Synthesis of Metal Selenide Semiconductor Nanocrystals Using Selenium Dioxide As Precursor

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SYNTHESIS OF METAL SELENIDE SEMICONDUCTOR NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR

XIAN CHEN

A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2007

To my parents

ACKNOWLEDGMENTS

Above all, I would like to thank my parents for what they have done for me through these years. I would not have been able to get to where I am today without their love and support.

I would like to thank my advisor, Dr. Charles Cao, for his advice on my research and life and for the valuable help during my difficult times. I also would like to thank Dr. Yongan Yang for his kindness and helpful discussion. I learned experiment techniques, knowledge, how to do research and so on from him. I also appreciate the help and friendship that the whole Cao group gave me.

Finally, I would like to express my gratitude to Dr. Ben Smith for his guidance and help.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...... 4

LIST OF FIGURES ...... 7

ABSTRACT...... 9

CHAPTER

1 SEMICONDUCTOR NANOCRYSTALS ...... 11

1.1 Introduction...... 11 1.2 General Synthetic Methods for Nanocrystals ...... 11 1.2.1 Injection-Based Synthetic Method...... 13 1.2.2 One-Pot Synthetic Method...... 14 1.3 Applications of Semiconductor Nanocrystals...... 16 1.3.1 Biological Detection ...... 16 1.3.2 Hybrid Electroluminenscent Device...... 17 1.3.3 Photovoltaic Device...... 18

2 SYNTHESIS OF CADMIUN SELENIDE NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR ...... 19

2.1 Introduction...... 19 2.2 Experimental Section...... 20 2.2.1 Materials ...... 20 2.2.2 Instrumentation ...... 20 2.2.3 Preparation of Cd-Precursors...... 22 2.2.3.1 Cadmium myristate (CdC14)...... 22 2.2.3.2 Cadmium stearate (CdC18) ...... 22 2.2.3.3 Cadmium docosanate (CdC22)...... 22 2.2.4 Preparation of CdSe Nanocrystals ...... 23 2.3 Results and Discussion ...... 23 2.3.1 Diol Effect...... 23 2.3.2 Precursor Effect ...... 32 2.3.3 Multiple-Addition Method...... 36 2.4 Conclusion ...... 37

3 SYNTHESIS OF METAL SELENIDE NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR ...... 39

3.1 Introduction...... 39 3.2 Experimental Section...... 40 3.2.1 Materials ...... 40 3.2.2 Instrumentation ...... 41

3.2.3 Preparation of Precursors...... 41 3.2.3.1 Gallium myristate...... 41 3.2.3.2 Silver oleate...... 41 3.2.3.3 Copper oleate...... 41 3.2.3.4 Nickel oleate...... 42 3.2.4 Preparation of Nanocrystals...... 42 3.2.4.1 Gallium selenide nanocrystals...... 42 3.2.4.2 Lead selenide nanocrystals...... 42 3.2.4.3 Silver selenide nanocrystals ...... 43 3.2.4.4 Copper selenide nanocrystals ...... 43 3.2.4.5 Nickel selenide nanocrystals ...... 43 3.2.5 Purification of Nanocrystals ...... 43 3.3 Results and Discussion ...... 44 3.4 Conclusion ...... 48

4 SUMMARY AND FUTURE WORK...... 50

4.1 Summary...... 50 4.2 Future work...... 50 4.2.1 Injection-Synthetic Method for CdSe ...... 50 4.2.2 Improvement of Other Metal Selenide Nanocrystals...... 51 4.2.3 Mechanism Study...... 51

LIST OF REFERENCES...... 52

BIOGRAPHICAL SKETCH ...... 56

LIST OF FIGURES

Figure page

1-1 Scheme of the formation of nanocrystals...... 12

1-2 LaMer Curve...... 13

1-3 Representation of the synthetic apparatus employed in the injection-based method...... 14

1-4 Absorption spectrum of CdS nanocrystals (d = 3.5 nm)...... 15

1-5 Representation of the synthetic apparatus employed in the one-pot synthetic method...... 16

2-1 Sizing curve of CdSe nanocrystals ...... 19

2-2 HWHF and peak sharpness used for size distribution determination...... 20

2-3 Schematic diagram of a UV-Vis microscope...... 21

2-4 Schematic diagram of a Fluorolog-3 Model FL3-12 spectrofluorometer...... 21

2-5 Molecular structures of organic solvents used...... 23

2-6 Temporal evolution of the absorption spectra during the CdSe synthesis ...... 24

2-7 Characterization of CdSe nanocrystals synthesized in ODE with reaction time of 40 minutes...... 25

2-8 Temporal evolution of CdSe nanocrystal concentration synthesized in ODE with different C16-diol/SeO2 ratios...... 26

2-9 CdSe particle growth rate in the synthesis with different C16-diol/SeO2 ratios ...... 27

2-10 CdSe particle size and normalized nuclei number in the synthesis with different C16-diol/SeO2 ratios...... 28

2-11 Temporal evolution of the absorption spectra during the CdSe synthesis with different diols...... 29

2-12 HWHM of CdSe during synthesis with different diols...... 29

2-13 Temporal evolution of CdSe nanocrystal concentration with different diols...... 30

2-14 CdSe particle size in the synthesis with different numbers of carbon atom per diol...... 30

2-15 Temporal evolution of the absorption spectra during the CdSe synthesis with different alcohols...... 31

2-16 Temporal evolution of the absorption spectra during the CdSe synthesis with different Cd precursors ...... 32

2-17 Multiple exiton peaks...... 33

2-18 Effect of Cd precursor on the nuclei concentration during the CdSe synthesis ...... 34

2-19 Effect of Cd precursor on the CdSe particle size in the synthesis...... 34

2-20 CdSe particle growth rate in the synthesis with different Cd precursors...... 35

2-21 Characterization of CdSe nanocrystals during the multiple-addtion synthesis ...... 36

2-22 Temporal evolution of the absorption spectra of the as-prepared CdSe nanocrystals...... 38

3-1 Evolution of absorption spectrum of GaSe nanocrystals...... 44

3-2 TEM image of GaSe nanocrystals...... 45

3-3 TEM image of PbSe nanocrystals...... 45

3-4 Evolution of absorption spectrum of AgSe nanocrystals...... 46

3-5 HR-TEM images of AgSe nanocrystals ...... 47

3-6 TEM image of CuSe nanocrystals...... 47

3-7 TEM image of NiSe nanocrystals...... 48

Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science

SYNTHESIS OF METAL SELENIDE SEMICONDUCTOR NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR

Xian Chen

August 2007

Chair: Y. Charles Cao Major: Chemistry

Nanotechnology has been one of the most popular research areas in these two decades. The semiconductor nanocrystals, which are also called quantum dots, are of the great interest because of their unique size-dependent properties. The nanomaterials have wide applications, including light emitting diodes, solar cells, biological labeling, and so on. The critical part in the use of quantum dots is to prepare monodispersed nanocrystals. The methods to synthesize high-qulity nanocrystals have been well developed.

Selenium element was used in most method for synthesizing high quality metal selenide nanocrystals. However selenium element is toxic and unstable in the air, thus requires complicated operations. Herein, we developed a new approach for using selenium dioxide to replace the selenium element. Selenium dioxide is very stable and nontoxic. It is found that when adding of 1,2-hexadecanediol (C16-diol) the quality of nanocrystals can be improved.

