SUPPRESSED CARRIER SCATTERING IN CADMIUM SULFIDE- ENCAPSULATED LEAD SULFIDE NANOCRYSTAL FILMS

Upendra Rijal

A Thesis

Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

August 2014

Committee:

Mikhail Zamkov, Advisor

Lewis P. Fulcher

Alexey Zayak ii ABSTRACT Mikhail Zamkov, Advisor

One of the main problems on the development of efficient nanocrystal devices is the weak strategy to deposit colloidal nanocrystals into films. The current work demonstrates the comparative study of the several mechanisms to make nanocrystal films with the use of all-optical approach to examine the dynamics of the electron transport process. The mechanism of carrier scattering on trap states/defects is distinguished exclusively from other charge or energy transfer processes through exciton dissociation by measuring the lifetimes for excited charges in matrix encapsulated or ligand linked PbS nanoparticle solids, featuring a varying amount of ZnS nanocrystals. The carrier mobility and the diffusion lengths for all types of solids within the hoping transport regime were then estimated using the measured lifetimes. The matrix capped nanoparticle solids have shown a smaller probability for the carrier scattering than that of nanoparticle films cross linked with 1,2-ethanedithiol molecule or 3- mercaptopropionic acid. The density of the traps on the surface of nanoparticles/matrix interfaces are comparatively lower due to the suppressed charge scattering in matrix capped nanoparticle films. iii

ACKNOWLEDGMENTS

I would like to extend my sincere gratitude to my supervisor, Dr. Mikhail Zamkov for his incredible support, guidance, and encouragement to enhance my graduate experience as good researcher and graduate student. I am glad to a part of his research group, which has opened a new way in my life.

I am also thankful to other committee members, Dr. Lewis Fulcher and Dr. Alexey

Zayak, for the time they spent to go through my manuscript.

Additionally, I am thankful to all the members of Dr. Zamkov research group, especially

Pavel, Geethika, and Prakash for their amiable collaboration and help. I would also like to thank

Matthew for his help during the work.

Lastly, I would also like to thank my family for the support and, in particular, my wife,

Padma, without whose love, encouragement, and editing assistance I would not have finished this thesis. v

TABLE OF CONTENTS

Page

CHAPTER 1. INTRODUCTION ...... 1

CHAPTER 2. EXPERIMENTAL ...... 7

2.1 Materials ...... 7

2.2 Methods ...... 7

2.2.1 Synthesis of PbS quantum dots ...... 7

2.2.2 Preparation of CdCl2 precursor for the synthesis of hybride passivated

PbS quantum dots ...... 8

2.2.3 Synthesis of the hybride passivated PbS quantum dots ...... 8

2.2.4 Synthesis of PbSCdS coreshell quantum dots ...... 9

2.2.5 Synthesis of ZnS quantum dots ...... 9

2.2.6 Preparation of the FTO/glass substrate ...... 10

2.2.7 Fabrication of the nanocrystal films ...... 10

2.2.8 In-filling of SMENA pores with ZnS ...... 11

2.3 Characterization ...... 12

CHAPTER 3. RESULTS AND DISCUSSION ...... 13

CHAPTER 4. CONCLUSION ...... 33

REFERENCES ...... 34 vi

LIST OF FIGURES

Figure Page

1.1 Band structure of type I and type II nanocrystal core/shell heterostructures. (a)

Type I Nanocrystal (b) Type II nanocrystal ...... 1

1.2 The matrix encapsulated method to determine the carrier trapping rates in colloidal

semiconductor nanocrystal films nanocrystal film ...... 4

3.1 (a) Sketch of the general strategy for encapsulation of colloidal quantum dots into

matrices. (b-c) Typical TEM images of PbS/CdS core/shell and ZnS NCs used as

as nanoparticle precursors during film assembly...... 14

3.2 Absorbance spectra of the PbS cores (3.2-nm-black) and PbSCdS coreshell NCs

(3.2-nm-brown) featuring 3.0 nm core diameter ...... 15

3.3 Different stages of NC films seen on XPD spectrum ...... 16

3.4 (a). Emission and FL intensity decay of PbSCdS core(shell) nanocrystals in

solution(ΔHCdS = 0.32 nm). (b). FL intensity decay of CdS embedded in PbS NCs

into matrices (Redge = 0.64 nm) with increasing amount of ZnS NCs in the film.

(c). Biexponential curve of the Fluorescence intensity decay indicating fast and

slow components ...... 17

3.5 FL intensity decay of oleic acid-capped PbS nanocrystals in solution (black) and .

in a film ...... 20

3.6 The dynamics of the fluorescence decay intensity CdS embedded PbS nanocrystal

films. (a). Development of fast component of the fluorescence Intensity Decay

with growing amount of ZnS nanocrystals in the solid. The FL lifetime

(without-ZnS) of PbS films gives the exciton dissociation time. (b). Slow vii

component of fluorescence intensity decay evolved with increasing amount of

ZnS nanocrystals in the solid. (c) Photoconductivity . measured on the same solids

as in (a) and (b) ...... 21

3.7 The positions of the excited energy levels of PbS QDs and bulk CdS ...... 22

3.8 (a). FL Intensity decay of weakly-coupled PbS NCs embedded into CdS matrices

(Redge = 2.7 nm) with increasing amount of ZnS NCs in solid. (b). Development

of the fast component of Flourescence decay with increasing amount of ZnS

nanoparticles in solid. (c) Development of the slow component of flourescence

decay with increasing amount of ZnS nanoparticles in solid ...... 24

3.9 Flourescence decay of CdS encapsulated PbS nanoparticle solids (Redge = 0.64 nm)

before (black) and after (red) the in filling ...... 25

3.10 (a). Sketch of SILAR method to fill the pore with ZnS. (b). The result of

implementation of the SILAR mechanism on Fl decay of CdS encapsulated

PbSZnS NC solids having lowered energy and charge transfer. The reduction of

rate of charge trapping resulted in the higher luorescence lifetime of the pore filled

NC film ...... 25

3.11 The FL intensity decay curve of PbS nanoparticles capped with MUA

(in methanol) ...... 27

3.12 (a-c). Fluorescence intensity curve of EDT passivated PbS nanoparticles in

solution (a), MPA (b), MPA/Cl (c) different environment (d-f). Pl intensity decay

of PbS nanoparticle solids with EDT (d), MPA (e), and MPA/Cl (f) Functions

of raised amounts of ZnS NCs in the solid. (g-j) Fast and slow components Pl

intensity lifetime of PbS NCs ...... 28

viii

LIST OF TABLES

Table Page

T3.1 FL Fluorescence intensity lifetime data and calculated transport properties of

matrix-encapsulated and ligand-linked PbS NC films ...... 32

T3.2 Investigated drift scattering lengths for CdS matrix encapsulated and ligand linked

PbS NC films ...... 32 1

CHAPTER 1. INTRODUCTION

A quantum dot is defined as a semiconductor material of nanoscale size, which confines the motion of the electrons in the conduction band, of the holes in the valence band, or excitons in all three spatial directions. Quantum dots demonstrate very interesting characteristics because of their small size. These particles have recently proved themselves as promising materials with applications such as solar cells, LEDs, LASERs, bio-imaging and other medical applications. A hetero-structured semiconductor quantum dot is formed by developing one type of semiconductor enclosed by different type of semiconductor. Such hetero-structure quantum dots show unique optical and electronic properties which are not possible to achieve from either of the semiconductors. A quantum dot core that is completely covered by the shell is called coreshell quantum dot. The relative position of the energy band level of the coreshell determines the optical and electronic properties. On the basis of the relative position of the conduction band and valence band of semiconductor in the coreshell, the coreshells are classified into three types.

If the energy band gap of the shell is larger than that of core, and it lies completely outside, then those coreshells are called type I coreshells (figure 1.1 a). If the bandgap of the core is wider than

Figure 1.1: Band structure of type I and type II nanocrystal core/shell heterostructures. (a) Type I nanocrystal (b) Type II nanocrystal 2 that of the shell (exact opposite of type I), the coreshell is called a reverse type I coreshell.

Whereas if both valence and conduction band of shell lie above that of the core, then the coreshell is known as type II (figure 1.1b).

