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Solution-phase synthesis of metal chalcogenide nanocrystals at low temperatures using dialkyl dichalcogenide precursors

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Solution-phase synthesis of metal chalcogenide nanocrystals at low temperatures using dialkyl dichalcogenide precursors

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SOLUTION-PHASE SYNTHESIS OF METAL CHALCOGENIDE NANOCRYSTALS
AT LOW TEMPERATURES USING DIALKYL DICHALCOGENIDE PRECURSORS

by

Matthew A. Franzman
________________________________________________________________________

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)

May 2010

Copyright 2010 Matthew A. Franzman
ii

Table of Contents
Acknowledgements
List of Figures
List of Schemes
Abstract
Chapter 1. Low-Temperature Solution-Phase Synthesis of Metal
Chalcogenide Nanocrystals
1.1. Significance and Applications of Semiconductor
Nanocrystals
1.2. Current State-of-the-Art in Low-Temperature Synthesis
1.2.1. Copper and Silver Chalcogenides
1.2.2. Lead and Bismuth Chalcogenides
1.2.3. Zinc, Cadmium, and Mercury Chalcogenides
1.3. Conspectus
1.4. References
Chapter 2. Peroxide-Mediated Synthesis of Indium Oxide
Nanocrystals
2.1. Introduction
2.2. Results and Discussion
2.2.1. Nanocrystal Synthesis
2.2.2. Structural Characterization
2.2.3. Spectroscopic Characterization of Surface-Bound
Ligands
2.2.4. Reaction Growth and Kinetics
2.3. Conclusions and Future Work
2.4. Experimental Details
2.4.1. General Considerations
2.4.2. Synthesis of In
2
O
3
Nanocrystals
2.4.3. Instrumentation
2.4.4.
1
H NMR Analysis of Surface-Bound Ligands
2.4.5. FT-IR Analysis of Reaction Mixture
2.4.6. TEM and XRD Analysis of Nanocrystal Growth
2.5. References
Chapter 3. Solution-Phase Synthesis of Well-Defined Indium Sulfide
Nanorods
3.1. Introduction
3.2. Results and Discussion
3.2.1. Structural Analysis
iv
v
vi
ix
x
1

1

3
4
8
13
17
19

25

25
28
28
30
32

35
42
44
44
45
45
46
47
47
47
52

52
53
53
iii

3.2.2. Synthetic Methodology
3.2.3. Electronic Characterization
3.3. Conclusions and Future Work
3.4. Experimental Details
3.4.1. Synthetic Procedure
3.4.2. Structural Characterization
3.4.3. Electronic Characterization
3.5. References
Chapter 4. Growth Kinetics of Wurtzite Cu-In-S Nanocrystals
4.1. Introduction
4.2. Results and Discussion
4.2.1. Nanocrystal Characterization
4.2.2. Statistical Analysis of Cu-In-S Growth
4.2.3. Nanocrystal Growth Mechanism
4.2.4. Elemental Composition of Growing Nanocrystals
4.2.5. Departure from Focusing-Defocusing Growth
Mechanism
4.3. Conclusions and Future Work
4.4. Experimental Details
4.4.1. General Considerations
4.4.2. Cu-In-S Nanocrystal Synthesis
4.4.3. Instrumentation
4.4.4. TEM and Statistical Analysis of Nanocrystal
Growth
4.5. References
Chapter 5. Phase, Size, and Shape Control of Tin Selenide and Tin
Sulfide Nanocrystals using Dialkyl Dichalcogenides
5.1. Introduction
5.2. Results and Discussion
5.2.1. Synthetic Methodology
5.2.2. Structural Analysis
5.2.3. Optoelectronic Analysis
5.3. Conclusions and Future Work
5.4. Experimental Details
5.4.1. General Considerations
5.4.2. Synthesis of Di-tert-Butyl Diselenide
5.4.3. SnSe Nanocrystal Synthesis
5.4.4. SnS Nanocrystal Synthesis
5.4.5. Structural Characterization
5.4.6. Electronic Characterization
5.5. References

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108
iv

Appendix. Spectroscopic Evidence of the Formation of Goldfingers
A.1. Introduction
A.1.1. Gold-based Drugs
A.1.2. Zinc Fingers as Potential Au(I) Coordination
Sites
A.2. Results and Discussion
A.2.1. Formation and Stoichiometry of Goldfingers
A.2.2. Secondary Structure of Goldfingers
A.2.3. Biological Implications of Goldfinger Formation
A.3. Conclusions and Future Work
A.4. Experimental Details
A.4.1. General Considerations
A.4.2. Peptide Purification
A.4.3. Synthesis of Et
3
P-Au-Cl
A.4.4. UV/Vis Titrations
A.4.5. Circular Dichroism
A.5. References
Bibliography

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133
v

Acknowledgements

Without the constant support, guidance, and leadership from my research advisor
Professor Richard Brutchey, my achievements at USC would not have been possible.
Richard has always been patient with me, from my first synthesis to the final words of
my last manuscript. For this I am truly appreciative. I thank Professor Amy Barrios for
her leadership and support as my advisor when I first arrived at USC. I would like to
thank all the members of the Brutchey and Barrios lab groups for their fellowship,
camaraderie, and friendship as well.
I thank Professors Mark Thompson, Noah Malmstadt, Peter Qin, Surya Prakash, Kyung
Jung, Thomas C. Flood and Ralf Langen for serving as committee members during the
various steps toward earning my doctorate. I must acknowledge the chemistry department
at USC for their support, especially Heather Connor and Michele Dea for their support. I
am very thankful for the help, knowledge and efforts given to me by my collaborators on
various projects.
I would also like to thank my parents, sisters, brother, niece and nephews for their
constant love. My parents never ceased to ask ―when will you graduate?‖ every time we
spoke on the phone. Though I viewed it as badgering then, that question has served as
my motivation to remain committed and focused on my work, and for that I am forever
grateful. Thank you to James Finlay for always having confidence in me. Thank you to
all my friends who have made my time at USC unforgettable.
vi

List of Figures

Figure 1.1: Tunable band gaps of semiconductor metal chalcogenide
nanocrystals.
Figure 1.2: Effects of precursor concentration and reaction
temperature on the size and shape of Ag
2
S nanocrystals.
Figure 1.3: Shape evolution of glucose-mediated Cu
2
O microcrystals
synthesized at room temperature.
Figure 1.4: TEM and SEM images of surfactant-assisted shape
control of PbS nanocrystals.
Figure 1.5: TEM images of low-aspect ratio (a) and high-aspect ratio
(b) PbS nanocrystals synthesized at low temperature.
Figure 1.6: TEM and SEM images of Bi
2
S
3
rods (a), Bi
2
Te
3
wires (b)
and Bi
2
Te
3
particles.
Figure 1.7: TEM images of ZnO (a) and ZnS (b) nanocrystals.
Figure 1.8: Solutions of CdSe synthesized by chemical etching
fluoresce at different wavelengths (a). TEM image of
CdSe nanocrystals (b).
Figure 1.9: TEM images of CdS (a), PbS(b), and CuS (c) nanowires
synthesized via a template-directed technique at room
temperature.
Figure 2.1: XRD pattern (a) and TEM image (b) of crystalline indium
oxide nanocrystals synthesized via a peroxide-mediated
route at 120 °C.
Figure 2.2: XRD pattern of In
2
O
3
nanocrystals synthesized using
benzoyl (a), lauroyl (b), and tert-butyl (c) peroxides.
Figure 2.3: TEM images of cubic-In
2
O
3
nanocrystals synthesized
using di-tert-butyl (a), benzoyl (b), and lauroyl (c)
peroxides.
Figure 2.4: HRTEM image (a), SAED pattern (b), and XPS (c)
spectrum of In
2
O
3
nanocrystals synthesized using di-tert-
butyl peroxide.
Figure 2.5:
1
H NMR of nanocrystals (a) and mixture of lauric acid
and oleylamine representative of the reaction solution (b).

2

5
8

9

11

12

14
15

17

29

30

31

32

33
vii

Figure 2.6: C 1s XPS spectrum (a) and ATR-IR spectrum (b) of
surface bound carboxylates.
Figure 2.7: FT-IR spectra of the nanocrystal growth solution before
(a) and after injection of di-tert-butyl peroxide (b).
Figure 2.8: FT-IR spectra of the nanocrystal growth solution before
(a) and after (b) indium acetylacenate decomposition.
Figure 2.9: Powder XRD patterns and TEM images of c-In
2
O
3

nanocrystals.
Figure 2.10: Size distribution histograms of the indium oxide
nanocrystals as determined by TEM.
Figure 2.11: Statisitcal analysis of nanocrystal growth.
Figure 3.1: XRD pattern of the -In
2
S
3
nanorods.
Figure 3.2: High-resolution (a) In 3d, (b) S 2p, (c) C 1s, and (d) N 1s
XPS spectra of the nanorods.
Figure 3.3: TEM image of the nanorods, HRTEM shown in the inset.
Figure 3.4: Left, histogram showing the relative distribution of the
nanorods by width. Right,histogram showing the
distribution by length.
Figure 3.5: Initial optoelectronic characterization.
Figure 3.6: Photoluminescence spectra of a similar solution with
lifetime decay shown in the inset.
Figure 3.7: Cyclic voltammetry curves of the nanorods (orange),
nanorods plus 5.25x10
-4
M disulfide (green), nanorods
plus 1.05x10
-3
M disulfide (blue).
Figure 4.1: XRD (a), SAED (b), and HRTEM of wz-Cu-In-S
nanocrystals.
Figure 4.2: Representative TEM micrographs demonstrating the
temporal evolution of the size and shape distribution of
Cu-In-S nanocrystals taken from the reaction mixture.
Figure 4.3: Size distribution histograms of the wz-Cu-In-S
nanocrystals as determined by TEM.
Figure 4.4: Statistical analysis of the evolving wz-Cu-In-S
nanocrystals.
34

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40

41
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57

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75

76

77

viii

Figure 4.5: EDX analysis of 6.9 nm copper-rich wz-Cu-In-S
nanocrystals.
Figure 4.6: Structural and compositional analysis of large wz-CuInS
2

nanocrystals.

Figure 5.1: XRD pattern of SnSe (a) and SnS (b) nanocrystals
synthesized using di-tert-butyl dichalcogenides at 180 °C.
Figure 5.2: EDX spectra of SnSe (a) and SnS (b) products
synthesized using the respective dichalcogenide
precursor.
Figure 5.3: TEM images of SnSe nanocrystals (a), HRTEM of one
SnSe nanocrystal showing the [111] crystal plane (b) and
SAED pattern of a cluster of nanocrystals (c).
Figure 5.4: XPS spectra of SnSe nanocrystals. Survey (a) and high-
resolution Sn 3d (b) and Se 3d (c) spectra.
Figure 5.5: TEM (a) and SEM (b) images of crystalline SnS
products.
Figure 5.6: UV-vis-NIR spectrum of SnSe nanocrystals with band
gap determination shown in the inset.
Figure A.1: Peptide sequence of the model zinc finger peptides
named CCHH, CCHC, and CCCC.
Figure A.2: Titrations of reduced zinc finger peptides with Et
3
P-Au-
Cl monitored by UV-vis.
Figure A.3: Electronic investigations of reaction solution.
Figure A.4: CD spectra of zinc fingers (a) CCHH, (b) CCCC, and (c)
CCHC.
Figure A.5: CD spectra of CCHH (a), CCCC (b), and CCHC (c)
goldfingers formed using aurothiomalate.

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102

103

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116

117
120

121
ix

List of Schemes

Scheme 1.1: Selective synthesis of silver sulfide (Ag
2
S) nanoscale
shapes
Scheme 1.2: Aqueous synthesis of crystalline cuprous oxide (Cu
2
O)
Scheme 1.3: General synthesis of anisotropic zinc oxide (ZnO)
nanocrystals from an organozinc precursor
Scheme 1.4: Synthesis of cadmium sulfide nanocrystals at the
water/toluene interface
Scheme 2.1: Peroxide-mediated synthesis of In
2
O
3
nanocrystals
Scheme 3.1: Disulfide-mediated synthesis of In
2
S
3
nanorods
Scheme 5.1: Diselenide-mediated synthesis of tin selenide (SnSe)
nanocrystals
Scheme 5.2: Disulfide-mediated synthesis of tin sulfide (SnS)
nanocrystals
Scheme A.1: Chemical structures of auranofin (left) and aurothiomalate
(right)
Scheme A.2: Metabolish of auranofin in stomach acid

4

4

7
13

16

28
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96

112

115

x

Abstract

Solution-phase synthetic reactions have proven to be viable routes toward semiconductor
metal chalcogenide nanocrystals; however, these reactions are often reliant upon high
temperatures, designer single-source precursors, or environmentally harmful reagents.
To overcome these obstacles, a versatile method for the moderately low temperature
synthesis of metal chalcogenide nanocrystals using dialkyl dichalcogenides as the
chalcogen source has been developed. These precursors decompose in solution to
promote the growth of kinetically controlled nanoscale products. Well-defined, 1-D
indium sulfide (In
2
S
3
) nanorods are synthesized using di-tert-butyl disulfide as the sulfur
source at 180 °C. In a similar fashion, monodisperse wurtzite copper indium sulfide
(wz-CuInS
2
) and tin sulfide (SnS) nanocubes can be synthesized by using di-tert-butyl
disulfide. This method proves equally effective toward the synthesis of novel
nanocrystals of tin selenide (SnSe) using di-tert-butyl diselenide and cubic indium oxide
(In
2
O
3
) nanocrystals using a family of peroxides. To better understand the role of these
dialkyl dichalcogenide precursors, the nanocrystal growth mechanisms have been
explored for the dichalcogenide-mediated synthesis of wz-CuInS
2
and c-In
2
O
3
.
1

Chapter 1. Low-Temperature Solution-Phase Synthesis of Metal
Chalcogenide Nanocrystals

1.1. Significance and Applications of Semiconductor Nanocrystals
Recent advances in nanotechnology have the potential to make significant contributions
to the fields of medicine, electronics, and solar energy conversion.
1a-d
Nanocrystals
composed of an ever-expanding repertoire of different material families are currently
being developed as the basic units of optoelectronic devices with superior performance
capabilities. In the very near future, the production and sale of nanocrystals and
nanocrystal-based devices is expected to be a multibillion dollar industry in the United
States alone.
2,3

The world is beginning to recognize that traditional petrochemical sources of energy
cannot fuel our ever-growing population.
4,5
Renewable energy sources like solar energy
are becoming increasingly important as demand for energy will inevitably increase.
4,5

Currently, commercial solar panels are mainly composed of highly-purified solar-grade
silicon (c-Si) which is generally produced by an expensive and wasteful process.
Amorphous, polycrystalline, or thin films of silicon decrease the overall production cost,
but are far less efficient compared to c-Si-based solar cells.
6
Solar cells composed of
light absorbing semiconductor nanocrystals could drastically improve the cost, efficiency
and reliability of solar energy.
7
In addition to solar energy, new materials are being
developed to increase the quality, efficiency, and durability of a broad range of
optoelectronic devices.
2

The demand for compatible materials in optoelectronic devices provides the impetus to
explore new materials. Semiconductor nanocrystals that are small enough to exhibit
quantum confinement have different electronic properties compared to their bulk
counterparts. When excited by phonons from light, nanocrystals may have improved
quantum yields or may carry current more efficiently; properties that can be further tuned
by adjusting the nanocrystal’s composition, size, and shape.
7
As shown in Figure 1.1, the
optical band gap (the energy required to produce an electron-hole pair) of a
semiconductor can be tuned in the nanoregime.
8
Nanocrystals are therefore viewed as
promising and versatile components for a broad range of optoelectronic components that
may prove to be a solution to the current problems in solar energy conversion.

Figure 1.1. Tunable band gaps of semiconductor metal chalcogenide nanocrystals. The band gaps of the
bulk material (●) increase due to quantum confinement up to a maximal limit (▼). Modified from
reference 7.

Nanocrystals are often fabricated using energy-intensive and expensive techniques such
as laser ablation or molecular beam epitaxy that rely on hazardous and/or
environmentally harmful metal precursors. Chemists have made substantial advances in
3

solution-phase nanocrystal syntheses; however, they often rely on high temperatures
(>300 °C) and environmentally hazardous reagents (flammable hydrocarbons and
phosphines, harsh reducing/oxidizing agents, pyrophoric metal precursors, etc.). These
drawbacks significantly increase the cost of manufacturing nanocrystal-based devices and
therefore limit their broad range use. If semiconductor nanocrystal-based optoelectonics
are to become mass produced in a cost-effective manner, then low temperature and
environmentally benign techniques must be developed.

1.2. Current State-of-the-Art in Low-Temperature Synthesis
Over the past decade increasing emphasis has been placed on developing low-
temperature synthetic routes toward semiconducting nanocrystals, but many routes lead
to products that are poorly crystalline and lack morphological control. Nanocrystalline
semiconducting binary metal chalcogenides (M
2
E, ME, M
2
E
3
; M = metal; E = O, S, Se,
Te) represent a promising family of components for optoelectronic devices. For example,
cadmium telluride (CdTe) exhibits excellent power conversion efficiencies in solar cells
and lead sulfide (PbS) is commonly used in infrared detectors. As a result, most
synthetic studies are aimed at tuning the electronic properties of these materials using
solution-phase methods that rely on high temperatures or caustic reagents. Described in
the following subsections is the current state-of-the-art in low-temperature (25 ºC to 100
ºC) synthesis of metal chalcogenide nanocrystals. Emphasis is placed on synthetic routes
that achieve highly crystalline products with strict morphological control.

4

1.2.1. Copper and Silver Chalcogenides
There have been significant advancements in solution-phase synthesis of monovalent
coinage metal chalcogenides (Cu
2
S, Cu
2
Se, Ag
2
S, Ag
2
Se, Ag
2
Te) at moderately high
temperatures (150 °C to 200 °C) to produce well-defined, single-phase nanoscale
products.
9-11
Exceptional morphological control over Ag
2
S—a narrow band gap material
for photovoltaic cells, photoconductors, and IR detectors—has been demonstrated at
temperatures below 100 °C.
12
As described in Scheme 1.1, Chin et al. have synthesized
Ag
2
S nanocrystals via thermal decomposition of silver(I) thiobenzoate in a concentrated
solution of trioctylphosphine and hexadecylamine; simple modifications of the reaction
scheme yielded a variety of well-defined unique products.

Scheme 1.1. Selective synthesis of silver sulfide (Ag
2
S) nanoscale shapes

Small, facetted nanocrystals (14.5 nm ± 6.2%) were synthesized from a solution
containing a 1:16 molar ratio of Ag(SCOPh) to amine for 15 minutes and heating to 80
ºC. Larger cubes (44.0 nm ± 4.3%) could be synthesized by slightly increasing the
reaction temperature and using a more concentrated solution. In fact, their size, shape,
and uniformity were further tuned by subtle changes in the reaction scheme as illustrated
in Figure 1.2. Smaller hexagonal nanocrystals seem to be favored at lower temperatures
5

(80 ºC), while cube-shaped nanocrystals are favored at 120 °C. Interestingly, it is
difficult to achieve shape selectivity at moderate reaction temperatures (100 ºC) as both
nanocrystals and nanorods can be seen.

Figure 1.2. Effects of precursor concentration and reaction temperature on the size and shape of Ag
2
S
nanocrystals. Modified from reference 12.

Long-chain amines weakly coordinate to the surface of the growing metal chalcogenide
nanocrystals; they are therefore frequently used as both a solvent and a ligand to control
the growth of nanocrystals in solution. It has since been realized that the shape of Ag
2
S
nanocrystals may be governed by an amine-promoted reduction of Ag(I) to Ag(0)
mechanism at temperatures above 100 °C.
10
Zero-valent silver is speculated to have
different chemical reactivity compared to monovalent silver; therefore Ag
2
S nanocrystals
that have a core composed of Ag(0), or are born form Ag(0) nucleation, will have
different size and morphology compared to those formed from Ag(I). In the above
synthetic scheme, the amine acts as the solvent, surface-pacifying ligand and also
6

facilitates the decomposition of the silver(I) precursor by weakening the Ag—S bond,
thus drastically reducing the reaction temperature.
Silver chalcogenide nanocrystals can be synthesized at room temperature using a variety
of techniques, but at great sacrifice to quality. Nanofibers composed of Ag
2
S or Ag
2
Se
have been synthesized using a polymer directed growth method in which Ag
2
C
2
O
4

nanofiber bundles were dissolved in ethanol and treated with either thioacetamide or
sodium selenite to form the corresponding sulfide or selenide, respectively.
13
While the
syntheses can be achieved at room temperature, the method results in polycrystalline
bundles which lack defined shape or smooth surfaces. These defects may severely affect
the electrical charge transport capabilities of the material.
Ag
2
Se polydispersed nanospheres have been created from solution-phased alloying of
silver(0) and selenium(0) nanoparticles at room temperature in different volume ratios.
14

The silver(0) nanoparticles were synthesized by reduction of silver nitrate with sodium
borohydride in water near 0 ºC, while selenium(0) nanoparticles were synthesized using a
similar method at 70 °C. Once combined, the particles alloyed to form nanospheres
composed of Ag
2
Se. The formation mechanism was followed spectrophotometrically
and found to exhibit Arrhenius-type behavior as diffusion of the spheres was dependent
on temperature. Increasing the ratio of silver(0) to selenium(0) particles created larger,
more spherical nanocrystals, but the sample lacked monodispersity.
Monodisperse -Ag
2
Te

nanowires have been synthesized by reacting premade Te
nanowires with AgNO
3
at room temperature.
15
Tellurium nanowire templates were
synthesized by reduction of tellurium dioxide in excess hydrazine hydrate then treated
7

with sodium dodecyl sulfate to control the growth rate. Once purified, silver nitrate was
added to the Te templates in water at room temperature and the resultant product was
isolated by lyophilizing. HRTEM confirmed that the -Ag
2
Te nanowires were single
crystalline, while elemental mapping analysis indicated an even distribution of Ag and Te
throughout the 1D structures. This reaction system favors the formation of the low-
energy phase of Ag
2
Te as silver readily displaces Te in the crystal structure. The product
could be converted to the high-temperature face centered cubic phase ( -Ag
2
Te) by
heating to 417 K.
While silver chalcogenides appear to nucleate and crystallize at fairly low temperatures,
there has been less success with copper(I) chalcogenides. Excellent morphological and
phase control have been achieved using a variety of solution-phase synthetic techniques,
but most require temperatures above 200 ºC.
16-20
Gao et al. found that reduction of
CuSO
4
by glucose in an alkaline H
2
O/ethanol/oleic acid solvent system at 100 ºC formed
microcrystalline shapes composed of Cu
2
O described in Scheme 1.2.
21
Similar to the
method used to synthesize Ag
2
S described above, this method proved highly versatile.

