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Synthesis and characterization of metal chalcogenide semiconductor nanocrystals using dialkyl dichalcogenide precursors

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Synthesis and characterization of metal chalcogenide semiconductor nanocrystals using dialkyl dichalcogenide precursors

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Content SYNTHESIS AND CHARACTERIZATION OF METAL CHALCOGENIDE
SEMICONDUCTOR NANOCRYSTALS USING DIALKYL DICHALCOGENIDE
PRECURSORS

by

Michelle E. Norako

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)

December 2013

Copyright 2013 Michelle E. Norako

ii
Acknowledgements
Over the past 5 years I have received support and encouragement from a great number of
individuals. Professor Richard Brutchey has been my research advisor throughout my
entire graduate school career and his guidance has made this journey and my
achievements possible; for this I am truly grateful. I would like to thank my committee
members: Dan Dapkus, Barry C. Thompson, Mark Thompson, Alex Benderskii and of
course Richard Brutchey for their support during the many steps toward earning my
degree. I also acknowledge the USC chemistry department and all of the past and present
members of the Brutchey group with whom I have had the pleasure to work. I cannot
finish without thanking my undergraduate academic and research advisors, Thomas A.
Jackman and Steven A. Hendrix, who mentored and encouraged me throughout my
undergraduate and graduate careers. Kira, Sirish, Jackie, Federica, and Manny: thank you
for always being there for me, from near and far, through all the ups and downs, for
picking up late night phone calls, and for never giving up on me; with out these closest
friends I would not have made it through. And last but not least, I am most thankful to
my family, especially my parents Mary and Stephen Norako for supporting me in
everything I do; my brothers Brian and Stephen J. Norako for their words of
encouragement when they were most needed; and my grandfather Col. Vincent W.
Norako who passed away while I was in graduate school, who never let me forget how
proud he was of me and who set an amazing example for how to live with honor,
integrity, faith, hope, and love.

iii

Table of Contents
Acknowledgements ii
List of Tables vi
List of Figures vii
Abstract xii
Chapter 1. Synthesis of Metastable Semiconductor Nanocrystal Phases 1
1.1. Introduction 1
1.2. Metastable II-VI Metal Chalcogenides 1
1.3. Metastable I-VI Metal Chalcogenides 5
1.4. Metastable I-III-VI Metal Chalcogenides 6
1.5. Alloyed Metastable II-VI:I-III-VI and I-VI:I-III-VI nanocrystals 8
1.6. Metastable I-IV-VI Metal Chalcogenides 9
1.7. Metastable Copper Zinc Tin Sulfide (Cu
2
ZnSnS
4
, CZTS) 10
1.8. References 12
Chapter 2. Growth Kinetics of Monodisperse Cu-In-S Nanocrystals Using a Dialkyl
Disulfide Sulfur Source 21
2.1. Abstract 21
2.2. Introduction 21
2.3. Experimental Details 23
2.3.1. General Considerations 23
2.3.2. Cu-In-S Nanocrystal Synthesis 24
2.3.3. Instrumentation 25
2.3.4. TEM Analysis of Nanocrystal Growth 26
2.4. Results and Discussion 26
2.4.1 Nanocrystal Synthesis and Characterization 26
2.4.2. Nanocrystal Growth Kinetics 31
2.4.3. Band Gap Determination 38
2.5. Conclusion 40
2.6. Acknowledgement 41

iv
2.7. References 42
Chapter 3. Synthesis of Metastable Wurtzite CuInSe
2
Nanocrystals 46
3.1. Introduction 46
3.2. Experimental Details 47
3.2.1. General Considerations 47
3.2.2. Synthesis of CuInSe
2
Nanocrystals 48
3.2.3. Material Characterization 48
3.3. Results and Discussion 49
3.3.1. Nanocrystal Synthesis 49
3.3.2. Simulation of Wurtzite Diffraction Pattern 50
3.3.3. Nanocrystal Characterization 51
3.3.4. Band Gap Determination 59
3.4. Conclusion 60
3.5. Acknowledgment 60
3.6. References 61
Chapter 4. Synthesis and Characterization of Wurtzite-Phase Copper Tin Selenide
Nanocrystals 64
4.1. Abstract 64
4.2. Introduction 64
4.3. Experimental Details 66
4.3.1. General Considerations 66
4.3.2. Synthesis of Di-tert-Butyl Diselenide (
t
Bu
2
Se
2
) 66
4.3.3. Copper Tin Selenide Nanocrystal Synthesis 67
4.3.4. Nanocrystal Characterization 68
4.4. Results and Discussion 69
4.4.1. Nanocrystal Synthesis 69
4.4.2. Simulation of Wurtzite Copper Tin Selenide Diffraction Pattern 70
4.4.3. Nanocrystal Characterization 70
4.4.4. Band Gap Determination 76
4.5. Potential Application for Photovoltaics 78
4.6. Conclusion 80
4.7. Acknowledgment 80
4.8. References 81
Chapter 5. Synthesis and Characterization of Five Different Metal Chalcogenides 86
5.1. Introduction 86
5.2. Experimental Details 89
5.2.1. General Considerations 89
5.2.2. Synthesis of di-tert-butyl diselenide (
t
Bu
2
Se
2
) 89

v
5.2.3. Nanocrystal Syntheses 90
5.2.3.1. Synthesis of Cu
2
GeSe
3
Nanocrystals 90
5.2.3.2. Synthesis of Cu
2
SnS
3
Crystals 91
5.2.3.3. Synthesis of Ni
0.95
Se Crystals 92
5.2.3.4. Synthesis of CuSbS
2
92
5.2.3.5. Synthesis of PbS 93
5.2.4. Nanocrystal Characterization 94
5.3. Results and Discussion 94
5.3.1. Cu
2
GeSe
3
Nanocrystals 94
5.3.2. Cu
2
SnS
3
Crystals 96
5.3.3. Ni
0.95
Se Crystals 99
5.3.4. CuSbS
2
Crystals 100
5.3.5. PbS Crystals 102
5.4. Conclusion 103
5.5. References 103
Bibliography 109

vi
List of Tables
Table 3.1: Wurtzite CuInSe
2
Atomic Coordinates 50
Table 3.2: Comparison of experimental and simulated d-spacing values with
respect to their corresponding diffraction peaks. 53
Table 4.1: Wurtzite CTSe Atomic Coordinates 70
Table 4.2: Comparison of experimental and simulated d-spacing values with
respect to their corresponding diffraction peaks. 72
Table 5.1: Wurtzite Cu
2
SnS
3
Atomic Coordinates 97
Table 5.2: Comparison of experimental and simulated d-spacing values with
respect to their corresponding diffraction peaks. 98

vii
List of Figures
Figure 1.1: Structure of (a) zinc blende and (b) wurtzite ZnS (Zn – black; S –
white). 2
Figure 1.2: Double unit cells of zinc blende ZnSe, zinc blende CuInSe
2
, and
chalcopyrite CuInSe
2
that also pertain to CuInS
2
. 7
Figure1.3: Structures of (a) zinc blende CdTe, (b) chalcopyrite CuInSe
2
, and
(c) kesterite Cu
2
ZnSnSe
4
, showing the derivation of the kesterite
structure from the zinc blende via the chalcopyrite structure. 12
Figure 2.1: (a) TEM micrograph of 6.9-nm Cu-In-S nanocrystals with
representative SAED pattern displayed as inset. (b) HRTEM
micrograph of an individual Cu-In-S nanocrystal. 27
Figure 2.2: XRD pattern of the wurtzite Cu-In-S nanocrystals synthesized
with di-tert-butyl disulfide in oleylamine at 180 ˚C. 28
Figure 2.3: (a) XRD pattern of nanocrystals resulting from the reaction done
in squalane instead of oleylamine under otherwise identical
conditions. The tetragonal chalcopyrite phase of Cu-In-S is
formed as a result. (b) TEM image of the resulting agglomerated
nanocrystals. 29
Figure 2.4: (a) XRD pattern of nanocrystals resulting from the reaction
performed with tert-butyl mercaptan as the only sulfur source
under otherwise identical conditions. Phase impure chalcopyrite
Cu-In-S is formed as a result. (b) TEM image of the resulting
agglomerated nanocrystals. 30
Figure 2.5: Representative TEM micrographs demonstrating the temporal
evolution of the size and size distribution of Cu-In-S
nanocrystals taken from the reaction mixture at t = (a) 1, (b) 18,
(c) 23, (d) 28, (e) 33, (f) 68, (g) 128, and (h) 188 min. 32

viii
Figure 2.6: Quantitative statistical results for the nanocrystal growth
represented in Figure 2.5. Temporal evolution of the (a) mean
diameter (m
D
), (b) distribution about the mean (s
D
/m
D
), and (c)
skew (g) for the Cu-In-S nanocrystals. A vertical line is drawn at
t = 33 min. 33
Figure 2.7: EDX spectra of monodisperse wz-Cu-In-S (a) and large
hexagonal platelets of wz-Cu-In-S (b). Peaks at 0.26, 0.52, and
1.48 keV are assigned to the C K, O K, and Al K lines,
respectively. Nanocrystal composition as a function of time for
the growth of monodisperse wz-Cu-In-S nanocrystals, as
determined by EDX (c). Each point represents the average of at
least 10 randomly sampled areas. 35
Figure 2.8: XRD patterns of the wz-Cu-In-S nanocrystals taken from the
reaction mixture at t = 15, 25, 60, and 180 min. 36
Figure 2.9: TEM micrograph of wurtzite Cu-In-S hexagonal platelets formed
without 1-dodecanethiol, with representative SAED pattern
displayed as inset. 37
Figure 2.10: XRD pattern of the hexagonal wz-Cu-In-S platelets formed in the
absence of 1-dodecanethiol. 38
Figure 2.11: (a) UV-vis absorption spectrum and (b) cyclic voltammetry
curve for the 6.9-nm Cu-In-S nanocrystals (sweep rate = 10 mV
s
-1
). 39
Figure 2.12: (a) CV curves of the electrolyte blank (black), 1 µL of 1-
dodecanethiol (blue), and 2 µL of 1-dodecanthiol (red). (b) CV
curves of Cu-In-S nanocrystals (purple) with 1 µL of 1-
dodecanethiol (red) and 2 µL of 1-dodecanethiol (green). 40
Figure 3.1: Simulated XRD pattern of wurtzite-CuInSe
2
. 50
Figure 3.2: Simulated wurtzite-CuInSe
2
unit cell. 51

ix
Figure 3.3: Experimental and simulated XRD patterns of wurtzite-CuInSe
2

nanocrystals. 52
Figure 3.4: XRD pattern of nanocrystals resulting from the reaction done in
squalane instead of oleylamine under otherwise identical
conditions. The tetragonal chalcopyrite phase of CuInSe
2
is
formed as a result. 54
Figure 3.5: XRD pattern of nanocrystals resulting from the reaction done
with elemental selenium instead of the diselenide under
otherwise identical conditions. Chalcopyrite CuInSe
2
and Cu
2
Se
are formed as a result. 54
Figure 3.6: (a) Low-res TEM micrograph of CuInSe
2
nanocrystals with
representative SAED pattern displayed as inset. (b) High-res
TEM micrograph of an individual CuInSe
2
nanocrystal. 55
Figure 3.7: Indexed SAED pattern of an ensemble of wz-CuInSe
2

nanocrystals. 56
Figure 3.8: Hi-res TEM micrograph of two differently sized wz-CuInSe
2

nanocrystals. The (002) lattice planes are displayed for both
nanocrystals (d = 0.33 nm). 56
Figure 3.9: Representative EDX spectrum of wz-CuInSe
2
nanocrystals. 57
Figure 3.10 High-resolution XPS spectra of Cu 2p, In 3d, and Se 3d regions
taken of wz-CuInSe
2
nanocrystals. 58
Figure 3.11. Visible-NIR absorption spectrum of the wurtzite-CuInSe
2

nanocrystals. The small features from 1200-1400 nm are related
to absorption from cyclohexane. 59
Figure 4.1: Experimental XRD patterns of (a) wurtzite and (b) cubic CTSe
nanocrystals. The simulated XRD patterns of wurtzite (red) and
cubic (black) CTSe are shown for reference. 71

x
Figure 4.2: XRD patterns in the 40-60˚ 2! range illustrating the peak
symmetry of the (110) reflection for the CTSe nanocrystals. 73
Figure 4.3: XRD patterns of nanocrystals resulting from the reactions
performed with (a) 8 equivalents of oleic acid (versus SnI
4
); (b)
elemental selenium used instead of the
t
Bu
2
Se
2
chalcogen source;
and (c) TOPSe used instead of the
t
Bu
2
Se
2
chalcogen source. All
reactions were otherwise performed identically at 180 ˚C for 5
min. The simulated diffraction patterns for wurtzite CTSe
(black), cubic CTSe (maroon), and orthorhombic SnSe (orange)
are given as reference. 74
Figure 4.4: (a) HRTEM image of a CTSe nanocrystal with an interplanar
spacing of 3.26 Å; (b) low resolution TEM image of CTSe
nanocrystals with an average diameter of 15 nm; (c) SAED
pattern of nanocrystals indexed to wurtzite CTSe; and (d) UV-
vis-NIR absorption spectrum of wurtzite CTSe nanocrystals. 75
Figure 4.5: Representative EDX spectrum of wurtzite CTSe nanocrystals. 76
Figure 4.6: UV-visible-NIR absorption spectrum of cubic CTSe
nanocrystals. 77
Figure 4.7: Differential pulse voltammograms of cubic (orange) and wurtzite
(red) CTSe nanocrystals, both treated with hydrazine on ITO in
0.1 M TBAPF/CH
3
CN. DVPs were obtained with a scan rate of
10 mV s
-1
using a Pt wire counter electrode and Ag wire pseudo-
reference electrode, calibrated against Fc/Fc
+
. The onsets of the
reduction (-0.4 V for wurtzite CTSe and -0.28 V for cubic CTSe)
and oxidation (1.3 V for wurtzite CTSe and 1.2 V for cubic
CTSe) peaks were used to determine the electrochemical band
gaps of ca. 1.7 eV (wurtzite) and 1.5 eV (cubic). The shoulders
observed immediately prior to the onsets for cubic CTSe may be
attributable to shallow trap states present from the remaining
native ligands. The lack of such shoulders for the wurtzite CTSe
may possibly be due to trap state passivation by thiols, a
phenomenon that has been reported in other nanocrystal systems 78

xi
Figure 4.8: Transient photocurrent response of a wurtzite CTSe nanocrystal
film showing clear p-type behavior. The nanocrystal film was
spun-cast onto ITO, treated with hydrazine, and photocurrent
was measured under nitrogen in aqueous 0.01 M Eu(NO
3
)
3
/0.1 M
KCl using 472 nm chopped illumination with a Ag wire pseudo-
reference electrode and a Pt wire counter electrode. The
potential values are given relative to NHE. 79
Figure 5.1: Experimental XRD pattern of cubic Cu
2
GeSe
3
nanocrystals, the
known XRD pattern – PDF#01-089-2878 – (green) is shown for
reference. 95
Figure 5.2: TEM image of cubic Cu
2
GeSe
3
nanocrystals with diameters of
ca. 10-20 nm. HR-TEM image of a single nanocrystal shown as
inset. 95
Figure 5.3: Experimental XRD pattern of wurtzite Cu
2
SnS
3
crystals, the
simulated wurtzite XRD pattern is shown underneath in black. 96
Figure 5.4: TEM image of Cu
2
SnS
3
crystals. 98
Figure 5.5: Experimental XRD pattern of hexagonal Ni
0.95
Se, the reference
XRD pattern – JCPDS card # 01-072-2546 – is shown in green. 99
Figure 5.6: Experimental XRD pattern of CuSbS
3
, the reference XRD pattern
– JCPDS card # 01-073-3954 – is shown in green. 100
Figure 5.7: TEM images of polydisperse cubic CuSbS
2
crystals. 101
Figure 5.8: Experimental XRD pattern of PbS with the reference rock salt
structure shown in black. 102

xii
Abstract
Metastable semiconductor nanocrystals have been shown to possess new and interesting
properties that are highly reliant upon their synthetic reaction parameters. A versatile
method for a relatively low temperature synthesis of metastable 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. Monodisperse metastable copper indium sulfide (CuInS
2
)
nanocrystal were synthesized using di!tert!butyl disulfide as the sulfur source at 180 °C.
In a similar fashion, metastable wurtzite copper indium selenide (CuInSe
2
) and
metastable wurtzite copper tin selenide (CTSe) were synthesized and characterized for
the first time. This method was further expanded to synthesize Cu
2
GeSe
3
, Cu
2
SnS
3
,
Ni
0.95
Se, CuSbS
2
, and PbS also using dialkyl dichalcogenide precursors. To further study
these products, the nanocrystal growth mechanism was explored for the
dichalcogenide!mediated synthesis of wz!CuInS
2
and the potential applicability of the
wurtzite CTSe nanocrystals as a photovoltaic material was assessed.

