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Chapter 1

General introduction

1.1 Introduction

Colloidal semiconductor nanocrystals (quantum dots, QDs) have generated great

fundamental and technical interest due to their novel size-dependent properties.1-4

This new class of nanomaterials has come under intensive investigation for their linear

and nonlinear optical properties in connection to optoelectronic devices and biomedical

tags.5-10

The most striking feature of nanomaterials is that their chemical and physical

properties differ markedly from those of the corresponding bulk solids. This behavior can

be ascribed to two basic reasons as follows: The first reason is the high surface to volume

ratio (i.e., the number of atoms at the surfaces or grain boundaries of the crystalline

regions is comparable to the number of those that are located in the crystalline lattice

itself). Smaller nanoparticles have higher surface to volume ratio. The second one is that

the de Broglie wavelength of electrons and/or holes becomes comparable to the crystallite

size of the nano-sized semiconductors. Under such conditions, the charge carriers can be

determined by the classical particle-in-a-box quantum model, where the band gap is

inversely proportional to the dimensions of the nanocrystals. The band gap increases if

semiconductor particles become smaller than the Bohr radius of the exciton, and a

transition from a continuous distribution of energy levels to discrete energy levels is

caused. In this regime, the clusters still possess short-range structures that essentially

mimic the bulk semiconductors from which they are derived.

Currently, the development of general techniques for the fabrication of high-quality

nano-sized semiconductor has been a major goal of material chemists. The following

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subsections will provide a review and highlight recent progress of these synthetic

methods.

1.2 Synthetic methods

The preparation of nearly monodisperse nanoparticles is essential to distinguish the

truly novel properties inherent to the nano-structure from those associated with structural

heterogeneity or polydispersity. Samples with standard deviations ca.5% in diameter are

referred to as monodisperse in this thesis. Many synthetic methods for the preparation of

monodispersed material have been reported, and the following subsection will review

several of these approaches.

1.2.1 Arrested precipitation

Perhaps the simplest method for preparing small particles is arrested precipitation

from a solvent as a colloid. In turn, the simplest manifestation of this approach involves

the solvent itself acting as the surface stabilizer of the small clusters, leading to so-called

organosols. For example, stable colloids of TiO2 can be generated by the hydrolysis of

TiCl4 or equivalent alkoxides in water with the surface hydroxyl groups on the TiO2

clusters acting as the colloid stabilizer.11, 12

In addition, the unique luminescent properties

of TiO2 prepared by this route have been reported.13

Other types of metal oxide

nanoparticles such as ZnO14

and SiO215

have also been prepared by similar techniques of

hydrolysis of the corresponding metal salts or metal alkoxides in alcohol or aqueous

solvents. The challenge of this method is to obtain a solid material that is dense,

inorganic and amorphous from a room-temperature liquid (not a melt), which is generally

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a homogeneous mixture of organic compounds. A similar approach has been explored to

synthesize piezoelectric lead zirconate titanate (PZT)16

and ferroelectric barium titanate

nanoparticles.17

As for the synthesis of metal chalcogenide nanoclusters, the addition of organic

molecular capping agents is an easy technique to control the growth of nanoparticles.

These agents, typically anionic, are added to a semiconductor precipitation reaction. They

intercept the growing clusters and prevent further growth by covalently binding to the

cluster surface. Thiolate are the most commonly used capping agent since SH has a high

affinity with most of the metal ions. In most cases, the metal salt is mixed with the

capping agent and then the chalcogenide source is introduced either by bubbling

hydrogen chalcogenide or adding the corresponding solution of chalcogenide salt into the

mixture. CdS, CdSe, CdTe, ZnS and Ag2S have been synthesized by using different types

of thiols.18-24

Other capping agents, such as long chain arachidic acid, have been used to

provide a suitable organic matrix for the epitaxial growth of CdS nanoparticles.25

1.2.2 Microemulsion

Microemulsion is thermodynamically stable, isotropic dispersion of oil and water

with a thin film of surfactant molecules adsorbed at the water and oil interface. By

varying the composition of suitable components and the HLB (hydrophilic-lipophilic

balance)value of the surfactants, one can obtain oil-swollen micelles dispersed in water

(known as oil-in-water microemulsion), or water-swollen micelles dispersed in oil

(known as water-in-oil microemulsion). The latter is also called an inverse

microemulsion. Due to the small dispersion sizes, usually 5 to 20 nm in diameter,

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microemulsions are transparent or translucent. In contrast to the colloidal approach, the

micellar reagent acts as a physical boundary rather than a true surface capping agent.