Experiments were carried out to test the results of using different amount of C16-diol. It turns out that adding more C16-diol can result in smaller size nanocrystals, higher nuclei number, and slower growth rate. Different cadmium precursors were used and the results show that with longer carbon chains in cadmium precursor, smaller size CdSe nanocrystals can be obtained.

CdSe nanocrystals with diameters of 4.5 nm by carrying out multiple-addition experiment were

generated.

SeO2 was also employed to prepare other metal selenide nanocrystals. GaSe nanocrystals

with diameters of 2.0 nm were formed. AgSe nanocrystals with diameters of around 7.4 nm and a lattice spacing of 0.21 nm were obtained. The absorption spectrum shows that during the formation of AgSe nanocrystals, Ag nanocrystals were formed first and then gradually reacted with SeO2 to form AgSe nanoparticles. PbSe aggregates consisting of uniform nanocubes were

observed.

CHAPTER 1 SEMIDONDUCTOR NANOCRYSTALS

1.1 Introduction

In the last two decades, there has been an increasing progress in the synthesis and

characterization of semiconductor nanocrystals. They are of great interest for both fundamental

research and industrial development because of their unique properties. In the nanometer range,

the properties of semiconductor nanocrystals are strongly dependent upon their size, shape, and

crystal structure, which make them differ substantially from the corresponding molecular and

bulk materials.1,2 Thus controlling the physical size of materials can be used to tune materials

properties. These novel properties lead to many applications such as light emitting diodes

(LEDs), biological fluorescent labels, lasers and solar cells.3-17

Efforts to explore structures on the nanometer scale combine the material science,

chemistry, physics and engineering. Studying size-dependent materials properties requires

synthetic routes to prepare homologous size series of monodisperse nanometer size crystals.18

1.2 General Synthetic Methods for Nanocrystals

Synthesis of high-quality semiconductor nanocrystals is the key element for studying the

size-dependent properties in the nanometer scale. This has been a very active area of research.

Colloidal methods are of most interested because the optical and electrical properties of semiconductor nanocrystals made by these methods can be tuned by changing the physical size of the nanocrystals. Synthesis of high-quality colloidal nanocrystals have been reported by several groups. The research group of Alivisatos and Bawendi developed methods of using

6,19 molecular precursors. In early 1990s, Cd(CH3)2 as precursor and technical-grade trioctylphosphine oxide (Tech TOPO) as the reaction solvent were used to synthesize

6 high-quality CdSe nanocrystals. But Cd(CH3)2 is extremely toxic, expensive and unstable, and

this synthesis is not very reproducible. Since 2001, CdO, CdCO3 and Cd(OOCCH3)2 precursors

with functionalized organic ligands have been used to replace Cd(CH3)2 precursor, for a

“greener” approach and noncoordinating solvents, such as 1-octadecene, were used to replace

TOPO.20-23 This thermal decomposition method has also been extended to the synthesis of ZnS

and ZnSe nanocrystals.24

In a typical colloidal synthesis there are three components: precursors, surfactants and

solvents. In some cases, solvents also serve as surfactants. When the system is heated to a

sufficiently high temperature, the precursors chemically transfer to active atoms or molecules,

which are called monomers. The monomers then form nanocrystals whose subsequent growth is

greatly affected by the presence of surfactants. The formation of the nanocrystals involves two

steps: nucleation of an initial “seed” and growth. In the nucleation step, precursors decompose at

a high temperature to form a supersaturation of monomers followed by a burst of nucleation of

nanocrystals. These nuclei then grow by incorporating additional monomers still present in the

reaction solution.25 The scheme of the formation of nanocrytals is shown in Figure 1-1.

Figure 1-1. Scheme of the formation of nanocrystals.

1.2.1 Injection-based Synthetic Method

In colloidal synthesis, chemists developed a method to separate the nucleation stage from

the nanocrystal’s growth stage6-8 as described by LaMer Curve (Figure 1-2). Rapid injection of

metal-organic precursors into a vigorously stirred flask containing a hot coordinating solvent can

form the supersaturation and subsequent nucleation. A short nucleation burst partially relieves the supersaturation. As long as the consumption of feedstock by the growing colloidal nanocrystals is faster than the rate of precursor addition to solution, no new nuclei form.18

Growth rate can be controlled by diffusion rate and/or reaction rate. Finally, the growth will be

balanced by the solubility.

Figure 1-2. LaMer Curve.

Figure1-3 illustrates a synthetic apparatus employed in the injection-based method. This method has led to synthesis of a variety of high-quality nanocrystals ranging from II-VI (e.g.,

CdS and CdSe) and III-V (e.g., InP and InAs) to IV-VI (e.g., PbS and PbSe) semiconductors.6,7,20,26-30

Figure 1-3. Representation of the synthetic apparatus employed in the injection-based method.

However, the injection-based synthetic method is not suitable for large-scale, industrial preparation. It is very difficult to inject precursors rapidly because industrial preparation may use hundreds of kilograms of precursors. 27 In the laboratory, nucleation time is determined by the rate of the injection and the mass transfer in the reaction systems, and the temperature is very hard to control. So this injection based synthesis method is not ideal for mechanistic mechanism studies that require a highly reproducible system for quantitative measurement.

Therefore, methods that do not require the injection of precursors have to be developed.

1.2.2 One-Pot Synthetic Method

Several groups have reported the one-pot synthesis of semiconductor nanocrystals without

the injection of precursors. However, the quality of their nanocrystals was not comparable to that

of the nanocrystals made by the injection-based method. Typically, they do not exhibit as many

multiple-exciton absorption peaks, 31-34 while high-quality nanocrystals with multiple exciton-absorption peaks are critical for the applications in advanced optical and electronic devices.35 Recently, the Cao group has developed a new non-injection synthesis for making CdS,

CdSe and CdTe nanocrystals.27,28 The quality of the nanocrystals made by this new synthesis is

at least comparable to the best particles made by injection-based methods.Without size-selective

separation, the nanocrystals formed by this new synthesis exhibit up to four exciton-absorption

peaks, indicating their very narrow size distribution and excellent optical properties (Figure 1-4).

The set-up for the one-pot synthesis is shown in Figure 1-5.

Figure 1-4. Absorption spectrum of CdS nanocrystals (d = 3.5 nm).

The one-pot synthetic method is based on a new concept of controlling the

thermodynamics and kinetics of the nanocrystal nucleation stage.27 The precursors are thermal

decomposed when heat up to a sufficiently high temperature, so more and more monomers are

produced as time passes, when the concentration of monomers increased to supersaturation,

nucleation happens. As the monomer concentration drops lower than the nucleation

concentration, the nucleation stops, followed by the nuclei formed growth. When the

concentration of monomer drops to saturation concentration, the particles stop growing.

Figure 1-5. Representation of the synthetic apparatus employed in the one-pot synthetic method.

Although compared to the injection method, one-pot synthesis has the advantages of

reproducibility, capability of large-scale and industrial preparation, when dealing with different

materials core-shell nanocrystals and doped nanocrystals, the injection method is the only choice.

The one-pot synthesis can only be employed when same material core-shell nanocrystals are

desired.

