO2 and CdSe) together. We revealed that that the aminolysis reaction of titanium oleate can lead to the formation of anatase TiO2 NRs with different NR lengths. We found that the as–prepared TiO2 seeds determined the morphology of TiO2/CdSe NRs. By changing the synthetic recipe, the different shape of NRs, either isolated

(long or short rods) or entangled TiO2/CdSe NRs was obtained. We show that the initial excited state of the sensitizer CdSe QDs is efficiently quenched by the presence of TiO2 cores.

Femtosecond transient absorption revealed that exciton states decays ps-to-ns time scale most probably due to the surface trapping at the interface of the heterostructures and the decay kinetics of the heterostructures are similar to that of CdSe QDs. The efficient PL quenching suggests that the ultrafast exciton dissociation over our experimental time resolution occurs in TiO2/CdSe NRs.

Chapter (5) took the thesis in a slightly different approaches than the conventional surfactant- assisted synthesis approach for synthesizing QDs used in chapter (4). Instead, this chapter explored a novel approach of directly synthesized CdSe NCs with electroactive ligands wherein the electroactive ligands chemically adhere to the NC surface through the favored phosphine oxide functional group that maintains a surface-to-ligand short distance.

This chapter builds on the well recorded observation in literature of that facile transport of charge carriers between individual NCs is an essential requirement in optoelectronic devices based on these materials. However, the conventional surfactant-assisted synthesis yields NCs with surfaces

226 Chapter 7 Summery and Conclusions functionalized with insulating organic ligands. These insulating ligands act as a barrier for charge transport between NCs. Electro-active (reducing/oxidizing) ligands like ferrocene and cobaltocene, however, due to their potential for extraordinarily large conductance are particularly appealing for applications as photo-excited hole conductors and photoredox systems. Although ferrocene ligands anchored to the NC surface through insulating long-chain hydrocarbon spacers have previously been reported, this approach is not ideal because the rate of charge transfer is known to be highly sensitive to the distance between donor and acceptor. We report in chapter (5) ferrocene directly bound to the inorganic core of the NC and as a result the distance between the

NCs and the electro-active moiety is minimized.

In order to ensure a short surface-to-ligand distance, we used ferrocene-based ligands containing phosphine and phosphine oxide functional groups without an intervening alkyl chain spacer. We characterized the morphology of the NCs using transmission electron microscopy and X-ray diffraction and find that the NCs are highly monodisperse and have high crystalline quality of

Wurtzite structure. Optical, nuclear magnetic resonance, mass and X-ray photoelectron spectroscopies confirm that the novel electro-active functional ligands are directly attached to the

NC surface. In order to gauge their potential as a photo-excited hole conductor and photoredox system, we performed cyclic voltammetry on the ferrocene-capped nanocrystals and find that the electronic structure is appropriate for hole transfer from the nanocrystalline core to the electro- active ligand. Complete fluorescence quenching and accelerated exciton decay dynamics suggest a high rate of hole transfer from the NC to electro-active ligand. These results suggest that the ferrocene-capped NCs represent promising photo-excited hole conductor and photoredox systems.

The last chapter, chapter (6), focuses on a new class of organic-inorganic metal halide-perovskite materials (known as perovskites). Inspired by our experience on synthesizing different types of

227 Chapter 7 Summery and Conclusions

QDs using hot injection technique, we presented our discovery of a simple method (bottom-up) to synthesize colloidally stable methyl ammonium lead halide perovskite nanocrystals seeded from high quality PbX2 NCs (X = I or Br) with a pre-targeted size. This chapter reports advances in preparation of both these materials (PbX2 nanocrystals and 2D and 3D lead halide perovskite nanocrystals). We reported the preparation and characterization of these materials. According to ligand choice and ratio, these nanocrystals were prepared in different layered crystal structures denoted R2(CH3NH)3n-1PbnI3n+1; n = 1, 2, 3 where R is the ligand (butyl, octyl, or oleylammonium iodide) and the excitonic gap was thus tuned stepwise. Our femtosecond transient absorption measurements indicate that the photophysical properties and excited-state dynamics of the perovskite nanocrystals are very similar to those observed in perovskite films. Therefore, the spectacular excitonic features and the reproducibility of this simple method with the pre-targeted size make these nanocrystals well suited for systematic investigations of the intrinsic photophysics and spectroscopy of organic-inorganic metal halide-perovskites. This method produces perovskite

NCs with controlled size with no traps and represent a potential platform that will improve device performance stability.