Scheme 1.2. Aqueous synthesis of crystalline cuprous oxide (Cu
2
O).

The reaction mechanism takes advantage of mild, environmentally friendly reaction
conditions. Water and ethanol serve as the solvents, and glucose was able to effectively
8

reduce Cu(II) to Cu(I). As described above, subtle changes in the silver-amine ratio
shifted the reaction products from small hexagons to larger cubes. Similarly, the
presence or absence of oleic acid (a surfactant) in Scheme 1.2 directly altered the
morphology of the product. Oleic acid preferentially coordinates to specific faces of
cubic Cu
2
O in the following order: {100} > {111} > {110}. Thus, adding 1 mL of oleic
acid capped the {100} crystal face and stunted growth along the <100> direction. Further
increases in the concentration of oleic acid affected the vulnerable faces of the crystal and
slowed growth in those directions as well, the results can be seen in Figure 1.3. On the
other hand, eliminating oleic acid completely allowed for unrestricted crystal growth and
lowered the reaction temperature to 60 °C.

Figure 1.3. Shape evolution of glucose-mediated Cu
2
O microcrystals synthesized at room temperature.
Modified from reference 21.

1.2.2. Lead and Bismuth Chalcogenides
Lead chalcogenides (PbS and PbSe) are frequently studied for their optoelectronic
properties and represent promising components for a wide range of applications from
near-infrared photodetectors to biological labeling materials.
22
Low-temperature
9

solvothermal
24
and aqueous
23,25-27
methods have been utilized to synthesize crystalline
PbS products of varying quality. Uniform star-shaped nanocrystals of PbS (Figure 1.2a)
have been synthesized from the thermal decomposition of lead acetate and thioacetamide
in an aqueous solution of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl
sulfate (SDS) at 80 °C.
23
Qi et al. found that the size and morphology of the resultant
star-shaped nanocrystals could be easily modified by adjusting the reaction time; the stars
increased in size and their shapes became less facetted as reaction time progressed to
produce large polyhedron after two days. Adjusting the solvent mixture also directly
influenced nanocrystal growth. Similar to Schemes 1.1 and 1.2 for the synthesis of nano-
Ag
2
S and Cu
2
O, CTAB and SDS acted as capping agents that coordinate selectively to
different faces of the growing PbS crystal. It was determined that while both CTAB and
SDS preferentially cap the Pb-rich {111} surface, the effect was more predominant for
SDS. The various shapes that result can be seen in Figure 1.4.

Figure 1.4. TEM and SEM images of surfactant-assisted shape control of PbS nanocrystals. [CTAB] = 5.7
mM, (a); [SDS] = 1.1 mM, (b); [SDS] = 11 mM, (c) and (d). Reproduced from reference 23.

Elongated PbS nanocrystals have been synthesized at low temperatures using either
hydrothermal or organic routes. Low-aspect nanorods have been made by Wang et al
from a simple hydrothermal route whereby Pb(AOT)
2
(AOT = sodium dioctyl
sulfosuccinate) was dissolved in isooctane and added to a solution containing SDS and
10

thiourea.
24
This heterogeneous solution was sealed in an autoclave and heated to 85 ºC
for 12 hours. An example of low-aspect PbS nanorods is shown in Figure 1.5a. As seen
above for the synthesis of star-shaped PbS nanocrystals, the concentration of SDS can be
increased to truncate growth along a particular crystallographic direction. Increasing the
concentration of SDS effectively caps the {111} Pb-rich PbS surface causing anisotropic
growth along the <100> direction to form nanorods. Thioacetamine once again proves to
be an effective sulfur source likely due to its thermal decomposition to hydrogen sulfide.
A route using organic solvents developed by Golan et al. led to the formation of ultrathin
(1.8 nm) PbS nanowires.
28
The nanowires were synthesized from an organometallic lead
precursor (lead hexadecylxanthate) dissolved in trioctylamine and heated to 90 ºC (Figure
1.5b).
28
Trioctylamine acts as both solvent and surfactant that strongly promotes growth
of the nanowires perpendicular to the [200] rock salt crystal phase, resulting in very
narrow, high-aspect PbS nanorods.
Lead selenide (PbSe) quantum dots were also formed at low temperatures—however,
given that low-temperature methods often cause surface defects that affect the electronic
properties of quantum dots—synthetic techniques that afford nanocrystals with high
quantum efficiencies are stressed above all else. Therefore, methods that use
temperatures below 100 ºC
29-31
have received little attention compared to the vast amount
of PbSe syntheses that are currently published. Despite their low synthetic temperature,
magic-sized water-soluble nanodots composed of PbSe were synthesized by Krauss et al.
at room temperature boasting quantum efficiencies greater than 50%.
29
In this method,
lead oxide was heated in the presence of oleic acid, then trioctylphosphine selenide was
11

added in the presence of air at room temperature to produce nanocrystals that ranged in
size from 1 to 2 nm in diameter.

Figure 1.5. TEM images of low-aspect ratio (a) and high-aspect ratio (b) PbS nanocrystals synthesized at
low temperatures. Reproduced from references 24 and 28, respectively.

While there have been many attempts to synthesize phase-pure nanocrystals composed of
Bi
2
S
3
,
32-34
Bi
2
Se
3
,
35-38
and Bi
2
Te
3
39-43

using a variety of wet, sonochemical,
electrochemical, or microwave techniques, these methods often lack control over
morphology and size resulting in large, ill-defined microstructures composed of
nanoscale polycrystalline domains. Although growth along a specific metal-rich
crystallographic direction for Ag, Cu, and Pb-based nanocrystals can be controlled, this
level of synthetic control for Bi-based nanocrystals is only in its primitive stages.
Orthorhombic Bi
2
S
3
prefers to grow along the [001] crystallographic direction to produce
rods and wires that originate from a central nucleation point, often forming large flower-
like superstructures.
33,34
Currently, the most effective way to avoid this growth
mechanism is to use temperatures above 130 °C;
44-46
however, a Qian et al. successfully
yielded the individual Bi
2
S
3
nanorods shown in Figure 1.6a via a hydrothermal method.
32

Briefly, aqueous solutions of bismuth nitrate pentahydrate and sodium sulfide were added
12

to an autoclave and heated to 100 °C for 12 to 24 hours. Low temperature conversion to
Bi
2
S
3
was possible as Bi(NO
3
)
3
was hydrolyzed in water to form BiONO
3
which easily
reacted with S
2-
ions in solution. The rods appeared to grow along the <220> direction,
but it was unclear how trioctylamine facilitated this type of growth.
A novel room-temperature sonoelectrochemical route was devised by Burda et al. to
construct heterostructured nanowires of Bi
2
Se
3
.
35
An electrochemical method has been
applied to the synthesis of individual Bi
2
Te
3
nanowires from Bi
3+
and TeO
2
2-
in solution
(Figure 1.6b).
41
Badding et al. utilized a solution-phase route using long-chain thiols as
capping agents to construct relatively monodisperse Bi
2
Te
3
nanoparticles at temperatures
as low as 50 ºC with lower thermal conductivity as compared to bulk (Figure 1.6c).
43

Bismuth neodecanoate was dissolved and heated in diphenyl ether, then various long-
chain thiol capping agents are added along with elemental tellurium. The resulting
nanocrystals do not appear to favor growth along one particular crystallographic
direction.

Figure 1.6. TEM and SEM images of Bi
2
S
3
rods (a), Bi
2
Te
3
wires (b) and Bi
2
Te
3
particles. Reproduced
from references 32, 41, and 43, respectively.

13

1.2.3. Zinc, Cadmium, and Mercury Chalcogenides
Semiconductor zinc chalcogenides ZnO,
47-51
ZnS,
47,52-56,59
and ZnSe,
54,57,58
have been
synthesized using a variety of low temperature techniques; however, the quality of these
materials still lags behind those that were previously discussed. A variety of
monodisperse nanocrystalline ZnO structures were created by Chaudret et al. at room
temperature by a highly versatile synthetic scheme employing an organometallic zinc
precursor (dicyclohexyl zinc), long-chain amine, and an organic solvent as shown in
Scheme 1.3.
48,49
Organozinc compounds such as dicyclohexyl zinc readily decomposed
at room temperature in the presence of water vapor to form zinc alkoxide, which then
hydrolyzed to the corresponding zinc oxide. Organic solvents (THF, toluene, anisole,
heptanes, ether) were coupled with long-chain amines (hexadecylamine, octadecylamine,
dodecylamine) to afford various products. For example, high-quality, monodisperse
(11.4 ± 5.7 × 2.8 ± 0.7 nm) nanorods are shown in Figure 1.7a.
49

Scheme 1.3. General synthesis of anisotropic zinc oxide (ZnO) nanocrystals from an organozinc precursor

In a different method by Zhu et al., short-chain amines (ethanolamine and
ethanediamine), zinc nitrate, and L-cysteine were dissolved in water and heated to 95 °C
to afford crystalline ZnS (Figure 1.7b).
53
While many of the aforementioned synthetic
14

schemes rely on various surfactants to control morphology, the short-chain amines used
here were able to control the crystal phase of ZnS. Hexagonal ZnS was created using
ethanolamine, while ethanediamine produced cubic ZnS. It has been speculated that the
hydroxyl group on ethanolamine coordinates tightly to zinc, thus influencing the
crystallization mechanism.

Figure 1.7. TEM images of ZnO (a) and ZnS (b) nanocrystals. Reproduced from references 49 and 53,
respectively.

Cadmium based nanocrystals, CdS, CdSe, CdTe, are explored for a variety of
applications including light-emitting diodes, lasers, biological labeling and photovoltaic
devices. Since size and shape drastically affect their electronic properties, there have
been significant advances in achieving strict morphological control. However, most low-
temperature synthetic methods of CdS, CdSe and CdTe produce only ill-defined, highly
agglomerated nanocrystals.
57,60-62
Papadimitrakopoulus et al. found that post-synthetic
chemical etching of CdSe proved to be an effective method to create nanocrystals of
desired sizes. Initial CdSe nanocrystals were synthesized by the reaction between
EDTA-stabilized CdCl
2
and a basic aqueous solution of elemental selenium at room-
temperature.
63
Chemical etching was performed by simply dissolving the CdSe
nanocrystals in 3-amino-1-propanol and acetone for 10 to 90 hours at 80 °C. This
15

method proved successful in creating water-soluble CdSe nanocrystals that emitted a
broad spectrum of colors and exhibited quantum efficiencies as high as 50% as shown in
Figure 1.8. Nanocrystals composed of mercury telluride (HgTe), another material studied
for its optoelectronic properties, have been synthesized from mercury acetate and
trioctylphosphine telluride in a mixture of hexadecylamine and ethanol at -78 ºC,
64
but
lacked defined shape and monodispersity.

Figure 1.8. Solutions of CdSe synthesized by chemical etching fluoresce at different wavelengths (a).
TEM image of CdSe nanocrystals. Reproduced from reference 63.

Cadmium sulfide and cadmium selenide nanocrystals can also be made at fairly low
temperatures at the organic/aqueous interface of a reaction mixture. This technique
pioneered by An et al. allows for superior control over nucleation and growth rates
resulting in small, monodisperse nanocrystal products while eliminating the need for
water- or air-free conditions and high reaction temperatures.
65
Triangular, monodisperse,
trioctylphosphine-capped CdS nanocrystals were created based on the simplified reaction
shown in Scheme 1.4. Thiourea readily dissolved in water while cadmium myristate and
trioctylphosphine oxide self-orient so that their polar moieties were in close proximity to
the aqueous phase and their hydrophobic tails remain in toluene. At 100 °C, thiourea
16

decomposes to release hydrogen sulfide which readily deprotonates and attacks the
cadmium located at the interface. As cadmium sulfide forms, trioctylphosphine
coordinates to the surface, bringing the final crystalline product into the organic phase
and thus eliminates further growth. The formation of monodisperse nanocrystals is
usually dependant on a fast nucleation rate; however, this two-phase approach physically
separates nucleation and growth events thus making the reaction scheme more versatile.
For example, monodisperse, oleic acid-capped CdS, CdSe, and CdSe/CdS core-shell
nanocrystals can be created via similar two-phase approaches using either fast-reacting
chalcogenide precursors (NaHSe or Na
2
S) or slow-reacting precursors (Na
2
SeSO
3

selenourea, or thiourea) below 100 °C.
66-68
Scheme 1.4. Synthesis of cadmium sulfide nanocrystals at the water/toluene interface (reproduced from
reference 65)

A generalized synthesis has been developed by Wong et al. for CdS, PbS and cupric
sulfide (CuS) monodisperse nanowires at room temperature by a template-directed
technique using sodium sulfide and common metal salts.
69
For the case of CdS, aqueous
solutions of 0.01 M Na
2
S at pH 6 and 0.01 M Cd(NO
3
)
2
were added to separate ends of a
17

U-shaped tube that was divided by pretreated polycarbonate membranes with pore sizes
ranging from 50 to 200 nm. The solutions were allowed to diffuse and crystallize within
the pores of the membrane for 12 hours at temperatures ranging from 25 °C to 80 °C.
This procedure resulted in monodisperse bundles of individual nanorods that have similar
diameters as the pores of the parent membrane. Interestingly, increasing the temperature
to 80 °C led to the formation of bristles on the surface of the nanowires. TEM images of
bundles of CdS, PbS, and CuS nanowires formed at 25 °C are shown in Figure 1.9a-c.

Figure 1.9. TEM images of CdS (a), PbS(b), and CuS (c) nanowires synthesized via a template-directed
technique at room temperature

1.3. Conspectus
While the examples described in the previous section represent the most successful
attempts toward low-temperature solution-phase synthesis of metal chalcogenide
nanocrystals, high-temperature routes remain the most dependable and popular methods
to produce monodisperse, high-quality nanocrystals. Most low-temperature synthetic
methods rely on poorly reactive reagents, such as thiourea, to serve as the chalcogenide
source which often results in small, ill-defined, polydispersed nanocrystals. Additionally,
18

reaction conditions whereby controlled anisotropic growth is favored at lower
temperatures are rare.
The demand for nanocrystal-based technologies is steadily growing. Therefore, there is a
strong need for dependable and versatile synthetic strategies that avoid high-
temperatures, thus decreasing the energy and cost required to create nanocrystals. The
following chapters describe the synthesis of semiconductor metal chalcogenides using
dialkyl dichalcogenide precursors at moderate temperatures. This method serves as a
bridge between high and low-temperature routes affording a variety of high-quality
nanocrystals that will provide insight into developing even lower temperature reaction
schemes in the future.
In the second chapter, a family of peroxides (R
2
O
2
) is used to decompose indium
acteylacetonate to form crystalline cubic indium oxide (In
2
O
3
) at temperatures as low as
120 ºC. Dialkyl disulfides are explored in the third and fourth chapters as reactive sulfur
sources for the formation of crystalline In
2
S
3
nanorods in the third chapter and metastable
wz-CuInS
2
nanocrystals in the fourth chapter. The fifth chapter further expands the use
of dialkyl dichalcogenides as di-tert-butyl disulfide is used to form nanosquares and
cubes of tin sulfide (SnS) and di-tert-butyl diselenide is used to form nanocrystals
composed of tin selenide (SnSe).
It appears that dialkyl dichalcogenides cause nanocrystal growth by a kinetic mechanism.
Spectroscopic and structural analyses were used to determine that peroxide mediated
formation of In
2
O
3
nanocrystals evolve according to a steady-state Ostwald ripening
mechanism. In addition a tandem size-focusing/Ostwald ripening growth mechanism
19

caused by the decomposition of di-tert-butyl disulfide leads to the kinetically-favored wz-
Cu-In-S product. The spectroscopic and structural analytical techniques used to form
these conclusions are discussed in the relevant chapters. This manuscript proves the
utility of dialkyl dichalcogenides toward the synthesis of various metal chalcogenide
nanocrystals at moderate temperatures.

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65. Pan, D.; Jiang, S.; An, L.; Jian, B. Controllable Synthesis of Highly Luminescent
and Monodisperse CdS Nanocrystals by a Two-Phase Approach under Mild Conditions
Adv. Mater. 2004, 16, 982.
66. Wang, Q.; Pan, D.; Jiang, S.; Ji, X.; An, L.; Jiang, B. ―A New Two-Phase Route
to High-Quality CdS Nanocrystals. Chem. Eur. J. 2005, 11, 3843.
67. Pan, D.; Wang, Q.; Jiang, S.; Ji, X.; An, L. Synthesis of Extremely Small CdSe
and Highly Luminescent CdSe/CdS Core-Shell Nanocrystals via a Novel Two-Phase
Thermal Approach. Adv. Mater. 2005, 17, 176.
68. Pan, D.; Wang, Q.; Jiang, S.; Ji, X.; An, L. Low-Temperature Synthesis of Oil-
Soluble CdSe, CdS and CdSe/CdS Core-Shell Nanocrystals by Using Carious Water-
Soluble Anion Precursors. J. Phys. Chem. C 2007, 111, 5661.
69. Zhang, F.; Wong, S. S. ―Controlled Synthesis of Semiconducting Metal Sulfide
Nanowires.‖ Chem. Mater. 2009, 21, 4541.
25

Chapter 2. Peroxide-Mediated Synthesis of Indium Oxide Nanocrystals*
*Published in part in J. Phys. Chem. C 2009, 113, 630.

2.1. Introduction
The controlled synthesis of binary metal oxide nanocrystals continues to be of interest
because of the wide range of technologically important properties these materials
possess.
1,2
Although high quality nanostructures can be synthesized using metal-organic
chemical vapor deposition (MOCVD) and pulsed laser ablation, these methods are
generally energy intensive (requiring temperatures between 450-900 ˚C) and face
limitations in terms of cost, yield, and scalability.
3
In order to address these issues,
solution-phase synthetic routes have been developed over the last several years that offer
lower reaction temperatures and the potential for scale-up.
4
One of the most versatile and
successful methods to date for the synthesis of binary metal oxide nanocrystals is the
―heating-up‖ process, whereby metal carboxylates are heated to high temperatures in the
presence of surfactants to form the desired oxide nanocrystals. A variety of nanocrystals
(e.g., copper oxide, gadolinium oxide, indium oxide, iron oxide, manganese oxide, zinc
oxide, etc.) have been made from the direct thermal decomposition of metal carboxylate
precursors, but in every case their success is limited to high reaction temperatures (250-
320 ˚C).
5-13

The reaction and growth mechanisms of the ―heating-up‖ process were not studied in
great detail until recently; however, several mechanistic investigations have begun to
better define what occurs during the ―heating-up‖ process. Hyeon and co-workers
26

demonstrated that the decomposition of iron carboxylate precursors occurs via an
autocatalytic thermal pyrolysis mechanism to form iron oxide nanocrystals.
11
In regards
to indium oxide (In
2
O
3
), Park and co-workers reported the synthesis of highly crystalline
In
2
O
3
using the ―heating-up‖ process in oleylamine at 250 ˚C.
14
In a related system, Peng
and co-workers showed that the synthesis of In
2
O
3
nanocrystals from indium carboxylate
precursors occurs via hydrolysis or alcoholysis pathways, rather than a thermal pyrolysis
mechanism.
9
If the growing In
2
O
3
were to undergo a thermal pyrolysis mechanism, the
concentration of the indium carboxylate precursor would decrease over the course of the
reaction as In
2
O
3
is produced. Instead, a constant relative concentration between the
metal carboxylate precursor and unreacted carboxylic acid ligands was observed by FT-
IR, providing evidence of equilibrium between the hydrolysis of metal precursors and the
dissolution of In
2
O
3
by unreacted acid. It was also found that by adding alcohol, the
reaction can be forced to completion via the irreversible formation of esters from the
carboxylic acid ligands. Through a detailed understanding of the reaction mechanism of
nanocrystal formation, Peng and co-workers obtained excellent control over the size and
morphology of the In
2
O
3
nanocrystal products.
There is considerable interest in the solution-phase synthesis of transparent
semiconductor oxide nanocrystals, such as In
2
O
3
,

which is an n-type semiconductor with
a direct band gap of ca. 3.6 eV. Indium oxide nanocrystals have been synthesized in a
variety of morphologies, including: dots,
9,10,15,16
flowers,
9
rods,
10
lotus roots,
17
wires,
18

and cubes.
19,20
As a result of its transparency in the visible region and high electrical
conductivity, In
2
O
3
nanocrystals have a variety of potential uses in solar cells, flat-panel
displays, and computer touch screens.
21
In addition, In
2
O
3
has proven utility as a
27

chemical and biological sensing material.
22-24
Solution-phase routes toward In
2
O
3
are
hampered by the same limitations as mentioned above; they often require temperatures
>250 ˚C and, as a result, employ environmentally harmful solvents. With growing
emphasis on green chemistry principles, it is becoming increasingly important to develop
more environmentally friendly, less energy intensive methods of nanocrystal
synthesis.
4,25-28

The potential exists to lower the reaction temperature of solution-phase routes toward
binary metal oxide nanocrystals, such as In
2
O
3
, by employing rational oxygen transfer
reactions for oxide formation rather than direct thermal pyrolyses. This chapter describes
a new solution-phase synthesis of In
2
O
3
nanocrystals via the addition of thermally
reactive oxygen-containing species (i.e., a series of organic peroxides), which promote
oxide nanocrystal formation at temperatures governed by the decomposition temperature
of the organic peroxide, which is well under the thermal decomposition temperature of
the In(acac)
3
(acac = acetylacetonate) precursor. As such, addition and decomposition of
the organic peroxide allows nanocrystal nucleation and growth to occur at lower
temperatures than the aforementioned ―heating-up‖ process. To determine if different
mechanistic conclusions for the ―heating-up‖ process can be drawn for the more complex
peroxide-mediated system, the reaction was followed over time by FT-IR spectroscopy,
powder X-ray diffraction (XRD), and transmission electron microscopy (TEM). This
investigation provides valuable insight into the reaction pathway and growth mechanism.

