Energy Transport in Colloidal Inorganic Nanocrystals

ENERGY TRANSPORT IN COLLOIDAL INORGANIC NANOCRYSTALS

Mingrui Yang

A Dissertation

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

DOCTOR OF PHILOSOPHY

May 2021

Committee:

Mikhail Zamkov, Advisor

Jenny Toonstra Graduate Faculty Representative

Liangfeng Sun

Alexey Zayak

Mingrui Yang

iii

ABSTRACT

Mikhail Zamkov, Advisor

Excitonic energy transfer (ET) represents the primary step of energy conversion during

photosynthesis and is the key mechanism of the energy flow in excitonic solids and organic crystals.

Unlike the charge-mediated energy transfer in bulk semiconductors, energy transfer in most

nanoscale systems proceeds through the electrically excitons transport. The donor and acceptor

can be any optical materials such like quantums, metal ions and organic crystals. Here, we

demonstrate a strategy that can be utilized as a simple post synthetic procedure for controlling the

surface chemistry, adjusting the average particle size and reducing the particle size dispersion of

semiconductor colloids. The low dispersion of nanocrystal shapes can facilitate the energy transfer

efficiency between donor and acceptor nanoscale materials. After making particles uniform, we

show the energy transport in both metal and semiconductor systems. Typically, the interaction of

molecular fluorophores with surface plasmons in metals result in either quenching or enhancement

of the dye excitation energy, we demonstrate that fluorescent molecules can also engage in a

reversible energy transfer with proximal metal surface, the quenching of the dye emission via the

energy transfer to localized surface plasmons can trigger delayed ET from metal back to the

fluorescent molecule. The resulting two-step process leads to the sustained delayed

photoluminescence (PL) in metal-conjugated fluorophores. In the meantime, artificial solids of

CsPbBr3 perovskite nanocrystals are well known for their promising charge transport characteristics, we show that the same set of electronic properties allows CsPbBr3 NC solids to act

as superior energy transport materials, which support a long-range diffusion of electrically neutral

excitons. By performing time-resolved bulk quenching measurements on halide-treated CsPbBr3

NC films, we observed average exciton diffusion lengths of 52 and 71 nm for I−- and Cl− -treated iv solids, respectively. Steady-state fluorescence quenching studies have been employed to explain such a large diffusion length as due to a high defect tolerance and a low disorder of exciton energies in CsPbBr3 NC solids. The long-range exciton transport ability of halide treated CsPbBr3 NC solids could be beneficial for applications in light energy concentration, as was demonstrated in this work through energy transfer measurements in assemblies of perovskite NC donors and CdSe quantum dot acceptors.

ACKNOWLEDGMENTS

Foremost, I would like to express my sincere gratitude to my advisor Dr. Mikhail Zamkov for his patience, motivation, enthusiasm, and immense knowledge. His guidance helped me in all the time of research and writing of this dissertation. Without his continuous support of my Ph.D. study and research in the past few years, it would be impossible for me to complete my study. I would also like to thank my dissertation committee members: Dr. Liangfeng Sun, Dr. Alexey

Zayak and Dr. Jenny Toonstra, for their encouragement and insightful comments.

I would like to offer my special thanks to my labmates in our group: Pavel Moroz, Natalia

Razgoniaeva, Natalia Kholmicheva, Dmitry Porotnikov, James Cassidy, Dulanjan Harankahage,

Nida Sundrani, Emily Miller, Holly Eckard, Cole Ellison and Jacob Bettinger, for their assistance at every stage of the research projects, the stimulating discussions, the insightful comments and suggestions and their unwavering support and belief in me. This work could not be completed without my lab mates who over the course of this journey became good friends of mine.

Last but not the least, I would like to express my gratitude to my family members, for their love, understanding, prayers and continuing support to help me all the time during my Ph.D. study and complete this research work.

TABLE OF CONTENTS

Page

INTRODUCTION ...... 1

CHAPTER I. SIZE FOCUSING OF SEMICONDUCTOR NANOCRYSTALS ...... 5

1.1 Introduction ...... 5

1.2 Size focusing and coalescence of CdS nanocrystals ...... 7

1.2.1 Key factors in growing uniform semiconductor nanocrystals ...... 7

1.2.2 Size focusing and coalescence in DR process ...... 13

1.3 Ion-mediated ligand exchange and size focusing of CdSe nanocrystals ...... 20

1.3.1 Size focusing with two different sizes CdSe ...... 20

1.3.2 Ligand exchange during DR process ...... 23

1.4 Conclusion ...... 32

CHAPTER II. ENERGY TRANSPORT IN INORGANIC NANOCRYSTALS ...... 34

2.1 Reverse energy transfer in metal-conjugated fluorophores ...... 34

2.1.1 Introduction ...... 34

2.1.2 Delayed photoluminescence in metal-conjugated fluorophores ...... 37

2.1.3 STEP measurements of the reverse energy transfer ...... 48

2.1.4 Model calculations for the conjugated systems ...... 57

2.2 Energy transport in CsPbBr3 perovskite nanocrystals solids ...... 63

2.2.1 Introduction ...... 63

2.2.2 Homogeneous energy transfer in assemblies of perovskites ...... 67

2.2.3 Heterogeneous energy transfer in CsPbBr3 with CdSe ...... 78

2.3 Conclusion ...... 86 vii

CONCLUSION...... 89

REFERENCES ...... 92

APPENDIX A. COPYRIGHT PERMISSION ...... 133 viii

LIST OF FIGURES

Figure Page

1 Possible mechanisms of excitonic energy transfer involving semiconductor

nanocrystals ...... 2

1.1 Illustration of the digestive ripening process in samples of CdS semiconductor

nanocrystals ...... 9

1.2 The absorption profile changes of 4.6 nm CdS430 nanocrystals observed in the

solvent-only reaction mixture (octadecene) ...... 10

1.3 Effect of heating 4.6 nm CdS NCs in a OLAM/ODE mixture for 60 min at T = 260

℃...... 11

1.4 A comparison of the digestive ripening dynamics in oleic acid and oleylamine

ligand environments...... 12

1.5 Exploring the mechanism of digestive ripening using a blend of 3 and 14 nm CdS

nanocrystals in a OLAM/ODE = 60:40 ligand/solvent reaction mixture ...... 15

1.6 Digestive ripening of CdS semiconductor nanorods via the intraparticle growth ...... 17

1.7 TEM analysis of the reaction temperature effect on digestive ripening dynamics of

CsPbBr3 and CdS NCs ...... 19

1.8 OLAM-induced digesting ripening of a CdS470 + CdSe520 nanocrystal mixture

containing two sizes of CdSe nanocrystals (d = 2.1 nm and d = 2.6 nm,

respectively) ...... 21

1.9 The evolution of the emission spectral profile during OLAM-induced digesting

ripening of a CdS470 + CdSe520 nanocrystal mixture (d = 2.1 nm and 2.6 nm,

respectively) ...... 22 ix

1.10 The mechanism of ligand displacement and the changes in absorption and PL

lifetime spectra...... 24

1.11 X-ray powder diffraction analysis and NMR (1H-NMR) spectra ...... 25

1.12 The absorption profile of OLAM-ripened CdSe NCs before and after recapping

with Cd(O2CR)2 (blue and red curves, respectively) ...... 27

1.13 Oleic acid-induced digesting ripening of a CdSe470 + CdSe520 nanocrystal mixture

containing two sizes of CdSe nanocrystals (d = 2.1 nm and d = 2.6 nm)...... 28

1.14 The absorption profile of OA-ripened CdSe NCs before and after recapping with

Cd(O2CR)2 (blue and red curves, respectively)...... 30

1.15 Energy dispersive X-ray characterization, Changes in the exciton absorption and the

proposed scheme of surface ligand exchanges in CdSe NCs corresponding to a

consecutive application of OA and OLAM DR treatments...... 31

2.1 Illustration of the forward (dye → AuNP) and reverse (AuNP → dye) energy

transfer ...... 36

2.2 Synthesis, TEM image and size distribution of Au NPs ...... 39

2.3 Absorption, emission and PL lifetime of three Dye-AuNP assemblies ...... 40

2.4 Synthesis and measurements of dye-AuNP conjugates ...... 41

2.5 Summary of ultrafast transient absorption (TA) measurements ...... 43

2.6 The summary of transient absorption measurements ...... 45

2.7 Spectrally resolved transient absorption measurements of LA-PIMA-PEG-capped

Au NPs showing a long-lived photoinduced absorption feature (positive ΔA) ...... 48

2.8 Illustration of the STEP technique for measurements of the Au → dye energy

transfer efficiency ...... 50 x

2.9 The experimental protocol for STEP measurements ...... 52

2.10 Comparison of surface plasmon electric field amplitudes for aligned and

isotopically oriented dye molecules and the modulation of PL lifetime based on the

rate equations ...... 55

2.11 Fitting the delayed PL in Alexa 488 - AuNP assemblies (purple circles) with the

solution of coupled rate equations (Eqs. 3 & 4) based on the reverse ET model

(purple curve)...... 61

2.12 Δt contour plot showing possible changes in the PL lifetime of a metal-conjugated

dye (Δt= tdye-metal/tdye) as a function of kET and kPIRET rates, respectively ...... 63

2.13 The steady-state emission of CsPbBr3 NC solids blended with Au nanoparticles ...... 65

2.14 Spatial profile of exciton diffusion in CsPbBr3 QD solids ...... 67

2.15 Illustration of the fluorescence (bulk)-quenching approach for estimating the

exciton diffusion length in CsPbBr3 NC solids, which relies on blending

investigated films with FL-quenching Au nanoparticles...... 69

2.16 Illustration of the general strategy for determining the effective mean free path of

excitons in a Au-doped nanocrystal solid ...... 71

2.17 TEM images and changes of the FL intensity decay corresponding to the increasing

concentration of Au NPs in CsI and CdCl2 treated (Au, CsPbBr3) solids ...... 73

2.18 The photoluminescence and absorbance of CsPbBr3 NC solids without surface

treatment and after being treated with CsI and CdCl2 molecular halides...... 74

2.19 Illustration of the exciton population for CsI- and CdCl2- treated CsPbBr3 NC

solids...... 75 xi

2.20 Illustration of the STEP technique for measurements of the CsPbBr3 with CdSe

energy transfer efficiency ...... 79

2.21 The excitation scheme used in CsPbBr3 to CdSe STEP measurements ...... 82

2.22 The experimental protocol for CsPbBr3 to CdSe STEP measurements...... 83

2.23 The energy diagram comparing the alignment of conduction and valence band

edges for 8-nm CsPbBr3 (left) and 4-nm CdSe (right) nanocrystals, reconstructed

from the Ref. [256] ...... 86

xii

LIST OF TABLES

Table Page

1 Summary of Model Calculations for the Three Conjugated Systems, Showing

-1 -1 -1 k spon, k ET and k PIRET Inverse Rates ...... 59

2 Summary of Exciton Transport Characteristics for CsPbBr3 Nanocrystal Solids

Obtained Using the Fluorescence (Bulk) Quenching Approach ...... 76

3 Summary of FL Lifetime and STEP Measurements for (CdSe, CsPbBr3) Blended Solids

...... 84 1

INTRODUCTION

Unlike the solvent-mediated relaxation of charge transfer in bulk semiconductions, energy transfer in most nanoscale systems proceeds through the electrically neutral excitons transport.

Cascade-like exciton transport is the first step in energy conversion in photosynthesis[1] and is the

primary mechanism of energy flow in excitonic solids or organic crystals. The donor and acceptor

in the energy transport systems can be any optical materials such like quantum dots, metal ions

and organic crystals. This process including both radiative and nonradiative mechanism can be

summarized by a chemical reaction, �∗ + � = � + �∗ , where the asterisk indicates the excited state of the ion or molecules. These energy transfer systems have enabled a wide development of technological applications in solar energy production, near-field optical imaging, sensing, photovoltaic[2]-[14] and solid-state light-emitting.[2],[15]-[22] For instance, The interaction of

fluorescent dyes with proximal metal surfaces can result in either the enhancement[23]-[27] or

quenching28-37 of the dye photoluminescence which determined by the specific nature of electric dipole−dipole interactions at the molecule−metal interface that can shift the energy transfer (ET) balance in either direction. Such metal-induced quenching of the dye PL has recently evolved into a popular strategy of signal transduction in biosensing[38]-[47]and near-field imaging

applications.[38], [48]-[50]The realization of the metal-enhanced fluorescence,[51]-[60] on the other hand,

the excitation energy in a metal could be transferred to a proximal dye through the plasmon-

induced resonant energy transfer (PIRET),[26],[61]-[65] also known as the field enhancement mechanism.[56],[66]-[68] Examples of the PIRET process in dye−metal assemblies have been

witnessed in photocatalytic,[65],[68]-[76] photovoltaic,[77]-[83] and biosensing[38]-[40],[43],[51],[84]

applications of metal nanoparticles.

Semiconductor nanocrystals, also known as colloidal quantum dots, have optical and 2

electronic properties that intermediate between bulk semiconductor and discrete atoms or

molecules, these materials are famous as highly efficient fluorophores with strong luminescence

which are tunable by their own size. Semiconductor NCs are well known as an artificial system

that could support energy transport process and engage in a variety of energy transfer process with

other nanoscale materials such as metal nanocrystals, other semiconductors and organic molecules.

(Figure 1)

Figure 1 Possible mechanisms of excitonic energy transfer involving semiconductor nanocrystals.

For example, the assembly of semiconductor nanocrystals with surface-anchored molecules

are one of the well-known energy transport systems.[86]-[88] By modulating the photoluminescence intensity, the excitonic energy transfer in such assemblies could provide widespread applications in the area of biology and biochemistry.[89]-[93] On the other hand, the assembly of semiconductor nanocrystals with metal nanocrystals has also shown the ability to support photoinduced energy 3

transfer. The interactions of semiconductors with proximal metal surfaces can result in either the

enhancement of the semiconductor photoluminescence that represents a popular strategy to

increase the optical extinction of photocatalytic and photovoltaic absorbers[94]-[96] or the quenching

of the semiconductor photoluminescence which could be used as a biosensing strategy.[97]-

[99]Similar to molecular solids,[100] ET processes in semiconductor nanocrystal films are mediated

by quasi dipole-dipole coupling[101] and proceed via an interparticle transfer of neutral excitons, and the interparticle distance is tunable via the use of different binding motifs. which represents a potentially useful mechanism of energy concentration in photovoltaic,[102]-[106] solid-state lighting,[107]-[110] and photocatalytic[111],[112] applications.

Here, we discuss the energy transport in colloidal inorganic nanocrystals, including both

metal and semiconductor systems, we report on the experimental observation of the reversible

energy transfer in assemblies of Au nanoparticles and fluorescent molecules (Alexa 488, Cy3.5,

Cy5). The present study reveals that quenching of the dye emission via FRET or NSET to localized

surface plasmons in Au nanoparticles is followed by the backward transfer of metal excitations to

dye molecules. Such a delayed repopulation of the dye excitation energy was manifested in this

work through the observation of an increased PL lifetime of Au-conjugated dyes in comparison to

nonconjugated molecules. Ultrafast transient absorption measurements have confirmed the

delayed rise of the excitation energy in Au-conjugated fluorophores, which was temporally

correlated with the excited-state decay in polymer-capped Au nanoparticles. Notably, relative

amplitudes of the reverse metal → dye ET in each of the three investigated samples were found

proportional to the respective metal induced PL enhancement factors, determined using

polarization-averaged sample transmitted excitation photoluminescence (STEP) spectroscopy.

Theoretical calculations based on the reversible ET model were subsequently employed to explain 4

the observed PL lifetime enhancement in dye−AuNP assemblies, suggesting that such processes

could be ubiquitous in many other dye−metal systems.

The low dispersion of nanocrystal shapes can facilitate the energy transfer efficiency

between semiconductor nanocrystals. we demonstrate that ligand-mediated digestive ripening of

semiconductor nanocrystals can be used to control both the average particle size and the ensuing

size dispersion. A set of control experiments has confirmed that such behavior was the result of

two competing processes that included particle coalescence and the concurrent size focusing. After

making particle uniform, we discuss the energy diffusion processes in electrically coupled CsPbBr3

nanocrystal solids. By blending CsPbBr3 NC films with fluorescence (FL)-quenching Au nanoparticles, we were able to deduce the exciton diffusion length of ldiff ≈ 52−71 nm. Steady-state

FL-quenching measurements were able to attribute such a large exciton diffusion volume to a low

disorder of exciton energies in perovskite QD films, which was found to be less than 5 meV at

room temperature. In addition to the homogeneous energy transfer in assemblies of perovskite

QDs, we have explored the possibility of inducing heterogeneous ET processes in a blended film

of CsPbBr3 and CdSe NCs and indicate that perovskite solids could be developed to support a two- step energy concentration process, where a cascade-like energy transfer across the population of

CsPbBr3 NCs is followed by the delivery of a photoinduced charge to incorporated CdSe acceptors.