1.3 Applications of Semiconductor Nanocrystals

1.3.1 Biological Detection

In 1998, both Alivisatos group16 and Nie17 group first reported the use of colloidal quantum

dots for biological labeling. They suggested that due to the photochemical stability and the

tubable luminescence of the quantum dots, they may make these materials extremely useful for

biolabeling. Compared to regular organic dyes, quantum dots have the advantages of tunable

luminescence, high quantum yield, broad light absorption, narrow emission spectra and high stability. Since 2002, there has been development of a wide range of methods for bio-conjugating

colloidal quantum dots36-40 in diverse areas of application: cell labeling41, cell tracking42, in vivo

imaging43, DNA detection44, and multiplexed beads45.

Colloidal quantum dots with a wide range of bio-conjugation and with high quantum yields

are now available commercially, so that it is no longer necessary for each experimenter to grow

their own or to become lost in the myriad discussions concerning the best way to render colloidal dots water soluble and bio-compatible.46

1.3.2 Hybrid Electroluminenscent Device

A light-emitting diode (LED) is a semiconductor device that emits incoherent

narrow-spectrum light when electrically biased in the forward direction of the p-n junction. This

effect is a form of electroluminescence. An LED is a small extended source with extra optics

added to the chip that makes it emit a complex radiation pattern.46 Since the first observation of

light emission from organic materials by Tang et al.48, continuous and rapid improvement in

device performance have enabled organic light emitting devices (OLEDs) to compete with

existing technologies. However, there are still many problems to be overcome, such as

improving device stability and color purity. Full width at half maximum (FWHM) of

photoluminescence of colloidal semiconductor nanocrystals is about 30nm which is narrower

than those from organic materials. Moreover, these inorganic nanocrystals are much more stable

and robust than organic molecules. So hybrid OLEDs using semiconductor nanocrystals as an

emission layer have to been to have good stability and efficiency. The first demonstration of a

hybrid OLED was reported by Colvin et al in 1994.49 In order to enhance the quantum efficiency

of hybrid OLED devices, several problems must be solved including more efficient charge

transfer between the organic layer and nanocrystals, the imbalance of injected conduction

through nanocrystals, a high density of pinhole defects in the nanocrystal layer, uniformity of

nanocrystals in the deposited layer, and optimization of interlayer structure of device.3,50

1.3.3 Photovoltaic Device

Inorganic solar cells that have limitations due to the high costs of fabrication have power

conversion efficiencies of 10%. While organic solar cells that use polymers which can be

processed from solution have been investigated as a low-cost alternative have solar power

efficiencies of up to 2.5%.51 One way to overcome these limitaions is to combine polymers with inorganic semiconductors Because of the nanoscale nature of light absorption and photocurrent

generation in solar energy conversion, the advent of methods for controlling inorganic materials

on the nanometer scale opens new opportunities for the development of future generation solar

cells. Alivisatos group used colloidal semiconductor nanorods as the inorganic phase in the

construction of these solar cells via solution-phase nano-assembly. By varying the radius of the

rods, the quantum size effect can be used to control the band gap; furthermore, quantum

confinement leads to an enhancement of the absorption coefficient compared with the bulk, such

that devise can be made thinner. One-dimensional (1D) nanorods are preferable to quantum dots

or sintered nanocrystals in solar energy conversion, because they naturally provide a directed

path for electrical transport. The length of the nanorods can be adjusted to the device thickness

required for optimal absorption of incident light.5

CHAPTER 2 SYNTHESIS OF CADMIUM SELENIDE NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR

2.1 Introduction

In chapter 1, we have mentioned that CdO was used to replace Cd(CH3)2 in the molecular

precursor synthesis because Cd(CH3)2 is extremely toxic, unstable and expensive. Actually,

selenium element has the same problem. Se is unstable in the air, and it is toxic. Here, we present

a method of synthesizing CdSe nanocrystals using SeO2 to replace Se element. We found that

with the presence of C16-diol and using cadmium precursors with longer carbon chain, the

quality of the CdSe quantum dots formed by this method is comparable to that of the best CdSe

nanocrystals reported in the literature, and some nanocrystals are even better.

To characterize the nanocrystals, the diameters of the nanocrystals were calculated from

the wavelength of first exciton peak using the CdSe sizing curve (Figure 2-1). The size distribution of the particle was evaluated by measuring the HWHM of first exciton peak, which

is a well accepted method to estimate nanocrystals’ size distribution.52 In our previous work, we

also found that peak sharpness is also a way to evaluate the size distribution (Figure 2-2).

Fluorometer and TEM was also employed.

Figure 2-1. Sizing curve of CdSe nanocrystals

Figure 2-2. HWHM and peak sharpness used for size distribution determination.

2.2 Experimental Section

2.2.1 Materials

1-octadecene (ODE, 90%), 1-tetradecene, (TDE, 92%), squalene (98%), dioctyl ether

(99%), docosanoic acid (CH3(CH2)20COOH, 99%), selenium dioxide (SeO2, 99.9+%) and

1,2-hexadecanediol (C16-diol, 90%), 1,2-decanediol (C10-diol, 98%) , 1,2-octanediol (C8-diol,

98%), 1-octadecanol (C18-OH, 99%), and phenol were purchased from Sigma-Alrich. Methanol

(99.9%), toluene (99.9%), acetone (99.8%) were purchased from Fisher. Sodium myristate

(CH3(CH2)12COONa), sodium stearate (CH3(CH2)16COONa) were purchased from TCI.

Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O) was purchased from Alfa Aesar.

Tetrabutylammonium hydroxide (1M in methanol) was purchased from Acros. All chemicals

were used without further purification.

2.2.2 Instrumentation

Absorption spectra of aliquots were collected by a Shimadzu UV-1700 UV-Visible

Spectrophotometer (Figure 2-3). The wavelength, absorption and the half width at half maximum

(HWHM) of first exciton peak for each aliquot were recorded. This method also was compared

with our previous one-pot synthesis method and other current injection method.

Figure 2-3. Schematic diagram of a UV-Vis microscope.

Photoluminescence (PL) was measured at room temperature from nanocrystals suspended in toluene using a JOBIN YVON HORIBA Fluorolog-3 Model FL3-12 spectrofluorometer

(Figure 2-4).

Figure 2-4. Schematic diagram of a Fluorolog-3 Model FL3-12 spectrofluorometer.

High resolution transmission electron microscopy (HR-TEM) images were obtained using a JEOL 2010F microscope for lattice imaging and crystal size determination. TEM samples were prepared by dispersing the nanocrystals in toluene and depositing them onto formvar-coated copper grids.

2.2.3 Preparation of Cd-Precursors

2.2.3.1 Cadmium myristate (CdC14)

1.5g cadmium nitrate was dissolved in 75ml methanol, while sodium myristate solution

was prepared by dissolving 3.8g sodium myristate in 550ml methanol. Then the cadmium

nitrate solution was dropped slowly into sodium myristate solution under magnetic stirring

conditions. The observed white precipitate was washed with methanol 4-5 times to get rid of

impurities and dried under vacuum overnight to remove all solvents.

2.2.3.2 Cadmium stearate (CdC18)

0.3g Cadmium nitrate was dissolved in 40ml methanol, Sodium stearate solution was prepared by dissolving 0.90g sodium stearate in 600ml methanol. Then the cadmium nitrate solution was dropped slowly with into sodium stearate solution under magnetic stirring conditions. The observed white precipitate was washed with methanol 2-3 times get rid of impurities then put back in flask with 600 ml methanol and ultrasonicated. Wash the product again and dried under vacuum overnight to remove all solvents.