28

2.2. Results and Discussion
2.2.1. Nanocrystal Synthesis
Indium oxide nanocrystals were synthesized via the addition of di-tert-butyl peroxide to a
mixture of In(acac)
3
, oleylamine, and lauric acid at temperatures between 120 and 180 ˚C
(Scheme 2.1). By cycling the mixture of In(acac)
3
, oleylamine, and lauric acid between
nitrogen and vacuum at 95 ˚C, adventitious water was removed from the system.
Decreasing the concentration of water in the reaction system prevents (i) the formation of
indium oxide via a hydrolysis mechanism,
9
(ii) premature decomposition of the organic
peroxide (especially for the diacyl peroxides),
29
and (iii) production of crystalline InOOH
or In(OH)
3
intermediates.
10,17,19,30
As mentioned above, In
2
O
3
nanocrystals have been
produced previously by the direct decomposition of In(acac)
3
at 250 ˚C in the presence of
oleylamine.
14
Since the In(acac)
3
precursor is known to be thermally stable at 180 ˚C,
31

adding di-tert-butyl peroxide substantially decreases the reaction temperature, allowing
well-defined particles to be produced at temperatures as low as 120 ˚C, as shown by the
XRD pattern and TEM image in Figure 2.1. Importantly, crystalline In
2
O
3
is not
produced in the absence of peroxide under identical conditions at either 120 ˚C or 180 ˚C,
illustrating the essential role of the peroxide in nanocrystal formation at lower
temperatures.
Scheme 2.1. Peroxide-Mediated Synthesis of In
2
O
3
Nanocrystals

29

Decomposition of metal carboxylates into binary metal oxides via the ―heating-up‖
process typically requires high temperatures, and therefore high-boiling point petroleum-
derived solvents. In a further effort to lessen the environmental impact of nanocrystal
synthesis, non-petroleum derived oleylamine (derived from naturally occurring oleic
acid) and lauric acid (derived commercially from coconut oil) were added in
stoichiometric amounts and serve as both ―green‖ reaction solvents and nanocrystal
surfactants.
32,33
Nanocrystal formation was found to be dependent on the molar ratio of
oleylamine to lauric acid. A molar excess of lauric acid prevented the growth of In
2
O
3

nanocrystals, most likely through dissolution processes.
9
Conversely, decreasing or
eliminating the amount of lauric acid resulted in grossly agglomerated nanocrystals. As
such, it appears that the initial molar ratio of oleylamine and lauric acid strongly
influences nanocrystal synthesis.

Figure 2.1. Structural analysis of nanoscale c-In
2
O
3
synthesized at 120 °C. XRD pattern (a) and TEM
image (b).

30

2.2.2. Structural Characterization
This peroxide-mediated reaction works for a series of organic peroxides including,
benzoyl peroxide, and lauroyl peroxide, and tert-butyl peroxide (the model peroxide in
this study). The XRD analysis shown in Figure 2.2 revealed that the nanocrystals
synthesized according to Scheme 2.1 are composed of phase-pure bixbyite-type, cubic-
In
2
O
3
, with no evidence of crystalline In(OH)
3
or InOOH intermediates. The lattice
constant of a = 10.17±0.03 Å calculated for the nanocrystals synthesized using tert-butyl
peroxide is in good agreement with the literature value of a = 10.12 Å for cubic-In
2
O
3

Figure 2.2. XRD pattern of In
2
O
3
nanocrystals synthesized using benzoyl (a), lauroyl (b), and tert-butyl (c)
peroxides.
31

(JCPDS no. 06-416), while benzoyl and lauroyl peroxide-mediated nanocrystals both had
similar lattice constants that correspond well with bixbyite-type, cubic-In
2
O
3
.
The TEM images in Figure 2.3 demonstrate that the peroxide-mediated synthetic method
results in quasi-spherical nanocrystals, possibly caused by different peroxide
decomposition rates. TEM analysis indicates the different peroxides result in differently
sized particles. Di-tert-butyl peroxide produces cubic-In
2
O
3
that are 7.1±1.2 nm (Figure
2.3a) in diameter after 330 min, while benzoyl peroxide (Figure 2.3b) and lauroyl
peroxides (Figure 2.3c) produce particles that are 7.3 and 12.1 nm in diameter,
respectively. These calculated diameters match well with the particle size determined by
Scherrer analysis of the XRD patterns.

Figure 2.3. TEM images of cubic-In
2
O
3
nanocrystals synthesized using di-tert-butyl (a), benzoyl (b), and
lauroyl (c) peroxides.

An HRTEM image of an apparent single crystalline particle produced using di-tert-butyl
peroxide, with the (222) lattice planes displayed, is displayed in Figure 2.4a. The lattice
constant from randomly selected SAED patterns (Figure 2.4b) agrees with that calculated
from the XRD results. Furthermore, XPS confirms that the In
3+
oxidation state of the
32

nanocrystals. The In 3d
5/2
binding energy measured for the nanocrystals was 444.3 eV,
which is in excellent agreement with reported values (Figure 2.4c).
10,17

Figure 2.4. HRTEM image (a), SAED pattern (b), and XPS (c) spectrum of In
2
O
3
nanocrystals synthesized
using di-tert-butyl peroxide.

2.2.3. Spectroscopic Characterization of Surface-Bound Ligands
A combination of spectroscopic techniques was used to probe the surface environment of
the In
2
O
3
nanocrystals. Solution
1
H NMR spectroscopy has become an increasingly
useful tool in identifying the ligands bound to the surface of nanoparticles.
34
The
1
H
NMR spectrum of the purified In
2
O
3
nanocrystals and a
1
H NMR spectrum of olelyamine
and lauric acid (3:1 mol/mol)—a mixture representative of the ligands present in the
reaction mixture—are shown in Figure 2.5a and b, respectively. The most notable
difference between the two spectra is the substantial peak broadening observed for the
suspended nanocrystals that is characteristic of surface-bound ligands, rather than free
ligands.
34-37
It has been proposed that the observed line broadening is a result of either (i)
slower nanoparticle rotation compared to the time required to achieve isotropic
averaging, or (ii) chemical shift heterogeneity on the crowded surface of the
nanoparticle.
36
In the representative reaction mixture solution (Figure 2.5b) the
33

multiplets present at 2.59 and 1.93 ppm can be assigned to the methylene protons of the 
carbons of oleylamine and lauric acid, respectively, while the multiplet at 5.50 ppm can
be assigned to the vinylic protons of oleylamine. The spectrum of the suspended
nanocrystals (Figure 2.4a) features a broad multiplet at 5.50 ppm for the vinylic protons,
confirming the presence of bound oleylamine; however, the peaks for the methylene
protons of the  carbons of both oleylamine and lauric acid are shifted upfield. This
upfield shift most likely occurs because the ligands are coordinated to metal sites on the
surface of the nanocrystals, thereby changing their chemical environment. Also, it is
interesting to note that the double bond of oleylamine, which can be sensitive toward
oxidation by peroxides under certain conditions,
38
is not compromised during synthesis.

Figure 2.5.
1
H NMR of nanocrystals (a) and mixture of lauric acid and oleylamine representative of the
reaction solution (b).

XPS proves equally useful at determining the surface composition of the nanocrystals as
it does for studying the inorganic core. The C 1s binding energies of the organic ligands
bound to the nanocrystal surface are shown in Figure 2.6a. The peak at 285.0 eV
34

represents the aliphatic carbon chains from both ligands while the small shoulder located
at 288.7 eV arises from the carboxylate group of the lauric acid, which confirms that the
ligand is bound to the nanocrystal surface.
39-41
Carboxylates are known to coordinate to
the surface of nanocrystals by either monodentate, bidentate, or ionic chelation modes.
42

It is possible to qualitatively distinguish between these different binding modes by FT-IR
spectroscopy based on the frequency difference between the asymmetric and symmetric
(CO
2
–
) stretches.
42-44
The ATR-IR spectrum (Figure 2.6b) of the isolated nanocrystals
shows a 112 cm
-1
difference between the 
a
and 
s
(CO
2
–
) stretches located at 1547 cm
-1

and 1435 cm
-1
, respectively, which is indicative of a bidentate chelation of the
carboxylate group to the nanocrystal surface. Further evidence of aliphatic capping
agents present on the surface of the indium oxide nanocrystals is found by the –CH
3
and
–CH
2
bending modes located at 1377 cm
-1
and 1454 cm
-1
, respectively. In addition the
bending mode from the -CH
2
of the lauric acid can be observed at 1403 cm
-1
.
Thermogravimetric analysis of a dried sample indicates that the nanocrystals contain
approximately 26 wt% organic species on their surface (data not shown).

Figure 2.6. C 1s XPS spectrum (a) and ATR FT-IR spectrum (b) of surface bound carboxylates.
35

2.2.4. Reaction and Growth Kinetics.
FT-IR spectroscopy was used to study changes in nanocrystal growth solution over time,
giving a qualitative picture of the reaction kinetics for this reaction. Distinct
spectroscopic handles for In(acac)
3
and di-tert-butyl peroxide were identified in the IR
spectrum allowing the disappearance of these reagents to be accurately tracked over time.
Di-tert-butyl peroxide features a weak absorbance located at 877 cm
-1
, which can be
assigned to the O-O skeleton vibration from the peroxide bond.
46-48
A sample of the
reaction mixture taken at 95 ˚C (t = 0), before the addition of di-tert-butyl peroxide is
analyzed using FT-IR and the resultant spectrum is shown in Figure 2.7a. Clearly, there
are no other species in the nanocrystal growth solution that absorb between 850 and 900
cm
-1
. Immediately after adding di-tert-butyl peroxide, samples were taken at specified
time intervals and analyzed. These spectra were subtracted from the spectrum at t = 0
(Figure 2.7a), and then normalized based on the CH
3
bending mode at 1378 cm
-1
. As
shown in Figure 2.7b, the temperature of the system rises to the final temperature of 180
˚C and the O-O skeleton vibration decreases as time progresses. After 23 min, the band
completely disappears, signifying that all of the peroxide has been spent. Di-tert-butyl
peroxide is particularly stable compared to many dialkyl peroxides. Homolytic cleavage
of the peroxide bond does not occur until over 250 ˚C, well above the boiling point of the
compound;
48,49
however, in the presence of Lewis acids (such as Group XIII metals) O-O
cleavage occurs at far lower temperatures.
29
Once all the peroxide has been depleted, the
metal precursor begins to undergo decomposition, which is necessary for In
2
O
3

nanocrystal nucleation.
36

Figure 2.7. FT-IR spectra of the nanocrystal growth solution before (a) and after injection of di-tert-butyl
peroxide. The concentration of the peroxide steadily decreases and temperature progresses and time
progress as evidenced by the weakening absorbance at 877 cm
-1
. Samples taken at t = 0, 4, 7, 14, 23, and
34 min.

The decomposition of indium acetylacetonate was monitored in a similar fashion. As
illustrated in FT-IR spectrum in Figure 2.8a, the initial nanocrystal solution has a broad
absorbance peak located at 1394 cm
-1
that corresponds to a combination of vibrational
modes in the metal coordinated acetylacetonate ligand.
45
The sharp absorbance peak at
1378 cm
-1
is attributed to the CH
3
bending mode from the lauric acid and oleylamine.
Once again, the concentration of these species does not change throughout the course of
the reaction and therefore this absorbance peak is used to normalize the time-lapsed
spectra. After the initial 23 minutes, and after the nanocrystal growth solution has
reached 180 °C, the band at 1394 cm
-1
begins to decrease with the concomitant increase
of a band at 1366 cm
-1
(corresponding to the free, protonated acetylacetone).
After 180 minutes, there are no further changes in the intensities of these two bands,
indicating that the consumption of the In(acac)
3
monomers and the release of free acetyl-
37

Figure 2.8. FT-IR spectra of the nanocrystal growth solution before (a) and after (b) indium acetylacenate
decomposition. The decreasing band shown in (b) located at 1362 cm
-1
corresponds to decomposing
peroxide; spectra correspond with those shown in Figure 2.6b. The bands shown in (b) located at 1394 and
1366 cm
-1
begin to change at 43, 80, 150, 210, 270, 330 min and illustrative the progressive release of free
acetylacetone into the nanocrystal growth solution.

acetonate have reached completion or equilibrium. It has been observed by Niederberger
et al. that metal acetylactonates can decompose when heated to high temperatures (> 200
°C) in the presence of amines to yield a variety of organic products, such as acetamides
and N-isopropyldieneamines.
50
It is speculated that the oxygen to form crystalline metal
oxides is donated directly from the acetylacetonate ligand. However, in the peroxide-
mediated synthesis of metal oxides, the FT-IR bands assigned to free acetylacetone
remain present in the spectrum throughout the course of this reaction, implying that the
ligand does not decompose in the presence of reactive oxygen species or oleylamine. In
the absence of di-tert-butyl peroxide, decomposition of In(acac)
3
and release of free
acetylacetonate occur over a similar timescale under otherwise identical conditions
(results not shown); however, the nucleation of crystalline In
2
O
3
does not occur. This
suggests that the peroxide is playing a vital role in the formation of the oxide, perhaps via
38

transfer of oxygen atoms from reactive tert-butoxy species generated via decomposition
of the peroxide. Indeed, tert-butoxy species are known to transfer oxygen through the
elimination of isobutylene at low temperatures (<200 ˚C) in the presence of Lewis
acids.
51
Addition of tert-butanol, a plausible byproduct of di-tert-butyl peroxide
decomposition, leads to oxide formation under similar conditions; however, this result is
mitigated by the fact that addition of alcohols can lead to oxide formation through a
separate, kinetically rapid alcoholysis pathway in the presence of carboxylic acids.
7,9

Diacyl peroxides, which do not thermally decompose to give alcohols, also lead to oxide
formation at reaction temperatures ≤180 ˚C (vide supra), which suggests yet a different
oxygen transfer pathway for these reagents.
Indium oxide crystal growth was monitored by XRD to determine evolution of the
nanocrystal phase and size as a function of time. Figure 2.9a-d shows the XRD patterns
from nanocrystals isolated from the reaction mixture at t = 120, 180, 240, and 330 min.
At 95 ˚C, before the addition of peroxide, no product could be isolated from the reaction
mixture by precipitation. After 120 min, crystalline In
2
O
3
is formed and the high
intensity (211), (222), (400), (440) and (622) reflections assigned to the cubic phase of
In
2
O
3
are clearly visible. The nanocrystals continue to grow as time progresses,
demonstrated by the gradual sharpening of the XRD reflections. At 240 min, the weaker
(411), (332), (431), and (611) reflections are detected and they sharpen along with the
rest of the reflections for the course of the reaction. By applying the Scherrer equation,
the growth of the nanocrystals was indirectly monitored. At 120 min, the nanocrystals
are ca. 3.6 nm in diameter. Growth continues until the end of the reaction time for a final
particle diameter of over 7 nm at 330 min. Previously studied synthetic methods for
39

making In
2
O
3
often have crystalline In(OH)
3
or InOOH intermediates, which are then
converted to In
2
O
3
upon heating to higher temperatures; these intermediates were not
observed at any point during the course of the reaction.
10,17,19,30
However, this technique
cannot disprove the presence of amorphous indium hydroxide or oxohydride impurities.
Representative TEM images that illustrate the shape and morphology of the growing
nanocrystals are shown in Figure 2.9e-g.

Figure 2.9. Powder XRD patterns and TEM images of c-In
2
O
3
nanocrystals. Samples are taken from the
reaction mixture at t = 120 (d), 180 (c), 240 (b), and 330 (a) min. Representative TEM images of the
samples taken at 120, 180, and 240 min are shown in (g), (f), and (e), respectively.

In order to more quantitatively study the peroxide-mediated formation of In
2
O
3

nanocrystals, samples were taken from the reaction mixture at various times and
statistically analyzed by TEM. Particles large enough to be detected by TEM are first
40

observed after 120 min of the reaction. Based on the size histograms from this analysis,
the nanocrystals steadily increase in mean diameter over the course of the reaction until
the reaction is stopped at 330 min (Figure 2.10). Assuming a spherical geometric model
for the nanocrystals, the nanocrystal volume increases linearly as a function of time
(Figure 2.11a), and the absolute standard deviation of the size distribution also increases
during the course of the reaction (Figure 2.11b). The nanocrystals formed after 120 min
possess a relatively narrow size distribution; however, as heating continues and the
nanocrystals grow in size, they become increasingly polydisperse in size and ultimately
reach a maximum standard deviation during the final stages of the reaction.

Figure 2.10. Size distribution histograms of the indium oxide nanocrystals as determined by TEM. Each
histogram represents the relative population of nanocrystals at the designated time.
41

Figure 2.11. Statisitcal analysis of nanocrystal growth. Average particle volume as a function of time (a);
standard deviation of the size distribution as a function of time (b); standard deviation of the size
distribution as a function of time, in terms of percent (c).

Using TEM to statistically analyze nanocrystal size, Peng et al. identified a ―self-focusing
via ripening‖ mechanism to explain the growth of MnO nanocrystals from a manganese
stearate precursor at 310 ˚C.
7
In that system, nanocrystal size increases slowly and
nonlinearly during the first 30 min of the reaction, with the size distribution of the
nanocrystals decreasing, or focusing, during these first 30 min. After 30 min, the
nanoparticle volume increases linearly (accompanied by an increase in absolute standard
deviation of the size distribution) for the course of the reaction in accordance with a
classic steady-state Ostwald ripening mechanism.
52
For the peroxide-mediated synthesis
of In
2
O
3
at lower temperatures, the nanocrystals grow linearly in size from 120 min into
the reaction (at which point nanocrystals of sufficient size to be imaged by TEM are first
observed) until its completion, and the standard deviation from the mean size also
increases over time. This linear change in volume is indicative of a steady and sustained
Ostwald ripening growth mechanism.
52
Closer inspection of the histogram shown in
Figure 2.9 further supports this hypothesis. During a typical steady-state Ostwald
ripening mechanism, smaller nanocrystals are consumed to feed the growth of larger
42

nanocrystals, thus favoring the formation of an ever-larger nanocrystal size regime.
53
As
smaller nanocrystals are spent, their relative population in relation to larger nanocrystals
decreases. In the peroxide-mediated growth of In
2
O
3
nanocrystals at 150 min, there exist
two main populations of nanocrystals between 4.0 and 4.8 nm. By 210 min, this
population of smaller particles noticeably decreases while the size of the larger
nanocrystal regime increases (Figure 2.10). This pattern continues throughout the course
of the reaction; the population of smaller nanocrystals decreases in favor of the formation
of larger nanocrystals. Since nanocrystal growth does not ―self-focus,‖ the standard
deviation of the nanocrystal size distribution steadily increases during the reaction
(Figure 2.11c); beginning at ca. 0.5 which is above the lower limit of standard deviation
predicted by recent theoretical studies of Ostwald ripening.
54,55

2.3. Conclusions and Future Work
Indium oxide nanocrystals were successfully synthesized in the solution-phase using a
new and reproducible peroxide-mediated reaction at lower temperatures. Organic
peroxides proved to be effective and versatile oxygen-sources that promote nanocrystal
formation at temperatures as low as 120 ˚C, instead of ≥250 ˚C for the traditional
―heating-up‖ process. Importantly, crystalline In
2
O
3
does not nucleate under these
conditions without the addition of organic peroxide. Slow growth on the time-scale of
several hours is observed, which stands in contrast to the fast growth observed in
reactions using the ―heating-up‖ process, which typically reach comparable nanocrystal
sizes within several minutes.
9

43

Through a combination of FT-IR, XRD, and TEM, changes in the reaction were
monitored over the course of the reaction. FT-IR spectroscopy established that the di-
tert-butyl peroxide immediately begins to decompose upon injection until it is completely
spent after 23 min. At this point, the In(acac)
3
precursor begins to release the
acetylacetone ligand, which continues until 180 min into the reaction. It is reasonable to
surmise that peroxide decomposition leads to reactive tert-butoxy species that could
transfer oxygen via the elimination of isobutylene, which would allow for nucleation
events to occur during this time interval up to 120 min into the reaction. Only after 120
min, once a significant amount of In(acac)
3
is consumed and nucleation occurs, are small
nanocrystals detectable by TEM and XRD. XRD analysis indicates that these initial
crystalline products are pure cubic-In
2
O
3
, not In(OH)
3
or InOOH as is often the case in
hydrolysis pathways. Both XRD and TEM demonstrate that the In
2
O
3
nanocrystals
continue to grow as long as heat is supplied to the reaction. This growth occurs as a
linear increase in nanocrystal volume as a function of time and the nanocrystal population
becomes increasingly polydispersed as the reaction continues, which are indicative of a
steady-state Ostwald ripening growth mechanism whereby large particles are formed at
the expense of smaller particles. Lastly, the mechanism presented here differs from
known alcoholysis pathways that form nanocrystals more rapidly and do not grow via
Ostwald ripening mechanisms.
7,9
As such, it appears that even if decomposition of di-
tert-butyl peroxide passes through an alcohol intermediate, the mechanism of oxide
formation is different for the peroxide-mediated synthesis. Further analysis of this
system my reveal the exact mechanism of oxygen transfer from the growth solution to
44

indium to create the corresponding metal oxide. This mechanistic understanding may
help determine if metals other than indium will act similarly.
While most mechanistic investigations have focused on the thermal decomposition of
metal carboxylates at high temperatures to form the corresponding binary oxide, this is
one of the first attempts to examine the more complex, chemically-induced conversion of
a metal precursor to the nanocrystalline metal oxide product.
50,56,57
Further understanding
of the reaction mechanisms responsible for metal oxide formation will help to improve
the synthesis and extend the reaction scope to other technologically interesting metal
oxide nanocrystals at low temperatures. This account also represents the first application
of a dialkyl dichalcogenide precursor toward the synthesis of inorganic nanocrystals. As
the following chapters will describe, other members of the dialkyl dichalcogenide family
will prove advantageous for the synthesis of a variety of metal chalcogenide nanocrystals.