Normal vesicles using dihexadecyl phosphate, cetyltrimethylammonium chloride

(CTAB) or dioctadecyldimethylammonium chloride (DODAC) can be produced in water

with diameters of the order of 150 to 300 nm. Dissolution of metal ions in these vesicles

followed by a precipitation with H2S or Sodium Sulfide can lead to semiconductors of up

to 5 nm inside the micelles.26

Reverse micelles using bis(2-ethylhexyl) sulfosuccinate

salts (AOT) allow for the formation of small water pools (< 10 nm and the size of water

pools is dependent on the water/AOT ratio) in nonpolar solvents, such as hexane,

cyclohexane or 2,2,4,trimethyloctane. Then the metal ions can be encapsulated into the

pools followed by the chalcogenide treatment. This process can generate semiconductor

nanoparticles with a relatively monodisperse distribution. In most cases, the pools formed

by this technique result in spherical nanoparticles. However, it is found that the shape of

the pools can be controlled by incorporating some other additives, which can produce

different shapes of nanoparticles.27

CdSe and ZnSe nanorod,28

triangular CdS

nanoparticles29

have been reported by using this method. In addition, many ultrafine

powders of metal oxides such as ZnO, PbO, CdO, perovskite-type mixed oxides and

magnetic cobalt ferrite can also be derived from such methods.

Nonionic surfactants are also excellent templates to synthesize semiconductor

nanoparticles. With Igepal CO 520 as the surfactant, monodisperse silica-CdS

nanocomposite spheres can be prepared and then etched with strong acid. The etched

silica particles have geometrically tailorable voids and can be used in catalyst support

media.30-31

Stupp first reported the growth of stable CdS-organic superlattice obtained by

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a liquid-crystalline phase formed from nonionic amphiphiles oligoethylenexoide oleyl

ether.32

The hydrophilic parts can conjugate with the cadmium ions, while the

hydrophobic group can self-assemble into a hexagonal mesophase, which plays the major

role in the formation of the superlattice morphology.

1.2.3 Passivation in polymer matrix

As compared to inorganic materials, polymers have the advantages of superior

mechanical properties and excellent film processability. Nanocomposites consisting of

inorganic nanoparticles and organic polymers exhibit a host of mechanical, electrical,

optical and magnetic properties, which are far superior as compared with those of the

individual components.33

These desirable properties are derived from a complex interplay

between the building blocks and the interfaces separating the building blocks.

Two main approaches have been used to produce these nanocomposites. The first

approach is an in situ precipitation method. In a typical experiment, the matrix material

and metal ions are mixed in solution and then exposed to the counterion (S2-, Se

2-) in the

form of gas or as ions dissolved in solution. The composite can be cast as a film before or

after exposure to the counterion.33

Take the PbS nanoparticles for example, if

poly(vinyl pyrrolidone) (PVP) is the matrix material for PbS nanoparticles, a solution of

PVP and Pb2+

can be prepared in water. This solution can then be exposed to H2S gas in

order to form the PbS and cast as a film. In this particular reaction, the crystalline

semiconductor forms extremely quickly and yields a wine-red solution. An alternative in

situ polymerization method is also developed; a well-defined nano-sized semiconductor

is first prepared using the monomer as a capping agent, then the nanoparticles undergo

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the homopolymerization or copolymerization process with other monomers to get

polymer nanocomposites.34

The second one is an ex situ method. In this method, semiconductor clusters are first

prepared by using capping agents as the stabilizers, and then the nanoparticles are

dissolved in a solvent along with a soluble polymer. This mixed solution can be cast to

produce a polymer film doped with the semiconductor cluster. This simple approach

provides some new examples of interesting photoconductive or photovoltaic

nanocomposites. For instance, CdS/PVK35

and CuS, ZnS, CdS in polyacrylonitrile36

have

been reported.