The application of promising semiconductor nanocrystals has grown rapidly over the past several years due to their properties such as quantum confinement and size tunability.1 This process helps to optimize the film’s main characters, either increasing the quantum yield using the core shell,2 managing to adjust the band gap,3 or adjusting the driving force at the junction of the heterostrucutre.4 The thin film making process from the colloidal nanocrystal (quantum dot) solution is cheaper than the regular process of making films, which requires extreme conditions, namely high vacuum and high temperature. Due to these advantages over traditional methods, different experimental works have been performed in the past to develop better methods of making thin films from colloidal nanocrystals, and to improve the applications such as LASERs,5-7 Photovoltaics (solar cells),8-27 LEDs,26-28 and FETs.29-35

The main reason behind low efficiency of semiconductor nanocrystal films is poor conductivity, which is basically due to either energy disorder36 or trap states.37,38 It has already been shown that the unpassivated anions form trap states above the highest level of valence band and the cationic dangling bonds develop traps below the lowest level of the conduction band.39 Due to significantly high surface-volume ratio of the semiconductor quantum dots, the trap states can have substantial density. The charge transport in quantum dots is hence affected by the recombination of electric charges on the surface of the dot or on the surface of other dots, which is harmful to the efficiency of the device27.

The semiconductor quantum dots thin films are traditionally made by joining adjacent quantum dots with inorganic or organic ligands. One of the major drawbacks of this method is 3 that the number of ligands must be equal to number of surface atoms. The inequality between number of ligands and number of surface atoms results in the high density of trap states within the nanocrystal band gap, which causes charge scattering to trap states. The density of trap states on semiconductor nanocrystals for either organic or inorganic ligand nanocrystal films is of the order of 1017 cm-3eV-1 as shown by the recent work of the Sargent group. 40 This result is higher than the density of trap states of most crystalline semiconductors by at least three orders of magnitude. The hybrid passivation method27, on the other hand, employs both organic and inorganic ligands, which reduces the density of the trap states to about 2×1016 cm-3eV-1. Thin film nanocrystal devices still need thorough treatment for the unpassivated surfaces to increase the efficiency of the devices. It is therefore obvious to have trap states on the thin film if the traditional ligand based strategy is followed to make the films.

The substitute for the traditional method is given by a ligand free film deposition method of colloidal semiconductor nanocrystal.41-45 It is based on the inorganic ligands for the surface passivation of NCs in a solid. Currently, we are particularly interested in a recently developed matrix-encapsulation method, where CdS or ZnS quantum dot cores, which have wide band gap are used to fill the gap between the PbS cores.43,44 The CdS or ZnS quantum dots nanocrystals inserted between the PbS nanocrystals preserves the quantum confinement effectively by protecting the quantum dot surface from interacting with neighboring quantum dots. The main benefit of the matrix encapsulated method is that the ions (cations or anions) on the surface of the quantum dots are forced to develop bonds with respective cations and anions of the CdS or ZnS matrix. It stops the direct formation of trap states. The tunneling of the charge can still occur on the matrix boundaries. However, the possibility of the charge trapping is relatively low because of the wide band gap of CdS matrix. Hence, the formation of trap 4 states can be lowered by using the matrix encapsulated method over the traditional ligand based methods.

Figure 1.2: The matrix encapsulated method to determine the carrier trapping rates in colloidal semiconductor nanocrystal film.

In the current work, the electrical transport characteristics of matrix encapsulated and ligand linked NC films will be compared by measuring the rate of charge trapping to trap states, neighboring dots, and radiation recombination. In order to compare the dynamics of defects from radiation recombination, insulating ZnS quantum dot cores are inserted in between NCs of a PbS film in a controlled way, which helps to reduce the transfer of charges or energy to adjacent dots 5

(Fig. 1.2). The ligand-free film deposition method proves itself as a potential alternative to the ligand based film deposition method, where a stable inorganic medium is used to passivate the surface of the nanocrystal film. In this work, we are particularly interested in the recently reported matrix encapsulated method, which uses materials having large band gap such as CdS or ZnS quantum dots to separate the CdS quantum dots.45 The middling matrix protects the quantum confinement of the surrounded quantum dots while preserving the surfaces from possible interactions with surroundings. The main advantage of such method is the idea of surface passivation, which forces all anions and cations on the outer surface of the semiconductor quantum dots to stay heteroepitaxially bonded with their respective counter ions of CdS matrix. This method prevents formation of trap states directly on the surface of the quantum dots or reduces the trap states. Besides, the charge trapping can still occur at the junction between the CdS matrix and quantum dots arrays. The possibility of the charge trapping is comparatively low because of the quantum confinement produced by the potential barrier developed by matrix. As a consequence, the development of trap states are expected to be more suppressed in matrix encapsulated quantum dot films compared to ligand-linked quantum dot films.

It can be proven that the rate of tunneling of the charge on the trap states is less for solids made from the matrix encapsulated method than in the solids made from traditional methods, where the QDs are connected by ligands, if the amount of ZnS QDs in the PbS QD films is varied. Apart from the rate of charge scattering, the dynamics of charge decay can also be used to investigate the exciton dissociation rate. The measured parameters are then changed to diffusion length and charge mobilities. It has been found that the films of PbS quantum dots covered by CdS shells produced the highest carrier diffusion length with 3- 6 mercaptopropionic acid (MPA)-linked PbS films containing the surfaces passivated by Cl. In the meantime, the carrier diffusion with the shortest length was given by the 1,2ethanedithiol

(EDT)-linked QD films.

In general, a relative examination of charge dynamics in nanocrystal films involving different types of matrices and ligands have revealed the fact that the matrix encapsulation method might eventually offer better strategies to passivate the surface of quantum dots leading to greater suppression of carrier scattering. 7

CHAPTER 2. EXPERIMENTAL

2.1 Materials

Chemicals. 1-octadecene (ODE, 90% Aldrich), oleic acid (OA, 90% Aldrich), cadmium oxide (CdO, 99.99%, Aldrich), lead(II) oxide powder (PbO, 99.999% Aldrich), sodium sulfide nonanhydrate (Na2S•9H2O, 98% Alfa Aesar), sulfur (S, 99.999% Acros), ethanol (anhydrous, 95%

Aldrich), hexane (anhydrous, 95% Aldrich), methanol (anhydrous, 99.8% Aldrich), toluene

(anhydrous, 99.8% Aldrich), isopropanol (anhydrous, 99.8% Acros), octane (anhydrous, 99%

Aldrich), 3-mercaptopropionic acid (MPA, 99% Alfa Aesar), 11-Mercaptoundecanoic acid (MUA,

95% Aldrich), Diethylzinc (Et2Zn, 15% wt., 1.1 M solution in toluene, Aldrich), bis(trimethylsilyl) sulfide ((TMS)2S, Aldrich, synthetic grade), Tri-n-octylphosphine (TOP, 97% Strem), cadmium chloride (CdCl2, 99.99% Aldrich), acetone (anhydrous, Amresco, ACS grade), zinc acetate (98+%

Acros), Tetradecylphosphonic Acid (TDPA, 97% Aldrich), 1,2-ethanedithiol (EDT, 98+% Fluka), and triton X-100 (Alfa Aesar) were used as purchased. Argon atmosphere was provided using the standard Schlenk method to all the reactions.

Fluorine-doped tin oxide (FTO) glass (TEC 15, 12-14 Ohm/sq) was received from Pilkington

Glass.

2.2 Methods

2.2.1 Synthesis of PbS quantum dots

For the synthesis of PbS NCs, the procedure developed by Hines, et al46 was adopted. First of all, 0.49 gm of PbO, 18 ml of ODE, and 1.5 ml of OA were placed in a 100 ml three neck flask containing a magnetic stirrer. The mixture was then degassed for two hours at 1200C. The temperature of the mixture was further lowered to 1100C by setting up Argon flow. At the same time, 10 ml of ODE was degassed for 2 hours at 1200C using a magnetic stirrer in another 50 ml 3 8 neck flask. After 2 hours, the mixture was pumped off, lowered to room temperature, and Argon gas was pumped in. When the temperature of the second flask reached room temperature, 0.21 ml of

(TMS)2S (source of Sulfur) was added to the flask. It was stirred for five minutes and the mixture from the second flask was added to the first flask. At that point, the mixture in first flask was cooked for three minutes, and the temperature of the flask was reduced to room temperature using a water bath.

When the mixture in the first flask was reduced to room temperature, the PbS quantum dot solution was poured in equal amounts into 6 centrifuge tubes. 14 ml of Acetone were added in each tube and centrifuged for 5 minutes. The PbS QDs were redispersed in hexane after separating the

QDS from solution containing from the tubes. Again, Acetone was added on each tubes up to 14 ml and centrifuged for 5 minutes as before. The QDs are further separated from the solution containing it and dispersed with hexane. Finally the PbS QDs were placed in a cleaned vial and labeled it.