CHAPTER I. SIZE FOCUSING OF SEMICONDUCTOR NANOCRYSTALS

1.1 Introduction

Colloidal semiconductor nanocrystals (NCs) have attracted considerable attention as possible candidates for developing a wide variety of optoelectronic materials.[121],[122] The

synthesis of colloidal semiconductor nanocrystals (NCs) has been continuously perfected over the

course of several decades.[123]-[125] The primary goal behind these synthetic efforts was aimed at achieving a narrow distribution of nanoparticle sizes, which pertains to many useful characteristics of colloidal nanocrystals and their solids.[126]-[132] For instance, the low dispersion of nanocrystal

shapes can facilitate the assembly of inorganic colloids into superlattices[133] or reduce the dispersion of excited state energies across the film.[134] The fundamental advances resulting from

an improved particle homogeneity could be harnessed toward diverse applications, including

enhancing the color purity in light-emitting applications,[135],[136] increasing the charge extraction efficiency in photovoltaic devices,[137]-[139] or processing of nanocrystals into long-range ordered solids exhibiting excitonic bands.[140]

Controlling the nucleation rate has been recognized as one of the most effective tools for

achieving a narrow distribution of nanoparticle sizes during growth.141,142 At the current state of the art, hot injection strategies can be used to obtain nanocrystal samples with the standard size deviation within a 5-10%[126],[143],[144] range, which could be further reduced through the size- selective precipitation techniques.[145]-[147] It is, however, a highly parametrized strategy that

requires timing the precursor decomposition to achieve size focusing.[143],[148]-[154] In addition to

nucleation-based approaches, a narrow 3-5% particle size dispersion was recently realized[155]

through a sequential application of the colloidal atomic layer (c-ALD)[156]deposition for a stepwise synthesis of half-monolayers. This method, however, required a rather laborious synthetic protocol. 6

Considering the complex nature of hot-injection strategies, post-synthetic treatments have

been developed as alternative methods for improving the particle homogeneity.[153] Among those, digestive ripening (DR) has emerged as a facile strategy for narrowing the size distribution of metal colloids. First proposed in 2000 by Lin, Sorensen, and Klabunde,[157] the DR process is typically initiated by the addition of free ligand molecules to the nanoparticle solution at a relatively high concentration and is catalyzed by the application of heat.[158],[159] The increase in the solvent temperature results in a ligand mediated dissolution of larger-diameter nanocrystals, which leads to size focusing. In this regard, the DR mechanism is sometimes viewed as being opposite to the processes of Ostwald ripening,[160] where small particles, placed in a ligand deprived environment, dissolve in favor of larger ones. Thus, far, the DR method has been successfully applied to narrowing the size dispersion of many metal colloids, including Au,[161],[162]

Ag,[163] Cu,[164] and Ru,[165] as well as metal oxides[166] and semiconductor clusters.[167] The

application of digesting ripening to semiconductor nanocrystal colloids, however, has been limited

to a few early papers[168],[169] that reported an improved size dispersion in treated CdSe NCs.

Here, we demonstrate that ligand-mediated digestive ripening of semiconductor nanocrystals can be used to control both the average particle size and the ensuring size dispersion.

In contrast to the size-focusing mechanism of metal nanoparticles, the digestive ripening of semiconductor nanocrystals was found to be accompanied by a significant increase in the average particle diameter at temperatures above a certain thermal threshold, Tth = 200-220℃. A set of control experiments has confirmed that such behavior was the result of two competing processes that included particle coalescence and the concurrent size focusing.[170] Both reactions were initiated by the addition of free ligand molecules, such as oleylamine (OLAM) or oleic acid (OA) to nanocrystal solutions at 160-260℃, which promoted ion solubility and interparticle monomer 7

exchange. Notably, the coalescence process was prevalent only if the reaction temperature was

greater than Tth. The existence of such an activation threshold for a particle coalescence allowed controlling the ultimate size of semiconductor nanocrystals through the DR treatment, a functionality not easily available in the case of metal nanoparticles.[171] In this work, the digestive

ripening approach was demonstrated for CdS, CdSe, ZnSe, CsPbBr3, and CuSnZnS4 nanocrystals.

One of the unexpected outcomes of size-focusing experiments was the observation that digestive

ripening enabled complex ligand exchange processes that required restructuring of nanocrystal

surfaces. In particular, the digestive ripening treatment has been shown to promote a classically

forbidden L → X ligand exchange[172] in CdSe NCs, which was made possible due to the desorption of surface Se, mediated by ligand saturated. Overall, we expect that the demonstrated strategy can be utilized as a simple post synthetic procedure for controlling the surface chemistry, adjusting the average particle size and reducing the particle size dispersion of semiconductor colloids.

1.2 Size focusing and coalescence of CdS nanocrystals

1.2.1 Key factors in growing uniform semiconductor nanocrystals

In the case of metal nanoparticles, digestive ripening is usually initiated with the addition of ion-solubilizing ligand molecules into the reaction solvent, such as toluene or octadecene. In a ligand-rich environment, nanocrystals attain an enhanced level of fluidity that allows annealing and rearrangement of lattice atoms.[171] These processes are catalyzed by heating the solution to

60−120 °C, which preferentially affects small-diameter nanoparticles featuring lower melting

temperatures compared to their bulk.[173] Refluxing of the reaction mixture for several hours results

in reshaping of nanoparticles toward a narrower size distribution.

In light of the limited literature available on the DR process in semiconductor colloids, we

have chosen to explore the fundamental effects of ligand-mediated ripening in well investigated 8

examples of semiconductor NCs, including cadmium chalcogenides and CuZnSnS4 (CZTS) quantum dots.

4.6 nm CdS NCs featuring size distribution of ≈6% were investigated first (Figure 1.1b).

To this end, as-prepared nanoparticles were cleaned and reloaded into a flask containing an octadecene (ODE) solvent. Without the addition of free ligands, heating of the reaction mixture to

260 ℃ did not produce significant changes in the nanoparticle shape even after several hours (see

Figures 1.1 and 1.2). Some mild size increase, tentatively attributed to Ostwald ripening, could be observed after 7 h of heating (Figure 1.1). The addition of oleylamine ligands to the ODE solution of nanocrystals (OLAM: ODE = 60:40), on the other hand, has enabled a rapid growth of CdS nanoparticles causing the average diameter to reach 18.1 nm in just 4 h. The final product exhibited a narrow size distribution of less than 3%. 9

Figure 1.1 Illustration of the digestive ripening process in samples of CdS semiconductor nanocrystals. (a). ATransmission Electron Microscopy (TEM) image and statistical analysis of the particle size distributions of a control sample of CdS NCs after heating the original 4.6 nm CdS seeds for 7 h in octadecene (no ligands were added). A minor 4.6 nm → 4.7 nm size increase was observed. (b) TEM image of the original CdS NC sample prior to heating. (c). TEM image of CdS

NC after heating for 4 h in the presence of oleylamine ligand (OLAM: ODE =60:40). A significant size growth was attributed to digestive ripening. (d). A scheme summarizing the differences between digestive and Ostwald ripening processes. Adapted with permission from ref. [180].

Figure 1.2 The absorption profile changes of 4.6 nm CdS430 nanocrystals observed in the solvent-only reaction mixture (octadecene). The absence of ligand molecules in solution suppresses the digestive ripening process resulting in negligible transformations of a particle shape during heating (absorption changes during the 1st hour of heating). Adapted with

The particle diameter evolution accompanying the digesting ripening of 4.6 nm CdS NCs

was inferred from the absorption profile changes in Figure 1.3e. The addition of OLAM ligands to

the ODE solution of CdS NCs (OLAM/ODE = 60:40) signified the start of the nanoparticle growth

as was evident by the characteristic red shift of the exciton absorption feature.

The growth rate appeared to be dependent both on the ligand concentration and the

concentration of CdS seeds. For instance, the 60:40 OLAM/ODE reaction mixture promoted a

notably faster CdS particle growth than in the case of 10:90 OLAM/ODE combination, as evident 11 from the TEM size analysis in Figures 1.3a-d. Likewise, a greater concentration of CdS seeds has given rise to a product with a relatively larger diameter (Figures 1.3a, 1.3c). Along these lines, 24.2 nm CdS particles could be grown from 4.6 nm CdS seeds via the DR process in just 60 min provided that [OLAM] = 60% and [CdS430] = 2 g/L; however, in the case of lower ligand and seed concentrations, [OLAM] = 20% and [CdS430] = 1 g/ L, only a moderate increase d = 4.6 → 8.7 nm was observed after 9 h of refluxing. In all cases, the dispersion of particle diameters was found to decrease with heating time. Based on averaging the results of several experiments, it was concluded that the CdS nanocrystal growth saturated at d ≈ 25 nm, regardless of the ligand concentration. Upon reaching this size, the product became prone to precipitation and destabilization.

Figure 1.3 Effect of heating 4.6 nm CdS NCs in a OLAM/ODE mixture for 60 min at T = 260 ℃.

TEM images illustrate the effect of ligand and CdS NC concentrations, [OLAM] and [CdS430], 12

respectively, on the ultimate shape of nanocrystals. (a). [OLAM] = 10%, [CdS430] = 1 g/L. (b).

[OLAM] = 60%, [CdS430] = 1 g/L. (c). [OLAM] = 10%, [CdS430] = 2 g/L. (d). [OLAM] = 60%,

[CdS430] = 2 g/L. The scale bars are 20 nm. (e). The evolution of the CdS NCs absorption profile resulting from heating of 4.6 nm CdS seeds in a 60:40 OLAM: ODE mixture for 60 min. [CdS430]

= 2 g/L. (f). The evolution of the exciton absorption in CdS NCs accompanying the digestive ripening process at different reaction conditions. Adapted with permission from ref. [180].

Replacing OLAM with oleic acid (OA) as a DR-initiating ligand has produced similar increases in the average nanoparticle size at T = 260 ℃ (see Figures 1.4c, 1.4d) but led to predominantly cubic shaped nanostructures (Figure 1.4d). The splitting of the exciton absorption feature into two was also observed during the OA-initiated DR process (Figures 1.4b).[174] The apparent difference in nanoparticle shapes between OLAM and OA induced ripening, however, was not associated with the difference in the crystalline structure, which, in both cases, was wurtzite (Figure 1.4a). Notably, the original CdS seeds inhibited a zinc blende crystal structure

(Figure 1.4a, red curve), suggesting the DR-triggered phase transition of the CdS lattice.

Figure 1.4 A comparison of the digestive ripening dynamics in oleic acid and oleylamine ligand environments. (a). X-ray powder diffraction analysis of CdS NCs produced through digestive 13 ripening in OA/ODE = 60:40 (blue) and OLAM/ODE = 60:40 (green) ligand/solvent reaction mixtures. Both structures exhibited a characteristic wurtzite lattice pattern. The Bragg lines of the original 4.4 nm CdS NCs (red) indicated a zinc blende crystal structure. (b). The evolution of the

CdS NC absorption profile resulting from digestive ripening in a OA/ODE = 10:90 mixture at T =

260 ℃. (c, d). TEM images of CdS NCs grown in a OA/ODE = 60:40 mixture for 1 h. The scale bars are 20 and 5 nm in (c) and (d), respectively. (e, f). TEM images of CdS NCs grown in a

OLAM/ODE = 60:40 mixture for 1 h. The scale bars are 20 and 5 nm in (e) and (f), respectively.

1.2.2 Size focusing and coalescence in DR process

To explore the key factors contributing to digestive ripening of semiconductor nanocrystals, we have designed a control sample comprising a mixture of small and large diameter CdS NCs.

By core acting a small amount of 3 nm CdS with larger, 14 nm CdS NCs, we were able to distinguish between three separate growth scenarios: (i) the redissolution of small diameter nanoparticles in favor of large ones (Ostwald ripening), (ii) the redissolution of larger particles in favor of small ones (conventional DR), and (iii) the interparticle coalescence. Figure 1.5b shows the absorbance profile of the (3 nm CdS + 14 nm CdS) nanocrystal mixture. Small-diameter CdS

NCs can be identified by a distinguishable excitonic feature at l = 385 nm, while 14 nm CdS nanostructures give rise to a shoulder-like step at l = 470 nm. In order to suppress the process of small-to-small particle coalescence, the number of small-diameter nanoparticles was reduced to yield a minimally observable absorbance feature. Heating the two nanoparticle types in a 60:40

OLAM/ODE ligand/solvent combination promoted a slow growth of CdS385 crystallites at temperatures below T = 220 ℃, as was evident from the excitonic red shift in Figures 1.5c and

1.5d. At T = 220 ℃, the nanoparticles growth has accelerated resulting in an abrupt shift of the 14

excitonic peak from 390 to 430 nm (Figure 1.5d) followed by either the loss of the quantum

confinement or inhomogeneous broadening of this feature at T > 240 ℃. Importantly, the observed

red shifting of the CdS385 exciton edge prior to the accelerated growth phase strongly suggests that smaller-diameter dots did not dissolve, such as in the process of Ostwald ripening, but rather grew in size. This behavior is consistent with the conventional DR mechanism, where larger nanoparticles slowly dissolve in favor of smaller ones. The abrupt size increase at T = 220 ℃ could be attributed to either the continuing redissolution of larger particles in favor of small ones (classic digestive ripening) or the attachment of small particles to surfaces of larger ones (coalescence).

Out of the two scenarios, the most likely was revealed by TEM images of CdS385-CdS470 dimer

structures in Figure 1.5e showing the attachment of small particles to larger ones during the

accelerated particle growth stage, T > 220 ℃. The continuing red-shifting of the CdS470 shoulder

accompanying the T = 220-240 ℃ temperature growth corroborates the attachment hypothesis.

Consequently, we conclude that the abrupt red shift of the nanoparticle absorption edge at T = 220 ℃

should be attributed to the particle coalescence. We note that this process is not commonly

observed during digestive ripening of metal colloids, as these are typically grown at lower

temperatures. 15

Figure 1.5 Exploring the mechanism of digestive ripening using a blend of 3 and 14 nm CdS nanocrystals in a OLAM/ODE = 60:40 ligand/solvent reaction mixture. (a). The schematics of the observed two-step digestive ripening process, which involves coalescence and intraparticle growth 16

mechanisms. (b). The evolution of the CdS385 + CdS470 NC mixture absorption profile during the

1-h reaction accompanied by the T = 100 → 260 °C temperature increase. (c). The magnified portion of the absorption profile in (b) shows the gradual red-shift of the CdS385 exciton feature with the reaction temperature. The associated changes in the spectral position of the peak are plotted in (d). (e). TEM evidence of the particle coalescence during digestive ripening. Scale bar is 5 nm. (f). Reshaping of the coalesced nanoparticle dimers at higher temperatures (T > 240 °C) from dimer-like to hexagonal. Scale bar is 5 nm. Adapted with permission from ref. [180].

The formation of dimer structures during the coalescence stage (Figure 1.5e) is eventually followed by the formation of spherical (or hexagonal) nanoparticles with a narrow size distribution

(Figure 1.5f). This observation suggests that some degree of the intraparticle growth may be contributing at this stage as a mechanism, which transforms the dimers into thermodynamically stable shapes. To test this hypothesis, we have explored the dynamics of digestive ripening in CdS nanorods featuring high energy facets (Figure 1.6a). Heating these structures in a 90:10

OLAM:ODE solution resulted in a gradual red-shift of the exciton absorbance reflecting the average diameter increase. TEM images of the DR product confirm the reduction in the aspect ratio of nanorod structures after 30 min of the reaction (Figure 1.6b) and the eventual formation of spherical nanoparticles with an average volume being approximately equal to that of original rods

(Figure 1.6c). Based on these observations, we conclude that the presence of ion solubilizing ligands, in this case, causes some degree of the intraparticle growth, a process which was originally reported for semiconductor colloids by Peng et al.[170]

Figure 1.6 Digestive ripening of CdS semiconductor nanorods via the intraparticle growth. (a).

TEM image of original CdS nanorods. (b). The product forming after 30 min of digestive ripening at T = 260 °C in a 90:10 OLAM/ODE mixture. (c). Spherical nanoparticles form after 60 min of digestive ripening at T = 260 °C. (d). The corresponding evolution of the CdS nanorod absorption profile during 60 min of digestive ripening in a 90:10 OLAM/ODE mixture. Adapted with permission from ref. [180]. Copyright 2018 American Chemical Society.

The aforementioned experiments imply that the process of digestive ripening in CdS NCs leads to (i) focusing of nanoparticle sizes toward the average ensemble diameter at temperatures below the thermal threshold for coalescence and (ii) coalescence-driven growth of larger diameter

structures at higher temperatures. To verify the threshold-like behavior of the DR mechanism in

semiconductor NCs, we have explored both low- and high-temperature regimes using two model

systems: CsPbBr3 perovskite nanocrystals and large-diameter nonuniform CdS NCs. In both cases, the starting nanoparticles were loaded into a flask containing a 60:40 OLAM/ODE mixture and slowly heated up in order to reveal the corresponding shape evolution.

