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Nanocrystal Superlattices

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Title:
Nanocrystal Superlattices Synthesis and Characterization under High-Pressure
Physical Description:
1 online resource (200 p.)
Language:
english
Creator:
Nagaoka, Yasutaka
Publisher:
University of Florida
Place of Publication:
Gainesville, Fla.
Publication Date:

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Chemistry
Committee Chair:
Cao, Yunwei Charles
Committee Members:
Murray, Leslie Justin
Martin, Charles R
Christou, George
Xue, Jiangeng

Subjects

Subjects / Keywords:
highpressure -- nanocrystal -- nanomaterials -- nanotechnology -- superlattice -- superstructure
Chemistry -- Dissertations, Academic -- UF
Genre:
Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract:
Nanocrystals are very small crystals, which contains from a few hundred to thousands of atoms, depending on the volume of the nanocrystals.  Nanocrystals have tunable physical properties depending on their size and/or shape, resulting in applications in many areas of modern science and technology.  High-quality colloidal nanocrystals are self-assembled to form superlattices.  In nanocrystal superlattices, the properties of the building block nanocrystals often change through interparticle interactions.  Thus, further development in the synthesis and characterization of nanocrystal superlattices will promote research of nanocrystals to be more practical research of future significance. The first accomplishment of this dissertation on self-assemblies of nanocrystals is development of a new concept to control the structure of nanocrystal superlattice, “guest molecule inclusion.”  Using this method, one can controllably create nanocrystal superlattices which adopt non-closed packed structures such as bcc, to compare with fcc nanocrystal superlattices with closest packed structures whose synthetic methods are already known.  This methodology may also provide a new way to synthesize novel organic/inorganic composite materials. Next, binary assemblies from crystals with very different shapes such as CdSe/CdS semiconductor nanorods and spherical metal nanoparticles are created.  The key to this methodology is organic additives with suitable polarity and strong affinity to spherical nanoparticles having both a high dielectric constant and a large Hamaker constant.  Finally, we study the superstructural effect on nanocrystal superlattice behavior under high pressure.  We have developed one combined technique that allows simultaneous collection of synchrotron SAXS/WAXS under high-pressure.  Using this technique, (i) the mechanical behavior of nanocrystal superlattices under high-pressure and (ii) the superstructural effect on phase-transition pressure of the building block nanocrystals(tuning by as much as 5 GPa), were investigated.  The results show that the mechanical properties of nanocrystals are determined by the superstructures, and in turn,the mechanical properties of nanocrystal superlattices are also decided by the building block nanocrystals.   This dissertation demonstrates the fundamental importance of the design of the superstructures in nanocrystal materials.  Nanocrystals have attracted much attention as promising materials, and therefore, the importance of this research should be emphasized.
General Note:
In the series University of Florida Digital Collections.
General Note:
Includes vita.
Bibliography:
Includes bibliographical references.
Source of Description:
Description based on online resource; title from PDF title page.
Source of Description:
This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility:
by Yasutaka Nagaoka.
Thesis:
Thesis (Ph.D.)--University of Florida, 2013.
Local:
Adviser: Cao, Yunwei Charles.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2015-08-31

Record Information

Source Institution:
UFRGP
Rights Management:
Applicable rights reserved.
Classification:
lcc - LD1780 2013
System ID:
UFE0045882:00001

MISSING IMAGE

Material Information

Title:
Nanocrystal Superlattices Synthesis and Characterization under High-Pressure
Physical Description:
1 online resource (200 p.)
Language:
english
Creator:
Nagaoka, Yasutaka
Publisher:
University of Florida
Place of Publication:
Gainesville, Fla.
Publication Date:

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Chemistry
Committee Chair:
Cao, Yunwei Charles
Committee Members:
Murray, Leslie Justin
Martin, Charles R
Christou, George
Xue, Jiangeng

Subjects

Subjects / Keywords:
highpressure -- nanocrystal -- nanomaterials -- nanotechnology -- superlattice -- superstructure
Chemistry -- Dissertations, Academic -- UF
Genre:
Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract:
Nanocrystals are very small crystals, which contains from a few hundred to thousands of atoms, depending on the volume of the nanocrystals.  Nanocrystals have tunable physical properties depending on their size and/or shape, resulting in applications in many areas of modern science and technology.  High-quality colloidal nanocrystals are self-assembled to form superlattices.  In nanocrystal superlattices, the properties of the building block nanocrystals often change through interparticle interactions.  Thus, further development in the synthesis and characterization of nanocrystal superlattices will promote research of nanocrystals to be more practical research of future significance. The first accomplishment of this dissertation on self-assemblies of nanocrystals is development of a new concept to control the structure of nanocrystal superlattice, “guest molecule inclusion.”  Using this method, one can controllably create nanocrystal superlattices which adopt non-closed packed structures such as bcc, to compare with fcc nanocrystal superlattices with closest packed structures whose synthetic methods are already known.  This methodology may also provide a new way to synthesize novel organic/inorganic composite materials. Next, binary assemblies from crystals with very different shapes such as CdSe/CdS semiconductor nanorods and spherical metal nanoparticles are created.  The key to this methodology is organic additives with suitable polarity and strong affinity to spherical nanoparticles having both a high dielectric constant and a large Hamaker constant.  Finally, we study the superstructural effect on nanocrystal superlattice behavior under high pressure.  We have developed one combined technique that allows simultaneous collection of synchrotron SAXS/WAXS under high-pressure.  Using this technique, (i) the mechanical behavior of nanocrystal superlattices under high-pressure and (ii) the superstructural effect on phase-transition pressure of the building block nanocrystals(tuning by as much as 5 GPa), were investigated.  The results show that the mechanical properties of nanocrystals are determined by the superstructures, and in turn,the mechanical properties of nanocrystal superlattices are also decided by the building block nanocrystals.   This dissertation demonstrates the fundamental importance of the design of the superstructures in nanocrystal materials.  Nanocrystals have attracted much attention as promising materials, and therefore, the importance of this research should be emphasized.
General Note:
In the series University of Florida Digital Collections.
General Note:
Includes vita.
Bibliography:
Includes bibliographical references.
Source of Description:
Description based on online resource; title from PDF title page.
Source of Description:
This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility:
by Yasutaka Nagaoka.
Thesis:
Thesis (Ph.D.)--University of Florida, 2013.
Local:
Adviser: Cao, Yunwei Charles.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2015-08-31

Record Information

Source Institution:
UFRGP
Rights Management:
Applicable rights reserved.
Classification:
lcc - LD1780 2013
System ID:
UFE0045882:00001


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1 NANOCRYSTAL SUPERLAT TICES: SYNTHESIS AND CHARAC TERIZATION UNDER HIGH PRESSURE By YASUTAKA NAGAOKA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013

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2 2013 Y asutaka Nagaoka

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3 To my family and my friends

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4 ACKNOWLEDGMENTS First of all, I would like to express my sincere gratitude to my supervis or, Dr. Y. Charles Cao. His perspective toward science always motivates me. It was my best luck that I could join involved in such intere sting research for my Ph.D My gratitude also goes to my Ph.D. committee members: Dr. Charles Martin, Dr. George Christou, Dr. Leslie J. Murray, and Dr. Jiangeng Xue. I am so grateful for their kind support and stimulating comments. I would also like to thank all of the people who I met through my research. Dr. Zhongwu Wang, who is a res earch scientist at Cornell University, is a great collaborator. I had a lot of fun working with him on high pressure experiments I regret that I cannot ; however, I am grateful to all, e spec ially, Derek LaMontagne and Dr. Kathyn Williams who helped me with my writing Finally, I would like to express my appreciation to my friends and family, especially, my mother and sister, Ikuko Nagaoka and Rikako Nagaok a, and my father Haruo Nagaoka.

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5 T ABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 12 LIST OF ABBREVIATION S ................................ ................................ ........................... 16 ABSTRACT ................................ ................................ ................................ ................... 17 CHAPTER 1 BACKGROUND ................................ ................................ ................................ ...... 19 1 1 Nanocrystals ................................ ................................ ................................ .... 19 1 1 1 Synthesis of Nanocrystals ................................ ................................ ...... 19 1 1 1 1 Theoretical background of nanocrystal synthesis ......................... 20 1 1 1 2 Experime ntal background of nanocrystal synthesis (using PbSe nanocrystals as an example) ................................ ................................ .. 22 1 1 2 Properties of Nanocrystals ................................ ................................ ..... 25 1 1 2 1 Spec ial properties of nanocrystals ..................... 26 1 1 2 2 Special properties of nanocrystals ................. 29 1 2 Nanocryst al Superlattices ................................ ................................ ................ 31 1 2 1 Synthesis Methods of Nanocrystal Superlattices ................................ ... 33 1 2 1 1 Evaporation of carrier solvents ................................ ..................... 33 1 2 1 2 Destabilization driven crystallization ................................ ............. 35 1 2 1 3 Liquid air interface ................................ ................................ ........ 35 1 2 2 Interparticle Forces in Nanocrystal Superlattices ................................ ... 36 1 2 2 1 General ideas ................................ ................................ ............... 37 1 2 2 2 Van der Waals forces ................................ ................................ ... 38 1 2 2 3 Electrostatic interactions ................................ ............................... 40 1 2 2 4 Magnetic interactions ................................ ................................ .... 41 1 2 3 Properties of Nanocrystal Superlattices ................................ ................. 41 1 2 3 1 Optical properties ................................ ................................ ......... 42 1 2 3 2 Magnetic properties ................................ ................................ ...... 4 2 1 2 4 Nanocrystals and Nanocr ystal Superlattices Under High P ressure ....... 43 1 2 4 1 Size shrinkage ................................ ................................ .............. 44 1 2 4 2 So lid solid crystal phase transition ................................ ............... 45 1 2 4 3 Morphological change through local sintering .............................. 48 1 3 Summary of the Present Research ................................ ................................ .. 50 2 STRUCTURAL CONTROL O F NANOCRYSTAL SUPERL ATTICES USING ORGANIC GUEST MOLECU LES ................................ ................................ ........... 57

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6 2 1 Introduction ................................ ................................ ................................ ...... 57 2 2 Experimental ................................ ................................ ................................ .... 58 2 2 1 Chemicals ................................ ................................ .............................. 58 2 2 2 Synthesis of Nanocrystals ................................ ................................ ...... 59 2 2 2 1 PbSe nanocrystals ................................ ................................ ........ 59 2 2 2 2 Gold nanocrystals ................................ ................................ ......... 59 2 2 2 3 Iron oxide nanocrystals ................................ ................................ 60 2 2 3 Preparation of Nanocrystal Superlattices ................................ ............... 60 2 2 4 Characterizations ................................ ................................ ................... 61 2 2 4 1 Transmission electron microscope measurements ....................... 61 2 2 4 2 X ray diffraction measurements ................................ .................... 61 2 2 4 3 IR measurements ................................ ................................ ......... 62 2 3 Results and Discussion ................................ ................................ ................... 62 2 3 1 Superstructural Control of PbSe NC superlattices ................................ 62 2 3 1 1 Calculation of the lattice constants and packing density of the fcc superlattices made from PbSe NCs ................................ .................. 66 2 3 1 2 Calculation of the lattice constants and packing density of the bcc superlattices made from PbSe NCs ................................ ................. 68 2 3 2 Mechanistic Study of PbSe NC Superlattice Formation ......................... 69 2 3 3 Superstructural Co ntrol of Binary Nanocrystal Superlattices .................. 77 2 3 3 1 Calcu lations of the packing density of the binary nanocrystal superlattices with AlB 2 structure ................................ ............................. 78 2 3 3 2 Calcu lations of the packing density of the binary n anocrystal superlattices with A B 13 structure ................................ ............................. 79 2 4 Conclusions ................................ ................................ ................................ ..... 79 3 BINARY ASSEMBLY OF C OLLOIDAL SEMICONDUCT OR NANORODS WITH SPHERICAL METAL NANO PARTICLES ................................ ............................... 80 3 1 Introduction ................................ ................................ ................................ ...... 80 3 2 Exp erimental ................................ ................................ ................................ .... 80 3 2 1 Chemicals ................................ ................................ .............................. 80 3 2 2 Nanocrystal Synthesis ................................ ................................ ............ 81 3 2 2 1 CdSe/CdS nanorods ................................ ................................ ..... 81 3 2 2 2 Palladium nanoparticles ................................ ............................... 82 3 2 2 3 Gold iron oxide and lead selenide n anocrystals .......................... 82 3 2 3 Preparation of Nanocrystal Assemblies ................................ ................. 82 3 3 Results and Discussion ................................ ................................ ................... 83 3 3 1 Pre paration of Binary Nanocrystal Assemblies from CdSe/CdS Nanorods and Gold Nanospheres ................................ ................................ 83 3 3 2 Additive Effect ................................ ................................ ........................ 87 3 3 3 Mechanistic Studies on the Formation of the Binary Nanocrystal Assemblies ................................ ................................ ................................ .... 88 3 3 3 1 Detailed calculations of interactions and collision between nanocrystals ................................ ................................ ............................ 91 3 3 3 2 Dipole dipole interaction ................................ ............................... 91 3 3 3 3 Van der Waals interactions ................................ ........................... 92

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7 3 3 3 4 Size dependent brownian collisio n kernel ................................ .... 94 3 3 4 Required Conditions for Intercalated Nanocrystals ................................ 94 3 4 Conclusions ................................ ................................ ................................ ..... 96 4 NANOCRYSTAL SUPERLAT TICES UNDER HIGH PRESSURE .......................... 97 4 1 Introduction ................................ ................................ ................................ ...... 97 4 2 Experimental ................................ ................................ ................................ .... 99 4 2 1 Chemicals ................................ ................................ .............................. 99 4 2 2 Synthesis of Nanocrystal Superlattices ................................ ................ 100 4 2 3 Characterizations ................................ ................................ ................. 100 4 2 3 1 In situ high pressure small angle and wide angle X ray scattering (SAXS and WAXS) measurement ................................ ........ 100 4 2 3 2 Electron micro scope measurements ................................ .......... 102 4 2 4 Peak Analysis ................................ ................................ ...................... 102 4 3 Results and Discussion Pressure Driven Superstructural Transformation of PbSe N anocrystal Superlattices ................................ ................................ .... 103 4 3 1 Preparation of Nanocrystal Superlattices from 5.3 nm PbSe Nanocrystals ................................ ................................ ............................... 103 4 3 2 General Descript ion of PbSe Nanocrystal Superlattices under High Pressure ................................ ................................ ................................ ...... 104 4 3 3 Superstructural Transformation of the PbSe Superlattices Driven by Pressure ................................ ................................ ................................ ...... 109 4 3 3 1 Stage I: From body centered cubic to body centered tetragonal at pressures between 0 GPa and 6.5 GPa ................................ ........... 109 4 3 3 2 Stage II: from body centered tetragonal to simple cubic f or pressures between 6.5 GPa and 9.0 GPa ................................ ............ 113 4 3 3 3 Stage III: two dimensional squared and rectangular arrays through a columnar alignment under pressures between 10.5 GPa and 13.4 GPa ................................ ................................ ........................ 116 4 3 3 4 Stage IV: lamellar superstructures from nanoplates resulting from nanocrystal fusion at 14.1 GPa and subsequent pressure release ................................ ................................ ................................ .. 119 4 3 4 Discussion 1: Nanocrystal Superlattices as Elastic Materials .............. 123 4 3 5 Discussion 2: Size dependency of the Superstructural Transformation ................................ ................................ ............................ 126 4 3 6 Summary of the Section 4 3 ................................ ................................ 127 4 4 Results and Discussion Strengthening Nanocrystal Superlattices by the Superstructure with Respect to the Phase Transition Pressu re ........................ 127 4 4 1 Preparation of Nanocrystal Superlattices with Bcc and Fcc Superstructures from 8.2 nm PbSe Nanocrystals ................................ ....... 128 4 4 2 Press ure Driven Phase Transition in the Bcc and Fcc Nanocrystal Superlattices ................................ ................................ ............................... 131 4 4 3 Pressure Medium Effects on the Phase Transition Pressures of Nanocrystal Superlattices ................................ ................................ ............ 136 4 4 4 Backstroke Phase Transition in the Depressurization Process ............ 138 4 4 5 Discussions: Strengthening by the Superstructures ............................. 141

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8 4 4 6 Summary of the S ection 4 4 ................................ ................................ 146 4 5 Conclusions ................................ ................................ ................................ ... 147 5 CONCLUSIONS AND PERSPECTIVES ................................ ............................... 148 APPENDI X: THE COMPLETE SAXS DATA FROM SECTION 4 3 ............................ 151 LIST OF REFERENCES ................................ ................................ ............................. 188 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 200

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9 LIST OF TABLES Table page 1 1 Nonretarded hamaker constants for two identical media interactin g in vacuum at room temper ature ................................ ................................ .............. 52 1 2 List of van der Waals equations for various geometries. ................................ .... 53 1 3 Equation of dipole dipole, dipole charge, and charge charg e interactions. ........ 55 1 4 Crystal structures of ionic semiconductors under high pressure. ........................ 56 2 1 Structural information for fcc PbSe NC superlattices from TEM and XRD measurements. ................................ ................................ ................................ ... 67 2 2 The structural information of bcc PbSe NC superlattices from TEM and XRD measurements. ................................ ................................ ................................ ... 68 2 3 The lattice constants of bcc PbSe superlattices made with the following additives: squalane, squalene and polyisoprene. ................................ ............... 75 3 1 The calculated energies of the vdW interactions and dipole dipole interactions in a binary assembly and a nanorod assembly. .............................. 89 3 2 The energies of dipole dipole interactions in the binary assemblies and the assemblies with only CdSe/CdS nanorods. ................................ ........................ 92 3 3 The energies of vdW interactions between two neighboring particles. ............... 93 4 1 Detailed information of the SAXS under ambient pres sure .............................. 110 4 2 Detailed information of the SAXS under 2.8 GPa ................................ ............ 110 4 3 Summary of the pressure driven phase transitions of the examined PbSe nanocrystal superlattices ................................ ................................ .................. 143 4 4 Summary of the superstructural changes under pressure. ............................... 143 4 5 Summary of the structural inform ation at the start of phase transition. ............. 145 A 1 Detailed information of the experimentally obtained and simulated spectra in Figure A 1B. ................................ ................................ ................................ ..... 152 A 2 Detailed information of the experimentally obtained and simulated spectra in Figure A 2B. ................................ ................................ ................................ ..... 154 A 3 Detailed information of the experimentally obtained and simulated spectra in Figure A 3B. ................................ ................................ ................................ ..... 156

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10 A 4 Detailed information of the experimentally obtained and simulated spectra in Figure A 4B. ................................ ................................ ................................ ..... 158 A 5 Detailed information of the experimentally obtained and simulated spectra in Figure A 5B. ................................ ................................ ................................ ..... 160 A 6 Detailed information of the experimentally obtained and simulated spectra in Figure A 6B. ................................ ................................ ................................ ..... 162 A 7 Detailed information of the experimentally obtained and simulated spectra in Figure A 7B. ................................ ................................ ................................ ..... 164 A 8 Detailed information of the experimentally obtained and si mulated spectra in Figure A 8B. ................................ ................................ ................................ ..... 166 A 9 Detailed information of the experimentally obtained and simulated spectra in Figure A 9B. ................................ ................................ ................................ ..... 168 A 10 Detailed information of the experimentally obtained and simulated spectra in Figure A 10B (i) and Figure A 10C (ii) ................................ ............................ 170 A 11 Detailed information of the experimentally obtained and simulat ed spectra in Figure A 11B. ................................ ................................ ................................ ... 173 A 12 Detailed information of the experimentally obtained and simulated spectra in Figure A 12B. ................................ ................................ ................................ ... 175 A 13 Detailed information of the experimentally obtained and simulated spectra in Figure A 13B. ................................ ................................ ................................ ... 177 A 14 Detailed information of the experimentally obtained and simulated spectra in Figure A 14B. ................................ ................................ ................................ ... 179 A 15 The results of the experimentally obtained spectrum at the 12.7 GPa and the comparison with a square model. ................................ ................................ ..... 181 A 1 6 Detaile d information of the experimentally obtained and simulated spectra in Figure A 15B. ................................ ................................ ................................ ... 181 A 1 7 The results of the experimentally obtained spectrum at 13.5 GPa and the comparison with a square mode l. ................................ ................................ ..... 183 A 1 8 Detailed information of the experimentally obtained and simulated spectra in Figure A 16B. ................................ ................................ ................................ ... 183 A 1 9 Detailed information of the experimentally obtained and simulated spectra in Figure A 17B. ................................ ................................ ................................ ... 185

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11 A 20 Detailed information of the experimentally obtained and simulated spectra in Figure A 18B. ................................ ................................ ................................ ... 187

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12 LIST OF FIGURES Figure page 1 1 LaMer Plot: Change of the degree of supersaturation as a function of reaction time.. ................................ ................................ ................................ ..... 20 1 2 PbSe nanocrystals. ................................ ................................ ............................. 24 1 3 The colors and shapes o f gold and silver nanoparticles ................................ ..... 26 1 4 The color of CdSe nanocrystals depends on nanocrystal size.. ......................... 27 1 5 TEM images and the lattice schematics of nanocrystal superlattices. ................ 32 1 6 A prototypical colloid al potential derived by intergrating the atomic Lennard Jones interaction over the volume of the two spheres. ................................ ....... 38 1 7 Crystal phase transition behavior in CdSe nanocrystals.. ................................ ... 47 1 8 Schematic illustration for the formation model of oriented attachment i n PbS nanocrystal superlattices ................................ ................................ .................... 49 2 1 TEM images of nanocrystals as building blocks for superlattices. ...................... 60 2 2 Choice of guest organic molecules.. ................................ ................................ ... 61 2 3 SAXS spectra of the nanocrystal superlattices made of 8.3 nm PbSe NCs ....... 63 2 4 TEM images of the NC superlattices. ................................ ................................ 65 2 5 Schematic illustrations of the nanocrystal superlattices and the formin g process ................................ ................................ ................................ ............... 69 2 6 PbSe NC superlattices made under a slow solvent evaporation rate. ................ 72 2 7 TEM images of PbSe NC superlattices made wit h octacosane. ......................... 72 2 8 TEM images of PbSe NC superlattices made with 1 octadecene. ...................... 73 2 9 TEM images of PbSe NC superparticles made with squalene. .......................... 73 2 10 NC superlattices made with squalane. ................................ ............................... 74 2 11 TEM images of superparticles made of 8.3 nm PbSe NCs in the presence o f polyisoprene. ................................ ................................ ................................ ...... 76 2 12 A TEM image of bcc pentyl 4 biphenylcarbonitrile. ................................ ................................ .............. 77

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13 2 1 3 TEM images of binary superlattices made of 4.8 nm gold and 9.2 nm iron oxide NCs. ................................ ................................ ................................ .......... 78 3 1 TEM images of assemblies of CdSe/CdS nanorods and gold nanoparticles prepared in the presence of TPP. ................................ ................................ ....... 85 3 2 TEM images of assemblies of CdSe/CdS nanorods and gold nanoparticles with or without additives. ................................ ................................ .................... 86 3 3 The formation of 2D binary assembly of CdSe/CdS nanorods with spherical gold nanoparticles. ................................ ................................ ............................. 90 3 4 TEM images of nanocrystal assemblies of the CdSe/CdS nanorods with spherical nanoparticles. ................................ ................................ ...................... 95 4 1 A schematic illustration of an X ray scattering measurement with DAC under high pressure. ................................ ................................ ................................ ... 101 4 2 Typical X ray scattering patterns collected on the Mar3450. ............................ 101 4 3 A schematic illustration of a peak analysis. ................................ ...................... 102 4 4 TEM pictures the PbSe nanocrystals. ................................ ............................... 104 4 5 A series of SAXS and WAXS patterns of the PbSe nanocrystal superlattices under different pressures. ................................ ................................ ................. 105 4 6 Schematic illustration of the superstructural transfor mation of the PbSe nanocrystal superlattices under high pressure. ................................ ................ 106 4 7 Plot of the d spacing value of the first peaks in the series of the SAXS patterns. ................................ ................................ ................................ ........... 107 4 8 Models of PbSe nanocrystals. ................................ ................................ .......... 108 4 9 Plot of the fraction of the high pressure phase of the PbSe crystals vs. the applied pressure. ................................ ................................ .............................. 109 4 10 SAXS patterns of the PbSe nanocrystal superlattices in Stage I. ..................... 110 4 11 Transformation of superlattices in Stage I. ................................ ....................... 112 4 12 SAXS pattens of PbSe nanocrystal superlattices in Stage II. ........................... 114 4 13 Two proposed mechanisms of the transformation from bct to sc ..................... 115 4 14 SAXS patterns of the PbSe nanocrystal superlattices in Stage III. ................... 116

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14 4 15 The a tetragonal and b tetragonal values versus the applied pressure in the square and tetragonal mod els in Stage III ................................ ................................ .... 117 4 16 The PbSe nanocrystal superlattices in the pressurization process up to 11.5 GPa. ................................ ................................ ................................ ................. 118 4 17 SAX S patterns of the PbSe nanocrystal superlattices in Stage IV. ................... 119 4 18 Electron microscope measurements of the pressurized nanocrystal superlattices ................................ ................................ ................................ ..... 121 4 19 Schematic illustration of the lamellar structure formation associated with nanoplate generation as a result of nanocrystal fusion. ................................ .... 121 4 20 Schematic illustrations of the lamellar structures. ................................ ............. 123 4 21 ........................ 123 4 22 SAXS and WAXS patterns of the b cc nanocrystal superlattices from 8.2 nm PbSe nanocrystals under different pressures. ................................ .................. 126 4 23 TEM picture of 8.2 nm PbSe nanocrystals. ................................ ...................... 128 4 24 Detailed observations of a 8.2 nm PbSe nanocrystal. ................................ ...... 129 4 25 Characterization of two nanocrystal superlattices ................................ ............. 130 4 26 A s eries of SAXS and WAXS patterns in the pressurization process. .............. 131 4 27 Plots of the high pressure ratio and the d spacing of the first SAXS peak. ...... 133 4 28 The experimental and simulated SAXS patterns ................................ ............ 134 4 29 Plots of the ratio of the atomic unit cell volume to the initial volume vs. the applied pressure. ................................ ................................ .............................. 135 4 30 A series of SAXS and WAXS patterns in the pressurization process in the presence of pressure medium. ................................ ................................ ......... 136 4 31 Plots of the percentage of the hig h pressure phase and the d spacing of the first SAXS peak in the presence of the pressure medium.. .............................. 137 4 32 Plots of the atomic unit cell volume over the initial volume vs. the applied pressure i n the presence of the pressure medium ................................ ............ 137 4 33 A series of SAXS and WAXS patterns in the stepwise depressurization process ................................ ................................ ................................ ............. 139