2.2.2 Preparation of CdCl2 precursor for the synthesis of hybrid passivated PbS NCs.

For additional passivation of PbS quantum dots, the metal-halide precursor was prepared as follows. 0.033gm (0.12 mmol) of TDPA and 0.30 gm (1.64 mmol) were placed in 5 ml Oleylamine and degassed for 18 hours at 1000 C. Then the temperature was lowered to 800 C and the Argon gas flow was set up to avoid solidification.

2.2.3 Synthesis of the hybrid passivated PbS nanocrystals.

Procedure reported by Ip et al.27 was used to prepare the hybrid passivaed PbS quantum dots.

PbS quantum dots with OA as ligands were synthesized by the regular procedure as explained above.

The synthesis process was further followed by reaction mixture while cooling. When the flask was removed from the heating mantle, 1 ml solution from the CdCl2 solution was added in the reaction mixture. The temperature was raised to 300C. As the temperature reached 300C, the quantum dots 9 were precipitated adding 50 ml Acetone. After that, the quantum dot solution was centrifuged and nanocrystals were separated from other solvents and unwanted materials. The nanoparticles were then redispersed in 15 ml of Hexane. This cleaning cycle was repeated two times to clean the quantum dots better. Finally, the quantum dots were redispersed in hexane (5-6 ml).

2.2.4 Synthesis of PbSCdS coreshell quantum dots

PbSCdS nanoparticles were synthesized using the procedure reported in Ref. 47 by cation exchange method. First of all, 0.30 gm of CdO, 10 ml of ODE, and 4 ml of OA were taken in a 100 ml three neck flask containing a magnetic stirrer and flow of Argon was set up. The mixture was then heated for two and half hours at 2350C. When the solution in the flask became clear, the temperature of the flask was reduced to 1350C. 4 ml of PbS NCs were then added to the flask and cooked for 2 minutes. Samples of PbSCdS core shells from the flask were taken and the fluorescence peak was checked constantly. Once the desired size of the quantum dots were formed, the reaction was stopped using a water bath.

When the mixture in the first flask reached room temperature, the PbS quantum dot solution was poured into 6 centrifuge tubes in equal amounts. 14 ml of Ethanol were added in each tube and centrifuged for 5 minutes. The PbS QDs were redispersed in hexane after separating the QDs from solution containing from the tubes. Again, Ethanol was added in each tube up to 14 ml and centrifuged for 5 minutes as before. The QDs were further separated from the solution and dispersed within hexane. Finally the PbS QDs were placed in a cleaned vial and labeled.

2.2.5 Synthesis of ZnS core quantum dots

ZnS quantum dots were synthesized using the typical procedure as follows. First of all, 2 ml

TOP were placed in a 3 neck flask and degassed at 1200C for 2 hours. After 2 hours, it was switched with Argon flow. 0.2ml (TMS)2S and 1.0 ml diethyl zinc were separately injected into the flask 10 containing TOP. The temperature of the solution was reduced to 900C and stirred for 30 minutes. In the meantime, 5 ml Butanol were injected into the flask to prevent the solidification of TOPO. The solution containing ZnS quantum dots in the flask was cooled down to room temperature. The QD solution was poured into 6 centrifuged tubes, and 14 ml of Acetone were added to each tubes. Then the solution in the tubes was centrifuged for 5 minutes and the quantum dots were separated from the solvent and other unwanted solutions. The quantum dot solution was centrifuged one more time to make it cleaner. Finally, the quantum dots were redispersed in Hexane (5-6 ml) and kept in a clean vial.

2.2.6 Preparation of the FTO/glass substrate.

For the preparation of the glass substrate, FTO coated glass was chosen and cut into 2.5 cm x

2.5 cm pieces. The glass pieces were then cleaned with detergent (Alconox) and rinsed with deionized water. Soon after, the glass pieces were sonicated with Methanol, Acetone, and

Isopropanol each for five minutes.

2.2.7 Fabrication of nanocrystal films.

Standard Layer by Layer Techniques48 were used to deposit ligand linked Quantum dot films inside the glove box with Argon flow. Using the procedure43, complete inorganic QD films

(SMENA) were deposited.

For the deposition of layer by layer complete inorganic PbSCdS coreshell QD films, 6-7 drops of PbSCdS QD solution made in Hexane (10 mg/ml) were injected dropwise on the surface of

FTO/glass substrate. The glass was then spun for 10 seconds at 3000 rpm. 8-10 drops of mixture of

MPA and Methanol (1:4) were placed at the center of the glass in order to exchange native OA ligands with thermally degradable ligands MPA. The mixture on the glass was drenched for 10 seconds and spun for 10 seconds. After the replacement of OA ligands with MPA, 10 drops of 11

Methanol were placed on the glass and spun for 10 seconds to wash the film. The film was further rinsed with Octane in the same fashion. Once two layers of film were deposited, the film was heated at 1200C-1400C for 15 minutes. 6-7 layers of PbSCdS QD films in total were placed on the glass substrate. The ratios of optical density of ZnS QDs (λ = 270 nm) and that of PbSCdS coreshells (λex at ≈ 850 nm - 900 nm) were calculated. The originally prepared PbSCdS coreshell solution was mixed with ZnS quantum dot solution in the different ratio as calculated before.

The Layer by Layer Spincoating Technique was used to deposit ligand (MPA)-linked PbS quantum dot films in Argon flow. 6-8 drops of PbS quantum dots dispersed in Hexane were deposited on the surface of FTO glass and spun for 10 seconds at 3000 rpm. Afterward, 8-10 drops of mixture of Methanol and MPA (4:1) were placed on the glass, saturated for 10 seconds, and spun for 10 seconds at 3000 rpm. Total 6-10 layers of NCs were made as per the procedure explained before. ZnS cores were mixed with PbS NCs in different ratios, which were obtained by dividing the optical density of the ZnS cores (λ = 270 nm) by that of PbS QDs. For the deposition of the ethanedithiol (EDT)-linked QD films, 5-7 drops of PbS nanoparticles were placed on the FTO/glass, spun for 10 seconds at 3000 rpm. The film was then adsorbed in 0.1 M EDT solution prepared in

Acetonitrile for one minute. After that, the film was dried and cleaned with 10 drops of Acetonitrile.

8-10 layers, in general, were needed for the fabrication of EDT-cross-linked PbS QDs and PbS QDs with ZnS films.

2.2.8 In-filling of SMENA pores with ZnS.

The SILAR Method was employed49 for the pore filling process. An additional amount of

ZnS QDs, cores with a wide band gap, was injected in among NCs by soaking the heated NC films containing thermally degradable ligands successively, completely inorganic QD film in solution of

Methanol with Zn and S precursors. 12

For this process, Sulfur and Zinc bath were used. 0.10 gm Zinc Acetate was dissolved in 20 ml of Methanol to produce a Zinc bath and 0.098 gm Na2S. 9 H2O was dissolved in 20 ml Methanol to prepare the Sulfur bath. For a complete SILAR cycle, the film was soaked in the zinc bath for a minute, washed with methanol for a minute, and soaked in a sulfur bath for a minute. Subsequently, the film was rinsed with methanol. For the films, 2-10 SILAR cycles were completed for inorganic films having longest PL-lifetime. Then, the films were heated at 1500 C for 15 minutes.

2.3 Characterization

Simadzu UV-3600 UV-vis-NIR and CARY 50 scan spectrophotometers were used to investigate absorption spectra. To record the Photoluminescence spectra, Jobin Yvon Fluorolog FL3-

11 fluorescence spectrophotometer was used. JEOL 3011UHR and 2010 transmission electron microscopes were used for the measurements of High-resolution transmission electron microscopy

(HR-TEM), which were operated at 300 and 200 kV respectively. A small piece of nanocrystal film was scratched for the preparation of the TEM sample. The scratched piece of film was dissolved in

Toulene and sonicated. The solution was then dropped on a carbon-coated copper grid and was allowed to dry. Besides the instruments, a Scintag XDS-2000 X-ray powder diffractometer was used to measure X-ray powder diffraction (XRD). To measure the FL lifetime, a time-correlated single photon counting setup utilizing SPC-630 single-photon counting PCI card (Becker & Hickle GmbH) was used, where a picosecond diode laser operating at 400 nm acted as an excitation source

(Picoquant), an id50 avalanche photodiode (Quantique), and long pass filters on 400nm, 532nm and

750nm. 13

CHAPTER 3.RESULTS AND DISCUSSION

The rate of charge trapping in quantum dot films is determined by uniquely distinguishing the dynamics of this process from other processes of exciton decay such as variable range hopping (VRH),47 charge tunneling between neighboring quantum dots, and the resonant energy transfer to a dark state.48

These mechanisms have resulted in the dissociation of NC excitons, which cause a significant drop in fluorescence life time. The binding energy of the exciton is remarkably smaller than thermal energy of

Photoinduced charges whereas the time of charge carrier hopping or tunneling is about the same as the carrier ionization time. The measured FL lifetime of PbS quantum dots is, thus, useful to examine the accumulative rate by which charge carriers are removed from the excited state. The rate of FL intensity decay of solid is given as follows:

Γ = Γ + Γ FL decay rad non−rad ------(1) = Γrad + Γtrapping + Γtunneling + ΓVRH + Γenergy transfer

The rate of radiative decay becomes negligible in comparison to the carrier removal rate through the transfer processes if electrical coupling between adjacent quantum dots is strong, Γrad <<

Γnon-rad. Consequently, the band edge emission from each of the QDs in the film can be suppressed.