2.4. Experimental Details
2.4.1. General Considerations.
Indium(III) acetylacetonate (99.99%; Strem Chemicals), lauric acid (dodecanoic acid;
99%; MP Biomedicals), oleylamine (cis-9-octadecenylamine; TCI America), benzoyl
peroxide (97%; Sigma-Aldrich), di-tert-butyl peroxide (98%; Sigma-Aldrich), and
lauroyl peroxide (didodecanoyl peroxide; 97%; Sigma-Aldrich), were all purchased and
used without further purification. Nanocrystal syntheses were performed under nitrogen,
in the absence of water, using standard Schlenk techniques.
45

2.4.2. Synthesis of In
2
O
3
Nanocrystals.
In a typical synthesis, In(acac)
3
(0.30 g, 0.73 mmol), lauric acid (0.58 g, 2.91 mmol), and
oleylamine (2.32 g, 8.66 mmol) were added to a three-neck round bottom flask fitted with
a reflux condenser, glass stopper, and rubber septum. Prior to heating, the system was
cycled three times with vacuum and nitrogen to eliminate adventitious water. The
mixture was heated (10 ˚C min
-1
) to 95 ˚C and vacuum and nitrogen were again cycled.
Di-tert-butyl peroxide (4.0 mL, 2.18 mmol) was quickly injected into the system under
flowing nitrogen, and the temperature was increased (10 ˚C min
-1
) to 180 ˚C and allowed
to react for 330 min with stirring. After cooling to room temperature, the reaction
mixture was dissolved in 2 mL of dichloromethane; then 15 mL of ethanol was added and
the mixture was sonicated and centrifuged (4000 rpm for 15 min) to yield a white solid.
Precipitation was repeated with toluene (0.5 mL) and ethanol (15 mL) to yield the
purified product. When benzoyl peroxide (0.26 g, 2.16 mmol) or lauroyl peroxide (0.86
g, 2.16 mmol) were used, the oxidants were funneled into the reaction mixture at 95 ˚C
under flowing nitrogen, followed by application of vacuum. The resulting In
2
O
3

nanocrystals form suspensions in organic solvents such as hexane, toluene, and benzene
that are stable for several weeks.

2.4.3. Instrumentation.
Powder X-ray diffraction (XRD) analyses were performed on a Rigaku Rotaflex RTP 300
X-ray diffractometer using a Cu K  radiation source (  = 1.54 Å). Transmission electron
46

microscopy (TEM) and selected area electron diffraction (SAED) were performed on a
JEOL 1011 microscope. SAED patterns were collected at a camera distance of 60 cm.
X-ray photoelectron spectra (XPS) were acquired on a VG Escalab II using a
monochromated aluminum anode. The binding energy of aliphatic C 1s in all spectra
was standardized to 285.0 eV. Fourier-transformed infrared (FT-IR) and attenuated total
reflectance (ATR) FT-IR spectra were collected on a Perkin-Elmer Spectrum 2000 at a
scanning interval of 0.5 cm
-1
with a resolution of 1 cm
-1
under flowing nitrogen. ATR-
FT-IR spectra were taken from concentrated solutions of purified nanocrystals in hexanes
that were dried on the MIRacle ATR crystal (Pike). Solution
1
H NMR spectra were
acquired at 400 MHz using a Varian Mercury spectrometer at room temperature and were
internally referenced to a tetramethylsilane standard. Thermogravimetric analysis (TGA)
was performed on a TA Instruments 2900 Series instrument by heating the sample to 800
˚C (10 ˚C min
-1
) under nitrogen.

2.4.4.
1
H NMR Analysis of Surface-Bound Ligands.
Nanocrystal samples (ca. 5 mg) for
1
H NMR analysis were dried and rinsed repeatedly in
benzene-d
6
(99.96% D, Cambridge Isotopes) to eliminate any residual dichloromethane,
toluene, or ethanol from the precipitation procedure. The dried nanocrystals were then
diluted with benzene-d
6
to a final volume of approximately 500 L.

47

2.4.5. FT-IR Analysis of the Reaction Mixture
Hot aliquots (0.1 mL) of the reaction mixture were taken by syringe at specified times
and deposited between two NaCl windows without purification or the addition of any
solvent. Absorption spectra were taken immediately and subtracted from the spectrum of
a sample taken at t = 0 (95 ˚C) before the addition of peroxide. Data was normalized
based on the band located at 1378 cm
-1
attributed to the CH
3
bending mode.

2.4.6. TEM and XRD Analysis of Nanocrystal Growth
Aliquots (0.1-0.25 mL) of the reaction mixture were taken via syringe at specified times
and quenched via cooling. Nanocrystals were precipitated from the reaction mixture as
described above and dissolved in toluene. TEM samples were prepared on 300 mesh
carbon-coated copper grids (Ted Pella, Inc.). Each data point in the statistical analysis
represents over 200 particles. For XRD analysis, each sample was prepared by casting
the purified nanocrystal suspensions on glass substrates at room temperature.

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18. Li, C.; Zhang, D.; Han, S.; Liu, X.; Tang, T.; Zhou, C. Diameter-Controlled
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Nanowires and Their Electronic Properties. Adv.
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2
Down to ppb Levels Using Individual and Multiple In
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Nanowire Devices.
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Nanoparticles. Chem. Mater. 2007, 19, 2519.
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3
Nanoparticles via a
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52

Chapter 3. Solution-Phase Synthesis of Well-Defined Indium Sulfide
Nanorods*

*Published in part in Chem. Mater. 2009, 21, 1790.

3.1. Introduction
Indium sulfide (In
2
S
3
), an n-type III-VI semiconductor material with a mid-band gap (E
g

= 2.0-2.2 eV for bulk), is currently being explored for a wide range of potential
applications including, solar cells, fluorescent labels for biological imaging, and
phosphors for color displays.
1-8
Inorganic solar cells with In
2
S
3
buffer layers were
reported to have power conversion efficiencies of up to 16.4%.
4,7,9
Indium sulfide also
represents a promising alternative to CdS and CdSe because it is elementally nontoxic,
thermally stable, can have comparable quantum yields, and possesses a band gap in the
same range as these important II-VI semiconductors.
1-4,10
Consequently, there is
increasing interest in the synthesis of In
2
S
3
nanocrystals and thin films.
Hydrothermal,
3-5,11
solvothermal,
6,12
sonochemical,
13
and ―heating-up‖
2,7
methods have
proven successful in the synthesis of In
2
S
3
; however, many of these methods require high
temperatures and/or extreme pH conditions, and result in products with irregular
morphology.
It is known that low dimensional semiconductor nanocrystals possess size and shape
dependent optoelectronic properties; strict control over morphology is therefore of critical
importance when designing novel nanocrystals. Large aspect ratio nanorods possess
unique electronic properties not only as a result of quantum confinement, but also
because their 1-D architecture directs electron flow.
14,15
One dimensional Cu
2
S-In
2
S
3

53

heterostructures were recently reported whereby In
2
S
3
grows off a 0-D Cu
2
S nanocrystal
in solution at 200 °C.
16
There are very few synthetic routes to high-aspect ratio In
2
S
3

nanorods. Room temperature sulfurization of electrodeposited metallic indium yielded
In
2
S
3
nanorods with large diameters (>50 nm), although In(OH)
3
impurities are present
when carried out at low voltages.
17
Indium sulfide nanorods with diameters of ca. 20 nm
were also synthesized via AACVD (aerosol-assisted chemical vapor diffusion) using a
designer single source precursor, but this method required temperatures >375 ˚C to yield
nanorods with good uniformity.
18
Given the success of using peroxides to make In
2
O
3
nanocrystals,
19
organodisulfides are explored here as potential sulfur sources to make
In
2
S
3
nanostructures. This chapter describes the synthesis of well-defined In
2
S
3

nanorods prepared using di-tert-butyl disulfide as the sulfur source in the presence of an
aliphatic amine. This investigation describes the first example of a solution-phase
synthesis of cubic -In
2
S
3
nanorods and characterization of their optoelectronic
properties.

3.2. Results and Discussion
3.2.1. Structural Analysis
Indium sulfide crystallizes in three different phases. The defect cubic -In
2
S
3
phase
features a random arrangement of cation sites and vacancies that transforms into -In
2
S
3
at 420 °C and -In
2
S
3
, a layered hexagonal structure, at 754 °C.
6,12
-In
2
S
3
crystallizes as
a defect spinel with a high concentration of ordered vacancies located at the tetrahedral
cation sites.
6
Above 420 °C the ordered vacancies begin to randomize creating either a
54

tetragonal -In
2
S
3
(JCPDS no. 33-1375) or cubic -In
2
S
3
(JCPDS no. 32-0456) phase.
Various methods to synthesize of In
2
S
3
, including solution phase synthesis, generally
favor the formation of tetragonal -In
2
S
3
;
3,7,8,11-13,18,20-22
however, cubic -In
2
S
3
can be
formed in some instances.
1,2,4-6,23,24
The optoelectronic properties of either phase of -
In
2
S
3
are directly dependant on its inherent vacancies and surface defects.
As will be described in depth below, In
2
S
3
nanorods were readily prepared by the fast
injection of di-tert-butyl disulfide into a solution of indium acetylacetonate in oleylamine
under anhydrous conditions. Nanorod formation occurred over the course of 7 hours at
180 ˚C, resulting in a bright yellow-orange product that formed optically translucent and
stable suspensions in organic solvents, such as toluene and hexanes. The nanocrystalline
product was confirmed to be in the cubic -phase of In
2
S
3
by powder X-ray diffraction
(XRD). The XRD pattern of the nanorods shown in Figure 3.1 clearly shows the (211),
(311), (222), and (440) reflections of cubic -In
2
S
3
, and the general line shape and
breadth of the diffraction peaks resemble those found of the diffraction peaks resemble

55

Figure 3.1. XRD pattern of the -In
2
S
3
nanorods.
those found for other anisotropic nanostructures.
2,4,5,20
A lattice parameter of a = 10.7 ±
0.3 Å was calculated for the nanorods, which is in excellent agreement with the literature
value (JCPDS no. 32-0456). Impurities such as In
2
O
3
, In(OH)
3
, InS, or other phases of
In
2
S
3
are not observed. As mentioned above, compared to the tetragonal form of -In
2
S
3
,
the cubic phase remains relatively unexplored.

Figure 3.2. High-resolution (a) In 3d, (b) S 2p, (c) C 1s, and (d) N 1s XPS spectra of the nanorods.

56

X-ray photoelectron spectroscopy (XPS, Figure 3.2) was used to confirm the elemental
composition of the In
2
S
3
nanorods. Peaks at 444.7 and 452.6 eV match the reported
binding energies of In 3d
5/2
and In 3d
3/2
for In
2
S
3
, respectively, and an asymmetric peak at
162.0 eV is representative of the S 2p binding energy for lattice S
2–
.
4,23
A weak N 1s
peak at 399.8 eV and a prominent C 1s peak at 285.0 eV are assigned to the primary
amine nitrogen and aliphatic carbon chain, respectively, of the oleylamine bound to the
surface of the purified nanorods. The surface atomic concentration was determined to be
ca. 37% In and 63% S, which is close to the expected stoichiometry of In
2
S
3
.

Figure 3.3. TEM image of the nanorods, HRTEM shown in the inset.

Transmission electron microscopy (TEM) analysis revealed the anisotropic nanorod
morphology of the -In
2
S
3
nanocrystals (Figure 3.3). The nanorods have an average
diameter of 11.5 ± 1.3 nm ( /  = 11.3%) as illustrated by the histogram shown in Figure
3.4. Although they are quite monodisperse in cross-sectional diameter, the nanorods
57

vary greatly in length (mean length = 171.7 nm) as can also be seen in Figure 3.4. In
addition to nanorods, there are also large, thin sheets of cubic -In
2
S
3
present. The
concentration of these sheets decreases significantly throughout the course of the
reaction, so that after 7 hours the primary product is the nanorods shown above. Some
TEM images show the nanorods splintering off from these large sheets. Although
unconfirmed it is possible that the nanorods are born from these large sheets. As
described above, nanorods composed of In
2
S
3
have been synthesized previously;
however, those synthetic schemes resulted in nanorods that were either phase-impure,
16,17

or wider than the Bohr radius of In
2
S
3
.
18
The nanorods described herein are the thinnest
In
2
S
3
nanorods reported thus far.

Figure 3.4. Representative population of nanorods from the reaction solution. Histogram showing the
relative distribution of the nanorods by width (left). Histogram showing the distribution by length (right).

58

3.2.2. Synthetic Methodology
The synthesis of -In
2
S
3
nanorods was adapted from the previously described synthesis
for In
2
O
3
nanocrystals.
19
Briefly, di-tert-butyl disulfide is injected into a hot, anhydrous
solution of oleylamine containing indium acteylacetonate (Scheme 3.1). This solution is
heated to 180 °C and allowed to stir under nitrogen for 7 hours. In this reaction system,
oleylamine is present in stoichiometric amounts (i.e., 14.2 equiv based on In
3+
) and acts
as both the reaction solvent and surface pacifying ligand for the nanorods. This solution
is rather concentrated compared to most solution phase syntheses of inorganic
nanocrystals in organic media. It is speculated that a high initial monomer concentration
can promote the formation of anisotropic structures, such as nanorod and nanowires.
25

In fact, decreasing the initial indium acetylacetonate concentration from 0.19 M to
0.10 M causes nanorods to not be formed under otherwise identical conditions. A
stoichiometric amount of oleylamine is essential for product formation, but a small
amount of lauric acid (2 equiv based on In
3+
) also yields the desired -In
2
S
3
nanorods
with no detectable crystalline In
2
O
3
or In(OH)
3
byproducts present at this temperature.

Scheme 3.1. Disulfide-mediated synthesis of In
2
S
3
nanorods

Proper phase formation is also dependant on the quality of the oleylamine. It is known
that long-chain aliphatic amines can decompose in the presence of carbon dioxide to form
59

alkylammonium and alkylcarbamate byproducts.
26
These byproducts have been
determined to hinder the efficiency of nanocrystal synthesis. Using oleylamine that has
been exposed to air for long periods of time prevents the proper formation of -In
2
S
3
. In
addition, water saturated oleylamine favors the formation of poorly crystalline In
2
O
3
via a
hydrolysis mechanism.
The presence of the dialkyl dichalcogenide reagent, di-tert-butyl disulfide, proved
essential for nanorod formtation and offers many advantages compared to other sulfur-
containing reagents. Di-tert-butyl disulfide, like all other disulfides, is stable in air and at
ambient temperatures. In basic solution, such as a solvent system made from amines, the
disulfide form is further stabilized, preventing protonation to yield tert-butyl mercaptan
in situ. Disulfides are not readily reactive toward metals, such as indium. Therefore, the
formation of unwanted reaction intermediates is avoided by choosing a disulfide reagent.
In addition, di-tert-butyl disulfide, unlike diphenyl or dibenzyl disulfide, is a liquid at
room temperature, allowing for easy injection into the reaction mixture under nitrogen.
Thioethers share many of the same benefits, but decompose at higher temperatures
compared to the corresponding disulfides.
27
While crystalline In
2
S
3
is formed at 180 °C
under above reaction conditions, this temperature would likely increase if di-tert-butyl
disulfide is replaced with di-tert-butyl sulfide.
Replacing di-tert-butyl disulfide with other sulfur-containing reagents does not allow for
the successful synthesis of the desired nanorods. For example, adding an equimolar
amount (per sulfur atom) of tert-butyl mercaptan produces cubic -In
2
S
3
under otherwise
identical conditions, however, ill-defined nanoparticulates are formed instead of
60

nanorods. Elemental sulfur also fails to produce any crystalline products at 180 ˚C under
these conditions. Moreover, reducing the sulfur loading by using only half the required
amount of di-tert-butyl disulfide results in the formation of mainly ill-defined
particulates, rather than nanorods.
Given that elemental sulfur does not produce crystalline indium sulfide and that tert-butyl
mercaptan does not produce the desired anisotropic nanostructures, the formation of -
In
2
S
3
nanorods is likely a consequence of the unique chemistry between di-tert-butyl
disulfide, indium acetylacetonate and oleylamine and at elevated temperature. In this
reaction scheme, -In
2
S
3
is formed at 180 °C, but the thermal decomposition of di-tert-
butyl disulfide begins at ca. 375 ˚C, which suggests that sulfur is not expelled as a result
of the simple thermal decomposition of the disulfide to H
2
S and 1/8 S
8
.
27,28
Sulfur
extraction at this significantly lower reaction temperature is likely facilitated by the
presence of oleylamine in the reaction mixture.
29
It is known that primary amines can
react with elemental sulfur to form reactive polysulfides and H
2
S.
30,31
The oleylamine
employed in this reaction may promote the decomposition of di-tert-butyl disulfide in
order to provide the necessary kinetically controlled conditions for the formation of -
In
2
S
3
nanorods at 180 ˚C.
The exact mechanism of sulfur donation from di-tert-butyl disulfide remains speculative.
Di-tert-butyl disulfide may decompose symmetrically to form (CH
3
)
3
CS
.
( H
b
= 226 kJ
mol
-1
), which can then attack indium to form indium sulfide—the energy to cleave the
sulfur-sulfur bond could be lessened in the presence of amine.
27
It may be also possible
that di-tert-butyl disulfide decomposes asymmetrically to form (CH
3
)
3
C-S-S
.
and
61

(CH
3
)
3
C
.
( H
b
= 272 kJ mol
-1
).
28
The naked disulfide radical formed in situ could also
readily attack indium acetylacetonate to form indium sulfide. The slow formation of this
relatively unstable species may promote the kinetically-favored growth of the anisotropic
nanorods and explain why the reaction requires over 7 hours to form the desired product.
Furthermore, it may be possible that H
2
S
2
formed by a two step decomposition of the
disulfide.
28
One hydrogen is abstracted from one tert-butyl group to form hydrogen tert-
butyl disulfide, (CH
3
)
3
C-S-S-H. A second hydrogen is abstracted from the remaining
tert-butyl group to yield H
2
S
2
and two equivalents of isobutene. Regardless of the exact
route of indium sulfide formation, the unique decomposition of dialkyl disulfides cannot
be replicated by thioethers, thiols, or elemental sulfur.

3.2.3. Electronic Characterization
Indium sulfide has a relatively large Bohr radius of ca. 34 nm, which is greater than the
cross-sectional diameter of the nanorods – suggesting that quantum confinement may
substantially alter the optoelectronic properties of this material.
1,3,8
The nanorods absorb
strongly in the ultraviolet region of the spectrum with significant absorption in the early
visible region resulting in the yellow-orange color of the product (Figure 3.5a). The UV-
vis spectrum of the -In
2
S
3
nanorods is shown in Figure 3.5b. Noticeable step-like
contours in the absorption spectrum are observed for the valence to conduction band
transition in In
2
S
3
.
2,3,11
The band gap of the In
2
S
3
nanorods was determined from the
onset of the absorption spectrum to be E
g
= 2.94 eV, while bulk In
2
S
3
has a reported band
gap between 2.0-2.2 eV.
3-7,10
The blue-shifted band edge observed for the nanorods is
indicative of quantum confinement resulting from their relatively small diameter.
62

Figure 3.5. Initial optoelectronic characterization. Photograph of the In
2
S
3
nanorods dissolved in
cyclohexane (a). Absorbance spectra of a dilute solution shown in (a) with the calculated optical band gap
shown in the inset (b).

The steady-state photoluminescence (PL) spectrum (25 °C, λ
ex
= 367 nm) of the In
2
S
3

nanorods is given in Figure 2. The nanorods produce a single broad emission centered at
λ
max
= 445 nm (quantum yield Φ < 1%), which corresponds well with the emission
characteristics of previously reported In
2
S
3
nanostructures.
5,8,11
The Stokes shift of
approximately 154 meV is greater than that typically observed for near band-edge
emission in semiconductor nanocrystals.
32
Moreover, time-resolved PL measurements
show a nonlinear decay best modeled by a double exponential fit with observed lifetimes
of τ = 2.9 and 8.7 ns (  = 0.32 and 0.68, respectively; Figure 3.6, inset). These data
suggest that the emission may be a result of trap states and/or defects in the In
2
S
3

structure.
1,5,8
Di-tert-butyl disulfide is photoluminescent, but the emission profile is
significantly different than the In
2
S
3
nanorods. Oleylamine possesses a similar emission
profile to that of the In
2
S
3
nanorods, but much higher concentrations of oleylamine are
needed to get emission at λ
ex
= 367 nm than would be present in the purified sample of
In
2
S
3
. Likewise, In
2
O
3
nanocrystals exhibit a very weak and broad emission centered at
385 nm, ruling out the possibility that the emission observed for the In
2
S
3
nanorods is a
63

result of oxide impurities. Further investigation is needed to determine if organic surface
species, or impurities from the organic species, are acting as surface trap states.