According to Figure 1.7, the low-temperature DR regime results in the reduction of particle size dispersions for both CsPbBr3 (Figure 1.7a) and CdS (Figure 1.7e) NCs in comparison with the respective original samples in Figures 1.7b and 1.7f. Interestingly, the morphology of CsPbBr3 18

NCs has evolved from cubic to nearly spherical without significant changes in the average particle

size. In the case of CdS NCs, the original shape dispersion of 11% was reduced to about 6%,

consistent with the expected size-focusing effect. In both materials, the low-temperature regime

was similar to a classic DR growth, typically observed in metal nanoparticles. At higher DR

temperatures (TCdS = 260 ℃ and TCsPbBr3 = 200 ℃), the particle shape evolution was driven primarily by coalescence, as evident from TEM images in Figures 1.7c, 1.7d, and 1.7g. The samples of coalesced nanoparticles were obtained by stopping the reaction shortly after the maximum temperature was reached in order to minimize the subsequent intraparticle growth. In the case of perovskite nanocrystals, a one-directional assembly of starting cubic seeds into nanowires was observed (Figure 1.7c). When ODA molecules were used as DR-initiating ligands,

CsPbBr3 nanocrystals were found to form three-dimensional superstructures, as shown in Figure

1.7d. Evidence of the interparticle coalescence was also observed in the case of CdS NCs that were found to aggregate into flower-shaped structures at T = 260 ℃ (Figure 1.7g). This morphology,

however, was only reproducible when the original sample contained fairly polydisperse seeds.

Overall, the data in Figure 1.7 suggests that the DR process in semiconductor nanocrystals exhibits

a threshold-like temperature dependence with low-T regime resembling the classical DR process

in metal nanoparticles (size focusing toward the ensemble average size), while the high-T mode

promotes interparticle coalescence. The coalesced structures, in the latter case, become more

uniform with the extended heating time due to an intraparticle growth (see Figure 1.6). 19

Figure 1.7 TEM analysis of the reaction temperature effect on digestive ripening dynamics of

CsPbBr3 and CdS NCs. (a). Low-temperature DR of CsPbBr3 NCs in a OLAM:ODE = 60:40 mixture (T = 160 ℃) preserves the average size of original cubic-shaped CsPbBr3 seeds shown in

(b). (c-d). High-temperature digestive ripening of CsPbBr3 NCs (T = 200 ℃) in the presence of (c)

- OLAM and (d) - ODA ligands leads to nanoparticle assembly into ultralong wires and three- dimensional superstructures, respectively, suggesting a significant impact of the interparticle coalescence. (e). Low-temperature DR of CdS NCs in a OLAM:ODE = 60:40 mixture (T = 180 ℃) preserves the average size of original seeds shown in (f), which is accompanied by the reduction in the size dispersion from 11% to 6%. (h). High temperature digestive ripening of CdS NCs (T =

Society. 20

1.3 Ion-mediated ligand exchange and size-focusing of CdSe nanocrystals

1.3.1 Size focusing with two different sizes CdSe

Based on the aforementioned findings, it was reasonable to assume[175] that below the critical temperature for coalescence, one can expect to see a reduction in the size dispersion without significant changes in the average particle diameter.

As a model system of a polydisperse nanoparticle sample, we have employed a mixture of the two different-diameter CdSe NCs (d = 2.1 and d = 2.6 nm) exhibiting a corresponding sample size dispersion of 22%. Both nanoparticle types were prepared by means of traditional hot- injection routes[176],[177] that can result in three different types of surface ligands, including

[178] cadmium carboxylate Cd(O2CR)2, phosphine (PR3) complexes, and X-type acids (oleic,

phosphonic, or steric acids) donating one electron to a cadmium bond. The presence of L-type

phosphine passivation in investigated samples was partly supported by the fact that the exposure

of as-prepared CdSe colloids to a concentrated solution of Cd(O2CR)2 produced a 3-5 nm redshift of the exciton absorption, indicating the presence of surface Se sites that are coordinated to Cd-L complexes.

The absorption profile of the CdSe470-CdSe520 mixture in Figure 1.8a (bottom black curve)

exhibits distinguishable excitonic features of the smaller and larger species corresponding to

average particle sizes[179] of 2.1 and 2.6 nm, respectively. The presence of the two different particle sizes is also evident in the photoluminescence spectrum of the mixed sample (Figure 1.9) as well as in the transmission electron microscopy (TEM) image of the mixture. To initiate digestive ripening, a mixed-diameter sample was transferred to a reaction flask containing 60% of OLAM ligands by volume and heated to about 150-180 ˚C. This temperature interval represents the low- temperature range where the processes of interparticle coalescence are suppressed (see Ref. [180]). 21

During the DR treatment, the particle size evolution was then monitored through the changes in

the absorption profile, as shown in Figure 1.8b.

Figure 1.8 OLAM-induced digesting ripening of a CdS470 + CdSe520 nanocrystal mixture containing two sizes of CdSe nanocrystals (d = 2.1 nm and d = 2.6 nm, respectively). (a). The associated low-energy absorbing transitions are labelled as green and yellow dashed lines. Heating of the polydisperse nanoparticle sample in the OLAM:ODE = 60:40 reaction mixture at T = 150-

180 ˚C results in the gradual convergence of the two absorbing features into one, corresponding to the reduction in the particle size dispersion from 22% to about 5%. (b). The corresponding evolution of the FWHM for the combined 1S(e)1S(h) absorption feature in a mixed-diameter CdSe

American Chemical Society.

Figure 1.9 The evolution of the emission spectral profile during OLAM-induced digesting

ripening of a CdS470 + CdSe520 nanocrystal mixture (d = 2.1 nm and 2.6 nm, respectively). (a). The

PL profile of the original mixture prior to the digestive ripening treatment. The broad spectral feature in the λ=600-800 nm range was attributed to the trap state emission of 2.1-nm CdSe NCs

(see insert). (b). The PL profile of the size-focused CdSe NCs, resulting from the digestive ripening treatment in the OLAM:ODE = 60:40 reaction mixture at T = 160-180˚C. Adapted with permission from ref. [195]. Copyright 2018 American Chemical Society.

Figure 1.8 summarizes the DR dynamics of a mixed-diameter sample containing two CdSe

NC types in a OLAM:ODE = 60:40 mixture. During a ≈ 14-hour DR treatment at temperatures not exceeding 180 ℃, the lowest-energy excitonic features of the two nanoparticle types appear to gradually converge into a single-particle profile, indicating the reduction in the sample size dispersion. The spectral position of the 1S3/2(h)1S(e) transition in the final product corresponds to a particle diameter of about 2.45 nm,[179]which is close to the expected volume-weighted average

" " "#!$#" of the original sizes in the mixture (� = = 2.37��). Notably, the total concentration of %

CdSe nanoparticles in the flask did not appear to change significantly, such that processes of 23

precipitation or dissolution of nanocrystals during the digestive ripening reaction were deemed

minimal. The crystal structure of CdSe NCs was also found to be preserved during the DR

treatment (see XDR analysis in Figure 1.11a) consistent with the absence of phase-changing

coalescence processes.[180] The size dispersion analysis indicates the reduction in the standard

deviation of particle diameters from 22% in the original sample to about 5% in the final product.

1.3.2 Ligand exchange during DR process

In addition to size focusing, digestive ripening of CdSe NCs was accompanied by the

enhancement in PL lifetimes (Figure 1.10c) corresponding to a 4% → 38% increase in the emission

quantum yield (QY). This trend was consistent with previous reports of the increased PL intensity

in phosphonate- or cadmium carboxylate-capped CdSe NC upon the displacement of original

ligands with amines.[181]-[185] For instance, Talapin et al.[182] has reported an emission QY = 50%

for CdSe NCs exposed to a high concentration of alkylamines at 240 ˚C. Another study

corroborated this result[181] reporting a 48% emission QY in OLAM-treated CdSe NCs.

Interestingly, a few other works have reported the reduction in the CdSe photoluminescence upon

treatment with amines (see for example Ref. [186]). The existence of conflicting observations

points to a possibility that treatment with amines may heal certain traps that are not involved in

the Z-type displacement. Further research is needed to reconcile these contradictory reports on the

emission changes in amine treated CdSe.

Digestive ripening of CdSe NCs in the OLAM:ODE mixture is likely to cause the

displacement of Z-type Cd(O2CR)2 ligands with L-type OLAM (Figure 1.10a), a reaction that was

thoroughly investigated by the Owen group.[183],[186],[187]This process is accompanied by the

removal of surface cadmium, which could be detected as a blue–shift of the exciton

absorption.[183]Along these expectations, the 1S(h)3/21S(e) excitonic feature of 3.0-nm CdSe NCs was found to blueshift by about 8 nm (d = 3.0 → 2.85 nm) upon digestive ripening in a 24

OLAM:ODE = 60:40 solution, which corresponds to the loss of about 1/4 monolayer (Figure

1.10b). The Z → L displacement scenario was also corroborated by the fact that 2S(h)3/21S(e) excitonic transition, which profile is dependent on the type of surface ligands bound to the nanocrystal,[183] noticeably diminished upon a short exposure to OLAM:ODE = 60:20 at T=180

˚C (Figure 1.10b). Proton NMR spectra of OLAM-treated CdSe NCs have also confirmed the displacement of original ligands in CdSe with oleylamine, as discussed below (Figure 1.11b, c).

These surface changes did not seem to affect the lattice bulk structure of CdSe, which, based on the PXRD analysis (Figure 1.11a), appeared to be nearly unchanged after 14 hours of DR treatment.

Figure 1.10 The mechanism of ligand displacement and the changes in absorption and PL lifetime spectra. (a). The proposed scheme for the displacement of Z-type ligands with L-type OLAM, 25 which results in the removal of Cd cations from nanocrystal surfaces. (b). Changes in the absorption spectra of 3.0-nm CdSe in the first 20 min of digestive ripening reaction in the

OLAM:ODE = 60:40 mixture. The blue-shift of the excitonic absorption feature and the reduced intensity of the 2S(h)3/21S(e) transition are consistent with the removal of Cd cations via the Z →

L ligand displacement. During this process, any phosphine ligands on the surface of original nanocrystals are believed to be eventually exchanged with amines. (c). The evolution of the CdSe band gap PL lifetime during the digestive ripening of the two-diameter CdSe sample at T < 180

Figure 1.11 X-ray powder diffraction analysis and NMR (1H-NMR) spectra. (a) X-ray powder diffraction analysis of CdSe NCs produced through digestive ripening in OLAM/ODE = 60:40 26

(green) ligand/solvent reaction mixtures. Both samples exhibit a wurtzite lattice pattern. (b) Proton

NMR (1H-NMR) spectra recorded at 298 K. Proton NMR chemical shifts (δ) are reported in parts per million (ppm) relative to residual solvent signals in CDCl3 (δ = 7.26). it shows the 1H NMR

1 spectrum of CdSe NCs after recapping with CdOA2, and Free oleic acid (OA). (c) Stacked H

NMR spectra of [CdSe]-CdOA2 nanocrystals in CDCl3 (δ = 7.26) resulting from a DR treatment

1 in OLAM, it shows the H NMR spectrum of the alkene resonance of CdOA2 (gray) before and

(black) after the OLAM DR treatment, [CdSe]-CdOA2 with increasing the OLAM DR treatment time, original [CdSe]-CdOA2 nanoparticles (after recapping with cadmium oleate) prior to the

OLAM DR treatment, free oleic acid (OA) and free oleylamine (OLAM), respectively. Adapted

The Z→L ligand displacement during DR appeared reversible, as the addition of

Cd(O2CR)2 to OLAM-treated CdSe (Figure 1.12) partly restored the original position of the CdSe exciton feature (evident from a 6-nm red-shift) while increasing the amplitude of the 2S3/2(h)1S(e) absorbing transition (red arrow, Figure 1.12). It should be noted that the replacement of nanocrystal ligands with n-alkylamines is generally not expected to be 100% efficient unless OA impurities are first etched from the surface.[187] In this regard, the DR-treatment may prove to be an effective

tool for the passivation of colloids with a high percentage of L-type surface coverage, as this

process is facilitated by the surface ion diffusion and reabsorption. Taken all together, the above

observations suggest that OLAM-induced DR reaction removes cadmium from Cd-rich

nanoparticles leaving NH2R terminated nanocrystals. 27

Figure 1.12 The absorption profile of OLAM-ripened CdSe NCs before and after recapping with

Cd(O2CR)2 (blue and red curves, respectively). The red shift of the lowest-energy exciton

transition indicates the increase in the nanocrystal volume, while the enhanced intensity of the

2S(h)1S(e) feature is consistent with binding of cadmium carboxylates. Adapted with permission

It is expected that the DR process can be driven by a variety of free ligands as long as these

molecules can efficiently solubilize ions and monomers in the growth solution. To test this

hypothesis, OLAM was replaced with oleic acid as a DR initiating agent. The dynamics of CdSe 28

NC shape evolution during the DR reaction in the presence of OA was investigated by monitoring

the absorption profile of a mixed CdSe NC sample containing two different particle sizes (d = 2.1

and d = 2.6 nm). The temperature of the OA:ODE = 70:30 reaction mixture was kept below 170

˚C to suppress interparticle coalescence. According to Figure 1.13a, the absorption features of the

two nanocrystal types gradually converge into a single peak upon heating, consistent with the

reduction in the particle size dispersion. Similarly to the case of OLAM induced ripening, the

employment of a OA:ODE reaction mixture allowed reducing the standard deviation of CdSe sizes

from 22% to about 5-6% in 10 hours.

Figure 1.13 Oleic acid-induced digesting ripening of a CdSe470 + CdSe520 nanocrystal mixture containing two sizes of CdSe nanocrystals (d = 2.1 nm and d = 2.6 nm). (a).The associated low energy absorbing transitions in the starting sample are labelled with a green and yellow dashed line. Heating of the polydispersed nanoparticle sample in the OA:ODE = 70:30 reaction mixture at T ≤ 170 ˚C resulted in a gradual convergence of the two absorbing features into a single profile, corresponding to the reduction in the particle size dispersion from 22% to about 5-6%. (b).

Changes in the absorption spectra of CdSe NCs in the first 20 min of digestive ripening reaction in the OA:ODE = 70:30 mixture. (c). The evolution of the CdSe band gap PL lifetime during the 29 digestive ripening reaction at 170 ˚C. The 10-hour sample exhibited the emission QY of 5-7%.

Digestive ripening of CdSe NCs in a OA:ODE solution was expected to produce rather complex changes in the nanoparticle surface chemistry made possible by the diffusion and reabsorption of ligated surface ions and monomers. Considering that the replacement of Z-type and L-type (TOP) ligands in original CdSe NCs with an X-type OA would require a change in the surface charge, one can expect this process to be accompanied by changes in the surface cation- to-anion ratio. Indeed, for the surface charge neutrality of CdSe NCs to be preserved upon binding of electron-donating X-type OA (donating a -1 charge to the bond) to surfaces L-type passivated nanocrystals (contributing zero surface charge), the proportion of negative ions on the surface should be reduced. This could be achieved though the addition of metal cations or removal of anions. The former appears to be less likely since no Cd precursors are introduced during the DR procedure.

To explore whether surface anions are removed during digestive ripening of CdSe NCs to balance the charge of oleic acid ligands, we first examine the evolution of the exciton absorption in 4.1nm CdSe NCs caused by the DR treatment at T = 170 ˚C. According to Figures 1.13b and

1.14b, the DR reaction in a OA:ODE = 70:30 mixture results in a blue shift of the exciton feature by 5-10 nm, which supports the ion removal hypothesis. These absorption changes could not be reversed by the addition of Cd(OA)2 as evidenced from the absence of a red-shift in Figure 1.14a, which is consistent with Cd-rich surfaces of OA-treated CdSe NCs due to removal of Se. 30

Figure 1.14 The absorption profile of OA-ripened CdSe NCs before and after recapping with

Cd(O2CR)2 (blue and red curves, respectively). (a). Since the reaction of OA-capped NCs with

Cd(O2CR)2 does not produce a red shift in the lowest-energy exciton absorption and or changes in the intensity of the 2S(h)1S(e) transition, we conclude that cadmium carboxylate did not bind to nanocrystal surfaces. This behavior is consistent with having a large Cd-to-Se surface ion ratio in original NCs. (b). A significant blue-shift is observed upon digestive ripening of as-fabricated

CdSe NCs in a 70% OA reaction mixture. Adapted with permission from ref. [195]. Copyright

2018 American Chemical Society.

To further characterize the stoichiometric changes accompanying the ligand exchange processes for OA- and OLAM-treated CdSe samples, we have performed the energy dispersive X- ray (EDX) analysis of nanoparticle elemental compositions for each case. To this end, a Cd:Se X- ray emission ratio in original CdSe NCs (r ≈ 1.75), obtained by integrating the area of the SeL and

CdL x-ray emission peaks in Fig. 1.15a (red curve), was compared to Cd:Se ratios in DR-treated

nanoparticles. According to Fig. 1.15a, the digestive ripening in OLAM reduces the Cd:Se ratio in

nanocrystals (r ≈ 1.51), which supports the proposed of Z to L ligand displacement (loss of Cd).