The FL lifetime (τFL = 1/ ΓFL decay) then is approximately the same as the nonradiative exciton decay time as τFL ≈ τnon-rad = 1/(Γtrapping+ Γtunneling+ ΓVRH + Γenergy transfer). On the other hand, the charge and energy transfer among PbS quantum dots in the film have comparatively lower probability when the electrical coupling between adjacent QDs is weak. The role of radiative decay makes the rate of total exciton decay significant. This decay is further demonstrated with the improvement of the FL lifetime and the parallel growth in the emission quantum yield of nanocrystal films having large interparticle distances.44 14

Figure 3.1: (a) Sketch of the general strategy for encapsulation of colloidal quantum dots into matrices. (b-c) Typical TEM images of PbS/CdS core/shell and ZnS NCs used as nanoparticle precursors during film assembly. Equation 1 can be used to investigate average charge trapping time when the electrical coupling between adjacent quantum dots is strong (Γrad << Γnon-rad), whereas the other mechanisms of exciton dissociation are negligible, Γtunneling+ ΓVRH + Γenergy transfer → 0. By inserting wide band gap materials such as ZnS semiconductor quantum dots in between the PbS NCs, this type of situation can be created as shown in fig. 1(ii). It also increases the interparticle distance among the PbS quantum dots, which basically lowers the rate of short-range hoping, energy transfer, and tunneling. The amplitude of the energy transfer phenomenon is inversely proportional to the fourth power of interparticle separation, which is reduced significantly when PbS QDs are separated by ZnS QDs. In the meantime, ZnS QDs having wide band gap develop a considerable potential barrier for valence and conduction band charges 15 in the film as the PbS-PbS tunneling is also assumed to be insignificant. The VRH process, coupling the resonance states of PbS QDs is lowered as well with the presence of insulating ZnS QDs. The short range hopping rate, specifically, among the adjacent quantum dots can certainly be lessened by using a higher amount/fraction of ZnS QDs in the film. In contrary, the rate of long range hopping among resonant states with non-adjacent QDs is moderately suppressed since the hopping of excited charges in resonance via longer distances can still exist.

Several mixed NC films with different ratios of ZnS to PbS QDs were fabricated to examine if the ZnS NCs added to PbS NCs actually help to suppress the charge and energy transfer with neighboring PbS QDs. Afterwards, the corresponding FL intensity lifetimes were measured. ZnS NCs to

PbS NCs were incorporated into CdS matrices (Fig. 3.1a) employing the semiconductor matrix- encapsulated

Figure 3.2: Absorbance spectra of the PbS cores (3.2-nm-black) and PbSCdS coreshell NCs (3.2- nm -brown) featuring 3.0 nm core diameter. 16 nanocrystal arrays (SMENA) technique43. Further, PbS QD cores covered with CdS semiconductor QD shells (Fig. 3.2) are mixed with ZnS QDs. The mixture was then spincoated on an FTO/glass, native ligands were exchanged with thermally degradable MPA ligands, and the adjacent QD Shells were fused. It was finally heated at 1200C for 15 minutes. FTIR measurements were carried out to confirm the removal of the native organic ligands.

Figure 3.3: Different stages of NC films seen on XPD spectrum. (a) Bragg’s peaks displayed by PbS NCs (4.0-nm) for rock salt. (b) Fused PbSCdS coreshell (c) PbSCdS coreshell and ZnS NCs before pore filling process. (d) Matching of the lattice structure at the boundary between zinc blende CdS and rock-salt PbS crystals. (e) A small piece of PbSCdS SMENA film.

Additional wide band gap materials such as CdS or ZnS NCs were used to fill the pores of the matrices via ionic layer adsorption and reaction (SILAR) method. Fig. 3.3 clearly indicates the iffraction spectra of PbS QDs, (Fig. 3.3a), PbS QDs in CdS (3.3b), and the mixture of PbS NCs and ZnS NCs in 17

CdS matrix (Fig. 3.3c). A remarkable band edge emission of PbS QDs obtained from the NC films of

PbS and Zns demonstrating the ZnS diffraction pattern, which indicates the successful encapsulation of the both type of QDs into CdS NCs. Fig. 3.3e shows the TEM image of PbS QDs with embedded CdS.

The result of the increasing portion of ZnS NCs in CdS encapsulated PbSCdS NC films (without pore filling) is demonstrated in Fig. 3.4b below. The biexponential decay (Fig. 3.4b black curve) having slow and fast components almost equal to 50 ns and 1.6 ns respectively was shown by the FL lifetime of the band edge excitons in PbS QDs before adding ZnS NCs. The higher rate of energy and charge transfer to other particles resulted in a comparatively short lifetime of the fast component. It further causes the detachment of excitons which suppresses the band gap emission. The dissociation process is reserved due to the addition of increasing amount of ZnS QDs changing the increment of the fast component from 1.6 ns to 8.9 ns (Fig. 3.4b and 3.6 a). In the meantime, the slow component of the PbS exciton decay also grows with the addition of higher amount of ZnS QDs but at a lower rate ranging from 50 ns to 87 ns. Eventually, both components saturate showing no further changes with the addition of ZnS NCs.

Figure 3.4: (a). Emission and FL intensity decay of PbSCdS core(shell) nanocrystals in solution (ΔHCdS = 0.32 nm). (b). FL intensity decay of CdS embedded in PbS NCs into matrices (Redge = 0.64 nm) with increasing amount of ZnS NCs in the film. (c). Biexponential curve of the Fluorescence intensity decay indicating fast and slow components. 18

The ZnS nanocrystals that insulate the film are responsible for the origin of fast and slow components of the fluorescence decay of the PbS excitons. In order to describe the significant rise in the fast decay component of the NC films saturated with ZnS, it is important to remember that before the addition of ZnS nanocrystals, the edge-to-edge distance between adjacent PbS nanocrystals, Redge= 2 x

0.32 nm = 0.64 nm, was short enough for the process of charge tunneling. Most photoconducting NC devices43 use seperation distances similar to this because larger distances of the Redge make the material exhibit insulating properties. As the ratio of ZnS to PbS increases, the PbS nanocrystals become more covered by ZnS. As a result, the separation distance Redge reaches Redge = 2DH + dZnS , where ΔH is the thickness of the CdS shell, and dZnS is the typical diameter of the ZnS nanocrystals (d=4.6nm). Redge increases from 0.64nm to at least 5.2 nm, when the PbS(CdS) matrix is saturated with ZnS NCs. This causes the PbS to PbS transport to be intensively suppressed. Since the PbS NCs are greatly separated by the ZnS NCs insulator, all interparticle interactions such as tunneling, short-range hopping, and resonant energy transfer will be suppressed at the saturation fraction of ZnS NCs. Phototcurrent measurements of the same films backed up this hypothesis by showing that when ZnS concentration is increased, the suppression of photoinduced current is increased as well (Fig. 3.6c). On the basis of the observations, the exciton dissociation processes are therefore associated with the fast component of the FL decay as demonstrated in Fig. 3.4c, this causes the charge and energy transfer between adjacent PbS NCs.