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15 4 34 Pl ots of the percentage of the hi gh pressure phase and the d spacing of the first SAXS peak in the depressurization process. ................................ ............. 140 4 35 Schematic illustrations of the nanocrystal superlattices. ................................ ... 141 A 1 SAXS patterns of the nanocrystal superlattices before the pressurization process.. ................................ ................................ ................................ ........... 151 A 2 SAXS patterns of the nanocrystal superla ttices under 0.3 GPa ........................ 153 A 3 SAXS patterns of the nanocrystal superlattices under 0.5 GPa ........................ 155 A 4 SAXS patterns of the nanocrystal sup erlattices under 0.9 GPa ........................ 157 A 5 SAXS patterns of the nanocrystal superlattices under 1.8 GPa ........................ 159 A 6 SAXS patterns of the nanocrystal superlattices under 2.8 GPa. ....................... 161 A 7 SAXS patterns of the nanocrys tal superlattices under 3.5 GPa ........................ 163 A 8 SAXS patterns of the nanocr ystal superlattices under 4.9 GPa.. ...................... 165 A 9 SAXS patterns of the nanocrystal superlattices under 6.5 GPa. ....................... 167 A 10 SAXS patterns of th e nanocrystal superlattices under 7.4 GPa. ....................... 169 A 11 SAXS patterns of the nanocrystal superlattices under 9.0 GPa ........................ 172 A 12 SAXS pattern s of the nanocrystal supe rlattices under 10.5 GPa ...................... 174 A 13 SAXS patterns of the nanocrystal superlattices under 11.2 GPa. ..................... 176 A 14 SAX S patterns of the nanocrystal superlattices under 11.9 GPa. ..................... 178 A 15 SAXS patterns of the nanocrystal superlattices under 12.7 GPa.. .................... 180 A 16 SAXS patterns of the nanocrystal superlattice s under 13.5 GPa ...................... 182 A 17 SAXS patterns of the nanocrystal superlattices under 14.1 GPa. ..................... 184 A 18 SAXS patterns of the nanocrystal superlattices after releasing pressure. ........ 186

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16 LIST OF ABBREVIATION S BCC Body Centered Cubic BCT Body Centered Tetragonal DAC Diamond Anvil Cell FCC Face Centered Cubic HCP Hex agonal Closed Pack P B S E Lead Selenide SAXS Small Angle X ray Scattering S C Simple Cubic SEM Scanning Electron Microscope TEM Transmitting Electron Microscope WAXS Wide Angle X ray Scattering 5CB 4 pentyl 4 biphenylcarbonitrile

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17 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy NANOCRYSTAL SUPERLAT TICES: SYNTHESIS AND CHARACTERIZATION UND ER HIGH PRESSURE By Yasutaka Naga oka August 2013 Chair: Y. Charles Cao Major: Chemistry Nanocrystals are very small crystals, which contains from a few hundred to thousands of atoms depending on the volume of the nanocrystals. N anocrystals have tunable physical properties depending o n their size and/or shape, resulting in applications in many areas of modern science and technology. High quality colloidal nanocrystals are self assembled to form superlattices. In nanocrystal superlattices, the properties of the building block nanocry stals often change through interparticle interactions Thus, f urther development in the synthesis and characterization of nanocrystal superlattices w ill promote research of nanocrystals to be more practical research of future significance The first acco mplishment of this disserta tion on self assemblies of nanocrystals is development of a new concept to control the structure of nanocrystal superlattice Using this method, one can controllably create nanocrystal superlattices w hich adopt non closed packed structures such as bcc to compare with fcc nanocrystal superlattices with clos est packed structures whose synthetic methods are already known. This methodology may also provide a new way to synthesize novel or ganic/inorganic composite materials.

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18 Next, binary assemblies from crystals with very different shapes such as CdSe/CdS semiconductor nanorods and spherical metal nanoparticles are created. The key to this methodology is organic additives with suitable polarity and strong affinity to spherical nanoparticles having both a high dielectric constant and a large Hamaker constant. Finally, we study th e superstructural effect on nanocrystal superlattice behavior under high pressure. We have developed one combined technique tha t allows simultaneous collection of synchrotron SAXS/WAXS under high pressure. U sing this technique, (i) the mechanical behavior of nanocrystal superlattices under high pressure and (ii) the superstructural effect on phase transition pressure of the build ing block nanocrystals (tuning by as much as 5 GPa), w ere investigated The results show that the mechanical properties of nanocrystals are determined by the superstructures, and in turn, the mechanical properties of nanocrystal superlattices are also dec ided by the building block nanocrystals. This dissertation demonstrates the fundamental importance of the design of the superstructures in nanocrystal materials. Nanocrystals have attracted much attention as promising materials, and therefore, the impo rtance of this research should be emphasized.

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19 CHAPTER 1 BACKGROUND 1 1 Nanocrystals The focus of this dissertation work is nanocrystal superlattice formation and characterization. This chapter introduces fundamental concepts of the s e materials. Colloida l nanocrystals which are used as building blocks to create nanocrystal superlattices, have a sma ll (~3 to 30 nm) inorganic core and a layer of surfactant molecules on the surface. Research on colloidal nanocrystals has been one of the most influential to pics in science and society today, due to the unique and/or tunable physical properties of these materials Further advancement of technology and accumulation of knowledge of colloidal nanocrystals will lead to the future design and construction of novel devices and materials based on nanostructures. Section 1.1 provides a brief overview of: (i) a general methodology for successful synthesis of high quality colloidal nanocrystals; and (ii) characteristic phenomena that can be observed with nanocrystals. 1 1 1 Synthesis of N anocrystals For meaningful investigation, nanocrystals of high quality are necessary. uniformity. Properties of nanocrystals are usually s ensitive to these factors, so development of a synthetic methodology for high quality nanocrystals has been always an important issue. Among the many methodologies available to create nanocrystals, this dissertation focuses on o rganic phase synthesis. Org anic phase synthesis is one of the most established methods both theoretically and experimentally to produce high quality nanocrystals 1 10 The basic idea

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20 is that inorganic sources of or ganometallic compounds reac t in an organic sol vent at high temperature causing monomer production and subsequent nucleation and growth The key to success for high quality nanocrystal synthesis is to control the dynamics and kinetics by adopting judicious choices of precursors and reaction conditions. Figure 1 1 LaMer Plot: Change of the degree of supersaturation as a function of reaction time. Stage I: no nucleation or growth occurs; Stage II: rapid nucleation occurs and this is followed by parti cle growth; Stage III: growth of nuclei 1 1 1 1 Theoretical background of nanocrystal s ynthesis The most accepted theory for organi c phase nanocrystal synthesis was originally proposed by LaMer et al. 1 3,11 Accor ding to their theory, g eneral nanocrystal formation mechanism involves three stages: (i) monomer production from precursor molecules, (ii) nucleation, an d (iii) growth (Figure 1 1). In the first stage, monomers are produced rapidly through chemical conv ersion from precursor molecules often triggered by thermal energy The thermal energy is supplied through a rapid temperature increase of the solution containing precursor molecules (as known as the or injection of precursor mole cules into a hot solvent (as known as the The concentration of the

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21 monomers constantly increases until spontaneous homogeneous nucleation happens as the reaction proceeds. When the monomer concentration crosses a threshold, s pontane ous nucleation occur s ( stage II in Figure 1 1). Subsequently, in stage III, the stable nuclei grow to larger crystals through consumption of the monomers remaining in the solution. When the nanocrystals grow to a desirable size, the heat source is remove d and the reaction is thermally quenched. Then, t he resulting nanocrystals are isolated by adding a bad solvent and subsequent centrifugation Typically, the nanocrystals created through organic phase synthesis are stable in a non polar solvent, such as toluene or hexane. The importance of a temporal separation of nucleation and growth is emphasized. 5,11 On one hand, in nanocrystal synthesis reaction solutions, the monomers are used for nucleation and growth. On the other hand, however, monomers are continuously produced until all the precursor molecules are consumed The over all rate of monomer concentration is determined by the balance between consumption and production of monomer species. As the concentration of the monomer species is continuously decreased after the nucleation event, the nuclei are all created in the same temporal regime, thus resulting in monodispersed nanocrystals. This is called a exceeds the consumption rate, the concentration of monomer increases until the concentra tion reaches a threshold, when nucleation occur s again. This route results in nanocrystals with poor size distribution because nanocrystals with nuclei from different time regimes ha ve different lengths of growth time

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22 Particle growth can be governed by b oth diffusion and reaction kinetics. In order to successfully control the kinetics of the growth, the importance of the energy between the core crystal and surfactant molecules is emphasized. In the growth process, surfactant molecules continuously excha nge between the surface and solution while the mo nomer species is being depleted. T hus a judicious choice of surfactant molecule is needed for controllable synthesis. 1 1 1 2 Experimental background of nanocrystal s ynthesis ( using PbSe n anocrystals as an e xample ) In order to understand the mechanism of nanocrystal formation, PbSe nanocrystals which are studied in detail in this dissertation are used as an example 12 15 PbSe nanocrystals are useful III V type s emiconductor materials with a long Bohr radius, making them a promising candidate materia l s for photo detector devices 16 and light emitting diodes 17 PbSe nanocrystals are typically synthesized by injecting tri n octylphosphine selenide (TOPSe) into a hot organic solution containing a source of lead, such as Pb(oleate) 2 molecules. The choice of the precursors was based on thier balanced moderate stability. I f the stability of the precursors is too high, the reaction w ill not occur In contrast, when the stability is too low, the en ergetic barrier is so low that monomer production and nucleation overlap, leading to poor size distribution. These precursors react with each other to create monomer at the reaction temperature (120C 200C), followed by nucleation and growth. The mon omer is a transient species likely a PbSe unit stabilized by a number of coordinating molecules in solution. The proposed mechanism of the monomer (PbSe) formation was two competing reactions expressed as below. 15

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23 (Oleate) 2 Pb + TOP Se [PbSe] + O=TOP + O(Oleate) 2 (i) (Oleate) 2 Pb + PH(R) 2 [Pb 0 ] + O=P(R) 2 + O(Oleate) 2 [Pb 0 ] + TOP Se [PbSe] + TOP (ii) wh ere TOP is tri n octylphosphine, (Oleate) 2 Pb is lead oleate, [PbSe] is monomer of PbSe, O=TOP is tri n octylphosphine oxide (TOPO), O(Oleate) 2 is oxidized oleate, a nd PH(R) 2 is di n octylphosphine (an impurity of TOP). Because of the weak bond between TOP and Se, TOPSe can be either as a source of Se 0 or Se 2 making these two reaction pathways available. 15 In the first mechanism (i ), TOPSe deliv ered Se 2 species, resulting in the production of PbSe monomer and TO PO In the second mechanism (ii ), Pb(oleate) 2 was reduced by di n octylphosphine, which usually exist in commercially available TOP as a n impurity creating un charge d Pb 0 The Pb 0 species reacted with TOPSe and the monomer [ PbSe ] was generated. The balance between the mechanism (i) and (ii) has profoun d effects on the kinetics of the monomer formation and hence the resulting PbSe nanocrystals. Thus, the chemicals used here require careful handling, especially for TOP containing differing amount of di n octylphosphine in different lots of TOP. In our laboratory, it was also found that the freshness of TOP is the important factor, presumably because decomposed molecules from TOP such as di n octylphosphine, influenced the balance between the mechanism s (i) and (ii) Another impurity that may change the reaction of the PbSe nanocrystal synthesis is acetate Acetate is generated as the byproduct of a synthetic reaction of Pb(oleate) 2 molecules formed by a reaction of Pb(acetate) 2 with oleic acid. The resulting acetate was removed under vacuum at a high temperature, but if the drying process was not

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24 thorough and the acetate was remained in the reaction system, it changed the reaction completely. Vanmaekelbergh et al. pointed out that acetate promote d more rapid crystal growth in the <100> atomic direction leading to large octahedral or star shaped crystals. 13,14 Figure 1 2 PbSe nanocrystals. A) TEM image. B) I llustration of a PbSe nanocrystal model Thus, in order to reproducib ly synthesize monodispersed and (quasi) spherical PbSe nanocrystals based on the LaMer theory, a careful usage of chemicals and an extensive removal of unnecessary byproducts are critical. The TEM image of successfully synthesized (quasi) spherical PbSe n ano crystals is shown in Figure 1 2A The PbSe nanocrystals have a uniform shape (sperical) and monodispersed size distribution, forming two dimensional superlattices. The resulting PbSe nanocrystals adopt a r ock salt (a NaCl type) crystal structure in wh ich Pb and Se atoms form separate face centered cubic lattice s which interpenetrat e to form a three dimensional che ckerboard pattern (Figure 1 2 B ), which can be identified using typical diffraction techniques such as X ray or electron diffractions.

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25 Inter estingly, it is known that (quasi) spherical PbSe nanocrystals possess a strong dipole moment although PbSe has a centrosymmetric rocksalt crystal structure 18 According to Murray et al 13 t he PbSe nanocrystals adopt a shape with ma ny facets as seen in Figure 1 2B Then, the PbSe nanocrystals lack central symmetry due to a noncentrosymmetric arrangement of Pb and Se terminated {111} facets and hence possess a strong dipole moment. The synthesis of PbSe nanocrystals with anisotropically elongated shapes such as rods and wires has been explored as well 13 Generally speaking, one dimensional nanocrystals (nanocrystals elongated in one direction) are created through an anisotrop ic growth of nanocrystals and there are two ways to make it happens : (i) anisotropic crystal growth 19 and (ii) oriented attachment. 13,20 Anisotropic crystal growth may occur due to differ ent growth speeds between crystal planes due to different surface energies. In the case of PbSe, the crystal structure is a centrosymmetric rocksalt type, thus there is no way to grow the crystal in one specific direction. Oriented attachment, attaching crystals through a specific crystal face, may be used to create one dimensional nanocrystals of PbSe Oriented attachment of nanocrystals is induced by surface interactions or anisotropic interactions such as the intrinsic dipole moment of the crystal str ucture Murray et al. reported that through oriented attachment, PbSe nanocrystals with various shapes, including nanorods 21 and nanowires 13 can be synthesized 1 1 2 Properties of N anocrystals Nanocrystals exhibit interest ing properties different from conventional materials, such as the o ptical, catalytic, magnetic and thermodynamic properties making nanoscience research fertile The unique properties can be derived from the nanosize

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26 inorganic core and the organic surfac e molecules In this section, some of representi ve special properties of nanocrystals are reviewed. 1 1 2 1 Special properties of nanocrystals art Figure 1 3 The colors and shapes of gold and silver nanoparti cles 22,23 Small is beautiful. Simply downsiz ing is sometimes beneficial for certain applications. For memory devices, when one unit is smaller in a memory cell, it can inversely store more information. 24 For nanomedicine, smaller therapeutic s can physically penetrate into a cell more deeply and can be delivered to places where the medicine is needed more effectively. 25 For these purposes, being small is beneficial. However some properties of inorganic crystals can have a size and shape dependenc e, called the makes nanocrystal research one of the hottest scientific topics. The most well known example of the nanosi z e effect is the characteristic colors of metal nanoparticles like gold and silver 22,23,26 28 Their colors are different from the

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27 bulk state and can be tuned by their size and shape as shown in Figure 1 3 The co lors are result ed from attenuation of incident light through harmonic resonance with localized electrons on the surface of nanocrystals, a process called surface plasmon resonance. This interaction of light (electromagnetic waves) with electrons from meta ls induces many intriguing phenomenon such as surface enhancement Raman spectra and Frster resonance energy transfer 29 34 Figure 1 4. The color of CdSe nanocrystals depends on nanocrystal size. A) Photo image of CdSe nanocrystals under UV irradiation. B) Absorption spectra (left) and emission spectra of different sized CdSe nanocrystals.

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28 Semiconductor nanocrystals, known as quantum dots, also possess distinct nanosize effe cts and the color of the nanocrystals strongly correlates with their size. 2,35 38 The size dependence of the nanocrystal absorption and luminescence spectra is governed by the quantum size effect, which drastically changes the energy state of three d imensionally confined quasiparticles. Figure 1 4 shows the fluorescent properties of CdSe nanocrystals with different sizes. By changing the diameter of CdSe nanocrystals, the energy gap of CdSe varies, leading to tunable fluorescent emission 2 In addition to a broad range of accessible optical wavelengths, quantum dots have advantages 39 co mpared to conventional fluorescent molec ules in terms of narrow emission spectra, resistivity against burning, and high quantum yield. With these advantageous properties, semiconductor nanocrystals are ideal for biological marker materials 40 In the nano size regime, the electronic and magnetic properties of materials can be tuned. 37,41 53 It is known that ferromagnetic and ferrimagnetic materials with sizes smaller than a critical value show paramagnetic responses. 54,55 T he nanosize magnetic domain is so limited that the inf luence of temperature dominates over the e ffects of magnetism. As a result, magnetization can randomly flip, leading to the unique behavior named superparamagnetism. Magnetic nanocrystals can be used as magnetic imaging agents in clinical care, as well a s other medical applications. For instance, drug delivery systems might need the magnetism in order to deliver therapeutics to a difficult yet desirable location 54 Nanocrystals are also useful as heterogeneous catalyst materials because of the ir large surface/volume ratio. 56 61 Although r eactions proceed on the entire surf ace of

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29 the nanocrystals a preferential crystal surface should exist because each surface has a different state in terms of the packing factor and dangling bonds. 62,63 Th us, nanocrystal catalysts can have optimally designed structure s and shape s in order to maximize their catalytic properties. A large surface/volume ratio also modifies the i nternal energies of the nanocrystals and hence changes their melting point s 64,65 and phase transition pressure 66 69 as discussed in section 1.2.2.4. There are many more fa scinating examples of the nanosize effects determining physical properties. 70,71 Furthermore, collective properties through interparticle interactions from different nano crystals can be realized in assemblies of nanocrystals, leading to more complex and higher functions, which will be reviewed in section 1.2 1 1 2 2 Special properties of nanocrystals art Typically, colloidal nanocrystals are capped w ith hydrophilic or hydrophobic organic molecules. After ligand capping, the hydrophilicity (or hydrophobicity) of the surfactant molecules directly determines the stability of the nanocrystals in liquid phases. 72,7 3 In other words, surfactant molecules can endow their properties on the nanocrystals. F urthermore, surfactant molecule layers can change properties of inorganic core. 44,74,75 The surface of nanocrystals is an important playground for nanochemists, and many kinds of molecules are attached on nanoparticles to engineer their functions. One of the fanciest molecules used as a nanocrystal surfactant is deoxyribonucleic acid (DNA). In 1996, Mirkin et al and Alivi satos et al. simultaneously reported gold nanocrystals attached to DNA molecules. 76,77 Since then, various types of nanoparticles engineered with biological molecules (i.e. enzyme and peptide ) have

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30 been intensivel y developed. 40,78 81 The advantages of biomolecules lies in their binding selectivity. One application example is a DNA and RNA sensor using SERS active silver DNA nanocrystals reported by Cao et al 82,83 The reliable selectivity of a DNA strand for its complementary sequence DNA enables exceptional detection ability to realize multiplexing and rationing capabilities. Using this exceptional selectivity, r ecently, DNA nan ocrystals have been used as building blocks of assemblies with programmable structures. 84 88 By capping nanocrystals with photo isomer able molecules, one can endow photo functionality to nanocrystals or manipulate physical properties of nanocrystals by photo illumination. One example is magnetic nanocrystals (i.e. FePt) capped with photo functional molecules such as azobenzene derivatives Einaga et al reported the magnetism of nanocrystals can be tun ed upon phot o illumination associated with the cis/trans isomeriza tion of the azobenzene moietiy. 89 92 For another example, Grzybowski et al reported that the stabilities of gold nanocrystals capped with azobenzene derivativ e surfactant molecules can be change d by photo illumination which triggers isomerization of the azobenzene molecules, resulting in aggregation and associating color change of the nanocrystal solution. 93 The surface s of nanocrystals can also be replaced with inorganic compounds such as metal chalcogenide s 94 O rganic molecules are usually insulating, thus, nanocrystals with organic surfaces it is not suitable for usage in electronic materials such as LEDs or electrical circuits. When the organic molecules are replaced with inorganic specie s like Sn 2 Se 6 4 or S 2 94,95 the conductivity between nanocrystals is improved, hence maximizing the properties of nanocrystals.

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31 Although choice of surface molecule is an important factor when discussing properties of nan ocrystals, only a limited number of examples could be introduced here because of the limited s cope in this chapter. Finally, it is worth mentioning that, from the viewpoint of a researcher of nanocrystal assemblies, the surfactant molecules are one of the most decisive factors for the stability of nanocrystals in the liquid phase, and hence affect how the assembly process is changed. 93, 96 98 Usually, nanocrystals with shorter hydrocarbon chains (less than C8) are not stable in nonpolar solvent resulting in aggregation with non organized sturucture. In turn, longer hydrocarbon chains (more than C12) endows good stabilities in suspension, hence causing slow crystallization leading to periodically ordered s uperlatti ces 3,133 1 2 Nanocrystal Superlattices When colloidal nanocrystals are brought together, superstructures are generated as amorphous, quasi crystal like, 99 or periodically ordered structures. 100 A nanocrystal superlattice is an array of nanocrystals with a beautifully ordered structure (Figure 1 5). Studies on nanocrystal superlattices attract much attention due to the potent ial to expand their functionality though interparticle interactions and superstructural effects. The properties of the building block nanocrystals such as the size, shape, and surface dictate the resulting nanocrystal superlattices (Figure 1 5) In other words, by engineering these factors, created are various nanocrystal superlattices with newly manifesting properties. In addition nanocrystal superlattices can be also formed from two or three kinds of different nanocrystals, namely binary nanocrystal su perlattices (Figure 1 5D ) 101 103 and ternary nanocrystal superlattices 104 The polymorph of the

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32 available structures of the nanocrystal superla ttices makes the interactions inside the superlattices complex and expands the possible range of materials. Interparticle interactions are the key to both synthesis and the manifesting properties of nanocrystal superlattice. This section briefly review s : (1) Synthesis of nanocrystal superlatt ices; (2) Interparticle forc es in nanocrystal superlattices; (3) Properti es of nanocrystal superlattices; and finally (4) N anocrystal and nanocrystal superlattices under high pressure. Figure 1 5 TEM images and the lattice schematics of nanocrystal superlattices A) CdS nanocubes superlattice, B) CdSe nanopyramids superlattice. C ) Gd 2 O 3 nanoplates superlattice. D) G old and iron oxide binary superlattice with an AlB 2 superstructure

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33 1 2 1 Synthesis M ethods of Nan ocrystal Superlattices Colloidal nanocrystals with a narrow size distribution and appropriately d eriva rtized surface can be crystallized to form aggregations with a periodically ordered superstructure, namely nanoc rystal superlattices Typically, the n anocrystal superlattice formation starts with nucleation of nanocrystals and subsequently proceeds to a growth stage. The following sections describe several methodologies for creation of high quality nanocrystal superlattices high s to a large superlattice crystal domain s as well as structural uniformity and complexity. 1 2 1 1 Evaporation of carrier solvents The evaporation method is one of the most accepted methods for making nanocrystal superlattices. The basic idea is to cond ense nanocrystal dispersions via evaporation and crystallize the nanocrystals Some of the pioneering work was performed by Bawendi et al 105 whose typical procedure involves : (i) placing a nanocrystal suspensio n in a mixture of 90% octane and 10% octanol ; and (ii) subsequent slow evaporation at 80 C under controlled pressure. The initial sub 10 nm CdSe nanocrystals assembled int o large sub micrometer three dimensional nanocrystal superlattices having a face c entered cubic structure with a minor amount of a hexagonal closest packing. extend ed for the prepara tion of binary superlattices. 103 Control evaporation of the solvent consisting multiple kinds of nanocrystals leads to high quality binary nanocrystal superlattices of which super structures are analog ous to naturally occur ring ionic c rystals 99,101,102,104

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34 The resulting super lattices usually have the superstructure with the highest packing density. It is often asserted that the free volume per particle is inversely related to the packing den sity and that the entropy increases proportionally with the larger free volume. Thus, without any significant forces between nanocrystals, the superlattices adopt the highest packing density. The basic mechanism of the nanocrystal superlattice formation t hrough the evaporation methods are as follows. In a stable nanocrystal suspension, the c arrier solvent separate s individual nanocrystals from each other and screen s some intermolecular interactions. As the carrier solvent is evaporated and the distance b etween nanocrystals decreses the nanocrystals attract each other with greater strength, and when the interacting force reaches a threshold (c.a. kT ), nucleation starts. Subsequently, the nuclei attract and incorporate residual nanocrystals in solution, r esulting in growth to large superlattices. As expected, these events are basically driven kinetically, although certain types of processes can be explained solely using thermodynamic arguments 106 Thus, a ny conditions that can influence the kinetics of nucleation and growth can change the morphology and structure. Interactions such as intermolecular and interparticle forces between every combination of nanocrystals, substrates, solvent, and temperature ea ch can have a significant effect on nanocrystal superlattice formation. 106 111 Such interactions can be tuned by adding additional molecules to the evaporating solvent, thereby affecting the kinetics of superlatt ice formation. For example, it is known that 1 dodecanethiol induces the wettability between a nanocrystal and a substrate like silicon nitride. As a result, the quality of nanocrystal superlattices 110 and binary superlattices 111 can be significantly improved.