Conversely, when the DR reaction is performed in OA solutions, the ratio of the Cd to Se atoms 31 becomes slightly increased in the final nanoparticle product (r ≈ 1.97), suggesting that Se is in fact being removed from nanoparticle surfaces. The absolute changes in the Cd:Se ratio are fairly small as these represent contributions from surface and bulk atoms. Considering that the average diameter of investigated CdSe NC sample is 4.1 nm, we estimate that the fraction of surface atoms amounts to ~30%. Overall, the EDX measurements support the hypothesis that OLAM solutions remove Cd while OA solutions remove Se (see Figure 1.15c).

Figure 1.15 Energy dispersive X-ray characterization, Changes in the exciton absorption and the proposed scheme of surface ligand exchanges in CdSe NCs corresponding to a consecutive application of OA and OLAM DR treatments. (a). Energy dispersive X-ray characterization of the elemental composition for CdSe NCs ripened in OLAM (blue) and OA (green). The EDX of original CdSe NCs is shown in red. The observed changes in the Cd:Se X-ray emission ratios 32 supports the hypothesis that DR processes in OLAM and OA solutions result in a loss of surface

Cd and Se ions, respectively. (b). Changes in the exciton absorption of 4.1 nm CdSe NCs accompanied by the consecutive application of OA and OLAM digestive ripening treatments (T ≈

150-170 ℃). The total spectral blue-shift resulting from both applications corresponds to the 0.6- nm loss in the average particle diameter. (c). The proposed scheme of surface ligand exchanges in

The desorption of surface ions to balance the charge of incoming ligands in CdSe NCs can be observed directly through a consecutive application of OLAM and OA digestive ripening treatments. Figure 4b summarizes the changes in the absorption profile of 4.1 nm CdSe NCs upon the exposure of these nanoparticles to ODE:OA = 30:70 solutions at 160 ˚C, followed by a single wash with ethanol, and the subsequent exposure to a 70% OLAM solution in ODE at 160 ˚C. Each of the two DR processes resulted in a blue shift of the exciton absorption feature by 10-15 nm. The total blue shift of λ = 25 nm corresponds to the 0.6-nm reduction in the diameter of original nanocrystals, which is equivalent to nearly a full CdSe monolayer. Similar dynamics was observed for 3.2-nm CdSe where a reverse application of consecutive OLAM and OA DR processes (T <

150 ˚C) resulted in 0.15-nm reduction in the nanocrystal diameter.

1.4 Conclusion

In conclusion, we demonstrate that digestive ripening of semiconductor nanocrystals can be used to tuning the surface-ligand chemistry, control both the particle size and the corresponding size dispersion. In contrast to the well-studied ripening mechanism in metal nanoparticles, the digestive ripening of semiconductor nanocrystals leads to significant changes in the average particle diameter at reaction temperatures above a certain thermal threshold. Meanwhile, at low 33

temperatures, size focusing leading to an ensemble average diameter was observed. The existence

of a thermal threshold for a particle coalescence allowed controlling the ultimate size of

semiconductor nanocrystals through the DR approach, a functionally, which is not easily

accessible in the DR treatment of metal nanoparticles. The ability to tune nanoparticle diameter

while reducing the size dispersion has been demonstrated for samples of CdS, CdSe, CsPbBr3, and

CuSnZnS4 colloids. Digestive ripening was also shown to facilitate complex ligand exchange processes that require restructuring of nanocrystal surfaces. In particular, the enhanced diffusion of surface ions during the digestive ripening reaction has been shown to promote a classically forbidden L → X ligand exchange in CdSe NCs, which was possible due to the desorption of surface Se. Overall, we expect that the DR approach could be extended to many semiconductor materials as a simple tool for controlling the surface chemistry and advanced shape control of colloidal nanostructures.

CHAPTER II. ENERGY TRANSPORT IN INORGANIC NANOCRYSTALS

2.1 Reverse energy transfer in metal-conjugated fluorophores

2.1.1 Introduction

The interaction of fluorescent dyes with proximal metal surfaces can result in either the enhancement[23]-[27] or quenching[28]-[37] of the dye photoluminescence (PL). The ultimate change in the PL intensity is determined by the specific nature of electric dipole−dipole interactions at the molecule−metal interface that can shift the energy transfer (ET) balance in either direction. For instance, PL quenching[188]-[193] is commonly observed when the surface plasmon band of a metal

nanoparticle has a significant spectral overlap with the emission profile of a dye, favoring the

transfer of its excitation energy via Forster or nanosurface energy transfer mechanisms. Such

metal-induced quenching of the dye PL has recently evolved into a popular strategy of signal

transduction in biosensing[38]-[47] and near-field imaging applications.[38],[48]-[50] The realization of

the metal-enhanced fluorescence,[51]-[60] on the other hand, demands more stringent requirements

to be met by a dye−metal system, which include the spectral overlap in the excitation region,

spatial alignment of dye and metal electric dipoles, and the absence of the photoinduced charge

transfer between the two components. Under these conditions, the excitation energy in a metal

could be transferred to a proximal dye through the plasmon-induced resonant energy transfer

(PIRET),[26],[61]-[65] also known as the field enhancement mechanism.[56],[66]-[68] Examples of the

PIRET process in dye−metal assemblies have been witnessed in photocatalytic,[65],[68]-[76]

photovoltaic,[77]-[83] and biosensing[38]-[40],[43],[51],[84],[85] applications of metal nanoparticles.

From a theoretical standpoint, the interaction of fluorescent molecules with surface

plasmons in metals is usually treated as an incoherent process,[194] which excludes the possibility

of the reversible energy transfer between the two components. Consequently, quenching of the dye 35

PL is usually not considered to result in the enhancement of the excitation energy in the metal.

This scenario, however, is not characteristic of localized surface plasmons in metal nanoparticles,

where the energy accepted from a proximal dye could be resonantly transferred back to conjugated

molecules due to the absence of Stokes shifts associated with nonradiative emission by metals[61]

(no vibrational relaxation losses). Furthermore, an electric dipole of a metal nanoparticle excited

via the dye → metal ET would be aligned with a dipole of the energy donating molecule, such that

the rate of the corresponding reverse metal → dye energy transfer process could exceed that of the

polarization-averaged field enhancement in the same dye-metal system. In order to account for a

coherent excitation transfer in a metal-fluorophore assembly, the interaction of fluorescent

molecules with metal surfaces must be treated as reversible, which assumes that the loss of dye

excitations (-dNdye) augments the population of metal plasmons (+dNmetal) and vice versa. This scenario is possible when the system is excited in the vicinity of the exciton-plasmon resonance

[26],[193],[196],[197] (hωpl ≈ hωexciton), a condition met by many dye-metal assemblies.

Here, we report on the experimental observation of the reversible energy transfer in

assemblies of Au nanoparticles and fluorescent molecules (Alexa 488, Cy3.5, Cy5). The present

study reveals that quenching of the dye emission via FRET or NSET to localized surface plasmons

in Au nanoparticles is followed by the backward transfer of metal excitations to dye molecules

(Figure 2.1). Such a delayed repopulation of the dye excitation energy was manifested in this work

through the observation of an increased PL lifetime of Au-conjugated dyes in comparison to

nonconjugated molecules. Ultrafast transient absorption measurements have confirmed the

delayed rise of the excitation energy in Au-conjugated fluorophores, which was temporally

correlated with the excited-state decay in polymer-capped Au nanoparticles. Notably, relative

amplitudes of the reverse metal → dye ET in each of the three investigated samples were found 36 proportional to the respective metal induced PL enhancement factors, determined using polarization-averaged sample transmitted excitation photoluminescence (STEP) spectroscopy.

Theoretical calculations based on the reversible ET model were subsequently employed to explain the observed PL lifetime enhancement in dye−AuNP assemblies, suggesting that such processes could be ubiquitous in many other dye−metal systems. Considering the significant role that metal−dye assemblies play in many areas of science and technology, the present findings could have important implications for the development of biological sensors, light-emitting materials, and light-harvesting assemblies.[38],[40],[42],[51],[198]-[206]

Figure 2.1 Illustration of the forward (dye → AuNP) and reverse (AuNP → dye) energy transfer.

Quenching of the dye PL via Forster resonant energy transfer (FRET) or nanosurface energy transfer (NSET) results in the excitation of localized surface plasmons in Au NPs. The induced excitation energy in the metal can subsequently be transferred back to surface-anchored dyes (via the PIRET mechanism). The resulting two-step energy transfer process, therefore, can cause the repopulation of the dye excitation energy, which is evidenced as the delayed photoluminescence in conjugated dyes. Adapted with permission from ref. [331]. Copyright 2019 American Chemical

Society. 37

2.1.2 Delayed photoluminescence in metal-conjugated fluorophores

Early studies of the metal-enhanced fluorescence[207]-[216] have identified the existence of

several competing processes that contribute to the energy exchange between a metal nanoparticle

and a semiconducting fluorophore. In the absence of charge-transfer interactions, quenching of the

fluorophore emission is usually described by the dye to metal FRET,[217] NSET,[218] or the Gersten-

Nitzan[219] mechanisms (nonradiative quenching), whereas the PL enhancement process is

attributed to the metal-dye PIRET (field enhancement).[61],[62],[194] When the two processes are incoherent (absence of the back and forth transfer), their cumulative effect on emission changes in metal-coupled fluorophores (FL) can be expressed using a weak-coupling[220],[96] dipole-dipole approximation, as follows:[65]

��#&'()'*+, ∆� = = ∆��-./01 × ∆��01 = (1 + �-./01) × (1 − �01) ��#&'

� 1 1 ⎛ 2,+3)45 ⎞ ⎛ ⎞ = 1 + × 5 × 1 − 5 (1) �#&' � � 1 + 8 -./01: 1 + 8 01: ⎝ �6 ⎠ ⎝ �6 ⎠

where n = 4-6 depending on whether the dipoles are considered to be surface or point-like, α

01 represents the wavelength-dependent absorption coefficient, and �6 is the donor-acceptor

distance corresponding to the 50% efficiency for nonradiative dye energy quenching (via FRET

or NSET).

The competition of metal-induced FL quenching (FL < 1) and field enhancement (FL > 1)

processes is well documented in the literature. Typically, quenching prevails when the size of a

metal nanoparticle falls below 20 nm.[28]-[31],[221] In this size regime, quenching via FRET or NSET,

as well as the photoinduced charge transfer back to metal, overwhelm the effect of PIRET-based

enhancement.[192],[221] Meanwhile, the plasmon-enhanced fluorescence (FL > 1) is usually 38

observed in systems featuring large-diameter metal nanoparticles (>30 nm in

size)[26],[207],[212],[215],[222]-[227] and metal nanorods,[228]-[232] where slower dephasing surface

plasmons exhibit a greater probability of interacting with semiconductor excitons through the

PIRET mechanism. In the case of weakly emitting dyes, the PL gain may be further increased by

the plasmon enhancement of the semiconductor radiative rates,[30],[218],[233],[234] although a radiative

rate reduction has also been predicted for chemically conjugated dyes.[31]

In the present study, the interaction of fluorescent molecules with localized surface

plasmons was investigated by using three dye−metal assemblies comprising 14.4 nm Au

nanoparticles (a characteristic TEM image is shown in Figure 2.2) conjugated with Alexa 488,

Cy3.5, and Cy5 dye molecules. Au NP surfaces were capped with a lipoic acid (LA)-containing

polymer[235] with an average thickness of 4−6 nm. The three types of fluorophores were chosen to

represent different regimes of the spectral overlap between the molecular absorption range and the

surface plasmon band, as shown in Figure 2.3. The Cy3.5−AuNP assembly exhibited the strongest

overlap in the absorption range, which was expected to enhance the corresponding “reverse” AuNP

→ Cy3.5 energy transfer rate. Meanwhile, the absorption bands of Alexa 488 and Cy5 dyes were

offset toward the higher and lower energy regions of the Au NP absorption maximum, respectively.

In terms of PL quenching, the Alexa 488 assembly exhibited the strongest emission−absorption

overlap, favoring the Alexa → AuNP ET, while PL emission spectral ranges for Cy3.5 and Cy5

were redshifted from the Au absorption maximum. The conjugation protocol was performed

according to a previous report,[235] as illustrated in the diagram of Figure 2.2. The dye-AuNP product was characterized using 1H NMR, dynamic light scattering, and exclusion column filtration. (figure 2.4) 39

Figure 2.2 Synthesis, TEM image and size distribution of Au NPs. (a). A TEM image of 14.4-nm

Au NPs conjugated to Cy5 dyes. The inset shows a STEM image of citrate-capped Au colloids.

(b). A statistical analysis of the Au NP size distribution. (c) Schematics of LA-PIMA-PEG/TEG-

NH2 ligand synthesis and photo-induced cap exchange of citrate-AuNPs with the multi- coordinating ligand. Dye-AuNP conjugates synthesis relies on the amine-functionalized AuNPs reacting with NHS ester-modified dyes (Cy3.5/Cy5/Alexa 488). Adapted with permission from ref. [331]. Copyright 2019 American Chemical Society.

Figure 2.3 Absorption, emission and PL lifetime of three Dye-AuNP assemblies. (a−c) Absorption profiles of three dye−AuNP assemblies comprising 14.4 nm Au nanoparticles conjugated to (a)

Alexa 488, (b) Cy3.5, and (c) Cy5 dye molecules. The inserts show approximate dye to AuNP molar ratios estimated from known extinction coefficients. (d−f) Emission profiles of the three dye molecules before (dark red) and after (gray) conjugation to Au nanoparticles. The PL intensity quenching ratios for each case are shown. (g−i) PL lifetimes of the three investigated dye molecules before (gray) and after (red) conjugation to Au nanoparticles. Notably, the PL lifetimes of Cy3.5−AuNP in (h) and Cy5−AuNP in (i) are increased relative to those of isolated molecules.

Figure 2.4 Synthesis and measurements of dye-AuNP conjugates. (a) 1H NMR assignment and integration of ring-opened LA40%-PIMA-PEG50%/TEG10%-NH2 ligand (in DMSO-d6). The percent refers to the number of attached groups with respect to the total number of monomers in a polymer chain. (b-d) Purification of dye-AuNP conjugates: (b) AuNP-Alexa 488, (c) AuNP-Cy3.5,

(d) AuNP-Cy5. In step 1, the conjugates were purified from excess dyes using a PD-10 size exclusion column; in step 2, the first eluted dispersion was further purified from unbound and loosely attached dyes by two rounds of the membrane filtration. (e-f) 13-nm citrate-capped gold colloids (red) and the photoligated samples (blue) are characterized by (e) UV-vis absorption; and

(f) dynamic light scattering measurements. Adapted with permission from ref. [331]. Copyright

2019 American Chemical Society. 42

The conjugation of fluorescent dyes with Au nanoparticles has resulted in 7-8 fold quenching of the steady-state emission for all three investigated samples, as illustrated in Figure

2.3d-f. Considering the substantial spectral overlap between the surface plasmon band and emission profiles of Alexa488 (Figure 2.3d), Cy3.5 (Figure 2.3e), and Cy5 (Figure 2.3f), the reduction in the PL intensity in all cases was attributed primarily to dye → AuNP nonradiative quenching via resonant energy transfer (FRET, NSET). The photoinduced charge transfer from the excited dye to a metal surface was considered as another possible mechanism of PL quenching,[236] which affected the subpopulation of conjugated molecules exhibiting electrical coupling with Au.

Contrary to expectations based on dye-AuNP energy transfer scenario, the changes in the

PL lifetime of Au conjugated dyes (Figure 2.3g-i) were not proportional to their respective PL quenching efficiencies. The difference was particularly prominent in the case of Cy3.5-AuNP assemblies, where, despite an 88.2% reduction in the Cy3.5 PL intensity (ΔI = 0.12), the corresponding lifetime was increased almost 2-fold relative to that of isolated fluorophores, Δ� ≈

2 (Figure 2.3h). A similar trend was observed for Cy5-AuNP assemblies, where the PL lifetime was increased upon conjugation (Figure 2.3i), despite a 7-fold reduction in the total emission intensity (Figure 2.3f). The Alexa 488-AuNP assembly was the only system that showed a slight

PL lifetime reduction upon conjugation (Figure 2.3g). Nevertheless, the relative change in the PL lifetime of Au-anchored Alexa 488 (Δ� ≈ 1) did not correspond to the change in the emission intensity (ΔI = 0.14).

The increase in the excited-state lifetime of metal-conjugated Cy3.5 and Cy5 fluorophores was corroborated by ultrafast transient absorption (TA) measurements (Figure 2.5 and Figure 2.6).