2.2.3.3 Cadmium docosanate (CdC22)

0.3g Cadmium nitrate was dissolved in 40ml methanol. Sodium docosanate solution was prepared by dissolving 0.90g docosanoic acid in 600ml methanol and slowly adding 1.1 ml

tetrabutylammonium hydroxide (1M in methanol) dropwise. Then the cadmium nitrate solution

was dropped slowly into sodium docosanoicate solution under magnetic stirring conditions. The

observed white precipitate was washed with methanol 2-3 times get rid of impurities then put

back in flask with 600 ml methanol and ultrasonicated. Wash the product again and dried under vacuum overnight to remove all solvents.

2.2.4 Preparation of CdSe Nanocrystals

Cadmium precursor (0.1mmol), SeO2 (0.05mmol), C16-diol (0.05mmol) and non coordinating solvent (5g) were mixed in a three-neck flask equipped with condenser, magnetic stirrer, thermocouple, and heating mantle (as shown in Figure 1-5), degassed before heated to

265 oC with gentle stirring under vacuum to synthesize CdSe nanocrstals. Aliquots of the solution for each reaction were taken quantitatively with a syringe at different time intervals, and quickly cooled and diluted in toluene to stop further growth. These aliquots were employed to monitor the reaction via UV-Vis and photoluminescence measurement.

2.3 Results and Discussion

2.3.1 Diol Effect

C16-diol Effect on the Quality and Size of CdSe Nanocrystals

ODE TDE

Squalene

Octyl ether

Figure 2-5. Molecular structures of organic solvents used.

CdSe nanocrystals were formed using SeO2 compound instead of Se element in the solvent of ODE, which means that SeO2 is active at high temperature. However, the quality is not as

good. We found that for CdSe nanocrystals synthesized in ODE, addition of equal molar

amounts of C16-dio land SeO2 has several effects on the growth, including growth rates,

HWHMs, sharpness, optical densities, and final sizes. This phenomenon was also observed in

CdSe nanocrystals synthesized in TDE, squalene, and octyl ether. The molecular structures of these four solvents are shown in Figure 2-5.

Figure 2-6 shows the absorption spectra of CdSe nanocrystals made of 0.1 mmol CdC14,

0.05 mmol SeO2 and 0.05 mmol C16-diol in different solvents.

b) squalene, no C16-diol c) octyl ether, no C16-diol d) TDE, no C16-diol a) ODE, no C16-diol 10 8 10 8 8 6 8 6 30min 6 30min 10min 6

4 10min 4 30min 5 min 4 30min 5 min 4 0 min 10min 10min 0 min 2 5 min o 2

Absorbance (a.u.) 2 240 C o 2 5 min 240 C 0 min o 220oC 220 C 0 min 0 0 0 0

e) ODE, C16-diol added f)squalene, C16-diol added g)octyl ether, C16-diol added h)TDE, C16-diol added 10 8 10 8 8 6 8 30min 6 6 30min 10min 6

4 10min

4 5 min 4 30min 5 min 4 0 min 30min 10min 2 0 min 10min o 2 Absorbance (a.u.) 240 C 2 o 2 5 min 250 C 5 min o 220 C 220oC 0 min 0 min 0 0 0 0 400 500 600 700 400 500 600 700 400 500 600 700 400 500 600 700 wavelength (nm) wavelength (nm) wavelength (nm) wavelength (nm)

Figure 2-6. Temporal evolution of the absorption spectra during the CdSe synthesis in (a) and (e) ODE, (b) and (f) squalene, (c) and (g)octyl ether and (d) and (h)TDE; in syntheses (a), (b), (c) and (d), C16-diol was not added while in syntheses (e), (f), (g) and (h), C16-diol was added.

In Figure 2-7, we show the absorption and photoluminescent (PL) spectra and TEM image

of CdSe nanocrystals which were made in ODE and have a reaction time of 40 minutes. It can be

seen that the sample is nearly monodispersed. The average diameter is 3.3 nm, which is very close to the calculated diameter of 3.4 nm.

The CdSe nanocrystals formed with the presence of C16-diol have much better quality than

those formed without adding C16-diol. With the presence of C16-diol, the first peak is narrower

and deeper than that without the presence of C16-diol, which indicates that size distribution is

better. Thus one can conclude that adding C16-diol can improve the quality of CdSe nanocrystals.

It was also observed that the final sizes of CdSe nanocrystals are smaller and the ODs are higher in the case of adding C16-diol. To better understand the role that C16-diol assumes in the

synthesis, three syntheses with different amounts of C16-diol were performed. Nuclei

concentration, nuclei number, growth rate as well as peak sharpness and HWHM were employed

to analyze the data.

a b

1.0

0.8

0.6

Intensity 0.4

0.2

0.0

500 550 600 650 Wavelength (nm)

Figure 2-7. Characterization of CdSe nanocrystals synthesized in ODE with reaction time of 40 minutes. (a) Absorption (in blue) and photoluminescent (PL) (in red) spectra and (b) TEM image.

Nuclei concentrations are calculated by using the exciton energy to determine the particle

radius and then the extinction coefficient for each size to determine the particle concentration.52

CdSe : D = (1.6122×10−9 )λ4 − (2.6575×10−6 )λ3 + (1.6242×10−3 )λ2 − (0.4277)λ + 41.57 (2-1)

In the above equation, D (nm) is the diameter of a given nanocrystal sample, and λ (nm) is the

wavelength of the first excitonic absorption peak of the corresponding sample.

The extinction coefficient of CdSe is calculated as

ε = 5857(D) 2.65 (2-2)

Then using the Lambert-Beer’s law,

A = εCL (2-3)

The molar concentration C (mol/L) of the nanocrystals of the sample can be calculated. A is the absorbance at the peak for a given sample and L is the path length (cm). In our experiments, L was fixed at 1 cm.

M) 8 -6 c 7

CdSe nuclei concentration (10 concentration nuclei CdSe 2 b a 1 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 size of CdSe nanocrystals (nm)

Figure 2-8. Temporal evolution of CdSe nanocrystal concentration synthesized in ODE with different C16-diol/SeO2 ratios, (a) C16-diol/SeO2=0, (b) C16-diol/SeO2=1, and (c) C16-diol/SeO2=2.

The calculated temporal evolution of concentrations and growth rates of CdSe nanocrystals

made in ODE with different C16-diol/SeO2 ratio is shown in Figure 2-8 and Figure 2-9,

respectively. Figure 2-8 shows that the more C16-diol in the reaction solution, the higher the

CdSe nuclei concentration and the concentration dramatically increases when the C16-diol to

SeO2 ratio changes from 1 to 2. It can be seen in Figure 2-9 that the higher the ratio of C16-diol

to SeO2, the slower the growth rate.

8 a b

6 /min) 3

4 c

0 CdSeparticle grpwth rate (nm

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 CdSe particle size (nm3)

Figure 2-9. CdSe particle growth rate in the synthesis with different C16-diol/SeO2 ratios (a) C16-diol/SeO2=0, (b) C16-diol/SeO2=1, and (c) C16-diol/SeO2=2.

It was found that the C16-diol affects not only the quality and nuclei number of CdSe

nanocrystals, but also the final CdSe particle sizes, as shown in Figure 2-10. The more the

C16-diol in the reaction solution, the smaller the CdSe nanocrystals will be obtained.