Figure 3.6. Photoluminescence spectra of a similar solution with lifetime decay shown in the inset.

Cyclic voltammetry (CV) can be used to estimate the HOMO (valence band) and LUMO
(conduction band) energies for semiconductor nanocrystals.
33-35
A typical CV curve for
the In
2
S
3
nanorods deposited as a thin film on the carbon working electrode is given in
Figure 3.7. Relative to an Ag/Ag
+
reference electrode, the oxidation and reduction
potentials of the nanorods are 1.64 V and -0.91 V, respectively. The peak-to-peak
separation between the oxidation (E’
ox
) and reduction (E’
red
) peaks gives an
electrochemical band gap of E
g
= 2.55 V, which is smaller than the band gap estimated
from the absorption spectrum; however, smaller electrochemical band gaps have also
been measured for CdS,
33
CdSe,
34
and CdTe
36
nanocrystals. The HOMO and LUMO
levels were determined to be -6.3 eV and -3.8 eV, respectively. To confirm that these
oxidation and reduction peaks result from the purified nanorods and not residual di-tert-
butyl disulfide or tert-butyl mercaptan byproduct, CV curves were taken with varying
64

concentrations of disulfide added to the solution. As the concentration of disulfide
increases, a peak at 1.88 V appears, increasing in peak current with increasing
concentration. This peak is assigned to the oxidative scission of the C–S bond in the
disulfide.
37
An additional peak with an onset at 1.03 V is observed in all cases; however,
it can be assigned to the one electron oxidation of the disulfide to form a disulfide radical
cation.
37
Furthermore, the small reduction peak at -0.91 V is not observed in the CV
analysis of neat disulfide or the electrolyte blank, suggesting this peak is due to the
nanorods.

Figure 3.7. Cyclic voltammetry curves of the nanorods. Nanorods alone (orange), nanorods plus 5.25 ×
10
-4
M disulfide (green), nanorods plus 1.05 × 10
-3
M disulfide (blue).

3.3. Conclusions and Future Work
In summary, this work represents a novel and facile solution-phase synthesis method
toward very well-defined In
2
S
3
nanorods at relatively low temperatures. The nanorod
synthesis is based on the use of a dialkyl disulfide reagent as the sulfur source under high
reaction concentrations in an aliphatic amine solvent; further proving the utility of dialkyl
65

dichalcogenide precursors as valueable reagents in nanocrystal synthesis. The resulting
nanorods possess a very monodisperse cross-sectional diameter (11.5 ± 1.3 nm, /  =
11.3% ) that is well below the Bohr radius of In
2
S
3
– resulting in a band gap energy that
is significantly blue shifted relative to the bulk material.
Given their unique size and morphology, the electronic properties of the as-synthesized
nanocrystals were studied. The optical band gap as determined by UV-vis spectroscopy
was determined to be approximately 2.94 eV, which is significantly greater than the band
gap of bulk In
2
S
3
. Photoluminescence experiments show that the nanorods are weakly
fluorescent, with a broad emission centered at 367 nm. It is currently unclear this
fluorescence is a direct result of trap states and/or defects in the In
2
S
3
crystal structure, or
if the surfactants contribute significantly. Cyclic voltammetry was used to determine the
HOMO and LUMO energy levels of the nanorods and were found to be -6.3 and -3.8 eV,
respectively. The unique electronic characteristics will help to prove their compatibility
in nanocrystal/conducting polymer composites.
31

Based on these intriguing results, further investigation into the fabrication and utility of
the nanorods is warranted. Incorporating the nanorods into solar cells will determine
their solar efficiency, durability, and charge transfer between the nanorods and the
polymer or electrodes. Further investigation into the mechanism of disulfide
decomposition may lead to more control over the synthesis of other metal sulfide
nanocrystals, or perhaps different morphologies of cubic -In
2
S
3
. Based on the results of
the In
2
O
3
and In
2
S
3
syntheses, it may be possible for nanocrystalline In
2
Se
3
to be
synthesized by a similar route from a dialkyl diselenide precursor.
66

3.4. Experimental Details
3.4.1. Synthetic Procedure
All reagents were purchased from commercial sources and used without further
purification. Indium (III) acetylacetonate (0.29 g, 0.71 mmol) and oleylamine (2.71 g,
10.13 mmol) were added to a three-neck round-bottom flask equipped with a reflux
condenser, glass stopper, rubber septum, and stir bar. The reaction system was cycled
between nitrogen and vacuum several times prior to heating to 95 °C (10 °C min
-1
). At 95
°C, the system was again cycled between nitrogen and vacuum several times to eliminate
adventitious water. Di-tert-butyl disulfide (0.37 g, 2.1 mmol) was injected into the flask
under flowing nitrogen, followed by heating (10 °C min
-1
) to 180 °C. After 7 hours, the
reaction mixture was slowly cooled to room temperature. After dissolving the crude
material in a minimal volume of dichloromethane, ethanol (ca. 15 mL) was added
followed by sonication and centrifugation (6000 rpm for 15 min) to isolate the material.
The yellow-orange solid was redissolved in a minimal volume of toluene and again
precipitated with ethanol to produce a yellow-orange solid that forms stable suspensions
in organic solvents.

3.4.2. Structural Characterization
Powder X-ray diffraction (XRD) analyses were performed on a Rigaku Rotaflex RTP 300
X-ray diffractometer using Cu Kα radiation (λ = 1.54 Å). X-ray photoelectron spectra
(XPS) were acquired on a VG Escalab II instrument using a monochromated aluminum
67

anode. The binding energy of aliphatic C 1s in all spectra was standardized to 285.0 eV.
For both XRD and XPS analysis, suspensions of the nanorods in toluene were deposited
on glass substrates and dried in air at room temperature. Transmission electron
microscopy (TEM) was performed on a JEOL 1011 microscope. Samples for TEM were
prepared from dilute purified samples of the nanorods dissolved in toluene and deposited
on 300 mesh carbon-coated copper grids (Ted Pella, Inc.). The length and diameter of
approximately 300 nanorods were counted to determine the size distributions.

3.4.3. Electronic Characterization
Samples of nanorods were suspended in hexanes in a quartz cell. UV-Vis spectra were
acquired on a Cary 14 spectrophotometer. The fluorescence spectrum of the same sample
was found using a Jobin Yvon Fluorolog FL-3-222 Tau fluorimeter at an excitation
wavelength of 367 nm. CV curves were acquired on a Princeton Applied Research
Potentiostat/Galvometer Model 283. The tetra-n-butylammonium hexafluorophosphate
(TBAPF6; 98%; Alfa-Aesar) electrolyte (0.3 M in dry, deoxygenated acetonitrile) was
added to a dry three-neck round bottom flask equipped with three rubber septa. Glassy
carbon, Pt, and Ag/Ag+ electrodes were inserted into the solution via the rubber septa and
served as the working, counter, and reference electrodes, respectively. Samples of
nanorods were dried in the presence of excess electrolyte on the surface of the carbon
electrode to form a thin yellow-orange film that adheres to the surface when submerged
in the acetonitrile. For experiments with added di-tert-butyl disulfide, 1 or 2 μL di-tert-
butyl disulfide were injected into the cell to give 5.25x10
-4
M or 1.05x10
-3
M solutions of
68

the disulfide in acetonitrile, respectively. All other experimental conditions were
constant. CV curves were acquired under an inert atmosphere at a scan rate of 10 mV s
-1
.

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2
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3
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3
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Interface
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3

Nanoparticles. J. Nanosci. Nanotechnol. 2007, 7, 2087.

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17. Datta, A.; Panda, S. K.; Gorai, S.; Ganguli, D.; Chaudhuri, S. Room Temperature
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21. Kim, W.-T.; Kim, C.-D. Optical Energy Gaps of -In
2
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3
Thin Films Grown by
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23. Xiong, Y.; Xie, Y.; Du, G.; Tian, X.; Qian, Y. A Novel in Situ Oxidation-
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3
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Di-t-butyl Disulphide. J. Chem. Soc., Perkin Trans. 2 1976, 1421.

29. Liu, J.; Yu, H.; Wu, Z.; Wang, W.; Peng, J.; Cao, Y. Size-tunable Near-infrared
PbS Nanoparticles Synthesized from Lead Carboxylate and Sulfur with Oleylamine as
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71

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72

Chapter 4. Growth Kinetics of Wurtzite Cu-In-S Nanocrystals*

*Published in part in Chem. Mater. 2009, 21, 4299.

4.1. Introduction
There is a growing need for highly efficient, low cost, and elementally non-toxic
components for photovoltaic devices, which continues to drive new research in the
development of inorganic semiconductor nanocrystals.
1
Solution routes to semiconductor
nanocrystals are particularly appealing because they are less energy intensive than
vacuum techniques, have the potential for scalability, and the nanocrystal suspensions
can be easily deposited via spin-coating or printing.
2
During recent years there has been
significant interest in the development of solution-phase routes to I-III-VI semiconductor
nanocrystals, such as CuInS
2
, CuInSe
2
, CuIn
x
Ga
1-x
Se
2
.
3-5
This family of semiconductors
has band gaps that match well with the solar spectrum that can be tuned by adjusting their
relative elemental compositions.
6,7
Of these materials, copper indium sulfide (CuInS
2
) is
particularly attractive because it possesses a favorable direct band gap (E
g
≈ 1.45-1.50
eV), has a large absorption coefficient, good photostability, has exhibited power
conversion efficiencies of 12% in photovoltaic devices, and avoids heavy elements such
as selenium.
8-11
Nanocrystals of CuInS
2
have been previously synthesized by heating
copper and indium salts with a sulfur source (e.g., dithiocarbamates,
6,12
thiourea,
13,14

thiols,
15,16
elemental sulfur,
7,17
and carbon disulfide
18
) at temperatures ranging between
180-250 ˚C, which typically yields the thermodynamically stable tetragonal chalcopyrite
phase.
6,7,15,17,18

73

Very recently, CuInS
2
nanocrystals were found to adopt a hexagonal wurtzite (wz) phase,
which was first observed by Lu and co-workers.
12
In the bulk, wz-CuInS
2
is a high
temperature phase stable between 1045-1090 ˚C, in which the copper and indium cations
randomly share common lattice sites.
19
As such, the wurtzite phase of CuInS
2
allows for
a high degree of compositional variability resulting in stable, non-stoichiometric Cu-In-S
nanocrystals. In fact, it was found by Lu et al. that the elemental composition of the Cu-
In-S nanocrystals is highly tunable; copper rich or indium rich nanocrystals can be made
by altering the relative ratio of copper to indium in the reaction mixture. Observation of
wz-Cu-In-S nanocrystals at room temperature proves that the metastable phase is
kinetically accessible under less energy intensive conditions,
12-14,16,20
and offers the
potential of discovering new optoelectronic and morphological properties specific to this
crystal phase.
The synthesis of chalcopyrite and wurtzite Cu-In-S nanocrystals continues to be an area
of active research, in which various reaction conditions and precursors have been used to
obtain nanocrystals with varying degrees of size and shape control. However, there have
been no investigations concerning the growth kinetics of this important material. As part
of an ongoing investigation into using dialkyl dichalcogenides as low temperature
chalcogen sources,
21-23
this investigation attempts to study the growth mechanism of
monodisperse, phase-pure nanocrystals composed of Cu-In-S.

74

4.2. Results and Discussion
4.2.1. Nanocrystal Characterization
Nanocrystals of Cu-In-S were synthesized via the fast addition of di-tert-butyl disulfide
into a mixture of CuCl, In(acac)
3
, oleylamine, and 1-dodecanethiol, which was then
heated to 180 ˚C. XRD analysis revealed that the nanocrystals are composed of
kinetically preferred wz-Cu-In-S with no evidence of crystalline Cu
2
S or In
2
S
3
impurities
(Figure 4.1a). The lattice constants of a = 3.9 ± 0.1 Å and c = 6.4 ± 0.1 Å calculated for
the nanocrystals from the XRD patterns are in good agreement with the values reported
by Lu and others (a = 3.9 Å and c = 6.4 Å).
12
These values derived from randomly
selected SAED patterns (Figure 4.1b) agree with those calculated from the XRD results
for wz-Cu-In-S. An HRTEM image of an individual nanocrystal with the wurtzite-
specific (102) lattice planes displayed (d = 0.25 nm) is shown in Figure 4.1c. An average
Cu:In:S composition as determined by EDS is found to be 0.51:0.11:0.38, which is
compositionally quite different from the expected 0.25:0.25:0.50 stoichiometry for
CuInS
2
. (The elemental composition will be discussed in greater detail in Section 4.2.2.)

Figure 4.1. XRD (a), SAED (b), and HRTEM (c) of wz-Cu-In-S nanocrystals.
75

4.2.2. Statistical Analysis of Cu-In-S Growth
There have been significant advances in the synthesis of I-III-VI semiconductor
nanocrystals; however, there have been no quantitative reports on the growth kinetics of
these nanocrystals in solution. Understanding the growth kinetics, as well as developing
a rational synthetic methodology, will ensure greater control over nanocrystal size and
shape. To investigate the growth kinetics of the Cu-In-S nanocrystals, aliquots were
taken throughout the course of the reaction and the corresponding TEM images were
statistically analyzed, thus creating a quantitative view of Cu-In-S nanocrystal growth.
Representative TEM images from each sample are shown in Figure 4.2 and the
corresponding histograms are displayed in Figure 4.3.

Figure 4.2. Representative TEM micrographs demonstrating the temporal evolution of the size and shape
distribution of Cu-In-S nanocrystals taken from the reaction mixture. Samples were taken at (a) 1, (b) 18,
(c) 23, (d) 28, (e) 33, (f) 68, (g) 128, (h) 188 min.

76

Figure 4.3. Size distribution histograms of the wz-Cu-In-S nanocrystals as determined by TEM. Each
histogram represents the relative population of nanocrystals at the designated time and correspond to the
images shown in Figure 4.3; (a) 1, (b) 18, (c) 23, (d) 28, (e) 33, (f) 68, (g) 128, (h) 188 min.

Nanocrystal nucleation was observed visually when the reaction solution rapidly changed
from yellow to black after several seconds of heating at 180 ˚C. Nanocrystals large
enough to be observed by TEM form 1 min after nucleation. As demonstrated in Figure
4.2a, these primitive nanocrystals are small, ill-defined in regard to shape, and
agglomerated. Growth is sustained during the initial stage, eventually producing
nanocrystals that appear uniform in size and shape after t = 33 min (Figure 4.2e). The
narrow, near-Gaussian population distribution shown in the histogram in Figure 4.3e is a
strong indication that the nanocrystals produced at this time are monodisperse. After 33
minutes have elapsed, the nanocrystals continue to grow in solution, almost doubling in
size after 180 minutes (Figure 4.2h, 4.3h). Although the population distribution remains
somewhat Gaussian, after 180 minutes the histogram appears to lean to the right,
77

indicating the population favors nanocrystals that are larger than the mean diameter
(Figure 4.3h).

Figure 4.4. Statistical analysis of the evolving wz-Cu-In-S nanocrystals. The average size of the
nanocrystals increases quickly until 33 minutes, then slows to a linear rate (a). Similarly, standard
deviation (in terms of nanometers) increases steadily throughout the course of the reaction. When standard
deviation is described in terms of percentage (c), before 33 minutes standard deviation decreases according
to a size focusing mechanism, then increases based on an Ostwald ripening mechanism. Skew (d) shifts
toward more positive values as growth continues as well, providing further evidence of an Ostwald
ripening mechanism.

The TEM images and histograms above provide an empirical and qualitative view of wz-
Cu-In-S nanocrystal growth. However a more statistical, quantitative view of nanocrystal
growth is required to determine the exact mechanism of nanocrystal growth. The
statistical analysis shown in Figure 4.4 consists of the first three standardized moments,
78

average, variance (standard deviation), and skewness of the evolving wz-Cu-In-S
nanocrystals.
As shown by the graph in Figure 4.4a, there are clearly two separate growth regimes.
From the nucleation event until t = 40 min, the nanocrystals grow quickly (k
1
= 12.5 ×
10
-2
nm min
-1
), then slow down significantly (k
2
= 2.2 × 10
-2
nm min
-1
) for the
continuation of the experiment. The histograms shown in Figure 4.3 narrow during this
time span, but then broaden throughout the course of the reaction. The variance
represented by standard deviation ( 
D
) does not seem to follow a distinguishable pattern
initially, but significantly increases after approximately 68 minutes (Figure 4.4b). By
expressing the variance as the ratio of standard deviation to mean diameter ( 
D
/ 
D
), there
appear to be two separate growth periods that coincide with the change in average
diameter. After the first minute of heating, the nanocrystals have a large standard
deviation ( 
D
/ 
D
= 24.8%) which decreases quickly until it reaches a minimum of 
D
/ 
D

= 9.1% at 33 minutes (Figure 4.4c). After this invariant region at t = 33 min, the standard
deviation of the wz-Cu-In-S nanocrystals begins to gradually increase, with the standard
deviation about the mean increasing from 
D
/ 
D
= 9.1% to 31.4% (Figure 4.4c). This
reversal directly coincides with the change in growth rate discussed above and shown in
Figure 4.4a.

4.2.3. Nanocrystal Growth Mechanism
As mentioned in Chapter 1 and again in Section 4.2.3, this investigation represents the
first quantitative investigation concerning the growth of ternary metal chalcogenide
79

nanocrystals. As a result, many of the conclusions drawn from this investigation have
been aided significantly by advancements made in simpler, binary metal chalcogenide
nanocrystals.
24,25,26
It is assumed that many of the same general principles, such as
classical nucleation theory, Gibbs-Thomson effect, diffusion-controlled growth, etc. are
still applicable and play significant roles in the evolution of more complex ternary metal
chalcogenide nanocrystals. Morphology, elemental composition, growth rate, and crystal
phase are therefore not directly caused by only one component in the synthetic scheme;
rather, a combination of kinetic variables governed by known principles leads to the
formation of monodisperse wz-Cu-In-S nanocrystals.
The statistical information presented in Figures 4.2 through 4.4 can be used to determine
the mechanism of wz-Cu-In-S nanocrystal growth. Once di-tert-butyl disulfide is
injected, the temperature is allowed to steadily increase, causing the metal and
dichalcogenide precursors to decompose and react in solution to form a supersaturated
solution of monomers. Nucleation is subsequently observed as the reaction solution
quickly turns black at 180 °C, initiating the growth of wz-Cu-In-S nanocrystals that are
observable by TEM only after 1 minute heating. The statistical analysis discussed above
during the initial growth period following nucleation reveals that (1) the average diameter
of the nanocrystals grows rapidly and (2) the variance represented by standard deviation
about the mean diameter ( 
D
/ 
D
) decreases substantially. These two observations are the
hallmarks of the size-focusing mechanism whereby rapid growth and narrowing variance
occurs simultaneously to create a monodisperse population of nanocrystals. This
mechanism was first observed by Peng as applied toward the synthesis of II-VI and III-V
nanocrystals.
27
In the case of CdSe synthesis, the nanodots had a large size distribution
80

( 
D
/ 
D
= 20%) then focus to 7.7% after 190 minutes. During this growth period, the
nanocrystals constantly grow by consuming monomers in solution while maintaining a
constant concentration of particles in solution. Refocusing can be induced simply by
replenishing the concentration of monomers in solution.
Following the initial size focusing growth period, the wz-Cu-In-S nanocrystals continue
to grow at a considerably slower rate, but their size distribution also increases. Slow
growth coupled with increasing variance is the hallmark of a steady-state Ostwald
ripening mechanism. This is the most common growth mechanism observed for metal
chalcogenide nanocrystals and has thus has been studied the most.
25,28-30
During steady-
state Ostwald ripening large particles grow by consuming smaller particles; the
population of larger nanocrystals therefore continues to grow at the expense of smaller
particles. Since smaller particles feed the growth during Ostwald ripening instead of
monomers as seen in size focusing, the standard deviation continually increases as each
particle is growing at substantially different rates. A narrow size distribution is sacrificed
in favor of size; nonetheless, high-quality nanocrystals have been synthesized by the
mechanism.
Peng witnessed a similar tandem focusing-defocusing mechanism in the growth of MnO
nanocrystals.
31
During the initial growth period, the nanocrystals increase in average size
as their standard deviation continually decreases from 
D
/ 
D
= 25% to 
D
/ 
D
<10%.
After this initial growth period (t = 30 min), the concentration of monomers has reached a
minimum. In the absence of monomers in solution, smaller particles are now consumed
by larger particles in order to continue growth. Therefore, the average size of the
81

particles continues to increase from ca. 25 nm to ca. 45 nm, but the variance significantly
broadens from <10% to >30%. Further evidence of this growth mechanism was observed
as the concentration of particles in solution rapidly decreased during the second growth
period. Although the concentration of monomers and particles could not be observed
under our reaction conditions, it is probable that similar conclusions can be drawn from
the wz-Cu-In-S system. After the initial size focusing growth period, the wz-Cu-In-S
nanocrystals continue to grow, almost doubling in size after the final sample is taken;
however, the standard deviation noticeably defocuses as it increases dramatically from
9.1% to 31.4%.
Closer analysis of the statistical data reveals further evidence of a steady-state Ostwald
ripening stage. After t = 33 min, the degree of asymmetry, or skew ( ), steadily shifts
toward more positive values. Skew represents a divergence from a normal Gaussian
distribution, and along with mean and standard deviation, is another standardized
moment used to describe the of the evolving nanocrystal population.
27
A positive skew
indicates an increased population of larger particles in the sample population relative to
the mean, while a negative skew indicates a greater population of smaller particles in the
sample population about the mean. During the final 210 min of nanocrystal growth, the
population of nanocrystals that are larger than the statistical average increases with time,
and the degree of asymmetry reaches more positive values (Figure 4.5d). Interestingly,
after 240 min of heating there are no particles that are more than double the mean particle
size; indicating that the Cu-In-S growth trend matches well with the predications
described by Lifshitz et al.
32
82

This trend can also be qualitatively visualized by the histograms shown in Figure 4.4.
The data shown in Figure 4.4f is distributed evenly about the mean (t = 68 min); there are
approximate equal amounts of nanocrystals that are smaller or larger than the average
value. The skew is therefore close to zero (  = -0.05). Once Ostwald ripening
mechanism begins to control the growth of the nanocrystals, the population of
nanocrystals that are larger than the average value increases. The histogram for the final
growth sample (Figure 4.4h, t = 188 min) is therefore considerably different compared to
the histograms that describe pre-Ostwald ripening growth. Instead of a Gaussian
distribution, there are substantially more nanocrystals that are larger compared to the
average value ( 
D
= 12.5 nm), forcing the skew to shift toward a positive value (  =
0.45).