To determine the lifetime of dye excitations in dye−AuNP and dye-only samples, chirp-corrected 43

TA spectra were recorded in the 0.3−2.5 ns time interval (see Figure 2.5a, b). Excitation at the

high-energy side of the dye absorption range (λexc = 590 nm for Cy3.5−AuNP and λexc = 640 nm for Cy5−AuNP) has caused an instantaneous bleach of both the dye and metal nanoparticle excitation transitions, as indicated in Figure 2.5a, b by red and blue arrows, respectively. The negative ΔA feature in the TA spectra of Au nanoparticles (blue arrow) is known to arise from excitation-induced broadening of the surface plasmon peak, which causes a characteristic spectral

“dip” at the plasmon wavelength sandwiched by the two positive “wings”. [237],[238] Such

broadening of the plasmon absorbance results from nondipolar plasma oscillations in Au

nanoparticles induced by the excitation pulse.

Figure 2.5 Summary of ultrafast transient absorption (TA) measurements. (a) Transient absorption

spectra of Cy3.5-AuNP assemblies. TA bleach features corresponding to AuNP and Cy3.5 dye

components are indicated by blue and red arrows, respectively. The pump excitation wavelength

was set to lexc = 590 nm. (b). Transient absorption spectra of Cy5-AuNP assemblies, where blue and red arrows indicate the TA bleach in AuNP and Cy5 dye, respectively. The pump excitation wavelength was set to lexc = 640 nm. (c). Integrated TA bleach recovery corresponding to Cy3.5

(red) and AuNP (blue) spectral ranges of a Cy3.5-AuNP system, shown in (a). The rise of the

Cy3.5 bleach amplitude at short pump-probe delays (< 3 ps) is attributed to the 3 ps recovery of 44 the spectrally broad photoinduced absorption in Au nanoparticles. (d) Integrated TA bleach recovery corresponding to Cy5 (red) and AuNP (blue) spectral ranges of the Cy5-AuNP system, shown in (b). (e). Comparison of the Cy3.5 bleach recovery before (red) and after (blue) conjugation with Au nanoparticles. The increased lifetime of the excited-state population in Au- conjugated dyes is consistent with the observation of increased PL intensity decay in Cy3.5-AuNP assemblies. Adapted with permission from ref. [331]. Copyright 2019 American Chemical Society.

Figure 2.6 The summary of transient absorption measurements. (a-e). Temporal evolution of the integrated TA bleach for metal (blue) and dye (red) components. (b). Spectrally resolved TA data corresponding to the dye-AuNP assemblies in the left column. Adapted with permission from ref.

According to the time-dependent evolution of the spectrally integrated Cy3.5 TA bleach in

Figure 2.5e, the recovery of excitations in dye-only Cy3.5 samples (red curve) was substantially faster than the corresponding bleach recovery in Cy3.5-AuNP assemblies (blue curve). The difference in the excited state decay time between dye-metal and dye-only samples was very similar to the difference observed in measurements of the PL intensity decay. Indeed, over the 2.5 ns pump-probe delay range, depicted in Figure 2.5e, the recovery of Au-conjugated Cy3.5 appears to be at least twice as slow as in isolated Cy3.5 molecules. This observation provides strong evidence that excited-state lifetimes of Cy3.5 dyes increase upon conjugation with Au NPs.

In addition to a slower decay of excited states in metal-conjugated dyes, the TA dynamics of Cy3.5-AuNP and Cy5-AuNP assemblies showed some other interesting features. At early pump-probe times (< 3 ps), metal-conjugated dyes exhibited a fast rise of the excited-state population, which was not observed in isolated molecules (Figure 2.5c, d). In this case, no correlation between the early onset of the dye bleach and the decay of plasmon excitations in Au

NPs was inferred. In particular, a careful analysis of TA traces was able to attribute the initial growth of the spectrally integrated bleach in Cy3.5 and Cy5 dyes (Figure 2.5c, d, red curve) to the early-time background contribution. According to Figure 2.5a, b and Figure 2.6, TA spectra of Au- conjugated dyes contained a spectrally broad, positive feature associated with the photoinduced absorption (PA) in metals (positive “wings”),[239] which effectively reduced the amplitude of the dye bleach in dye−metal assemblies. On the basis of these observations, we conclude that the early- time recovery of the TA bleach in Au domains (Figure 2.5a, blue circles) did not correlate with the rise of the TA bleach in Cy3.5 dyes (Figure 2.5a, red circles). This refers to both hot electron and resonant energy transfer processes, as the former mechanism does not enhance the Cy3.5 TA bleach signal beyond the aforementioned PA effect, while the AuNP → Cy3.5 PIRET process is 47

generally too fast (<50 fs) to be resolved in present TA measurements (pump pulse duration ∼120 fs).

The presence of a long-lived PL component in the decay of Cy3.5 and Cy5 dyes (and to a lesser extent in Alexa 488) suggests the existence of the excited-state repopulation process

(dNdye(t) > 0). As was stated above, the AuNP → dye PIRET resulting from the direct excitation of Au nanoparticles and the hot electron transfer have relatively short decay times to contribute to such a long-lived excited state of metal-conjugated fluorophores (>1 ns). In this regard, any possible mechanism that could result in a continuous repopulation of metal-conjugated dyes must involve an excited-state reservoir, which slowly feeds the excitation energy to a dye. Under these considerations, one possible mechanism of the delayed PL in metal-conjugated dyes could be attributed to a two-step excitation transfer process, involving the dye → metal energy transfer followed by the metal → dye PIRET. This scenario is schematically illustrated in Figure 2.1. The reverse PIRET process is triggered by the ET from a dye molecule and, therefore, is temporally delayed relative to the initial excitation event. The Au to dye energy transfer could be mediated by an LA-containing polymer shell, which accepts the initial excitation energy from a metal. The evidence in favor of such metal → polymer excitation transfer is seen in the TA spectra of

Au−polymer composites (Figure 2.7) as a spectrally broad photoinduced absorption. The long- lived positive ΔA feature implies that new states in a Au−polymer assembly become occupied as the plasmon excitation relaxes. The decay of these states can therefore induce a delayed excitation of conjugated dyes. Considering that surface plasmons induced by the energy transfer from conjugated dyes are dipole-aligned with the electric field of the donating molecule, the reverse plasmon → dye energy transfer could be more efficient than a “direct” PIRET process.

Figure 2.7 Spectrally resolved transient absorption measurements of LA-PIMA-PEG-capped Au

2.1.3 STEP measurements of the reverse energy transfer

In order to understand whether the proposed “reverse” energy transfer via the field enhancement is feasible in investigated dye−metal systems, we have performed sample transmitted excitation photoluminescence (STEP) measurements of the metal-enhanced PL in all three samples.

[240]-[243] STEP spectroscopy was recently introduced for measurements of the ET efficiency, ED→A,

in systems with nonemissive donor species (e.g., plasmonic nanoparticles). It is based on the 49

-8 assumption that the number of photons emitted by an acceptor fluorophore,�7 ,depends linearly

on the number of excited acceptor (A) and donor (D) molecules, NA and ND, respectively:

-8 �7 = ��7(�7 + �9→7�9) (2)

where QYA is the emission quantum yield of the fluorophore A in the presence of the donor

D (as measured in the donor−acceptor assembly). The ED→A parameter represents the percentage of donor excitations that are transferred nonradiatively to the acceptor moiety. To determine ED→A,

the donor−acceptor sample is excited using a broad-band light source and the emission intensity

-8 of the acceptor dye �7 (E) is recorded. The excitation light is then spectrally shaped using donor- like or acceptor-like filters (Figure 2.8a, b) designed to suppress the excitation of Au NP donor or dye acceptor species in the investigated sample (ND ≪ NA or NA ≪ ND, respectively). If the spectral

profile of the excitation light, n(l), and the optical density (OD) of the excitation filter are known,

one can predict the change in the acceptor emission as a function of a single parameter, ED→A (see

Figure 2.9). It should be noted that eq 2 does account for the loss of the acceptor (dye quenching)

excitations due to the dye-Au ET. This is because quenching of the dye emission lowers the

effective QYA in eq 2. The corresponding reduction, QYA, is the same with and without the

excitation filter present since Au nanoparticles do not need to be excited to accept energy from a

proximal dye. As a result, the value of QYA cancels out from the equation for ED→A, as shown in

the diagram of Figure 2.9.

Figure 2.8 Illustration of the STEP technique for measurements of the Au → dye energy transfer efficiency. which relies on either (a) Au-donor type or (b) dye-acceptor type excitation filters. The broad-band excitation light is passed through a respective excitation filter (Au-donor type, solutions of CdSe/CdS nanocrystals, which spectrally match the plasmon absorption of Au NPs; 51

-8 dye-acceptor type, Alexa 488, Cy3.5, Cy5), which causes the acceptor emission, �7 , to change

proportionally to the energy transfer efficiency, ED→A. To obtain the ED→A value, the measured ƒ

ratio (ƒA or ƒD, see text) is fitted with a model parametric curve, ƒtheor(ED→A), featuring a single fitting parameter, ED→A. (c) Summary of STEP measurements for all three investigated dye−Au assemblies. (d) STEP measurements of ED→A in Alexa 488−AuNP assemblies utilizing an

acceptor-type filter. The observed set of experimental fA is consistent with ED→A ≈ 0%. (e) STEP measurements of ED→A in Cy3.5−AuNP assemblies utilizing a donor-type filter. The observed experimental f D values are best fitted with a model curve utilizing ED→A = 3.6%. (f) STEP measurements of ED→A in Cy3.5−AuNP assemblies utilizing an acceptor-type filter. The observed

experimental fA values are best fitted with a model curve utilizing ED→A = 3.2%. (g) Donor-type

STEP measurements of ED→A in Cy5−AuNP assemblies resulting in ED→A = 0.1%. (h) Acceptor type STEP measurements of ED→A in Cy5−AuNP assemblies resulting in ED→A = 0.2%. Adapted with permission from ref. [331]. Copyright 2019 American Chemical Society.

Figure 2.9 The experimental protocol for STEP measurements. Adapted with permission from ref.

Figure 2.8ab illustrates the procedure for extracting the energy transfer efficiencies from

STEP measurements utilizing the two types of excitation filters. A donor-type excitation filter is 53

used to suppress the excitation of donor molecules in the sample, causing the acceptor emission to

change proportionally to ED→A (Figure 2.8a). These changes are best illustrated by plotting a

-8 normalized dye-acceptor emission, ƒD =�7 /NA, as a function of the donor-type filter optical density, as shown for the two limiting cases of ED→A:E = 100% and E = 0%. Conversely, an

acceptor-like excitation filter (e.g., a solution of acceptor dye molecules) can be used to selectively

suppress the excitation of acceptor molecules, such that the energy transfer efficiency, ED→A, could

-8 be obtained from the acceptor emission scaled by the number of donor excitations, ƒD =�7 /ND,

as illustrated in Figure 2.8b. To reduce an experimental error, present STEP measurements were

performed through sequential injections of the reference (dye-only) and dye−AuNP samples into

the target cuvette without realigning the optical setup or changing the filter solution. Such a

reference-based measurement strategy has allowed for the experimental uncertainty to be

contained within 1−2%. Figure 2.8 summarizes the results of STEP measurements. Out of three

investigated dye−AuNP systems, the Cy3.5−AuNP assembly resulted in the highest value of the

PIRET efficiency. According to the analysis in Figure 2.8e, the fD ratio corresponding to Cy3.5- only samples was essentially independent of the Au-donor type filter OD, as was expected in this case due to the lack of donor (Au) contribution into the dye emission. Conversely, for

Cy3.5−AuNP samples, the experimental fD ratio (blue circles) decreased with the filter OD, indicating the contribution of Au excitations to the Cy3.5 emission. The measured ƒD was fitted

with a model parametric curve, ƒtheor, featuring a single fitting parameter, ED→A, where ƒtheor(E) =

-8 �7 (E)/NA, was determined using eq 2 as a parametric function of the energy transfer efficiency,

E. As shown in Figure 2.8e, the average PIRET efficiency for Cy3.5−AuNP samples was determined to be 3.6%. This value represents the percentage of Au-absorbed excitations that were transferred to the entire population of proximal Cy3.5 dyes. In other words, the group of surface- 54

anchored dyes was treated as a single acceptor moiety. STEP measurements of the same sample,

utilizing an acceptor-like excitation filter (Figure 2.8f), have yielded a slightly lower value for

ED→A, which equaled 3.2%. Consequently, we conclude that the average metal → dye ET

efficiency for a Cy3.5−AuNP system was 3.4%, which represents the PIRET efficiency averaged

over different molecular orientations (κ2 ≈ 2/3). In the case of Cy5−AuNP samples, the PIRET efficiency was found to be 0.15% (Figure 2.8g, h). A relatively lower value of ED→A for

Cy5−AuNP assemblies (in comparison to Cy3.5−AuNP) was consistent with a reduced spectral overlap, F, between Au and Cy5 species in the excitation spectral range (FCy3.5/FCy5 ≈ 2.7). In the case of Alexa 488-AuNP samples, Au-donor-type STEP measurements could not be performed due to a significant spectral cross talk between the surface plasmon band and acceptor (Alexa 488) emission profile. Consequently, on the basis of only dye-acceptor type measurements, we determined that the metal to dye PIRET in this case was close to zero, ED→A ≈ 0.0 ± 0.1 (Figure

2.8d). It should be noted, however, that due to a relatively high ratio of optical extinction coefficients in Au and Alexa 488 dye (~1400), the transfer of up to 0.1% of the excitation energy from a Au nanoparticle to about 70 surface-appended dyes is sufficient to increase the Alexa 488 emission by 3-4%.

Figure 2.10 Comparison of surface plasmon electric field amplitudes for aligned and isotopically oriented dye molecules and the modulation of PL lifetime based on the rate equations. (a) The reverse PIRET is expected to result in a preferential alignment of metal and dye dipoles (κ2 ≈ 4).

The electric field intensity amplification was calculated for a spherical Au nanoparticle (ε =

−8.4953 + 1.6239i) using the T matrix linking of outgoing (Hankel) and incident (Bessel) fields, as detailed in refs [241] and [246]. (b) Fitting the delayed PL in Cy3.5−AuNP assemblies (purple circles) with the solution of coupled rate equations (eqs 3 and 4) on the basis of the reverse ET 56

−1 model (purple curve). The best fit is obtained for kPIRET = (0.29 ns) . The emission of

nonconjugated Cy3.5 is shown by gray circles. (c) Fitting the delayed PL in Cy5−AuNP assemblies

(purple circles) with the solution of coupled rate equations on the basis of the reverse ET model

−1 (purple curve). The best fit was obtained for kPIRET = (0.33ns) . The emission of nonconjugated

STEP measurements of the polarization-averaged ET efficiencies in Figure 2.8 suggest that

“instantaneous” plasmon → dye energy transfer can occur in all three systems with the highest rate expected in Cy3.5-AuNP assemblies and the lowest in Alexa 488-AuNP. The energy transfer efficiency measured by the STEP spectroscopy represents the percentage of metal-absorbed photons that is transferred to the entire population of conjugated dyes. In other words, the group of surface-anchored dyes is treated as a single acceptor moiety. Consequently, the Au/dye loading ratio is not used in calculations of the STEP energy transfer efficiency. However, this ratio is used in estimating the average number of excitation per dye received from a Au NP. Namely, the ratio of Au:dye extinction coefficients is used to estimate the average number of dye molecules per Au nanoparticle. For instance, in the case of AuNP-Cy3.5 assemblies this ratio is 1:173 (see Figure

2.3b). By analyzing the relative contributions of the Au and dye absorption profiles in the AuNP-

Cy3.5 assembly (Figure 2.3b), we conclude that for every 100 photons that a Au NP absorbs, a population of 173 Cy3.5 dyes receives 7.5 photons by means of a direct photon absorption. In addition to that, STEP measurements indicate that 3.4% of Au-absorbed photons are transferred to the entire population of conjugated dyes (Figure 2.8c). Consequently, 173 of Cy3.5 dyes receive a total of 3.4 + 7.5 = 10.9 excitations, an enhancement of 31% from the direct absorption of 7.5 photons. By taking into account the relative nanoparticle/dye extinction ratios and the average number of conjugated dyes, we estimate that, for a single direct excitation absorbed by one of the 57

Cy3.5 dyes on the surface of Au, it receives an average of 0.31 excitations from the metal

nanoparticle. For instance, if PL quenching due to FRET or NSET were negligible, the resulting

PL enhancement due to proximal Au would have been 31% (FL = 1.31). For Cy5 and Alexa 488,

the estimated increase in the exciton population due to the plasmon-induced energy transfer from

Au was ~6% (FL = 1.06) and ~3.5% (FL ≦ 1.035), respectively. Notably, the latter value represents

the upper bound rather than the average efficiency due to large experimental uncertainties.