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35 1 2 1 2 Destabilization driven c rystallization A ggregatio n of nanocrystals can also be induced by adding a poor solvent 112 called the 112 Rod shaped CdSe@CdS nanocrystals are crystallized at the interface of a nonpolar phase and a polar phase. The nonpolar phase contains the nanocrystals and toluene as a carrier solvent. The polar phase consists of methanol and isopropanol as a poor solvent. Th is system had the advantage of avoiding the development of temperature gradients a nd minimizing the local flux of the nanocrystals in the solution, thusly creating well ordered three dimensional superlattices. 113 In addition to polarity, other interparticle forces can induce destab ilization of nanocrystals in suspension. For example, Manna et al. fabricated large assemblies of semiconductor nanorods through the addition of excess amounts of oleic acid which triggered strong depletion for ces leading to format ion of nanorod superlattices. 114 Another important method is an approach for using solvophobic interactions to control the formation of nanoparticle assemblies, developed by Cao et al. 115 120 Th is approach includes two major steps: (i) synthesis of water soluble nanocrystal micelles, and (ii) growth of superparticles from nanocrystal micelles in an aqueous solution of ethylene glycol. Differing from conventional methods, this approach enable s on e to control the resulting morphology and size indicating their potential as a new class of building blocks in nanoscience 1 2 1 3 Liquid air i nterface When nanocrystal assemblies are used as materials, t he limitation of available substrates and the c ontinuous size of nanocrystal superlattices are often challenging issue s One of the circumventing methods is a liquid air interface synthesis of

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36 assemblies. The most well known example is the Langmuir Blodgett films. 121 This concept has been employed to prepar e nanocrystal assemblies, 122 however, high quality nanocrystal superlattices has not been achieved yet In 2010, Angang Dong et al published an excellent method to create high quality nanocrystal superlattices at liquid air interfaces. 123 They used diethylene glycol (DEG) as a liquid substrate and spread a drop of nanocrystal hexane suspension with selected size and concentration over the DEG in a Teflon well resulting in a solid film supported on the DEG. The resulting nanocrystal superlatti ces possess an exceptional quality in terms of: (i ) the highly ordered structures; (ii ) the morphology such as thickness and uniformities ; and (iii ) the size domain as large as centimeters allowing the fabrication of nanocrystal based devices 1 2 2 Int erparticle Forces in Nanocrystal S uperlattices 124 125 observed everywhere in the world. For scientists, it is meaningful to understand the underpinning p hysical interactions. 100,126 All structures of self assemb lies can be explained as a balance between attractive and repulsive forces. In the case of a self assembly of n anocrystal s, a steady state structure result s from many interactions including non covalent or weak covalent bonds such as van der Waals, electr ostatic, an d hydrophobic forces. One may hope to understand it in the context of conventional intermolecular (or interatomic) forces. When extrapolating the knowledge to the nanocrystal regime, there are both analogies and differences, so this section re view s these forces in order to understand the formation of nanocrystal superlattices

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37 1 2 2 1 General i deas In the process of superlattice formation from building block nanocrystal s in suspension t hermodynamics governs the formation pathways involved in n ucleation and growth As seen in conventional studies of inter molecular interactions, the fundamental significance of thermal energy, kT is emphasized when partitioning nanocrystals in carrier solvents among the different levels of systems. If the inter action energy exceeds kT it will dominate over thermally randomizing effects and create aggregations. This arises subsequent questions: (i) how is the interaction energy calculated?, and (ii) how and what parameters of the nanocrystal does effect on the s cale of the energy? In order to have general understanding of the thermodynamics here a simplified model where two nanocrystals are interacting through the van der Waals interaction is used. The interaction energy between the two nanoparticles are calcu lated as a sum of all atom atom pairs within the two particles, expressed as below for r << a1a2/a 1 +a2 (eq 1 1) where a 1 and a 2 are the nanoparticle diameters, is the depth of the potential well, is the chara cteristic atomic diameter, and r is the di stance between particles. The equation clearly shows the size dependency. The prototypical potential curves originated from equation 1. 1 are shown in Figure 1 6 They are expressed as an analogy of the ubiquitous Leonard Jones potentials, and the depth of the potential well and the distance between particles ( r ) are dependent on the nanoparticle size. In this way, the balance between nanocrystals is outlined thermodynamically. Of course the interparticle interactions vary depending on the working forc es. Furthermore,

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38 it is important to emphasize that the forces can have a profound influence on the kinetics of nucleation and growth of superlattices. In the following sections, several types of interparticle forces are reviewed. Figure 1 6 A prototy pical colloidal potential derived by intergrating the atomic Lennard 1 2 2 2 Van der W aals forces Van der Waals forces are forces working between un charge d substances and one of the most ubiquitous forms of nanoscale interactions The s e force s are additive and non retarded. The origin of van der Waals forces is the electromagnetic fluctuations withi n substances. The interaction between atoms may be simply expressed by equation 1.2 (see also Table 1 2) (eq. 1 2) where u is the interaction energy, A is the Hamaker constant of the system, and r is the distance between atoms. Hamaker constants are inherent numbers in systems and determine the intensity of the interactions. In order to strictly calculate van der Waals interactions between two crystals although the interaction is additive, the influence of neighboring atoms on the interaction

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39 between any pair of atoms must be considered not simply the sum of the van der Waals interac tions between each pair of atoms. The original theory on these long range complex interaction s was proposed by Lifshitz et al After that, many researchers have extended the theory to be easier and more practical, since the original theory requires a tho rough working knowledge of quantum field theory so it was not practical for general usage One of the most useful derived forms for nanocrystal assemblies is a model where phase 1 and phase 2 are interacting across medium 3 When forming nanocrystals, b etween two nanocrystal cores ( phase 1 and 2) there are solvent molecules and surfactant molecules ( medium 3) T he A (Hamaker constant) is expressed as shown in eq 1. 3 ( eq. 1 3) where the is the dielectric c onstant h is the Plank constant, and the is the main absorbance frequency. This equation can be further simplified as below. (eq. 1 4 ) where n is the refractive index and e is the main electronic absorption f requency in the UV typically around 3 10 15 s 1 In the case that same kind of nanocrystals are interacted (where two identical phase 1 interacting across medium 3 ) equation 1.4 can be further simplified as below. ( e q. 1 5 )

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40 where two identical phase 1 are interacting across medium 3 Experimentally and theoretically determined Hamaker constants in a vacu um (where n 3 = 3 = 0 ) are l isted in Table 1 1. It is known that the metal possesses high Hamaker constan t s compare d to semiconductors or organic compounds, hence greater van der Waals interactions. With an a ppropriate Hamaker constant the van der Waals forces can be calculated. T he interaction value s are obtained as a sum of every combination of the interaction between one atom in one nanocrystals and one atom in the other nanocrystals. T hus the interaction energy is dependent on their shapes, size, and orientation leading to a size selective sorting effect 127,128 Table 1 2 lists some of t he equations for various geometries of two bodies. 1 2 2 3 Electrostatic i nteractions Many colloidal nanocrystals possess electric charges and/or permanent electric dipole moments. 18,129 131 These electrical charg es can be endowed both by their intrinsic crystal structure and by the surfactant molecules. E lectrostatic interactions often compete with van d er Waals interactions during self assembly process. Electrostatic forces can be either attractive or repulsive and highly directional. The classic Coulomb equation between two charged bodies is given by: (eq. 1 6 ) where q is the charge on the substance, is the permittivity of the medium and r is the distance between two bodies. This equation indicates that the magnitude and the length scale of these electrostatic interactions can be tuned by the choice of the intermediate solvent.

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41 Nanocrystals with a noncentrosymmetric crystallographic lattice exhibit an elec tron polarization, leading to a dipole moment. For example, CdS and CdSe nanorods with a Wurzite type crystal structure have a permanent dipole along the c crystallographic axis. Their liquid crystal like behaviors is originated from their strong dipole dipole interactions. Practically speaking the dipole moment can be treated as two separated charges. Thus, the net dipole dipole interaction and charge dipole interaction can be expressed as a sum of electrostatic interactions. The equation s for each ca se of electrostatic interactions are list ed in Table 1 3 1 2 2 4 Magnetic i nteractions Magnetic nanocrystals are imp ortant materials. The magnetic and the effects between nanoparticles must be considered In the p resence of magnetic fields, particles te nd to align their magnetic moments in the direction of the local magnetic field. Magnetic interactions always compete with van der Waals forces and they become increasingly important with decreasing particle size. However, i n reality, without a strong ex ternal magnetic field, van der Waals and electrostatic interaction s are dominant over the effect of magnetic interactions in t he process of nanocrystal self assembly 1 2 3 Properties of Nanocrystal S uperlattices When nanocrystals are close together, their interparticle interactions may change their properties. 132 135 This phenomenon has attracted major interest in the designing new functional materials. This section outlines several reported nanocrystal superlatt ices with unique properties due to interparticle interactions.

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42 1 2 3 1 Optical p roperties Some metal nanocrystals, such as gold and silver, exhibit characteristic colors due to their localized surface plasmon resonance ( section 1.1.2). The absorption band is strongly dependent on their near field. Thus, when two metal nanocrystals are close together, the dielectric field that is affected by neighboring nanocrystals can influence the resonance leading to different color s Fluorescent properties of semico nductor nanocrystals change in superlattices as well. Bawandi et al reported that the emission spectra of CdSe nanocrystals close packed in the solid are red shifted compared with the same nanocrystals in a diluted matrix and yet there is no difference i n their absorption spectrum 105 Recently, Cao et al. repor ted that quantum yields of CdSe/ CdS (core/shell) nanorods are enhanced when in the f orm of superparticles 117 Different kinds of nanocrystals can also interact with each other in superlattices and tune their optical properties. For example, Talapin et al reported the fluorescence study of binary nanocrystal superlattices containing CdSe and gold na nocrystals. 136 The fluorescence properties of the CdSe nanocrystals were changed resulting in decreased quantum yield and shortened lifetime, indicating that the excitation energy was transferred to the surroun ding gold nanocrystals. 1 2 3 2 Magnetic p roperties It is also known that magnetisms of nanocrystals are significantly dependent on their environments, differing the magnetic properties in different nanocrystal superlattices. Co a s sembly of nanocrystals with hard and soft magnetism can produce high energy magnetic materials as shown in the pioneering work by Sun et al. 137 They

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43 made ferromagnetic FePt nanopart icles and paramagnetic Fe 3 O 4 nanoparticles and assembled them together. The subsequent high temperature annealing resulted in a solid composite with nanosized soft and hard magnet ic domains. These two phases interact though magnetic exchange coupling, re sulting in a very large magnetic hysteresis loop. This optimal high energy product from an optimal exchange coupling cannot be obtained with single component nanomaterials, thus, the importance of this bottom up approach using different nanocrystals is em phasized. For another example, Murray et al. demonstrated that the magnetic resistance of binary nanocrystal superlattices from magnetic nanocrystals is dependent on the lattice superstructure. They showed that the magnetresistance of binary superlattices from FePt and Fe 2 O 3 nanocrystals with a superstructure of AlB 2 type was higher than that of an ico AB13 type superstructure. 123,138 In conclusion, well designed nanocrystal superlattices can produce desirable func tions. To have further advances in materials from nanocrystals, continuous research is needed for characterization of the properties and a new methodology for creation of nanocrystal superlattices with new structures. 1 2 4 Nanocrystals and Nanocrystal S uperlattices Under H igh P ressure 68,139 Physical sta bility is an important characteristic. In nanocrystal science, melting wa s the first phenomenon to be studied in terms of the phase stability 65 The melting point of nanocrystals is dependent on their crystal size, and is usually lower than the melting point of the bulk state b ecause of their large surface energies. Similarly it is known that nanocrystals and bulk materials behave differen tly under a high pressure. In other words, nanocrystals exhibit a unique phase diagram different from that of the bulk state, of the temperature and pressure.

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44 Investigations on nanocrystals and nanoc rystal superlattices under high pressure are of fundame ntal importance in high pressure chemistry. Because a nanocrystal has a small and single crystalline crystal domain, the manner of the pressure driven crystal structure change (the phase transition) is unique, giving important insights into the phase stab ility. Furthermore, characteriz ation of the physical tolerance and structural strength as a function of pressure should provide a n optimal design strategy for high physical strength nanocrystal materials. Despite the importance, research on nanocrystal s and nanocrystal superlattices under high pressure is still in the early stag e s Prior to presenting our rese a r ch in Chapter 4, i n this section, we overview what can generally happen when nanocrystals and nanocrystal superlattices are subjected to high p ressures 1 2 4 1 Size s hrinkage When things are pressurized, the volume is decreased. T his principle can be applied to nanocrystals and nanocrystal superlattices as well for size shrinkage with respe ct to (i) the crystal structure of the nanocrystal c ore and (ii) the superstructure of the nanocrystal sup erlattice In nanocrystal superlattices, the relative position s of the atoms inside the nanocrystals and the nanoc rystals inside the superlattice are determined as a result of the balance of attractive an d repulsive forces under ambient condition s When pressure is applied these atoms and nanocrystals penetrate the repulsive region of interatomic and/or inter particle potentials. In other words, the balance point moves with the pressure as atoms get clo se to each other and are packed more closely. F or typical solid inorganic crystals with a large crystal domain the volume decreses by a factor of 1.5 5 under a pressure of 200 GPa because of the shortened

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45 chemical bond s 139 This triggers many events such as changes in the natur e of the chemical bonds, the phase transition s (explained in the next section) and the metallization under extremely harsh conditions. For nanosize crystals which are found in typi cal colloidal nanocrystals, the downsizing process is usually different f rom that in the bulk state. It is known that the values of the shrinking rate of nanocrys tals, which is quantified as a bulk modulus, are usually different from the bulk state because of the strong surface effects. 140 For the superposition of nanocrystals inside a superlattice the interparticle distances are also shorten ed by increased pressure. In a typ ical supramolecule the van der Waals space may be the easiest to shrink in pressure range s less than c.a. 10 GPa. 139 S imilar downsizings should occu r in nanocrystal superlattices. Non covalent bonds between n anoc rystals inside superlattices are shortened easily, leading to downsize of nanocrystal superlattices. Noteworthy the pr essure regimes of these events overlap, so atomic structures and supers tructures of nanocrystal superlattices shrink simultaneously and their movements correlate with each other Thus, in order to correctly understand the pressure response of nanocrystal superlattices, simultaneous in situ wide and small angle X ray scatte ring is needed. 1 2 4 2 Solid solid crystal phase t ransition A phase diagram of a substance show s the region s of pressure and temperature under which an observed phase of the substance is thermodynamically stable. The phases include gas, liquid, and sol id with many possible crystal structures. In terms of chemical potential the chemical potential of a substance with a n observ ed phase at the condition is the lowest of all the possible phases. By c hanging the pressure and

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46 temperature, a spontaneous conv ersion of one phase to another can occur namely a phase transition. 66,68,139,141 This dissertation focused on solid solid phase transition s in which one crystal structure is converted to another structure driven by pressure When the pressure exceeds the value at which the two che mical potentials of crystal structures are equal s olid solid crystal phase transition occurs Prewitt and Downs summarized several rules on crystal structure changes corresponding to pressure driven solid solid phase transition 142 Two of the important principals are: ( i) Increased pressure increases coordination number ; and ( ii) High pre s sure structure s tend to be composed of closest packed arrays of atoms Table 1 4 presents several examples of the phase transitions in ionic semiconductors. These transformations are all associated with a higher coordination number and denser atomic density Thermodynamically the high pressure phase can be expressed as a metastable phase with a high internal energy In other words, the internal energy of a material with a certain crystal structure is dependent on the pressure and temperature. The internal energy U for a crystal structure A is given by (eq. 1 7 ) where S is the entropy, is the chemical potential and N A is the Avogadro number. Alivisatos et al first reported that CdSe nanocrystal s have higher phase transition pressure s than that of the bulk CdSe crystals 66,68,143 The phase transition exhibits a size dependency (Figure 1 7) attributed to their non negligible surface energy Accordingly the equation 1.7 is modified for nanocrystals as below

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47 (eq. 1 8 ) where and SA are the surface tension and surface area, respectively 66 When comparing the high pressure (HP) and low pressure (LP) phase s it is a reasonable assumption that the condition for the phase transition to occur is HP = L P Substitution into equation 1.8 gives the necessary condition for this phase tran sition pressure P T as : (eq. 1 9 ) This equation reasonably descr ibes the dependency of the phase transition pressure on the surface area and hence on the nanocrystal size. Figure 1 7. 143 Crystal phase transition behavior in CdSe nanocrystals. The x axis is the applied pressure, and the y axis is the existing probability of the high pressure crystal structure phase (rock salt structure) at the relevant pressure. Reprinted with permission from Chen, C. C.; Herhold, A. B.; Johnson, C. S.; Alivisatos, A. P. Science 1997 276 398. Copyr ight 1997, American Association for the Advancement of Science Although the formalism thermodynamically predicts a general tendency of the phase transition event driven by pressure, in reality, there is an energy barrier to overcome between two different phases, so kinetic considerations are also involv ed. Usually, the energetic barrier to transition is explained in terms of nuclear dynamics

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48 associated with crystal transformation. As seen in Figure 1 7 there are big hysteresis curves between the upstrok e and backstroke phase transitions in the case of nanocrystals, which suggest that a big energetic barrier exists between the two phases. In the case of nanocrystal superlattices, in addition to their intrinsic structural strength and surface factors des cribed above the superstructural effect must be considered. 144 Chapter 4 extensively study the relationship between the superstructur e s of the superlsttices and the phase transition pressure of the constituent nanocrystals. 1 2 4 3 Morphological change through local sintering Increased pressure also results in the shape changes. I t is known that the shapes of nanocrystals are changed in association with the crystal structure change to the high pressure phase. 140 In addition, recently, some research groups have reported that nanocrystals in an assembly are fused together driven by the controlled applied pre ssure, leading to a different shape nanocrystals. One may expect t hat this strategy would become a clean and simple route t o create an anisotropic shape. In this section, several examples of nanocrystal fusions driven by pressure are introduced Wu et al. reported nanowires 145 and nanostructured hollows 146 as a result of fusions of gold nanocrystal s with a diameter of 5.3 nm. The original nanocrystals formed nanocrystal super lattices with a face ce ntered cubic superstructure in polymer films. They loaded the superlattices in a pressure chamber, called a d iamond anvil cell (DAC, explained in Chapter 4), and pressurized it up to 10 GPa. The superstructures were transformed as pressure was applied and above a threshold pressure (c.a. 7 GPa) the nanocrystals fused, creating a special shape such as nanowires. The gold nanowires reported by Wu et al. possess ed length s rang

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49 diameter s of 6.1 nm and th ey can form stable colloidal dispersion s in organic solvent s These results suggest the pressure driven method may have a priority over a conventional chemical route to 1 D nanostructures like nanowire s in terms of controllability, variety of available co mponents etc Wang et al. observed semiconductor, lead sulfide (PbS) nanocrystal fusion driven by pressure, creating single crystal PbS nanosheet s 147 (Figure 1 8). The original PbS nanocrystals with an average size of 3.5 nm were assembled into a face centered cubic nanocrystal superlattice. The deviatoric stress that a DAC generated prop agate s through the superlattices, leading their alignment and fusion with the same orientation. The mechanism to create the single crystalline can be considered as an or iented attachment driven by pressure In any case, the stress and strain existing ins ide nanocrystal assemblies play a role that results in the ultimate formation. In order to understand the mechanism and precisely and controllably transform any nanocrystals by this pressure method, a precise study on the correlation between their superst ructures and the stress response would be very useful. Figure 1 8. Schematic illustration for the formation model of oriented attachment in PbS nanocrystal superlattices. 147 Reprinted with permission from Wang, Z.; Schliehe, C.; Wang, T.; Nagaoka, Y.; Cao, Y. C.; Bassett, W. A.; Wu, H.; Fan, H.; Weller, H. Journal of the American Chemi cal Society 2011 133 14484. Copyright 2011, American Chemical Societ y.

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50 1 3 Summary of the Present Research This dissertation was untaken to study the formation mechanisms of nanocrystal superlattices and characterizations of nanocrystal superlattices und er high pressure. In chapter 2, a host guest chemistry approach to controlling structures of nanocryst al superlattices through a mole cular inclusion process are reported. U pon adding an appropriate amount of guest molecules into a nanocrystal suspension, the resulting nanocrystal superlattices adopted non closed packed structures (e.g., from face centered cubic to body centered cubic) and changed their morphologies to form superparticles. 148 Some of the figures and tables presenting in this chapter are reprinted wit h permission from Nagaoka, Y.; Chen, O.; Wang, Z.; Cao, Y. C. Journal of the American Chemical Society 2012 134 2868. Copyright 2012, American Chemical Society. Chapter 3 describes a preparation of binary assemblies of CdSe/CdS semiconductor nanorods wit h spherical metal nanoparticles where the spherical nanoparticles are intercalated into parallelly aligned nanorod arrays. 149 Some of the figures and tables presenting in this chapter are reprinted with permission from Nagaoka, Y.; Wang, T.; LaMontagne, D.; Ca o, Y. C. Small 2012 134 2868. Copyright 2012, Wiley. Chapter 4 presents studies on nanocrystal superlattices under high pressure. This chapter contains two sets of research. In the section 4.3, superstructural transformations of PbSe nanocrystal superl attices driven by pressure were investigated. We found various superstructures including unprecedented superstructures under ambient conditions, ending with nanocrystal fusion. The polymorph of superstructures seen under high pressure may expand availabl e nanocrystal assemblies. Section 4.4

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51 demonstrates superstructures of nanocrystals effects on the phase transition pressure of the building block nanocrystals. We found that the PbSe nanocrystals in the superlattices with a body centered cubic superstruc ture exhibited a higher phase transition pressure than the ones in the face centered cubic superlattices by as much as 5 GPa. Our mechanistic study demonstrated that the superstructures could dictate the direction of the pressure propagation along the sup erlattice network in the absence of a pressure medium. Finally, overall conclusions on the research introduced and suggestions of future directions are presented in chapter 5.

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52 Table 1 1. Nonretarded hamaker constants for two identical media interacting i n vacuum at room temperature. 150 Medium Dielectric Constant Refractive Index n Absorption Frequency e (10 15 s 1 ) Calculated Value A (10 20 J) Experimental Value A (10 20 J) Water n Octane n Dodecane Benzene Ethanol Teflon Silica Silicon ZnS CdS PbS Metals (Au, Ag, Cu) 80 1.95 2.01 2.28 26 2.1 3.8 11.6 8.5 9.97 169 1.333 1.387 1.411 2.28 1.361 1.359 1.448 3.44 2.26 2.261 4.005 3.0 3.0 3.0 2.1 3.0 2.9 3.2 0.80 1.6 3 5 3.7 4.5 5.0 5.0 4.2 3.8 6.3 18 16 11.4 8.17 25 40 5 6 Created with data from Israelachvili, J. N. In termolecular and Surface Forces, 3rd ed.; Academic Press: London, 2010.

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53 Table 1 2. List of van der Waals equations for various geometries. Geometry of bodi es with surfaces D apart Van der Waals Interaction Two identical atoms A/r 6 Two macroscopic spheres Two spheres with different sizes (R 1 R 2 >>d) Two parallel macroscopic rods Two parallel cylinders (or rods) with different radii (per length)

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54 Table 1 2. Continued Geometry of bodies with surfaces D apart Van der Waals Interaction Two perpendicular cylinders (or rods) with different radii (per length) A nanorod and a spherical particle (when the length of the rod >> R s ph ) Created with data from Israelachvili, J. N. In termolecular and Surface Forces, 3rd ed.; Academic Press: London, 2010.

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55 Table 1 3. Equation of dipole dipole, dipole charge, and charge charge interactions. Type of Interaction Interaction Energy w(r) Charge Charge q 1 q 2 /4 0 r Charge Dipole (Fixed dipole) qu cos /4 0 r 2 Charge Dipole (Freely rotating) q 2 u 2 / 6(4 0 ) 2 kTr 4 Dipole Dipole (Fixed dipole) u 1 u 2 / (2 cos 1 cos 2 sin 1 sin 2 cos ) /4 0 r 3 Dipole Dipole (Freely rotating) u 1 2 u 2 2 / 3 (4 0 ) 2 kTr 6 Created with data from Israelachvili, J. N. In termolecular and Surface Forces, 3rd ed.; Academic Press:: London, 2010.

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56 Table 1 4. Crystal structures of ionic semiconductors under high pressure. Materials Under am bient pressure Under high pressure (2 nd phase) Under high pressure (3 rd phase) Phase Transition Pressure 2 nd /3 rd 3 GPa/50 GPa (CdS) 5 GPa/20 GPa (PbSe) 7 GPa/9 GPa (InAs) 10 GPa (ZnTe) 20 GPa/60 GPa (GaAs) 7 GPa/70 GPa 20 GPa 1.5 GPa/8GPa (HgT e) CdX (X=S, Se, Te) PbX (X=S, Se, Te) InX (X=P, As) ZnX (X=S, Se, Te) GaX (X=P, As) GaSb HgS HgX (X=Se, Te) Wurzite (Zinc blend) Rock salt Zinc blend Zinc blend (Wurzite) Zinc blend Zinc blend HgS type Zinc blend Rock salt Orthorho mbic Pnma Rock salt Rock salt Orthorhombic (GaAs II & III) Sn Rock salt HgS type Orthorhombic Cmcm CsCl Orthorhombic Cmcm Simple Hexagonal Body Centered Cubic Rock salt Created with data from Mujica, A.; Rubio, A.; Munoz, A.; Needs, R. J. Rev. Modern Phys. 2003 75 863. Copyright 2003, American Physical Society.