1.2.4 Encapsulation in the dendrimer template

Dendrimers are a new class of three-dimensional, man-made macromolecules

produced by an unusual synthetic route, which incorporates branching groups to create a

unique novel architecture.37 Exceptional features of the dendritic architecture, include a

high degree of structural symmetry, a density gradient displaying an intra-molecular

minimum value and a well-defined number of terminal groups, which are chemically

different from the interior. The combination of these features creates an environment

within the dendrimer molecule that facilitates the trapping of guest species. Recently,

dendritic polymers have been used as soluble templates/unimolecular reactors from

which nano-clusters of inorganic compounds or elements can be synthesized. The basic

concept involves using dendrimers as hosts to pre-organize small molecules or metal ions,

followed by simple in situ reaction, which will immobilize and stabilize domains of

atomic or molecular guest components (inorganic compounds as well as elemental

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metals). Additionally, the dendrimer peripheral groups can be used to control the

solubility.

CdS/dendrimer nanocomposites are first reported by using PAMAM as the

stabilizing host. The synthetic conditions, such as the dendrimer type, solvent, pH value

can affect the optical properties of CdS nanoparticles.38

In addition, carboxy-terminated

PAMAM dendrimers have been utilized for the synthesis and stabilization of

ferrimagnetic iron oxide nanoparticles dendrimer.39

In the above mentioned efforts to

prepare dendrimer nanocomposites, primarily interdendrimer composites are obtained;

such materials have been shown to be agglomerates in which multiple dendrimers

stabilize relatively large CdS nanoparticles. Intradendrimer CdS nanocomposites have

been synthesized, and they are sequestered in individual dendrimers, which can prevent

the nanoparticles from aggregations. Meanwhile, different sizes of CdS nanoparticles are

prepared by changing the generation of the dendrimers.40

The approaches described here for preparing dendrimer-encapsulated nanoparticles

take advantage of each of the unique aspects of the dendrimer structure: the chemistry of

the terminal groups, the generation-dependent size, the three-dimensional structure, the

low-density core and endoreceptors present within the dendrimer interior. Due to the

dramatic developments in the design and characterization of dendrimers themselves,

nanocomposite of dendrimers will attract increasing attention for their potential

applications in biosensor, catalyst, self-assembly, engineering etc.

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1.2.5 Organometallic method

A powerful method for the preparation of semiconductor nanocrystallites has been

described by Bawendi et al.41

Solutions of dimethylcadmium ((CH3)2Cd) and tri-n-

octylphosphine selenide (TOPSe) are injected under Ar atmosphere into hot tri-n-

octylphosphine oxide (TOPO) in the temperature range 120-300 C. This results in

TOPO capped nanocrystallites of CdSe. The combination of tri-n-octylphosphine and tri-

n-octylphosphine oxide (TOP/TOPO) allows for slow and steady growth conditions

above 280C. This method has advantages over other synthetic methods; including the

near monodispersity and the fact that gram scale amount of material can be produced. In

addition, the nano-sized semiconductors obtained by this method have high luminescence

quantum yields and narrow luminescence width. CdS, CdSe, CdTe,41

CdSe/CdS42

and

CdSe/ZnS43

chalcogenide nanoparticles have been synthesized and well studied. Recently,

transition oxide nanoparticles have been prepared by a nonhydrolytic solution based

reaction at elevated temperatures.44

In addition to using TOPO as the coordinating

solvent, carboxylic acid45

has also been utilized to prepare high-quality and high-

luminescent III-V semiconductor nanocrystals. In addition, the shape of the colloidal

semiconductor nanocrystals can be tuned by the growth of nanoparticles in a mixture of

aliphatic phosphonic acid and TOPO. Nanorods, arrow-, teardrop-, tetrapod- and

branched tetrapod-shaped CdSe nanocrystals have been achieved.46, 47

The major limitation of this organometallic method is the use of hazardous

compounds such as dimethylcadmium especially at high temperatures. One approach to

overcome this problem is to use more environmental friendly chemicals, such as

cadmium oxide,48

cadmium carbonate, cadmium stearate and cadmium acetate.49

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It is found that high-quality nanocrystals can be produced from alternative route. For

example, Yu et al.50

produced high-quality CdS and other II-VI semiconductor

nanocrystals in noncoordinating solvents such as octadecene (ODE). The introduction of

noncoordinating solvents further enhances the possibility of implementing green-

chemical principles into the design of synthetic schemes for colloidal nanocrystals.