The experiment manifestation of the metal to dye energy transfer in Cy3.5-AuNP and Cy5-

AuNP assemblies supports our hypothesis that the reverse PIRET process is possible in these

systems. As was previously mentioned, the reverse PIRET rate is expected to be enhanced relative

to polarization averaged ET observed in STEP measurements due to the favorable orientation of

the surface plasmon and exciton electric dipoles. This concept is illustrated in Figure 2.10a, which

compares the amplitude of the plasmon electric field (dAu = 14.4 nm) for aligned and isotopically oriented molecules. Since quenching of the dye emission gives rise to surface plasmons that are dipole-aligned with the fluorophore electric field, the orientation factor κ2 for these molecules is assumed to be maximized (κ2 ≈ 4). In comparison to the isotropic orientation of dipoles observed in STEP measurements (κ2 ≈ 2/3), the corresponding enhancement of the Forster radius for the

+,;<5'# ;34*=42;> reverse PIRET �6 /�6 is expected to range between 1.35 and 1.56, depending on

whether metal dipoles are considered to be point- or surface-like.

2.1.4 Model calculations for the conjugated systems

To model the coherent energy exchange in Cy3.5−AuNP and Cy5−AuNP assemblies,

which includes the possibility of a reverse PIRET, we assume that the number of excitations in

Au-conjugated dye molecules (Ndye) can grow proportionally to the number of surface plasmon excitations in Au (Nmetal) caused by the reverse PIRET mechanism. Under this assumption, the 58

temporal evolution of Ndye and Nmetal populations can be determined by solving coupled rate equations that include the metal → dye energy transfer rate (kPIRET):

#? #$% = −� � − � � + � � (3) #* 3245 #&' 01 #&' -./01 )'*+,

#?&%'() = −� � − � � + � � (4) #* -./01 )'*+, )'*+,∗ )'*+, 01 #&'

where kET is the PL quenching rate due to the dye to metal FRET and NSET, kspon is the

rate of a spontaneous decay in nonconjugated dyes, and kmetal* is the rate of the excitation decay in metal. We assume that any modifications of kspon due to the stimulated-emission effect of metal surfaces are negligible in systems with strong emitters. Indeed, since the interaction of surface plasmons with proximal dyes is short-lived (<50 fs), a significant change in the value of kspon is

expected only if the nonradiative decay rate greatly exceeds the rate of radiative processes (weakly

emitting fluorophores). This is not the case for strongly emitting Cy3.5 and Cy5 fluorophores

exhibiting PL quantum yields over 30%.[244],[245]

The value of kmetal* is nominally determined by the rate of the plasmon energy relaxation in Au NPs. Due to the presence of a surrounding polymer matrix, however, the excitation energy in metal could be thermalized into long-lived states of a polymer or a polymer-dye interface. This could be attested by the long-lived photoinduced absorption feature observed in TA measurements of LA-PIMA-PEG-capped Au NPs. The relaxation of this PA feature (~100 ps) is more than 103

times slower than the plasmon dephasing time constant. We therefore propose that the presence of

the polymer matrix effectively extends the lifetime of plasmon excitations in Au NPs.

The results of model calculations for the PL decay in Cy3.5−AuNP and Cy5−AuNP

assemblies are summarized in Figure 2.10b, c, as well as in Table 1. Out of four independent

parameters entering eqs 3 and 4, kspon, kET, kPIRET, and kmetal*, the first two were acquired from experimental measurements. The value of kspon was obtained from the single-exponential fit of the 59

PL intensity decay for nonconjugated dyes (gray curve in Figure 2.10b, c), while the initial value

of kET was determined by assuming that nonradiative energy transfer was the sole process

responsible for PL quenching in Au-conjugated dyes. In the case of Cy3.5−AuNP assemblies, this

−1 yielded: kET = kspon × EQ/(1 − EQ) = (0.21 ns) , where EQ = 0.882 is the PL quenching efficiency.

In order to include any non-ET contributions to PL quenching (e.g., dye → Au photoinduced charge transfer), the above equation was modified to kET + knon-ET = kspon × EQ/(1 − EQ), which effectively reduced the value of the kET parameter. To allow for small adjustments of the ET rate

during the fitting procedure, both kET and kPIRET parameters were allowed to be varied in a self- consistent manner in order to obtain the best fit of the experimental PL intensity decay. In the case

−1 of Cy3.5−AuNP assemblies, the ultimate value of kET = (0.32 ns) , resulting from a model fit,

−1 was 34% lower than the initial kET = (0.21 ns) , suggesting that 34% of excitations in Cy3.5 are

likely to be depleted by non-ET processes (e.g., dye → Au charge transfer). The resulting PIRET

−1 rate, kPIRET = (0.29 ns) , was found to be similar to the kET value, indicating that the excitation energy transfer to and from the dye molecule occurred with comparable rates. The similarity of kET and kPIRET parameters could be explained by the nearly symmetric overlap integrals associated with ET (e.g., FRET, NSET) and PIRET processes (see eq 1).

-1 - Table 1 Summary of Model Calculations for the Three Conjugated Systems, Showing k spon, k

1 -1 ET and k PIRET Inverse Rates. 60

The results of model calculations for the Cy5-AuNP system are shown in Figure 2.10c and

-1 Table 1. The value of kspon = (1.23 ns) was determined by fitting the PL intensity decay in nonconjugated Cy5 samples (gray curve), while the initial value of kET = ksponEQ/(1 - EQ) =

(0.19ns)-1was estimated from the quenching efficiency. Coupled rate equations were then solved in a self-consistent manner to obtain the best fit of the experimental PL decay for the Cy5-AuNP

−1 system (Figure 2.10c, purple curve). The resulting ET rate, kET = (0.51 ns) , was found to be

−1 lower than its initial assessment of (0.19 ns) determined from EQ = 0.866. Accordingly, we

estimate that PL quenching in Cy5−AuNP assemblies could be contributed up to 63% by non-ET

processes. Such a non-ET contribution to Cy5 PL quenching could arise from Au-conjugated

molecules that share the electron density with the metal surface (e.g., molecules that are in direct

electrical contact with Au) and therefore essentially dark. Due to their absorption contribution,

these dyes are accounted for in calculations of PL intensity quenching (ΔI), but not in the PL

lifetime measurements (Δτ). Finally, the PL lifetime enhancement effect in model calculations

based on eqs 3 and 4 could be reproduced only by assuming that kmetal* was much lower than kspon.

This condition was necessary for the excited state “reservoir” that feeds the dye excitation

population to exist. The exact nature of such an excited state, however, could not be determined in

this work.

In the case of Alexa 488−AuNP colloids, the application of the fitting procedure based on

eqs 3 and 4 was only partially successful (see Figure 2.11). While the model reproduced the long-

term decay component accurately, the early-time PL decay could not be fit well using a reasonable

range of kET and kPIRET parameters. The discrepancy between the experimental and simulated PL intensities for t < 1 ns could be attributed to the contribution of short-lived processes that were not accounted for by the three rates in eqs 3 and 4. For instance, the dye → Au NP photoinduced 61

charge transfer affecting a subpopulation of dyes that are electrically coupled to a metal surface

represents one example of such processes. In regard to a long-term decay of the Alexa 488−AuNP

emission, the fitting procedure has yielded a comparatively large value of kPIRET (see Table 1) that exceeded the corresponding rates in Cy3.5−AuNP and Cy5−AuNP samples.

Figure 2.11 Fitting the delayed PL in Alexa 488 - AuNP assemblies (purple circles) with the

solution of coupled rate equations (Eqs. 3 & 4) based on the reverse ET model (purple curve). The

(@ -1 best fit is obtained for �-./01= (0.15 ns) . Adapted with permission from ref. [331]. Copyright

2019 American Chemical Society.

One of the key questions raised by the present investigation concerns the lack of previous reports of the reverse PIRET process in dye-metal assemblies. According to the majority of publications reporting PL intensity decay measurements, the conjugation of dyes with metal 62 nanoparticles results in the reduction of the PL lifetime.[31],[60],[221],[247]-[254] Only one other study[255] has reported a 1.5-fold enhancement in the PL lifetime of Au-conjugated Cy3 fluorophores despite

~41% PL intensity quenching. After a careful examination of the existing literature, we were not able to determine any particular condition that was responsible for a different trend observed in the PL lifetime by both the present work and ref [255], as similar dye-metal assemblies (e.g., Cy5-

DNA-AuNP) were also reported to result in the reduced PL lifetime by other groups.[249]

Nevertheless, the present observation of the enhanced PL lifetime was supported by transient absorption measurements, clearly showing that the excited-state population in Au-conjugated dyes exhibits a longer lifetime than in isolated molecules. Such PL lifetime enhancement in metal coupled fluorophores can be predicted for an arbitrary dye-metal assembly using the aforementioned coherent ET model (eqs 3 and 4) as a parametric function of kPIRET and kET rates.

As shown in Figure 2.12, the increase in the dye→metal ET rate relative to kspon leads to a reduction in the PL lifetime of a conjugated fluorophore, Δt= tdye-metal/tdye < 1 (blue-green color region), which is commonly observed in PL quenching measurements. Meanwhile, when both kET and kPIRET parameters are large (kET, kPIRET > kspon), an enhancement in the PL lifetime of a conjugated fluorophore Δt> 1 (orange red color region) is expected. 63

Figure 2.12 Δt contour plot showing possible changes in the PL lifetime of a metal-conjugated

dye (Δt=tdye-metal/tdye) as a function of kET and kPIRET rates, respectively. Both nonradiative quenching ET and PIRET transfer rates are normalized by the dye’s spontaneous decay rate (kspon).

The dotted red contour line encircles the parametric region where the PL lifetime of a conjugated

dye is enhanced, Δt > 1, relative to its PL intrinsic lifetime. Adapted with permission from ref.

2.2 Energy transport in CsPbBr3 perovskite nanocrystals solids

2.2.1 Introduction 64

Excitonic energy transfer (ET) represents the primary step of energy conversion during

photosynthesis[256] and is the key mechanism of the energy flow in excitonic solids[257]-[259] and

organic crystals.[260],[261] Recently, assemblies of colloidal semiconductor nanocrystals (NCs) have

emerged as another promising artificial system exhibiting cascade-like exciton energy transfer on

par with charge transfer (CT) processes.[257],[262]-[268] Similar to molecular solids,[100] ET processes in NC films are mediated by quasi dipole-dipole coupling[101] and proceed via an interparticle transfer of neutral excitons, which represents a potentially useful mechanism of energy concentration in photovoltaic,[102]-[106] solid-state lighting,[107]-[110] and photocatalytic[111],[112]

applications.

In general, energy transport in quantum dot (QD) solids is negatively affected by the

morphological diversity of semiconductor colloids.[269]-[272] The size dispersion of chemically

prepared NCs[273] can cause the photoinduced energy to be funneled into potential minima of larger size nanoparticles (e.g., due to Anderson localization[274]), where it is thermally or radiatively quenched. Likewise, the size-induced energy disorder can diminish the rate of the resonant energy transfer between proximal nanocrystals as a result of a reduced donor−acceptor energy overlap.

This may lead to a slower diffusion and, therefore, a smaller exciton diffusion volume. Another issue of energy transport in QD solids concerns surface defects in colloidal NCs.[102],[275]-[280] These

states can localize photoinduced carriers promoting exciton dissociation,[281],[282] which in turn

causes the exciton diffusion length to diminish.

The aforementioned issues of energy transport in quantum dot solids are potentially

avoidable in assemblies of lead halide perovskite nanocrystals (e.g., CsPbX3), the band gap of which is less sensitive to particle size variations.[283]-[285] For instance, a 10% size dispersion in

CsPbX3 NC solids corresponds to energy disorder of less than 1% relative to the nanoparticle band 65

gap value. As a result, the energy landscape across perovskite QD solids is expected to be flatter

than that of traditional semiconductors, such as cadmium chalcogenides (see Figure 2.13b).

Furthermore, both mixed and inorganic halide perovskites (e.g., CH3NH3PbX3 or CsPbX3) exhibit a high tolerance to surface defects (up to 1-2% of atoms),[286]-[289] as can be attested by the high

photoluminescence quantum yield (QY) of corresponding nanomaterials without any surface

treatment.[290]-[294] Such defect tolerance of lead halide perovskites has been attributed to the fact

that intrinsic defects primarily lead to shallow transition levels,[291] which are electrically benign.[295] Thus far, these unique electronic properties of metal halide perovskites have been utilized primarily in charge transfer applications of these materials, as evidenced by high- performance photovoltaic[296]-[298] and light-emitting devices.[293],[294],[299],[300] On the other hand,

the employment of halide perovskite QD solids as energy transport systems has received much

less attention.[301]-[304]

Figure 2.13 The steady-state emission of CsPbBr3 NC solids blended with Au nanoparticles. (a).

A small blue shift of the emission in (Au, CsPbBr3) solids is attributed to the shorter diffusion volume of CsPbBr3 excitons in Au-doped films, which prevents these excitations from reaching some of the potential minima in the film. The magnitude of the spectral shift indicates the energy disorder of less than 5 meV. (b). An estimated band gap energy distribution resulting from a 10% 66

size dispersion in 2.5-nm CdSe and 10-nm CsPbBr3 NCs. Adapted with permission from ref. [334].

Here, we use a time-resolved fluorescence quenching technique[263],[264] to explore the

dynamics of energy diffusion processes in electrically coupled CsPbBr3 nanocrystal solids. By

[299] [305],[306] blending CsPbBr3 NC films (passivated by short metal-halide CsI or CdCl2 ligands)

with fluorescence (FL)-quenching Au nanoparticles, we were able to deduce the exciton diffusion

length of ldiff ≈ 52-71 nm. The observed ldiff range is larger than typically reported for lead and

cadmium chalcogenide NC solids comprising short, electrically coupling ligands (e.g., PbS,

CdSe).[269],[280],[307]-[309] Steady-state FL-quenching measurements were able to attribute such a large exciton diffusion volume to a low disorder of exciton energies in perovskite QD films, which was found to be less than 5 meV at room temperature.

In addition to the homogeneous energy transfer in assemblies of perovskite QDs, we have explored the possibility of inducing heterogeneous ET processes in a blended film of CsPbBr3 and

CdSe NCs. To enable the energy exchange between the two materials, CdSe NCs were size-tuned

for a maximum spectral overlap of their exciton absorption feature with the perovskite QD

emission profile. Under these conditions, CdSe NCs could resonantly accept nonradiative energy

from proximal CsPbBr3 NCs. The exciton diffusion dynamics of (CdSe, CsPbBr3) blended films

was then analyzed using a combination of PL lifetime quenching and the sample transmitted

excitation photoluminescence (STEP) spectroscopy. It was found that the diffusion of CsPbBr3

NC excitons to CdSe sites was accompanied by a high probability of their dissociation into free

electron−hole pairs. In particular, STEP measurements have indicated that only 4.5% of the

excitation energy absorbed by the perovskite film was transferred to the subpopulation of CdSe

NCs in the form of excitons. Meanwhile, ∼70% of perovskite QD excitations were quenched as a 67

result of CdSe-induced non-ET processes, such as the CsPbBr3 → CdSe charge transfer. These

observations indicate that perovskite solids could be developed to support a two-step energy

concentration process, where a cascade-like energy transfer across the population of CsPbBr3 NCs is followed by the delivery of a photoinduced charge to incorporated CdSe acceptors.

Figure 2.14 Spatial profile of exciton diffusion in CsPbBr3 QD solids. (a) High-resolution transmission electron microscope (TEM) image of 11.1 ± 0.9 nm CsPbBr3 NCs. (b) Spatial profile

of the photoinduced energy diffusion in 11 nm CsPbBr3 NC quantum dot solids reconstructed from the measured probability of exciton dissociation in CdCl2-treated films. Adapted with permission from ref. [334]. Copyright 2019 American Chemical Society.

2.2.2 Homogeneous energy transfer in assemblies of perovskites

The dynamics of energy flow in nanocrystal solids reflects a complex interplay of bulk and molecular characteristics. Typically, the absorption of a photon leads to the formation of a weakly confined exciton that diffuses through a film at rates determined by the strength of the interparticle coupling.[310] Such bound electron−hole pairs in a quantum dot solid “hop” through the thermally

accessible energy landscape toward the potential minimum. The subsequent decay of excitons 68

occurs either by their recombination or dissociation into a free electron−hole pair. In the latter case,

emerging free carriers will drift through the solid with mobilities that depend on the strength of

interparticle coupling.[311]-[315]

Energy transfer processes in quantum dot solids are usually characterized using transient

absorption/time-resolved PL microscopy and other spectroscopy methods,[267],[280],[316]-[319] which

allow determining both the spatial extent of the energy migration (exciton diffusion length, ldiff)

and the rate of energy transfer between neighboring nanocrystals (khop). An important benefit of the present fluorescence-quenching approach lies in the ability to study solids with a flat energy landscape. Since the present technique does not track the changes in the energy of diffusing excitons, it is particularly well suited for systems exhibiting a negligible energy disorder, such as assemblies of CsPbBr3 NCs. Within this approach, the exciton diffusion length is determined by

analyzing the reduction in the emission lifetime of CsPbBr3 NC films, FL, resulting from the incorporation of fluorescence-quenching Au nanoparticles (Au NPs) into the film. If the FL lifetime of Au-doped CsPbBr3 films is not altered significantly, the exciton diffusion length in

CsPbBr3 NC solids is short, since most excitons dissociate before reaching Au NPs. Conversely,

if the exciton diffusion length is relatively large, the energy migration across the film is greatly

impeded by the recombination on Au sites, leading to a significant reduction in the PL lifetime of

Au doped CsPbBr3 solids relative to CsPbBr3-only samples.