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57 CHAPTER 2 STRUCTURAL CONTROL O F NANOCRYSTAL SUPERL ATTICES USING ORGANI C GUEST MOLECULES 2 1 Introduction The inclusion of guest molecule s into a host lattice through noncovalent forces is an ordinary phenomenon that takes place in the formation of molecular crystals 151 H ost guest inclusion chemistry has been widely used to design and construct molecular crystals with desired physical and chemical properties for the needs of various applica tions, for example host guest crystals with non central symmetry for the use in second harmonic generation, and co crystals containing active pharmaceutical ingredients for drug delivery and formulatio n. 152,153 As analogs of molecular crystals, three dimensional (3 D) n anocrystal (NC) superlattices are ordered assemblies comprising one or more types of NC building blocks 103,105 To date, many small molecules and polymers have been used to mediate the assembly of colloidal NCs, which have resulted in the preparation of a variety of organic/inorganic nanocomposites that exhibit unique optical, magnetic, electric, catalytic or m echanical properties. 154 158 However, because most of these organic/inorganic nanocomposites do not adopt a 3D ordered structure (e.g., a face centered cubic, fcc structure), little progress has so far been made in the stru ctural control of 3D NC superlattices using host guest inclusion chemistry. Recently, two research groups have found that solvent molecules can occupy the interstitial space in PbS NC superlattices and solvent vapor can be used to control the structure of these NC superlattices, leading to the formation of non closed packed superlattice structures such as body centered cubic ( bcc ). 159,160 However, these NC

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58 superlattices are not stable upon the evap oration of solvent molecules, 160 limiting the use of these nanostructures in pra ctical applications. Here we report that high boiling organic compounds can be used as guest molecules to tailor the structure and morphology of NC superlattices. The resulting guest host NC superlattices are stable under high vacuum conditions during T EM (transmission electron microscope) observations. Our results show that the inclusion of guest molecules such as squalane, squalene, and polyisoprene can lead to the formation of non closed packed single component or binary NC superlattices as well as t he formation of superparticles (i.e., supercrystalline collections of NCs in a form of particle 118,120 ). The insights gained in this study are not only important to making nanocrystal superlattices with desirable architectures, but also open a new way to synthesizing novel organic/inorganic composite materials. 2 2 Experimental 2 2 1 Chemicals Oleic acid (OAcid, 90%), 1 octadecene (ODE, 90%), gold(III) chloride (AuC l 3 99.99%), sodium borohydride (NaBH 4 99%), trio ctylphosphi ne (TOP, technical grade, 90%), squalane, 4 pentyl 4 biphenylcarbonitrile (5BC), polyisoprene ( cis MW = 40,000) and squalane (99%) were purchased from Aldrich. Selenium (Se, 99.99%), dodecyl dimethylammonium bromide (DDAB, 99%), and 1 dodecan ethiol (DDT, 98%) were purchased from Alfa Aesar. Sodium oleate (95%) was purchased from Tokyo Chemical Industry Co., Ltd. Iron chloride hexahydrate (FeCl 3 6H 2 O, 99%) was purchased from Acros Organic. Lead acetate trihydrate (Pb(C 2 H 3 O 2 ) 2 3H 2 O, ACS), and all the other solvents were purchased from Fisher Scientific International Inc.

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59 2 2 2 Synthesis of Nanocrystals 2 2 2 1 PbSe nanocrystals PbSe nanocrystals (NCs) were synthesized according to a literature method. 13 In a typical synthesis, lead acetate trihydrate (240 mg) w as dissolved in squalane (5 mL) in the presence of oleic acid (0.82 mL). After the resulting solution was degassed under vacuum at 80 C for 1 hour, the solution was heated up to 170 C, and a TOP solution of tri n octylpho sp hine selenide (2 mL, 1 M) was injected into the solution. After injection, the solution was cooled to c.a. 145 C, and the temperature was maintained for 2 min utes for nanocrystal growth. Then the reaction solution was cooled to room temperature and the resulting nanocrystal s were isolated from the solution using ethanol. TEM analysis showed that the PbSe core had a 8.3 nm diameter with a standard deviation of 6.0 %. 2 2 2 2 Gold nanocrystals Gold NCs were synthesized based on a method developed by Klaubunde et al 161 In a typical synthesis, AuCl 3 (34 mg) was dissolved in 10 mL of toluene in the presence of dodecyldimethylammonium bromide (92.5 mg). An aqueous solution of NaBH 4 DDT (0.8 mL) was added into the solution. Resulting gold nanocrystals were precipitated out from the solution by adding ethanol and centrifugation. Then the gold nanocrys tals were redispersed in 10 mL of toluene in the presence of DDT (0.8 mL), and the resultant solution was refluxed under argon atmosphere for 30 minutes. TEM analysis showed that the gold core had a 4.8 nm diameter with a standard deviation of 6.3 %. The effective diameter (the gold core and bilayer of the surfactant molecules of 1 dodecanethiol) was estimated to be 7.5 nm. (Figure 2 1B )

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60 2 2 2 3 Iron oxide nanocrystals Iron oxide NCs were prepared according to a literature method. 162 In a typical synthesis, iron oleate (0.9 g) and oleic acid (0.156 g) were dissolved in ODE (5.0 g). The mixture was stirred under an argon flow at room temperature for 10 minutes. Then, the reaction solution was heated up to 320 C at a rate of ~18 C/min and maintained at that temperature for 60 minutes. Afterwards, the reaction solu tion was cooled to room temperature and the resulting nanocrystals were isolated from the solution by adding ethanol. TEM analysis showed that the iron oxide core had a 9.2 nm diameter with a standard deviation of 4.3 %. The effective diameter (the iron oxide core and bilayer of the surfactant molecules of oleic acid) was es timated as 13.2 nm. (Figure 2 1C ). Figure 2 1. TEM images of nanocrystals as building blocks for superlattices. A) PbSe. B) Gold NCs. C) Iron Oxide NCs. 2 2 3 Preparation of Nano crystal Superlattices In a typical preparation, colloidal PbSe NCs ( 10 n mol) were dispersed in toluene (1 mL), and a co solvent was added into the nanocrystal solution in a concentration of 1% v/v. Then, 10 L of the resulting solution was drop cast onto a carbon coated TEM grid supported on a silic on substrate and was dried by natural evaporation.

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61 Figure 2 2 Choice of guest organic molecules A) Squalane. B ) Squalene. C ) 4 pentyl 4 biphenylcarbonitrile (5BC) D ) P olyisoprene 2 2 4 Characterizati ons 2 2 4 1 Transmission electron microscope m easurements Transmission Electron Microscope (TEM) measurements were performed on a JEOL 200CX and a JEOL 2010F operated at 200 kV. In order to see a particular zone of a nanocrystal superlattice, tilting expe riments were also conducted. The selected area electron diffraction patterns were acquired using the 200CX TEM operated at 200 kV with a magnification of 100,000 and a camera length of 82 cm. The area of the superlattice applied for the electron diffrac 2 where at least the 100,000 PbSe NCs existed. 2 2 4 2 X ray diffraction measurements Small angle X ray diffraction patterns were measured on a Philips MRD X'Pert System with Cu K an angle dispers ive synchrotron beamline at the Cornell High Energy Synchrotron Source (CHESS) 163

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62 The peak fitting were conducted using Peak Fitin g program, Multi Peak Fitting 2.0 in Igor Pro version 6.2. 2 2 4 3 IR measurements IR spectra were obtained using a Perkin Elmer 1600 FT IR spectrometer. The specimens were prepared by directly loading nanocrystal suspensions with or without guest molecule s to form superlattices on a NaCl window. 2 3 Results and Discussion 2 3 1 Superstructural Control of PbSe NC superlattices To study the effects of the inclusion of guest molec ules in NC superlattices, we ch ose PbSe NC superlattice and squalane as the mod el lattice and guest molecule, respectively. The PbSe NC building blocks used in this study were 8.3 nm oleate functionalized PbSe NCs, which were synthesized according to a literature method. 13 The choice of squalane as a guest molecule is due to its low melting point, high boiling typical experiment, PbSe NC assembly samples were prepared by drop casting a toluene solution containing PbSe NCs (5 M) in the presence (or absence) of 1% squalane onto the surface of substrates, and then the NC solution was dried under natural evaporati on at room temperature. The substrates used in this study include diamond (or silicon) substrates for small angle X ray scattering (SAXS) measurements and carbon coated TEM grids supported on silicon substrates. Our SAX S measurements show ed that the PbSe NC assemblies made in th e absence of squalane adopt a n fcc superlattice structure. T hese NC assemblies display a SAX S pattern with eight peaks that can be indexed as the (111), (200), (220), (222), (331), (420), (422) and (333) Bragg diffraction s of an fcc structu re with a lattice

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63 constant (a) of 16.8 0. 2 nm (Figure 2 3 A ) In contrast the PbSe NC assemblies prepared in the presence of squalane adopt a non close packed cubic superlattice structure whose SAX S pattern consists of five peaks wit h normalized peak positions at q/q 0 identifying the crystalline domains as possessing a bcc structure with a lattice constant (a) of 13.9 0. 1 nm (Figure 2 3 B ). The structural identification of these two types of PbSe NC superlattices wa further confirmed using TE M. Figure 2 3 SAX S spectra of the nanocrystal superlattices made of 8.3 nm PbSe NC s A) T he fcc superlattice. B ) The bcc superlattice. Under low resolution TEM, both of the se NC superlattices display cross fringe images which are i dentified as the on axis superlattice fringe patterns of the corresponding lattice structures (Figure 2 4 ) 164 The assignment of the zone axis of these TEM images is consistent with the corresponding fast Fourier transform (FFT) patterns (Figure 2 4 A F : bottom left panel). Like th e cross fringes of atomic lattice s taken under a high resolution TEM these superlattice fringe images are also acquired

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64 under an objective lens defocus ing 164 The origin of t hese superlattice fringes is likely from electron phase contrast due to the interference am ong the incident electron beam and small angle diffraction beams t hrough supercrystalline NC assemblies. 164 Indeed, the appearance of superlattice fringes in TEM images follows the same selection rule that governs the systematic absence of Bragg reflections of atomi c lattices with an identical symmetry. For example, an fcc NC superlattice display s lattice fringes from the corresponding scattering planes whose Miller indices (hkl) are all odd or all even, whereas superlattice fringes from a bcc NC superlattice appear when the sum of the Miller indices of the corresponding plane are even (Figure 2 4 ). The [ 100 ] image of the fcc superlattice (SL) shows the perpendicular cross fringes projected from the { 02 0} SL and { 022 } SL planes of the superlattice (Figure 2 4 A ) wher eas that of the bcc superlattice displays cross fringes projected from its { 0 11 } SL and { 02 0} SL planes (Figure 2 4 D ). The cross fringes in the [011] projection image of the fcc superlattice exhibit an angle of 70.5 o (Figure 2 4 C ), which is consistent with t he theoretically calculated value of 70.53 o between the ( ) SL and ( ) SL planes (Figure 2 3B ) On the contrary, the [110 ] image of the bcc superlattice shows the characteristic rectangular bcc cross fringes from its (002) S L and ( ) SL planes (Figure 2 4 E ). In addition, viewed along the [111] zone axis, both the fcc and bcc NC superlattices show the hexagonal cross fringes which are associated with the {022} SL of the fcc structure but the {0 11 } SL of the bcc structure (Figure 2 4C and 4 E ). The lattice const ant (a) determined using these TEM images is 17.0 0. 3 nm for the fcc PbSe NC superlattice and 13.8 0. 1 nm for the bcc superlattice, in good agreement with the results from SAXS measurements.

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65 Figure 2 4 TEM images of the NC superlattices. A C ) The fcc nanocrystal superlattice. D F ) The bcc NC superlattices. The left inset panels (L) are FFT patterns of the corresponding TEM images, and the right inset panels (R) are the electron diffraction patterns from taken from NC superlattices in the relevant projections. All the scale bars are 100 nm.

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66 The results from SAXS and TEM measurements unambiguous ly demonstrate that the presence of squalane significantly affect s the assembly of PbSe NCs, leading to the formation of a bcc superlattice structure. The bcc PbSe NC superlattice has a lower particle packing density than the fcc structure formed without squalane (22.8% vs. 24.8% of the volume occupied by inorganic PbSe cores in a unit cell). The nearest interparticle distance in the bcc superlattice (= a identical to that of the fcc distance corresponds to a nearest interparticle spacing of ~3.7 nm, in agreement with twice the length of an oleic acid molecule. 160 This result shows that squalane molecules may not occupy the space between neares t neighboring PbSe NCs but instead occupy the void spaces such as the gaps between the second nearest neighboring PbSe NCs in the bcc structure. This structural configuration can maximize the energetic interactions between NC building blocks in this non c losed packed structure, and the occupation of the interstitial void space with squalane molecules may further increase the stability of this superlattice structure. 2 3 1 1 Calculation of the lattice constants of the fcc superlattices m ade from PbSe NCs Th e superlattice fringes from TEM measurements and SAXS show the structural details of the super lattices. The lattice constant ( a ) was calculated from these data using the formula: Table 2 1 shows the d spacing between the superlattice fringes measured from the TEM images and SAXS spectra and the calculated lattice constant ( a ).

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67 Table 2 1 Structural information for fcc PbSe NC superlattices from TEM and XRD measurements From these data, the value for the lattice constant was determined to be 17.0 0.3 nm. In a fcc structure, the interparticle distance is expressed as where a fcc is a lat tice constant of fcc superlattice. Thus, the interparticle distance in the fcc superlattice was calculated as 12.0 0.2 nm. The volume of the unit cell in the fcc superlattice is calculated as follows; where a is the lattice constant of the unit cell. The density of the superlattice was calculated as below; where N is the number of nanocrystals in one unit superlattice (in the case of fcc N = 4) and Volume NC is the volume of one nanocr ystal core. TEM [100] zone d(200) = 8.6 nm d(220) = 6.1 nm a = 17.1 nm a = 17.2 nm TEM [110] zone d(200) = 8.6 nm d(111) = 9.9 nm d(220) = 6.1 nm a = 17.1 nm a = 17.1 nm a = 17.3 nm TEM [111] zone d(220) = 5.9 nm a = 16.7 nm S AXS peak(111) peak(200) peak(222) a = 17.0 nm a = 16.7 nm a = 16.6 nm

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68 2 3 1 2 Calculation of the lattice constants and packing density of the bcc superlattices m ade from PbSe NCs Table 2 2 below shows the d spacing between the superlattice fringes measured from the TEM images and SAXS spectra and the calculated l attice constant ( a ). Table 2 2 The structural information of bcc PbSe NC superlattices from TEM and XRD measurements TEM [100] zone d(110) = 9.8 nm d(200) = 6.8 nm a = 13.9 nm a = 13.6 nm TEM [110] zone d(110) = 9.8 nm d(200) = 6.9 nm a = 13. 9 nm a = 13.8 nm TEM [111] zone d(110) = 9.7 nm a = 13.7 nm SAXS peak(110) peak(200) peak(211) peak(220) a = 13.9 nm a = 14.0 nm a = 13.8 nm a = 13.9 nm From these data, the value for the lattice constant was determined to be 13.8 0. 1 nm. In a bcc structure, the interparticle distance is expressed as where a bcc is a lattice constant of bcc superlattice. Thus, the interparticle distance in the bcc superlattice was calculated as 12.0 0.1 nm. The volume of the unit cell in the bcc supe rlattice is calculated as below: In the case of bcc the number of nanocrystals in one unit superlattice ( N ) is two thus, the inorganic core density of the superlattice was calculated as below; Thus, the bcc superlattices have a less packing density by 2.0% than the fcc superlattices.

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69 2 3 2 Mechanistic Study of PbSe NC Superlattice Formation Figure 2 5. Schematic illustrations of the nanocrystal superlattices and the forming process. A ) F cc (the right panel) and bcc (the left panel) superlattices with identical coaxially aligned PbSe NC atomic lattice. B ) Proposed mechanism for NC superlattice formation Interestingly, viewed along the same zone axis, the fcc and bcc PbSe NC superla ttices display nearly the identical dot like electron diffraction (ED) patterns of a rock salt lattice (a type of fcc structure) suggest ing that the individual PbSe NCs exhibit 3D ordered atomic alignment with in these superlattices (Figure 2 4 A F bottom right panel) 12,164 In addition, the [ 100 ] [11 0], and [111] zone axis of the PbSe NCs in these superlattices are oriented coaxially with the corresponding superlattice zone axis (Figure 2 4 ), and thus the 3D atomic orientation of PbSe NCs are identical in both the fcc and bcc superlattices. Such a 3D atomic alignment of PbSe NCs in superlattices

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70 requires a non spherical interaction potential between neighboring NCs, which indicates the PbSe NCs made in this study may have a non spherical shape. 13,160 We propos e that these PbSe NCs possess a rhombicuboctahedron shape, a Wulff polyhedron enclosed by six {200} NC faces, twelve {220 } NC faces, and eight {111} NC faces (Figure 2 5 A ). This nearly spherical NC shape can impose non spherical interparticle interaction pot ential s that allow s for the coaxial alignment of the atomic lattice of these NCs in a perfect geometric arrangement in both the fcc and bcc superlattices : the NCs are tightly packed in 12 fold coordination through their {220 } NC faces along the [110] SL axes of the fcc superlattices, whereas, in the presence of squalane, the NCs are tightly packed through their { 111} NC faces along the [111] SL axes of the bcc superlattice with an 8 fold coordination (Figure 2 5 A ). During bcc superlattice formation, squalane ma y affect both the thermodynamics and kinetics of the assembly of PbSe NCs during solvent evaporation. Squalane can be absorbed onto the surface of PbSe NCs through van der Waals interactions and become a part of the solvent/ligand shell of these NCs i n a toluene solution (Figure 2 5 ). During the formation of NC superlattices, the system needs to pay a large energetic penalty to remove squalane molecules from the surface of NCs to maximize NC packing density, 165 and thus the formation of a bcc superlattice may be favored kinetically because it requires a smaller energetic penalty when compared to the formation of a n fcc structure (Figure 2 5 ). Thus, the formation of a bcc superlattice may be favored kinetically because the required energetic penalty is smaller than that for an fcc structure. In other words, this bcc superlattice likely is in a kinetically trapped st ate at a local free energy minimum. Indeed, the results of superlattice preparations in a partially

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71 closed evaporation chamber at a controlled temperature (20 0.5 C) are consistent with this mechanism (Figure 2 6 ). Also, the unremoved squalane molecules can occupy the interstitial space of the bcc superlattice, which can substantially increase the lattice energy and stability of this non close packed structure. In this mechanism, the guest molecule to be included into an NC superlattice should have a low melting point, as molecules with high melting points have a tendency to self nucleate and thus leave the NC surface during solvent evaporation. For example, we did not obtain a bcc PbSe NC superlattice when octacosane was used as the guest molecule (Figur e 2 7 ). The the bcc superlattice. In comparison with shorter guest molecules, longer ones can more effectively fill the interstitial space of the superlattice (e.g., cross linking the second ne arest neighbor NCs in the lattice through hydrocarbon chain interdigitation) and thus result in a more stable inclusion superlattice. Indeed, the use of 1 octadecene as the guest molecule resulted in the formation of PbSe NC superlattices with a mixture of the fcc and bcc structures (Figure 2 8 ). On the other hand, squalene is an unsaturated derivative of squalane with four trans double bonds, which make its effective length slightly longer than that of squalane. 166 Instead of the flat bcc superlattice thin layers made in the presence of squalane, the inclusion of squalene resulted in PbSe NC superparticles of a bcc structure with sizes (Figure 2 9 ), indicating stronger interparticle interactions in the superparticles with squalene inclusion. In addition, Fourier transform IR spectra suggested that squalene indeed exists in the bcc PbSe NC superlattces, as indicated by the appearance of the characteristic vibrational bands associated with

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72 squalene (e.g., the CH 3 asymmetric stretching vibration and the CH 3 asymmetric and symmetric be nding vibrations; see Figure 2 10 ). Figure 2 6 PbSe NC superlattice s made under a slow so lvent evaporation rate. A ) a typical TEM image of bcc superlattices for med in the presence of squalane. B ) a typical TEM image of fcc superlattices formed in the absence of squalane. In a typical experiment, a toluene solution containing PbSe NCs (5 M) with (or without) 1% squalane was drop cast onto the surface of carbon coated TEM grids supported on silicon substrates, and then the NC solution was dried in a partially closed evaporation chamber for 6 h at a controlled temperature of 20 0.5 o C Fig ure 2 7 TEM images of PbSe NC superlattices made with octacosane. A ) An amorphous structure B ) A mixture of fcc superlattice and amorphous structure

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73 Figure 2 8 TEM images of PbSe NC superlattices made with 1 octadecene. A ) Multiple domains of fc c [110] zone and bcc [110] zone B ) bcc superlattices [110] zone. Figure 2 9 TEM images of PbSe NC superparticles made with squalene. A) Low magnification image. B D) High magnification images.

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74 Figure 2 10 NC superlattices made with squalane. A ) FT IR Spectra of squalene (red), bcc PbSe NC superlattice (blue), and fcc PbSe NC superlattices (black). B,C) T he zoomed in IR spectra, in which P 1 :2966 cm 1 CH 3 asymmetric stretching vibration; P 2 :2920 cm 1 CH 2 asymmetric stretching vibration; P 3 :2851 cm 1 CH 2 stretching vibration; P 4 :1445 cm 1 CH 3 bending vibration; P 5 :1406 cm 1 COO symmetric stretching vibration; and P 6 :1380 cm 1 CH 3 symmetric bending vibration. D ) A typical TEM image of the fcc PbSe NC superlattices taken from the sam ple used for FTIR measurements. E ) A typical TEM image of the bcc PbSe NC superlattices taken from the sample used for FTIR measurements.

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75 Amazingly, with an even longer guest molecule, polyisoprene (M.W. ~400,000), we obtained bcc PbSe superparticles of 5 70 nm with a relative size distribution of 27% (Figure 2 11 ). Moreover, the spontaneous formation of bcc PbSe superparticles upon solvent evaporation could also be introduced by the presence of 4 pentyl 4 biphenylcarbonitrile (5CB), a widely used room temp erature nematic liquid crystal The resulting superparticles were 334 2 12 ). The lattice constants of the bcc PbSe superparticles made with squalene, polyisoprene, and 5CB are identical to that of the bcc superlattice made with squalane (Table 2 3 ), showing that these guest molecules modify just the morphology of the bcc NC superlattice but not its structure. This result is consistent with the proposed mechanism ( Figure 2 5 ), in which the energetic interactions between PbSe NCs also play a substantial role in the formation of the bcc superlattice. Table 2 3 The lattice constants of bcc PbSe superlattices made with the following additives: squalane, squalene and polyisopre ne. The lattice constants were determined from TEM images Additives Lattice Constant (a) Squalane a= 13.8 (0.1) nm Squalene a= 13.8 (0.1) nm Polyisoprene a= 13.8 (0.1) nm pentyl 4 biphenylcarbonitrile a= 13.7 (0.2) nm

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76 Figure 2 11 TE M images of superparticles made of 8.3 nm PbSe NCs in the presence of polyisoprene. The sca le bars are 2 micron meter in (A), and 200 nm in (B E )

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77 Figure 2 1 2 A TEM image of bcc PbSe NC superparticles prepared in the presence of pentyl 4 biphenylca rbonitrile B ) shows [110] zone image of a bcc PbSe NC superparticle. 2 3 3 Superstructural Control of Binary Nanocrystal Superlattices Moreover, to study the squalane inclusion effects on binary NC superlattice s we used 4.8 nm Au and 9.2 nm Fe 3 O 4 NC s w ith an effective particle size ratio of 0.57 When the ratio of Fe 3 O 4 / Au NC was 1: 5, the co assembly of these two types of NCs at room temperature resulted in Fe 3 O 4 / Au binary superlattices with AlB 2 as the dominant structure (Figure 2 1 3 ) 167 In this b inary structure Fe 3 O 4 NCs occupy the Al sites and Au NCs occupy the B sites. The superlattice exhibits a particle packing density of 22.9% and an effective space filling factor of 72.0%. In contrast, the presence of sq ualane led to the formation of binary Fe 3 O 4 / Au NC superlattices with a dominant structure of cub AB 13 superlattic e, where B spheres occupy the vert ices of cuboctahedron 168 The superlattice exhibits a volume packing density of 18.6% and an effective space filling factor of only 65.4%. This result clearly demonstrates that the guest molecule inclusion stabilized the binary NC superlattice with a low space filling factor, wh ich may also underwent a kinetically limited formation process.

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78 Figure 2 1 3 TEM images of binary superlattices made of 4.8 nm gold and 9.2 nm iron oxide NCs A ) I n the absence of squalane B ) I n the presence of squalane The scale bars are 100 nm. 2 3 3 1 Calculation s of the packing density of the binary nanocrystal s uperlattices with AlB 2 structure The unit cell for the AB 2 structure is hexagonal, thus the volume is calculated as The packing density (PD) was the one calculated by using the size of the inorganic core of the NCs (Fig S1). Thus, it was calculated as below: The effective filling factor (EFF) was the one calculated by using the size of the NCs with the surfactant shell. Thus, it was calculated as below:

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79 2 3 3 2 Calculation s of th e packing density of the binary n anocrystal superlattices with A B 13 structure The volume of the unit cell of the AB 13 superlattice is calculated as: The PD was calculated as below; The EFF was calculated as below; 2 4 Conclusions In conclusion, chapter 2 has described have reported an important study of the inclusion of gues t molecules in NC superlattices. Our results show that guest molecule inclusion leads to the formation of non close packed PbSe and binary Fe3O4/Au NC superlattices and enables control of the mor phology of the NC superlattices. W e have synthesized PbSe NC superparticles having a bcc structure using squalene and polyisoprene. By means of guest molecule inclusion, we have shown for the first time that both fcc and bcc NC superlattices with an identical 3D coaxially aligned NC atomic lattice can be prepared f rom identical PbSe NC building blocks. Moreover, one can expect to use guest molecules with specific optical, electronic, and magnetic properties to tailor the functionality of NC superlattices for applications such as biomedical diagnosis, solar cells, an d photodetectors.