1
Chapter 1. Synthesis of Metastable Semiconductor Nanocrystal Phases

1.1. Introduction
The synthesis of metastable nanocrystals has recently gained significant attention, as
these new metastable nanocrystalline materials can possess physical and optical
properties that differ from their thermodynamically stable structures. Metastable phases
have been recently reported in many semiconductor nanocrystals, including II-VI, I-VI, I-
III-VI and I-IV-VI (i.e., CdSe/S, AgSe, and copper-containing ternary and quaternary
metal chalcogenides, such as Cu
2
ZnSnS
4
, CuInS
2
, CuInSe
2
, Cu
2
SnSe
3
, and Cu
2
GeSe
3
).
1-81

These metastable phases are highly dependent on specific reaction conditions, where
slight changes in these reaction conditions can result in other phases and impurities.

1.2. Metastable II-VI Metal Chalcogenides
Zinc blende (zb) and wurtzite (wz) are the two common crystallite structures of zinc
sulfide (ZnS), both structures are four-coordinate, where each Zn atom is coordinated to
four S atoms and each S atom is coordinated to four Zn atoms.
1
The zinc blende structure
is the most common crystalline form of ZnS, which has essentially the same geometry as
diamond with alternating Zn and S layers (Figure 1.1a).
1
The zinc blende structure can
also be described as having zinc ions and sulfide ions that are in a combined face-
centered lattice, where each ion is a tetrahedral hole of its opposing lattice.
1
The wurtzite
structure of ZnS is a rare crystallite form and is obtained at higher temperatures than the

2
zinc blende structure.
1
Similar to the zinc blende structure, the wurtzite structure also
exhibits zinc ions and sulfide ions in the tetrahedral holes of the other lattice (Figure
1.1.b).
1
However, the wurtzite structure crystallizes with each ion forming a hexagonal
close-packed (hcp) lattice whereas zinc blende only has half of the tetrahedral holes
occupied in each lattice.
1
The rock salt (sodium chloride, NaCl) structure is the
combination of face centered cubic (fcc) sodium and chloride ions offset by a half unit
cell, where the sodium ions are centered on the edges of the chloride lattice and the
chloride ions are centered on the edges of the sodium ions.
1
Each sodium ion is
coordinated to six chloride ions and each chloride ion is also coordinated to six sodium
ions.
1

It has been previously discussed that there are kinetic pathways that allow nanocrystals to
form metastable phases that are not stable in bulk.
2-81
In the bulk, ZnS, ZnSe, ZnTe, and
CdTe crystals, form the zinc blende structure which is the thermodynamically stable
phase for each. Murray et al. reported the stabilization of the hexagonal CdTe

Figure 1.1. Stucture of (a) zinc blende and (b) wurtzite ZnS (Zn – black; S – white).
1

3
nanocrystals for the first time using Me
2
Cd as the cadmium source.
3
It was determined
that !G between wurtzite and zinc blende decreases as the D (diameter) decreases.
However, there exists a critical size (D
c
), where at D = D
c
, !G = 0. When D < D
c
, the
wurtzite phase becomes the more thermodynamically stable phase. The critical size was
determined to equal 7.02, 3.5, 5.5, and 5.8 nm for ZnS, ZnSe, ZnTe, and CdTe,
respectively. Thus, zinc blende is stable and wurtzite is metastable when D > D
c
. It was
observed that the surface energy and surface stress values of wurtzite II"VI nanocrystals
are lower than those of the zinc blende, and therefore leads to the stabilization of the
wurtzite structure as the diameter decreases. ZnS, ZnSe, and ZnTe crystals can also form
in the zinc blende and wurtzite structures, in which the zinc blende is stable and wurtzite
is metastable.
4-7

Li and Yang performed theoretical studies on seven different II-VI semiconductor
nanocrystals (ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe) using a thermodynamic
model on the basis of first-principles calculations; the surface energies of 2 – 50 nm
II"VI semiconductor nanocrystals were calculated using the modified broken bond model
and DFT.
8
#For all seven of these nanocrystals, the wurtzite structure was determined to
have a lower surface energy than that of the zinc blende structure. It was also determined
that the wurtzite phase is the more stable phase for ZnO, CdS, and CdSe. Li and Yang
discovered that the cause of the phase transition from wurtzite to zinc blende is not a
result of the decrease in size. The total energy of the zinc blende phase was determined
to always be greater than that of the wurtzite phase, thus it is the wurtzite phase that is the
stable phase and the zinc blende phase is the metastable phase.
8
Before these theoretical

4
studies by Li and Yang, Fedorov et al. reported that there was no agreement on which
phase was the thermodynamically stable phase for bulk CdSe.
9
But according to the
energy values calculated by Li and Yang, wurtzite is the more stable phase.
8

Conversely, for the zinc chalcogenides, it is the wurtzite phase of ZnS and ZnSe that is
metastable at room temperature (in ZnS, for example, the phase transition temperature
from face centered cubic to hexagonal is 1296 K).
10,11
In most cases, ZnS and ZnSe
nanocrystals yield the cubic zinc blende phase.
10,12
There are a few reports of ZnSe and
ZnS nanocrystals that form the hexagonal crystal structure and were synthesized at
relatively low temperature.
10,11,13-19

Dawood and Schaak studied the reaction pathway of the formation of wz-ZnS and wz-
ZnSe nanoparticles, and concluded that when ZnO nanoparticles are used as a structural
template, wz-ZnO forms first as a reaction intermediate, which provides a predictable
pathway for synthesizing wz-ZnS and wz-ZnSe nanoparticles.
20
The materials are
attractive because they are known to have wide band gaps and good luminescence
properties.
5,21
There have also been other reports on wz-ZnS and wz-ZnSe nanocrystals
using precursor decomposition via solvothermal reactions,
22,23
ultrasonication,
24
and
colloidal method
15,24,25
techniques, where the zinc blende structure is predominant. Slight
synthetic differences can shift selectivity to favor one crystal structure over the other, as
there are only slight structural differences between zinc blende and wurtzite.
11
Thus, it
remains a challenge to predict and produce the wurtzite structure and understand the key
factors that cause the production of one structure over the other.

5

Cation exchange was demonstrated by Pietryga et al. to yield nanocrystals with
metastable phases. Metastable rock salt CdSe (usually only stable at high pressures) can
be synthesized by partial cation exchange of the rock salt of PbSe, as the rock salt phase
of CdSe minimizes the strain at the PbSe–CdSe interface.
26,27
When Cd
2+
ions were
replaced with Cu
+
ions in hcp CdSe nanocrystals, the selenide ions remains in place and
the resulting nanocrystals are also hexagonal.
26
These nanocrystals were also found to
have a Cu:Se stoichiometry close to 2:1, and not 1:1 as for the stable hcp phase of bulk
copper selenide.
26
Hexagonal Cu
2
Se is therefore a metastable phase; before this report by
Li et al. the hexagonal phase had not been observed in the bulk but had been observed in
molecular clusters.
26,28-32
Metastable hcp Cu
2
Se nanocrystals can be converted into pure
hcp ZnSe nanocrystals and the resulting hcp phase of ZnSe is rare (i.e., the fcc phase is
known to be the more stable phase).
33,34
A few other reports have demonstrated the
synthesis of hcp ZnSe nanocrystals at low temperatures.
13,19,35
CdS and ZnS
nanaoparticles were doped with Mn ions in a few previous publications, where it was
observed that the doped state of the nanoparticles formed a metastable state.
36-38

1.3. Metastable I-VI Metal Chalcogenides
Ag
2
Se has been shown to exist in two low-temperature phases: orthorhombic (stable) and
tetragonal (metastable).
39-41
Norris et al. report a one-pot, single-step synthesis method of
metastable tetragonal Ag
2
E nanocrystals (lattice contents a = b = 0.706 nm and c = 0.498
nm).
39
It was concluded that the synthesized Ag
2
Se and Ag
2
Te nanocrystals undergo a
first-order phase transition upon cooling and that the Ag
2
Se are kinetically trapped in the

6
metastable tetragonal structure, which is not observed in the bulk. However, Ag
2
S and
Ag
2
Te have been reported to form stable low-temperature monoclinic phases also
observed in the bulk and no metastable phases were observed.
39

1.4. Metastable I-III-VI Metal Chalcogenides
The chalcopyrite phase is the most common crystal structure for A
+
B
3+
X
2
2–

semiconductors, where the A and B ions are ordered in the cation sublattice sites.
42

Random distribution of these cations forms the zinc blende structure.
43-45
The
orthorhombic phase is another phase of A
+
B
3+
X
2
2–
, where the A and B ions are ordered in
the cation sublattice, which has been observed in AgInS
2
and AgInSe
2
.
46-48
Similar to the
phase transition from chalcopyrite to zinc blende, when the A and B ions are disordered
in the cation sublattice sites, the orthorhombic phase converts to the wurtzite phase. Due
to the random distribution of A and B ions in the wurtzite phase, flexibility in
stoichiometry allows tunable band gaps,
49
making these materials applicable to the
fabrication of photovoltaics.
50

Bulk wz-CuInS
2
is a high-temperature phase stable between 1045 and 1090 °C, where the
lattice positions are substituted with a 50/50 occupancy probability of Cu
+
and In
3+

cations and these cations randomly share common lattice sites (see Figure 1.2).
42,53
Wz-
CuInS
2
nanocrystals have been observed to be accessible at room temperature
49,51-53
and in
one report, well-defined and monodisperse wurtzite Cu"In"S nanocrystals were
synthesized via a solution-based method using di-tert-butyl disulfide as the sulfur source.
The nanocrystals were shown to pass through a size focusing event before undergoing

7
steady-state Ostwald ripening at later stages of growth.
53
Reaction control (i.e.,
coordinating solvent, capping ligand, and sulfur source) was proven to play a key role to
form the proper environment for the selective formation of metastable wurtzite phase.
53

Figure 1.2. Double unit cells of zinc blende ZnSe, zinc blende CuInSe
2
, and chalcopyrite CuInSe
2
that
also pertain to CuInS
2
.
42,53

8
The observation of wz"Cu"In"S nanocrystals at room temperature proves that this
metastable phase is kinetically accessible under less energy intensive conditions.
49-52,54,55

In addition to synthesized metastable wz-CuInS
2
nanocrystals, metastable zinc blende
CuInSe
2
nanocrystals (Figure 1.2)
56
and metastable wz-CuInSe
2
nanocrystals have been
reported.
57
Specific reaction control was necessary to synthesize wz-CuInSe
2
; analogous
to wz-CuInS
2
, primary amines have been shown to influence the formation of other
wurtzite metal chalcogenides (i.e., wurtzite ZnS, CdSe, and CuInS
2
nanocrystals).
53,55,58,59

Monodisperse wurtzite CuIn
x
Ga
1–x
S
2
(CIGS) nanocrystals have been synthesized using a
facile solution-based method by varying the chemical composition and synthesis
conditions, the stoichiometry of the nanocrystals was be controlled from x = 0 – 1.
42
Bulk
chalcopyrite CIGS is thermodynamically more stable than bulk wurtzite CIGS, indicated
by both experimental results
54
and theoretical calculations.
60
Gupta et al. reported that a
particular combination of kinetic factors are responsible for the preferential growth and
stability of wurtzite CIGS.
42

1.5. Alloyed Metastable II-VI:I-III-VI and I-VI:I-III-VI nanocrystals
Alloyed metastable zinc blende (ZnSe)
x
(CuInSe
2
)
1–x
and metastable zinc blende
CuInSe
x
S
2–x
nanocrystals have been synthesized by a hot-injection approach.
61
Based on
the CuInSe
2
phase diagram, the tetragonal chalcopyrite structure is converted to a
metastable zinc blende structure at temperatures higher than 810 °C,
62,63
where Cu
+
and
In
3+
cations randomly occupy the same position in the zinc blende structure with 50%
occupancy of Cu
+
and In
3+
(see Figure 1.2). Similar to other metastable metal

9
chalcogenide nanocrystal synthesis, the reaction parameters play a critical role in
achieving the desired metastable phase of alloyed (ZnSe)
x
(CuInSe
2
)
1–x
and CuInSe
x
S
2–x

nanocrystals.
61

Rod-like Ag
2
S-AgInS
2
heterostructured nanocrystals have been reported as yielding the
monoclinic phase of Ag
2
S that coexists with the primitive orthorhombic phase of AgInS
2
.
The orthorhombic phase is a metastable state of AgInS
2
, which forms at high
temperatures (above 620 °C), and the tetragonal phase appears at lower temperatures
(below 620 °C).
64,65
The existence of the metastable orthorhombic phase of AgInS
2
was
reported to be observed at a drastically lower temperature (150 °C) and is attributed to the
chemical growth environment (i.e., solvent and surfactants).