According to Figure 2.15a and b, quenching of the CsPbBr3 NC emission in (Au, CsPbBr3)

blended films due to incorporate Au NPs is apparent. For a solid featuring a ∼1:100 ratio of Au to

CsPbBr3 colloids (based on the TEM particle count), the band gap emission of perovskite solids was reduced 10-fold relative to the FL intensity of original CsPbBr3 NC assemblies that received

the same photon flux. There are a few technical considerations that were implemented to ensure 69

that FL quenching was efficient. First, to prevent the phase separation between CsPbBr3 and Au colloids upon film processing, the colloidal stability of Au NPs was maximized by choosing sufficiently small nanocrystals (∼6 nm). A relatively small size of Au colloids was also beneficial for suppressing plasmon induced FL enhancement effects, which are often reported for Au NPs exceeding 20 nm in size.[320]-[322] Second, surfaces of metal NPs were stripped of long-chain

insulating ligands (OLAM) in order to facilitate the CsPbBr3 → Au charge and energy transfer processes.

Figure 2.15 Illustration of the fluorescence (bulk)-quenching approach for estimating the exciton diffusion length in CsPbBr3 NC solids, which relies on blending investigated films with FL-

quenching Au nanoparticles. (a) TEM image of (Au, CsPbBr3) blended solids exhibiting a

Au:CsPbBr3 = 1:100 nanoparticle ratio. The FL intensity is reduced relative to CsPbBr3-only

samples. (b) TEM image and a corresponding FL spectrum of CsPbBr3-only solids used as a

reference sample. (c) Scanning electron microscope (SEM) image of a (Au, CsPbBr3) film on FTO.

The scale bar is 100 nm. (d) Decay of exciton population, Nex, in a solid of identical quantum dots versus the number of interparticle energy transfer events (hops), which depends on a single-hop exciton dissociation probability, pdiss (1 hop). If Au NPs are introduced into a solid, quenching of excitons is induced at shorter distances (green dashed line), causing the reduction in the (Au, 70

CsPbBr3) emission intensity. By comparing the ratio of the two areas under the exciton population curve (bright-exciton population to the total exciton population) to the FL attenuation ratio in (Au,

According to the bulk quenching formalism,[262],[263],[323],[324] the incorporation of Au NPs

within a nanocrystal solid forces exciton dissociation at shorter distances from the excitation site.

The corresponding reduction in the exciton diffusion length is related to the Au-Au interparticle

separation, RAu-Au. To determine RAu-Au for a (Au, CsPbBr3) blended film, we followed a

[264] previously developed protocol, which assumes a simple cubic packing of CsPbBr3 NCs, such

* that �7A(7A = √�10B�C93 + �7A, where DQD and DAu are average sizes of CsPbBr3 and Au NPs, respectively, and nTEM is an apparent ratio of Au to CsPbBr3 nanoparticles on a TEM grid. A denser packing order of perovskite nanoparticles is possible and may result in the reduction of RAu-Au. In

the limiting case of closed packed ordering (face cubic centered, fcc), Au-to-Au interparticle

distance becomes RAu-Au(fcc) ≈ 0.89RAu-Au (simple cubic).

Quantitatively, to determine the exciton diffusion length in CsPbBr3 QD solids, we assumed that exciton diffusion in NC solids obeys the three-dimensional random walk approximation

'E> (Wiener formula) �#;DD = �F42I�F42/6 , where lhop is the average center-to-center spacing between adjacent dots and nhop is the number of interparticle hops. By setting the average

probability of exciton dissociation per single hop, pdiss(1 hop), to be a free parameter, we can

express the probability of exciton dissociation on its nth hop as �L�F42M = �#;33(1) × (1 −

5+,-(@ �#;33(1)) . The resulting probability distribution, shown as a black curve in Figure 2.15d, reflects the fraction of the exciton population in a solid, Nex(nhop), which remain excited

(unquenched) after nhop number of energy transfer events. Upon the incorporation of Au NPs 71 within the perovskite NC films, excitons become quenched at shortened distances given by a parameter lfree travel (Figure 2.15d dashed line), which is determined from RAu−Au (Figure 2.16). In this case, only a fraction of the initial exciton population will remain “bright”.

Figure 2.16 Illustration of the general strategy for determining the effective mean free path of excitons in a Au-doped nanocrystal solid. If we assume that a single photon is absorbed by a semiconductor NC in a film, the average scatter free diffusion of the resulting exciton can vary from 0 to about RAu-Au/2 (minus the diameter of a Au NP) depending on the exact location of the absorbing NC. The statistically averaged distance from the absorbing nanocrystal to the closest Au 72

nanoparticle is defined as the scatter free distance, lfree travel, for which half of semiconductor NCs

are located closer and the other half further away from a Au NP (volumes V2 and V3, respectively).

In other words, the total volume of a V2 sphere (excluding the effective volume of a Au

nanoparticle) should be equal to the interstitial volume (V3). Given the large number of

nanoparticles in the film, the statistical error of estimating lfree travel using the aforementioned

American Chemical Society.

Figure 2.17 illustrates the effect of Au NP concentration in (Au, CsPbBr3) blended solids on the perovskite QD FL intensity decay. The two types of ligand exchange treatments, utilizing either I− or Cl− halides, were applied in an effort to reduce interparticle spacing and prevent the formation of halide vacancies. Out of several molecular halides (containing formamidine, methylammonium, cesium, and cadmium cations), we have selected the best-performing (in terms of PL intensity) iodine and chloride passivation strategies, corresponding to CsI and CdCl2

treatments, respectively. In most tests, the exposure of CsPbBr3 NCs to molecular halide solutions

did not produce significant changes of the PL spectral position (see Figure 2.18a), indicating that

Br → I and Br → Cl halide exchanges in exposed nanoparticle films were minimal. 73

Figure 2.17 TEM images and changes of the FL intensity decay corresponding to the increasing

concentration of Au NPs in CsI and CdCl2 treated (Au, CsPbBr3) solids. (a) TEM image of a

- mixed, Au + CsPbBr3 nanoparticle sample used for developing CsI treated (Au, CsPbBr3) solids.

(b) TEM image of a mixed, Au + CsPbBr3 nanoparticle sample used for developing CdCl2-treated

(Au, CsPbBr3) solids. (c, d) Changes of the FL intensity decay corresponding to the increasing concentration of Au NPs in (c). CsI-treated (Au, CsPbBr3) solids and (d) CdCl2-treated (Au,

Society.

Figure 2.18 The photoluminescence and absorbance of CsPbBr3 NC solids without surface treatment and after being treated with CsI and CdCl2 molecular halides. (a). The emission profile of CsPbBr3 NC solids without surface treatment (black), and after being treated with CsI (red) and

CdCl2 (blue) molecular halides. (b). The absorption of CsPbBr3 NC solids without surface treatment (black) and after being treated with CsI (red) and CdCl2 (blue) molecular halides.

For both types of molecular halide treatments in Figure 2.17c and d, the increase in the

Au:CsPbBr3 ratio (expressed in terms of the relative particle count) resulted in the proportional

reduction of the PL lifetime. According to the bulk quenching strategy, the PL lifetime of Au-

doped films was divided by the PL lifetime of CsPbBr3-only NC solids in order to determine the

fraction of “bright” excitons in blended films (see Figure 2.15d). This procedure is illustrated in

Figure 2.19a for the case of CsPbBr3:Au ≈ 2000 blended solids, where Au-limited free travel of

excitons was estimated to be ∼66 nm. The blue and red curves in Figure 2.19a represent the decay

of exciton populations in CsPbBr3 NC solids treated with CsI and CdCl2 halide molecules,

respectively. The unshaded area under the Nex curve represents the exciton population that

recombines radiatively before reaching Au nanoparticles at an average diffusion distance of lfree

travel. 75

- - Figure 2.19 Illustration of the exciton population for CsI and CdCl2 treated CsPbBr3 NC solids.

- - (a) Fitting exciton population curves for CsI and CdCl2 treated CsPbBr3 NC solids using a single

fitting parameter, pdiss(1 hop). The fit is obtained for each (Au, CsPbBr3) solid by demanding that

the ratio of bright-to-total area under the Nex(nhop) curve matches the known FL attenuation ratio

for this solid. (b) Probability of exciton dissociation on each hop, pdiss(1 hop), determined

according to the procedure described in (a) using different concentrations of Au NPs in

(Au,CsPbBr3) solids. Average pdiss(1 hop) values for CsI- and CdCl2-treated CsPbBr3 NC solids

were determined to be 0.8% and 0.4%, respectively. (c) Spatial profiles of the energy diffusion for

7='+ ./01+'2-#0334 By solving = �G8(��, ��H)/�G8(��H) equation for each of the 7='+','()

investigated (Au, CsPbBr3) blends in Figure 2.17c and d, we find exciton dissociation probabilities

per single hop, pdiss(1 hop). The results of these calculations are summarized in Figure 2.19b, 76

showing the two sets of pdiss values obtained for CsI- and CdCl2-treated solids. After averaging the

experimental data over different Au:CsPbBr3 ratios, it was concluded that CsI-treated CsPbBr3 NC films had a greater probability of exciton dissociation per single energy transfer event (pdiss(CsI) =

0.008, pdiss(CdCl2) = 0.004). The extracted pdiss values were subsequently used to reconstruct the

evolution of the exciton migration in each film type, according to the following equation:

5+,-(@ [264] �'EL�F42M = �6 × �#;33 × (1 − �#;33) . These results are illustrated in Figure 2.19c,

which shows the exciton survival probability for CdCl2- and CsI- treated solids versus the

corresponding diffusion length. Based on the 1/e drop in the exciton population, we estimated that

the average diffusion lengths for CsI- and CdCl2- treated solids were 52 and 71 nm, respectively

(see Table 2), which corresponded to average “hop” numbers of CsI = 84 hops and CdCl2

= 260. Overall, the energy diffusion volume in CsPbBr3 NC solids was found to be greater than in the case of nanoparticle solids comprising lead and cadmium chalcogenide

NCs.[264],[273],[280],[313],[325] It should be noted, however, that the exciton diffusion in superlattices of

CdSe NCs featuring long-chain ligands has been reported to exceed this value.[324]

Table 2 Summary of Exciton Transport Characteristics for CsPbBr3 Nanocrystal Solids Obtained

Using the Fluorescence (Bulk) Quenching Approach.

Since the total time of the exciton diffusion is given by the FL lifetime in CsPbBr3-only

solids, FL, we can estimate the exciton diffusivity using the Einstein equation: 77

" " ', ', M5 N �'E> = +,- @ = +,- +,- (1) IJ1 K IL1 K56(P3-QR=*(45,&)

where lhop is the center-to-center interparticle distance, estimated to be ∼11.5 nm.

According to eq 1, the room temperature exciton diffusivities for CsI- and CdCl2-processed solids

were 0.009 and 0.018 cm2 s−1, respectively (see Table 2). The average rate of energy transfer

between neighboring CsPbBr3 nanocrystals in a solid was determined from the average energy

@ (@ transfer time, as follows:�01 = =< �F42 >/�G8�� . We found that CdCl2-treated films MK+,-N

-1 exhibited an approximately 2 times greater ET rate (kET = 1/0.065 ns ) than solids treated with CsI

-1 complexes (kET = 1/0.142 ns ). By using average hop times, we were able to draw a quantitative comparison of intradot coupling between halide perovskites and several chalcogenide semiconductor quantum dots, the exciton dynamics of which was obtained using the same bulk quenching strategy. In particular, the range of �G8 for CsPbBr3 NC solids (0.065-0.14 ns, Table 2) was found to be comparable to that of MPA-linked PbS NC films (0.089 ns, ref [264]), while somewhat lower than in oxalic acid-linked CdSe/CdS core/shell NCs (0.2 ns, ref [313]). Overall,

Forster resonance energy transfer (FRET) rates for CsPbBr3, PbS, and CdSe/CdS core/shell NCs, measured using the same experimental strategy, appeared to be rather similar.

One of the key factors responsible for the long-range diffusion of excitons in perovskite

QD solids was revealed by the steady-state PL measurements of Au-doped CsPbBr3 films. In

general, the spectral position of the band gap fluorescence in substrate-bound NCs is red-shifted

relative to the corresponding value in solution.[326] The spectral shift results from the exciton diffusion toward the potential energy minima of larger size nanoparticles in a film. The addition of Au NPs to the nanocrystal solid diminishes the excitons’ diffusion volume, which causes some of the potential minimum sites to become inaccessible. As a result, the emission profile of Au- 78

doped nanocrystal solids is expected to be blue-shifted relative to the emission of undoped

nanocrystal solids. This trend has been previously observed for solids of PbS NCs,[264] where an

increasing fraction of Au quenching sites was accompanied by the detectable blue-shift of the band

gap fluorescence. In the case of CsPbBr3 solids, the spectral position of the band gap emission did not produce any noticeable changes with the increasing concentration of Au colloids (Figure 2.13a).

This behavior is characteristic of a low-energy disorder in the film, as both unrestricted and Au- restricted exciton travel find the same minima of the potential energy. Taking into account an experimental uncertainty in the determination of the PL spectral position, we estimated that the energy disorder of CsPbBr3 QD solids was below 5 meV. This value is lower than the thermal

energy of room-temperature samples, ΔE = 2sqrt(2ΔkT) ≈ 30 meV, suggesting that the motion of

excitons in investigated CsPbBr3 NC films is consistent with a random walk approximation.

2.2.3 Heterogeneous energy transfer in CsPbBr3 with CdSe

The observed long-range energy diffusion in solids of CsPbBr3 NCs suggests the

possibility of using these assemblies as energy-concentrating materials. To explore this prospect,

CsPbBr3 NC solids were blended with energy accepting CdSe NCs, the exciton absorption of which was spectrally matched to the perovskite QD emission profile. Under these conditions,

CsPbBr3 NCs can resonantly transfer their nonradiative energy to CdSe dots. In order to enhance

the CsPbBr3 → CdSe energy transfer efficiency, the charge transfer coupling between CsPbBr3

and CdSe NCs in a solid was reduced by overcoating CdSe surfaces with a thin layer of CdS.

Furthermore, the fraction of CdSe NCs in blended films was maintained at a relatively high value

of 0.8 by the total particle count (see a characteristic TEM image in Figure 2.20b). While this

geometry is not practical for concentrating the photoinduced energy, it allowed for an accurate

measurement of the CsPbBr3 → CdSe ET efficiency, since each perovskite NC in the assembly 79

was coupled to multiple energy acceptors.

Figure 2.20 Illustration of the STEP technique for measurements of the CsPbBr3 with CdSe energy transfer efficiency. (a) FL spectrum of the (CsPbBr3, CdSe) blended film. CdSe NCs were overcoated with a thin layer of CdS to improve the emission quantum yield. (b) TEM image of a blended (CsPbBr3, CdSe) nanoparticle solid. (c) FL intensity decay of the CsPbBr3 NC band gap emission corresponding to CsPbBr3-only and (CsPbBr3, CdSe) nanoparticle solids. The FL quenching efficiency was determined using, �1T1 = 1 − �97/�9, where the respective lifetime

H H values were obtained from �+U = ∑;V@ �; �;/ ∑;V@ �; . (d) Absorption profiles of CsPbBr3 and

CdSe NCs along with the absorption profile of the excitation filter (Alexa 488) used in STEP

measurements. (e) Illustration of the STEP concept used for the measurements of the CsPbBr3 → 80

CdSe energy transfer efficiency. The broad-band excitation light was passed through an excitation

filter (a solution of Alexa 488 dye), which spectrally matches the donor (CsPbBr3 NC) absorption.

-8 Such an excitation filter causes the acceptor (CdSe) emission, �7 , to change proportionally to the energy transfer efficiency, ED→A. To obtain the ED→A value, the measured f-ratio (see text) was fitted with a model parametric curve, ftheor(ED→A), featuring a single fitting parameter, ED→A. (f)

STEP measurements of ED→A in (CsPbBr3, CdSe) nanoparticle solids. Observed experimental fD

values (blue circles) fall between the two model curves corresponding to ED→A = 3% and ED→A =

6%. Consequently, we estimate that ECsPbBr3→CdSe ≈ 4.5%. Adapted with permission from ref. [334].