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80 CHAPTER 3 BINARY ASSEMBLY OF C OLLOIDAL SEMICONDUCT OR NANORODS WITH SPHERICAL METAL NANO PARTICLES 3 1 Introduction The ability to assemble one or more types of colloidal nanoparticles into higher ordered architectures allows for the ratio nal control of the electronic, plasmonic, and/or magnetic coupling between nanoparticle building blocks, which can enable nanoparticle assemblies to exhibit unique and collective physical properties. 58,84,85,101,169 171 Thus far, a variety of binary nanoparticle superlattices have been prepared using building blocks with chemical compositions of semiconductors, metals, and metal oxides; these building blocks have primarily been spheres or spherical polyhedrons. 101,102 However, there have been fewer explorations of binary nanoparticle assemblies with anisotropic building blocks, such as the superlattice of LaF 3 triangular nanoplates with Au nanoparticles 101 and binary assemblies of gold nanowires with gold nanospheres and nanorods. 172 This chapter describe the preparation of binary assemblies of CdSe/CdS semiconductor nanorods with spherical metal nanoparticles where the spherical nanoparticles are intercalated into parallelly aligned nanorod arrays. Our mechanistic stu dies suggest that the formation of these binary assemblies is a kinetically limited process. Organic additives with suitable polarity and strong affinity to spherical nanoparticles having both a high dielectric constant and a large Hamaker constant play a n important role 3 2 Experimental 3 2 1 Chemicals Oleylamine (OAm 70%), oleic acid (OAcid, 90%), 1 octadecene (ODE, 90%), sulfur (99%), gold(III) chloride (AuCl 3 99.99%), sodium borohydride (NaBH 4 99%),

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81 trioctylphosphine oxide (TOPO, 99%), trioctylphos phine (TOP, technical grade, 90%), squalane (99%), decylamine (95%), benzyl alchol (99.8%), 1 octanethiol (98.5+%), and palladium(II) acetylacetonate (Pd(acac) 2 99%) were purchased from Aldrich. Cadmium oxide (CdO, 99.99 %), Selenium (Se, 99.99%), dodecy l dimethylammonium bromide (DDAB, 99%), 1 octanethiol (95%), and 1 dodecanethiol (DDT, 98%) were purchased from Alfa Aesar. Octadecylphosphonic acid (ODPA, 99%), and hexylphosphonic acid (HPA, 99%) were purchased from Polycarbon Inc. Triphenylphosphine ( 99%) was purchased from STREAM. Sodium oleate (95%) was purchased from Tokyo Chemical Industry Co., Ltd. Lead acetate trihydrate (Pb(C 2 H 3 O 2 ) 2 3H 2 O, ACS), and all the other solvents were purchased from Fisher Scientific International, Inc. 3 2 2 Nanocrysta l Synthesis 3 2 2 1 CdSe/CdS nanorods CdSe/CdS nanorods were prepared via a seeded growth method a ccording to a literature protocol 173 In a typical synthesis, CdSe nanoparticle seeds (2.3 nm in diameter) were synthesized as follows. TOPO (3 g), ODPA (0.28 g) and CdO (0.06 g) were mixed in a three neck flask and degassed under vacuum for 1 hour at 150 C The resulting solution was heated to 300 C and TOP (1.5 g) was added. The solution was heated to 370 C and a TOP solution of tri n oc tylphosphine selenide (0.43 mL, 1.7 M) was injected into the solution. The heating mantle is removed immediately after the injection, and then the solution was cooled to room temperature. Resulting nanoparticles were purified through three cycles of prec ipitation and redispersion using methanol and toluene, respectively. Then the CdSe nanoparticle seeds were dissolved in TOP and the concentration was adjusted to 400 M.

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82 CdSe/CdS nanorods were synthesized as follows. CdO (0.06 g), TOPO (3.0 g), ODPA (0.29 g), and HPA (0.08 g) were mixed and degassed under vacuum for 1 hour at 150 C. The solution was heated to 380 C under argon, and TOP (1.5 g) was added. After the temperature reached 380 C again, a TOP solution with sulfur and CdSe nanoparticle seeds (1.9 mL, 0.12 g of sulfur, and 80 nmol of CdSe nanoparticle seeds) was injected into the solution. The reaction temperature was maintained at 380 C for 8 min for th e growth of CdSe/CdS nanorods, and then the reaction solution was cooled to room temperature. The resulting CdSe/CdS nanorods were isolated from the reaction solution using ethanol. A typical nanorod sample from this synthesis exhibits a length of 74 5 nm and a diameter of 4.2 0.3 nm. 3 2 2 2 Palladium n anoparticles Palladium nanoparticles ( 4.0 0.3 nm) were synthesized according to a method developed by Hyeon et al 174 In a typical synthesis, Pd(acac) 2 (100 mg) was dissolved in TOP (1 mL) under arg on, and OAm (10 mL) was added to the solution. The resulting solution was heated to 250 C slowly (5 C per one min) and the temperature was maintained for 30 min. Then the reaction solution was cooled to room temperature, and the resultant nanoparticles were isolated from the reaction solution by adding ethanol. 3 2 2 3 Gold, iron oxide and lead s elenide n anocrystals Nanocrystals of g old ( 5. 0 0.6 nm ), 175 iron o xide ( 9.2 0.7 nm) 162 and lead s elenide ( 8.2 0.6 nm ) 13 were synthesized according to literature methods. The details of synthetic conditions were described in the previous chapter. 3 2 3 Preparation of Nanocrystal Assemblies In a typical preparation, colloidal CdSe/CdS nanorods (50 pmol) and spherical nanoparticles (500 600 pmol) were dispersed in toluene (1 mL), and an additive was

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83 added into the nanoparticl e solution in a concentration to give a final concentration 1% v/v for DDT, 1 decanethiol, 1 octanethiol, de cylamine, and benzy lalcohol, or 1 mM for triphenylphosphine. Then, 10 L of the resulting solution was drop casted onto a carbon coated TEM grid supported on a silicon substrate and was dried under natural evaporation. 3 3 Results and Discussion 3 3 1 Preparation of Binary Nanocrystal Assemblies from CdSe/CdS Nanorods and Go ld Nanospheres A critical step in the formation of binary nanoparticle superlattices is to simultaneous generation of nuclei containing two different types of building blocks This process requires a fine balance of the entropic and energetic interactions between these building blocks. However, this fine balance is difficult to achieve when the two building blocks have significant differences in their size and/or shape ( e.g., nanorods and nanospheres) which often resulting in the nucleation of the two bu ilding blocks separately. A number of experiments and the theoretical simulations have sho w n that binary colloidal mixtures of noninteracting hard spheres and hard rods give rise to entropically driven phase. 176 17 8 Recently, Manna et al. have found that CdSe/CdS nanorods can be selectively precipitated from a solution containing these nanorods and spherical CdSe nanoparticles by controlling entropy based depletion forces. In addition, the existence of strong ene rgetic interactions (e.g., dipole dipole and van der Waals interactions) between nanorods can further promote the phase separation of nanrods and nanospheres. 114 Therefore, overcoming these en tropically and energetically un favorable inte ractions should be critical for the formation of binary assemblies of colloidal nanorods with nanospheres.

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84 To explore this possibility, we used colloidal CdSe/CdS semiconductor nanorods and spherical gold nanoparticles as a model system to study the format ion of nanoro d/nanosphere binary assemblies. Using literature methods, we synthesized octadecylphosphonate functionalized CdSe/CdS nanorods (745 nm in length and 4.20.3 nm in diameter) and dodecanethiol ( DDT ) capped gold nanoparticle s of 5.0 nm in diame ter with a re lative standard deviation of 6% Since gold exhibits a very large Hamaker constant, we originally hypothesized that the strong van der Waals (vdW) attraction between CdSe/CdS nanorods and gold nanoparticles would prevent the phase separation of these two building blocks. 150 However, our experimental results show that the use of triphenylphosphine (TPP) as an additive is important to the formation of binary assemblies of CdSe/CdS nanorods with gold nanoparticles. In a typical assembly experi ment, TPP (1 mol) was added to a toluene solution (1 mL) containing a binary mixture of the Au nanoparticles (500 nM) and the CdSe/CdS nanorods (50 nM). The resulting solution was drop casted onto a carbon coated TEM grid supported on a silicon substrate and was dried under natural evaporation. TEM images in Fugure 3 1 show that 2 dimensional (2D) binary assemblies formed as gold nanoparticles intercalated into nearly parallel CdSe/CdS nanorod arrays. There are about eight to ten close packed gold nanop articles aligned linearly between two neighboring nanorods. These 2D binary assemblies exhibit a spacing of 2.0 nm between the close packed nanoparticles and a nanoparticle/nanorod spacing of 1.9 nm, which are consistent with those distances be tween the s urface ligands of two contacting particles. On the contrary, a phase separation of the two building blocks predominantly took place when TPP was absent (Figure 3 2A ). In this case, gold nanoparticles

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85 primarily formed close p acked monolayer islands, where as the CdSe/CdS nanorods were randomly orientated and partial ly parallel packed together; there was o nly a trace a mount of gold nanoparticles incorporate d between neighboring nanorods, (Figure 3 2A ). Figure 3 1 A ssemblies of CdSe/CdS nanorods and gold nanoparticles prepared in the presence of TPP A) Schematic illustration. B) TEM image

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86 Figure 3 2 TEM images of assemblies of CdSe/CdS nanorods and gold nanoparticles with or without additives. A) No additives. B) ODE C ) B enzylalc o hol D ) H exano l E ) 1 dodecanethiol F ) 1 decanethiol G ) 1 octanethiol H ) 1 dodecylamine

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87 3 3 2 Additive Effect TPP is a polar molecule with a high boiling point, and it can bind onto both CdSe/CdS nanorods and gold nanoparticles. 179,180 To better understand the functio n of TPP in the formation of our binary assemblies, we studied the assembly of these two building blocks in the presence of one of the following high boiling solvents as an additive at 1% v/v concentration: 1 octadecene (ODE, a non polar molecule), benzyl alcohol 1 hexanol (a highly polar molecule), and DDT (a molecule with moderate polarity similar to TPP). 181 TEM observations show that only drop casting of the nanoparticle solution with DDT resulted in 2D nanorod/nanosphere binary assemblies (Figure 3 2B E ). The edge to edge interparticle distances in th ese binary assemblies are almost identical to those in the binary assemblies made usin g TPP (Figure s 3 1 and 3 2 E ). The t ype s of these additive s do not substantially change the interparticle distances, indicating that the additives may only affect the nucleation of binary assemblies but may not be present in the final binary assemblies. Taken together, these results show th at appropriate polarity of the additive is important to the formation of the binary assemblies. Additional results further show that the binary assembly formation is indeed very sensitive to the polarity of additive s It is known that the length of the hy drocarbon chain slightly affects the polarity of alkanethiols; the polarity order is 1 octanethiol > 1 decanethiol > DDT 182 Remarkably, the use of 1 decanethiol as an additive led to the formation of binary assemblies (Figure 3 2 F ), whereas the presence of 1 octanethiol results in the phase separation of these two building blocks (Figure 3 2 G ). We attribute the phase separatio n to the strong non solvent effects induced by the high polarity of 1 octanethiol. In addition, the strong affinity of the additives to gold nanoparticles also

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88 seems important to the formation of the nanoparticle/nanorod binary assemblies. Our resul ts sh ow that do decylamine which has a similar polarity to DDT and can bind strongly onto the surface of CdSe/CdS nanorods but has a weaker affinity to gold nanoparticles as compared to DDT (or TPP) does not lead to the formation of nanorod/nanosphere binary ass emblies (Figure 3 2 H ). 3 3 3 Mechanistic Studies on the Formation of the Binary Nanocrystal Assemblies To gain insight into the function of these additives, we used the interparticle distances between CdSe/CdS nanorods and gold nanocrystals in their dry fo rms to calculate the major energetic interactions in the binary assemblies. According to Hamaker theory, 150,183 the vdW interaction energies between a nanorod and a gold nanoparticle, two gold nanoparticles, and two nanorods are 23 .1 meV (0.90 kT), 10.7 meV (0.42 kT), and 4.0 meV (0.12 kT) in toluene and 30.4 meV (1.18 kT), 11.6 meV (0.45 kT), and 6.4 meV (0.25 kT) in DDT respectively. These calculations were described in the next section. Using a dipole approximation, we obtaine d an electric dipole dipole interaction energy for two close packe d nanorods oriented in parallel of 977306 meV (38. 2 11.8 kT) in toluene and 832259 meV (32.510.1 kT) in DDT 184 Ignoring the dielectric effects of gold nanoparticles, the interaction energy for two neighboring nanorods in binary assemblies is 94.630.4 meV (3.81.2 kT) in toluene and 83.325.8 meV (3.31.0 kT) in DDT The energies of the interactions in the binary nanorod/nanoparticle assemblies and the assembies with only nano rods are listed in Table 3 1. These calculation results show that the formation of nanoparticle/nanorod binary assemblies is favored by vdW interactions. However, the free energy gain from vdW interactions is significantly smaller as compared to the l oss due to dipole dipole

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89 interac tions either in toluene or DDT. In addition, the nanoparticle/nanorod contact gives rise to a smaller reduction in the excluded volume when compared to the parallel nanorod/nanorod contact, 127 and thus the formation of binary assemblies needs to further overcome an unfavorable entropi c change. Taken altogether, these results suggest that the nanoparticle/nanorod binary assemblies should no t form under thermodynamic equilibrium conditions Table 3 1. The calculated energies of the vdW interactions and dipole dipole interactions in a binary assembly and a nanorod assembly. Assembly Binary Assembly Nanorod Assembly Solvent Toluene DDT Toluene DDT vdW Energy 9.0 kT 11.8 kT 0.12 kT 0.25 kT Dipole dipole Energy 3.8 1.2 kT 3.3 1.0 kT 38.2 12.1 kT 32.5 10.1 kT Sum 12.8 1.2 kT 15.1 1.0 kT 38.3 12.1 kT 32.8 10.1 kT We propose that the formation of the nanoparticle/nanorod binary assemblies is kinetically limited by solvent evaporation in the pre sence of appropriate additives. Before their concentration s reach the critical (or nucleation) point toward binary assembl y formation or phase separation on the surface of a TEM grid, the CdSe/CdS nanorods and gold nanoparticles are stabilized in a drop cast toluene solution by repulsive steric forces that are as sociated with the interactions between solvent molecules and their surface ligands. 165 Upon solvent evaporation, the repulsive steric forces decrease and the collision rates (or frequency) between these colloidal particles increase. Owing to the size dependent Brownian collision kernel, 185 the collision rate between the CdSe/CdS nanorods and the gold nanoparticles is larger than those

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90 between particles of the same type thus leading to the formation of loose, kinetically stable assemblies with individual nanorods surrounded by tens of gold nan oparticles in the presence of excess gold nanoparticles ( Figure 3 3 ). Indeed, some of these assemblies can be preserved on a TEM grid in dry form when the drop cast solution was diluted by 10 times (Figure 3 3 A ). Figure 3 3 The formation of 2D binary assembly of CdSe/CdS nanorods with sphe rical gold nanoparticles. A) CdS/CdSe nanorod. B ) D iscrete as sembly of gold nanoparticles with a si ngle CdSe/CdS nanorod. C ) B inary assembly of the nanorods and gold nanoparticles. During the nucleation of binary assemblies, these loose assemblies can be stabilized by a suitable additive because of the f ollowing m ajor effects: (1) the additive such as DDT is slightly more polar than toluene and introduces non solvent effects that can increase the attractive interactions between the nanorods and the gold nanoparticles, and (2) the additive may partially exchange wit h the surface ligands on gold nanoparticles and in turn attract the gold nanoparticles around the CdSe/CdS nanorods. In addition, the large vdW interactions between the nanorods and the gold nanoparticles are important to the stabilization of these kineti cally assembled nanostructures. Moreover, the infinite dielectric constant (low frequency) of gold nanoparticles can substantially screen and decrease the electric dipole dipole

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91 lies at a local free energy minimum in this kinetically limited process ( Figure 3 3 ). Therefore, beside suitable additives, the Hamaker constant and dielectric constant of the spherical nanoparticles are also important factors in the formation of binary a ssemblies of these nanoparticles with CdSe/CdS nanorods. 3 3 3 1 Detailed calculations of interactions and collision between n anocrystals This section describe s the detailed calculation of the interactions between the nanorods and/or the gold nanocrystal s, and the collision rate. The interactions calculated here include van der Waals interactions and electric dipole dipole interactions. The function of the size dependent Brownian collision kernel is also disscussed 3 3 3 2 Dipole dipole i nteraction (a) Dipole moments of the nanorods The CdSe/CdS nanorods used in this work exhibit a length of 745 n m, a diameter of 4.20.3 nm, with a 2.3 nm spherical CdSe core inside each nanorod According to a literature method, 129,186 we estimated the dipole moments of the CdSe/CdS nanorods to be (1.00.2) 10 3 D (b) Calculation of the Dipole Dipole Interaction When the nanorods with dipole moments are aligned side by side, the dipole dipole intera ction can be calculated acc ording to (eq. 3 1 ) where is the dipole moment, 0 is the vacuum permittivity, is the dielectric constant of the intervening medium, and r is the center to center separation.

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92 The dielectric constants of toluene and DDT are 2.3 and 2.7, respectively. 187 The center to center distance ( r ) here is 6.5 nm. Thus, the dipole dipole interactions in toluene and DDT were calculated as 38.211.8 kT (977 306 meV) and 32.5 10.1 kT (832 259 meV), respectively. In the case of binary assembly, the center to center distance was determined as 14.0 nm from TEM measurement. Ignoring the dielectric effects of gold nanoparticles, the interactions energy in toluene and DDT were calculated as 3.81.2 kT (97.730.4 meV) and 3.3 1.0 kT (83.3 25.9 meV), respectively. The results are summarized in Table 3 2. Table 3 2 The energies of dipole dipole interactions in the binary assemblies and the assemblies with only CdSe/ CdS nanorods. Assembly Binary Assembly Nanorod Assembly Solvent Toluene DDT Toluene DDT Dipole dipole Energy 3.8 1.2 kT (98 30 meV) 3.3 1.0 kT (83 26 meV) 38.2 12.1 kT (977 306 meV) 32.5 10.1 kT (832 259 meV) 3 3 3 3 Van der Waals i nt eractions (i ) Hamaker Constants The Hamaker constants ( A ) of gold and CdS in vacuum are 2.81 eV and 0.686 eV, respectively. The Hamaker constants of toluene and DDT in vacuum are calculated using and the ir dielectric constant s (2.3 for toluene, 2.7 for DDT ) and refractive indices (1.50 for toluene, 1.44 for DDT) According to Eq. 1.2.2.2 2 we obtained A toluene = 0.440 eV and A DDT = 0.384 eV The Hamaker constants between an inorganic material (1) through a liquid solvent (2) are expressed as

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93 (e q. 3 2 ) Thus, A gold in toluene is 1.03 eV, A gold in DDT is 1.12 eV, A CdS in toluene is 0.0272 eV, and A CdS in DDT is 0.0435 eV. The Hamaker constants between different materials (1 and 2) through a solvent (3) can be calculated using (e q. 3 3 ) Thus, A g old CdS in toluene is 0.167 eV and A gold CdS in DDT is 0.220 eV. (ii ) Calculation of the van der Waals Interactions Van der Waals interactions between two neighboring spherical gold nanoparticles, two neighboring CdSe/CdS nanorods, and CdSe/CdS nanorod/gol d particle were calculated using the equations presented in Table 1 2 On the basis of the sizes of the nanoparticles and interparticle distances determined by TEM measurements as well as the Hamaker constants calculated above, we obtained the van der Waa ls interactions (vdW) between the two gold nanoparticles (NPs), the two CdSe/CdS nanorods (NRs), and NP/NR which are placed in toluene or DDT. The calculated results are listed in Table 3 3 Table 3 3. The energies of vdW interactions between two neighbo ring particles. NP NP NR NR NP NR in toluene in DDT in toluene in DDT in toluene in DDT vdW (meV) 10.7 11.6 3.95 6.43 23.1 30.4 vdW (kT) 0.416 0.452 0.153 0.249 0.895 1.18

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94 3 3 3 4 S ize dependent b rownian c ollision k ernel Collision (agglom eration) rates are described by 185 (e q. 3 4 ) where N is the total number of particles per unit volume suspension and is the collision kernel rate. Here, is a size dependent term that is described as: (e q. 3 5 ) where is the viscosity of the solvent, 1/ W eff is the collision capture efficiency, and R 1 and R 2 are the size s of the two coll oid particles. As the equation indicates, the collision rate gets larger when R 1 and R 2 are different numbers than when they are similar. This suggests that the CdSe/CdS nanorods and the gold nanoparticles, that have big differences in the ir sizes, preferentially collide with each other in the suspension. 3 3 4 Required Conditions for Intercalated Nanocrystals To further examine this mechanism, we used oleate capped 9.2 nm Fe 3 O 4 and 8.2 nm PbSe nanoparticles instead of the 5.0 nm gold nanop articles to study their assembly with the CdSe/CdS nanorods in the presence of 1% DDT Our TEM observations show that both cases resulted in the phase separation of the nanorods and nanoparticles (Figure 3 4A B ) This is consistent with our mechanism be cause these two materials exhibit limited low frequency dielectric constants and they have smaller Hamaker constants than gold. In contrast, we obtained 2D binary assemblies of the CdSe/CdS nanorods with oleylamine functionalized palladium nanoparticles ( 4.0 nm in diameter) in the presence of 1% DDT (Figure 3 4C ). These results show that the

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95 presence of DDT is critical to the formation of these binary assemblies. Because like gold nanoparticles, palladium nanoparticle exhibit an infinite dielectric cons tant and a large Hamaker constant the formation of CdSe/CdS nanorods/palladium nanoparticle binary assemblies further supports our mechanism. Figure 3 4. TEM images of nanocrystal assemblies of the CdSe/CdS nanorods with spherical nanoparticles. A) Phase segregated assemblies of the nanorods with 8.2 nm PbSe nanoparticles. B) Phase segregated assemblies of the nanorods with 9 .2 nm iron oxide nanoparticles. C) B inary intercalated assemblies of nanorods with 4.0 nm palladium nanoparticles

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96 3 4 Concl usions In conclusion, this chapter report s a preparation of 2D binary assemblies with spherical metal nanoparticle s intercalated into parallelly aligned CdSe/CdS nanorod arrays. The results from our mechanistic studies suggest that the formation of these binary assemblies is a kinetically limited process, in which suitable additives and spherical nanoparticles with a high dielectric constant and a large Hamaker constant play important roles. This new understanding is important to the development of new me thod s for assembling multiple nanoparticle building blocks int o higher order nanostructures for use in the fields of plasmonics, phototcatalysis, and potentially many others.

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97 CHAPTER 4 NANO CRYSTAL SUPERLATTICES UNDER HIGH PRESSURE 4 1 Introduction Phase stability is an important physical property of any material. In the field of nanocrystal research, the melting point is the first p roperty used as an indicator of the phase stability 65,188 It is know n that nanocrystals change the ir melting point s depending on their crystal size s; and usually nanocrystal melting points are lower than the melting point of the bulk state because of the large surface energies. Furthermore nanocrystals possess a unique pressure dependency different from that in the bulk state. In other words, nanocrystals exhibit a unique temperatu re pressure phase diagram. Investigations on nanocrystals and nanocrystal superlattices under high pressure are of fundamental importance for high pressure chemistry. Because nanocrystal s possess a small and defect free crystal domain, the manner of the p ressure driven crystal structure change (the phase transition) is unique, giving important insights for the phase stability. 189 One of the most ex tensively studied phenomena is the solid solid phase transition of the semiconductor CdSe nanocrystal s 66 68,140,190 CdSe crystals adopt a Wurtzite type crystal structure under ambient pressure, changing to a a Rock salt type at high crystal field are : (i) a si ze dependency on the phase tra nsition pressure (pressure at which a phase transition occur s) 75 191 and (ii) the unique kinetics of the phase transition 192 Acc ording to Tolbert 68 t he smaller CdSe nanocrystals exhibit higher phase transition pressure s and the nature of the surface significantly affect s the phase transition.

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98 However, when it comes to nanocrystal superlattices, high pressure research ha s not yet fully developed One of the reasons for the slow progress is the technological difficulty in monitor ing the superstructures. For typical measurements on nanocrystals under high pressure, nanocrystals are loaded into a pressure chamber and their structura l and/or property changes under pressure are monitored using a synchrotron X ray scattering technique or optical measurements. With a technique like X ray scattering, based on Bragg diffractions the superstructural studies on the na nocrystal superlattic es require very small angle diffraction (scattering) cor responding to a large d spacing ( c.a. 10 nm and less.) Although superstructural data are important in the characterizatio n of nanocrystal superlattices thorough investigations are currently provin g difficult due to lack of techniques available Recently, a research group at Cornell University has develop ed a new technology, enabling simultaneous X ray scattering measurement s ranging from atomic structures (as small as 0. 1 ) and superstruc t ures ( as large as 50 nm) in a pressurizing chamber. 193 This is an ideal instrumentation for study o f nanocrystal superlattices, because an unambiguous relatio nship between the superstructures and their properties dictated by crystal structures can be observed This chapter describes two mechanistic studies on PbSe nanocrystal superlattices under high pressure. First, a study of pressure driven superstructur al transformation s of PbSe superlattices is presented. We found polymorphs of the superstructures of the na nocrystal superlattices from 5.3 nm PbSe nanocrystals at various pressures including body centered cubic, body centere d tetragonal, simple cubic, t wo

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99 dimensionally ordered square and tetragonal, as well as a lamellar structure from nanoplates resulting from the nanocrystal fusion. Th e s e pressure driven superstructural transition s are important physical character istics of nanocrystal superlattices S ome of the superstructures observed in this study ( e.g. the two dimensionally ordered square and tetragonal superstructures ) were created in this study for the first time ever to our best knowledge. Second, we present a study o f the effect of superstructu re on the phase transition pressure of the PbSe crystals Our mechanistic study shows that the superstructure of the PbSe nanocrystal superlattices can tune the phase transition pressure by as much as 5 GPa. The key to tun ing the phase transition pressur e is the direction of the propagating pressure We demonstrate that the superstructure could dicta te the pressure direction applied to the nanocrystal superlattices, leading to a different phase transition pressure s Both of the stud ies show the signific ance of superstructures in the nanocrystal superlattices in determin ing the mechanical properties. These studies c ould provide useful insight for material design with a strong phase stability. 4 2 Experimental 4 2 1 Chemicals Oleic acid (OAcid, 90%), t rioctylphosphine (TOP, technical grade, 90%), and squalane (99%) were purchased from Aldrich. Selenium (Se, 99.99%) w as purchased from Alfa Aesar. Lead acetate trihydrate (Pb(C 2 H 3 O 2 ) 2 3H 2 O, ACS), and all the other solvents were purchased from Fisher Scie ntific International Inc.