Addition of C16-diol can slow the CdSe particle growth remarkably with the nuclei

number increasing at the same time. It can be concluded that nuclei number is related the particle

growth rate. The faster the particles growth, the lower the nulclei number. This leads us to a

hypothesis that C16-diol acts as a reducing agent in the reaction, helping reduce the selenium in

+4 -2 SeO2 from Se to Se , helping increase the concentration of selenium monomers. This leads to

easier nucleation and a higher nuclei number and thus, a smaller size.

3.8

3.7

3.6

3.5

3.4

3.3

3.2

Size of CdSe nanocrystals (nm) nanocrystals of CdSe Size 3.1

3.0 0.0 0.5 1.0 1.5 2.0 Ratio of C -diol/SeO 16 2

Figure 2-10. CdSe particle size and normalized nuclei number in the synthesis with different C16-diol/SeO2 ratios.

Effect of Numbers of Carbon Atoms per Diol

Effect of different carbon chain length of diols (C8-diol, C10-diol and C16-diol) was studied. The temporal evolution of the absorption spectra is shown in Figure 2-11. As shown in

Figure 2-12, the HWHM of CdSe nanocrystals made with C16-diol is the best and this is

equivalent to a tighter size distribution. Figure 2-13 shows CdSe nanocrystal concentration with

different diols. The nuclei concentration with C10-diol is very close to but slightly higher than

that with C16-diol, while the concentration with C16-diol is higher than that of C8-diol. Figure

2-14 illustrates that with C10-diol and C16-diol, the final particle sizes are very similar and with

C8-diol slightly larger size particles can be obtained.

a) b) c) 8 8 8

30min 6 6 6 30min 30min 10min 10min 10min 4 4 4

5min 5 min 5 min

0min 0 min 0 min

Absorbance (a.u.) 2 2 o 2 240 C 250oC 240oC o o 220 C 230oC 220 C 0 0 0 400 500 600 400 500 600 400 500 600

wavelength (nm) wavelength (nm) wavelength (nm)

Figure 2-11. Temporal evolution of the absorption spectra during the CdSe synthesis with different diols: (a) C16-diol, (b) C10-diol and (c) C8-diol.

a 16

HWHM (nm) 15

b 14

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 Size of CdSe nanocrystals (nm)

Figure 2-12. HWHM of CdSe during synthesis with different diols: (a) C16-diol, (b) C10-diol and (c) C8-diol.

M) b -6 3

1 c CdSe nuclei concentration (10 nuclei CdSe

0 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 Size of CdSe nanocrystals (nm)

Figure 2-13. Temporal evolution of CdSe nanocrystal concentration with different diols: (a) C16-diol, (b) C10-diol and (c) C8-diol.

3.9

3.8

3.7

3.6

3.5

3.4 Size of CdSe nanocrystals (nm) Size of CdSe

3.3 8 9 10 11 12 13 14 15 16 17 number of carbon atoms per diol

Figure 2-14. CdSe particle size in the synthesis with different numbers of carbon atom per diol.

Based on all the information, one can conclude synthesis with C16-diol has the lowest

growth, smallest size. The possible explanation for this is that the longer the carbon chain in the diol molecule, the hydrogen bonding will be weaker, and thus the more active the diol would be.

So among all three, C16-diol is the most active.

Comparison of Alcohols and Diols

a) b) c) 8 8 8

6 30min 6 6 30min 30min 10min 10min 10min 4 4 4

5min 5 min 5 min

0min 0 min 0 min

Absorbance (a.u.) 2 2 2 o 240 C o 240oC 240 C 220oC 220oC 220oC 0 0 0 400 500 600 700 400 500 600 700 400 500 600 700

wavelength (nm) wavelength (nm) wavelength (nm)

Figure 2-15. Temporal evolution of the absorption spectra during the CdSe synthesis with different alcohols. (a) C16-diol, (b) C18-OH and (c) phenol.

Synthesis with alcohols was performed. The results are compared with diols. C18-OH and

phenol were used. The nanocrystals were formed by cadmium myristate (0.1 mmol) reacted with

SeO2 (0.05 mmol) and C18-OH or phenol (0.05 mmol). The absorption spectra are shown in

Figure 2-15. Compare HWHM and sharpness, one can conclude that the qualities CdSe

nanocrystals with C18-OH are better than that without alcohol, but not are not comparable to

those with C16-diol. The CdSe nanocrystals made with phenol did not get improved.

The possible reaction happened is proposed below. The SeO2 got reduced by the alcohol and selenium was formed. The active selenium then reacted with cadmium precursor to form

CdSe nanocrystals.

2.3.2 Precursor Effect

a) b) 8 8

6 30min 6 30min

10min 10min 4 4

5min 5 min

0min 0 min 2

Absorbance (a.u.) 2 o o 240 C 240 C o o 220 C 220 C 0 0 400 500 600 700 400 500 600 700 wavelength (nm) wavelength (nm)

c) d) 8 10

6 8 30min 30min 10min 6 4 10min

5 min 5min 4 0 min 0min 2 Absorbance (a.u.) 255oC 2 220oC o 240 C o 210 C 0 0 400 500 600 700 400 500 600 700 wavelength (nm) wavelength (nm)

Figure 2-16. Temporal evolution of the absorption spectra during the CdSe synthesis with different Cd precursors. (a) CdC14, (b) CdC18, (c) CdC22 and (d) CdC10.

Besides the diol effect, it was found that using precursors that have longer carbon chains

can also improve the quality of CdSe nanocrystals. Figure 2-16 shows the absorption spectra of

CdSe nanocrystals made of four different cadmium precursors (0.1 mmol) reacted with SeO2

(0.05 mmol) and C16-diol (0.05 mmol) in ODE. The particles made of CdC22 have the best

quality, followed by those made of CdC18, and particles formed by CdC10 have poor spectra. So

we can conclude that the longer carbon chain in the cadmium precursors, the higher-quality

nanocrystals can be obtained. The absorption spectrum of CdSe nanocrystals made of CdC18 and

CdC22 exhibit multiple exiton peaks (Figure 2-17).

a b

2.0 2.0

1.5 1.5

1.0 1.0

(a.u) Absorbance Absorbance (a.u.) Absorbance 0.5 0.5

0.0 0.0 300 400 500 600 300 400 500 600 wavelength (nm) wavelength (nm)

Figure 2-17. Multiple exiton peaks. (a) CdC18, (b) CdC22.

M) 20 -6 c

10 b

CdSe (10 nuclei concentration 5 a

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Diameter of CdSe nanocrystals (nm)

Figure 2-18. Effect of Cd precursor on the nuclei concentration during the CdSe synthesis. (a) CdC14, (b) CdC18 and (c) CdC22.

6.0

5.5

5.0

4.5

4.0

Size (nm) 3.5

3.0

2.5

2.0 8 1012141618202224 carbon atom number in precursor

Figure 2-19. Effect of Cd precursor on the CdSe particle size in the synthesis.

The calculated nuclei concentrations of syntheses with different cadmium precursors are

shown in Figure 2-18. The more carbon atoms in the cadmium precursors, the higher nuclei

concentration will be achieved. Figure 2-19 illustrates the relationship between the final particle

size and the number of carbon atoms per cadmium precursor. When a cadmium precursor with

longer carbon chains is used, CdSe nanocrystals with smaller size will be generated. Growth

rates are shown in Figure 2-20. The longer the carbon chains in the precursor, the slower the

growth rate. The result is that cadmium precursor and selenium precursor have more comparable

reactivity and this may cause high-quality nanocrystals.

