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Exciton Diffusion in Nanocrystal Solids EXCITON DIFFUSION IN NANOCRYSTAL SOLIDS Natalia Kholmicheva 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 August 2017 Committee: Mikhail Zamkov, Advisor Ron C. Woodruff Graduate Faculty Representative Ksenija Glusac Alexey Zayak © 2017 Natalia Kholmicheva All Rights Reserved iii ABSTRACT Mikhail Zamkov, Advisor This dissertation work is focused on exploring the unique optical and electronic properties of several semiconductor nanostructures and devices based on them. The present research demonstrates the near-field confinement of light achieved through the use of small-diameter Au nanoparticles embedded into a PbS nanocrystal solid. Using this strategy, we developed plasmonic solar cells that can harness the emission of Au nanoparticles by transferring the plasmon energy to band gap transitions of PbS semiconductor nanocrystals. The contribution of Au near-field emission toward the charge carrier generation was successfully proved through the observation of an enhanced short circuit current and improved power conversion efficiency of mixed (Au, PbS) solar cells, compare to PbS-only devices. Moreover, unique behavior of semiconductor nanocrystals makes them to be promising candidates not only for photovoltaics but for light-emitting applications as well. In light-emitting devices NC solids are designed to have large interparticle gaps that minimize exciton diffusion to dissociative sites. This strategy reduces electrical coupling between nanoparticles in a film, making the injection of charges inefficient. We demonstrated that bright luminescence from nanocrystal solids can be achieved without compromising their electrical conductivity. Our research showed that solids featuring low absorption-emission spectral overlap exhibit slower exciton diffusion to recombination centers, promoting longer exciton lifetimes. As a result, enhanced emission is achieved despite a strong electronic coupling. The inverse correlation between film luminescence and absorption-emission spectral overlap was verified by the iv comparison of CdSe/CdS and ZnSe/CdS solids and further confirmed in two control systems (ZnTe/CdSe and Mn2+-doped ZnCdSe/ZnS). Another challenging task in the development of quantum dot based solids lies in the studying the motion of neutral excitons. The nature of the exciton dissociation mechanism as well as exciton diffusion trajectories in nanocrystal solids remain poorly understood. We developed an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length and the exciton diffusivity. v My dissertation is dedicated to my parents, Elena Kholmicheva and Nikolay Kholmichev, for their endless love, support, and encouragement. vi ACKNOWLEDGMENTS I would like to thank my advisor Dr. Mikhail Zamkov for his constant guidance, personal attention, suggestions, and full support during my graduate study and research. I am sincerely grateful for the knowledge and scientific experience he shared with me. I would like to thank my committee members, Dr. Ksenja Glusac, Dr. Alexey Zayak, and Dr. Ron Woodruff for their time, valuable discussions, and for being my role models in the research world. I am also thankful to my former advisors Dr. Thomas Kinstle, Dr. Nataliya Chezhina, and Dr. Dmitry Korolev for inspiring me for future scientific career. Finally, I would like to acknowledge all the members of my research group, especially Dr. Pavel Moroz and Dr. Elena Khon for being extremely helpful in mentoring and introducing me into the basics of nanochemistry. vi TABLE OF CONTENTS Page CHAPTER I. PLASMONIC NANOCRYSTAL SOLAR CELLS UTILIZING STRONGLY CONFINED RADIATION .................................................................................................... 1 1.1 Abstract ............................................................................................................ 1 1.2 Introduction .......................................................................................................... 2 1.3 Experimental Section ........................................................................................... 7 1.3.1 Materials ............................................................................................... 7 1.3.2 Synthesis of PbS nanocrystals .............................................................. 7 1.3.3 Synthesis of PbS/CdS core/shell NCs through cation exchange .......... 8 1.3.4 Synthesis of Oleylamine-Capped Au nanocrystals (NCs) .................... 8 1.3.5 Synthesis of Oleylamine-Capped Au/Ag core/shell NCs ..................... 9 1.3.6 Synthesis of Au/Ag2S NCs from Au/Ag .............................................. 10 1.3.7 Synthesis of Au/CdS NCs from Au/Ag2S ............................................. 10 1.3.8 Preparation of the glass substrate .......................................................... 11 1.3.9 Fabrication of nanocrystal solids .......................................................... 11 1.3.10 Preparation of TiO2 electrodes ............................................................ 11 1.3.11 Deposition of mixed (Au, PbS) nanoparticle films from solution ...... 12 1.3.12 In-filling of SMENA pores with ZnS ................................................. 12 1.3.13 Deposition of Au/Pd counter electrodes ............................................. 13 1.3.14 Characterization .................................................................................. 13 1.3.15 Photoexcitation ................................................................................... 14 1.4 Results and Discussion ......................................................................................... 14 vii 1.5 Conclusions .......................................................................................................... 29 1.6 Supporting Information ........................................................................................ 30 1.7 References ............................................................................................................ 37 CHAPTER II. ENHANCED EMISSION OF NANOCRYSTAL SOLIDS FEATURING SLOWLY DIFFUSIVE EXCITONS ..................................................................................... 42 2.1 Abstract ............................................................................................................ 42 2.2 Introduction .......................................................................................................... 43 2.3 Experimental Section ........................................................................................... 46 2.3.1 Materials ............................................................................................... 46 2.3.2 Synthesis of ZnSe nanocrystals ............................................................ 47 2.3.3 Synthesis of ZnSe/CdS core/shell nanocrystals. ................................... 47 2.3.4 Synthesis of Oleylamine-Capped Au nanocrystals ............................... 48 2.3.5 Synthesis of CdSe nanocrystals. ........................................................... 48 2.3.6 Synthesis of CdSe/CdS core/shell nanocrystals .................................... 49 2.3.7 Synthesis of Zn0.87Cd0.11Mn0.02Se nanocrystals. ................................... 49 2.3.8 Synthesis of Zn0.87Cd0.11Mn0.02Se/ZnS core/shell nanocrystals. ........... 50 2.3.9 Synthesis of ZnTe NCs. ........................................................................ 50 2.3.10 Synthesis of ZnTe/CdSe core/shell NCs. ............................................ 51 2.3.11 Synthesis of CdS shell using ZnTe/CdSe nanocrystal seeds .............. 52 2.3.12 Fabrication of nanocrystal films ......................................................... 52 2.3.13 Ligand Exchange (OA to OX). ........................................................... 53 2.3.14 Characterization .................................................................................. 53 2.4 Results and Discussion ........................................................................................ 53 viii 2.5 Conclusions .......................................................................................................... 71 2.6 Supporting Information ........................................................................................ 72 2.7 References ............................................................................................................ 76 CHAPTER III. MAPPING THE EXCITON DIFFUSION IN NANOCRYSTAL SOLIDS 83 3.1 Abstract ............................................................................................................ 83 3.2 Introduction .......................................................................................................... 84 3.3 Experimental Section ........................................................................................... 87 3.3.1 Materials ............................................................................................... 87 3.3.2 Synthesis of PbS nanocrystals .............................................................. 87 3.3.3 Synthesis of Oleylamine-Capped Au
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