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100 4 2 2 Synthesis of Nanocrystal Superlattices PbSe nanocrystals (NCs) were synthesized according to a literature method with a minor modification. 133 In a typical synthesis, lead acetate trihydrate (240 mg) w as dissolved in squalane (5 mL) in the presence of OAcid (0.82 mL). After the resulting solution was degassed under vacuum at 80 C for 1 hour, the so lution was heated up to 140 C, and a TOP solution of tri n octylphosphine selenide (2 mL, 1 M) was injected into the solution. After injection, the temperature was maintained for 3 minutes for nanocrystal growth. Then the reaction solution was cooled to room temperature and the resulting nanocrystals were isolated from the solution using ethanol. TEM analysis showed that the PbSe core had a 5.3 nm (5.5 %) diameter NC superlattices with a body centered cubic ( bcc ) and a face centered cubic ( fcc ) super structure were prepared accordin g to a method describ ed in chapter 2. 148 Squalane or 5CB were used as a guest molecule for creation of bcc nanocrystal superlattices. The formed PbSe nanocrystal superlattices were loaded in a d iamond anvil cell (DAC shown in Figure 4 1 ) for further characterization under pressure. 4 2 3 Characterizations 4 2 3 1 I n situ high pr essure small angle and wide a ngle X ray scattering (SAXS and WAXS) m easurement The s amples were loaded in a stainless steel gasket with a hole of a bout 200 m diameter in a DAC (shown in Figure 4 1) Several ruby chips were loaded on the top of the sample for monitoring pressure. In this work, we did not use any pressure medium unless mentioned Accordingly, synchrotron SAXS and WAXS measurements were perf ormed at the B2 station at the Cornell High Energy Synchrotron Source (CHESS). 163 Two

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101 dimensional X ray scattering images were collected with a lar ge area MAR345 detector (Figure 4 2). The distances between the sample and the detector were calibrated by Ag behenate and CeO 2 for SAXS and WAXS, respectively. White synchrotron X rays were tuned to a monochromatic wavelength using monochromators. The diameter of the X ray beam was reduced to 100 m using a double pinhole collimated tube for illumination of the samples. A Fit2D program (www.esrf.eu/computing/scientific/FIT2D) was used to integrate a collected circular pattern into a one dimensional X ray scattering spectrum. Figure 4 1. A schema tic illustration of an X ray scattering measurement with DAC under high pressure. Figure 4 2 Typical X ray scattering patterns collected on the Mar3450. A) WAXS patterns. B ) SAXS patterns.

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102 4 2 3 2 Electron microscope measurement s TEM measurements we re performed on a JEOL 200CX and a JEOL 2010F operated at 200 kV. SEM measurements were performed on a Hitachi S 4000 FE SEM operated at 6 kV. 4 2 4 Peak Analysis Figure 4 3. A schematic illustration of a peak analysis. For SAXS and WAXS spectra anal ysis, some programs in Multi Peak Fitting 2.0 in Igor Pro version 6.2 were used. In the SAXS region, the peak positions and peak areas were determined using the Peak P icking p rogram The simulations of the spectrum decom position were conducted using Glob al F itting program The function used for the peaks was Gaussian, and for the baseline Cubic or PolyLog5 was used The parameters of the peak s e paration are shown in Figure 4 3. The peak position was determined as the center of the Gaussian curve. The peak intensity was determined as the peak area In the WAXS region, the peak positi ons were also determined using the same Peak Picking program In the phase transition study, the ratio between a high

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103 pressure phase and a low pressure phase were determin ed by comparing an experimentally obtained spectrum shape with a simulated spectrum The simulation spectr al was obtained t hrough a convolution of spectra of the high pressure and low pressure phase at a given ratio. 4 3 Results and Discussion Pressure Driven Superstructural Transformation of PbSe Nanocrystal Superlattices This section presents a study of the mechanical behavior of nanocrystal superlattices under high pressure, using the superlattices made from 5.3nm PbSe nanocrystals. In situ SAXS meas urements upon gradual pressure shows various superstructural transformation (i.e. from bcc to bct to sc to two dimensional square to two dimensional rectangular, and finally to a lamellar structure as a result of nanocrystal fusion.) Interestingly, the tw o dimensional square and rectangular superstructures were formed in a discotic liquid crystal like fashion. At the highest pressure, nanocrystals were fused to form large nanoplates, with a lamellar alignment. Our control experiment using 8.2 nm PbSe nan ocrystals suggests that the physical stability of the superstructures is dependent on the size of the building blocks. This study will be an important standard for characterizing the mechanical strength of nanocrystal superlattices. 4 3 1 Preparation of N anocrystal Superlattices from 5.3 nm PbSe Nanocrystals Figure 4 4A shows a TEM picture of individual nanocrystals (core diameter = 5.3 0.4 nm) used in this study as building blocks of the nanocrystal superlattices. Using these nanocrystals w e synthesiz ed the nanocrystal superlatti ces with a bcc superstructure according as described in chapter 2 148 Figu re 4 4B shows the TEM image of the nanocrystal superlattices Two orthogonal s uper lattice lattice fringes of 5.5

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104 nm and 7.5 nm were observed. They were indexed as (200) and (110), respectively, and the projection of this image was determined as [110]. These are consistent with a bcc superstructure and the lattice constant was calculated to be 10.9 nm. Figure 4 4 TEM pictures the PbSe nanocrystals A ) Building blocks of 5.3 nm PbSe nanocrystals B ) B cc nano crystal superlattices made from the nanocrysta ls from the [110] projection 4 3 2 General Description of PbSe Nanocrystal S uperlattices under H igh P ressure The nanocrystal superlattices were loaded into a DAC for pressure study First, the WAXS and SAXS measurements of the superlattices under ambient pressure were conduct ed (Figure 4 5 bottom line ). The SAXS pattern (Figure 4 5, bottom line, left) exhibits more than 8 peaks that could be indexed as the (110), (200), (211), (220), (310), (222), (321), and (420 ) Bragg diffractions The selection rul e of a bcc where the sum of the Miller indices is even is satisfied, and the lattice constant was calculated to be 10.8 nm. The s e result s show a good consistency with the TEM result s in Figure 4 4B

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105 The WAXS spectra (Figure 4 5, bottom line, right) show s a NaCl (Rock salt) type structure with a lattice constant of 6.13 , which is consistent with the crystal structure of PbSe. Figure 4 5. A series of SAXS and WAXS patterns of the PbSe nanocrystal superlattices under different pressures. (1GPa = 10 4 b ar = 1.013 10 4 atm) Next, we gradually applied a pressure using the DAC and the superstructure and crys tal structure were monitored th rough in situ SAXS and WAXS measurements. Figure 4 5 shows a series of SAXS (le ft) and WAXS (right) patterns from the PbSe nanocrystal superlattices under compression.

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106 The SAXS patterns varied in response to the applied pressure, indicating a fertile polymorphism of the superstructure in the compression process. T he original bcc superlattice transformed successively to b ody centered tetragonal ( bct ), simple cubic ( sc ), two dimensional square, two dimensional tetragonal structures and finally a lamellar structure from stacked nanoplates resulted from fusion of the PbSe nanocrystals (Figure 4 6). I dentification and detail ed structural transformations are discussed below For a step by step analysis, the SAXS spectra are divided into four stages (Figure 4 5): S tage I the pressure ranges from 0 GPa to 6.5 GPa where the superstructure adopts a bct structure; S t age II, the pressure ranges from 6.5 GPa to 9.0 GPa corresponding a bct and/or sc superstructure; Stage III the pressure ranges from 10.5 GPa to 13.5 GPa where the superstructure ta k es a two dimensional square or rectangular order with one disordered dimension, as seen in discotic liquid crystals; S tage IV the pressure is released from 14.1 GPa to ambient pressure , producing a lamellar structure. Figure 4 6 Schematic illustration of the superstructural transformation of the PbSe nanocrystal superlattices un der high pressure

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107 Figure 4 7 Plot of the d spacing value of the first peaks in the series of the SAXS patterns. The superstructural transformation was also indicated by the change in the d spacing s of the first peak s in the SAXS patterns as shown in F igure 4 7. The d spacing described with increasing pressure in S tage I. Then in Stage s I I and III the d spacing increased with higher pressure. In the pressure releasing process (Stage IV) the d spacing was not changed significantly except for the la st point where the d spacing dropped dramatically. The d spacing increase with increasing pressure as seen in Stage I I and III seems opposite to normal intuition We propose this is attributed to the superstructural transformation, which is discussed later. Compare d to the SAXS results, the WAXS showed a relatively simple transition (Figure 4 5). The PbSe nanocrystals adopted a different crystal structure at high pressure indexed it to the TlI type structure. Figure 4 8 shows models of a PbSe nanocr ystal with a low pressure phase of a NaCl type crystal structure and with a high

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108 pressure phase of a TlI type crystal structure. As seen in the f igure, the shape of an individual nanocrystal transforms in association with the crystal structural change. Figure 4 8. Models of PbSe nanocrystals. A) Low pressure phase of a NaCl type crystal structure. B) High pressure phase of a TlI type crystal structure. C F) The detailed crystal structures. C) Unit cell of the NaCl type crystal structure. D) NaCl t ype structure in a relevant TlI type unit cell. D) TlI type structure in a relevant NaCl unit cell. D) Unit cell of a TlI type structure. Figure 4 9 shows a plot of the fraction of the high pressure phase vs. the applied pressure. The high pressure rati o was quantified from the shape of the WAXS patterns which was described in the experimental section. The phase transition which started at 4.9 G Pa and was completed by 6.5 GPa, was reversible and t he nanocrystals adopted the original NaCl structure af ter the pressure was released. Figure 4 9 shows a large hysteresis loop, indicating the existence of an energetic barrier between these two crystal structures.

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109 Interestingly, the generation of the high pressure phase and the increase of the d spacing of the first peak in the SAXS patterns occurred simultaneously at 4.9 GPa. This coincidence can be explained as the transformation of the individual nanocrystals associated with the crystal structural change triggered the transformation of the superstructure of the nanocrystal superlattices. More discussion will be done in section 4.4. Figure 4 9 Plot of the fraction of the high pressure phase of the PbSe crystals vs. the applied pressure 4 3 3 Superstructural Transformation of the PbSe Superlattices Dr iven by Pressure 4 3 3 1 Stage I : From body centered cubic to bo dy centered tetragonal at pressure s between 0 GPa and 6.5 GPa The original su perstructure was identified as bcc from the results of both TEM and SAXS measurements (Figure s 4 4, 4 5, and 4 10). D etailed information of the SA XS peaks i s summarized in T able 4 1. The peaks were located at almost identical position s as calculated from the law of the Bragg diffractions where the (q/q 0 ) 2 ratio should be 1, 2, 3, 4, 5, 6 in the case of a bcc (note: t he (310) diffraction peak is very

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110 weak because of the small structural factor). The per fect consistency (within 0.01 in (q/q 0 ) 2 ratio ) indicate s that the superlattice adopted a very precise bcc superstructure. Figure 4 10 SAXS patterns of the PbSe n anocrystal superlattices in Stage I A) Under ambient pressure B ) At 2.8 GPa The blue dotted lines and the red solid lines are the experimental and simulated spectr a respectively. Table 4 1 D etailed information of the SAXS unde r ambient pressure (F igure 4 10A ) 0 GPa bcc (110) (200) (211) (220) (222) d spacing () 75.4 53.5 43.5 37.8 30.8 (q/q 0 ) 2 ratio 1.00 1.99 3.00 3.99 6.01 Table 4 2 D etailed information of the SAXS under 2.8 GPa (Figure 4 10B ) 2.8 GPa (110) (200) (211) (220) (222) d spac ing () 73.3 52.4 41.8 36.7 29.8 (q/q 0 ) 2 ratio 1.00 1.96 3.08 3.99 6.0 2 Next we applied a gradual pressure to this sample and the superstructural transformation was monitored with in situ SAXS ( see Figure 4 5 Figure 4 10 and Appendi x A for the comple te set of SAXS results ). In Stage I the shapes of the SAXS

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111 patterns were all similar with small peak shifts (Figure 4 7). However, detailed analysis of the SAXS pattern at 2.8 GPa (Figure 4 10B ) suggested that the superstructure was distorted in an anis otropic fashion. Table 4 2 shows that the (q/q 0 ) 2 ratio s of the SAXS pattern at 2.8 GPa w ere significantly off from the theoretical values, indicating an anisotropic distortion in the superstructure. We hypothesized that this distortion cam e from the dif ferent compression ratio of the a bcc and b bcc superlattice axes from the c bcc axis, leading to the transformation from the bcc superstructure to bct In order to test this idea, a peak convolution simulation were performed For a given a bct and c bct dist ance, we could calculate the d spacings from any diffraction index were calculated according to (eq. 4 1) Then, the peak positions were calculated from the d spacings. We convol uted these peaks usin g the Multi P eak Fitting 2.0 program in Igor Pro version 6.2, and tested if the experimentally obtained spectrum could be reasonably reproduced or not. Figure 4 10B shows the simulated spectrum (the red solid line) and all the diffraction peak s (the black lines) with a bct equaling 10.59 nm ( where a bct /a 0 is 99.1 % ) and c bct equaling 9.89 nm ( where c bct / c 0 is 92 .5 % ) As show n in Figure 4 10B the simulated spectrum reproduced the experimental spectra excellently. We applied this simulation for all the SA XS patterns in Stage I (the complete set of the results are presented in Appendix A), and determined the a bct and c bct values. The relusts are plotted in Figure 4 11A. As shown in Figure 4 11A in contrast to the continuous decrease of c bct value with th e increasing pressure, the a bct value increased after 3.5 GPa. These elastic behavi or is discussed in section 4 3 4.

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112 To further confirm this transformation the interparticle distances and the volume per one nanocrystal in the superlatt ices at each pre ssure were calculated The neighboring nanocrystals in both bcc and bct are packed in the (110) plain, and the interparticle distance between two neighboring particles can be expressed using the a bct and c bct values as Based on the a bct and c bct values presented in Figure 4 11, the calc ulated interparticle distance i s plotted with filled triangles in Figure 4 11B For the calculation of the volume per one nanocrystal in the superlattices, in both bcc and bct the number of the particles in a unit cell is two, thus the volume can be calculated as ( filled square s in Figure 4 11B ) As the f igure shows, b oth the interparticle distances and the volume per one nanocrystal w ere decreased in the stage I (until 6.5 GPa) which supporting the correctness of introducing the bct su perstructural model. Figure 4 11 Transformation of superlattices in Stage I. A ) The a and c values in the bct model vs. the applied pressure B ) The interparti cle distance (filled triangles ) and the volume per one nanocrystal (filled square s ) vs. the applied pressure, based on the bcc bct and sc models. The empty symbols a re the value s determined using only the bcc and bct models but not the sc model.

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113 4 3 3 2 Stage I I: f rom body centered tetragonal to simple cubic for pressure s between 6.5 GPa and 9.0 GPa The anisotr opic peak shifts and the d spac ing transition were successfully explained by introducing the bct model as shown in th e section 4 3 3 1. However, at pressures high er than 6.5 GPa, the simulation results indicated a necessity to introduce another model instead of the bct model. The fitting results using the bct model gave a large a value (Figure A 10 and Table A 10 in Appendix A), leading to the increase in the interparticle distance and the volume p er one nanocrystal (Figure 4 11B empty symbol s). This failure is primar il y attributed to the large increase in the d spacing at these pressure s (Figure 4 7) A simple cubic ( sc ) model may be a good candidate as an alternate model to cl arify up this issue In nature, i t is known that calcium metal crystal s go through a structural transformation with increasing pressure from fcc to bcc to sc 194,195 The transition sequence is opposite to normal intuition, as it is accompanied by a decrease in coordin a tion number 0.52). However, in reality, the sc can adopt a higher density because it has a shorter interatomic distance. By introducing the sc model, we could reasonably explain the d spacing inc rease seen in our work The selection rule for the Bragg diffraction from sc allows any h, k, l in contrast that the selection rule of the bcc which specifies that h + k + l = even. Thus, the first diffraction index of sc is (100) and the d spacing is equal to the interparticle distance. In contrast, the first diffraction peak of bcc is (110) of which d times the interparticle distance. Thereby, even though the first peak appears at a

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114 position of a larger d spacing, it is possible that the interparticle distance decrease s and the density increase s during the transition from bcc to sc We conducted the simulations using the bct and sc model Figure 4 12 shows the experime ntal and simulated SAXS spectra under 7.4 GPa and 9.0 GPa. The exp erimental and simulated spectra matched excellently using the sc model. The s imulation study showed that 30 % of the superlattices adopted a sc superstructure with a lattice constant of 7.8 nm at 7.4 GPa, and at 9.0 GPa their superstructures completely turned into sc ( Figure A 10&11 and Table A 10& 11 in Appendix A). With the obtained values, both the mean interparticle distances and the volume per one nanocrystal were indeed decreased with pre ssure (Figure 4 11B ), further support ing the introduction of the sc model. Figure 4 12 SAXS pattens of PbSe nanocrystal superlattices in Stage II A ) U nd er 7.4 GPa B ) U nder 9.0 GPa The blue dotted lines and the red solid lines are the exp erimental and simulated spectra respectively. The green and black peaks are peaks from the sc and bct models, respectively. Two mechanisms of the transformation from bct and sc are proposed in Figure 4 13. The t ransiti on pathway shown in Figure 4 13A is tak en from the phase transition

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115 mechanism of calcium metal from the bcc to the sc crystal structure. 196 In the mechanism, there is a decrease of the rhombohedral angle ( in Figure 4 13A ) of 109.47 in the primitive bcc struc tur e to the final angle 90 in the sc structure. With the bct model at 6.5 GPa with lattice constants a bct of 10.58 nm and c bct of 9.82 nm, the rhombohedral angle was 107. 58 which may be inte rpre ted as a middle point from bcc to sc In the proposed mechanism shown in Figure 4 13B the nanocrystals move in the same direc tion as they proceed from bcc to bct and subsequently to sc forms Thus far, we c an not rule out either of these mechanisms, but the mechanism shown in F igure 4 13A may be more likely because the movement of nanocrystals is more homogeneous, which would be more favorable in terms of the kinetic energy barrier. Figure 4 13 Two proposed mechanism s of the transformation from bct t o sc

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116 4 3 3 3 Stage I II: t wo dimensional squared and rectangular arrays through a columnar alignment under pressure s between 10.5 GPa and 13.4 GPa When the pressure reached 10.5 GPa, the SAXS patterns changed significantly in terms of the shape and the fi rst peak position, suggesting a drastic change in the superstructures. Figure 4 14. SAXS patterns of the PbSe nanocrystal superlattices in Stage III. A) Under 10.5 GPa. B) Under 12.7 GPa. The blue dotted lines and the red solid lines are the experi mental and simulated spectra, respectively. The two SAXS patterns at 10.5 GPa and 12.7 GPa are presented in Figure 4 14. In the SAXS spectra at 10.5 GPa, three distinct p eaks were observed (Figure 4 14A ). The q spacing ratio of the peak positions, q/q 0 the two dimensional square order ( Figure A 12 and Ta ble A 12 in Appendix A). SAXS pattern s with the s e c haracteristic peak positions were o b served at 11.2 GPa and 11.9 GPa as well ( Figure A 13&14 and Table A 13&14 in Appendix A). However, when the pressur e reached 12.7 GPa (Figure 4 14B ), the third peak (q 1 35 nm 1 in Figure 4 14 A ) disappeared, and the q/q 0 ratio diverged from the ratio broad ened and shifted to lower q value s ( Figure A 15&16 a nd Table A 15 1 8 in Appendix A). We explained t his distortion using a two dimensional rectangular model.

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117 Actually, the simulated spectrum us ing the rectangular model fit the experimental spectrum wel l (Figure 4 14B and Figure A 15&16 and Table A 15 1 8 in Appendix A). With the square and the rectangular model s the lattice constants of a tetragonal and b tetragonal (Figure 4 14, inset) were obtained from the SAX S patterns (Figure A 12 to A 16 and Table A 12 to A 16 in Appendix A). The transition s of the a t etragonal and b tetragonal vs. the pressure are plotted in Figure 4 15. Figure 4 15 The a tetragonal and b tetragonal values versus the applied pressure in the square and tetragonal model s in Stage I II The red and blue plots are the d spacings of a tetra gonal and b tetragonal respectively. In Stage I II, the SAXS patterns indicate that the superstructure are two dimensional structures such as square and rectangular. The change from the three dimensional sc structures to the two dimensional square structur es may occur via two mechanisms: (i) The nanocrystals are fused together along one direction and the resulting nanorods are ali gned in a square ordered array; or, (ii) in a three dimensional structure, the periodicity in one direct ion is disordered and the plana r order in the other two dimensions is reflected in the SAXS patterns. This phase symmetry appears in the arrays of discotic type liquid crystals. 197,198

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118 Figure 4 16. The PbSe nanocrystal superlattices in the pressurization process up to 11.5 GPa. A) SAXS pattern. B) TEM images of the PbSe nanocrystals of the superlattices before and after the pressurization process. In order to determine the structure s in Stage I II, we released the pressure right afte r the two dimensional order was achiev ed. After releasing the pressure, we measured the SAXS and the sample was imag ed by TEM. The results are shown in Figure 4 16. Surprisingly, the SAXS spectrum was reversible, meaning that a bcc (or sc ) superstructu re was recovered after rele asing the pressure (Figure 4 16A ). In the TEM measurement of the previously pressurized sample (Figure 4 16B ), only discrete nanocrystals with the original size are observed. From these results, the mechanism that the resulting nanorods formed two dimensional square array s can be rul ed out. Therefore, we conclude that the two dimensional orders were created through a s re quirement (discussed in section 4 3 4 ) the interparticle dis tance s in the c square axis w ere estimated to be less than 5.9 n m, 5.3 nm, and 5.2 nm under pressure s of 10.5 GPa, 11.2 GPa and 11.9 GPa, respectively. In addition, the WAXS data in Stage III show that t he mean diameter of the PbSe core was compressed to 5.1 nm. Thus, the re was very little ro om between two nanocrystals along the c square axis. Under such conditions, it would be reasonable

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119 that the periodicity in the c square axis be distorted, leading to a SAXS pattern reflecting a two dimensional order. To the best of our knowledge, s uch a liquid crystal like behavior of spherical nanocrystals has not been observed previously 4 3 3 4 Stage I V: lamellar superstructures from nanoplates resulting from nanocrystal f usion at 14.1 GPa and s ubsequent p ressure r eleas e Finally, we applied a pressure up to 14.1 GPa to the nanocrystal superlattices. The SAXS pattern exhibit s broad peaks (Figure 4 17A ), and the q/q 0 ratio for the peak positions is characteristic ratio for a lame llar structure (Figure 4 17, inset). The lamellar length, a is equal to the d spacing of the first peak position, calculated as 10.04 nm. (Figure A 17 and Table A 17 in Appendix A). It is note d that the transition of the first peak d spacing in Stage I V shown in Figure 4 7 is equal to the transition of the lamellar length. Figure 4 17. SAXS patterns of the PbSe nanocrystal superlattices in Stage IV. A) Under 14.1 GPa. B) After releasing pressure. The blue dotted lines and the red solid lines are the experimental and simulated spectra, respectively. Different from the case where the pressure was released in Stage I II (Figure 4 16), when the nanocrystal superlattices were compressed at 1 4.1 GPa, the SAXS

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120 patt ern is irreversible (Figure 4 1 7B ) Compare d to the SAXS pattern at 14.1 GPa, the SAXS pattern after releasing pressure exhibit s sharper peaks and the peak positions moved to smaller q value s while keeping the q/q 0 ratio : 3 : 4 (Figure 4 17B and Figure A 18 and Table A 18 in Appendix A). The lamellar length, a lamllar after releasing pressure was 8.00 nm (Table A 18 in Appendix A)) E lectron microscopy measurements were obtained for the previously pressurized sample s as shown in F igure 4 18 T he SEM image (Figure 4 18A ) indicates that multiple la yers are stacked together which is consistent with the lamellar structure indicated wit h the SAXS pattern. Large plana r crystals are seen in the TEM image with a low magnifica tion as well (Figure 4 18B ) The crystals are at least 100 nm in length, which is much larger than the original 5.3 nm PbSe nanocrystals. For further investigation, we obtained a high resol ution TEM images (Figure 4 18C,D ) after the samples were sonicate d to separate the stacking Atomic lattice fringes are observed in the center of the crystals and the domain size i s as large as 100 nm or more (Figure 4 18C ). From the d spacing of the lattice fringes and the fast Fourier transform (FFT) pa ttern of the image (Figure 4 18C inset), the crystal structure was identified as the low pressure phase (NaCl type) of the PbSe crystal and the projection of the image was assigned as the [110]. At the periphery of the crystal (Figure 4 18D ), dis continuous lattice fr inges with 3.50 0.1 were o bserved indexed as the latt ice fringe of the [111] are observed The dis continuity of the lattice fringes and the observed contrast inside the crystal suggest the existence of a large strain in the crystals. The elect ron d iffraction measurement gives weak circular patterns, which are assigned as the diffraction fro m the P bSe NaCl structure (Figure 4 18D inset).

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121 Figure 4 18 Electron microscope measurements of the pressurized nanocrystal superlattices. A) SEM image. B D) TEM images. The insets in ( C ) and ( D ) are the FFT pattern of the images and the electron diffraction pattern respectively. Figure 4 19. Schematic illustration of the lamellar structure formation associated with nanoplate generation as a result of nanocrystal fusion.