1.6. Metastable I-IV-VI Metal Chalcogenides
Palatnik et al.,
66
Rivet et al.,
67
and Sharma et al.
68
reported that Cu
2
SnSe
3
crystallizes in a
zinc blende structure with lattice parameters a = 5.688 – 5.696 Å. However, Rivet
69
later
proposed that up to about 450 °C Cu
2
SnSe
3
has an orthorhombic structure with lattice
parameters a = 4.028 Å, b = 5.696 Å and c = 12.084 Å and at 450 °C, and an order–
disorder transformation that was found to be zinc blende.
69
Marcano et al. subsequently
established that Cu
2
SnSe
3
crystallizes in a monoclinic structure, space group Cc, with
lattice parameters a = 6.5936(1) Å, b = 12.1593(4) Å and c = 6.6084(3) Å and ! =
108.56(2)°.
70
The first demonstration of wurtzite copper tin selenide (CTSe) nanocrystals
has been reported using a facile solution-phase synthesis method with dodecylamine and
1-dodecanethiol as coordinating solvents and di-tert-butyl diselenide (
t
Bu
2
Se
2
) as the

10
selenium source. Specific reaction control (i.e., a combination of 1-dodecanethiol with
t
Bu
2
Se
2
) was proven to be critical in order to obtain this new phase of CTSe, which was
verified by powder X-ray diffraction and selected area electron diffraction. The wurtzite
CTSe nanocrystals possess an optical and electrochemical band gap of 1.7 eV and display
an electrochemical photoresponse indicative of a p-type semiconductor.
71

Cu
2
GeSe
3
is known to have a disordered tetragonal unit cell where the unit cell symmetry
was been found to be highly sensitive to the concentration of Ge. A small deficiency in
the amount of Ge decreases the unit cell symmetry to monoclinic, and an increase in the
amount of Ge increases the unit cell symmetry to cubic. Thus, Cu
2
Ge
0.85
Se
3
is monoclinic
and Cu
2
Ge
1.55
Se
3
is cubic. The structure and stability of A
2
I
B
IV
C
3
VI
was reported to
depend on the valence state of the group IV elements, which are known to exhibit
variable valency, i.e. tetravalent and divalent. The tetravalent state favors a more
distorted structure thus the more stable phase, while the divalent state favors a less
distorted structure thus the less stable (metastable) phase. It was also concluded that the
lattice distortion decreases as the ionic character of the IV-VI bond increases.
72
Multiple
Cu
2
GeE
3
crystal structures have been reported for the nanocrystalline materials, including
zb-Cu
2
GeSe
3
,
72-74
tetragonal (chalcopyrite) Cu
2
GeSe
3
,
72,74
orthorhombic Cu
2
GeSe
3
,
73,74
wz-
Cu
2
GeSe
3
,
74
and zb-Cu
2
GeS
3
.
73

1.7. Metastable Copper Zinc Tin Sulfide (Cu
2
ZnSnS
4
, CZTS)
Ryan et al. synthesized stoichiometric CZTS nanorods using a mixture of copper(II)
acetylacetone, zinc acetate, tin(IV) acetate, trioctylphosphine oxide, and 1-octadecene

11
heated to 240-260 ºC.
75
A combination of 1-dodecanethiol and tert-dodecyl mercaptan
was then injected, resulting in metastable wurtzite CTZS nanorods as opposed to the
wurtzite phase.
75
The kesterite crystal structure of Cu
2
ZnSnSe
4
has been previously
reported and derived by Shenoy et al. from the zinc blende structure via the chalcopyrite
structure (of CuInS
2
) by using sequential cation cross-substitution.
76-79
Figure 1.3. depicts
the zinc blende CdTe, chalcopyrite CuInSe
2
, and kesterite Cu
2
ZnSnSe
4
.
76
The Cd and Te
ions in zinc blende CdTe occupy two interpenetrating fcc lattices independently.
76

Chalcopyrite CuInSe
2
was derived from the zinc blende structure by doubling the unit
cell along the c axis and replacing Cd with ordered Cu and In on the cation fcc lattice and
Te with Se. Kesterite Cu
2
ZnSnSe
4
was then obtained by an additional substitution of In
with ordered Zn and Sn, where Te and Se are located at one fcc lattice and the cations
share the other lattice.
76
The key to stabilizing the wurtzite phase as opposed to the
thermodynamically stable kesterite phase was the use of dodecanethiol in the synthesis,
which acts as both the sulfur source and surfactant.
75
Dodecanethiol is known to be a
robust coordinating ligand for the wurtzite copper-based chalcogenide nanocrystals, i.e.
CIS, CIGS and CZTS.
42,50,51,75-82
Additionally, it was observed that an injection
temperature greater than 200 ºC was required for the formation of single-phase CZTS
nanorods and to prevent Cu
2
S impurities.
75

12

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Figure 1.3. Structures of (a) zinc blende CdTe, (b) chalcopyrite CuInSe
2
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2
ZnSnSe
4
,
showing the derivation of the kesterite structure from the zinc blende via the chalcopyrite structure.
76

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(78) Lu, X.; Zhuang, Z.; Peng, Q.; Li, Y. Wurtzite Cu
2
ZnSnS
4
nanocrystals: a novel
quaternary semiconductor. Chem. Commun. 2011, 47, 3141.
(79) Li, J.; Mitzi, B. D.; Shenoy, V. B. Structure and Electronic Properties of Grain
Boundaries in Earth-Abundant Photovoltaic Absorber Cu
2
ZnSnSe
4
. ACS Nano,
2011, 5, 8613.
(80) Chen, S.; Gong, X. G.; Walsh, S.; Wei, S.-H. Electronic structure and stability of
quaternary chalcogenide semiconductors derived from cation cross-substitution of
II-VI and I-III-VI
2
compounds. Physical Review B: Condensed Matter and
Materials Physics 2009, 79, 165211.
(81) Goodman, L. C. H. The prediction of semiconducting properties in inorganic
compounds. Physics and Chemistry of Solids 1958, 6, 305.
(82) Pamplin, B. R. A Systematic Method of Deriving New Semiconducting
Compounds by Structural Analogy. J. Phys. Chem. Solids 1964, 25, 675.

21
Chapter 2. Growth Kinetics of Monodisperse Cu-In-S Nanocrystals
Using a Dialkyl Disulfide Sulfur Source*

*Published in Chem. Mater. 2009, 21, 4299-4304.
2.1. Abstract
Well-defined and monodisperse wurtzite Cu-In-S nanocrystals were synthesized via a
solution-based method using di-tert-butyl disulfide as the sulfur source. Reaction control
(i.e., coordinating solvent, capping ligand, and sulfur source) proved critical for providing
a kinetic pathway to the metastable wurtzite phase. The crystal phase was confirmed by
powder X-ray diffraction and selected area electron diffraction. Quantitative nanocrystal
growth kinetics were studied on a Cu-In-S system for the first time, clearly showing that
the nanocrystals pass through a size focusing event before undergoing steady-state
Ostwald ripening at later stages of growth. The size-focused 6.9 nm Cu-In-S nanocrystals
have an optical band gap of E
g
= 1.47 eV and an electrochemical band gap of E
g
= 1.84
eV.

2.2. 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

22
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, and they can be further tuned by
adjusting their relative elemental compositions.
6,7
Of these materials, copper indium
sulfide (CuInS
2
) is particularly attractive because it possesses a direct band gap (E
g
!
1.45-1.50 eV), has a large absorption coefficient and 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

Recently, CuInS
2
nanocrystals were found to also 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. 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.

23
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. As part of an
ongoing investigation into using dialkyl dichalcogenides as low temperature chalcogen
sources,
21-23
we report the rational synthesis of very well-defined, monodisperse
wz-Cu-In-S nanocrystals using di-tert-butyl disulfide as the sulfur source. Only by
exerting specific reaction control (i.e., coordinating solvent, disulfide sulfur source,
capping ligand) is the synthesis of monodisperse wz-Cu-In-S nanocrystals made possible.
Because the optoelectronic properties of semiconductor nanocrystals are highly
dependent on their size and distribution of size, we have also studied the quantitative
growth kinetics of Cu-In-S nanocrystals for the first time and found that they undergo a
size focusing event, which is highly dependent upon the identity of capping ligands
present in solution.

2.3. Experimental Details
2.3.1. 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

24
purification. Nanocrystal syntheses were performed under nitrogen, in the absence of
water and oxygen, using standard Schlenk techniques.

2.3.2. 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
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.

25
2.3.3. Instrumentation
Powder X-ray diffraction (XRD) analyses were performed on a Rigaku Ultima IV X-ray
diffractometer using a Cu K! radiation source (" = 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 kV. Samples were deposited on an aluminum tab and ten areas were
randomly analyzed. UV-vis-NIR spectra were acquired on a Cary 14 spectrophotometer
in dual beam mode using quartz cuvettes with 1 cm path lengths from nanocrystal
suspensions in cyclohexane. Cyclic voltammetry (CV) curves were acquired on a
Princeton Applied Research Potentiostat/Galvometer Model 283. The tetra-n-
butylammonium hexafluorophosphate (TBAPF
6
, Alfa-Aesar, 98%) 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. A 0.1 mL toluene suspension of the nanocrystals (30-40
mg/mL) mixed with the electrolyte (~1.0 mg) was added drop wise to the surface of the
carbon electrode to form a thin black film that adheres to the surface when submerged in
the acetonitrile. CV curves were acquired under an inert atmosphere at a scan rate of 10

26
mV s
-1
. The HOMO and LUMO energy levels were calculated from the oxidation
potential (E´
ox
) and reduction potential (E´
red
), respectively, according to:

E
HOMO
= -I
p
= -( E´
ox
+ 4.71) eV
E
LUMO
= -E
a
= -( E´
red
+ 4.71) eV,

where potential values are relative to the Ag/Ag
+
reference electrode.

2.3.4. 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
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.

2.4. Results and Discussion
2.4.1 Nanocrystal Synthesis and 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. The resulting 0-D nanocrystals are monodisperse with a mean diameter
of 6.9 ± 0.6 nm, as determined by TEM analysis (Figure 2.1). Rather than the
thermodynamically preferred chalcopyrite phase, XRD analysis revealed that the
nanocrystals are composed of wz-Cu-In-S with no evidence of crystalline Cu
2
S or In
2
S
3

27
impurities (Figure 2.2). The lattice constants of a = 3.9 ± 0.1 Å and c = 6.4 ± 0.1 Å
calculated for the nanocrystals are in good agreement with the values reported by Lu and
others (a = 3.9 Å and c = 6.4 Å).
12
The size of the nanocrystals estimated by the Scherrer
formula was 8.4 ± 0.3 nm, which is in close agreement with the TEM data.

High resolution TEM (HRTEM) and SAED analysis of individual Cu-In-S nanocrystals
is consistent with the wurtzite phase. An HRTEM image of an individual nanocrystal
with the wurtzite-specific (102) lattice planes displayed (d = 0.25 nm) is shown in Figure
1b. Moreover, the lattice constants from randomly selected SAED patterns (Figure 2.1
inset) agree with those calculated from the XRD results for wz-Cu-In-S. Since the
relative elemental composition of wz-Cu-In-S can vary greatly, the nanocrystals were

Figure 2.1. (a) TEM micrograph of 6.9-nm Cu-In-S nanocrystals with representative SAED pattern
displayed as inset. (b) HRTEM micrograph of an individual Cu-In-S nanocrystal.

28
analyzed by EDX. Analysis of ten randomly selected areas gave an average Cu:In:S
composition of 0.51:0.11:0.38, which is compositionally quite different from the
expected 0.25:0.25:0.50 stoichiometry for CuInS
2
. Because copper and indium share
lattice sites in the wurtzite structure, this Cu
4.6
In
1.0
S
3.5
composition still falls within the
phase space for wz-Cu-In-S.
19
Also, no other crystal phases were observed by SAED
analysis of randomly selected areas.

It was found that the particular reagents employed for this synthesis play a decisive role
in the formation of wz-Cu-In-S. In a result similar to those of Tang and Omata,
13,20
it was
observed that substituting an equal volume of squalane (i.e., a non-coordinating solvent)
for oleylamine under otherwise identical conditions produced the tetragonal chalcopyrite
phase of Cu-In-S (Figure 2.3). This suggests that the coordination of oleylamine to the

Figure 2.2. XRD pattern of the wurtzite Cu-In-S nanocrystals synthesized with di-tert-butyl disulfide in
oleylamine at 180 ˚C.

29
metal cations likely has a kinetic influence on phase determination. The choice of sulfur
source also plays a critical role in the formation of monodisperse wz-Cu-In-S
nanocrystals. Di-tert-butyl disulfide was chosen as the sulfur source because (i) it is a
liquid that is soluble in common organic solvents, and (ii) it possesses a relatively weak
S-S bond that should aid in relatively low temperature decomposition and sulfur transfer.
Di-tert-butyl disulfide is known to thermally decompose in the gas phase to give
isobutylene, hydrogen sulfide, and a small amount of elemental sulfur.
24
Under the
synthesis conditions, the disulfide is activated in solution to release sulfur species, which
can then react with the metal cations. As described previously for the synthesis of In
2
S
3

nanorods with di-tert-butyl disulfide, metal sulfide formation is relatively slow at 180
˚C,
22
which may be key in phase determination in the wz-Cu-In-S system.

Figure 2.3. (a) XRD pattern of nanocrystals resulting from the reaction done in squalane instead of
oleylamine under otherwise identical conditions. The tetragonal chalcopyrite phase of Cu-In-S is formed as
a result. (b) TEM image of the resulting agglomerated nanocrystals.

30
Elemental sulfur can serve as an effective sulfur source to create a variety of metal
sulfide nanocrystals;
25
however, sulfur is poorly soluble in most solvents at modest
reaction temperatures. When elemental sulfur was used in place of di-tert-butyl disulfide
(equimolar based on sulfur atoms), no crystalline product was observed under otherwise
identical conditions. When an equimolar amount of tert-butyl mercaptan is used (per
sulfur atom) as the sole sulfur source, the phase impure tetragonal chalcopyrite Cu-In-S is
observed (Figure 2.4). It is well known that thiols can act as sulfur sources for the
formation of metal sulfide nanocrystals;
26,27
however, the decomposition and sulfur
transfer rates of the disulfide and mercaptan are sufficiently different in order to exert a
kinetic influence on phase determination. The effect of adjusting the molar ratio
oleylamine to 1-dodecanethiol was also investigated. Results indicate that when molar
ratios are between 14:0 and 12:2 oleylamine/1-dodecanethiol, nanocrystals in the wurtzite

Figure 2.4. (a) XRD pattern of nanocrystals resulting from the reaction performed with tert-butyl
mercaptan as the only sulfur source under otherwise identical conditions. Phase impure chalcopyrite Cu-
In-S is formed as a result. (b) TEM image of the resulting agglomerated nanocrystals.

31
phase are formed. Higher concentrations of 1-dodecanethiol (e.g., 10:4 oleylamine/1-
dodecanethiol) resulted in no nanocrystalline product being formed. Preference for the
wurtzite phase over the chalcopyrite phase is not a direct result of any one component in
this system; rather, it is a combination of kinetic variables that leads to phase
determination. The decomposition rate of the disulfide, the presence of coordinating
solvents such as oleylamine, reaction temperature, and concentration of metal cations in
solution combine to form the proper environment for the selective formation of
metastable wz-Cu-In-S nanocrystals.

2.4.2. Nanocrystal Growth Kinetics
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 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 2.5 and the
corresponding statistical data is displayed in Figure 2.6.
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
3a, these primitive nanocrystals are small (3.7 ± 0.9 nm), ill defined in regard to shape,

32
and agglomerated. Growth is sustained during the initial stage, eventually producing
nanocrystals that are 6.9 nm in diameter (Figure 2.5e) after t = 33 min. From the initial
nucleation event, the nanocrystals size focus until this point in time where the size
distribution, as represented by standard deviation about the mean, reaches a minimum of
!
D
/µ
D
= 9.1% (Figure 2.6b). Such size focusing growth kinetics have been previously
observed in the synthesis of monodisperse manganese oxide and iron oxide
nanocrystals.
28,29
After this invariant region at t = 33 min, the standard deviation of the
Cu-In-S nanocrystals begins to gradually increase, with the standard deviation about the
mean increasing from !
D
/µ
D
= 9.1% to 31.4% (Figure 2.6b). In addition, after t = 33 min
the nanocrystals begin to grow at a slower linear rate, which suggests a steady-state
Ostwald ripening mechanism (Figure 2.6a).
30
At this stage of growth, smaller

Figure 2.5. Representative TEM micrographs demonstrating the temporal evolution of the size and size
distribution of Cu-In-S nanocrystals taken from the reaction mixture at t = (a) 1, (b) 18, (c) 23, (d) 28, (e)
33, (f) 68, (g) 128, and (h) 188 min.