4.2.4. Elemental Composition of the Growing Nanocrystals
Because copper and indium share lattice sites in the wurtzite structure, there is a large
degree of compositional variability for this phase. Monitoring the elemental composition
during the course of the reaction revealed that the evolving nanocrystals begin copper-
rich (Cu:In:S composition of 0.58:0.11:0.31) and remain so throughout the size-focusing
period (Figure 4.5a,b). After 33 min, the nanocrystals become more stoichiometric
during the steady-state Ostwald ripening stage of growth. The concentration of indium
and sulfur increase relative to the copper concentration until an average Cu:In:S
composition of 0.23:0.27:0.50 is reached at 188 min (Figure 4.5b). The initially high
83

concentration of copper relative to indium and sulfur may be a result of the copper
nucleating faster than the indium and sulfur.
It has been previously shown by difference FT-IR spectroscopy that In(acac)
3

decomposes slowly over the course of ca. 160 min in the presence of oleylamine at 180
˚C.
21
Similarly, the reaction between In(acac)
3
and di-tert-butyl disulfide does not form
nanocrystalline -In
2
S
3
until after 420 min in oleylamine at 180 ˚C, suggesting a similarly
slow rate of sulfur transfer.
22
Therefore, the rates of precursor decomposition may not
only influence the crystal phase and growth mechanism, but also the elemental
composition of the nanocrystals.

Figure 4.5. EDX spectrum of 6.9 nm copper-rich wz-Cu-In-S nanocrystals (a) and elemental composition
of the evolving nanocrystals determined by EDX. (b). Copper, ♦; sulfur, ●; indium, ■.

4.2.5. Departure from Focusing-Defocusing Growth Mechanism
The growth mechanism that controls the evolution of wz-Cu-In-S is dependent on the
synthetic conditions described below Section 4.4.2. Although an in-depth discussion of
84

these synthetic conditions is beyond the scope of this chapter, it is important to note that
phase-pure wz-Cu-In-S can be created independent of the focusing-defocusing
mechanism described above. The addition of 2 molar equivalents of 1-dodeceanethiol
relative to copper and indium is critical for the synthesis of monodisperse Cu-In-S
nanocrystals. When 1-dodecanethiol eliminated large, single-crystalline, hexagonal
platelets of wz-Cu-In-S are formed after 180 min (Figure 4.6a), which are polydisperse in
diameter (30-130 nm) but uniform in thickness (25.0 ± 1.9 nm). This is compared to the
size-controlled Cu-In-S nanocrystals (mean size = 12.5 nm after 180 min) when 1-
dodeceanethiol is used during synthesis. XRD analysis confirmed the large hexagonal

Figure 4.6. Structural and compositional analysis of large wz-CuInS
2
nanocrystals. Large hexagonal
platelets that are monodisperse in terms of width, but not diameter are formed without dodecanethiol and
are shown in (a). The hexagons are wurtzite phase (b) and have a near-stoichiometric ratio of Cu : In : S
(c). Each hexagonal platelet is single crystalline as evidenced by the SAED shown in (d).
85

platelets are also in the wurtzite phase (Figure 4.6b), and EDX gave an average Cu:In:S
composition of 0.29:0.24:0.47, which is very close to stoichiometric CuInS
2
(Figure
4.6c). The ratio of oleylamine/1-dodecanethiol strongly influences the overall phase and
morphology of the Cu-In-S nanocrystals. In the absence of 1-dodecanethiol, fast
nanocrystal growth is observed. This suggests that 1-dodecanethiol binds more strongly
than oleylamine to the exposed nanocrystal faces, resulting in an arrested growth
condition.
It is known for binary metal chalcogenide systems that a high initial concentration of
monomer present in the reaction solution often results in the formation of elongated or
anisotropic nanocrystals. In the synthesis of wz-Cu-In-S, the concentration of the metal
precursors are high initially ([Cu+In] = 0.44 M) and, in fact, yield the large plates shown
in Figure 4.6a. Though the plates vary significantly in diameter, they are monodisperse
in regard to their width, giving them an approximate aspect ratio (diameter/width) as high
as 5.2. Rapid, anisotropic growth is clearly preferred in this reaction system. Upon the
addition of dodecanethiol, small nanocrystals are created under otherwise similar reaction
conditions. Given that the metal precursor concentration ([Cu+In] = 0.45 M) and sulfur
loading are the same, dodecanethiol specifically acts as a retardant against rapid,
uncontrolled growth. It is possible that dodecanethiol has a lower dissociation constant
for the growing crystal compared to fatty amines.
33
The tightly bound thiolates on the
surface of the nanocrystal significantly decrease the permeability of the surrounding
diffusion sphere and impede nanocrystal growth.
34
Kinetic controlled growth of the wz-
Cu-In-S described in Sections 4.2.2 and 4.2.4 are dependent on the presence of the
growth retardant dodecanethiol.
86

4.3. Conclusions and Future Work
Di-tert-butyl disulfide proved to be an effective sulfur source to produce monodisperse
6.9-nm Cu-In-S nanocrystals in the metastable wurtzite phase. Based on the statistical
analysis shown in Figures 4.4 and 4.5, it was found that the growth kinetics described
above adhere nicely to the model proposed by Bawendi and Jensen.
35
From the
nucleation event to about t = 28 min, a size focusing event (Region III in the Bawendi-
Jensen model) occurs whereby the mean nanocrystal size increases with a concomitant
focusing of the standard deviation about the mean. At t = 33 min, a short invariant region
(Region IV) is observed where the minimum standard deviation about the mean is
achieved (i.e., 
D
/ 
D
= 9.1%). A steady-state Ostwald ripening pattern then begins to
emerge as the mean nanocrystal size increases along with a simultaneous increase in the
standard deviation about the mean (Region V). In addition to adhering to this theoretical
model, the data is qualitatively similar to the empirical growth kinetics described for
binary metal chalcogenide systems, such as (Fe
3
O
4
)
x
(Fe
2
O
3
)
1-x
,
37
MnO,
31
and CdSe
27

nanocrystals.
Given the promising future for copper indium sulfide and selenide as potential meaterials
in photovoltaic devices, substantial interest lies in understanding the synthetic and kinetic
factors that are necessary for phase-pure, monodisperse nanocrystalline products. More
specifically, understanding the decomposition mechanism of the dialkyl dichalcogenide
will allow for even greater synthetic control, and perhaps scale-up. Current progress is
being made toward applying di-tert-butyl diselenide as a potential reagent in the
kinetically controlled synthesis of Cu-In-Se. Whether the diselenide is able to promote
87

the formation of a kinetically-favored product remains to be seen. Future work will also
focus on exerting an extra level of reaction control to make compositional variants of this
Cu-In-S system to tune the band gap for potential photovoltaic applications.

4.4. Experimental Details
4.4.1. General Considerations. Copper(I) chloride (CuCl, Strem Chemicals, 99.999%),
indium(III) acetylacetonate (In(acac)
3
, Strem Chemicals, 98%), oleylamine (cis-9-
octadecenylamine, Aldrich, 70%), 1-dodecanethiol (Alfa Aesar, 98%), di-tert-butyl
disulfide (Chem Service, 99.3%), precipitated sulfur powder (Alfar Aesar, 99.5%), tert-
butyl mercaptan (TCI America, 98%), and squalane (Alfa Aesar, 98%) were all
purchased and used without further purification. Nanocrystal syntheses were performed
under nitrogen, in the absence of water and oxygen, using standard Schlenk techniques.

4.4.2. Cu-In-S Nanocrystal Synthesis.
In a typical synthesis, In(acac)
3
(0.30 g, 0.71 mmol) and CuCl (0.071 g, 0.71 mmol) were
added to a two-neck round-bottom flask fitted with a reflux condenser and rubber septum.
Oleylamine (2.32 g, 8.67 mmol) and 1-dodecanethiol (0.34 mL, 1.4 mmol) were added to
a Schlenk flask and were cycled between vacuum and nitrogen three times. The
oleylamine/1-dodecanethiol mixture was then transferred to the reaction flask containing
the metal salts via cannula. Prior to heating, the system was cycled between vacuum and
nitrogen three times, heated (10 ˚C min
-1
) to 95 ˚C, and again cycled three times between
vacuum and nitrogen to eliminate adventitious water and dissolved oxygen. Di-tert-butyl
88

disulfide (0.40 mL, 2.1 mmol) was quickly injected into the system under flowing
nitrogen, and the temperature was increased (10 ˚C min
-1
) to 180 ˚C and allowed to react
for 33 min with stirring. After cooling to room temperature, the reaction mixture was
dissolved in 2 mL of dichloromethane, sonicated, and centrifuged (6000 rpm for 15 min)
to yield a black solid. Precipitation was repeated with toluene (0.5 mL) and ethanol (10
mL) to yield the purified product. The resulting Cu-In-S nanocrystals form suspensions
in organic solvents such as hexane and toluene that are stable for several weeks.

4.4.3. Instrumentation
Powder X-ray diffraction (XRD) analyses were performed on a Rigaku Ultima IV X-ray
diffractometer using a Cu Kα radiation source (λ = 1.54 Å). Transmission electron
microscopy (TEM) and selected area electron diffraction (SAED) were performed on a
JEOL JEM-2100 microscope at an operating voltage of 200 kV, equipped with a Gatan
Orius CCD camera. SAED patterns were collected at a camera distance of 60 cm.
Energy dispersive X-ray spectroscopy (EDX) was performed on an EDAX Apollo silicon
drift detector (SDD) attached to a JEOL JSM-6610 scanning electron microscope
operating at 20 keV. Samples were deposited on an aluminum tab and ten areas were
randomly analyzed.

4.4.4. TEM Analysis of Nanocrystal Growth
Aliquots (0.10-0.25 mL) of the reaction mixture were taken by syringe at specified times
and quenched via cooling to room temperature. Nanocrystals were precipitated from the
89

reaction mixture as described above and dissolved in toluene. TEM samples were
prepared on 300 mesh carbon-coated copper grids (Ted Pella, Inc.). Each data point in
the statistical analysis represents over 350 particles.

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34. Pradhan, N.; Reifsnyder, D.; Xie, R.; Aldana, J.; Peng, X. Surface Ligand
Dynamics in Growth of Nanocrystals. J. Am. Chem. Soc. 2007, 129, 9500.
35. Rempel, J. Y.; Bawendi, M. G.; Jensen, K. F. Insights into the Kinetics of
Semiconductor Nanocrystal Nucleation and Growth. J. Am. Chem. Soc. 2009, 131, 4479.
36. Yen, B. K. H.; Stott, N. E.; Jensen, K. F.; Bawendi, M. G. A Continuous-Flow
Microcapillary Reactor for the Preparation of a Size Series of CdSe Nanocrystals. Adv.
Mater. 2003, 15, 1858.
37. Liu, Z.; Liang, J.; Xu, D.; Lu, J.; Qian, Y. A Facile Chemical Route to
Semiconductor Metal Sulfide Nanocrystal Superlattices. Chem. Commun. 2004, 2724.
92

Chapter 5. Phase, Size, and Shape Control of Tin Selenide and Tin
Sulfide Nanocrystals using Dialkyl Dichalcogenides.

5.1. Introduction
The need for efficient and cost effective photovoltaic materials continually fuels new
research in the field of metal chalcogenide semiconductor nanocrystal synthesis.
1-3

Cadmium and lead based sulfides and selenides have proven to be promising components
for photovoltaic devices; however, these materials are toxic and environmentally
hazardous. In addition, nanocrystals of CdSe are often synthesized using dimethyl
cadmium and trioctylphosphine oxide—reagents that are pyrophoric, toxic, and unstable.
4

Clearly, fabricating large quantities of these materials may have serious environmental
and safety implications. Copper and indium based nanocrystals have been utilized as mid
to low band gap photovoltaic materials.
5,6
These have shown great promise as highly
efficient components in solar cells, proving that environmentally friendly alternatives are
viable possibilities. However, given the increasing demand, cost, and scarcity of indium
and copper, the long-term feasibility of these materials as solar cell components is put
into question.
7,8

Recently, tin(II) sulfide (SnS) and tin(II) selenide (SnSe) have been explored as low cost,
nontoxic, and environmentally benign components for a variety of photovoltaic devices.
Both SnS and SnSe possess low band gaps and absorb in the infrared and near-infrared
spectral region. It may also be possible that SnS and SnSe exhibit multiexciton
generation (MEG) allowing them generate more than one electron-hole pair per absorbed
photon, potentially increasing their efficiency in solar cells.
9

93

Tin sulfide has an indirect and direct band gap of 1.09 and 1.33 eV, respectively, and can
switch between an n-type and p-type conductor based on the tin to sulfur elemental
composition.
10
Solar cells composed of SnS have a theoretical conversion efficiency of
25%.
11,12
Tin sulfide most commonly crystallizes in the orthorhombic (herzenbergite,
Pnma) structure, a severely distorted rocksalt structure composed of strongly bound
double layers that causes orthorhombic SnS to typically crystallize in large 2D sheets.
13

Thin, well-defined nanoscale square sheets composed of phase-pure orthorhombic SnS
can be formed by heating tin(II) chloride and elemental sulfur in oleylamine at 170 °C for
10 hours.
14
Adjusting the reaction conditions can afford orthorhombic SnS of different
shapes and sizes. For example, agglomerates of elongated SnS nanocrystals can be
created using metallic tin formed in situ and elemental sulfur dissolved in diglyme at 160
°C.
11
Monodisperse, spherical or triangular nanocrystals have been recently synthesized
using Sn[N(SiMe
3
)
2
]
2
, trioctylphosphine, oleic acid, octadecene and thioacetamide as the
sulfur source.
10
These small SnS nanocrystals exhibited curious optoelectronic properties
as their indirect band gap (E
g
= 1.6 eV) was determined to be significantly blue-shifted
compared to the bulk (E
g
= 1.09 eV).
10
Most recently, individual quantum dots of SnS
have been synthesized from tin(II) bromide, sodium sulfide, and various ethanolamines.
The individual nanocrystals are small enough (3.2 ± 0.5 nm) to cause quantum
confinement as evidenced by their large direct band gap (E
g
~ 1.65 eV).
15

The synthesis of nanoscale SnSe has received less attention compared to other low or mid
band gap semiconductor materials. Bulk SnSe has an indirect band gap of 0.90 eV and a
direct band gap of 1.30 eV, while SnSe thin films that exhibit quantum confinement
posses indirect and direct band gaps at approximately 1.23 eV and 1.74 eV,
94

respectively.
16,17
Tin selenide has been explored as a potential material for optical
recording, solar cells, sensors, lasers, and thermoelectric cooling.
16
Similar to SnS, SnSe
commonly crystallizes in the orthorhombic polymorph (Pnma) that consists of strongly
bound double layers.
13
Thin films composed of SnSe have been synthesized by chemical
bath deposition from tin(II) chloride and sodium selenosulfate precursors.
17
The material
was comprised of 14.8 nm SnSe nanocrystalline domains (as estimated by XRD patterns)
and showed indirect and direct band gaps that were blue-shifted compared to bulk values,
indicating that the nanocrystalline domains were small enough to achieve quantum
confinement.
17
To date, there have been no reports of individual, phase-pure SnSe
nanocrystals synthesized by a solution-phase technique that are small enough to exhibit
quantum confinement.
As the previous chapters have described, dialkyl dichalcogenides have proven to be
valuable precursors for solution-phase synthesis of a variety of metal chalcogenide
nanocrystals.
18-20
Tin telluride (SnTe) has been synthesized previously by a reaction
between metallic tin and diphenyl ditelluride; however, this method required isolating a
molecular precursor that needed to be decomposed at 300 ºC to afford a crystalline
product.
21
This chapter reports the synthesis and optoelectronic investigation of phase-
pure SnSe nanocrystals (19.0 ± 5.1 nm) using stoichiometric amounts of di-tert-butyl
diselenide as the selenium source that possesses a band gap (E
g
= 1.71 eV). This method
can be adapted for the synthesis of phase-pure nanocubes composed of orthorhombic
(herzenbergerite) tin sulfide ( -SnS) using di-tert-butyl disulfide as the sulfur source.

95

5.2. Results and Discussion
5.2.1. Synthetic Methodology
Peroxides and disulfides have proven to be potent and versatile chalcogenide sources for
the synthesis of III-VI and I-III-VI nanocrystals.
18-20
The results of this chapter indicate
that this methodology proves useful for the synthesis of tin-based IV-VI nanocrystals as
well. Di-tert-butyl disulfide and diselenide were chosen because (i) they are liquids that
are soluble in common organic solvents, and (ii) they possess relatively weak E-E and C-
E (E = S or Se) bonds that should aid in relatively low temperature decomposition and
chalcogenide transfer. The synthetic strategies employed in Chapters 2, 3 and 4 were
applied here, the successful reaction schemes that lead to high quality, phase-pure SnSe
and SnS are shown in Schemes 1 and 2, respectively. The presence of a dialkyl disulfide
chalcogenide source was found to play a critical role in the formation of well-defined
phase-pure SnSe and SnS. However, morphology, elemental composition, and crystal
phase are not a direct result of the presence of the dialkyl dichalcogenide—but, as will be
described in this section—are a combination of variables that lead to the formation of the
desired products.
Di-tert-butyl disulfide is known to thermally decompose in the gas phase to yield
isobutylene, hydrogen sulfide, and a small amount of elemental sulfur.
22
Under the
conditions shown in Scheme 5.2, the disulfide is activated in solution to release a sulfur
species which then reacts with Sn(II). As mentioned in Chapter 3, the exact mechanism

96

Scheme 5.1. Diselenide-mediated synthesis of tin selenide (SnSe) nanocrystals

Scheme 5.2. Disulfide-mediated synthesis of tin sulfide (SnS) nanocrystals

of disulfide decomposition remains unclear. The dialkyl disulfide may either cleave
symmetrically ( H
bS-S
= 226 kJ mol
-1
) or asymmetrically across the C-S bond (H
bC-S
=
272 kJ mol
-1
) to form a reactive sulfur species that can attack the metal precursors.
22,23

Given the longer reaction time (compared to Scheme 5.1) and excess disulfide required to
form SnS and -In
2
S
3
(see Chapter 3) it may be possible that the reactive sulfur species is
formed via C-S bond cleavage, the less thermodynamically preferred route.
In stark contrast, the synthesis of SnSe from di-tert-butyl diselenide required only 5
minutes to create a well-defined crystalline product. Selenols are more acidic and
nucleophilic compared to thiols, which may increase their reactivity toward metal
centers. Though there is little known about the decomposition of organodiselenides, the
average C-Se bond strength for alkyl selenides is lower compared to the C-S bond
strength ( H
bC-Se
= 234 kJ mol
-1
).
24
The short reaction time required to make SnSe
compared to SnS might indicate that the dialkyl dichalcogenides cleave along the C-E
97

bond. Thermodynamically, the formation of SnSe is favored ( H
fSnSe
= 35 kJ mol
-1
)
compared to SnS ( H
fSnS
= 108 kJ mol
-1
) which may also explain the difference in
reaction time.
24,25
However, given that both systems require heating to 180 °C to form a
crystalline product, this hypothesis is rather speculative. While excess disulfide is
required to form crystalline SnS and -In
2
S
3
, only a stoichiometric amount of diselenide
is required to form SnSe. In fact, adding excess diselenide results in the formation of
crystalline SnSe
2
and Se. It is interesting to note that the SnSe reaction solution turns
black at 125-130 °C after adding the diselenide, but a color change is not observed for the
analogous SnS system until approximately 1 hour heating at 180 °C. This suggests
relatively rapid nucleation for SnSe formation as compared to slow nucleation for SnS
formation.
Dodecanethiol has proven to be an effective yet passive surfactant in Chapter 4 that
efficiently retards the growth of Cu-In-S nanocrystals without directly donating sulfur.
20

Amines are known to form strong complexes with Sn(II) compounds, however a greater
level of control is desired to achieve smaller particles.
26
Dodecanethiol again proves to
be useful in controlling the growth of SnSe and SnS in solution. It was recently reported
that thiolates coordinate tightly (1283 kJ mol
-1
) to the surface of growing CdSe
nanocrystals compared to amines (86.8 kJ mol
-1
).
27
It is possible that long-chain thiolates
may also coordinate more strongly to SnSe than amines. To test this hypothesis
dodecanethiol is omitted from the reaction solution. The SnSe products become ill-
defined, microscale particulates that are poorly soluble in organic solvents. Similarly,
when dodecanethiol is omitted from the SnS synthetic scheme, the crystals tend to grow
in large 2D sheets. While dodecanethiol directly controlled the size of the growing Cu-
98

In-S nanocrystals, this same surfactant instead controls the shape and morphology of tin
chalcogenides.
Oleylamine is by far the most common alkyl amine used in nanocrystal synthesis.
However, long-chain amines are prone to decomposition in the presence of air; thus
substantially decreasing it reliablility.
28
Replacing dodecylamine with oleylamine in
Scheme 5.1 failed to produce a crystalline product. Similarly, when dodecylamine is
replaced by oleylamine in Scheme 5.2, large sheets of SnS are formed instead. It is
interesting to note that altering the surfactant mixture can also affect the phase of SnS.
When dodecanethiol is replaced with an equimolar amount of lauric acid, the zinc blende
phase of SnS is a significant byproduct. In addition, trioctylphosphine causes the
formation of a rare orthorhombic polymorph of SnS. Considering there are few reports
of these alternate phases of SnS,
14,29,30
this phenomenon requires future study.