In summary, this thesis has touches on a number of different aspects, but related, presenting research concern synthesis and simple preparation methods for novel unique light harvesting materials. Each of the chapters contributed another solution of the current challenges facing the world to move energy demands dependence on renewable resources, specifically solar energy.

228 Chapter 7 Summery and Conclusions

7.2 Future Directions and Closing Remarks

The work in this thesis has opened many avenues of research. As highlighted in the first chapter of this thesis, sections 1.4 and 1.5, the development of novel materials for solar energy conversion represents an outstanding challenge towards the development of alternative energy sources. The work of this thesis succeeded in introducing various innovative and novel synthetic methods of different types of colloidal nanocrystals (NCs) with attractive properties for energy applications.

Chapters 4, 5 and 6 in this thesis represent description of three different projects results. In the following paragraphs, I recommend some ideas to continue the development of the research based on the work done in this thesis.

7.2.1 TiO2/CdSe Heterostructure NRs

In chapter 4, we show a simple method for synthesizing colloidal linker–free TiO2/CdSe NRs heterostructures using seeded–growth type colloidal injection approach. According to our results, we found that the aminolysis reaction of titanium oleate can lead to the formation of anatase TiO2

NRs with different lengths. Moreover, we found that the as–prepared TiO2 seeds determined the morphology of TiO2/CdSe NRs. Typically by changing the synthetic recipe, the different shapes of NRs were obtained.

Accordingly, a further work can be done to explore the synthesis more different shapes of TiO2 heterostructured with QDs as well as their application in fabrication of efficient optoelectronic devices. For example:

1. Preparation of different shapes and sizes of TiO2-heterostrucutred with QDs. Typically controlling the precursor concentration in the aminolysis reaction as well as the reaction time will affect the resulted TiO2 size and shape.

229 Chapter 7 Summery and Conclusions

2. We believe that preparing TiO2/CdSe heterostructures by inverse injection (addition of

TiO2 seeds into Se-precursor instead of Cd-precursor) will give the chance of TiSe2 layer to facilitate the subsequent growth of CdSe.

3. Controlling the concentration of the Cd and Se-precursors in the heterostructure synthesis will affect the shell thickness of the QDs. Controlling the shell thickness affect the charge transfer between the core and the shell.

4. Investigating the reaction mechanism of TiO2-CdSe system. One can study the reaction kinetics and mechanism.

5. Exploring the ability of TiO2 to contact other surfaces like CdTe, CdS and PbS systems.

6. Exploring a new approach of QDs as core and TiO2 as shell.

7. Performing ligand exchange with these TiO2-QDs heterostructures using conductive polymers.

8. Utilization of these TiO2-QDs type II heterostructures in in fabrication of efficient optoelectronic devices.

7.2.2 Direct Synthesis of the QDs with Electro-active Ligands

In Chapter 5 in this thesis, we reported the synthesis and characterization of cadmium selenide nanocrystals (NCs) with electro-active ligands directly attached to the surface. Electro-active

(reducing/oxidizing) ligands like ferrocene have potential large conductance making them particularly appealing for applications as photo-excited hole conductors and photoredox systems.

In chapter 5 our approach of ferrocene directly bound to the inorganic core of the NC minimized distance between them improves the charge transfer. Facile transport of charge carriers between

230 Chapter 7 Summery and Conclusions individual NCs is an essential requirement in optoelectronic devices based on these materials.

However, further work can be conducted to prove the concept as well as to improve stability of these ferrocene derivative ligands.

A. In term of improving the stability and the inner charge transfer in the NCs directly

t synthesized in di-tert-butylphosphinylferrocene (FcP(O) Bu2), the following work can be

investigated:

t 1. The FcP(O) Bu2 is a bulky ligand and thus could inhibit the faster charge transport to the

NCs. We suggest replacing the di-tert-butyl alkyl chain by hexyl or octyl chains—i.e. dihexylphosphinylferrocene or dioctylphosphinylferrocene.