33
nanocrystals are absorbed by larger nanocrystals, causing the Cu-In-S nanocrystal
population to evolve into an more polydispersed ensemble as time progresses.

Figure 2.6. Quantitative statistical results for the nanocrystal growth represented in Figure 2.5. Temporal
evolution of the (a) mean diameter (µ
D
), (b) distribution about the mean (!
D
/µ
D
), and (c) skew (") for the
Cu-In-S nanocrystals. A vertical line is drawn at t = 33 min.

34
Closer analysis of the statistical data reveals further evidence of a late 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 one the first three standardized moments
of the growing population.
31
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 2.6c). 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.
30

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 (vide supra). After 33 min, the
nanocrystals slowly become more compositionally 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 2.7). The initially high 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

35
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, but also the elemental
composition of the nanocrystals.

Figure 2.7. EDX spectra of monodisperse wz-Cu-In-S (a) and large hexagonal platelets of wz-Cu-In-S
(b). Peaks at 0.26, 0.52, and 1.48 keV are assigned to the C K, O K, and Al K lines, respectively.
Nanocrystal composition as a function of time for the growth of monodisperse wz-Cu-In-S nanocrystals,
as determined by EDX (c). Each point represents the average of at least 10 randomly sampled areas.

36
Nanocrystal growth was also monitored by XRD to determine evolution of the
nanocrystal phase and size as a function of time. XRD patterns of the nanocrystals
isolated from the reaction mixture at t = 15 min, 25 min, 60 min, and 180 min show a
gradual sharpening of the reflections as time progresses, indicating nanocrystal growth
(Figure 2.8). By applying the Scherrer equation, the nanocrystal sizes at t = 15 min (5.1
nm), 25 min (6.8 nm), 60 min (9.0 nm), and 180 min (12.8 nm) were shown to
corroborate nanocrystal sizes measured directly by TEM analysis. Interestingly, the
wurtzite phase is observed through the entire reaction course without passing through any
intermediate phases.

Figure 2.8. XRD patterns of the wz-Cu-In-S nanocrystals taken from the reaction mixture at t = 15, 25,
60, and 180 min.

37
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 is
not used in the synthesis, large single-crystalline hexagonal platelets of wz-Cu-In-S are
formed after 180 min (Figure 2.9), 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 platelets are also in the wurtzite
phase (Figure 2.10), and EDX gave an average Cu:In:S composition of 0.29:0.24:0.47,
which is very close to stoichiometric CuInS
2
. The ratio of oleylamine/1-dodecanethiol
strongly influences the overall phase and morphology of the Cu-In-S nanocrystals (vide
supra). In the absence of 1-dodecanethiol, fast nanocrystal growth is observed. This

Figure 2.9. TEM micrograph of wurtzite Cu-In-S hexagonal platelets formed without 1-dodecanethiol,
with representative SAED pattern displayed as inset.

38
suggests that 1-dodecanethiol binds more strongly than oleylamine to the exposed
nanocrystal faces, resulting in an arrested growth condition.

2.4.3. Band Gap Determination
The band gap of the Cu-In-S nanocrystals was determined by UV-vis-NIR spectroscopy
and CV. The band gap of bulk chalcopyrite CuInS
2
is E
g
! 1.45 eV, with a Bohr radius of
4.0 nm,
15
which suggests that very small nanocrystals are needed to observe significant
quantum confinement effects. The as-synthesized 6.9-nm wz-Cu-In-S nanocrystals
absorb strongly through the entire visible region of the spectrum resulting in the black
color of the material (Figure 2.11a). The band gap of the 6.9-nm Cu-In-S nanocrystals
was approximated from the onset of the absorption spectrum to be E
g
= 1.47 eV,
consistent with the band gap of other chalcopyrite and wurtzite Cu-In-S nanocrystals and
close to that of bulk CuInS
2
.
6,14,16
Cyclic voltammetry can also be used to estimate the
band gap for semiconductor nanocrystals.
32
A typical CV curve for the 6.9-nm Cu-In-S

Figure 2.10. XRD pattern of the hexagonal wz-Cu-In-S platelets formed in the absence of 1-
dodecanethiol

39
nanocrystals deposited as a thin film on the carbon working electrode is given in Figure
2.11b. Relative to a Ag/Ag
+
reference electrode, the oxidation (E´
ox
) and reduction (E´
red
)
potentials of the nanocrystals are 1.13 and -0.71 V, respectively. The separation between
the E´
ox
and E´
red
peaks gives an electrochemical band gap of E
g
= 1.84 eV and HOMO
and LUMO levels of -5.84 and -4.00 eV, respectively. The electrochemical band gap is
greater than the band gap estimated from the absorption spectrum; however, higher
electrochemical band gaps have also been observed in the case of II-VI nanocrystals.
32

To confirm that these oxidation and reduction peaks result from the purified nanocrystals,
CV curves were taken of the blank electrolyte and varying concentrations of 1-
dodecanethiol added to the solution (Figure 2.12). As the concentration of 1-
dodecanethiol increases, peaks at ~1.68 and 0.47 V appear, increasing in peak current
with increasing concentration. These peaks are attributed to redox processes pertaining

Figure 2.11. (a) UV-vis absorption spectrum and (b) cyclic voltammetry curve for the 6.9-nm Cu-In-S
nanocrystals (sweep rate = 10 mV s
-1
).

40
to the thiol. Thus, it appears the E´
ox
and E´
red
peaks at 1.13 and -0.71 V, respectively, can
be assigned to the Cu-In-S nanocrystals. The reduction and oxidation peak assignments
correlate well with valence and conduction band energies previously calculated and
experimentally determined for the chalcopyrite phase of CuInS
2
.
33,34

2.5. Conclusion
A facile solution phase synthesis was developed using di-tert-butyl disulfide to produce
monodisperse 6.9-nm Cu-In-S nanocrystals in the metastable wurtzite phase. The keys to
wz-Cu-In-S formation appear to be coordination of the oleylamine solvent to the metal
cation precursors and relatively slow sulfur transfer from the disulfide. It was found that
the growth kinetics described above adhere nicely to a model recently 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

Figure 2.12. (a) CV curves of the electrolyte blank (black), 1 µL of 1-dodecanethiol (blue), and 2 µL of
1-dodecanthiol (red). (b) CV curves of Cu-In-S nanocrystals (purple) with 1 µL of 1-dodecanethiol (red)
and 2 µL of 1-dodecanethiol (green).

41
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
/µ
D
= 9.1%). Following this, the mean
nanocrystal size begins to increase again 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 recently for
(Fe
3
O
4
)
x
(Fe
2
O
3
)
1-x
,
28
MnO,
29
and CdSe
36
nanocrystals. Importantly, it was determined that
1-dodecanethiol was critical for the growth of monodisperse Cu-In-S nanocrystals under
these conditions. Taken together, the synthesis of monodisperse wz-Cu-In-S was
achieved through reaction control (i.e., choice of solvent, capping ligand, and sulfur
source). These nanocrystals were found to have an optical band gap of 1.47 eV that
matches well with the solar spectrum. Future work will 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.

2.6. Acknowledgement
This material is based upon work supported by the National Science Foundation under
DMR-0906745. The authors are thankful for the generous support provided by the
Department of Chemistry and the College of Letters, Arts & Sciences at USC.
Acknowledgement is also made to the Anton Burg Foundation for financial assistance
toward the purchase of the XRD. We also thank Prof. M. Thompson for use of his CV
apparatus.

42
2.7. References
(1) Greenham, N. C.; Peng, X. G. Alivisatos, A. P. Charge separation and transport in
conjugated-polymer/semiconductor-nanocrystal composites studied by
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2
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, CuInSe
2
, and
Cu(In
x
Ga
1-x
)Se
2
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, CuGaSe
2
, and CuInSe
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46

Chapter 3. Synthesis of Metastable Wurtzite CuInSe
2
Nanocrystals*

*Published in Chem. Mater. 2010, 22, 1613-1615.
3.1. Introduction
The ever increasing demand for energy resources continues to drive the development of
cost effective and higher efficiency photovoltaic devices.
1
The I-III-VI family of
semiconductors, including CuInS
2
, CuInSe
2
and CuIn
x
Ga
1-x
Se
2
, have band gaps that match
well with the solar spectrum, have large absorption coefficients and good photostability,
and avoid the use of overly toxic elements.
2
Thin films of I-III-VI semiconductors have
been used as the active layer for high efficiency solar cells, with CuIn
x
Ga
1-x
Se
2
-based
solar cells demonstrating power conversion efficiencies of nearly 20%.
3
One of the ways
to decrease the cost of these devices is to use solution-derived I-III-VI semiconductor
nanocrystals as the active layer, which is advantageous because solution-phase routes are
less energy intensive, have the potential for scalability, and nanocrystal suspensions can
easily be deposited via spin-coating or printing methods.
4

The tetragonal chalcopyrite phase of CuInSe
2
is thermodynamically preferred at room
temperature.
5
In the bulk, chalcopyrite CuInSe
2
possesses a desired band gap of E
g
!
1.04–1.10 eV, with a reported Bohr exciton radius of 10.6 nm.
6,7
A number of solution-
phase routes to high-quality chalcopyrite CuInSe
2
nanocrystals have been reported over
the last few years.
6,8-11
Since elemental stoichiometry, morphology, and crystal symmetry
are known to influence the optoelectronic properties of semiconductor nanocrystals, it is

47

of interest to explore new kinetically controlled pathways to unusual metastable crystal
phases. Recently, the hexagonal wurtzite (wz) phase of CuInS
2
nanocrystals was
discovered by Yang and Lu.
12
In the bulk, wz-CuInS
2
is a high-temperature phase stable
between 1045 and 1090 °C, where the copper and indium cations randomly share
common lattice sites. Because wz-CuInS
2
nanocrystals are accessible at room
temperature,
12-15
it follows that other types of I-III-VI nanocrystals may exist in non-
chalcopyrite phases at room temperature. Indeed, Hillhouse and Agrawal have recently
shown that nanocrystals of the metastable sphalerite phase of CuInSe
2
can be
synthesized.
6
Herein, we report the first example of the wurtzite phase for CuInSe
2

nanocrystals.

3.2. Experimental Details
3.2.1. General Considerations
Copper(I) chloride (CuCl, 99.999%; Strem Chemicals), indium(III) acetylacetonate
(98%; Strem Chemicals), oleylamine (cis-9-octadecenylamine, 70%; TCI America),
diphenyl diselenide (98%; Alfa Aesar), and squalane (98%; Alfa Aesar) were used
without further purification. Nanocrystal syntheses were performed under nitrogen, in
the absence of water and oxygen, using standard Schlenk techniques.

48

3.2.2. Synthesis of CuInSe
2
Nanocrystals
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. Degassed
oleylamine (2.83 mL, 8.52 mmol) was transferred to the reaction flask containing the
metal salts via cannula. Prior to heating, the system was cycled between vacuum and
nitrogen three times, then heated to 90 ˚C (10 ˚C min
-1
), and again cycled three times
between vacuum and nitrogen to eliminate adventitious water and dissolved oxygen.
Diphenyl diselenide (0.22 g, 0.71 mmol) dissolved in 0.85 mL of mesitylene was quickly
injected into the system under flowing nitrogen, and the temperature was increased to 180
˚C (10 ˚C min
-1
) and allowed to react for 180 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 (1 mL) and ethanol (10 mL) to yield the purified product. The resulting
CuInSe
2
nanocrystals form stable suspensions in common organic solvents, such as
hexane and toluene. In a result similar to Peng et al.,
16
it was found that the nanocrystals
are sensitive to oxidation when exposed to air over time, as evidenced by the appearance
of the (222) reflection for cubic-In
2
O
3
in the diffraction pattern.

3.2.3. Material Characterization
Powder X-ray diffraction (XRD) analyses were performed on a Rigaku Ultima IV X-ray
diffractometer using a Cu K! radiation source (" = 1.54 Å). The simulated CuInSe
2

powder XRD patterns were calculated using the Diamond 3.0 program. Transmission

49

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. Visible-NIR absorption spectra were acquired on a Cary 14 spectrophotometer in
dual beam mode using quartz cuvettes with 1 cm path lengths from nanocrystal
suspensions in cyclohexane. 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 kV. Samples were deposited on an
aluminum tab and ten areas were randomly analyzed. X-ray photoelectron spectra (XPS)
were acquired on a VG Escalab II using a monochromated aluminum anode.

3.3. Results and Discussion
3.3.1. Nanocrystal Synthesis
Diorganodichalcogenides have shown recent utility as facile and low temperature
chalcogen sources for semiconductor nanocrystal syntheses.
15,17-19
Here, CuInSe
2

nanocrystals were synthesized by injecting a solution of diphenyl diselenide into a hot
solution of CuCl and In(acac)
3
in oleylamine and heating to 180 °C for 3 hours. The
resulting CuInSe
2
nanocrystals form stable suspensions in common organic solvents, such
as hexanes and toluene.

50

3.3.2. Simulation of Wurtzite Diffraction Pattern
The powder X-ray diffraction (XRD) pattern of the CuInSe
2
nanocrystals prepared by this
procedure did not match the diffraction patterns for previously reported chalcopyrite
CuInSe
2
nanocrystals,
8-11
or those in the JCPDS database (JCPDS no. 00-040-1487). A
diffraction pattern was simulated starting from the wz-ZnSe crystal structure and
substituting the Zn
2+
lattice positions with a 50/50 occupancy probability of Cu
+
and In
3+

cations. The atomic coordinates of Cu, In, and Se for CuInSe
2
with the wurtzite crystal
Table 3.1. Wurtzite CuInSe
2
Atomic Coordinates
Atom Wyck. x/a y/b z/c
Cu 2a 0.33 0.67 0.3752
In 2b 0.33 0.67 0.3752
Se 2a 0.33 0.65 0.03

Figure 3.1. Simulated XRD pattern of wurtzite-CuInSe
2
.

51

structure and P6
3
mc (No. 186) space group are shown in Table 1. The lattice parameters
(a = 4.08 Å and c = 6.69 Å) used for the simulation were experimentally determined. The
resulting simulated wurtzite-CuInSe
2
XRD pattern and unit cell are shown in Figure 3.1
and Figure 3.2, respectively.

3.3.3. Nanocrystal Characterization
The simulated and experimental diffraction patterns match well (Figure 3.3), signifying
that these CuInSe
2
nanocrystals possess a wurtzite crystal structure. Differences in
intensity between the simulated and experimental diffraction patterns can be explained by
some degree of preferred orientation of the nanocrystals, which is corroborated by
selected area electron diffraction (vide infra). Lattice parameters of a = 4.08 Å and c =
6.69 Å were calculated from the experimental diffraction pattern for the wz-CuInSe
2

Figure 3.2. Simulated wurtzite-CuInSe
2
unit cell.

52

nanocrystals. The major diffraction peaks can be indexed to the (100), (002), (101),
(102), (110), (103), (112), and (202) reflections of the simulated wurtzite crystal
structure. The (200) and (211) reflections for chalcopyrite CuInSe
2
at 31° and 36° 2!,
respectively, are distinct in position from the wurtzite CuInSe
2
diffraction peaks and are
noticeably absent in the diffraction pattern. Likewise, the (200) diffraction peak for cubic
Cu
2
Se at 31° 2! (JCPDS no. 00-006-0680) and (090)/(620) diffraction peaks for
hexagonal Cu
2
Se at 40° 2! (JCPDS no. 00-047-1448), which are also be distinct, were
not observed. A comparison of the experimental and simulated d-spacing values is shown
in Table 2.