The relationship between the nuclei number and the particle growth rate is also shown

here. As has been discussed, the ratio of diol to SeO2 effect, the faster the particles growth, the

lower the nuclei number. Precursor with longer carbon chains causes slower growth rate and

higher nuclei number probably because its molecular size is larger and thus it is harder to get

activated. It takes a longer time to transfer to monomers and results in higher nuclei number.

14 a

12 /min)

3 10

8 b

4 c 2

CdSe particle growth rate (nm rate growth particle CdSe 0

4 6 8 10 12 14 16 18 20 22 CdSe particle size (nm3)

Figure 2-20. CdSe particle growth rate in the synthesis with different Cd precursors: a) CdC14, (b) CdC18 and (c) CdC22.

2.3.3 Multiple-Addition Method

With the above method, the acceptable nanocrystals with the largest size we can get is 3.4

nm. It was found that with the multiple-addition method CdSe nanocrystals with a size of 4.5 nm

were generated, which has an absorption peak at 582 nm. In the experiment, cadmium myristate

(0.1 mmol), SeO2 (0.05 mmol), and C16-diol (0.05 mmol) were added into a three-neck flask with

5 g ODE. The mixture solution was degassed for 10 min under vacuum (~16 mTorr) at room

temperature, and then the vacuum was removed. Under an argon flow, the solution was heated to

265°C (25°C /min) with gentle stirring and let to react for 1 hour. The reaction solution was

cooled down to room temperature, and then cadmium myristate (0.025 mmol), SeO2 (0.0125 mmol), and C16-diol (0.0125 mmol) were added. After degassing, the solution was heated to

265°C again and let react for 1 hour. Lastly, the second addition step and condition was repeated

twice. Aliquots were taken out for UV and photoluminescence (PL) measurement.

a b 12

Absorbance (a.u.) 2

0 400 500 600 700 wavelength (nm)

Figure 2-21. Characterization of CdSe nanocrystals during the multiple-addtion synthesis (a) Temporal evolution of the absorption spectra multiple-addition synthesis. (b) TEM image.

The temporal evolution of the absorption spectra during the CdSe synthesis is shown in

Figure 2-21(a). The TEM image of the final CdSe nanocrystals made by this method is shown in

Figure 2-21(b). It can be seen that the nanocrystals are uniformly spherical, and the size

distribution is good. The average size is around 4.5 nm.

2.4 Conclusion

In this part of work, it is showed that SeO2 can be used to replace selenium powder to synthesize CdSe nanocrystals. With the presence of C16-diol, the quality of CdSe nanocrystals

can be improved. The effects of the C16-diol to SeO2 ratio were studied. It was found that the

more diol added, the higher-quality and smaller-size CdSe nanocrystals will be obtained. The

quality of our product is comparable to the best results published and nanoparticles smaller than

3 nm are even better. C16-diol can make the CdSe particle growth slower with the nuclei number

increasing at the same time. Compared the quality of CdSe nanocrystals made with C8-diol,

C10-diol and C16-diol, these diols have similar effect. Results with C18-OH and phenol were

worse than those with diols. Cadmium precursor effect was studied. The results show that the

longer the carbon chains in cadmium precursor, the smaller size particles will be synthesized.

Cadmium precursors with longer carbon chains can retard the growth rate and increase the nuclei

number. Multiple-addition method was performed and obtained CdSe nanocrystals with size of

around 4.5 nm. The best spectrum of different peak position were chosen from several

experiments and shown in Figure 2-22.

Absorbance (a.u.) Absorbance 10

0 400 500 600 wavelength (nm)

Figure 2-22. Temporal evolution of the absorption spectra of the as-prepared CdSe nanocrystals. Black: made by CdC22+SeO2+C16-diol (0.1:0.05:0.05); Red:: made by CdC18 +SeO2+C16-diol (0.1:0.05:0.05); Blue: made by CdC14 +SeO2+C16-diol (0.1:0.1:0.05); Purple: CdC14 +SeO2+C16-diol (0.1:0.05:0.05); Green: motile-addtion reaction, CdC14 +SeO2+C16-diol (0.1:0.05:0.05).

CHAPTER 3 SYNTHESIS OF METAL SELENIDE NANOCRYSTALS USING SELENIUM DIOXIDE AS PRECURSOR

3.1 Introduction

In Chapter 2 it was that using SeO2 to replace selenium element, CdSe nanocrystals can be

obtained. Experiments where SeO2 was employed with gallium, lead, silver, copper and nickel

precursors were performed to synthesize metal selenide nanocrystals.

The Kelly group has done a lot of research on the GaSe nanoparticles, from synthesis to

physical properties.53-56 GaSe has a hexagonal layered structure57 consisting of Se-Ga-Ga-Se

sheets. GaSe is a semiconductor with indirect band gap58,59 having a 2.11 eV direct band gap.

Their GaSe synthesis is based on the reaction of an organometallic (GaMe3) with TOPSe in a

high-temperature solution of TOP and TOPO. The absorption spectra of GaSe nanoparticles have

an onset in the 400-500 nm region. The nanoparticle diameters range from 2 to 6 nm, with an

average size of about 4 nm. After chromatographic purification the average size is about 2.5

nm.53

Lead selenide nanocrystals have been widely studied. The popular method is colloidal

synthesis method. In the Murray group, PbSe nanocrystals is synthesized by rapidly injecting a

lead oleate and TOPSe dissolved in trioctylphosphine into a well-stirred solution of dioctylether

at 150oC.60 Temperature is tuned to control the size of the nanocrystals.

Ag2Se has two phases. The low-temperature phase (α-Ag2Se) is a narrow band-gap

semiconductor, and has been widely used as a photosensitizer in photographic flims to

thermochromic materials. β-Ag2Se is the high-temperature phase, and it is a superionic conductor

that is used a solid electrolyte in photochargable seconndary batteries.61 These two phases are

reversible. There are only a few reports on the preparation of Ag2Se nanocrystals. Yi Xie et .al synthesize Ag2Se nanocrystals at room temperature through the reaction of AgNO3, Se, and

62 KBH4 in pyridine. The Vittal group synthesized Ag2Se nanoparticles by thermolysis of silver

selenocarboxlyate in TOPO/TOP.61

CuSe is used in solar cells.63 The methods to synthesize CuSe nanocrystals include

thermolysis of Cu and Se powder mixtures64, the mechanical alloying of Se and Cu in a

high-energy ball mill, and the reaction of Se with Cu element in liquid ammonia65. The Vittal

group synthesized CuSe nanoparticles by thermolysis of copper selenocarboxlyate in

TOPO/TOP.66

NiSe is one of the typical Pauli paramagnets with metallic conductivity.67 These stoichiometric compounds and the solid solutions between them now have been regarded as typical materials for studies of the physical characteristics associated with a narrow band electron system.68-70 Meanwhile, transition metal dichalcogenides have extensive applications in energy areas such as electrochemistry and catalysis.71,72 The large surface areas and high activity

of nanomaterials will enhance their applications in these fields.73 Several methods have been

used to prepare NiSe nanocrystals, including elemental reactions74, organnometallic precursor

method65 and solvothermal processes76.