2. Synthesizing X-ligand types as ferrocene phosphonic acids and use them as a mixture with dihexylphosphinylferrocene in the synthesis of CdSe NCs.

3. Demonstrating if the directly synthesized ferrocene-capped NCs have better quality than those capped with ferrocene via post-synthesis ligand exchange or not. This can be proven via a control experiment that compare the directly synthesized NCs with the post-synthesis ligand exchanged NCs.

B. In term of application of these materials in devices,

1. These directly synthesized ferrocene-capped NCs could be used in electrochromics, known as an optical switching, in which the optical response of the nanocrystal is altered by the redox chemistry of the ferrocene moieties—i.e. fluorescent redox responsive NCs.

231 Chapter 7 Summery and Conclusions

2. Moreover, these QDs capped with ferrocene can be investigated in the context of fundamental nanoscale charge-transfer processes.

3. Referring to the sections (4.2.1, 4.2.2 and 4.2.4) in this thesis, QDs for many years known to be used for light harvesting assemblies, known as quantum dot sensitized solar cells (QDSSCs).

However, bifunctional linkers are needed to achieve the attachment of the QDs to the TiO2. For the last three decades these bifunctional linkers normally are mercaptopropionic acids. But, these linker molecules have substantially hindered the electron transfer yielding the low efficiency. On the other side, ferrocene has proven to be a good molecular wiring, therefore, we suggesting using the ferrocene derivatives as in the following cartoon.



Cartoon: A schematic diagram of the structure of the ferrocene-1-thiol-1’-carboxylate bifunctional linkers ligand used for the attachment of the QDs to the TiO2.

C. Motivated by the work done in chapter 5, one can investigate other metallocenes ligands like

cobaltocene. This is a great area of research which I believe it would be of interest to future

graduate students.

232 Chapter 7 Summery and Conclusions

7.2.3 Perovskite nanocrystallites

I reported in chapter 6 my discovery of a simple method to synthesize colloidally stable suspension of perovskite NCs with tunable size/excitonic gap using bottom-up hot injection technique. I strongly believe that there is a great deal of future potential in this project.

In order to further advance the proof of principle synthetic method and develop a platform for colloidal stable and controllable sized perovskite materials with no surface traps, I propose the following ideas:

1. Synthesizing of core-shell perovskite NCs and a novel prototype material that combines

perovskite and quantum dots (QDs) in a form of heterostructured materials—i.e. synthesize

new heterostructured materials centered on AMX3 perovskites. Shells improve the stability

and protect the colloid surface from the traps effect. Synthesizing type-II band alignment

heterostructures enables electron transfer reactions to be controlled and studied in depth,

so mechanisms of charge formation and dynamics can be elucidated.

2. Broadening my synthetic approach and pursue a fundamental research on the synthesis to

design a new family of colloidal perovskite NCs by varying the chemical composition. Full

characterization, air sensitivity and stability analysis needed to be embedded.

3. Finally, by collaborating with a team of multidisciplinary researchers with interests ranging

from thin film PV, WLED and EL to solution-based low threshold lasers will be

responsible for the production of tuned-size and surface traps free perovskite NCs based

optoelectronic devices and lasing perovskite materials, respectively.

The proposed research is meant to overcome the bottleneck of instability, uncontrolled size, and large variation in devices performance and elucidate mysteries of the intrinsic electronic properties of perovskites. It will involve developing original techniques to achieve the synthesis of new

233 Chapter 7 Summery and Conclusions perovskite designs and understanding the mechanisms of their performance. The aforementioned proposed research has impact is not limited to synthetic chemistry but rather extended to the state- of-the-art optoelectronic devices and lasing photophysics. This project is a significant step toward cheap, stable and efficient optoelectronic devices and could give the Canadian industry a competitive advantage internationally and promote academic and industrial collaboration in relevant fields.

234 Chapter 7 Summery and Conclusions

235