It was found that specific reaction control is necessary to synthesize wz-CuInSe
2
. When
an equal volume of squalane (i.e., a noncoordinating solvent) was substituted for

Figure 3.3. Experimental and simulated XRD patterns of wurtzite-CuInSe
2
nanocrystals.

53

oleylamine under otherwise identical conditions, the tetragonal chalcopyrite phase of
CuInSe
2
was produced (Figure 3.4). This suggests that the presence of oleylamine has an
influence on phase determination. Similar phenomena have recently been observed; that
is, primary amines have been shown to influence the formation of wurtzite ZnS, CdSe,
and CuInS
2
nanocrystals.
15,20-22
It is thought that amine coordination to the metal cations
has a kinetic effect on structure determination. In addition to amine, the use of diphenyl
diselenide as a selenium source was also necessary for the production of wurtzite
Table 3.2. Comparison of experimental and simulated d-spacing values with respect to their corresponding
diffraction peaks.
2! (degrees)
d-spacing
simulated (Å)
d-spacing
experimental (Å)
25.21 3.53 3.55
26.70 3.34 3.35
28.56 3.12 3.14
37.01 2.43 2.44
44.40 2.04 2.05
48.15 1.89 1.89
52.54 1.74 1.75
53.67 1.71 1.72
67.52 1.39 1.39
25.21 3.53 3.55

54

Figure 3.4. XRD pattern of nanocrystals resulting from the reaction done in squalane instead of oleylamine
under otherwise identical conditions. The tetragonal chalcopyrite phase of CuInSe
2
is formed as a result.

Figure 3.5. XRD pattern of nanocrystals resulting from the reaction done with elemental selenium
instead of the diselenide under otherwise identical conditions. Chalcopyrite CuInSe
2
and Cu
2
Se are
formed as a result.

55

nanocrystals. Use of elemental selenium under otherwise identical conditions at 180 °C
resulted in chalcopyrite CuInSe
2
in addition to Cu
2
Se impurities (Figure 3.5).
Interestingly, thiourea has been used to form wz-CuInS
2
nanocrystals in the presence of
an amine;
14
however, the analogous reaction with selenourea yielded chalcopyrite
CuInSe
2
nanocrystals rather than the wurtzite phase.
8
Thus, the rate of selenium transfer
from diphenyl diselenide appears to be just right to kinetically access the metastable
wurtzite phase at 180 °C.

The as-synthesized wz-CuInSe
2
nanocrystals are polydisperse with a mean diameter of
29.9 ± 6.6 nm, as determined by transmission electron microscopy (TEM) analysis
(Figure 3.6). The lattice parameters calculated from selected area electron diffraction

Figure 3.6. (a) Low-res TEM micrograph of CuInSe
2
nanocrystals with representative SAED pattern
displayed as inset. (b) High-res TEM micrograph of an individual CuInSe
2
nanocrystal.

56

(SAED) patterns (Figure 3.7) of several randomly chosen regions of the CuInSe
2

nanocrystals agree with the lattice parameters calculated from the XRD pattern for wz-

Figure 3.7. Indexed SAED pattern of an ensemble of wz-CuInSe
2
nanocrystals.

Figure 3.8. Hi-res TEM micrograph of two differently sized wz-CuInSe
2
nanocrystals. The (002) lattice
planes are displayed for both nanocrystals (d = 0.33 nm).

57

CuInSe
2
. A high-resolution TEM image of an apparent single crystalline particle with the
(002) lattice planes displayed (d = 0.33 nm) and of two differently sized wz-CuInSe
2

nanocrystals in Figure 3.8. Also, no other crystal phases were observed by SAED
analysis (Figure 3.7). Energy dispersive X-ray spectroscopy was used to analyze the
elemental composition of the wz-CuInSe
2
nanocrystals (Figure 3.9). Analysis of ten
randomly selected areas gave an average Cu:In:Se composition of 0.31:0.22:0.47. This
experimentally determined Cu
1.32
In
0.94
Se
2.00
elemental stoichiometry is consistent with a
Cu
x
In
y
Se
0.5x+1.5y
stoichiometry (within experimental error), and deviation from the ideal
CuInSe
2
stoichiometry is made possible because Cu
+
and In
3+
share lattice positions in the
wurtzite structure.
12
In the related system where wz-CuInS
2
nanocrystals were
synthesized using a dialkyl disulfide in oleylamine, it was thought that the slight

Figure 3.9. Representative EDX spectrum of wz-CuInSe
2
nanocrystals.

58

Figure 3.10. High-resolution XPS spectra of Cu 2p, In 3d, and Se 3d regions taken of wz-CuInSe
2

nanocrystals.

59

stoichiometric excess of copper in the nanocrystals resulted from copper nucleating faster
than indium and sulfur.
15
It is possible that a similar phenomenon is observed here. X-
ray photoelectron spectroscopy (XPS) was used to confirm elemental oxidation states for
the CuInSe
2
nanocrystals (Figure 3.10). The Cu 3p
3/2
peak at 931.8 eV matches well with
the reported binding energy for Cu
+
in CuInSe
2
, while the In 3d
5/2
peak at 444.2 eV is
representative of In
3+
in CuInSe
2
.
23
Finally, an asymmetric peak at 53.8 eV is
representative of the Se 3d binding energy for lattice Se
2–
.
23

3.3.4. Band Gap Determination
The wz-CuInSe
2
nanocrystals absorb strongly through the entire visible and into the near-

Figure 3.11. Visible-NIR absorption spectrum of the wurtzite-CuInSe
2
nanocrystals. The small features
from 1200-1400 nm are related to absorption from cyclohexane.

60

IR region of the spectrum (Figure 3.11), resulting in the black color of the material. The
band gap of the wz-CuInSe
2
nanocrystals was determined from the onset of the visible-
NIR absorption spectrum to be E
g
= 1.19 eV, while bulk chalcopyrite CuInSe
2
has a
reported band gap between 1.04–1.10 eV.
6,7
The reported Bohr exciton radius of 10.6 nm
for CuInSe
2
suggests that smaller nanocrystals are needed to observe significant quantum
confinement effects.
6,7

3.4. Conclusion
For the first time, a solution-phase synthesis of wz-CuInSe
2
nanocrystals was described.
The keys to wz-CuInSe
2
formation include the use of an amine solvent and facile
selenium transfer from a diselenide at relatively low temperatures. This metastable phase
may have important implications in photovoltaic device performance and heterojunction
formation with wz-CdS overgrowth layers.
24
Future work will focus on controlling the
size and shape of the CuInSe
2
nanocrystals and incorporating them into a functional
photovoltaic device.

3.5. Acknowledgment
This material is based on work supported by the National Science Foundation under
DMR-0906745. M.E.N. was supported as part of the Center for Energy Nanoscience, an
Energy Frontier Research Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under award number DE-SC0001013.

61

Acknowledgement is also made to the Molecular Materials Research Center of the
Beckman Institute at Caltech for use of XPS.

3.6. References
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2
solar cell with 81.2% fill
factor. Prog. Photovolataics 2008, 16, 235.
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2

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2009, 3, 325.
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2
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Cu
2
Se/In
2
Se
3
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62

(9) Panthani, M. G.; Akhavan, V.; Goodfellow, B., Schmidtke, J. P.; Dunn, L.;
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2
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Cu(In
x
Ga
1-x
)Se
2
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red to near-infrared. J. Am. Chem. Soc. 2008, 130, 9240.
(11) Tang, J.; Hinds, S.; Kelley, S. O.; Sargent, E. H. Synthesis of colloidal CuGaSe
2
,
CuInSe
2
, and Cu(InGa)Se
2
nanoparticles. Chem. Mater. 2008, 20, 6906.
(12) Pan, D.; An, L.; Sun, Z.; Hou, W.; Yang, Y.; Yang, Z.; Lu, Y. Synthesis of
Cu!In!S ternary nanocrystals with tunable structure and composition. J. Am.
Chem. Soc. 2008, 130, 5620.
(13) Connor, S. T.; Hsu, C.-M.; Weil, B. D.; Aloni, S.; Cui, Y. Phase transformation of
biphasic Cu
2
S!CuInS
2
to monophasic CuInS
2
nanorods. J. Am. Chem. Soc. 2009,
131, 4962.
(14) Koo, B.; Patel, R. N.; Korgel, B. A. Wurtzite!chalcopyrite polytypism in CuInS
2

nanodisks. Chem. Mater. 2009, 21, 1962.
(15) Norako, M. E.; Franzman, M. A.; Brutchey, R. L. Growth kinetics of
monodisperse Cu!In!S nanocrystals using a dialkyl disulfide sulfur source.
Chem. Mater. 2009, 21, 4299.
(16) Xie, R.; Rutherford, M.; Peng, X. Synthesis of Cu-doped InP nanocrystals (d-
dots) with ZnSe diffusion barrier as efficient and color-tunable NIR emitters. J.
Am. Chem. Soc. 2009, 131, 5691.
(17) Franzman, M. A.; Pérez, V.; Brutchey, R. L. Peroxide-mediated synthesis of
indium oxide nanocrystals at low temperatures. J. Phys. Chem. C 2009, 113, 630.
(18) Franzman, M. A.; Brutchey, R. L. Solution-phase synthesis of well-defined
indium sulfide nanorods. Chem. Mater. 2009, 21, 1790.

63

(19) Webber, D. H.; Brutchey, R. L. Photolytic preparation of tellurium nanorods.
Chem. Commun. 2009, 5701.
(20) Li, Y.; Li, X.; Yang, C.; Li, Y. Ligand-controlling synthesis and ordered assembly
of ZnS nanorods and nanodots. J. Phys. Chem. B 2004, 108, 16002.
(21) Nose, K.; Soma, Y.; Omata, T.; Otsuka-Yao-Matsuo, S. Synthesis of ternary
CuInS
2
nanocrystals; phase determination by complex ligand species. Chem.
Mater. 2009, 21, 2607.
(22) Mahler, B.; Lequeux, N.; Dubertret, B. Ligand-controlled polytypism of thick-
shell CdSe/CdS nanocrystals. J. Am. Chem. Soc. 2010, 132, 953.
(23) Kazmerski, L. L.; Jamjoum, O.; Ireland, P. J.; Deb, S. K.; Mickelsen, R. A.; Chen,
W. Initial oxidation of CuInSe
2
. J. Vac. Sci. Technol. 1981, 19, 467.
(24) Olekseyuk, I. D.; Parasyuk, O. V.; Piskach, L. V. The reciprocal
CuInS
2
+2CdSe!CuInSe
2
+2CdS system. Part I. The quasi-binary CuInSe
2
–CdSe
system: Phase diagram and crystal structure of solid solutions. J. Solid State
Chem. 2006, 179, 315.
!
!
"#!
Chapter 4. Synthesis and Characterization of Wurtzite-Phase Copper
Tin Selenide Nanocrystals*

$%&'()*+,-!).!!"#$%"#&'(%"#)*+"#!"#!/!,-./!0120"3!
4.1. Abstract
A new wurtzite phase of copper tin selenide (CTSe) was discovered and the resulting
nanocrystals were synthesized via a facile solution phase method. The wurtzite CTSe
nanocrystals were synthesized with dodecylamine and 1-dodecanethiol as coordinating
solvents and di-tert-butyl diselenide (
t
Bu
2
Se
2
) as the selenium source. Specific reaction
control (i.e., a combination of 1-dodecanethiol with
t
Bu
2
Se
2
) was proven to be critical in
order to obtain this new phase of CTSe, which was verified by powder X-ray diffraction
and selected area electron diffraction. The wurtzite CTSe nanocrystals possess an optical
and electrochemical band gap of 1.7 eV and display an electrochemical photoresponse
indicative of a p-type semiconductor.

4.2. Introduction
The need for low-cost, scalable, and solution-processible photovoltaic materials is a
leading impetus for new research in the field of semiconductor nanocrystal synthesis.
1-3

While great advancements in photovoltaic technologies have been made with CdTe, PbSe,
and CuIn
x
Ga
1-x
S
2
(CIGS) nanocrystals,
4-6
there is a current trend to replace materials
containing environmentally harmful (e.g., Cd and Pb) and scarce (e.g., Te, In, Ga)
elements with environmentally benign photovoltaic materials comprised of more earth
!
!
"#!
abundant elements.
7,8
Along these lines, there has been a recent focus on the
development of Cu
2
ZnSnS
4
(CZTS) and Cu
2
ZnSnSe
4
(CZTSe) nanocrystals,
9-11
in
addition to the less compositionally complex binary tin monochalcogenide (SnS and
SnSe) nanocrystals.
12-16
These materials all possess band gaps in the range of E
g
= 1.3-1.6
eV in addition to relatively high absorption coefficients (~10
4
cm
-1
), which make them
attractive candidates as absorber layers in photovoltaic devices.

Another potentially attractive, yet not well studied, material is Cu
2
SnSe
3
(CTSe). CTSe
is a ternary I-IV-VI semiconductor that possesses a direct band gap in the range of E
g
=
0.8-1.1 eV, a high absorption coefficient (~10
4
-10
5
cm
–1
), and electron and hole mobilities
on the order of ~2 and ~870 cm
2
V
–1
s
–1
, respectively, for bulk material.
17-20
While it is
often found as an impurity in CZTSe, there have been only a few examples where phase-
pure CTSe has been prepared – as bulk single-crystals,
21-23
as thin films,
17,19,20
and most
recently in colloidal nanocrystal form.
24
These examples of CTSe were reported to
crystallize in a cubic sphalerite-like phase (a = 5.69 Å) or in a monoclinic phase with a
sphalerite superstructure (a = 6.59, b = 12.16, c = 6.61 Å; ! = 108.6˚).
23
Since crystal
symmetry is known to influence the properties of materials, there have been a number of
recent reports describing the use of kinetic control to determine crystal phase in
nanocrystals. For example, CuInS
2
, CuInSe
2
and CIGS nanocrystals have been prepared
in wurtzite and sphalerite crystal structures rather than the typical chalcopyrite
structure.
25-29
Similarly, CZTS nanocrystals were recently reported in a wurtzite phase
rather than the expected kesterite phase.
30
These investigations have shown that reaction
control (including temperature, solvent, capping ligands, and chalcogenide source) is
!
!
""!
critical for crystal phase determination. Herein, we present the first report of a wurtzite
phase for CTSe, in the form of colloidal nanocrystals, and establish its potential viability
as a photovoltaic material by demonstrating clear electrochemical photocurrent upon
illumination.

4.3. Experimental Details
4.3.1. General Considerations
Copper(I) chloride (CuCl, Strem Chemicals, 99.999%), tin(IV) iodide (SnI
4
, Aldrich,
95%), 1-dodecanethiol (Alfa Aesar, 98%), and selenium powder (Alfa Aesar, 99.999%)
were all purchased and used without further purification. Dodecylamine (Alfa Aesar,
(98+%) was distilled from CaO prior to use. Nanocrystal syntheses were performed
under nitrogen, in the absence of water and oxygen, using standard Schlenk techniques.