As mentioned before, the growth of the fast component of the FL intensity decay and the drop in the photoconductivity of the films are caused by the destruction of exciton dissociation in the PbS nanocrystal films saturated with ZnS NCs (Redge ≈ 0.64 nm). Despite the reduction of film photoconductivity, the slow component only changes slightly. The slow component only increases from

50 to 87 ns (Fig. 3.6b). Concurrently, the lifetime of the slow component of FL intensity decay of PbS

NC films saturated with ZnS NCs is still much smaller (3.3 nm) than that of the PbSCdS coreshell NCs in solution which have lifetimes greater than 1120 ns. This lowerbound estimate was found from the 19 emission lifetime of the NCs in chloroform (Fig. 3.4a). There was a difference between τrad and

τFL,slow of at least 20 times. This points to the another source of non radiative decay, which is unaffected by seperation of nanoparticles. This phenomenon is being caused by charges being trapped on the surface of the nanocrystals as per equation 1. As all the energy and charge transfer processes are suppressed, the slow component of the Fluorescence lifetime of of PbS nanocrystal solids becomes approximately equal to the timescale of the carrier trapping, τtrapping ≈ τFL, slow = 87 ns.

The dependence of the slow component of fluorescence decay on the ratio of ZnS NCs present in the film further explains the trapping processes in PbS NC films. Figure 3.6b shows that τFL, slow is increasing from 50 ns to 87 ns, showing that the some charge trapping actions are also being prevented by the ZnS, whereas other mechanisms do not depend on the ZnS insulating effect. The concepts of global and local traps can explain these phenomenon as demonstrated by Fig. 3.6. As the photoinduced charges are trapped on the surface of the same PbS NCs (local trap state), the trapping rate is not affected by the amount of ZnS, however if the charge is trapped on nearby dots and requires global trap states, the amount of ZnS plays a significant role to reduce the trapping rate. Accordingly, both trap states (Fig. 3.6b) are shown by the slow component of fluorescence intensity decay in PbS NC solids

(without ZnS NCs), whereas the local trap states in the NC films saturated with ZnS NCs primarily determine the carrier decay.

In order to confirm the existence of global trap states in PbS NC solids, the fluorescence intensity decay of PbS NC films with OA ligands was investigated. It is expected that new trap states are not created when PbS NCs capped with OA ligands in solution are transferred into a solid. Therefore, the local trap states stay constant for both solution and film if it solely determines the slow component of the fluorescence decay. Measured data (Fig. 3.5), however, shows that the fluorescence lifetime of PbS

NCs drops at least by a factor of 30% with the deposition of film. The fluorescence intensity decay of a

PbS NC films picks up a rapid component (dissociation) over the slow one falling from 380 ns to 290 ns 20 unlike the PbS nanocrystal solution, where the fluorescence lifetime decay curve is single exponential.

Some global trap states, thus, take place between adjacent particles. This mechanism appears to be viable for NC solid although the new trap states are not formed but clearly repressed for quantum dots in solution.

Figure 3.5. FL intensity decay of oleic acid-capped PbS nanocrystals in solution (black) and in a film.

The fast component and slow component of the fluorescence intensity decay of PbS(ZnS) films evolves as being consistent with the dissociation and charge trapping process. Further explanation is required for the lack of saturation of the fluorescence lifetime in NC films containing ZnS NCs.

Specifically, when the ratio of ZnS NCs volume with PbS(ZnS) solid approaches 10-15 range, all the charge transfer and energy transfer mechanisms are expected to be prevented. Nonetheless, the fast decay component can still be seen in the ZnS dominated PbS(ZnS) (Fig. 3.4b and 3.4c) solids.

Additionally, the photoconductivity of NC films cannot completely reach zero even if insulating ZnS

NCs cover the entire PbS NCs (Fig. 3.6c). The long range hopping process can explain this mechanism.

Higher amount of ZnS nanocrystals might not even be able to dissociate excitons with the long range 21 hopping process as in fig. 3.4b. In addition to this, the nonvanishing carrier conductivity of the solid can also be explained by the long range hoping, which is resulted from the remaining photocurrent of the NC films saturated with ZnS NCs.

Figure 3.6: The dynamics of the fluorescence decay intensity of CdS embedded PbS nanocrystal films. (a). Development of fast component of the fluorescence Intensity Decay with growing amount of ZnS nanocrystals in the solid. The FL lifetime (without-ZnS) of PbS films gives the exciton dissociation time. (b). Slow component of fluorescence intensity decay evolved with increasing amount of ZnS nanocrystals in the solid. (c) Photoconductivity measured on the same solids as in (a) and (b). The dynamics of fluorescence intensity decay of the solids in weakly coupled structure of PbS films were examined to check additional information that could strengthen the hypothesis presented for slow and fast components to the trapping process and dissociation process. Soon after, the CdS thickness 22 was increased to 1.3 nm within the PbSCdS film-precursor and Redge distance of the PbSCdS matrix was raised from 0.64 nm to 2.7 nm. The thickness of the shell is inversely proportional to the charge trapping on the surface of the PbSCdS coreshell and the dissociation of the exciton (PbS-PbS) because of the potential barrier offered by the CdS NCs to both carriers (Fig. 3.7). If charge carriers being tunneled on the surface of the coreshell is considered as the main process of carrier trapping, then

Wentzel-Kramers-Brillouin (WKB) approximation49 can be used to calculate the probability of the trapping process, by means of single exponential dependence on the thickness of the shell, Γthick/Γthin

= exp(-ΔHthick)/exp(-ΔHthin), where the charge trapping rate is given by Γ. The lattice defects which are formed on the coreshell boundaries also contribute to the exciton dissociation. However, the characteristic time of the dissociation process is bigger than other trapping processes. The fluorescence lifetime measurement of the coreshell nanocrystals in solution (Fig. 4a, τ = 1120 µs) gives the lower limit of the charge trapping time of the interface. The magnitude of this lifetime is higher than the exciton lifetime of the PbS NCs bound in film by order of two. It is expected that the lattice structure of

Figure 3.7: The positions of the excited energy levels of PbS QDs and bulk CdS. zinc blend CdS and rock salt PbS (Fig. 3.3d, strain ≈ 1.7%) is approximately matched and it has caused a low probability of charge trapping on the interface of the PbSCdS coreshell. 23

The effects of the rising amount of ZnS NCs in the film of PbSCdS are demonstrated in figure

3.8 as the changes in the fluorescence intensity lifetime of PbSCdS NC film (Redge ≈ 2.7 nm). The fast decay component limited within PbS-PbS exciton transfer (Eq. 1) is 11.6 ns before adding ZnS nanoparticles. This value is remarkably higher than that of strongly coupled PbS nanoparticles in solids having Redge= 0.64 nm. The fast component, according to WKB approximation, is exponential with

Redge, exp(-0.64)/exp(-2.7) = 7.8. This value is approximately equal to the observed Fl lifetime ratio,

τ2.7 nm/τ0.64 nm = 11.6/1.6 = 7.25. The charge transfer mechanism is limited even more with the addition of the ZnS nanoparticles in the film, which caused the fast component to remain higher and lowering the amplitude (Fig. 3.8b). The fast decay component of the fluorescence decay curve in the

PbS NC solid with the value of Redge equal to 2.7 nm is ultimately overcome by the slow decay component like in case of Redge=0.64 nm. When the amount of ZnS nanoparticles within the NC films are constantly increased, the slow decay component is finally saturated at τFL, slow = 370 ns. At this point, trap states on the native surface are the only reason for exciton dissociation due to the strong suppression of the energy transfer and charge transfer on these NC solids, τtrapping (local) ≈ τFL, slow

= 370 ns. The charge trapping time in case of strongly coupled PbS nanocrystal films (Redge=0.64 nm) is nearly 87/370 = 1/(4.25) times that of weakly coupled PbS nanoparticle solids.

Charge transfer to the local trap states is further expected to be suppressed as a result of implementing SILAR mechanism in matrix encapsulated NC solids. The boundaries of the PbSCdS coreshells are in general defectless, (Γtrap(interfacial defects) < 1/450 ns), therefore the charge trapping must have resulted from the tunneling of the charges onto the local surface of the CdS NC matrix. By enhancing the size of the potential barrier that separates PbS localized charge carrier from trap states, the probability of the carrier trapping on surface could be reduced, for eg., more layers of Zns or CdS NCs could be deposited on the surface. Many layers of ZnS nanoparticles are deposited on the ZnS saturated 24 and ZnS non saturated PbS nanoparticle matrix through the SILAR mechanism to examine the result of such a surface treatment. The fluorescence lifetime of the slow decay component is increased by 35% by

Figure 3.8: (a). FL Intensity decay of weakly-coupled PbS NCs embedded into CdS matrices (Redge = 2.7 nm) with increasing amount of ZnS NCs in solid. (b). Development of the fast component of Flourescence decay with increasing amount of ZnS nanoparticles in solid. (c) Development of the slow component of flourescence decay with increasing amount of ZnS nanoparticles in solid. adding 6 monolayers of ZnS nanoparticles in case of no-ZnS NC films, where both global and local charge trapping is present (Fig. 3.9). Such intermediate rise of time for the charge trapping is the evidence for the existence of the global trap states. The SILAR mechanism on the other hand is more effective with the ZnS saturated NC films that have only local trap states. The slow decay component has three-fold rise, τFL, slow = 87 → 257 ns (Fig. 3.10b), which shows that the trap states at the local 25 surface are effectively passivated by the atomic layer deposition. But, the SILAR mechanism did not fully inhibit the fast component of the fluorescence intensity decay in the ZnS-riched PbS NC solids.