3.2 Experimental Section

3.2.1 Materials

Gallium nitrate hydrate (Ga(NO3)3·xH2O, 99.999%), lead oxide (PbO, 99%), silver nitrate

(AgNO3, 99%), copper nitrate trihydrate (Cu(NO3)2·3H2O, 99 %) , nickel nitrate hexahydrate

(Ni(NO3)2·3H2O, 99.999%), selenium dioxide (SeO2, 99.9+%) ,1-octadecene (ODE, 90%),

trioctylphosphine oxide (TOPO, 90%), trioctylphosphine (TOP), 1,2-hexadecanediol (C16-diol,

90%), oleyamine (OAm, 70%), octadecylphosphonic acid (ODPA), tributylphosphine (TBP,

97%) were purchased form Aldrich. Sodium myristate (CH3(CH2)12COONa), sodium oleate

(CH3(CH2)7CH=CH(CH2)7COONa) were purchased from TCI. All chemicals except oleyamine

and TOPO were used without further purification. Methanol (99.9%), toluene (99.9%), acetone

(99.8%) were purchased from Fisher.

3.2.2 Instrumentation

Absorption spectra of aliquots were collected by a Shimadzu UV-1700 UV-Visible

Spectrophotometer. High resolution transmission electron microscopy (HR-TEM) images were obtained using a JEOL 2010F microscope for lattice imaging and crystal size determination.

TEM samples were prepared by dispersing the nanocrystals in toluene and depositing them onto formvar-coated copper grids.

3.2.3 Preparation of Precursors

3.2.3.1 Gallium myristate

0.6 g gallium nitrate was dissolved in 20 ml methanol, while sodium myristate solution was prepared by dissolving 1.5 g sodium myristate in 100 ml methanol. Then the gallium nitrate solution was dropped slowly into sodium myristate solution under magnetic stirring conditions. The observed white precipitate was washed with methanol 4-5 times and dried under vacuum overnight to remove all solvents.

3.2.3.2 Silver oleate

0.8 g silver nitrate was dissolved in 40 ml methanol, while sodium myristate solution was prepared by dissolving 0.6 g sodium oleate in 100 ml methanol. Then the silver nitrate solution was dropped slowly into sodium oleate solution under magnetic stirring conditions. The observed white precipitate was washed with methanol 4-5 times and dried under vacuum overnight to remove all solvents.

3.2.3.3 Copper oleate

1.2 g copper nitrate was dissolved in 30 ml methanol, while sodium myristate solution was

prepared by dissolving 1.2g sodium oleate in 150 ml methanol. Then the copper nitrate

solution was dropped slowly into sodium oleate solution under magnetic stirring conditions. The observed blue precipitate was washed with methanol 4-5 times and dried under vacuum overnight to remove all solvents.

3.2.3.4 Nickel oleate

0.6 g nickel nitrate was dissolved in 30 ml methanol, while sodium myristate solution was

prepared by dissolving 1.2g sodium oleate in 200 ml methanol. Then the nickel nitrate solution was dropped slowly into sodium oleate solution under magnetic stirring conditions. Hexane was added into extract the nickel oleate and collected into a flask. The nickel oleate hexane solution was evaporated by a rotary evaporator. The green product was dried under vacuum overnight.

3.2.4 Preparation of Nanocrystals

3.2.4.1 Gallium selenide nanocrystals

Gallium myristate (0.1 mmol), SeO2 (0.05 mmol), and C16-diol (0.05 mmol) were added

into a three-neck flask with 5 g ODE. The mixture solution was degassed for 10 min under

vacuum (~16 mTorr) at room temperature, and then the vacuum was removed. Under an argon

flow, the solution was heated to 285°C with gentle stirring and let to react for 2 hours. The color

of the reaction solution changed from colorless to light yellow.

3.2.4.2 Lead selenide nanocrystals

Lead oxide (0.1 mmol), ODPA (0.2 mmol) were added into a three-neck flask with 3 mL

ODE. The mixture solution was degassed for 10 min under vacuum (~16 mTorr) at room

temperature, and then the vacuum was removed. Under an argon flow, the solution was heated to

160°C with gentle stirring for 1 hour until PbO dissolved. The solution was cooled to 120°C and

degassed at this temperature to get rid of water. When the solution was cooled down to room

temperature, SeO2 (0.05 mmol), C16-diol (0.05 mmol) and 3.3 mL ODE was added. The mixture

was degassed for 10 min and then heat up to 180 °C under argon flow.

3.2.4.3 Silver selenide nanocrystals

SeO2/TOP was obtained by dissolving SeO2 (0.1 mmol) in TOP (1.2 mmol, 0.54 mL).

Silver oleate (0.1 mmol) was added into a three-neck flask with 5 g purified TOPO. The

mixture solution was degassed for 10 min under vacuum (~16 mTorr) at room temperature, and

then the vacuum was removed. Under an argon flow, the solution was heated with gentle stirring.

When the temperature reached 120°C, 0.27 mL SeO2/TOP (0.05 mmol / 0.6 mmol) was quickly

injected to the solution and let react for 40 min.

3.2.4.4 Copper selenide nanocrystals

SeO2/TOP was obtained by dissolving SeO2 (0.1 mmol) in TOP (1.2 mmol, 0.54 mL).

Copper oleate (0.1 mmol) and 0.27 mL SeO2/TOP (0.05 mmol / 0.6 mmol) were added into a

three-neck flask with 5 g purified OAm. The mixture solution was degassed for 10 min under

vacuum (~16 mTorr) at room temperature, and then the vacuum was removed. Under an argon

flow, the solution was heated to 220°C with gentle stirring and allowed react for 90 min.

3.2.4.5 Nickel selenide nanocrystals

Nickel oleate (0.1 mmol) and SeO2 (0.1 mmol) were added into a three-neck flask with 5 g

purified OAm. The mixture solution was degassed for 10 min under vacuum (~16 mTorr) at

room temperature, and then the vacuum was removed. Under an argon flow, the solution was

heated with gentle stirring. TBP (1.2 mmol) was quickly injected to the solution when temperature reached 170°C. The solution was kept at this temperature to react for 30 min.

3.2.5 Purification of Nanocrystals

Nanocrystals were purified by precipitation in excess acetone followed by centrifugation.

The supernatant contains molecular reaction byproducts and was discarded. The nanocrystals

were redispersed in toluene and centrifuged again. The well-capped nanocrystals remained

dispersed while poorly-capped nanocrystals settled. The precipitate was discarded. The

toluene-dispersed nanoparticles were reprecipitated in excess acetone. The supernatant was

discarded after centrifugation. The purified nanocrystals were redispersed in toluene.

3.3 Results and Discussion

3.3.1 Gallium Selenide

The time evolution of the absorption spectrum is shown in Figure 3-1. The spectrum of

GaSe nanocrystal at 30 minutes starts to have an onset at around 340 nm, which means that the

GaSe nanocrystals made by our method are smaller in size than those of Kelly’s. This is proved

by the TEM image (Figure 3-2). The average size of our GaSe nanocrystals is around 2.0 nm. It

was found that the as-prepared GaSe nanocrystals have blue emission. We didn’t collect the

emission spectra of the nanoparticles and this will be done later. The size is small probably

because there are too many nuclei. To get larger size particles, one should try to decrease the

nuclei number.