4.3.2. Synthesis of Di-tert-Butyl Diselenide (
t
Bu
2
Se
2
)
Di-tert-butyl diselenide was synthesized according to an improved version of a
previously published method.
31
Briefly, magnesium turnings (10.75 g, 0.44 mol) were
allowed to stir 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 min,
producing a gray solution. Selenium (31.74 g, 0.40 mol) was subsequently added and
allowed to stir for an additional 30 min. The solution was 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 min, 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 ˚C (0.05 mmHg), the product was characterized by
1
H,
13
C, and
77
Se
NMR in CDCl
3
.
1
H: ! = 1.46 (s); {
1
H}
13
C: ! = 41.7 (s), 32.5 (s); {
1
H}
77
Se: ! = 488 (s)
ppm.

4.3.3. Copper Tin Selenide Nanocrystal Synthesis
In a typical synthesis, SnI
4
(0.125 g, 0.200 mmol) and CuCl (0.040 g, 0.404 mmol) were
added to a two-neck round-bottom flask fitted with a reflux condenser, stir bar, and
rubber septum and heated (10 ˚C min
-1
) to 40 ˚C. Dodecylamine (0.56 mL, 2.4 mmol)
and 1-dodecanethiol (0.10 mL, 0.42 mmol) were added to the reaction flask via syringes
and then cycled between vacuum and nitrogen three times. Then, the system was heated
(10 ˚C min
-1
) to 95 ˚C, and was again cycled three times between vacuum and nitrogen to
eliminate adventitious water and dissolved oxygen. The temperature was then increased
(10 ˚C min
-1
) to 180 ˚C and
t
Bu
2
Se
2
(0.11 mL, 0.56 mmol) was quickly injected into the
system under flowing nitrogen, and allowed to react for 5 min with stirring. After being
cooled to room temperature, the reaction mixture was dissolved in 5 mL of toluene and
precipitated with 10 mL of ethanol, sonicated, and centrifuged (6000 rpm for 15 min) to
yield a black solid. Dispersion/precipitation was repeated three times with toluene (1
mL) and ethanol (4 mL) to yield the as-prepared product. When oleic acid was added to
!
!
"#!
the reaction medium, varying amounts (0.13-0.51 mL, 0.4-1.6 mmol) were added along
with the dodecylamine and 1-dodecanethiol.

4.3.4. Nanocrystal Characterization
Powder X-ray diffraction (XRD) analyses were performed on a Rigaku Ultima IV X-ray
diffractometer using a Cu K! radiation source (" = 1.54 Å). The simulated CTSe
diffraction patterns were calculated using the Diamond 3.0 program. 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
20–30 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 kV. Samples were deposited on a silicon wafer and several
areas were randomly analyzed. UV-vis-NIR absorption spectra were acquired on a
Perkin-Elmer Lambda 950 spectrophotometer using dilute nanocrystal suspensions in
organic solvents. Electrochemical studies were carried out using a BASi Epsilon-EC
potentiostat using nanocrystal films on ITO (7 ! 50 ! 0.7 mm, R
s
= 5-15 ", Delta
Technologies Ltd., Loveland, CO). A suspension of the nanocrystals was spun-cast at
3000 rpm for 30 s to form a thin black film. Hydrazine treatment was carried out under
ambient atmosphere by dipping a CTSe nanocrystals film into a 0.10 M hydrazine
solution in CH
3
CN for 20 s, and then rinsing with neat CH
3
CN. Tetra-n-butylammonium
hexafluorophosphate (Alfa-Aesar, 98%), twice recrystallized from absolute ethanol, was
!
!
"#!
employed as the supporting electrolyte at concentrations of 0.10 M in acetonitrile (BDH,
99.9%) for DPV analysis. DPV was performed with a scan rate of 10 mV s
-1
under
nitrogen using a Pt wire counter electrode and Ag wire pseudo-reference electrode,
calibrated against Fc/Fc
+
. Transient photocurrent measurements were conducted in a
quartz cell under nitrogen in aqueous 0.01 M Eu(NO
3
)
3
/0.1 M KCl using 472 nm LED
illumination with a Ag wire pseudo-reference electrode and a Pt wire counter electrode.
The photoelectrochemical response of the CTSe nanocrystal films was assessed at several
potentials under 10 s on/off chopped illumination.

4.4. Results and Discussion
4.4.1. Nanocrystal Synthesis
Diorganodichalcogenides have shown recent utility as facile and low temperature
chalcogen sources for semiconductor nanocrystal syntheses.
12,26,27,32-35
Here, CTSe
nanocrystals were synthesized via the fast addition of di-tert-butyl diselenide (
t
Bu
2
Se
2
,
0.56 mmol) into a solution of CuCl (0.40 mmol) and SnI
4
(0.20 mmol) in dodecylamine
and 1-dodecanethiol (with or without oleic acid) at 180 ˚C, followed by heating for 5 min.
The powder X-ray diffraction (XRD) pattern of the resulting CTSe nanocrystals did not
match the diffraction pattern for previously reported CTSe nanocrystals or those in the
JCPDS database (JCPDS no. 03-065-7524).
23
A diffraction pattern was simulated
starting from the wurtzite ZnSe crystal structure and substituting the Zn
2+
lattice positions
with a 2:1 occupancy probability of Cu
+
and Sn
4+
cations.

!
!
"#!
4.4.2. Simulation of Wurtzite Copper Tin Selenide Diffraction Pattern
The diffraction pattern was simulated starting from the wurtzite ZnSe structure and
substituting the Zn
2+
lattice positions with a 2/1 occupancy probability of Cu
+
and Sn
4+

cations. The lattice parameters (a = b = 3.9772(6) Å, c = 6.655(2) Å) used for the
simulation were experimentally determined by fitting the diffraction maxima
corresponding to the (100) and (002) reflections, respectively, with a pseudo-Voigt
function. The atomic coordinates of Cu, Sn, and Se for Cu
2
SnSe
3
with the wurtzite crystal
structure and P6
3
mc (No. 186) space group are shown in Table 4.1.

4.4.3. Nanocrystal Characterization
The lattice constants calculated from the experimental diffraction pattern (a = b =
3.9772(6) Å and c = 6.655(2) Å) were used in this simulation. The d-spacings of the
experimentally observed reflections match well with the simulated reflections, suggesting
that the CTSe nanocrystals adopt a wurtzite crystal structure (see Figure 4.1); however,
the experimentally observed diffraction intensities match less well with the simulated

Table 4.1. Wurtzite CTSe Atomic Coordinates
Atom Wyck. x/a y/b z/c
Cu 2b 0.33 0.67 0.3752
Sn 2b 0.33 0.67 0.3752
Se 2b 0.33 0.67 0
!
!
!
"#!
diffraction pattern likely because of the elemental nonstoichiometry in the CTSe
nanocrystals (vide infra). Moreover, the variable diffraction peak widths suggest some
degree of particle anisotropy. The major diffraction peaks can be indexed to the (100),
(002), (101), (102), (110), (103), (112), (201), (202), and (203) reflections of the
simulated wurtzite crystal structure. The (100), (101), (102), (103), (202), and (203)
wurtzite reflections are distinct from cubic CTSe (Table 4.2). There is a significant 2!
shift between the second most intense reflections of each phase, which are (110) for
wurtzite CTSe (2! "45.6˚) and (220) for cubic CTSe (2! "44.8˚). Therefore, if a
significant amount of the cubic phase was present in the wurtzite CTSe, the shape of the
diffraction maximum corresponding to the (110) reflection should be asymmetric, with a
!

Figure 4.1. Experimental XRD patterns of (a) wurtzite and (b) cubic CTSe nanocrystals. The simulated
XRD patterns of wurtzite (red) and cubic (black) CTSe are shown for reference.
!
!
!
"#!
shoulder on the low angle side; however, such an asymmetry was not observed (see
Figure 4.2). This implies that if the cubic phase is present in the wurtzite CTSe, the
amount is not significant by XRD analysis. Moreover, the (400) reflection of the cubic
structure is distinct from with wurtzite structure and is not observed in the XRD pattern
of wurtzite CTSe.

Specific reaction control is crucial to synthesize CTSe in the wurtzite phase. Under
otherwise identical conditions, the nanocrystal synthesis without the addition of 1-
dodecanethiol results in a phase-pure cubic CTSe

product (Figure 4.1). Similar
phenomena have recently been observed; that is, capping ligands have been shown to
Table 4.2. Comparison of experimental and simulated d-spacing values with respect to their corresponding
diffraction peaks.
2! (degrees)
d-spacing
simulated (Å)
d-spacing
experimental (Å)
(h k l)
25.85 3.44 3.39 (100)
26.78 3.33 3.25 (002)
29.17 3.06 3.05 (101)
37.56 2.39 2.39 (102)
45.58 1.99 1.99 (110)
48.80 1.86 1.86 (103)
53.65 1.71 1.71 (112)
55.03 1.67 n/a (201)
60.48 1.53 n/a (202)
68.99 1.36 1.37 (203)
!
!
!
"#!
influence the formation of metastable crystal phases.
36-38
Interestingly, addition of
increasing amounts of oleic acid (with 1-dodecanethiol and dodecylamine) appears to
yield mainly cubic CTSe (see Figure 4.3a). When elemental selenium was substituted for
t
Bu
2
Se
2
under otherwise identical conditions (i.e., equimolar based on selenium in the
presence of 1-dodecanethiol and dodecylamine), a phase-impure cubic CTSe was
produced (Figure 4.3b). When TOPSe was used in an analogous way, phase-pure
orthorhombic SnSe was formed, with no crystalline copper containing phases being
present (Figure 4.3c). Thus, the chalcogenide source also seems to have a strong
influence on phase determination. Preference for the wurtzite phase over the cubic phase
appears to not be a direct result of any one component in this system; rather, it is a
combination of variables (i.e., a particular combination of capping ligands and selenium
source) that leads to phase determination.

Figure 4.2. XRD patterns in the 40-60˚ 2! range illustrating the peak symmetry of the (110) reflection for
the CTSe nanocrystals.
!
!
!
"#!

The resulting CTSe nanocrystals have a mean diameter of 15.1 ± 2.9 nm, as determined
by TEM analysis (Figure 4.4). The lattice parameters calculated from selected area
electron diffraction (SAED) patterns of several randomly chosen regions of the CTSe
nanocrystals agree with the lattice parameters calculated from the XRD pattern for
wurtzite CTSe. A high-resolution TEM (HRTEM) image of an apparent single
crystalline particle with the (002) lattice planes displayed (d = 3.26 Å) is shown in Figure
4.4a. Energy dispersive X-ray spectroscopy (EDX) was used to analyze the elemental
composition of the wurtzite CTSe nanocrystals (Figure 4.5). Analysis of several
!
!
!
Figure 4.3. XRD patterns of nanocrystals resulting from the reactions performed with (a) 8 equivalents of
oleic acid (versus SnI
4
); (b) elemental selenium used instead of the
t
Bu
2
Se
2
chalcogen source; and (c) TOPSe
used instead of the
t
Bu
2
Se
2
chalcogen source. All reactions were otherwise performed identically at 180 ˚C
for 5 min. The simulated diffraction patterns for wurtzite CTSe (black), cubic CTSe (maroon), and
orthorhombic SnSe (orange) are given as reference.
!
!
!
"#!
randomly selected areas gave an average Cu:Sn:Se composition of 1.57:0.92:3.00. This
stoichiometry is slightly tin and selenium-rich (Cu/Sn = 1.71; Se/(Cu + Sn) = 1.14) and
deviates from the expected Cu
2
SnSe
3
stoichiometry; however, no additional crystalline
phases of tin or selenium (e.g., SnSe or SnSe
2
) were observed by XRD.

!
!
!
Figure 4.4. (a) HRTEM image of a CTSe nanocrystal with an interplanar spacing of 3.26 Å; (b) low
resolution TEM image of CTSe nanocrystals with an average diameter of 15 nm; (c) SAED pattern of
nanocrystals indexed to wurtzite CTSe; and (d) UV-vis-NIR absorption spectrum of wurtzite CTSe
nanocrystals.
!
!
"#!
4.4.4. Band Gap Determination
The wurtzite CTSe nanocrystals absorb through the visible region of the solar spectrum,
resulting in the black color of the material. UV-visible-NIR absorption spectroscopy was
used to estimate a direct optical band gap of E
g
= 1.7 eV for the wurtzite CTSe
nanocrystals (Figure 4.4d), which is higher in energy than the values previously reported
for cubic Cu
2
SnSe
3
thin films (E
g
= 0.8-1.1 eV).
17
Interestingly, a direct optical band gap
of E
g
= 1.5 eV was estimated for the colloidal cubic CTSe (see Figure 4.6), suggesting
that these nanocrystals are in the quantum confined regime.

Figure 4.5. Representative EDX spectrum of wurtzite CTSe nanocrystals.
!
!
!
""!
Differential pulse voltammetry can be used to electrochemically estimate the band gap
for semiconductor nanocrystals.
39,40
Differential pulse voltammograms (DPVs) of
hydrazine treated cubic and wurtzite CTSe nanocrystal films deposited on ITO are
compared in Figure 4.7. The separation between the E´
ox
and E´
red
peak onsets gives
electrochemical band gaps of E
g
= 1.5 and 1.7 eV for the cubic and wurtzite phase
nanocrystals, respectively, which is in agreement with the estimated optical band gaps.
In the absence of hydrazine treatment, the as-made nanocrystals can be difficult to
electrochemically characterize due to the insulating nature of the native capping ligands.
It was found that briefly (20-30 s) dipping films of cubic or wurtzite CTSe in a 0.10 M
hydrazine solution in acetonitrile enabled the collection of DPVs that appear much
!
!

Figure 4.6. UV-visible-NIR absorption spectrum of cubic CTSe nanocrystals.
!
!
!
"#!
cleaner and possess stronger signals than those obtained without post-deposition
hydrazine treatment.

4.5. Potential Application for Photovoltaics
To assess the potential applicability of the wurtzite CTSe nanocrystals as a photovoltaic
material, the transient photocurrent of hydrazine-treated wurtzite CTSe nanocrystal films
were evaluated in an aqueous photoelectrochemical cell containing 0.01 M Eu(NO
3
)
3
and
!
!
Figure 4.7. Differential pulse voltammograms of cubic (orange) and wurtzite (red) CTSe nanocrystals,
both treated with hydrazine on ITO in 0.1 M TBAPF/CH
3
CN. DVPs were obtained with a scan rate of 10
mV s
-1
using a Pt wire counter electrode and Ag wire pseudo-reference electrode, calibrated against Fc/Fc
+
.
The onsets of the reduction (-0.4 V for wurtzite CTSe and -0.28 V for cubic CTSe) and oxidation (1.3 V for
wurtzite CTSe and 1.2 V for cubic CTSe) peaks were used to determine the electrochemical band gaps of
ca. 1.7 eV (wurtzite) and 1.5 eV (cubic). The shoulders observed immediately prior to the onsets for cubic
CTSe may be attributable to shallow trap states present from the remaining native ligands. The lack of
such shoulders for the wurtzite CTSe may possibly be due to trap state passivation by thiols, a phenomenon
that has been reported in other nanocrystal systems.
41-43

!
!
!
"#!
0.1 M KCl. Under illumination using an LED with peak intensity of !
max
= 472 nm,
which is greater than the measured optical and electrochemical band gap, the
nanocrystals produce a cathodic photocurrent that increases gradually with increasing
negative potential (Figure 4.8), indicating that the wurtzite CTSe nanocrystals exhibit p-
type behavior.
44-46
For p-type semiconductors, electrons are transferred from the
conduction band to a solution phase oxidant and from the back ohmic contact into the
semiconductor, resulting in the observation of cathodic currents. At 0 V bias vs. NHE,
the measured photocurrent is only 0.10 µA cm
-2
(measured as the difference between

Figure 4.8. Transient photocurrent response of a wurtzite CTSe nanocrystal film showing clear p-type
behavior. The nanocrystal film was spun-cast onto ITO, treated with hydrazine, and photocurrent was
measured under nitrogen in aqueous 0.01 M Eu(NO
3
)
3
/0.1 M KCl using 472 nm chopped illumination with
a Ag wire pseudo-reference electrode and a Pt wire counter electrode. The potential values are given
relative to NHE.
!
!
!
"#!
current density immediately prior to and at the end of an illumination cycle). As the bias
was decreased to -250 mV the photocurrent increased more than an order of magnitude to
1.8 µA cm
-2
.