5.2.1. Structural Analysis
To confirm the composition of the SnSe nanocrystals, XRD was performed and the
corresponding diffraction pattern is shown in Figure 5.1a. SnSe commonly crystallizes in
the orthorhombic polymorph (Pnma), which consists of a series of strongly bound double
layers.
13
The resulting diffraction pattern shown in Figure 5.1a (a = 11.55 Å, b = 4.16 Å,
c = 4.45 Å) matches well with the literature values for this phase of SnSe (PDF# 00-048-
1224), without evidence of crystalline SnO, SnO
2
, SnSe
2
or Se

byproducts. As described
above, an analogous synthetic scheme produced crystalline SnS using di-tert-butyl
disulfide as the chalcogenide source. XRD was again used to confirm that the
99

orthorhombic (herzenbergite, PDF# 01-073-1859) phase of SnS (a = 11.24 Å, b = 4.37 Å,
c = 4.02 Å) was formed without the presence of other crystalline byproducts (Figure
5.1b).

Figure 5.1. XRD pattern of SnSe (a) and SnS (b) nanocrystals synthesized using di-tert-butyl
dichalcogenides at 180 °C. Calculated peak positions for SnSe (red) and SnS (blue) are shown and the
high-intensity experimental peaks are labeled appropriately.

The elemental composition of both SnSe and SnS were confirmed to be stoichiometric
according to EDX. The EDX spectrum of the SnSe nanocrystals provided in Figure 5.2a
shows they are composed of 51.9 mol% Sn and 48.1 mol% Se. Establishing a 1:1 ratio is
important considering Sn-rich tin chalcogenides are known to have different electronic
properties compared to their stiochiometric counter parts.
10
Since thiols have been
proven to be effective sulfur donors for nanocrystal synthesis,
31,32
the concentration of
sulfur was also determined relative to the concentration of Sn and Se. Sulfur was found
only to comprise 1.73 mol % in relation to Sn and Se, indicating that dodecanethiol likely
coordinates to the surface of the nanocrystals, rather than donating the sulfur to form
SnS
x
Se
1-x
. EDX again confirms that the SnS product is stoichiometric (51.9 mol% Sn,
48.1 mol% S). Carbon and nitrogen are also apparent in the both EDX spectra (ca. 0.27
100

and 0.39 keV, respectively) which are likely due to the presence of dodecylamine and
dodecanethiol capping agents.

Figure 5.2. EDX spectra of SnSe (a) and SnS (b) nanocrystals synthesized using the respective
dichalcogenide precursor.

Even though SnSe and SnS were synthesized using similar schemes and have the same
orthorhombic crystal structure, they are morphologically quite different. TEM images of
SnSe and SnS nanocrystals are shown in Figures 5.3 and 5.4. As shown by the TEM
image in Figure 5.3a, the SnSe nanocrystals produced by the Scheme 5.1 are somewhat
elongated structures that are polydisperse by length, but fairly monodisperse by width (19
± 5.1 nm; = 26.8%) The high-resolution TEM image in Figure 5.3b displays the [111]
zone axis with lattice spacings that conform well to the literature values of the
orthorhombic crystal structure. In addition, the SAED pattern in Figure 5.3c also
matches well with known values and the XRD pattern in Figure 5.1a.
101

Figure 5.3. TEM image of SnSe nanocrystals (a), HRTEM of one SnSe nanocrystal showing the [111]
crystal plane (b) and SAED pattern of a cluster of nanocrystals (c).

As mentioned above, using excess diselenide in the synthesis of SnSe nanocrystals
resulted in the formation of SnSe
2
. To further confirm that the nanocrystals produced by
Scheme 5.1 are composed of SnSe, XPS analysis was also performed. As shown in the
survey spectrum in Figure 5.4a, all the peaks of the XPS spectrum can be assigned to tin,
selenium, carbon, or nitrogen. A relatively weak O 1s peak (~531 eV) is likely due to
slight amounts of atmospheric oxygen absorbed by the sample during preparation or
atmospheric oxygen trapped in the sample chamber. The high-resolution spectrum of the
shown in Figure 5.4b provides further proof that the nanocrystals are composed of
divalent tin as the assigned Sn 3d
5/2
peak is located at 487.50 eV and separated from the

Figure 5.4. XPS spectra of SnSe nanocrystals. Survey (a) and high-resolution Sn 3d (b) and Se 3d (c)
spectra.
102

Sn 3d
3/2
peak by 8.48 eV. Tetravalent tin peaks lie at high energies compared to divalent
peaks; no peaks in Figure 5.4b can be assigned to this oxidation state of tin. A high-
resolution spectrum of selenium peak (Figure 5.4c) clearly shows both the Se 3d
5/2
and Se
3d
3/2
near 55.5 eV, which is typical for metal diselenides.
The analogous reaction using di-tert-butyl disulfide shown in Scheme 5.2 produce much
larger SnS nanocrystals. As shown by the TEM image in Figure 5.5a, SnS crystallizes as
nanoscale squares and rectangles. Though they are uniform in shape, they vary
significantly by size, which is found to be highly sensitive toward the reaction conditions
outlined in Scheme 5.2. For example, decreasing the dodecanethiol by one third results
in the formation of substantially larger squares, cubes and rectangles that are large
enough to be seen by SEM (Figure 5.5b). Further investigation as to the mechanism of
controlled growth is clearly warranted.

Figure 5.5. TEM (a) and SEM (b) images of crystalline SnS products.

103

5.2.3. Optoelectronic Analysis
As noted above, thin films composed of SnSe (14.8 nm nanocrystalline domains) were
synthesized by chemical bath deposition.
17
The optical band gap of these films was
determined to be greater than the bulk band gap energies for SnSe (E
g
= 1.74 eV),
indicating that the nanocrystalline domains were small enough to achieve quantum
confinement. SnSe synthesized using di-tert-butyl diselenide were dissolved in
cyclohexane to form a brown solution. According to the spectrum shown in Figure 5.6,
the SnSe nanocrystals have a direct optical band gap of E
g
= 1.71 eV, which is blue-
shifted compared to bulk SnSe direct band gap values (E
g
= 1.30 eV), proving these
individual nanocrystals are small enough to exhibit quantum confinement.

Figure 5.6. UV-vis-NIR spectrum of SnSe nanocrystals with band gap determination shown in the inset.

5.3. Conclusions and Future Work
Chapters 2, 3, and 4 successfully describe the ability of dialkyl dichalcogenides to serve
as effective precursors for indium-based chalcogenide nanocrystals. Clearly, well-
104

defined, phase-pure nanocrystals composed of tin selenide and tin sulfide can be made
efficiently by employing these same dichalcogenide precursors. The differences in
chemical reactivity between the disulfide and diselenide are clearly shown by the
substantially different SnS and SnSe nanocrystal products. This chapter represents the
basic structural and optoelectronic characterization of the SnSe and SnS nanocrystals.
Similar to previous chapters, the mechanism of growth can be determined for these
systems to further understand the origin of their unique products. More specifically,
better control of the SnS synthetic pathway may produce smaller, more monodisperse
SnS nanocubes. Likewise, the synthesis of SnSe could be further improved by increasing
the time between nucleation and crystal growth. As mentioned above, tin-based
chalcogenides are currently being explored as an intriguing environmentally-friendly
components for solar cells. Based on the preliminary optoelectronic characterization
(Section 5.2.3), fabrication of hybrid SnSe-based solar cells is currently underway.

5.4. Experimental Details
5.4.1. General Considerations
Magnesium turnings (Mallinkrodt, 99.99%), tert-butyl bromide (Alfa Aesar, 98%),
ammonium chloride (EMD Chemicals Inc., 98%), anhydrous tin chloride (SnCl
2
, Strem,
98%), dodecylamine (Alfa Aesar, 98+%), dodecanethiol (Alfa Aesar, 98+%), and di-tert-
butyl disulfide (Chem Service, 99.3%) were all purchased and used without further
purification. Nanocrystal syntheses were performed using standard Schlenk techniques
under nitrogen and in the absence of water and oxygen.
105

5.4.2. Synthesis of Di-tert-Butyl Diselenide
Di-tert-butyl diselenide was synthesized according to an improved version of a
previously published method.
33
Briefly, magnesium turnings (10.75 g, 0.44 mol) were
allowed to stir for 48 hours in diethyl ether (200 mL) under nitrogen. tert-Butyl bromide
(50 mL, 0.44 mol) was added slowly under nitrogen to the ether solution and allowed to
stir for 30 minutes, producing a gray solution. Selenium (31.74 g, 0.40 mol) is
subsequently added and allowed to stir for an additional 30 minutes. The solution is
chilled in ice and an aqueous solution of ammonium chloride (17 g, 0.33 mol dissolved in
50 mL distilled water) was slowly added in air. After 10 minutes, the excess solid
magnesium was filtered off and the organic solution was isolated from the aqueous layer
in a separatory funnel by combining with hexanes (50 mL), washing four times with
additional aqueous ammonium chloride and drying over magnesium sulfate. The organic
solvents were removed by evaporation under reduced pressure to produce a pungent
yellow-orange liquid. After vacuum distillation at 40 ºC, the product was characterized
by
1
H,
13
C, and
77
Se NMR in CDCl
3
.
1
H:  = 1.45 (s);
13
C:  = 41.6 (s), 32.5 (s);
77
Se:  =
488.1 (s). GC-MS confirms the molecular weight (M
+
-H = 273.9 m/z).

5.4.3. SnSe Nanocrystal Synthesis
In a typical synthesis, SnCl
2
(0.14 g, 0.74 mmol) was added to a two-neck round-bottom
flask equipped with a reflux condenser, stir bar, and rubber septum. Deaerated and dried
dodecylamine (2.50 mL, 10.86 mmol) and dodecanethiol (0.50 mL, 2.10 mmol) were
added to the SnCl
2
under nitrogen and heated (10 ºC min
-1
) to 95 ºC. The colorless
106

solution is briefly degassed under vacuum prior to adding di-tert-butyl diselenide (70 L,
~0.38 mmol) via syringe under nitrogen. The temperature is increased to 180 ºC and
allowed to react for 5 minutes with constant stirring to produce a black solution. After
cooling to room temperature, the reaction mixture is dissolved in dichloromethane (2
mL), then ethanol (15 mL) was added and the mixture was sonicated and centrifuged
(6000 rpm for 10 minutes) to yield a black solid. Precipitation was repeated with toluene
(1 mL) and ethanol (15 mL) to yield the purified product which is easy dispersed in a
variety of organic solvents.

5.4.4. SnS Nanocrystal Synthesis
Similar to above, SnCl
2
(0.14 g, 0.74 mmol) was added to a two-neck round-bottom flask
equipped with a reflux condenser, stir bar, and rubber septum. Deaerated and dried
dodecylamine (2.00 mL, 8.70 mmol) and dodecanethiol (0.35 mL, 1.47 mmol) were
added to the SnCl
2
under nitrogen and heated (10 ºC min
-1
) to 95 ºC. The colorless
solution is briefly degassed under vacuum prior to adding di-tert-butyl disulfide (0.40
mL, 1.91 mmol) via syringe under nitrogen. The temperature is increased to 180 ºC and
allowed to react for 6 hours with constant stirring to produce a dark gray solution. After
cooling to room temperature, the reaction mixture is dissolved in dichloromethane (2
mL), then ethanol (15 mL) was added and the mixture was sonicated and centrifuged
(6000 rpm for 10 minutes) to yield a dark gray solid. Precipitation was repeated with
toluene (1 mL) and ethanol (15 mL) to yield the purified product which is easy dispersed
in a variety of organic solvents.
107

5.4.5. Structural Characterization
Powder X-ray diffraction (XRD) analyses were collected on a Rigaku Ultima IV
diffractometer using Cu Kα radiation source (λ = 1.54 Å). Transmission electron
microscopy (TEM) and selected area electron diffraction (SAED) analysis were
performed on a JEOL JEM-2100 microscope at an operating voltage of 200 kV equipped
with a Gatan Orius CCD camera. SAED patterns were collected at a camera distance of
60 cm. Samples for TEM and SAED were prepared from dilute purified samples
dissolved in toluene and deposited on 300 mesh Formvar-coated copper grids (Ted Pella,
Inc.). Energy dispersive X-ray spectroscopy (EDX) was performed on a JEOL JSM-6610
scanning electron microscope operating at 20 keV and equipped with an EDAX Apollo
silicon drift detector (SDD). Multiple regions of a sample deposited on an aluminum
stub were analyzed and averaged. XPS analysis was performed on a Phi ESCA2703
equipped with an Al monochrometer and flood gun.

5.4.6. Electronic Characterization
Nanocrystal suspensions in cyclohexane were analyzed by UV-vis-NIR spectroscopy in 1
cm path length quartz cuvettes. Spectra were acquired on a Cary 14 spectrophotometer in
dual beam mode.

108

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(PhSe)
4
Sn.‖ Inorg. Chem. 2002, 41, 6001.
22. Bock, H.; Mohmand, S. Unstable Intermediates in the Gaseous Phase: The
Thermal Decomposition of Alkyl Sulfides RS
2
R. Angew. Chem., Int. Ed. 1977, 16, 104.
23. Martin, G.; Barroeta, N. Gas-Phase Thermolysis of Sulphur Compounds. Part I.
Di-t-butyl Disulphide. J. Chem. Soc., Perkin Trans. 2 1976, 1421.
24. Chizhikov, D. M.; Shchastlivyi, V. P. Selenium and Selenides. Collet’s Publishers
Ltd. London, 1968.
25. Mills, K. C. Thermodynamic Data for Inorganic Sulphides, Selenides and
Tellurides. Butterworths. London, 1974.
26. Kovalenko, M. V.; Heiss, W.; Shevchenko, E. V.; Lee, J.-S.; Schwinghammer,
H.; Alivisatos, A. P.; Talapin, D. V. ―SnTe Nanocrystals: A New Example of Narrow-
Gap Semiconductor Quantum Dots.‖ J. Am. Chem. Soc. 2007, 129, 11354.
27. Schapotschnikow, P.; Hommersom, B.; Vlugt, T. J. H. ―Adsoprtion and Binding
of Ligands to CdSe Nanocrystals.‖ J. Phys. Chem. C 2009, 113, 12690.
28. Belman, N.; Israelachvili, J. N.; Li, Y.; Safinya, C. R.; Bernstein, J.; Golan, Y.
―Reaction of Alkylamine Surfactants with Carbon Dioxide: Relevance to Nanocrystal
Synthesis.‖ Nano Lett. 2009, 9, 2088.
29. Brownson, J. R. S.; Georges, C.; Levy-Clement, C. ―Synthesis of a -SnS
Polymorph by Electrodeposition.‖ Chem. Mater. 2006, 18, 6397.
110

30. del Bucchia, P. S.; Jumas, J.-C.; Maurin, M. ―Contribution à l’Etude de Composés
Sulfurés d’Etain(II) Affinement de la Structure de SnS.‖ Acta Cryst. 1981, B37, 1903.
31. Choi, S. H.; Kim, E. G.; Hyeon, T. ―One-Pot Synthesis of Copper-Indium Sulfide
Nanocrystal Heterostructures with Acorn, Bottle, and Larve Shapes.‖ J. Am. Chem. Soc.
2006, 128, 2520.
32. Han, W.; Yi, L.; Zhao, N.; Tang, A.; Gao, M.; Tang, Z. ―Synthesis and Shape-
Tailoring of Copper Sulfide/Indium Sulfide-Based Nanocrystals.‖ J. Am. Chem. Soc.
2008, 130, 13152.
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J.; Uzar, H.; Zhang, X.; Zhu, Z. ―Allium Chemistry: Synthesis, Natural Occurrence,
Biological Activity, and Chemistry of Se-Alk(en)ylselenocysteines and Their -Glutamyl
Derivatives and Oxidation Products.‖ J. Agric. Food Chem. 2001, 49, 458.

111

Appendix. Spectroscopic Evidence of the Formation of Goldfingers*
*Published in part in Inorg. Chem. 2008, 47, 3928.

A.1. Introduction
A.1.1. Gold-based Drugs
Au(I) complexes, such as those shown in Scheme A.1, have been used in the treatment of
rheumatoid arthritis for over 80 years,
1
but their mechanism(s) of action are still only
poorly understood.
2-5
Au(I) is a soft, thiophilic metal ion that preferentially coordinates
to soft nonmetals, such as sulfur. Therefore, Au(I) drugs readily undergo ligand
exchange to interact with cysteine under physiological conditions.
4,6,7
As an example,
serum albumin, an abundant transport protein known to carry metal ions and other drugs
throughout the body, coordinates to Au(I) through the cysteine residue at position 34.
8,9

The ability of gold to inhibit thiolate-dependent enzymes has been demonstrated as well.
Au(I) inhibits the cathepsin family of lysosomal cysteine proteases,
10,11
disulfide
reductases,
12-14
kinases
15-16
and protein tyrosine phosphatases
17,18
at physiologically
relevant concentrations. Crystal structures of several Au(I)-enzyme adducts have been
solved, indicating that Au(I) binds to the active site cysteine residues of these enzymes
and exerts its inhibitory effect by directly blocking the activity of the enzymes.
19,20
For
example, the crystal structure of myochrysin in complex with cathepsin K shows that
Au(I) retains one of its original thiomalate ligands while exchanging the other for the
active site cysteine thiolate from the enzyme.
20
The crystal structure of Au(I) bound to
human glutathione reductase provides another illustrative example.
19
In this case, both of
112

the original ligands are exchanged for protein-derived thiolate ligands, resulting in a
Au(I)-enzyme complex where the Au(I) is bound in a linear, two-coordinate geometry to
cysteine 58 and cysteine 63 from the enzyme active site. Although interactions of Au(I)
with thiolate-dependent enzymes can account for many of the biological effects of the
Au(I) drugs, it is likely that there are other biological targets of Au(I) as well.

Scheme A.1. Chemical structures of auranofin (left) and aurothiomalate (right).

A.1.2. Zinc Fingers as Potential Au(I) Coordination Sites
The biological effects of Au(I) are consistent with the decreased transcription of several
genes
21
and there is growing evidence that the anti-inflammatory effects of Au(I) may be
mediated through interactions with transcription factors. Transcription factors are
proteins that bind to specific regions of DNA thorugh DNA binding domains in order to
regulate transcription—the translation of a DNA sequence into mRNA. Au(I) has been
shown to interfere with the activity of both metal-independent transcription factors and
metal-responsive transcription factors. For example, Au(I) promotes DNA binding of the
Nrf2/small Maf transcription factor dimer, resulting in an antioxidative stress response in
the cell, although it is not clear how the Au(I) compounds effect this activation.
22
In
addition, auranofin and aurothiomalate have been shown to inhibit AP-1 and NF- B
113

transcription factors which directly control the production of proinflammatory genes.
22

On the other hand, Au(I) activates the copper-responsive promoter copA,
23
presumably
by mimicking biological Cu(I) and coordinating to its binding site. Cu(I) efflux is
therefore promoted, potentially lowering the concentration or reactive oxygen species in
arthritic tissue.
23
Au(I) has also been shown to interact with zinc finger transcription
factors, inhibiting DNA binding.
24,25

The zinc finger family of transcription factors employ a tetrahedrally coordinated zinc ion
to achieve the secondary structure necessary for DNA binding.
26-30
The zinc binding site
in canonical zinc fingers has two cysteine-derived thiolate ligands and two histidine-
derived nitrogen ligands coordinated to the metal ion (a CCHH zinc finger).
31

Biologically relevant variations of this sequence include CCHC, where the Zn(II) ion is
tetrahedrally bound to three cysteines and one histidine, and CCCC, where all four
binding sites are occupied by cysteine residues.
31,32
Based on the thiophilicity of Au(I),
as well as its tendency to form 2-coordinate, linear complexes, it is reasonable to propose
that the Au(I) ion would coordinate to the two cysteine residues in the CCHH zinc finger,
distorting the canonical zinc finger secondary structure and abrogating DNA binding
activity. Zinc fingers with CCHC and CCCC coordination sites could also presumably
bind Au(I), although the stoichiometry of these interactions has not been studied. A 26-
amino acid consensus zinc finger sequence has been identified as a model for metal
binding to the full length transcription factors and is often used in studies of metal
binding and the effect of metal ions on zinc finger secondary structure.
31
The consensus
sequences for the CCHH, CCHC and CCCC zinc fingers are shown in Figure A.1.
114

Figure A.1. Peptide sequence of the model zinc finger peptides named CCHH, CCHC, and CCCC.
Secondary structure is shown below: blue arrows = beta sheet, red arrows = a-helix. The tetrahedral zinc
coordination sphere is created by the cysteine (C) and histidine (H) residues. Conserved loop is denoted by
a curved, black line.