The existence of this fast component is further validated by the long range hopping process, which primarily depends upon interdot distance than passivation of the surface.

Figure 3.9: Flourescence decay of CdS encapsulated PbS nanoparticle solids (Redge = 0.64 nm) before (black) and after (red) the in filling.

Figure 3.10: (a). Sketch of SILAR method to fill the pore with ZnS. (b). The result of implementation of the SILAR mechanism on Fl decay of CdS encapsulated PbSZnS NC solids having lowered energy and charge transfer. The reduction of rate of charge trapping resulted in the higher fluorescence lifetime of the pore filled NC film.

The surface of the ligand-linked PbS nanocrystal films are passivated with the molecules having short chain. By changing the amount of ZnS NCs used in the NC solid, we investigated whether or not the both CdS encapsulated nanocrystal films and the NC film linked with ligands show the same exciton dissociation and charge trapping rates. For this particular study, the choice of ligands have been 26 narrowed down to three types : MPA, EDT, and a combination of MPA/Cl molecules. The materials have already been used as electric conducting surfactants in NC devices with excellent performance46.

First of all, the PbS NC (with MPA ligands) solids were fabricated using the spincoating process46. When ZnS nanoparticles are added into PbS NC (with MPA ligands) solids (d=3.2 nm), the charge transfer process is suppressed similar to the case of matrix encapsulated nanocrystal films. The fast decay component of fluorescence intensity lifetime of all PbS NC films before adding ZnS NCs was

1.04 ns whereas FL intensity lifetime of PbS NC films (with OA ligands) having non-polar solvents was

480 ns (Figures 3.12e, and 3.12h). The lower fluorescence lifetime intensity of PbS NCs obtained while transferring them from solution phase into a solid is further attributed to the resonant energy transfer, resonant charge transfer, charge trapping, and hopping. In principle, the carrier ionization can also be contributed by the photoinduced hole transfer of the MPA ligands. The rate of this process was not fast enough to be competent with other processes like energy transfer and charge transfer because of the fluorescence lifetime of PbS nanoparticles (with MUA ligands) in water/methanol solution, which is 220 ns (Fig. 3.11).

When the amount of ZnS nanoparticles in PbS dots linked with MPA ligands was increased, the fast component of the fluorescence intensity decay was enhanced. This enhancement also shows the reduction in the charge transfer between adjacent particles. The slow component of the fluorescence intensity decay of the charges trapped on the local trap states increases from τ = 35 ns (no-ZnS) solids

MPA (global + local traps) to a saturation level of τtrap = 60.5 ns (averaged over 3 films). This clearly indicates charge carrier decay only in local trap states. This latest value is about four times shorter

CdS than τtrap, local of matrix encapsulated PbS NC solids filled with ZnS NCs, which is τtrap = 257 ns. The results for the EDT-linked solids were also similar (Table 3.1), where the time for the charge trapping grew from 25 ns to 45 ns. The combination of MPA and halide anions recently was used as ligands on NC solids to develop one of the best efficient solar cell/photovoltaic devices. The halide 27 elements such as chlorine are dense enough to penetrate the difficult-to-access spots of NCs, which

Figure 3.11: The FL intensity decay curve of PbS nanoparticles capped with MUA (in methanol).

provide improved surface passivation and lead towards the decreased density of mid gap trap

states.27 The exciton dissociation dynamics in the PbS nanoparticle films with MPA/Cl ligands (Fig.

3.12f, 3.12j) was measured to study the effects of halide passivation. The fluorescence lifetime

intensity and FL quantum yield of the PbS nanoparticles has enhanced by 10% (Fig. 3.12c) and 20-

25% respectively when chlorine ligands were added on the surface of the NCs during the solution

growth process. For no-ZnS NC solids, the fast component (3.1 ns) and slow components (30 ns) of

the fluorescence intensity decay were represented by the biexponential character shown by the

MPA/Cl-passivated PbS nanoparticles. By adding ZnS NCs further, the later lifetime was enhanced

to 193 ns, which is the carrier decay time in local trap states. As expected, the lower rate of charge

trapping on the trap states (τtrap = 193 ns) was resulted from the use of Cl-capped NCs in

comparison to the MPA linked PbS nanocrystal solids, agreeing well with the estimate of the newly 28

developed diffusion model.50

Figure 3.12: (a-c). Fluorescence intensity curve of EDT passivated PbS nanoparticles in solution (a), MPA (b), MPA/Cl (c) different environment (d-f). Pl intensity decay of PbS nanoparticle solids with EDT (d), MPA (e), and MPA/Cl (f) Functions of raised amounts of ZnS NCs in the solid. (g-j) Fast and slow components Pl intensity lifetime of PbS NCs.

When the fluorescence lifetimes of the matrix encapsulated PbS nanoparticle films and ligand crossed PbS nanoparticle films with same interparticle distance are compared, an important trend is revealed. The rate of photoinduced charge trapping in CdS encapsulated PbS nanoparticles was found to be reduced with respect to ligand linked PbS NC solids. The observed charge transport characteristics are reviewed for a p-n junction for the better understanding of observed charge transport, where the carrier transport completely depends upon the drift in the depletion region and diffusion elsewhere.

The characteristics length of the diffusion and drift can be calculated by the equation given below. 29

ldrift = µEτ ; ldiffusion = √ Dτ (2)

Where, charge carrier mobility is represented by µ, diffusion coefficient by D, and the electric field by

E. In both no-ZnS NC films and ZnS saturated NC films, charge trapping is the main mechanism of charge scattering. The lifetime of the minority charge carrier is about the same as the trapping time τ ≈

τtrap. The carrier mobility (µ), and the diffusion coefficient (D) are required to be determined to find out the values of ldrift and ldiff for tested NC solids.

Einstein’s relation of mobility and diffusion can be used for a NC solid in the regime of hoping transport to determine the value of µ and D as follows:

µ = (ed2) ; D = µkT (3) 6kTτ e

where, d: center-center distance between PbS nanoparticles in a solid.

1/τ :cumulative charge carrier diffusion rate having charge transfer and energy transfer

It is considered that the charge carrier diffusion occurs with tunneling of the excitation energy on adjacent nanocrystals. This method, therefore, cannot be used to describe the band transport regime. The diffusion-limited carrier mobility in closest adjacent hopping approximation is represented by the resulting diffusion charge carrier mobility (µdiff). It should be noted that the µdiff is different than the field effect transistor (FET) mobility. By determining the charge carrier diffusion rate (1/τdiff) first, the

µdiff for the tested solids is estimated. The fast component of the fluorescence intensity decay of the

PbS nanoparticle solids was measured to achieve 1/τdiff..In the absence of ZnS nanoparticles, all the carrier transfer mechanisms occur. Hence the rate of the fluorescence intensity decay is given by:

Γ = Γ + Γ + Γ (Γ ;Γ ;Γ ) FL decay trapping rad diff tunneling VRH energy transfer (4)

where, the τdiff is characterized by the fast decay component. τdiff ≈ τex.dissociation 30

τFL, fast for no ZnS NC solids. The equation 3 is therefore reduced to

2 2 µ ed 1 µkT d diff = 6kT τ no ZnS ; D = e = 6τ no ZnS (5) FL , fast FL , fast

Hence, the carrier diffusion length becomes,

l = √ Dτ = d×√τ 6τ = d×√τ / 6τ × τ / 6τ (6) diffusion trap trap/ FL,fast FL,slow FL,fast FL,slow FL, fast

The slow component of the fluorescence intensity decay in both ZnS saturated films (local trap states only) and no ZnS (local and global trap states) in above equation is represented by τFL,slow.