0.25

0.20 120 min

0.15

0.10 30 min Absorbance

0.05 0 min

0.00 300 400 500 600 wavelength (nm)

Figure 3-1. Evolution of absorption spectrum of GaSe nanocrystals. Black: 0 min; pink: 30 min and red: 2 h.

Figure 3-2. TEM image of GaSe nanocrystals.

3.3.2 Lead Selenide

The PbSe nanocrystals cannot disperse in toluene, hexane, chloroform or other organic solvents, which means that what was formed is aggregated PbSe nanocrystals. The TEM image of PbSe nanocrystal is shown in Figure 3-3. It can be seen that the aggregated nanocrystals consists of nanocubes whose edge is around 74 nm.

Figure 3-3. TEM image of PbSe nanocrystals.

3.3.3 Silver Selenide

The absorption spectrum of AgSe nanocrystals during the synthesis is shown in Figure 3-4.

At the beginning, there is an absorption peak at around 407 nm, which belongs to Ag nanoparticles. The peak got weaker and weaker and disappeared at the reaction time of 40 minutes. This means that Ag nanoparticle were formed first, and then gradually reacted with

SeO2 to form AgSe. To prove this, X-ray diffraction patterns of samples taken at different

occasions should be obtained.

0.6

0.4

10 min

Aborbance 0.2

1 min 40 min

0.0 300 400 500 600 700 wavelength (nm)

Figure 3-4. Evolution of absorption spectrum of AgSe nanocrystals.

The high resolution TEM images of AgSe nanocrystals made by our method are shown in

Figure 3-5. The shape of the nanocrystals is uniform, and they are all spherical. But the size is not uniform, ranging from 4.3 nm to 12.2 nm, with an average size of 7.4 nm and the lattice spacing is 0.21 nm. Size distribution should be improved.

a b

Figure 3-5. HR-TEM images of AgSe nanocrystals. The lattice spacing in (b) is 0.21 nm.

3.3.4 Copper Selenide

Figure 3-6. TEM image of CuSe nanocrystals.

The TEM image of CuSe nanocrystals is shown in Figure 3-5. Interestingly, some hollow

nanocrystals were found but the size distribution is large and the shape is not uniform. To get

more uniform nanocrystals, one may try to anneal the precursors at 160oC for longer time first to

generate more active monomers.

3.3.5 Nickel Selenide

Needle-shape NiSe nanocrystals were obtained. The TEM image of NiSe nanocrystals is shown in Figure 3-6. Similar to AgSe and CuSe, the problem is that non-uniform shape and size

NiSe nanocrystals were formed.

Figure 3-7. TEM image of NiSe nanocrystals.

3.4 Conclusion

In this part of the work, experiments to prepare GaSe, PbSe, AgSe, CuSe and NiSe nanocrystal were performed. For GaSe, spherical nanocrystals were obtained and the average size is around 2.0 nm. The product has blue emission. To get larger size GaSe nanocrystals, one should try to decrease the nuclei number. For PbSe, the nanocrystals aggregated. But the

aggregates consist of nanocubes that are uniformly in size and shape. To avoid aggregation, one

might try to anneal the precursors for a longer time to get more active monomers before heating

to the reaction temperature. For AgSe, spherical nanocrystals were obtained but the size distribution is poor. The diameters range from 4 nm to 12 nm. The high resolution TEM image showed that the crystalline AgSe nanoparticles have a lattice spacing of 0.21 nm. The absorption spectrum of the nanocrystals shows that Ag nanoparticles were formed at the beginning and then gradually reacted with SeO2 to form AgSe nanocrystals. As for CuSe and NiSe, hollow nanocrystals and needle-shape nanocrystals were got, respectively. But their size and shape distribution are still poor. To improve the quality, one can try to generate active monomers first before the reaction.

CHAPTER 4 SUMMARY AND FUTURE WORK

4.1 Summary

It has been demonstrated that SeO2 can be used to replace selenium element to synthesize

metal selenide semiconductor nanocrystals. For CdSe, one-pot synthetic method was used. It is

found that when equal amount of C16-diol as SeO2 was added, the quality of CdSe nanocrystals

can be improved. Effect of the ratios of C16-diol to SeO2 was studied and the result shows that

the higher the ratio of C16-diol as SeO2, the better nanocrystals can be obtained, and the higher

nuclei number and the slower growth. Experiments using different cadmium precursors were

performed and it was found that the longer the carbon chains in cadmium precursor, the better

the quality of CdSe nanocrystals were got, and the higher the nuclei number and the slower the

growth. Multiple-addition reaction was employed to prepare larger size nanocrystals.

It was also proved that using SeO2 instead of selenium element, GaSe, PbSe, AgSe, CuSe

and NiSe nanocrystals were obtained. GaSe nanocrystals were uniform in size and shape, but the

size is small. PbSe nanocube aggregates were obtained, and each nanocube is uniform in size and

shape. Crystalline AgSe nanoparticles were obtained with an average size of 7.4 nm and a lattice

spacing of 0.21 nm. Uniform CuSe and NiSe nanoparticles have not been formed yet.

4.2 Future work

4.2.1 Injection-Synthetic Method for CdSe

So far the largest acceptable CdSe nanocrystals obtained by our method have a diameter of

around 4.5 nm. To get larger high-quality CdSe nanocrystals, injection method can be employed.

By quickly injecting precursors at high temperature, fewer nuclei will be formed in a very short

time, resulting in larger, more uniform CdSe nanocrystals.

Cadmium myristate (0.1 mmol) will be added into a three-neck flask with 4.3 g ODE.

SeO2 (0.1 mmol) and C16-diol (0.1 mmol) will be added in 2 mL ODE. The two mixture solutions

will be degassed for 10 min under vacuum (~16 mTorr) at room temperature, and then the vacuum

will be removed. Under an argon flow, the SeO2 and C16-diol solution will be heated to 100°C and SeO2 and C16-diol will dissolve and form yellow solution. Under an argon flow, the cadmium

solution will be heated with gentle stirring. When the temperature reaches 265°C, 1 mL of SeO2

(0.05 mmol) and C16-diol (0.05 mmol) ODE solution will be quickly injected to the cadmium

solution. The temperature will keep at 265°C.

4.2.2 Improvement of Other Metal Selenide Nanocrystals

For GaSe, PbSe, AgSe, NiSe and CuSe, acceptable results haven’t been obtained yet. One

can try to generate active monomers before heating to the reaction temperatures for PbSe, NiSe

and CuSe. Injection method and lower reaction temperature may be used for GaSe to get larger

size nanoparticles. X-ray diffraction patterns should be got to identify the crystal structures of

these nanocrystals.

4.2.3 Mechanism Study

The mechanism of SeO2 reacting with C16-diol can also be studied to understand how

1 13 31 C16-diol improve the quality of CdSe nanocrystals using H, C, and P NMR spectroscopy and mass spectrometry to confirm our hypothesis.

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BIOGRAPHICAL SKETCH

Xian Chen was born in Xiamen, a beautiful city on the southeast coast of China. In 1999, she started her college life at the University of Science and Technology of China (USTC). After

5 years of study in the Department of Polymer Science and Engineering, she received her bachelor’s degree in engineering in 2004. Then, she joined the Department of Chemistry at the

University of Florida. She would like to pursue a Ph.D. degree after graduation.