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122 With these results, we proposed that the nanoplates were formed through a fusion driven by pressurized stress as shown in Figure 4 19. The resulting nanoplates were piled together; hence a lamellar structure was formed. The large c rystal domain may be explained by fusion of the nanocrystals through an orient ed attachment. A similar pressure driven fusion from individual nanocrystals to single crystalline nanoplates was reported for PbS nanocrystal superlattices. 147 The PbS nanocrystals in the superlattice adopted a fcc superst ructure and changed to a lamellar superstructure after nanocrystal fusion under pressure The authors proposed that the nanocrystals were rotated as the configuration was aligned, and subsequently the nanocrystals were fused. Our result s support this mec hanism Furthermore, no linear crystals were observed in the compressed sample, although they went through the columnar alignment. This result suggests that the nanocrystal fusion occurs after completion of the configuration arrangement in two dimension s was not by one by one formation of nanoplates through the fusion of the in dividual nanocrystals Referring to Figure 4 7, the lamellar length remains the same in the de pressurization process and at the last point after the pressure i s completely release d the lamellar length drops to a much small number. This lamellar length transition can be explained as follows. The nanoplates were created through a pressure propagating in the in plain direction. Thus, the nanoplates could stand with a distance from each other in the presence of a pressure in the direction perpendicular to the lamellar order (Figure 4 20A ). In contrast, after the complete release of the pressure, the nanoplates c an move close r together due to the absence of the p ressure previously pr event ing the

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123 movement (Figure 4 20B ). As a result, the lamellar length is shorter and a more rigid lamellar structure i s realized, as indicated in the SAXS spectrum (Figure 4 17). Figure 4 20. Schematic illustrations of the lamellar structures. A) Und er some pressure. B) Under no pressure. 4 3 4 Discussion 1: Nanocrystal Superlattices as Elastic Materials Figure 4 Illustration of elastic materials. B) Plots of Poisson ratio vs pressure in Stage I. Our extensive study on nanocrystal superlattices under high pressure has witnessed a fertile polymorphism Through the transformation process a unique anisotropic movement was observed in the bct model as the superlattice c axis di rection was compressed while the a and b axes direction was expand ed with increasing

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124 pressure superlattices. This section quantitatively analy zes the elasticity using o. (eq. 4 1 ) where is ratio, trans is the transversal strain, and axial is the axial strain (see also Figure 4 21A) With the obtained a and c values in the tetragonal model (Figure 4 11A ), w vs. each pressure (Figure 4 21 B ). superlattices were compressed three dimensionally. This negative value lead s to two important conclu sion s; (i) t here was a space to be s queezed out until this pressure was reached (p resumably, it would be a van der Waals space between the nanocrystals); and (ii) t he nanocrystals inside the superlattices were connected, and the nanocrystals moved close t ogether when the pressure was applied. In contrast, after 4.8 GPa, the pos itive as much as 0.5. This is the maximum value of due to the requirement tha t Young's modulus, the shea r modulus and bulk modulus have posit ive values This elasticity is the main reason for the larger d spacing at higher pressure s in Stage I Shevchenko et al also reported a mechanistic study of nanocrystal superlattices from PbS n anocrystals under high pressure; however, their nanocrystal superlattices underwent simple downsizing and no superstructural transformation nor an increase in the d spacings was observed 199 mean ing that their nanocrystal superlattices ke pt exhibited thr oughout the whole process. In their experiment, n eon gas was use d a s a pressure medium, which create d a homogeneous

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125 pressure pr opagation, and that was the presumable reason for thes e different behaviors. In contrast, our experiment did not use a pressure medium, which would create more directional pressure, leading to the superstructural phase transition and the elastic behavior. The superstru ctural phase transition from sc to square and tetragonal in our study should be also associate d with the elastic behavior. In Stage I II, the nanocrystals are not yet fused together (Figure 4 16); thus the minimum interparticle distance possible along the c axis wa s the size of the PbSe core From the WAXS spectrum at the relevant pressure, the unit cell v olume i s c ompressed to c.a. 88 % compared to the original state, leading to the estimated core diameter of c.a. 5.1 nm. The lattice constant of the sc superlattice s is 7.8 nm at 9.0 GPa, and the length of the squa re structure i s 8.68, 8.97, and 9.03 nm at 10.5 GPa, 11.2 GPa, and 11.9 GPa, respective ly (Table A11 to A 14 in Appendix A). With these numbers, we can calculate the trans of the transversal strain through this pressure range Accordingly, because of the han 0.5, the equation calculates the minimal axial for the axial strain and the maximum interparticle distance along the c axis. The calculated possible maximum interparticle distances are 5.9 nm, 5.3 nm, and 5.2 nm under pressure s of 10.5 GPa, 11.2 GPa and 11.9 GPa respectively. These values are very close to the core size The PbSe nanocrystal cores have oleate molecules on the surface as a surfactant Thus even if the surfactant molecules were extensively compressed or moved away from the space b etween the two touching nanocrystals, it would be a reasonable assumption that the ac tual interparticle distances are very close to the calculated values. In turn, this suggests

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126 to 0.5 in the process. This shows that the nanocrystal superlattices would respond in a very elastic fashion under high pressure 4 3 5 Discussion 2: Size dependency of the Superstru ctural Transformation Nanocrystals have different physical properties including their physical stability d epending on their size,. 65 This begs the question: d o nanocrystal superlattices from different size d nanocrystals have different physical properties ? Figure 4 22. SAXS and WAXS patterns of the bcc nanocrystal superlattices from 8.2 nm PbSe nanocrystals under different pressures. In order to answer t he question, we synthesized nanocrystal superlattices with a bcc superstructure using 8.2 nm PbSe nanocrystals Subsequently, the superlattices were compressed under 15.7 GPa in the D AC and the WAXS and SAXS patterns were monitored. Figure 4 22 shows the SAXS and the WAXS patterns of the nanocrystal s uperlattices under ambient pressure (bottom), 15.7 GPa (middle), and after releasing the pressure (top). The SAXS pattern did not exhib it a si gnificant change in the spectral

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127 shape throughout the whole process, suggesting that the superstructure did not change. This result implies that the structural strength of the nanocrystal superlattices is highly dependent on the size of the buildin g block We speculate that the strength of the nanocrystal superlattices may be tuned by the superstructure, the inclusion molecules, the configuration, and even the atomic alignment. Section 4 4 present s a detailed study of the effect of superstructure on phase transition pressure. 4 3 6 Summary of the S ection 4 3 In section 4 3, we presented a study of the pressure driven superstructural transformation of the nanocrystal superlattices made from 5.3 nm PbSe nanocrystals. Upon pressure, the superstructur e changed from bcc to bct to sc to two dimensional square to two dimensional rectangular and finally to a lamellar structure as a result of nanocrystal fusion Interestingly, the two dimensional square and rectangular superstructures were form ed in a dis cotic liquid crystal like fashion. At the highest pressure, nanocrystals were fused to form large nanoplates, with a lamellar alignment. Our control experiment using 8.2 nm PbSe nanocrystals suggest s that the physical stability of the super structures is dependent on the size of the building blocks. This study will be an important standard for characterizing the mechanic al strength of nanocrystal superlattices. 4 4 Results and Discussion Strengthen ing Nanocrystal Superlattices by the Superstructure with Respect to the Phase Transition Pressure This section present s a mechanistic study of the effect of nanocrystal superstructure on the phase transition pressure of the building block nanocrystal s specifically for 8.2 nm PbSe nanocrystal superlattice s We found that the PbSe nanocrystals in the superlattices with a body centered cubic superstructure exhibited a

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128 higher phase transition pressure than the ones in the face centered cubic superlattices by as much as 5 GPa. Our mechanistic study demonstrated th at the superstructures could dictate the direction of the pressure propagation along the superlattice network in t he absence of a pressure medium, making the phase transition pressure of the building block nanocrystals higher or lower. The preparation of t he PbSe nanocry stal superlattices is described in section 4 4 1. The pressure driven structura l transformations were monitored by synchrotron X ray scattering a nd the results are shown in section s 4 4 2 to 4 4 4 Based on the results, we discuss the mec hanism for tuning the phase transition pressure according to that the superstructure in the nanocrystal superlattices in section 4 4 5. 4 4 1 Preparation of Nanocrystal Superlattices with B cc and F cc S uperstruct ures from 8.2 nm PbSe Nanocrystals Figure 4 23. TEM picture of 8.2 nm PbSe nanocrystals. PbSe nanocrystals with a diameter of 8.2 nm were synthesized as building blocks of the superlattices according to the reported method with a minor modification 12

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129 The TEM image in Figure 4 23 shows that the sy nthesized nanocrystals have a diameter of 8.2 0.6 nm and a nea r ly spherical shape. Figure 4 24. Det ailed observations of a 8.2 nm PbSe nanocrystal. A) From [100] projection. B) From [110] projection. C) From [111] projection. i) Illustrations of a model. (ii iv) High resolution TEM images of synthesized PbSe nanocrystals. (v) The FFT patterns of t he atomic fringes inside the nanocrystals. High resolution TEM images were acquired for further characterization s (Figure 4 24). It is prop osed that the PbSe nanocrystals adopt a quasi spherical sha pe with 26 faces consisting of six [100] faces, twelve [110] faces, and eight of [111] face s (Figure 4 24A ). Based on this model, because of the non spherical shape, the projected

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130 outlines should be dependent on the projection direction. We conducted high resolution TEM measurement s for the synthesized nanoc rystals from [100] (Figure 4 24A), [110] (Figure 4 24B), and [111] (Figure 4 24B) projections were compared with the outline s of the model. The projections were determined with the lattice fringes of PbSe and the rel evant FFT patterns (Figure 4 24E ). Ind eed, the out lines of the model (Figure 4 24 A ) and the TEM images (Figure 4 24 B to D ) were well matched, thereby support ing the correctness of the proposed model. The nanocrystal sizes in three directions were determined as d 100 = 7.7 0.5 nm, d 1 1 0 = 8. 2 0.5, and d 1 11 = 8.3 0.6. Figure 4 25. Characterization of two nanocrystal superlattices. A D) The fcc superlattices. E H) The b cc superlattices. A,E) SAXS patterns. B D, F H) TEM images. The TEM images and electron diffraction patterns (inset s) were taken from three projections: [100] (B, F), [110] (C, G), and [111] (D, H). With these nanocrystals, two kinds of nanocrystal superlattices were fabricated according to the method reported in c hapter 2 : the superlattices with a bcc superstructure u sing 5CB as guest molecules and with a fcc superstructure. The structures of the synthesized nanocrystal superlattices were confirmed by TEM and

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131 SAXS (Figure 4 25). The TEM and SAXS results consistently verified the successful creation of both the fcc an d bcc superstructure s T he lattice constants of the fcc and bcc superlattices were 16.7 nm and 13.5 nm where the interparticle distances were 11.82 nm and 11.52 nm respectively (See also Table 4 4) We further confirm ed the atomic alignment s by the ele ctron diffraction measurement from the [100], [110], and [111] projections (Figure 4 25 B D and 4 25 F H, inset s ). The electron diffracti on patterns showed that both the fcc and bcc nanocrystal superlattices possessed atomic alignment within these superlatt ices and the [100], [110], and [111] zone axes of the PbSe NCs in these superlattices are oriented coaxially with the corresponding superlattice zone axis (s ee also Figure 4 35) 4 4 2 Pressure Driven Phase Transition in the B cc and F cc Nanocrystal Superl attices Figure 4 26. A series of SAXS and WAXS patterns in the pressurization process. A) The bcc superlattices. B) The fcc superlattices. The peaks pointed to by the arrows correspond to the (131) diffraction peak from the PbSe high pressure phase ( TlI type), used as an indicator of the start of the phase transition. The peak marked with the asterisk is from the iron of the gasket.

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132 The bcc and fcc nanocrystal superlattices were loaded into the DAC, and the in situ WAXS and SAXS were monitored at a series of pressures as shown in Figure 4 26. The phase transition of the PbSe crystal structure can be seen in the WAXS regions, where the q value range is from 15 nm 1 to 40 nm 1 As a general tendency, under ambient pressure, the PbSe nanocrystals in t he superlattices adopt a NaCl type atomic structure (the low pressure phase), and under a high pressure, it turns to a TlI type atomic structure (the high pressure phase), as illustrated in Figure 4 8. The start of the phase transition can be monitored b y the generation of the (131) diffraction peak of the high pressure phase at around 27 nm 1 marked by arrows in Figure 4 26. This peak is a good indicator of the start of the phase transition, because no peaks from the low pressure phase overlap the (131) diffraction peak. The phase transition of the PbSe nanocrystals starts at 10.2 GPa in the bcc superlattices and at 5.3 GPa in the fcc superlattices. The fraction of the high pressure phase was determined by the comparison between the shapes of the WAXS spectrum and the simulated patterns at a given ratio. (The details are described in the experimental section.) The percentage of the high pressure phase is plotted vs. the applied pressure in Figure 4 27, top. The plot shows that the phase transition in the bcc superlattices starts at 10.2 GPa and ends at 13.1 GPa, and the phase transition in the fcc superlattices starts at 5.3 GPa and ends at 7.5 GPa. The difference in phase transition pressure in the two superlattices (varies as much as c.a. 5 GPa) is significant compared to a reported study (1.5 GPa using PbS nanocrystal superlattices), 200 indicating that the superlattices studied here are good models for examining how a superstructure of nanocrystals tunes the mechanical properties of the nanocrystals.

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1 33 In order to se e a superstructural change, the peak position of the first peak of the SAXS pattern is a convenient ind icator, as demonstrated in section 4 3. The plots of the d s pacings of the first SAXS peaks v s the applied pressure are shown in the bottom graph s of F igure 4 27 The d spacings continuously decreased until the y reached the pressure at whi ch the phase transition started. At this point and beyond they the d spacing showed an upward trend in both the fcc and bcc superlattices (Figure 4 27 ). At the phas e transition, t he PbSe nanocrystals change their shape in association with the crystal structure change. In the compressed nanocrystal superlattices, a tiny shape change would break the balance, and hence trigger the superstructural change. Figure 4 2 7. Plots of (upper) the high pressure ratio and (lower) the d spacing of the first SAXS peak. A) The bcc superlattices. B) The fcc superlattices. The pressure range corresponding to the phase transition is highlighted in blue. This increase in the d sp acings correlates with the superstructural transformations. In the case of the bcc superlattices a sc model was introduced to explain the d spacing increase, as s hown in section 4 3 T o further verify the

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134 introduction of the sc model, t wo SAXS patterns were chosen for peak fitting (Figure 4 28) corresponding to pressure of 13.1 and 15.1 GPa T he SAXS patterns were simulated using bcc and sc models and compared with the experimental ly obtained pattern s. In Figure 4 28 the SAXS patterns at 13.1 GPa and 15.1 GPa and the simulated patterns are presented. Indeed, both were well reproduced with the simulation when using the bcc and s c models with the same interparticle distance of 10.8 nm. The sc fractions in the superstructures were estimated from the ar eas of the deconvoluted peaks and they were calculated as 43% and 65 % at 13.1 GPa and 15.1 GPa, respectively. In the case of the fcc superlattices, we could not conduct the simulation because the SAXS patterns were too broad under high pressure and it wa s difficult to fit the diffraction peaks. However, the significant increase in the d spacing after the phas e transition pressure indi cate s that the fcc superlattices transformed into a sc superstru cture or hexagonal closed packest ( hcp ) superstructure Figure 4 28 T he experimental (red dotted line s, in the middle) and simulated (blue solid line, in the middle) SAXS patterns A) SAXS spectrum a t 13.1 GPa B ) SAXS spectrum at 15.1 GPa

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135 Figure 4 29 Plots of the ratio of the atomic unit cell volume t o the initial volume vs. the applied pressure A) The bcc superlattices. B) The fcc superlattices. The pressure range corresponding to the phase transition is highlighted in blue Moreover, i n order to study the phase transition in details, we calculate d the unit cell volumes V, of the PbSe crystal structure from the peak positions in the WAXS patterns, and plotted V/V 0 (V 0 : initial cell volume) vs. the applied p ressure ( Figure 4 29 ) In both the bcc and fcc superlattices the unit cell volume decreas ed continuously with increasing pressure. However, c omparing the unit cell volumes at the pressure corresponding the start of the phase transition the nanocrystals in the bcc superlattices were more compressed than those in the fcc (the unit cell ratio, V/V 0 of 86.6 % at 10.9 GPa in the bcc (Figure 4 29 A ) vs. the V/V 0 of 92.1 % at 5.3 GPa in the fcc (Figure 4 29 B ) ) In other words, the crystal lattices of the nanocrystals in the bcc superlattices require to be compressed more in order to facilitate the phase transition pressure than those in the fcc superlattices. This result gives un ambiguous proof that the PbS e nanocrystals eas il y underwent phase transition in the fcc superlattices relative to the bcc superlattices.

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136 4 4 3 Pressure Medium Effects on th e Phase Transition Pressures of Nanocrystal Superlattices In order to determine why the PbSe nanocrystals easily under go a phase transition in the fcc superlattices more easily than those in the bcc superlattices, we performed control experiments using a m ethanol/ethanol mixed solution as a liquid pressure medium. Figure 4 30 shows a series of WAXS and SAXS spectra under applied pressure for the bcc (a) and fcc (b) nanocrystal superlattices in the presence of the pressure medium. Different from the case i n the absence of the pressure medium, the phase transition started at 8.3 GPa in both the bcc and fcc superlattices and ended around 10 GPa. The fraction of the high pressure phase and the d spacing of the first SAXS peak s were determined from these measu rements, and are plotted it as in Figure 4 31 The phase t ransition is observed as a steep increase in the fraction of the high pressure phase (highlighted in blue in Figure 4 31). Figure 4 30 A series of SAXS and WAXS patterns in the pressuriz ation p rocess in the presence of pressure medium. A) The bcc superlattices. B) The fcc superlattices. The peaks pointed to by the arrow s is the (131) diffraction peak from the PbSe high pressure phase (TlI type), used as an indicator of the start of the phase transition.

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137 Figure 4 31 Plots of (upper) the percentage of the high pressure phase and (lower) the d spacing of the first SAXS peak in the presence of the pressure medium. A) The bcc superlattices. B) The fcc superlattices. The pressure correspond ing to the phase transition is highlighted in blue Figure 4 32 Plots of the atomic unit cell volume over the initial volume vs. the applied pressure in the presence of the pressure medium. A) The bcc superlattices. B) The fcc superlattices. The pre ssure corresponding to the phase transition is highlighted in blue.

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138 Figure 4 32 shows the plots of the atomic unit cell volume over the initial volume vs. the applied pressure in the bcc (A ) and the fcc (B ) superlattices in the presence of the pressure med ium. The V/V 0 values at the pressure where the pha se transition started were 88.5% and 88.8 % for the bcc and fcc superlattices, respectively, suggesting that the crystal lattices both in bcc and fcc superlattices indeed felt the same pressure at the press ure of the start of the phase transition. T hese results demonstrate that the pressure medium mediates the relative difficulty of the phase transition of the PbSe nanocrystals in the bcc and in the fcc superlattices and cause the phase transition to occur in the same manner regardless of the superstructure being bcc or fcc 4 4 4 Backstroke Phase Transition in the Depressuriz ation Process Previous sections have focused on nanocry s tal superlattices in the compression process. T his section examines the st ructural changes in the de pressurization process Figure 4 33 shows a series of SAXS and WAXS patterns obtained while releasing the pressure in a stepwise manner Generally, in the WAXS patterns, the high pressure phase (TlI type) disappears and in turn the low pressure phase (NaCl type) re appears as the pressure is released Almost all the SAXS patterns a re broad ly shaped suggesting th at the superstructures were damaged by the applied high pressure. As an exception, the bcc superlattices without pre ssure medium (Figure 4 33 A ) maintained the characteristic bcc (or sc ) SAXS spectrum for the entire process. Figure 4 34 shows plots of the high pressure phase fraction (top) and the d spacing of the first SAXS peak position (bottom) vs. the applied pressur e For all four sets of samples, the d spacings increased upon releasing the pressure and at pressures near the downstroke phase transition pressure the d spacing dropped

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139 Figure 4 33. A series of SAXS and WAXS patterns in the stepwise depressurizat ion process (bottom to top). A) The bcc superlattices in the absence of the pressure medium. B) The fcc superlattices superlattices in the absence of the pressure medium. C) The bcc superlattices in the presence of the pressure medium. D) The fcc superl attices superlattices in the presence of the pressure medium. Range of phase transition pressure observed during the ascending pressure changes for these PbSe superlattices was sizeable (6.1 GPa to 11.5 GPa without pressure medium), but the phase transit ions observed during the descending pressure

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140 ramp occurred at almost the same (Table 4 3) This would be beca u s e the superstructu res were corrupted after applying high pressure and hence exhibited no superstructural effect when the pressure was released Figure 4 34 Plots of (upper) the percentage of the high pressure phase and (lower) the d spacing of the first SAXS peak in the de pressurization process A) The bcc superlattices in the absence of the pressure medium. B) The fcc superlattices super lattices in the absence of the pressure medium. C) The bcc superlattices in the presence of the pressure medium. D) The fcc superlattices superlattices in the presence of the pressure medium. The pressure corresponding to the phase transition is highli ghted in blue

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141 4 4 5 Discussions: Strengthening by the Superstructures Figure 4 35. Schematic illustrations of the nanocrystal superlattices. A) The bcc superlattices. B) The fcc nanocrystal superlattices. The lower illustrations (ii) show two touc hing PbSe nanocrystals in the relevant superlattices It was demonstrated that the PbSe nanocrystals eas ily underwent the phase transition in the fcc superlattices relative to the bcc superlattices in the absence of a pressure medium, and with the pressure medium the superstructural effect disappeared. The values of the pha se transition pressure s are summa rized in Table 4 3. In the absence of the pressure medium, the bcc superlattices exhibited the largest gap between the ascending and descending phase tra nsition pressure, known as a hysteresis loop, by as much as 9.5 GPa. In contrast, the nanocrys tals in the fcc

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142 superlattices displayed a small hysteresis loop with the width of 5.6 GPa. The hysteresis loop of t he phase transition pressure originates from the energy barrier between the low and high pressure state s The energetic barrier determines the kinetics of the phase transition and hence the phase transition pressure 66,67,143 Thus the nanocry s tals in the bcc and fcc superlattices must have a different kinetic energy barrier. The difference of the kinetic energy barrier in the bcc and fcc superlattices was neutralized by the presence of the pressure medium We hypothesized that the pressure propagation ma y be the key to tuning the phase transition pressure. It is known that i n the pressure medium, the pressure propagates through the pressure medium, so substance s are compressed homogeneously In the absence of a pressure medium, in contrast, the pressure is applied directly to the substance In the case of the nanocrystal superlattices, the connection between adjacent nanocrystals should allow for pressure to transfer through the network To test this concept, the interparticle distances and the gap bet ween two neighboring nanocrystal s were examined in detail Th e nanocrystal superlat tices using PbSe models with 26 faces are shown in Figure 4 24 Since the fcc and bcc superlattices both possess atomic alignment (Figure 4 25), we can strictly determine t he configurations of the nanocrystals and hence the pressure direction applied to the nanocrystals In the case of the bcc superlattices, the neighboring nanocrystals are placed in the [111] direction of the superstructural axes and the touching facets a re the (111) face (Figure 4 35 A ). In contrast, in the fcc superlattices, the neighboring nanocrystals are placed in their [110] direction in the superlattice axes and the neighboring nanocrystals are facing each other through their

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143 (110) facets (Figure 4 35 B ). T he nanocryst al size s in three directions were d 100 = 7.7 0.5 nm, d 1 1 0 = 8.2 0.5, and d 1 11 = 8 .3 0.6 With the values, the interparticle distances and the surface to surface distances were obtained from the SAXS result s ( Table 4 4. ) Table 4 3 Summary of the pressure driven phase transitions of the examined PbSe nanocrystal superlattices Ascending PTP 1 (50%) 3 Descending PTP (50%) 3 H yster e sis width H yster e sis center position bcc without p.m. 2 11.5 Gpa 2.0 Gpa 9.5 Gpa 7.8 Gpa fcc with out p.m. 6.1 Gpa 0.5 Gpa 5.6 Gpa 3.3 Gpa bcc with p.m. 9.0 Gpa 1.0 Gpa 8.0 Gpa 5.0 GPa fcc with p.m. 9.3 Gpa 1.9 Gpa 7.4 Gpa 5.5 Gpa (*1) PTP: Phase transition pressure. (*2) p.m.: the pressure medium, (*3) The phase transition pressure (50%) were deter mined as the pressure where the crystal structures of the high pressure and low pressure phases evenly exist. Table 4 4 S ummary of the superstructural changes under pressure (*1) ipd distance: the interparticle distance (dis tance between two centers of neighboring nanocrystals) ; (*2) s to s distance: the surface to suface distance (distance between two fac ing faces of the neighboring nanocrystals) ; (*3) ptp: phase transition pressure; and (*4) p.m.: the pressure medium First peak in the SAXS @ ambient ipd distance* 1 @ ambient s to s distance* 2 @ ambient ipd distance @ ptp* 3 s to s distance @ ptp bcc without p.m 4 d SL (110) 9.57 nm 11.52 nm 3.2 nm 10.80 nm 2.5 nm fcc without p.m. d SL (111) 9.62 nm 11.82 nm 3.6 nm 11.27 nm 3.0 nm bcc with p.m. d SL (110) 9.50 nm 11.63 nm 3.3 nm 11.02 nm 2. 7 nm fcc with p.m. d SL (111) 9.64 nm 11.80 nm 3.6 nm 11.41 nm 3.2 nm

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144 First, under ambient condition s it was found that the surface to surface distance in the bcc superlattices is shorter than the one in the fcc superlattices by 0.3 0.4 nm (Table 4 4, columns 3 and 4) This is attributed to the surfactant density on the facing plane, leading to the different degree of interdigitation. Presumably, the (111) plane had less surfactant population compared to the (110) plane, leading to deeper interdigitation, resulting in the shorter surface to surface distances in the bcc superlattice Second, comparing of the surface to surface distances at the phase transition pressure in the presence and absence of the pressu re medium, the distances were shorter in the absence of the press ure medium by c.a. 0.2 nm in both the case of bcc and fcc superlattices (Table 4 4, columns 5 and 6) W ithout the pressure mediu m, the propagating pressure was more efficiently used to compa ct the interparticle distance than with the pressure medium. This result is consistent with the concept that the pressure pr opagates through out the nanocrystal superlattice networks in the absence of the pressure medium. To investigate the superstructur e effect on the phase transition pressure the structural information at the phase transition pressure is summarized in Table 4 5. C omparison of the superlattice volumes and the V SL /V 0 SL between with and without the pressure medium shows that in the prese nce of the pressure medium, the superstructure s were not squeezed as much (by ~ 5 6 % ) for either the bcc or fcc superlattices. Considering the fact that the compressibility of the P bSe nanocrystals does not change regardless of the superstructure or the pr esence of the pressure medium (Figure 4 29, 32 and Table 4 5), the s e results suggest that the manner in which