The summary of the fluorescence intensity lifetime data for both type of films is presented in table

T3.1. As seen as in table T3.1, the suppressed charge trapping detected in case of matrix encapsulated

NC solids is the most important outcome of the all compared data. Specifically, the rates for the charge carrier trapping in strongly coupled PbS(CdS) nanocrystal solids are 1.5 to 4 times shorter than that of three different tested ligand linked NC films. PbS nanocrystal films passivated with Cl were also equally efficient to discourage carrier tunneling on local traps with the characteristic scattering time (τtrap = 193 ns) above that for MPA linked NC solids by the factor of 3. Hence it is indirectly evidenced from the above explanation that Cl passivated nanoparticle films and matrix encapsulated nanoparticle film reveal relatively lower densities of traps on surface. Alternatively, µdiff for complete inorganic nanocrystal solids was found to be 0.57*10-3 cm2/V/s, which is less than that for PbS nanocrystals with MPA ligands

(µdiff = 0.9*10-3 cm2/V/s). The electrical coupling of the CdS capped nanocrystal solids resulting from

PbS-PbS interdot distance has been found to be less than its maximum value stated previously corresponding Redge= 0.5 nm. If we compare Redge = 0.64 nm and Redge = 2.7 nm PbS/CdS solids

(lines 1 and 2 in Table T3.1), it can be concluded that Redge has important role in the resulting value of

µdiff. To use the semiconductor nanocrystal quantum dots in photovoltaic/solar cell applications, charge 31 mobility over 10−2 cm2/V/s does not benefit the performance of the device because of the significant exciton dissociation.46 It has been demonstrated from this study that the carrier trapping can be significantly lowered by employing the matrix encapsulation mechanism instead of cross linking method. It is certainly disclosed from the compared values of ldrift (Table T3.2) and ldiff (Table T3.1) that CdS caped NC films show larger scatter free charge travel in comparison to other type of NC films. 32

Table T3.1: FL Fluorescence intensity lifetime data and calculated transport properties of matrix- encapsulated and ligand-linked PbS NC films.

Num τtrap(ns) τtrap(ns) ldiff (nm) ber of τdiss(ns) global+loc local τslow; µ (cm2/V/s) Type of NC film the τfast, no al ZnS Sat (Eq. 4) films ZnS τslow; no glob+loc local tested ZnS CdS-encapsulated 2 1.6 65 257.0 0.57*10-3 9.8 20.1 PbS NCs (with (with (Redge = 0.64 nm) SILAR) SILAR)

CdS-encapsulated 2 11.6 171 370 1.8*10-4 9.0 13.3 PbS NCs (Redge = 2.7 nm)

MPA-linked 3 1.04 35 60.5 0.9*10-3 9.0 11.8 PbS NCs EDT-linked 1 3.4 25 45 0.2*10-3 4.2 5.7 PbS NCs Hybrid (MPA/Cl)-linked 1 3.1 33 193 0.3*10-3 5.1 12.3 PbS NCs

Table T3.2: Investigated drift scattering lengths for CdS matrix encapsulated and ligand linked PbS NC films

ldrift (local +global traps) ldrift (local traps only)

Type of NC film (nm*10-3*E (V/cm)) (nm*10-3*E (V/cm))

CdS-encapsulated PbS NCs

19.0 75.04

(Redge = 0.5 nm) CdS-encapsulated

PbS NCs 9.4 20.3

(Redge = 1.5 nm)

MPA-linked

15.0 25.9 PbS NCs

EDT-linked 1.9 3.5

PbS NCs

Hybrid (MPA/Cl)- 6.6 38.7 linked PbS NCs

33

CHAPTER 4. CONCLUSION

In conclusion, the characteristics of the exciton dissociation and carrier scattering mechanisms were examined in many PbS nanoparticle films employing the fluorescence lifetime spectroscopy, absorption spectroscopy, and TEM image techniques. Controlled amounts of ZnS NCs were inserted into the films of PbS quantum dots to investigate the scattering time for the charge carrier for each type of film. Both of the charge and energy transfer mechanisms between the PbS nanocrystals are suppressed by the addition of the insulator ZnS into the nanocrystal films, resulting the charge carrier to decay mostly on trap states. Afterward, the properties of the photoinduced charge carrier were measured separately from that of other processes of charge transfer and energy transfer by using the fluorescence lifetime technique. The exciton dissociation rate and charge trapping rate were investigated for many types of nanocrystal solids on the basis of the observed relaxation times, which includes both matrix encapsulation and cross linked methods. The scattering lengths and diffusion mobility for the different types of films in the hoping regime were determined using the observed rates of carrier decay again. The matrix encapsulate PbS nanocrystal solids, in general, demonstrated the smaller carrier trapping rate for the trap states and larger diffusion lengths than the crossed linked PbS NC solids. 34

REFERENCES

1 Murray, C. B.; Nirmal, M.; Norris, D. J.; Bawendi, M. Synthesis and Structural Characterization of II-VI Semiconductor Nanocrystallites (Quantum Dots). G. Z. Phys. D 1993, 26, S231–233.

2 Beard, M. C. Multiple Exciton Generation in Semiconductor Quantum Dots. J. Phys. Chem. Lett. 2011, 2, 1282–1288

3 Pal, B. N.; Ghosh, Y.; Brovelli, S.; Laocharoensuk, R.; Klimov, V. I.; Hollingsworth, J.; Htoon, H. ‘Giant' CdSe/CdS core/shell nanocrystal quantum dots as efficient electroluminescent materials: Strong influence of shell thickness on light-emitting diode performance. Nano Lett., 2012, 12, 331–336.

4 Kramer, I. J.; Levina, L.; Debnath, R.; Zhitomirsky, D.; Sargent, E. H. Solar cells using quantum funnels. Nano Lett. 2011, 11, 3701-3706.

5 Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.; Bawendi, M. G. Optical gain and stimulated emission in nanocrystal quantum dots. Science 2000, 290, 314-317.

6 Klimov, V. I. Mechanisms for Photogeneration and Recombination of Multiexcitons in Semiconductor Nanocrystals: Implications for Lasing and Solar Energy Conversion J. Phys. Chem. B 2006, 110, 16827- 16845.

7 Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I.; McGuire, J. A.; Piryatinski, A. Single-Exciton Optical Gain in Semiconductor Nanocrystals. Nature 2007, 447, 441-446.

8 Hillhouse, H. W.; Beard, M. C. Solar Cells from Colloidal Nanocrystals: Fundamentals, Materials, Devices, and Economics. Curr. Opin. Coll. Int. Sci. 2009, 14, 245-259.

9 Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Hybrid nanorod-polymer solar cells. Science 2002, 295, 2425-2427.

10 McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Solution-processed PbS quantum dot infrared photodetectors and photovoltaics. Nat. Mater. 2005, 4, 138-142.

11 Ellingson, R. J.; Beard, C. M.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Highly efficient multiple exciton generation in colloidal PbSe and PbS quantum dots. Nano Lett. 2005, 5, 865-871.

12 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 Lett. 2007, 7, 1793.

13 Maria, A.; Cyr, P. W.; Klern, E. J. D.; Levina, L.; Sargent, E. H. Solution-processed infrared photovoltaic devices with > 10% monochromatic internal quantum efficiency. Appl. Phys. Lett. 2005, 87, 213112.

14 Kim, S. J.; Kim, W. J.; Cartwright, A. N.; Prasad, P. N. Carrier multiplication in a PbSe nanocrystal and P3HT/PCBM tandem cell. Appl. Phys. Lett. 2008, 92, 191107.

35

15 Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Air-stable all-inorganic nanocrystal solar cells processed from solution. Science 2005, 310, 462-465.

16 Guo, Q.; Kim, S. J.; Kar, M.; Shafarman, W. N.; Birkmire, R. W.; Stach, E. A.; Agrawal, R.; Hillhouse, H. W. Development of CuInSe2 Nanocrystal and Nanoring Inks for Low-Cost Solar Cells. Nano Lett. 2008, 8, 2982–2987.

17 Leschkies, K. S.; Beatty, T. J.; Kang, M. S.; Norris, D. J.; Aydil, E. S. Solar Cells Based on Junctions between Colloidal PbSe Nanocrystals and Thin ZnO Films. ACS Nano 2009, 3, 3638– 3648.

18 Ma, W.; Luther, J. M.; Zheng, H. M.; Wu, Y.; Alivisatos, A. P. Photovoltaic Devices Employing Ternary PbSxSe1-x Nanocrystals. Nano Lett. 2009, 9, 1699–1703.