4.6. Conclusion
In summary, a facile synthesis of colloidal CTSe nanocrystals was described. Through a
combination of 1-dodecanethiol capping ligands and a
t
Bu
2
Se
2
selenium source, a
metastable wurtzite phase of CTSe has been accessed for the first time. A direct band
gap of E
g
= 1.7 eV matches reasonably well with the solar spectrum, making these
nanocrystals potential candidates as photovoltaic materials. Indeed, preliminary
experiments show that solution-deposited CTSe nanocrystal films exhibit a clear
photoresponse. Future work will focus on examining the viability of this material for
nanocrystal-based photovoltaic devices.

4.7. Acknowledgment
The material discovery is based on work supported by the National Science Foundation
under DMR-0906745. M.E.N. and M.J.G. and the photoelectrochemical characterization
were supported as part of the Center for Energy Nanoscience, an Energy Frontier
Research Center funded by the U.S. Department of Energy, Office of Science, and Office
of Basic Energy Sciences under Award Number DE-SC0001013. R.L.B. also
acknowledges the Research Corporation for Science Advancement for a Cottrell Scholar
Award.
!
!
"#!
4.8. References
(1) Sargent, E. H. Infrared photovoltaics made by solution processing. Nat. Photonics
2009, 3, 325.
(2) Hillhouse, H. W.; Beard, M. C. Solar cells from colloidal nanocrystals:
Fundamentals, materials, devices, and economics. Curr. Opin. Colloid Interface
Sci. 2009, 14, 245.
(3) Kamat, P. V. Quantum dot solar cells. Semiconductor nanocrystals as light
harvesters. J. Phys. Chem. C 2008, 112, 18737.
(4) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Air-stable all-inorganic
nanocrystal solar cells processed from solution. Science 2005, 310, 462.
(5) Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.;
Nozik, A. J. Schottky solar cells based on colloidal nanocrystal films. Nano Lett.
2008, 8, 3488.
(6) Guo, Q.; Ford, G. M.; Hillhouse, H. W.; Agrawal, R. Sulfide nanocrystal inks for
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1!x
Ga
x
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1!y
Se
y
)
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ZnSnS
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!
!
"#!
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ZnSnS
4
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ZnSnS
4
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SnSe
3
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sputtered Cu
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SnSe
3
films using the targets composed of Cu
2
Se and SnSe
2
. Thin
Solid Films 2010, 518, 7218.
!
!
"#!
(20) Chandra, G. H.; Kumar, O. L.; Rao, R. P.; Uthanna, S. Influence of substrate and
selenization temperatures on the growth of Cu
2
SnSe
3
films. J. Mater. Sci. 2011,
46, 6952.
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Crystal growth and structure, electrical, and optical characterization of the
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2
SnSe
3
. J. Appl. Phys. 2001, 90, 1847.
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of the semiconducting compound Cu
2
SnSe
3
from X-ray powder diffraction data.
Mater. Res. Bull. 2003, 38, 1949.
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and structure of the semiconductor Cu
2
SnSe
3
. Mater. Lett. 2002, 53, 151.
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Colloidal synthesis of Cu
2
SnSe
3
nanocrystals. Mater. Lett. 2010, 64, 2043.
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(27) Norako, M. E.; Brutchey, R. L. Synthesis of metastable wurtzite CuInSe
2

nanocrystals. Chem. Mater. 2010, 22, 1613.
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2
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x
Ga
1–x
S
2
semiconductor nanocrystals with
tunable band gap. J. Am. Chem. Soc. 2011, 133, 11072.
!
!
"#!
(30) Lu, X.; Zhuang, Z.; Peng, Q.; Li, Y. Wurtzite Cu
2
ZnSnS
4
nanocrystals: a novel
quaternary semiconductor. Chem. Commun. 2011, 3141.
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indium sulfide nanorods. Chem. Mater. 2009, 21, 1790.
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Chem. Commun. 2009, 5701.
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nanocrystals in an aqueous micellar solution. Inorg. Chem. 2011, 50, 723.
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CuInS
2
nanocrystals; phase determination by complex ligand species. Chem.
Mater. 2009, 21, 2607.
(38) Mahler, B.; Lequeux, N.; Dubertret, B. Ligand-controlled polytypism of thick-
shell CdSe/CdS nanocrystals. J. Am. Chem. Soc. 2010, 132, 953.
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Reiss, P.; Pron, A. Electronic, electrochemical, and spectroelectrochemical
properties of hybrid materials consisting of carboxylic acid derivatives of
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!
"#!
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Thiols passivate recombination centers in colloidal quantum dots leading to
enhanced photovoltaic device efficiency. ACS Nano, 2008, 2, 2356.
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processed colloidal PbSe nanocrystal infrared photodetectors. Chem. Mater. 2010,
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ZnSnS
4
as an alternative absorber
material. Phys. Status Solidi B 2008, 245, 1772.
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B. A.; Bard, A. J. Photoelectrochemical characterization of CuInSe
2
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1!x
Ga
x
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2
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Photoelectrochemical characterization of nanocrystalline thin-film Cu
2
ZnSnS
4

photocathodes. ACS Appl. Mater. Interfaces 2010, 3, 58.

86
Chapter 5. Synthesis and Characterization of Five Different Metal
Chalcogenides*

*Unpublished results.
5.1. Introduction
Metal chalcogenide nanomaterials have garnered great interest over recent years due to
the possibility of tailoring their physical and optical properties, i.e. particle size and the
band gap.
1-14
In this chapter, five different metal chalcogenide syntheses will be reported
that utilize diorganodichalcogenide reagents that have been used to synthesize other
chalcogenide nanocrystals reported here-in.

Cu
2
GeE
3
(copper germanium sulfide, selenide; E = S, Se) is a potentially interesting
photovoltaic material, being an intrinsic p-type semiconductor with a bulk direct band
gap (E
g
) of 0.78 eV
11,15,16
for Cu
2
GeSe
3
and 1.5 eV
1,17
for Cu
2
GeS
3
. Cu
2
GeSe
3
has a
melting temperature of 770 ºC, low density (ñ = 5.6 g cm
-3
),
11,18
diffraction index of n ~
3.2,
11,15,16,19
thermal expansion coefficient of 8.4 x 10
-6
K
-1
, heat capacity of 0.34 J g
-1
K
-1
,
and thermal conductivity of 2.4 W m
-1
K
-1
at 300 ºC.
11,19,20
Two very recent reports
describe multiple Cu
2
GeE
3
crystal structures for the nanocrystalline materials, including
cubic (zinc blende, ZB) Cu
2
GeSe
3
,
1,11,21
tetragonal (chalcopyrite) Cu
2
GeSe
3
,
11,22-24

orthorhombic Cu
2
GeSe
3
,
1,11,25,26
wurtzite (WZ) Cu
2
GeSe
3
,
11
and ZB-Cu
2
GeS
3
.
1,17
Ibàñez et
al. reported the first solution-phase synthesis of polytypic Cu
2
GeSe
3
nanoparticles,
including ordered single-phase orthorhombic and disordered polytypic WZ-ZB Cu
2
GeSe
3

87
nanoparticles that were studied for their potential thermoelectric properties.
11
Most
recently, Yang et al. reported syntheses for Cu
2
Ge(S
3-x
Se
x
) colloidal nanocrystals, where
the Cu
2
GeS
3
and Cu
2
Ge(S
2
Se) nanocrystals are indexed to cubic Cu
2
GeS
3
and the
Cu
2
GeSSe
2
and Cu
2
GeSe
3
nanocrystals are orthorhombic Cu
2
GeSe
3
with optical band
gaps of 1.20, 1.61, 1.84, and 2.16 eV for Cu
2
GeSe
3
, Cu
2
GeS
1
Se
2
, Cu
2
GeS
2
Se
1
, and
Cu
2
GeS
3
nanocrystals, respectively.
1
The Cu
2
GeS
2
Se
1
nanocrystals were used to fabricate
a solar cell device with an efficiency of 0.2%.
1
In an early article by Sharma and Singh,
the lattice parameters of the unit cells for the different phases of Cu-Ge-Se were
determined.
24
Stoichiometric Cu-Ge-Se with a ratio of Cu:Ge:Se of 2:1:3 has a
disordered tetragonal unit cell a = b = 5.591 Å and c = 5.485 Å. The symmetry of the
Cu-Ge-Se unit cells was shown to be quite sensitive to the concentration of Ge within the
compound. A slight deficiency of Ge decreases the symmetry of the unit cell to the
monoclinic phase and when the amount of Ge increases the unit cell symmetry also
increases to the cubic structure. Non-stoichiometric Cu
2
Ge
0.85
Se
3
is monoclinic with
lattice parameters of a = 5.512 Å, b = 5.598 Å, c = 5.486 Å, and b = 89.7º and
Cu
2
Ge
1.55
Se
3
is cubic with a = 5.569 Å.
24
Copper Tin Sulfide (Cu
2
SnS
3
) has been
reported to exist in the monoclinic crystal structure that can be derived from CuFeS
2
and
has a reported bang gap of 0.9 eV.
27

Nickel selenide (NiSe) is a p-type semiconductor and has a bulk band gap of 2.0 eV (620
nm).
28-30
Ni
0.95
Se micrographs have been synthesized in a hexagonal phase, Ni
3
Se
4

particles have been reported as monoclinic, and Ni
6
Se
5
and Ni
5
Se
5
has been described as

88
orthorhombic.
28
Nanoscale hexagonal Ni
0.95
Se was reported having an atomic ratio of
48.72:51.28 Ni:Se, with lattice parameters a = b = 10.01 Å and c = 3.315 Å.
31

Copper antimony sulfide (Cu-Sb-S) is a p-type semiconductor with varying band gaps
depending on the stoichiometry and crystal phase.
32
Cu
12
Sb
4
S
13
has a reported band gap
of 1.72 eV,
32,33
Cu
3
Sb
4
S
4
and Cu
3
SbS
3
have reported direct band gaps of 0.47
32,34,35
and
1.84 eV, respectively.
32,36
Crystalline thin films of CuSbS
2
have a reported tunable direct
band gap ranging from 0.9 – 1.9 eV, which is characteristic of quantum
confinement.
32,37,38
CuSbS
2
has a high absorption coefficient (~4 x 10
5
cm
-1
) and a low
lattice conductivity.
32,38-40
Famatinite Cu
3
SbS
4
was recently synthesized that have a zinc-
blende related body-centered tetragonal until cell with lattice parameters a = b = 5.391 Å
and c = 10.764 Å and a band gap of 0.9 eV, which is characteristic of quantum
confinement.
32,38,41,42

Lead sulfide (PbS) is a direct band gap material that has been extensively studied due to
the narrow band gap (0.41 eV) and large Bohr radius (18 nm) it possesses.
43-46
PbS
nanocrystals with sizes smaller than the Bohr radius observe quantum confinement
effects allowing the band gap to blue shift to ~1.5 eV, making the band gap highly
tunable.
45,47

In this chapter, syntheses of Cu
2
GeSe
3
, Cu
2
SnS
3
, Ni
0.95
Se, CuSbS
2
, and PbS are described
that use diorganodichalcogenide reagents. These preliminary syntheses and
characterization data are the foundation of potentially useful photovoltaic materials,
based on their bulk and nanoscale properties. The syntheses described below contain

89
materials with relatively high earth abundances, low reaction temperatures, and facile
procedures and purifications.

5.2. Experimental Details
5.2.1. General Considerations
Copper(I) chloride (CuCl, Strem Chemicals, 99.999%), germanium(IV) iodide (GeI
4
,
Strem Chemicals, 99.999%), tin(IV) chloride bis(2,4-pentanedionate) (Alfa Aesar, 95%),
nickel(II) acetylacetonate (Ni(acac)
2
, Strem Chemicals, 95+%, anhydrous), lead(II)
acetylacetonate (Pb(acac)
2
, Strem Chemicals, 95+%), 1-dodecanethiol (Alfa Aesar, 98%),
mesitylene (Alfa Aesar, 98+%), lauric acid (Sigma-Aldrich, 98+%), n-heptadecane (Alfa
Aesar, 99%), tri-n-octylphosphine oxide (TOPO, 98%, Alfa Aesar), di-tert-butyl
disulfide (Chem Service, 99.3%), diphenyl diselenide (98%, Alfa Aesar) were all
purchased and used without further purification. Dodecylamine (Alfa Aesar, (98+%) was
distilled from CaO prior to use. Nanocrystal syntheses were performed under nitrogen, in
the absence of water and oxygen, using standard Schlenk techniques.

5.2.2. Synthesis of di-tert-butyl diselenide (
t
Bu
2
Se
2
)
Di-tert-butyl diselenide was synthesized according to an improved version of a
previously published method (see Block, E.; Birringer, M.; Jiang, W.; Nakahodo, T.;
Thompson, H.; Toscano, P. J.; Uzar, H.; Zhang, X.; Zhu, Z. J. Agric. Food Chem. 2001,
49, 458-470). Briefly, magnesium turnings (10.75 g, 0.44 mol) were allowed to stir in
diethyl ether (200 mL) under nitrogen. tert-Butyl bromide (50 mL, 0.44 mol) was added

90
slowly under nitrogen to the ether solution and allowed to stir for 30 min, producing a
gray solution. Selenium (31.74 g, 0.40 mol) was subsequently added and allowed to stir
for an additional 30 min. The solution was 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 min, 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 °C (0.05 mmHg), the product was characterized by
1
H,
13
C, and
77
Se
NMR in CDCl
3
.
1
H: ! = 1.46 (s); {
1
H}
13
C: ! = 41.7 (s), 32.5 (s); {
1
H}
77
Se: ! = 488 (s)
ppm.

5.2.3. Nanocrystal Syntheses
5.2.3.1. Synthesis of Cu
2
GeSe
3
Nanocrystals
In a typical synthesis, CuCl (0.040 g, 0.404 mmol) and GeI
4
(0.126 g, 0.217 mmol) were
added to a two-neck round-bottom flask fitted with a reflux condenser, stir bar, and
rubber septum and heated to 40 ˚C. Dodecylamine (0.56 mL, 2.4 mmol) and 1-
dodecanethiol (0.10 mL, 0.42 mmol) were added to the reaction flask via syringes and
then cycled between vacuum and nitrogen three times. Then, the system was heated (10
˚C min
-1
) to 95 ˚C, and was again cycled three times between vacuum and nitrogen to
eliminate adventitious water and dissolved oxygen. The temperature was then increased

91
(10 ˚C min
-1
) to 180 ˚C and
t
Bu
2
Se
2
(0.11 mL, 0.56 mmol) was quickly injected into the
system under flowing nitrogen, and allowed to react for 3 h and 40 min with stirring.
After cooling to room temperature, the reaction mixture was dissolved in 5 mL of toluene
and precipitated with 10 mL of methanol, sonicated, and centrifuged (6000 rpm for 5
min) to yield a black solid. Dispersion/precipitation was repeated three times with
toluene (1 mL) and methanol (4 mL) to yield the as-prepared product.