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145 the pressure propagates is the important factor in determining the phase transition pressure of the nanocrystals in superlattices Table 4 5. Su mmary of the structural information at the start of phase transition. Phase Transition Pressure Atomic unit cell volume Atomic unit cell volume ratio (V/V 0 ) SL* 1 volume per one NC* 2 S L unit cell volume ratio (V SL /V SL 0 ) bcc without p.m.* 3 10.2 GPa 199 3 86.6 % 990 nm 3 80 % fcc without p.m. 5.3 GPa 212 3 92.1 % 1012 nm 3 86 % bcc with p.m. 8.3 GPa 204 3 88.5 % 1030 nm 3 85 % fcc with p.m. 8.3 GPa 206 3 88.8 % 1050 nm 3 90 % (*1) SL: Superlattice; (*2) NC: Nanocrystal; and (*3) p.m.: the pressure medi um T he mechanism of effect of the superstructure change on the phase transition pressure can now proposed In the absence of the pressure medium, the pressure propagates throu gh the nanocrystal superlattice network and passes through the touching faces. Thus, in the bcc superlattices, each nanocrystal experiences the pressure applied from the adjacent nanocrystal in the [111 ] direction of the superlattice axes. In contrast in the fcc superlattices, the pressure arises from the [110] direction where the adjacent nanocrystals are placed. Because both the bcc and fcc superlattices possess the atomic alignment within these superlattices and the atomic axes of the PbSe are oriented coaxially with the corresponding superlattice zone axis the pressure along the [111] direction of the bcc superlattices presses the (111) faces of the nanocrystals from the [111] d irection of the atomic crystals. Likewise, in the fcc superlattices, the [110] pressure direction pressurize s the (110) faces from the [110] direction of the PbSe atomic crystals (Figure 4 35) There must be a preferential

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146 pressure direction causing the phase transition, because the main energetic barrier between two crystal phases is the movement and rearrangement of the constituent atoms in the cryst als. Presumably, the PbSe crystals have less tolerance against the phase transition from the [111] directional pressure than from the [110]. Thus, the nanocrystal s exhibited higher phase transition pressure s in the bcc superlattices compared to in the fc c superlattices. In the presence of the pressure medium, the pressure propagates in a different way. The pressure medium molecules of ethanol and methanol were everywhere in the pressure chamber even inside the nanocrystal superlattices, which create d a homogeneous pressure propagation. Under this condition, the pressure appl ied to the individual nanocrystals is homogeneous Therefore, the nanocrystals experience the pressure in the same way regardless of being in fcc or bcc superlattices. As a resul t, the phase transition occurr s at the same pressure in the fcc and bcc superlattices. The theoretical study of the detailed mechanisms is still under investigation. However, the experimental results presented so far gi ve un ambiguous proof that in the abse nce of pressure medium, the superstructur es of the superlattices dictate the pressure pathway, resulting in the change in the phase transition pressure of the constituent nanocrystals. 4 4 6 Summary of the S ection 4 4 Section 4.4 presented a study of the e ffect of superstructure of nanocrystal superlattices on the phase transition pressure of the building block nanocrystals T he PbSe nanocrystals in the superlattices with a body centered cubic superstructure exhibited a higher phase transition pressure (by as much as 5 GPa) compared to those in the face centered cubic superlattices. Our mechanistic study demonstrated that the

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147 superstructures could dictate the direction of the pressure propagation along the superlattice network in the absence of a pressure medium. In the body centered cubic superlattices, the nanocrystals experienced pressure applied from the [111] direction, creating a larger energetic barrier of the phase transition, and hence the higher phase transition pressure w as observed. The kn owle dge obtained in this study c ould be beneficial for an optimal material design using nanocrystals with a high resistance to phase change 4 5 Conclusions This chapter described a study on nanocrystal superlattices under high pressure. The results demonstra ted that the mechanic al properties of nanocrystals are determined by the superstructures, and in turn, the mechanic al properties of nanocrystal superlattices are also decided by the building block nanocrystals. These insights can help in the future de sign of materials from nanocrystals.

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148 CHAPTER 5 CONCLUSIONS AND PERSPECTIVES This dissertation presented resear ch on nanocrystal superlattices, demonstrating several methodologies to control the structure of nanocrystal superlattices and prov ing that the prope rties of the nanocrystals can be controlled by designing the ir superstrucrures The m ethodology presented in C hapter 2 enables control of the structures of nanocryst al superlattices through a mole cular inclusion process. Using this method, one can control lably create nanocrystal superlattices which adopt non closed packed structures such as bcc to compare with fcc n anocrystal superlattices with clos est packed structures whose synthetic methods are already known This may also provide a new way to synthes ize novel or ganic/inorganic composite materials. Wi th the insight gai ned in C hapter 3, binary assemblies from crystals with very different shapes such as CdSe/CdS semiconductor nanorods and spherical metal nanoparticles were created. The ability to design the superstructures of nanocrystal assemblies expand s the potential of nanocrystals as materials because the superstructures may dictate the interparticle interaction s between the nanocrystals inside a system. In Chapter 4, we proved that the superstruct ure indeed determin e s the properties in particular the phase transition pressure, of the nanocrystals in the superlattice The mechanistic study described in Chapter 4 showed how the superstructures of the nan ocrystal superlattices respond to the applied pressure and how the superstructures affect the phase stability of the nanocrystals against the

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149 applied pressure. The knowledge gai ned here would be important in the optimal design of materials base d on nanocrystals. As a conclusion, the research prese nted in this dissertation demonstrates the importance of the design of the superstructures in nanocrystal materials and expands the availability of techniques to synthesize nanocrystal superlattices. Nanocrystals have attract ed much attention as promising materials, and therefore, the importance of this research should be emphasized in this sense We end this dissertation with my perspective for research on nanocrystals and self assemblies of nanocrystals. It has been a while since nanocr ystals became one of the most exciting research topics in chemistry, and at this time research on nanocrystals and nanocrystal assemblies is s till highly active. One reason is the importance of the size regime in other fi e l ds outside of chemistry. The 1 10 nm size regime appears as the smallest unit in areas such as biological mechanisms and electronic materials and as the largest unit for many accumulation s of atoms Since the essence of chemistry lies in the ability to connect one field to another in my opinion nanochemistry potentially continues to be d s in other field s are all clarified. So, i n order for nanochemist r y to be beneficial to other fields, two important points are raised First, the openness to the novelty should be needed Nan ochemistry often deals with subject s originating from other fields, so the curiosity and bravery to seek knowledge in an unknown field may be a key to pioneer ing research in an other area Second, the impor tance of fundamental study should be pointed out. R esearch on nanocry stals and nanosized substances c ould be p otentially influential and relevant

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150 to other areas of science; however, the scientific logic must be solid in order to influence and sustain im portance in another fi eld. Thug a sincere attitude for learning and understanding the fundamental must be maintained

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151 APPENDIX T HE COMPLE TE SAXS DATA FROM SECTION 4 3 A 1 : 0 GPa Figure A 1 SAXS patterns of the nanocrystal superlattices before the pressurization process. A ) Two dimensional SAXS pattern collected in the Mar 3450 detector B ) T he converted one dimensional spectrum In F igure B the black peaks, the red solid line, and the blue dotted line are the deco nvolut ed peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

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152 Table A 1 Detailed information of the experimentally obtained and simulated spectra in Figure A 1 B Peak No. P osition (q/q 0 ) 2 ratio Assigned peak 1 0.5518 1. 00 bcc (110 ) 2 0.7796 2.00 bcc (2 00 ) 3 0.9585 3.01 bcc (2 11 ) 4 1.1049 4.01 bcc (22 0 ) 5 1.2332 4.99 bcc ( 310 ) 6 1.3549 6.02 bcc (222) 7 1.4615 7.01 bcc ( 321 ) The assigned structure is body centered cubic with a lattice constant ( a ) = 10.69 nm

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153 A 2 : 0.3 GPa Figure A 2 SAXS patterns of the nanocrystal superlattices under 0.3 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

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154 Table A 2 Detailed information of the experimentally obtained and simulated sp ectra in Figure A 2 B Peak No. position decomposition 2 theta peak width peak area (a.u.) 1 0.559 bct (110) 0.55 4 0.073 3.3 bct (101),(011) 0.5 63 0.0 68 7. 1 2 0.78 4 bct (200),(020) 0.78 2 0.06 2 2. 4 bct (002) 0. 808 0.060 1.0 3 0.97 5 bct (211),(121) 0.96 8 0.056 1.4 bct (112) 0.9 83 0.0 60 0.8 4 1.11 6 bct (220) 1.10 9 0.0 63 0. 9 bct (202),(022) 1.1 27 0.078 1. 8 6 1.370 bct (222) 1.363 The assigned structure is body centered tetragonal with a lattice constant ( a ) = 10.65 nm and ( c ) = 10.32 nm

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155 A 3 0.5 GPa Figure A 3 SAXS patterns of the nanocrystal superlattices under 0.5 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively

PAGE 156

156 Table A 3 Detailed information of the experimentally obtained and simulated spectra in Figure A 3 B Peak No. position decom position 2 theta peak width peak area (a.u.) 1 0. 560 bct (110) 0.55 4 0.0 66 3. 0 bct (101),(011) 0.5 6 7 0.0 66 6.8 2 0.78 2 bct (200),(020) 0.78 3 0.0 91 3.3 bct (002) 0. 8 17 0.0 84 1.0 3 0.9 80 bct (211),(121) 0.9 70 0. 100 2.7 bct (112) 0.9 91 0.0 65 1.0 4 1.1 18 bct (220) 1.1 10 0.0 58 1.1 bct (202),(022) 1.1 34 0.078 2.2 6 1.37 5 bct (222) 1.36 5 The assigned structure is body centered cubic with a lattice constant ( a ) = 10.64 nm and ( c ) = 10.20 nm

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157 A 4 0.9 GPa Figure A 4 SAXS patterns of the nanoc rystal superlattices under 0.9 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

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158 Table A 4 Detailed information of the experimentally obtained and simulated spectra in Figure A 4B Peak No. position decomposition 2 theta peak width peak area (a.u.) 1 0.5 6 3 bct (110) 0.55 5 0.07 5 3. 4 bct (101),(011) 0.5 6 8 0.0 75 8.0 2 0.78 2 bct (200),(020) 0.78 4 0.0 95 2. 9 bct (002) 0. 8 21 0.0 81 0.9 3 0.9 83 bct (211),(121) 0.9 71 0.0 83 1.6 bct (112) 0.9 946 0.0 87 1.3 4 1.1 18 bct (220) 1.1 10 0.0 98 1.6 bct (202),(022 ) 1.1 375 0. 114 2.4 6 1.37 9 bct (222) 1.3 7 3 The assigned structure is body centered tetragonal with a lattice constant ( a ) = 10.64 nm and ( c ) = 10.15 nm

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159 A 5 1.8 GPa Figure A 5 SAXS patterns of the nanocrystal superlattices under 1.8 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the experimentally obtained spectrum, and the s imulated spectrum, respectively.

PAGE 160

160 Table A 5 Detailed information of the experimentally obtained and simulated spectra in Figure A 5 B Peak No. position decomposition 2 theta peak width peak area (a.u.) 1 0.567 bct (110) 0.55 6 0.07 9 3. 4 bct (101),(01 1) 0.5 77 0.07 6 7. 0 2 0.7 89 bct (200),(020) 0.78 6 0.0 91 3.4 bct (002) 0. 8 43 0.0 92 1.0 3 0.9 92 bct (211),(121) 0.9 7 8 0.0 93 2.4 bct (112) 1.014 0.0 89 1. 3 4 1.1 31 bct (220) 1.1 14 0.08 3 1.5 bct (202),(022) 1.1 55 0. 133 2.7 6 1.3 9 0 bct (222) 1.3 82 The assigned structure is body centered tetragonal with a lattice constant ( a ) = 10.60 nm and ( c ) = 9.90 nm

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161 A 6 2.8 GPa Figure A 6 SAXS patterns of the nanocrystal superlattices under 2.8 GPa. A) Two dimensional SAXS pattern collected in the Mar 3 450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

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162 Table A 6 D etailed information of the experimentally obtained and simulated spectra in Figure A 6 B Peak No. position decomposition 2 theta peak width peak area (a.u.) 1 0.5 68 bct (110) 0.55 2 0.07 6 3. 8 bct (101),(011) 0.5 7 0 0.07 5 7. 4 2 0.7 89 bct (200),(020) 0.7 8 0 0.0 84 2.6 bct (002) 0. 8 33 0.0 85 1. 2 3 0.9 9 4 bct (211),(121) 0.9 7 9 0.0 83 2.1 bct (112) 1.013 0.0 78 1. 2 4 1.1 3 3 bct (220) 1.1 15 0.0 90 1.6 bct (202),(022) 1.1 4 8 0. 100 2.3 6 1.3 97 bct (222) 1.3 87 The assigned structure is body centered tetra gonal with a lattice constant ( a ) = 10.59 nm and ( c ) = 9.89 nm

PAGE 163

163 A 7 3.5 GPa Figure A 7 SAXS patterns of the nanocrystal superlattices under 3.5 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

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164 Table A 7 Detailed information of the experimentally obtai ned and simulated spectra in Figure A 7B Peak No. position decomposition 2 theta peak width peak area (a.u.) 1 0.5 70 bct (110) 0.55 9 0.07 5 2.9 bct (101),(011) 0.5 79 0.07 1 6.4 2 0. 794 bct (200),(020) 0.78 9 0.0 86 3.5 bct (002) 0. 8 46 0.0 95 1. 5 3 0. 9 9 7 bct (211),(121) 0.9 82 0.0 98 3.0 bct (112) 1.017 0.0 88 1.2 4 1.1 3 6 bct (220) 1.1 1 8 0.08 9 1.5 bct (202),(022) 1.1 59 0. 101 2.1 6 1. 401 bct (222) 1.3 88 The assigned structure is body centered tetragonal with a lattice constant ( a ) = 10.56 nm an d ( c ) = 9.86 nm

PAGE 165

165 A 8 4.9 GPa Figure A 8 SAXS patterns of the nanocrystal superlattices under 4.9 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the r ed solid line, and the blue dotted line are the deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

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166 Table A 8 Detailed information of the experimentally obtained and simulated spectra in Figure A 8B Pea k No. position decomposition 2 theta peak width peak area (a.u.) 1 0.5 6 9 bct (110) 0.5 58 0.0 80 3. 6 bct (101),(011) 0.5 7 9 0.0 8 0 7. 2 2 0.7 9 1 bct (200),(020) 0.78 8 0.0 87 3.5 bct (002) 0. 8 47 0.0 87 1. 1 3 0.996 bct (211),(121) 0.9 82 0.0 85 1.6 bct (11 2) 1.018 0.0 8 5 1.0 4 1.1 3 5 bct (220) 1.1 71 0.0 67 1. 0 bct (202),(022) 1.1 60 0.0 86 1. 5 6 1.3 97 bct (222) 1.3 88 The assigned structure is body centered tetragonal with a lattice constant ( a ) = 10.57 nm and ( c ) = 9.83 nm

PAGE 167

167 A 9 6.5 GPa Figure A 9 S AXS patterns of the nanocrystal superlattices under 6.5 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are th e deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

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168 Table A 9 Detailed information of the experimentally obtained and simulated spectra in Figure A 9B Peak No. position decomposition 2 theta peak width peak area (a.u.) 1 0.565 bct (110) 0.5 58 0.063 3.8 bct (101),(011) 0.5 79 0.07 9 6.5 2 0.78 5 bct (200),(020) 0.7 88 0.0 88 3.7 bct (002) 0. 8 48 0.0 89 1. 1 3 0.9 90 bct (211),(121) 0.9 81 0.0 7 0 1.7 bct (112) 1.019 0.0 7 3 1. 0 4 1.1 2 8 bct (220) 1. 117 0.0 7 9 1.3 bct (202),(022) 1.1 60 0 .115 1. 9 5 1.3 91 bct (222) 1.3 88 The assigned structure is body centered tetragonal with a lattice constant ( a ) = 10. 58 nm and ( c ) = 9.82 nm

PAGE 169

169 A 10 7.4 GPa Figure A 10 SAXS patterns of the nanocrystal superlattices under 7.4 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B,C) The converted one dimensional spectrum. In Figure B,C, the black peaks, the green peaks, the red solid line, and the blue dotted line are the deconvoluted peaks fro m the bct model, the deconvoluted peaks from the sc model, the experimentally obtained spectrum, and the simulated spectrum, respectively

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170 Table A 10 Detailed information of the experimentally obtained and simulated spectra in Figure A 10B (i) and Fig ure A 10C (ii) Table A 10 i. The assigned structure is body centered tetragonal with a lattice constant ( a ) = 10.69 nm and ( c ) = 9.93 nm Peak Number position decomposition 2 theta peak width peak area (a.u.) 1 0.5 54 bct (110) 0.5 40 0.0 84 3.4 bct (101),(011) 0.5 58 0.07 6 7. 5 2 0.78 1 bct (200),(020) 0.7 65 0.0 75 2.5 bct (002) 0. 813 0.0 66 1. 2 3 0.9 73 bct (211),(121) 0.9 47 0.0 77 0. 5 bct (112) 0.976 0.0 77 1. 6 4 1.1 04 bct (220) 1. 082 0.0 79 1.0 bct (202),(022) 1.1 16 0.0 79 1. 6 6 1.3 58 bct (222) 1.3 87

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171 Table A 10 ii. The assigned structure is a mixture of body centered tetragonal with a lattice constant ( a ) = 10.63 nm (70%) and ( c ) = 10.01 nm and simple cubic with a lattice constant ( a ) = 7.81 nm (30%) Peak No. position decomposition 2 theta peak width peak area (a.u.) 1st peak 0.554 sc ( 10 0) 0.534 0.0 79 4.3 bct ( 110 ) 0.555 0.0 64 2.3 bct ( 101 ) ,(0 11 ) 0.573 0.0 65 4.4 2nd peak 0.781 sc ( 11 0) 0.755 0.0 97 1.5 bct (20 0),(020 ) 0.785 0.0 8 4 2.4 bct ( 00 2) 0.834 0.0 84 1.1 3rd peak 0.973 sc ( 111 ) 0.925 0.0 84 0. 8 bct (2 11 ),( 121 ) 0.972 0.0 68 1. 2 bct ( 112 ) 1.002 0.0 68 0.6 4th peak 1.104 sc ( 200 ) 1. 068 0. 116 0.9 bct (2 20 ) 1.11 0 0.0 89 0.8 bct (2 0 2),(022) 1. 145 0.091 1.0

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172 A 11 9.0 GPa Figure A 11 SAXS patterns of the nanocryst al superlattices under 9.0 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the exp erimentally obtained spectrum, and the simulated spectrum, respectively.

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173 Table A 11 Detailed information of the experimentally obtained and simulated spectra in Figure A 11B Peak number P osition (q/q 0 ) 2 ratio Assigned peak 1 0.5397 1. 00 sc ( 100 ) 2 0.7587 1.98 sc ( 110 ) 3 0.9357 3.00 sc ( 111 ) 4 1.0754 3.97 sc (2 00 ) The assigned structure is simple cubic with a lattice constant ( a ) = 7.75 nm

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174 A 12 10.5 GPa Figure A 12 SAXS patterns o f the nanocrystal superlattices under 10.5 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvolute d peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

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175 Table A 12 Detailed information of the experimentally obtained and simulated spectra in Figure A 12B Peak position d spacing (nm) (q/q 0 ) 2 ratio assigned plane 1s t 0.480 8.68 1.00 square ( 10 ) 2nd 0.676 6.17 1.98 square ( 11 ) 3rd 0.959 4.35 3.99 square ( 20 ) 4th 1.082 3.85 5.08 square ( 21 ) The assigned structure is square with a lattice constant ( a ) = 8.6 8 nm

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176 A 13 11.2 GPa Figure A 13 SAXS patterns of the na nocrystal superlattices under 11.2 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

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177 Table A 13 Detailed information of the experimentally obtained and simulated spectra in Figure A 13B Peak position d spacing (nm) (q/q 0 ) 2 ratio assigned plane 1st 0.465 8.97 1.00 square ( 10 ) 2nd 0.670 6.23 2.08 square ( 11 ) 3rd 0.935 4.46 4.05 square ( 20 ) 4th 1.039 4.01 5.00 square ( 21 ) The assigned structure is square with a lattice constant ( a ) = 8.97 nm

PAGE 178

178 A 14 11.9 GPa Figure A 14 SAXS patterns of the nanocryst al superlattices under 11.9 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the ex perimentally obtained spectrum, and the simulated spectrum, respectively.

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179 Table A 14 Detailed information of the experimentally obtained and simulated spectra in Figure A 14B Peak position d spacing (nm) (q/q 0 ) 2 ratio assigned plane 1st 0.462 9.03 1. 00 square ( 10 ) 2nd 0.655 6.37 2.01 square ( 11 ) 3rd 0.935 4.46 4.10 square ( 20 ) 4th 1.033 4.03 5.01 square ( 21 ) The assigned structure is square with a lattice constant ( a ) = 9.0 3 nm.

PAGE 180

180 A 15 12.7 GPa Figure A 15 SAXS patterns of the nanocrystal super lattices under 12.7 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the experiment ally obtained spectrum, and the simulated spectrum, respectively.

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181 Table A 15 The results of the experimentally obtained spectrum at the 12.7 GPa and the comparison with a square model. Peak position d spacing (nm) (q/q 0 ) 2 ratio assigned plane 1st 0.4 50 9.26 1.00 square ( 10 ) 2nd 0.687 6.07 2.33 square ( 11 ) 3rd ------square ( 20 ) 4th 1.156 3.61 6.59 square ( 21 ) A square model is not fitted with this experimental spectrum Table A 1 6 Detailed information of the experimentally obtained and si mulated spectra in Figure A 15B D ecomposition 2 theta d spacing (nm) Rectangular (1 0) 0. 4355 9.57 Rectangular ( 0 1 ) 0.5 07 8.22 Rectangular (11 ) 0.6693 6.23 Rectangular (1 0) 0. 8624 4.84 Rectangular (1 0) 1.0004 4.17 The superstructure is assigned as a rectangular with a lattice constant ( a ) = 9. 57 nm and (b) = 8.22 m,

PAGE 182

182 A 16 13.5 GPa Figure A 16 SAXS patterns of the nanocrystal superlattices under 13.5 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

PAGE 183

183 Table A 1 7 The results of the experimentally o btained spectrum at 13.5 GPa and the comparison with a square model. Peak position d spacing (nm) (q/q 0 ) 2 ratio Assigned plane 1st 0.446 9.35 1.00 square ( 10 ) 2nd 0.688 6.05 2.39 square ( 11 ) 3rd ------square ( 20 ) 4th 1.148 3.63 6.63 square ( 21 ) A square model is not fitted with this experimental spectrum Table A 1 8 Detailed information of the experimentally obtained and simulated spectra in Figure A 16B D ecomposition 2 theta d spacing (nm) Rectangular (1 0) 0. 427 9.76 Rectangular ( 0 1 ) 0. 5 396 7.73 Rectangular (11 ) 0.6875 6.07 Rectangular (2 0) 0. 8551 4.88 Rectangular ( 0 2 ) 1.08 3.86 The superstructure is assigned as a rectangular with a lattice constant ( a ) = 9. 76 nm and (b) = 7.73 m,

PAGE 184

184 A 17 14.1 GPa Figure A 17 SAXS patterns of the nanocrystal superlattices under 14.1 GPa. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are the deconvoluted peak s, the experimentally obtained spectrum, and the simulated spectrum, respectively.

PAGE 185

185 Table A 1 9 Detailed information of the experimentally obtained and simulated spectra in Figure A 17B Peak position d spacing (nm) 1/Q ratio Assigned plane 1st 0.415 10 .04 1.00 lamella ( 1 ) 2nd 0.793 5.26 1.91 lamella ( 2 ) 3rd 1.210 3.45 2.91 lamella ( 3 ) 4th 1.712 2.44 4.12 lamella ( 4 ) The assigned structure is lamella with a lattice constant ( a ) = 10.04 nm

PAGE 186

186 A 18 After releasing the pressure Figure A 18 SAXS patter ns of the nanocrystal superlattices after releasing pressure. A) Two dimensional SAXS pattern collected in the Mar 3450 detector. B) The converted one dimensional spectrum. In Figure B, the black peaks, the red solid line, and the blue dotted line are t he deconvoluted peaks, the experimentally obtained spectrum, and the simulated spectrum, respectively.

PAGE 187

187 Table A 20 Detailed information of the experimentally obtained and simulated spectra in Figure A 18B Peak position d spacing (nm) q/q 0 ratio Assigne d plane 1st 0.522 8.00 1.00 lamella ( 1 ) 2nd 1.058 3.94 2.03 lamella ( 2 ) 3rd 1.571 2.65 3.01 lamella ( 3 ) 4th 1.981 1.98 4.04 lamella ( 4 ) The assigned structure is lamella with a lattice constant ( a ) = 8.0 nm

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200 BIOGRAPHICAL SKETCH Yasutaka Nagaoka was born in 1983 in Tok yo, Japan. After graduating from Komaba Toho high school in 2002 in Tokyo, Japan, he attended Keio University in Yokohama, Japan. He completed his B.S. in March 2006 and M.S in March 2008 in Chemistry under the supervision of Prof. Yasuaki Einaga. In Au gust 2008, Yasutaka moved to Gainesville, Florida and began his Ph.D studies in the Chemistry Department at the University of Florida. He performed his dissertation research in the laboratory of Prof. Y. Charles Cao and complete d his Ph.D study in August 2013 His research area in the Cao resear ch group included synthesis of nanocrystals and nanocrystal superlattices, and high pressure chemistry of nanocrystals.