The interaction of CsPbBr3 and CdSe NCs in blended solids can result in the transfer of

both photoinduced charges and neutral excitons. The combined quantum efficiency of the two

processes, Etot = ECT + EET, can be determined by measuring the donor emission quenching in the

presence of the acceptor moiety. If �9 is the FL lifetime of CsPbBr3-only solids and �97 is the FL

lifetime of CsPbBr3 in the presence of CdSe acceptors, then �1T1 = 1 − �97/�9. To determine the FL lifetime value, the FL intensity decay was fitted using a triple exponential function and

H H averaged according to �+U = ∑;V@ �; �;/ ∑;V@ �;. Figure 2.20c compares the fluorescence intensity decay in CsPbBr3-only and CsPbBr3-CdSe (donor-acceptor) films, corresponding to Etot = 76%

(Figure 2.20c). To determine the fraction of Etot, which was contributed by the CsPbBr3 → CdSe energy transfer process, we have employed STEP spectroscopy.

STEP spectroscopy[327]-[331] was recently developed for measurements of the donor →

acceptor energy transfer efficiency, ED→A. It is based on the assumption that the number of photons

-8 emitted by an acceptor fluorophore, �7 , depends linearly on the number of excited acceptor (A)

and donor (D) molecules, NA and ND, respectively: 81

-8 �7 = ��7(�7 + �9→7�9) (2)

Where QYA is the emission quantum yield of the fluorophore A in the presence of the donor D (as measured in the donor−acceptor assembly). The ED→A parameter represents the

percentage of donor excitations that are transferred nonradiatively to the acceptor moiety. To

determine ED→A, the donor−acceptor sample was excited using a broad-band light source, and the

-8 emission intensity of the acceptor dye �7 (E) was recorded. The excitation light was then spectrally shaped using donor-like filters (Figure 2.19d, e) designed to suppress the excitation of perovskite NC donors in the investigated sample (ND ≪ NA). As a viable strategy to achieving

such a suppression, we used a solution of organic molecules (Alexa 488 dye) for shaping the

incident light. The filter selection was justified by numerical simulations, which demonstrated that

an Alexa 488-based excitation filter causes the ratio of excited acceptor to donor molecules in the

sample to drop significantly with the increasing filter optical density. From the experimental

standpoint, tuning the filter optical density (OD) in the 0−3 range was realized through a stepwise

dilution of the Alexa 488 dye solution in the excitation filter cuvette (see Figure 2.21). This simple

strategy helped maintain the same excitation/detection optical geometry throughout STEP

measurements, circumventing the need for beam realignment in-between runs. The energy transfer

efficiency, ED→A, was then determined using the spectral profile of the excitation light, n(λ), and the OD of the excitation filter, as detailed in Figure 2.22. 82

Figure 2.21 The excitation scheme used in CsPbBr3 to CdSe STEP measurements. The excitation

light is passed through the excitation filter (Alexa 488). The emission of (CdSe, CsPbBr3) solids is collected at a 90-degree angle relative to the excitation beam direction using a TIRF geometry.

American Chemical Society.

STEP measurements of the CsPbBr3 → CdSe energy transfer efficiency in (CdSe, CsPbBr3)

blended solids are summarized in Figure 2.22e,f. Application of the donor-type excitation filter 84

-8 (Alexa 488 dye) has resulted in the reduction of the scaled CdSe acceptor emission (fD = �7 /NA,

red dots in Figure 2.22f) with the increasing filter OD. The measured values of fD(OD) were compared with two model parametric curves, ftheor(EET), corresponding to the energy transfer

efficiencies of 3% and 6% (blue curves in Figure 2.22f). The experimental uncertainty of STEP

measurements was estimated using the baseline run (gray dots) performed on CdSe-only solids.

Notably, fD measured for a film of CdSe NCs (a baseline sample) revealed no dependence on the

donor-filter optical density, consistent with the absence of the energy transfer in CdSe-only films.

On the basis of these measurements, we estimate that the energy transfer efficiency in the

investigated (CdSe, CsPbBr3) blended solids was 4.5%. This value represents the percentage of photons absorbed by the population of CsPbBr3 NCs that were ultimately transferred to CdSe

acceptors as excitons (regardless of the CdSe emission quantum yield). The difference between

EET = 4.5% and ETOT = 76% was attributed to the non-ET fluorescence quenching, which could be

the result of the CsPbBr3 → CdSe charge transfer, ECT.

Table 3 Summary of FL Lifetime and STEP Measurements for (CdSe, CsPbBr3) Blended Solids.

According to STEP measurements, the transfer of the photoinduced energy absorbed by

CsPbBr3 NCs into the subpopulation of CdSe acceptors was only 4.5% efficient. It appears that the greater portion of the photoinduced energy that was lost by perovskite QDs due to proximal

CdSe NCs did not result in the formation of CdSe excitons. Generally speaking, quenching of the 85

CsPbBr3 NC fluorescence by proximal CdSe NCs could be caused by three CdSe-induced

processes: exciton transfer, exciton dissociation, and exciton recombination through nonradiative

channels. The latter mechanism is the most convoluted, as it accounts for such processes as PL

quenching due to CsPbBr3−CdSe ion exchange, halide vacancy formation, loss of passivation, etc.

Notably, when CdSe-induced exciton recombination is negligible, the remaining two processes of perovskite PL quenching are the exciton transfer and the exciton dissociation, which correspond to CsPbBr3 → CdSe energy transfer and CsPbBr3 → CdSe charge transfer processes, respectively.

If this is the case, the 4.5%-efficient energy transfer would imply that 76% − 4.5% = 71.5% of quenched CsPbBr3 excitons are due to the CsPbBr3 → CdSe charge transfer mechanism. This scenario is consistent with a previously reported interplay of ET and CT processes[330] in a more

controlled assembly of CdSe NCs and Cy7 dyes, where the donor → acceptor charge transfer

accounted for 99% of PL quenching. A high probability of the CdSe-induced exciton dissociation

(charge transfer) is also consistent with the relative positions of the potential energy minima at the

[332],[333] CsPbBr3/CdSe boundary (Figure 2.23). Nevertheless, since CdSe-induced exciton recombination cannot be ruled out, the actual CsPbBr3 → CdSe charge transfer efficiency is likely to be less than 71.5%. Even under these conditions, the rate of the CsPbBr3 → CdSe charge transfer could still be greater than that of the CsPbBr3 → CdSe ET process. This scenario parallels the

photoinduced dynamics of plants, where the absorption of light by multiple chlorophylls is

followed by the cascade resonant energy transfer toward a specific chlorophyll pair in the reaction

center of the photosystem. This pair promotes the charge separation leading to biosynthesis. In this

analogy, CsPbBr3 NCs mimic the role of chlorophylls, which absorb the photoinduced energy,

passing it across multiple CsPbBr3 sites with a high efficiency (exciton diffusion length 71 nm) toward the CdSe NC that receives the photoinduced hole. Ultimately, the demonstrated ability of 86

(CdSe, CsPbBr3) assemblies to funnel the photoinduced energy of CsPbBr3 NC solids into a charge separated pair at the acceptor sites makes this material system an attractive candidate for light- harvesting applications.

Figure 2.23 The energy diagram comparing the alignment of conduction and valence band edges for 8-nm CsPbBr3 (left) and 4-nm CdSe (right) nanocrystals, reconstructed from the Ref. [256].

2.3 Conclusion

In conclusion, we explored two different types of energy transfer processes in metal conjugated fluorophores and perovskite quantums. Our work demonstrates that this is not the case when the metal to dye energy transfer is significant. Under these conditions, the energy transferred 87

from the photoexcited fluorophore to a metal nanoparticle could be transferred back to the

conjugated molecule, causing an enhancement in the PL lifetime. From a theoretical standpoint,

the present observation of the enhanced PL lifetime in metal-conjugated dyes reveals the

underlying coherent interaction between metal and molecular electric dipoles. By using a

combination of time-resolved spectroscopy and rate-equation calculations, we showed that

fluorescent molecules can engage in a reversible energy transfer with proximal metal surfaces,

during which quenching of the dye emission via the energy transfer to localized surface plasmons

triggers the delayed ET from the metal back to the fluorescent molecule. The reverse ET was

shown to occur concurrently with PL quenching, which causes the PL lifetime of metal-conjugated

fluorophores to increase despite an overall reduction of the dye emission intensity. The amplitude

of the reversible ET in dye-metal assemblies was found to be proportional to the metal to dye

energy transfer efficiency, determined by means of polarization-averaged STEP spectroscopy

measurements. Theoretical simulations based on the coherent ET model were employed to explain

the observed PL lifetime enhancement in Alexa 488-AuNP, Cy3.5-AuNP, and Cy5-AuNP

assemblies, indicating that such processes could be pervasive in other dye-metal systems.

We have experimentally determined energy diffusion characteristics of CsPbBr3

nanocrystal solids. Electrically coupled assemblies of perovskite QDs have long been investigated

in relation with their excellent charge transport characteristics, recently evidenced in numerous

reports of high performance CsPbX3 nanocrystal solar cells. In this work, we demonstrate that quantum solids of CsPbBr3 NCs can also support efficient, long-range exciton diffusion processes,

which makes these materials promising candidates for energy concentration and energy-

conversion applications. An important benefit of CsPbBr3-based energy transport assemblies lies

in the low interparticle energy disorder and their high tolerance to surface defects. By using 88 fluorescence bulk quenching measurements, we demonstrate that the exciton diffusion length in

CdCl2-treated CsPbBr3 NC films averages 71 nm, which is greater than usually reported for electrically coupled solids of PbS and CdSe NCs. Such a long-range energy diffusion could be beneficial for light-harvesting applications of CsPbBr3 NC assemblies, as was demonstrated in this work through energy transfer measurements in mixed solids of perovskite NC donors and CdSe quantum dot acceptors. The present findings demonstrate that in addition to excellent charge transfer properties, assemblies of halide perovskite NCs exhibit promising energy transport characteristics.

Considering a broad scientific interest in metal-conjugated dye assemblies, the observation of long-range energy diffusion in CsPbBr3 NC solids, the present findings could have an important scientific and technological impact on the development of biological sensors, light-emitting materials, and light-harvesting assemblies.

CONCLUSION

In conclusion, we discuss the energy transport in colloidal inorganic nanocrystals including both semiconductor and metal systems. In semiconductor energy transfer systems, based on the special size-dependent properties of semiconductor nanocrystals, it offers opportunities to control the energy transfer dynamics on the nanoscale and it could be applied to the development of nanocrystal-dye biosensors and light-emitting materials and beyond these applications, the energy transfer in semiconductor nanocrystal assemblies is becoming increasingly attractive in the areas of photovoltaics and photocatalysis.

To make the semiconductor nanocrystal a better energy transfer material and increase the energy transfer efficiency, we firstly demonstrate that digestive ripening of semiconductor nanocrystals can be used to control both the particle size and the corresponding size dispersion. In contrast to the well-studied ripening mechanism in metal nanoparticles, the digestive ripening of semiconductor nanocrystals leads to significant changes in the average particle diameter at reaction temperatures above a certain thermal threshold. Meanwhile, at low temperatures, size focusing leading to an ensemble average diameter was observed. The existence of a thermal threshold for a particle coalescence allowed controlling the ultimate size of semiconductor nanocrystals through the DR approach, a functionally, which is not easily accessible in the DR treatment of metal nanoparticles. The ability to tune nanoparticle diameter while reducing the size dispersion has been demonstrated for samples of CdS, CdSe, CsPbBr3, and CuSnZnS4 colloids.

Then we have experimentally determined energy diffusion characteristics of CsPbBr3

nanocrystal solids. Electrically coupled assemblies of perovskite QDs have long been investigated

in relation with their excellent charge transport characteristics, recently evidenced in numerous

reports of high performance CsPbX3 nanocrystal solar cells. We demonstrate that quantum solids 90

of CsPbBr3 NCs can also support efficient, long-range exciton diffusion processes, which makes

these materials promising candidates for energy concentration and energy-conversion applications.

An important benefit of CsPbBr3-based energy transport assemblies lies in the low interparticle energy disorder and their high tolerance to surface defects. By using fluorescence bulk quenching measurements, we demonstrate that the exciton diffusion length in CdCl2-treated CsPbBr3 NC films averages 71 nm, which is greater than usually reported for electrically coupled solids of PbS and CdSe NCs. Such a long-range energy diffusion could be beneficial for light-harvesting applications of CsPbBr3 NC assemblies, as was demonstrated in this work through energy transfer

measurements in mixed solids of perovskite NC donors and CdSe quantum dot acceptors. Overall,

the present findings demonstrate that in addition to excellent charge transfer properties, assemblies

of halide perovskite NCs exhibit promising energy transport characteristics. The observation of

long-range energy diffusion in CsPbBr3 NC solids suggests that these materials could be utilized for the concentration of the photoinduced energy in a variety of light-harvesting applications.

These include multicharged photocatalytic processes (e.g., H2 production or water oxidation), which require multiple excitons to be funneled to a particular site, and processes relying on multiple exciton generation, where energetic contributions from different nanostructures are funneled into a single quantum dot. According to the present study, assemblies of halide perovskite

QDs can offer great potential in performing such energy concentrating functions.

For metal system energy transport, we demonstrate the reverse energy transfer in metal- conjugated fluorophores. Quenching of the dye fluorescence by proximal metal surfaces represent a popular strategy of determining intermolecular distances in biosensing and near-field imaging applications. The vast majority of experimental measurements rely on the reduction in the PL intensity of metal-conjugated fluorophores, ΔI, assuming that corresponding changes in the PL 91 lifetime, Δτ, are proportional. Our work demonstrates that this is not the case when the metal to dye energy transfer is significant. Under these conditions, the energy transferred from the photoexcited fluorophore to a metal nanoparticle could be transferred back to the conjugated molecule, causing an enhancement in the PL lifetime. The difference between ΔI and Δτ is expected to be particularly large when both the nonradiative quenching and PIRET rates are greater than the rate of the spontaneous decay. Consequently, metal−dye assemblies featuring a strong plasmon−exciton coupling are likely to exhibit different values of ΔI and Δτ. From a theoretical standpoint, the present observation of the enhanced PL lifetime in metal-conjugated dyes reveals the underlying coherent interaction between metal and molecular electric dipoles. By using a combination of time-resolved spectroscopy and rate-equation calculations, we showed that fluorescent molecules can engage in a reversible energy transfer with proximal metal surfaces, during which quenching of the dye emission via the energy transfer to localized surface plasmons triggers the delayed ET from the metal back to the fluorescent molecule. The reverse ET was shown to occur concurrently with PL quenching, which causes the PL lifetime of metal-conjugated fluorophores to increase despite an overall reduction of the dye emission intensity. The amplitude of the reversible ET in dye−metal assemblies was found to be proportional to the metal to dye energy transfer efficiency, determined by means of polarization-averaged STEP spectroscopy measurements. Theoretical simulations based on the coherent ET model were employed to explain the observed PL lifetime enhancement in Alexa 488−AuNP, Cy3.5−AuNP, and Cy5−AuNP assemblies, indicating that such processes could be pervasive in other dye−metal systems.

Considering a broad scientific interest in metal-conjugated dye assemblies, the present findings could have an important scientific and technological impact on the development of biological sensors, light-emitting materials, and light-harvesting assemblies.

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Reproduced with permission from [YANG, M.; MOROZ, P.; JIN, Z.; BUDKINA, D. S.;

SUNDRANI, N.; POROTNIKOV, D.; CASSIDY, J.; SUGIYAMA, Y.; TARNOVSKY, A. N.;

MATTOUSSI, H.; ZAMKOV, M. DELAYED PHOTOLUMINESCENCE IN METAL-

[2019] American Chemical Society.

134

Reproduced with permission from [YANG, M.; MOROZ, P.; MILLER, E.; POROTNIKOV, D.;

CASSIDY, J.; ELLISON, C.; MEDVEDEVA, X.; KLINKOVA, A.; ZAMKOV, M. ENERGY

TRANSPORT IN CsPbBr3 PEROVSKITE NANOCRYSTAL SOLIDES. ACS PHOTONIC. 2020,

7, 154−164.] Copyright [2019] American Chemical Society.

135

Reproduced with permission from [RAZGONIAEVA, N.; YANG, M.; GARRETT, P.;

KHOLMICHEVA, N.; MOROZ, P.; ECKARD, H.; ROYO ROMERO, L.; POROTNIKOV, D.;

KHON, D.; ZAMKOV, M. JUST ADD LIGANDS: SELF-SUSTAINED SIZE FOCUSING OF

COLLOIDAL SEMICONDUCTOR NANOCRYSTALS. CHEM. MATER. 2018, 30, 1391-1398.]

136

Reproduced with permission from [KHOLMICHEVA, N.; YANG, M.; MOROZ, P.; ECKARD,

H.; VORE, A.; CASSIDY, J.; PUSHINA, M.; BODDY, A.; POROTNIKOV, D.; ANZENBACHER,

P.; ZAMKOV, M. ION-MEDIATED LIGAND EXCHANGE AND SIZE-FOCUSING OF

SEMICONDUCTOR NANOCRYSTALS IN LIGAND-SATURATED SOLUTIONS. J. PHYS.