Zinc fingers have been shown to coordinate to a wide variety of biologically relevant
metals, including Co(II)
31
, Fe(II)
33
, and Ca(II)
34
with varying K
d
values. The interactions
of exogenous metal ions with zinc fingers have also been explored to better understand
the toxicity of metals such as Cd(II)
31
, Ni(II)
33,35
Pb(II)
36,37
, and Hg(II)
38
. Recently, Au(I)
from aurothiomalate (AuTM) has been shown to coordinate to a CCHH zinc finger
peptide and to inhibit formation of a zinc finger-DNA complex.
25
In addition, ESI-MS
experiments indicate that Au(I) has a stronger affinity for this zinc finger than Zn(II)
(K
d
Zn
/K
d
Au
= 4.2).
25
Based on these studies, it has been proposed that Au(I) could replace
Zn(II) in zinc finger proteins at physiologically relevant concentrations.
Although Au(I) has been shown to interact with the CCHH zinc finger motif and inhibit
the DNA binding activity of the transcription factor, many questions about their
relationship still remain. Given that Au(I) binds specifically to Cys 34 in serum albumin,
a protein with many cysteine residues, can Au(I) coordinate to CCHC and CCCC zinc
fingers? What is the stoichiometry of these interactions? How does Au(I) coordination
affect the secondary structure of the zinc finger? In order to better understand the Au(I)-
zinc finger interaction and to address the questions posed here, a spectroscopic
115

investigation was performed to determine the stoichiometry and structure of biologically
relevant Au(I)-zinc finger complexes.

A.2. Results and Discussion
A.2.1. Formation and Stoichiometry of Goldfingers.
One of the major challenges in the study of Au(I)-biomolecule adducts is the
―spectroscopically quiet‖ d
10
electron configuration of the ion.
39
However, absorbances
attributable to ligand to metal charge transfer bands have been identified at short
wavelengths for several d
10
metal ions bound to cysteine residues in proteins.
36,40-42

Au(I)-thiolate complexes have also been shown to exhibit ligand to metal charge transfer
(LMCT) bands in the UV,
43
therefore the formation of Au(I)-thiolate complexes can be
followed by monitoring the formation of a shoulder at 310 nm. In order to follow the
formation of goldfingers, we chose to use Et
3
P-Au-Cl, a known metabolite of auranofin
that does not contain a thiolate ligand (see Scheme A.2).
2

Scheme A.2. Metabolism of auranofin in stomach acid.

The phosphine-based reductant tris(2-carboxyethyl)phosphine (TCEP) was used to
maintain the fully reduced state of the cysteine residues in the peptides without addition
of exogenous thiolates that would interfere with the electronic spectra. As illustrated in
116

Figure A.2, Et
3
P-Au-Cl binds readily to the zinc finger peptides that have been reduced
with excess TCEP. Initially, no increase in absorbance was observed until all of the
unreacted TCEP becomes saturated with Au(I). A steady increase in absorbance was
then observed as more Au(I) was added the peptide. When the peptide becomes saturated
with Au(I), the absorbance remains constant. As shown in Figure A.2a, the absorbance
of an Au(I)-CCHH mixture increases until a one to one molar ratio was reached, after
which, further additions of Et
3
P-Au-Cl result in no subsequent increase in absorbance.
The observation that one equiv of Au(I) can bind to each equivalent of CCHH is
consistent with the data in the literature.
25

Figure A.2. Titrations of reduced zinc finger peptides with Et
3
P-Au-Cl monitored by UV-vis. The insets
show the increase in absorbance at  = 310 nm as a function of Au(I) added: (a) a solution of 1.34 × 10
-4
M
CCHH coordinates 1 mol equiv of Au(I); (b) CCCC (1.42 × 10
-4
M) binds 2 molar equivalents of Au(I),
while (c) CCHC (1.50 × 10
-4
M) binds 1.5 equivalents of Au(I).

Control experiments where Et
3
P-Au-Cl was titrated into a solution containing oxidized
CCHH peptide (where the two cysteine residues are involved in disulfide bonds) show
little change in the absorption spectrum, indicating that Au(I) coordination to CCHH is
dependent on the presence of free thiols (data not shown). To ensure that growing
absorbance at 310 nm was a result of Au(I)-peptide coordination, Et
3
P-Au-Cl was titrated
into a solution of reduced and oxidized TCEP. A solution of Et
3
P-Au-Cl added to a
solution of fresh, non-oxidized TCEP (one to one molar ratio) produces the spectra
117

shown in Figure A.3a. A simple ligand replacement producing either TCEP-Au-Cl or
[Et
3
P-Au-TCEP]
+
[Cl]
-
likely causes the absorbance peak located below 240 nm;
however, this absorbance is far weaker and considerably blue-shifted compared to the
spectra produced by Au(I)-peptide coordination. Likewise, Et
3
P-Au-Cl titrated into a
solution of oxidized TCEP to produce a weak, broad absorbance at 335 nm shown in
Figure A.3b. In addition, Et
3
P-Au-Cl alone does not produce significant absorbance and
TCEP does not absorb at all from in the range of 240-490 nm (see Figures A.3c,d).

Figure A.3. Electronic investigations of reaction solution. (a) Et
3
P-Au-Cl is titrated into a solution of
TCEP (1.60 × 10
-4
M). Absorbance increases until a one to one (Et
3
P-Au-Cl to TCEP) ratio is reached.
Equimolar amounts of Et
3
P-Au-Cl is added to a solution of oxidized TCEP (1.60 × 10
-4
M) to produce the
spectrum shown in (b). Absorbance spectra of 5.22 × 10
-5
M Et
3
P-Au-Cl (c) and 5.22 × 10
-5
M TCEP (d).

Previous investigations of metal binding to CCCC zinc fingers invariably show that
metals coordinate to this peptide model in a one to one ratio, as seen with CCHH.
32,36,37,44

The same has also been observed for CCHC.
31-33,36,37,44
However, Au(I) has a strong
118

tendency to form two-coordinate, linear complexes.
4
The titration of the reduced CCCC
peptide with Et
3
P-Au-Cl is shown in Figure A.2b. In this case, a two to one Au(I) to
CCCC ratio was observed, independent of peptide concentration, TCEP concentration
and titration increment. To our knowledge this is the first example of multiple metal ions
binding to one zinc finger peptide. Previous studies of the metal binding properties of
CCCC have focused on metal ions capable of adopting a four-coordinate ligand
environment. Although tetrahedral Au(I) complexes are known, linear conformations are
strongly preferred, especially in the presence of anionic ligands such as thiolates.
4

The titration of reduced CCHC with Et
3
P-Au-Cl is shown in Figure A.2c. The binding
curve indicates that Au(I) coordinates to CCHC in a non-integer ratio. The absorbance at
310 nm increases until approximately 1.5 equivalents of Et
3
P-Au-Cl have been added.
One possible explanation for the observation that two peptides seem to bind three Au(I)
ions is the formation of two intramolecular Cys-Au-Cys complexes and one
intermolecular Cys
A
-Au-Cys
B
complex. This type of coordination has been observed in
metal binding studies of flexible peptide models.
45
For all three peptides the ratio of
Au(I) coordination was maintained regardless of the starting TCEP concentration or
peptide concentration, as shown in Table A.1.
Although Au(I) is known to bind histidine residues as well as cysteine residues in
proteins,
46,47
the affinity of Au(I) for histidine residues is considerably lower. In fact, it
has been found that histidines can be replaced by norleucine residues in a CCHH binding
motif with no decreased effect on Au(I) binding.
23
However, because the UV-vis data
discussed above employ a thiolate to gold charge transfer band to measure the
119

stoichiometry of goldfinger formation, we cannot rule out the possibility that additional
equivalents of Et
3
P-Au-Cl might react with the histidine residues from the CCHH and
CCHC peptides. Based on the consistent stoichiometries obtained for the three peptides
(one equivalent of gold binds for each two equivalents of cysteine in each case,
regardless of the availability of histidine) and previous ESI-MS studies of gold binding to
CCHH peptides, the predominant adducts seem to be cysteine-Au(I)-cysteine adducts.
25

Table A.1. Coordination of Au(I) is Independent of Peptide and TCEP Concentration.
Variant Trial [Peptide] (M) mol eq. TCEP Au(I) bound
CCHH A 3.29 × 10
-4
1.5 1
B 7.11 × 10
-5
1.5 1
C 9.67 × 10
-5
3.0 1
D 2.75 × 10
-4
1.0 1
E* 1.34 × 10
-4
1.0 1
CCCC A 7.73 × 10
-5
3.0 2
B 1.31 × 10
-4
2.0 2
C 8.24 × 10
-4
2.0 2
D* 1.42 × 10
-4
2.0 2
CCHC A 1.78 × 10
-4
1.5 1.5
B* 1.50 × 10
-4
2.0 1.5
C 1.09 × 10
-4
1.0 1.3
D 1.43 × 10
-4
1.0 1.8
*Trial is shown in Figure A.3.

120

A1.2.2. Secondary Structure of Goldfingers.
It is well known that zinc finger model peptides have no defined secondary structure if
there is no metal present.
25,44
The CD spectra of these random coils have distinct minima
at approximately 200 nm. Upon coordination to zinc, the peptide folds into the canonical
zinc finger fold with N-terminal beta sheets and a C-terminal alpha helix. The CD
spectra of properly folded zinc finger peptides show a maximum at approximately 190
nm and a minimum at approximately 205 nm.
25,44
CD spectra of the reduced and
oxidized peptides alone and in the presence of zinc and gold are shown in Figure A.4a.
The reduced (unstructured) zinc fingers have pronounced minima near 200 nm. The
oxidized peptides are slightly more rigid due to the formation of disulfide bond(s), but
also have distinct minima near 200 nm and little secondary structure. As expected, the
Zn(II)-peptide adducts have strong maxima located at approximately 190 nm and broad
minima at 205 nm. These trends are consistent for all three of the zinc finger peptide
motifs investigated (see Figure A.4b,c).

Figure A.4. CD spectra of zinc fingers (a) CCHH, (b) CCCC, and (c) CCHC. The data represent zinc
fingers (blue), gold fingers (green), reduced peptides (red), and oxidized peptides (orange).

When the reduced peptides are treated with Et
3
P-Au-Cl, distinct spectra are obtained.
The maximum at 190 nm, which corresponds to the formation of alpha helical secondary
121

structure, is decreased significantly as compared to the parent zinc finger. It is clear that
the alpha helix is compromised upon gold binding. In addition, the minimum seen at 205
nM in the intact zinc finger peptides is blue-shifted by approximately 5 nm upon Au(I)
coordination. Based on these data, we conclude that the goldfingers have different
secondary structures than their Zn(II)-bound counterparts. This observation correlates
well with the biological data indicating that goldfingers are no longer able to bind to
DNA.
25
Again, these trends are consistent for peptides with a CCHH metal binding motif,
a CCHC motif and a CCCC motif. It is important to note that the goldfingers, although
they have less secondary structure than the corresponding zinc fingers, are more ordered
than the metal-free peptides, either in the oxidized or reduced state. In addition, no
change is seen when an oxidized peptide is exposed to Au(I), providing further evidence
that the thiol ligands must be available in order for gold to bind.

Figure A.5. CD spectra of CCHH (a), CCCC (b), and CCHC (c) goldfingers formed using aurothiomalate
(purple). CD spectra of goldfingers formed by Et
3
P-Au-Cl shown originally in Figure A.3 are shown for
comparison (green).

Similar circular dichroism experiments were performed using aurothiomalate instead of
Et
3
P-Au-Cl to determine if this gold-based drug has a similar effect on the secondary
structure of the zinc finger peptide variants. As shown in Figure A.5 the goldfingers
produced by aurothiomalate do not have the same secondary structure as Zn(II)-bound
122

zinc fingers shown in Figure A.4. It is interesting to note that the CD spectra of the
goldfingers formed by Et
3
P-Au-Cl differ from those formed by aurothiomalate. Since
thiomalate and thiomalic acid are achiral, and would not likely produce a significant
signal, it may be concievable that a disulfide formed between two free thiomalate ligands
could disrupt the CD spectra. It is possible that the thiomalate ligands remain
coordinated to the Au(I) to form a peptide-Au(I)-thiomalate complex, similar to what has
been observed with cathepsin K,
20
or that the thiomalate may bind to a cysteine causing a
significant change in the peptide folding. Further investigation is required in order to
determine the reason behind the CD spectra; however, it is clear that the CD spectra of
these goldfingers are significantly difference compared to Zn(II)-bound zinc fingers.

A.2.3. Biological Implications of Goldfinger Formation.
The secondary structure of zinc finger transcription factors is integral to their ability to
recognize and bind DNA. As illustrated above, the secondary structure of the goldfinger
peptides is significantly different compared to the parent zinc finger. The prominent
difference is the lack of alpha-helical character as evidenced by loss of the maximum at
190 nm in the CD spectra. The data suggest that Au(I) could interfere with the DNA
binding activity of zinc finger peptides by coordinating to the cysteine residues in the
zinc binding site and inducing a conformational change. The spectroscopic evidence
presented here provides further support for the hypothesis that gold-based drugs elicit
their antiarthritic effect by coordinating to zinc fingers, down regulating the production of
proinflammatory proteins.

123

A.3. Conclusions and Future Work
Although Au(I) is often described as ―spectroscopically silent‖ a spectroscopic method is
described here to determine Au(I) binding to cysteine containing peptides. By
monitoring the increase in absorption caused by Au(I) coordinating to the free thiolate
ligands of zinc finger peptides, the stoichiometry of the reaction has been determined.
Au(I) coordination is dependent on the amount of free thiolate ligands on a zinc finger
model peptide. Zinc fingers with one pair of free thiolates (CCHH) can coordinate one
molar equivalent Au(I), while zinc fingers with two pairs of free thiolates (CCCC) are
able to coordinate two equivalents of Au(I). This is the first instance of multiple metals
coordinating to one zinc finger. Interestingly, the peptide CCHC coordinates 1.5
equivalents of Au(I), implying a possible interaction between multiple peptides with one
Au(I) metal center. Circular dichroism provided a qualitative comparison between Au(I)-
bound zinc fingers versus reduced, oxidized and Zn(II)-bound zinc fingers. In all cases,
the secondary structure of the goldfingers differed significantly from the Zn(II)-bound
zinc fingers which would drastically disrupt the zinc finger’s ability to effectively
transcribe DNA.
Although there have been recent significant advances, there is still little known about the
therapeutic mechanism of gold-based drugs. The above investigation focused on a
metabolite of auranofin, an anti-arthritic drug, but gold-based drugs are also currently
being explored to determine their effectiveness against bacteria, cancer, and HIV/AIDS.
By determining how this important class of drugs elicit their effect in the body it may one
day be possible to rationally design Au(I) drugs to target specific proteins or enzymes.
124

A.5. Experimental Details
A.5.1. General Considerations
Sodium tetrachloroaurate (III) dihydrate was purchased from Premion and Tris(2-
carboxyethyl)phosphine (TCEP) was purchased from Molecular Probes. Consensus zinc
finger peptides were purchased from Genemed Synthesis, Inc. of San Antonio, TX and
purified as described below. All other reagents were purchased from commercial sources
and used as received. All studies were carried out under ambient conditions. Mass data
were collected using an Applied Biosystems Voyager DESTR MALDI-TOF MS.
1
H and
31
P NMR spectra were obtained using a Varian Mercury 400 MHz NMR. UV-vis spectra
were obtained using a Varian Cary 50 Bio spectrometer and CD spectra were obtained
using a Jasco J-810 Spectropolarimeter.

A.4.2. Peptide Purification
50 mg each of the following consensus sequence zinc finger peptides were obtained from
Genemed Synthesis, Inc. in crude form; CCHH = PYKCPECGK-
SFSQKSDLVKHQRTHTG, CCHC = PYKCPECGKSFSQKSDLVKHQRTCTG, and
CCCC = PYKCPECGKSFSQKSDLVKCQRTCTG. The crude peptides were reduced
with excess dithiolthreitol (DTT) and purified thrice on a reverse phase C18 column. The
purified products were characterized by MALDI-TOF MS. CCHH m/z calc’d (+ 3 H
2
O)
3017, found 3019; CCHC m/z calc’d 2929, found 2929; CCCC m/z calc’d 2895, found
2894.
125

A.4.3. Synthesis of Et
3
P-Au-Cl
Et
3
P-Au-Cl was synthesized as described previously. Briefly, an excess of
triethylphosphine (104 mg, 0.886 mmol) dissolved in 1.0 mL cold ethanol, serving as
both the reducing agent and the ligand, was added in a dropwise manner to a cold
solution of sodium tetrachloroaurate (168 mg, 0.425 mmol) in 0.5 mL ethanol to produce
the desired white solid product. After column chromatographic purification over silica
gel, the product was characterized by
1
H and
31
P NMR.
31
P NMR (methanol-d
4
): δ 32.20,
s.
1
H NMR (methanol-d
4
): δ 1.20 (3H, t), 1.92 (2H, q).
31
P and
1
H NMR shifts correlated
well with reported values for Et
3
P-Au-Cl.
48

A1.4.4. UV-vis Titrations
Stock solutions of CCHH, CCCC, and CCHC were prepared by dissolving the purified
peptide in an aqueous buffer containing 100 mM HEPES (4-(2- hydroxyethyl)-1-
piperazineethanesulfonic acid) and 50 mM NaCl at pH 7.0. A stock solution of 20 mM
TCEP was made using the same buffer. TCEP was chosen as the reductant in these
experiments to ensure that any absorbance we saw was due to Au(I)-zinc finger thiolate
interactions. Et
3
P-Au-Cl was dissolved in DMSO to a final concentration of 8.0 mM.
Prior to the titrations, the peptide stock solutions were diluted using the same aqueous
buffer to a final concentration of approximately 1.4 × 10
-4
M and separated into 200 μL
aliquots. One 200 μL aliquot was pipetted into a quartz cuvette used for the titration
while the others are stored at -4 °C for future titrations. The concentrations of the
126

peptides in solution were verified using the calculated extinction coefficients ε
280
= 1520
cm
-1
M
-1
for CCHH, ε
280
= 1760 cm
-1
M
-1
for CCCC, and ε
280
= 1640 cm
-1
M
-1
for CCHC.
49

Excess TCEP (between 1-3 equiv) was added to the cuvette and incubated for 15 minutes
at room temperature to reduce the peptide and the UV-vis spectrum was taken. A 0.10
equiv aliquot of Et
3
P-Au-Cl was added to the reduced peptide solution and mixed
thoroughly. After 5 minutes, the UV-vis spectrum of the peptide-Au(I) adduct was
acquired, followed by addition of a second aliquot of Et
3
P-Au-Cl. This procedure was
repeated until the absorbance ceased to increase. The background was subtracted from
each spectrum using a blank sample consisting of an equivalent amount of oxidized
TCEP and Et
3
P-Au-Cl in the aqueous buffer. One molar equivalent of TCEP was
required to reduce one mol of disulfide into two mols of thiol. Titrations were performed
using between 1-3 equiv of TCEP and the results were independent of the starting TCEP
concentration. Initially, no increase in absorbance was observed until all of the unreacted
TCEP became saturated with Au(I). A steady increase in absorbance was then observed
as more Au(I) was added the peptide. When the peptide became saturated with Au(I), the
absorbance remained constant. The observed ratio of Au(I) to peptide was maintained
regardless of the starting TCEP concentration, peptide concentration, or titration
increment. Control experiments where Et
3
P-Au-Cl was titrated into a solution containing
oxidized CCHH peptide (where the two cysteine residues are involved in disulfide bonds)
show little change in the absorption spectrum, indicating that Au(I) coordination to
CCHH is dependent on the presence of free thiolates (data not shown).

127

A.4.5. Circular Dichroism
Circular dichroism experiments were performed using a Jasco J- 810 Spectropolarimeter.
Prior to spectral acquisition, the sample chamber was purged with N
2
for 10 minutes to
eliminate O
2
noise. The stock solutions of the purified CCHH, CCCC, and CCHC
peptides in buffer (5 mM NH
4
+
OAc
-
, 5% MeOH, pH 6.8) were prepared as outlined
above. Peptides were incubated under the following conditions to study metal binding.
Oxidizing: 25 μM peptide in buffer. Zinc binding: 25 μM peptide, 100 μM DTT, 100 μM
zinc chloride. Et
3
P-Au-Cl binding: 25 μM peptide, 100 μM DTT, 100 μM Et3P-Au-Cl.
For the CD studies, Et
3
P-Au-Cl was dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol
(HPF) and then added to the peptide solution (5% HPV v/v).

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Abstract (if available)

Abstract Solution-phase synthetic reactions have proven to be viable routes toward semiconductor metal chalcogenide nanocrystals

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Creator Franzman, Matthew A. (author)

Core Title Solution-phase synthesis of metal chalcogenide nanocrystals at low temperatures using dialkyl dichalcogenide precursors

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 01/15/2010

Defense Date 12/08/2009

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag auranofin,aurothiomalate,CIS,copper indium sulfide,CuInS₂,dichalcogenide,diselenide,disulfide,gold drugs,green chemistry,growth kinetics,In₂O₃,In₂S₃,indium oxide,indium sulfide,inorganic chemistry,myochrysine,nanocrystal,nanocrystal growth,nanocrystal synthesis,nanorods,OAI-PMH Harvest,peroxide,semiconductor,SnS,SnSe,synthesis,tin selenide,tin sulfide,zinc fingers

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Brutchey, Richard L. (committee chair), Malmstadt, Noah (committee member), Thompson, Mark E. (committee member)

Creator Email franzman@usc.edu,matthewfranzman@yahoo.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m2797

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Document Type Dissertation

Rights Franzman, Matthew A.

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Repository Location Los Angeles, California

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