5.2.3.2. Synthesis of Cu
2
SnS
3
Crystals
CuCl (0.141 g, 1.42 mmol) and tin(IV) chloride bis(2,4-pentanedionate) (0.28 g, 0.71
mmol) and were added to a two-neck round-bottom flask fitted with a reflux condenser,
stir bar, and rubber septum and heated to 40 ˚C. Dodecylamine (2.0 mL, 8.5 mmol) was
added to the reaction flask via syringe and then cycled between vacuum and nitrogen
three times. Then, the system was heated (10 ˚C min
-1
) to 95 ˚C, and was again cycled
three times between vacuum and nitrogen to eliminate adventitious water and dissolved
oxygen. At this time,
t
Bu
2
S
2
(1.0 mL, 5.2 mmol), was quickly injected into the system
under flowing nitrogen. The temperature was increased (10 ˚C min
-1
) to 220 ˚C and
allowed to react for 3 h with stirring. After the reaction cooled to room temperature, the
mixture was dissolved in 4 mL of toluene and precipitated with 10 mL of ethanol,
sonicated, and centrifuged (6000 rpm for 5 min) to yield a black solid.
Dispersion/precipitation was repeated three times with toluene (2 mL) and ethanol (20
mL) to yield the as-prepared product.

92
5.2.3.3. Synthesis of Ni
0.95
Se Crystals
Ni(acac)
2
(0.166 g, 0.646 mmol) was added to a two-neck round-bottom flask fitted with
a reflux condenser, stir bar, and rubber septum and heated to 40 ˚C. Dodecylamine (1.4
mL, 6.0 mmol) and 1-dodecanethiol (0.32 mL, 1.0 mmol) were added to the reaction
flask via syringes and then cycled between vacuum and nitrogen three times. Then, the
system was heated (10 ˚C min
-1
) to 95 ˚C, and was again cycled three times between
vacuum and nitrogen to eliminate adventitious water and dissolved oxygen. The
temperature was then increased (10 ˚C min
-1
) to 180 ˚C and a solution of Ph
2
Se
2
(0.25
mmol) dissolved in 0.5 mL mesitylene was injected and allowed to react for 6 min. After
cooling to room temperature, the reaction mixture was dissolved in 6 mL of toluene and
centrifuged (6000 rpm for 5 min) to yield a black solid. Precipitation was repeated with
toluene (2 mL) and ethanol (10 mL) to yield the purified product.
Dispersion/precipitation was repeated three times with toluene (1 mL) and methanol (4
mL) to yield the as-prepared product.

5.2.3.4. Synthesis of CuSbS
2

CuCl (0.05 g, 0.50 mmol), SbI
3
(0.25 g, 0.50 mmol), and tri-octyl-phosphine oxide
(TOPO, 2.4 g, 6.1 mmol) were added to a two-neck round-bottom flask fitted with a
reflux condenser, stir bar, and rubber septum and was cycled between vacuum and
nitrogen three times. Then, the system was heated to 95 ˚C, and was again cycled three
times between vacuum and nitrogen to eliminate adventitious water and dissolved
oxygen. The temperature was then increased (10 ˚C min
-1
) to 300 ˚C and di-tert-butyl

93
disulfide (
t
Bu
2
S
2
, 1.5 mmol) was quickly injected into the system under flowing nitrogen,
and allowed to react for 3 h and 6 min with stirring. After cooling to room temperature,
the reaction mixture was dissolved in 10 mL of toluene and precipitated with 20 mL of
methanol, sonicated, and centrifuged (6000 rpm for 5 min) to yield a black solid.
Dispersion/precipitation was repeated three times with toluene (2 mL) and methanol (10
mL) to yield the as-prepared product.

5.2.3.5. Synthesis of PbS
Pb(acac)
2
(0.29 g, 0.71 mmol), dodecylamine (1.6 g, 8.5 mmol), lauric acid (0.29 g, 1.42
mmol) and heptadecane (1.42 mL) were added to a two-neck round-bottom flask, fitted
with a reflux condenser, stir bar, and rubber septum, and heated (10 ˚C min
-1
) to 95 ˚C.
The System was then cycled three times between vacuum and nitrogen to eliminate
adventitious water and dissolved oxygen. Then
t
Bu
2
S
2
(4.26 mmol) was quickly injected
into the system under flowing nitrogen. The temperature was then increased (10 ˚C min
-
1
) to 250 ˚C and allowed to react for 2 h with stirring. After being cooled to room
temperature, the reaction mixture was dissolved in 2 mL of toluene and precipitated with
10 mL of ethanol, sonicated, and centrifuged (6000 rpm for 5 min) to yield a black solid.
Dispersion/precipitation was repeated three times with toluene (2 mL) and methanol (10
mL) to yield the purified product.

94
5.2.4. Nanocrystal Characterization
Powder X-ray diffraction (XRD) analyses were performed on a Rigaku Ultima IV X-ray
diffractometer using a Cu K! radiation source (" = 1.54 Å). Transmission electron
microscopy (TEM) was performed on a JEOL JEM-2100 microscope at an operating
voltage of 200 kV, equipped with a Gatan Orius CCD camera.

5.3. Results and Discussion
5.3.1. Cu
2
GeSe
3
Nanocrystals
Cu
2
GeSe
3
nanocrystals were synthesized via the injection of di-tert-butyl diselenide
(
t
Bu
2
Se
2
) into a reaction mixture of CuCl and GeI
4
in dodecylamine and 1-dodecanethiol
at 180 ˚C and allowed to react for 3 h and 40 min. After natural cooling to room
temperature, the reaction mixture was dissolved in 5 mL of toluene precipitated with 10
mL of methanol, sonicated, and centrifuged (6000 rpm for 5 min) to yield a black solid.
Precipitation was repeated with toluene (1 mL) and methanol (4 mL) to yield the purified
product.

The powder X-ray diffraction (XRD) pattern (Figure 5.1) of the resulting Cu
2
GeSe
3

nanocrystals matched well to the cubic phase of Cu
2
GeSe
3
(JCPDS card # 01-089-2878),
which has also been observed in recent reports.
1,11
The lattice parameters were calculated
from experimental values to be: a = b = c = 5.50 Å. TEM (Figure 5.2.) shows nearly
monodisperse nanocrystals with diameters ranging from ca. 10-20 nm that are highly
crystalline as shown in the inset HR-TEM in Figure 5.2.

95

Figure 5.2. TEM image of cubic Cu
2
GeSe
3
nanocrystals with diameters of ca. 10-20 nm. HR-TEM
image of a single nanocrystal shown as inset.

Figure 5.1. Experimental XRD pattern of cubic Cu
2
GeSe
3
nanocrystals, the known XRD pattern –
PDF#01-089-2878 – (green) is shown for reference.

96
5.3.2. Cu
2
SnS
3
Crystals
Cu
2
SnS
3
crystals were synthesized using di-tert-butyl disulfide (
t
Bu
2
S
2
) as the chalcogen
source, which was quickly injected into a solution of CuCl and tin(IV) chloride bis(2,4-
pentanedionate) in dodecylamine and reacted at 220 °C for 3 h. After cooling to room
temperature, the reaction mixture was dissolved in 4 mL of toluene and precipitated with
10 mL of ethanol, sonicated, and centrifuged (6000 rpm for 5 min) to yield a black solid.
Precipitation was repeated with toluene (2 mL) and ethanol (10 mL) to yield the purified
product.

Figure 5.3. Experimental XRD pattern of wurtzite Cu
2
SnS
3
crystals, the simulated wurtzite XRD pattern is
shown underneath in black.

97
The XRD pattern (Figure 5.3) of the as-synthesized Cu
2
SnS
3
crystals matched partially to
the cubic phase of Cu
2
SnS
3
(JCPDS card # 01-089-4714); however, there are additional
diffraction peaks that cannot be assigned. The diffraction peaks were however similar to
the peaks of wz-CuInS
2
, wz-CuInSe
2
, and wz-Cu
2
SnSe
3
,
12-14,48-50
indicating that Cu
2
SnS
3

also forms the wurtzite structure. Therefore the diffraction pattern of wz-Cu
2
SnS
3
was
simulated starting from the wurtzite ZnS structure and substituting the Zn
2+
lattice
positions with a 2/1 occupancy probability of Cu
+
and Sn
4+
cations. The lattice
parameters (a = b = 3.76 Å, c = 6.25 Å) used for the simulation were experimentally
determined by fitting the diffraction maxima corresponding to the (100) and (002)
reflections, respectively, with a pseudo-Voigt function. The atomic coordinates of Cu,
Sn, and S for Cu
2
SnS
3
with the wurtzite crystal structure and P6
3
mc (No. 186) space
group are shown in Table 5.1. Table 5.2. shows the comparison of the experimental and
simulated d-spacing values with respect to their corresponding diffraction peaks and
TEM (Figure 5.4) images show polydisperse crystals with diameters ranging from ca. 100
nm to over 200 nm.

Table 5.1. Wurtzite Cu
2
SnS
3
Atomic Coordinates
Atom Wyck. x/a y/b z/c
Cu 2b 0.33 0.67 0.3752
Sn 2b 0.33 0.67 0.3752
S 2b 0.33 0.67 0

98
Table 5.2. Comparison of experimental and simulated d-spacing values with respect to their corresponding
diffraction peaks.
2! (degrees)
d-spacing
simulated (Å)
d-spacing
experimental (Å)
(h k l)
27.37 3.26 3.26 (100)
28.54 3.13 3.12 (002)
30.94 2.89 2.89 (101)
39.95 2.25 2.26 (102)
48.38 1.88 1.89 (110)
52.07 1.75 1.76 (103)
57.13 1.61 1.62 (112)
58.54 1.58 1.58 (201)

Figure 5.4. TEM image of Cu
2
SnS
3
crystals.

99
5.3.3. Ni
0.95
Se Crystals
Ni
0.95
Se crystals were synthesized using the same general metal chalcogenide synthesis
where diphenyl diselenide (Ph
2
Se
2
) was used as the chalcogen source. The Ph
2
Se
2

precursor was dissolved in 0.5 mL mesitylene and injected into a solution of Ni(acac)
2
in
dodecylamine and 1-dodecanethiol at 180 ˚C before allowing to react for 6 min. After
cooling to room temperature, the reaction mixture was dissolved in 6 mL of toluene and
centrifuged (6000 rpm for 5 min) to yield a black solid. Precipitation was repeated with
toluene (2 mL) and ethanol (10 mL) to yield the purified product. The powder X-ray
diffraction (XRD) pattern (Figure 5.5) of the as-synthesized Ni
0.95
Se crystals is well
matched to the reference XRD pattern of hexagonal Ni
0.95
Se (JCPDS card # 01-072-
2546). The lattice parameters were experimentally determined (a = b = 3.61 Å and c =
5.32 Å).

Figure 5.5. Experimental XRD pattern of hexagonal Ni
0.95
Se, the reference XRD pattern – JCPDS card #
01-072-2546 – is shown in green.

100
5.3.4. CuSbS
2
Crystals
CuSbS
2
crystals were also synthesized via the fast injection of di-tert-butyl disulfide into
a solution of CuCl and SbI
3
in TOPO at 300 ˚C, and allowed to react for 3 h and 6 min.
After cooling to room temperature, the reaction mixture was dissolved in 10 mL of
toluene, precipitated with 20 mL methanol, and centrifuged (6000 rpm for 5 min) to yield
a black solid. Precipitation was repeated with toluene (2 mL) and methanol (10 mL) to
yield the purified product. The powder X-ray diffraction (XRD) pattern (Figure 5.6) of
the as-synthesized CuSbS
2
crystals matched well to the orthorhombic phase (JCPDS card
# 01-073-3954), and the lattice parameters were experimentally determined (a = 5.97 Å,
b = 3.80 Å, and c = 14.48 Å). TEM shows large CuSbS
2
crystals with smaller aggregated
particles (see Figure 5.7).

Figure 5.6. Experimental XRD pattern of CuSbS
3
, the reference XRD pattern – JCPDS card # 01-073-
3954– is shown in green.

101

Figure 5.7. TEM images of polydisperse cubic CuSbS
2
crystals.

102
5.3.5. PbS Crystals
PbS was synthesized using di-tert-butyl disulfide as the chalcogen source that was
quickly added to a solution of Pb(acac)
2
in dodecylamine, lauric acid, and heptadecane at
95 ˚C, and then the temperature was increased to 250 °C and allowed to react for 2 h.
After cooling to room temperature, the reaction mixture was dissolved in 2 mL of toluene
and centrifuged (6000 rpm for 5 min). Precipitation was repeated with toluene (2 mL)
and ethanol (10 mL) to yield the purified product. The powder X-ray diffraction (XRD)
pattern (Figure 5.8) of the as-synthesized PbS matched well to the rock salt structure of
PbS (JCPDS card # 01-078-1057) and the lattice parameters were experimentally
determined (a = b = c = 5.93 Å).

Figure 5.8. Experimental XRD pattern of PbS with the reference rock salt structure shown in black.

103
5.4. Conclusion
The described syntheses of Cu
2
GeSe
3
, Cu
2
SnS
3
, Ni
0.95
Se, CuSbS
2
, and PbS all utilize
diorganodichalcogenide precursors. These materials are the foundation to potentially
useful photovoltaic materials. This method resulted in the production of ZB-Cu
2
GeSe
3
,
WZ-Cu
2
SnS
3
, hexagonal-Ni
0.95
Se, orthorhombic-CuSbS
2
, and rock salt-PbS. These
syntheses demonstrate the proof of concept of this synthesis method and how it can be
expanded to synthesize a variety of semiconductors.

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Abstract (if available)

Abstract Metastable semiconductor nanocrystals have been shown to possess new and interesting properties that are highly reliant upon their synthetic reaction parameters. A versatile method for a relatively low temperature synthesis of metastable 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. Monodisperse metastable copper indium sulfide (CuInS₂) nanocrystal were synthesized using di-tert-butyl disulfide as the sulfur source at 180°C. In a similar fashion, metastable wurtzite copper indium selenide (CuInSe₂) and metastable wurtzite copper tin selenide (CTSe) were synthesized and characterized for the first time. This method was further expanded to synthesize Cu₂GeSe₃, Cu₂SnS₃, Ni₀.₉₅Se, CuSbS₂, and PbS also using dialkyl dichalcogenide precursors. To further study these products, the nanocrystal growth mechanism was explored for the dichalcogenidemediated synthesis of wz-CuInS₂ and the potential applicability of the wurtzite CTSe nanocrystals as a photovoltaic material was assessed.

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Asset Metadata

Creator Norako, Michelle E. (author)

Core Title Synthesis and characterization of metal chalcogenide semiconductor nanocrystals using dialkyl dichalcogenide precursors

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 08/15/2013

Defense Date 05/09/2013

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

Tag copper germanium selenide,copper indium selenide,copper indium sulfide,copper tin selenide,dialkyl dichalcogenide precursor,metastable,nanocrystals,OAI-PMH Harvest,wurtzite

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Brutchey, Richard L. (committee chair), Dapkus, Paul Daniel (committee member), Thompson, Barry C. (committee member)

Creator Email michelle.norako@gmail.com,norako@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-323984

Unique identifier UC11287830

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Legacy Identifier etd-NorakoMich-2001.pdf

Dmrecord 323984

Document Type Dissertation

Rights Norako, Michelle E.

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

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