19 Riha, S. C.; Fredrick, S. J.; Sambur, J. B.; Liu, Y.; Prieto, A. L.; Parkinson, B. A. Photoelectrochemical Characterization of Nanocrystalline Thin-Film Cu2ZnSnS4 Photocathodes. ACS Appl. Mater. Interfaces 2011, 3, 58-66.

20 Mora-Sero, 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. J. Phys. Chem. C 2007, 111, 14889-14892.

21 Yu, P. R.; Zhu, K.; Norman, A. G.; Ferrere, S.; Frank, A. J.; Nozik, A. J. Nanocrystalline TiO2 solar cells sensitized with InAs quantum dots. J. Phys. Chem. B 2006, 110, 25451-25454.

22 Luther, J. M.; Law, M.; Song, Q.; Reese, M. O.; Beard, M. C.; Ellingson, R. J.; Nozik, A. J. Schottky Solar Cells Based on Colloidal Nanocrystal Films. Nano Lett. 2008, 8, 3488–3492. 23 Debnath, R.; Tang, J.; Barkhouse, D. A.; Wang, X.; Pattantyus-Abraham, A. G.; Brzozowski, L.; Levina, L.; Sargent, E. H. Ambient-Processed Colloidal Quantum Dot Solar Cells via Individual Pre-Encapsulation of Nanoparticles. J. Am. Chem. Soc. 2010, 132, 5952–5953.

24 Ma, W.; Swisher, S. L.; Ewers, T.; Engel, J.; Ferry, V. E.; Atwater, H. A.; Alivisatos, A. P. Photovoltaic Performance of Ultrasmall PbSe Quantum Dots. ACS Nano 2011, 5, 8140–8147.

25 Acharya, K. P.; Khon, E.; O’Conner, T.; Nemitz, I.; Klinkova, A.; Khnayzer, R. S.; Anzenbacher, P.; Zamkov, M. Heteroepitaxial Growth of Colloidal Nanocrystals onto Substrate Films via Hot-Injection Routes. ACS nano 2011, 5, 4953-4964.

26 Wang, Q.; Zhu, K.; Neale, N. R.; Frank, A. J. Constructing Ordered Sensitized Heterojunctions: Bottom-Up Electrochemical Synthesis of p-Type Semiconductors in Oriented n-TiO2 Nanotube Arrays. Nano Lett. 2009, 9, 806-813.

27 Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; Kemp, K. W.; Kramer, I. J.; Ning, Z.; Labelle, A. J.; Wei Chou, K.; Amassian, A.; Sargent, E. H. Hybrid passivated colloidal quantum dot solids. Nature Nanotechnology 2012, 7, 577-582 28 Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Electroluminescence from single monolayers of nanocrystals in molecular organic devices. Nature 2002, 420, 800-803.

29 Ridley, B. A.; Nivi, B.; Jacobson, J. M. All-inorganic field effect transistors fabricated by printing. Science 1999, 286, 746-749. 36

30 Lee, S.; Jeong, S.; Kim, D.; Park, B. K.; Moon, J. Fabrication of a solution-processed thin-film transistor using zinc oxide nanoparticles and zinc acetate J. Superlattices Microstruct. 2007, 42, 361-368.

31 Schneider, J. J.; Hoffmann, R. C.; Engstler, J.; Soffke, O.; Jaegermann, W.; Issanin, A.; Klyszcz, A. A. A printed and flexible field-effect transistor device with nanoscale zinc oxide as active semiconductor material. Adv. Mater. 2008, 20, 3383-3387.

32 Talapin, D. V.; Mekis, L.; Gotzinger, S.; Kornowski, A.; Benson, O.; Weller, H. CdSe/CdS/ZnS and CdSe/ZnSe/ZnS core-shell-shell nanocrystals. J. Phys. Chem. B 2004, 108, 18826-18831.

33 Talapin, D. V.; Murray, C. B. PbSe Nanocrystal Solids for n- and p-Channel Thin Film Field- Effect Transistors. Science 2005, 310, 86-89.

34 Lee, J. S.; Shevchenko, E. V.; Talapin, D. V. Au-PbS Core-Shell Nanocrystals: Plasmonic Absorption Enhancement and Electrical Doping via Interparticle Charge Transfer. J. Am. Chem. Soc. 2008, 130, 9673-9675.

35 Lee, J-S; Kovalenko, M. V.; Huang, J.; Chung, D. S.; Talapin, D. V. Band-Like Transport, High Electron Mobility and High Photoconductivity in All-Inorganic Nanocrystal Arrays. Nat. Nanotech. 2011, 6, 348−352.

36 Guyot-Sionnest, P. Electrical Transport in Colloidal Quantum Dot Films. J. Phys. Chem. Lett. 2012, 3, 1169−1175. 37 Tang, J.; Sargent, E. H. Infrared Colloidal Quantum Dots for Photovoltaics: Fundamentals and Recent Progress. Adv. Mater. 2011, 23, 12–29.

38 Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of colloidal nanocrystals for electronic and optoelectronic applications. Chem. Rev. 2010, 110, 389-458. 39 Gai, I.; Peng, H.; Li, J. Electronic Properties of Nonstoichiometric PbSe Quantum Dots from First Principles. J. Phys. Chem. C 2009, 113, 21506–21511.

40 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. 2011, 10, 765– 771.

41 Liu, Y.; Gibbs, M.; Perkins, C. L.; Tolentino, J.; Zarghami, M. H.; Bustamante, J.; Law, M.Robust, Functional Nanocrystal Solids by Infilling with Atomic Layer Deposition. Nano Lett. 2011, 11, 5349– 5355.

42 Mashford, B.; Baldauf, J.; Nguyen, T. L.; Funston, A. M.; Mulvaney, P. Synthesis of quantum dot doped chalcogenide glasses via sol-gel processing. J. Appl. Phys. 2011, 109, 094305.

43 Kinder, E.; Moroz, P.; Diederich, G.; Johnson, A.; Kirsanova, M.; Nemchinov, A.; O’Connor, T.; Roth, D.; Zamkov, M. Fabrication of All-Inorganic Nanocrystal Solids through Matrix Encapsulation of Nanocrystal Arrays. J. Am. Chem. Soc. 2011, 133, 20488-20499.

44 Khon, E.; Lambright, S.; Khon, D.; Smith, B.; O’Connor, T.; Moroz, P.; Imboden, M.; Diederich, 37

G.; Perez-Bolivar, D.; Anzenbacher, P.; Zamkov, M. Inorganic solids of CdSe nanocrystals exhibiting high emission quantum yield. Adv. Funct. Mater. 2012, 22, 3714-3722.

45 Choi, J.-H.; Fafarman, A. T.; Oh, S. J.; Ko, D.-K.; Kim, D. K.; Diroll, B. T.; Muramoto, S.; Gillen, J. G.; Murray, C. B.; Kagan, C. R. Bandlike Transport in Strongly Coupled and Doped Quantum Dot Solids: A Route to High-Performance Thin-Film Electronics. Nano Lett. 2012, 12, 2631– 2638.

46 Pattantyus-Abraham, A. G.; Kramer, I. J.; Barkhouse, A. R.; Wang, X.; Konstantatos, G.; Debnath, R.; Levina, L.; Raabe, I.; Nazeeruddin, M. K.; Gratzel, M.; Sargent E. H. Depleted- Heterojunction Colloidal Quantum Dot Solar Cells. ACS Nano 2010, 4, 3374-3380.

47 Chandler, R. E.; Houtepen, A. J.; Nelson, J.; Vanmaekelbergh, D.: Electron transport in quantum dot solids: Monte Carlo simulations of the effects of shell filling, Coulomb repulsions, and site disorder. Phys. Rev. B 2007, 75, 085325-10.

48 Zhitomirsky, D.; Voznyy, O.; Hoogland, S.; Sargent, E. H. Measuring Charge Carrier Diffusion in Coupled Colloidal Quantum Dot Solids. ACS Nano, ACS Nano, 2013, 7 (6), pp 5282–5290.

49 Chandler, R. E.; Houtepen, A. J.; Nelson, J.; Vanmaekelbergh, D. Electron Transport in Quantum Dot Solids: Monte Carlo Simulations of the Effects of Shell Filling, Coulomb Repulsions, and Site Disorder. Phys. Rev. B 2007, 75, 085325-10. 50 Zhitomirsky, D.; Voznyy, O.; Hoogland, S.; Sargent, E. H. Measuring Charge Carrier Diffusion in Coupled Colloidal Quantum Dot Solids. ACS