1.2.6 Single molecular precursor thermolysis method

Another approach for overcoming the above problems involves the use of the single

molecule precursor, a single compound containing all elements required within the

nanocrystallite, such as alkyldiseleno- or alkyldithio-carbamate complexes.51, 52

The

fabrication of semiconductor nanocrystallites from single molecule precursors is a one

step process, typically carries out at temperatures in the range of 200-250 C, using tri-n-

octylphosphine oxide (TOPO) as the coordinating solvent. CdS,51, 53

CdSe,51, 53

ZnS53

and

CuSe54

nanoparticles have been prepared by using the dithio- and diseleno-carbamate

complexes. A similar approach in which non-airsensitive lead(II) alkyldithio-

carbamates55

are thermally decomposed under controlled conditions, leads to the

synthesis of cubic phase PbS nanocrystallites. Self-capped CdS nanoparticles have also

been synthesized by heating a novel cadmium dithiocarbamate complex,

Cd(S2CNMe(C18H37))2, at high temperature in a nitrogen atmosphere.56

Cheon et al. utilized long alkyl amines instead of TOPO as the capping agent to

prepare the II-VI ZnSe and ZnTe nanocrystals.57, 58

The single-molecular precursor is an

air-stable [Zn(SePh)2][TMEDA] or [Zn(TePh)2][TMEDA] complex, which effectively

produces different sizes of ZnSe or ZnTe QDs depending on the growth temperatures.

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The shape of the nanocrystals can be tuned by varying the composition of the capping

surfactants. In addition, they provide a novel method to control the CdS nanorod

architecture by using a monosurfactant hexadecylamine (HDA) system.59

Different

shapes of CdS nanocrystals can be obtained by simple adjustment of the reaction

temperature or the precursor concentration. It is found that different phases of

nanoparticles can be obtained at different temperature stages, which can control the

formation of the nanocrystals. Capping agents with bifunctional groups such as

ethylendiamine are also used to prepare CdS nanowires with quantum confined diameters

and lengths from 150 nm to 250 nm from a cluster precursor Cd2(S2Et2)4.53

Nucleophilic

attacked by the diamine groups at the thione can remove the capping groups.60

Alivisatos first reported a new nonhydrolytic single precursor approach to synthesize

transition metal oxide nanocrystals by thermolysis of metal cuperferron complexes.61

Compared with those transition metal oxides nanoparticles synthesized from the

traditional wet colloid method, metal oxide nanoparticles prepared by this method have

no hydroxyl groups and less surface defects, which have significant applications in

catalysis, magnetic data storage and so on. In addition, nano-sized Cd3P2 semiconductors

have been prepared by using a novel single source precursor.62

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1.3 Potential applications

1.3.1 Optoelectronics devices

The large surface area of a nanocrystal and the surrounding medium, such as the

capping agent, can have a profound effect on the properties of the particle. Defects within

the particle act as electron/hole traps which lead to nonlinear optical effects. For example,

the size-dependence of the third-order nonlinear absorption of CdS nanocrystals is

studied by using nanosecond Z-scan method.63

Polymer CdS nanocomposite has been

prepared and the nonlinear optical properties have been studied. It is found that the

nonlinear absorption is greatly enhanced as the particle size decreases, which is predicted

by the quantum size effect.64

A design for a solid-state laser, based on the luminescence

properties of metal chalcogenide quantum dots (CdSe, CdTe, ZnSe, ZnTe), in a host

material such as poly(methyl methacrylate) has been described.65

The wavelength of the

emitted light is determined by the size of the nanocrystallites chosen.

Bulk ZnS is an important inorganic material for light emitting applications. ZnS

doped with various transition metal ions such as manganese is an efficient light emitting

material. If such dopants are inserted in the nano-sized ZnS matrix, they can exhibit

interesting magnetic and optical properties. The photophysical properties of ZnS:Mn66

and ZnS:Cu67

have been reported. These works suggest that the fabrication of the

luminescent ZnS devices, with distinct optical properties, can be achieved by using ZnS

nanocrystallites possessing spatially localized metal ions.

Solar cells based on large band gap semiconductors usually TiO2 sensitized to

visible light with dyes have recently emerged as promising inexpensive alternatives to

conventional photovoltaic solar cells.68

The semiconductor is deposited onto a transparent

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conductive oxide (TCO) electrode, through which the cell is illuminated. The TiO2 pores

are filled with a redox electrolyte (I-/I3

-) that acts as a conductor and electrically connects

to a platinum electrode. Upon absorption of light, the dye adsorbed on the surface of the

semiconductor injects an electron into the conduction band. The oxidized dye molecule is

then regenerated by accepting an electron from a reducing agent present in solution. The

cell open-circuit photovoltage is limited to the difference between the quasi-Fermi level

for electrons in the semiconductor and the redox potential in the electrolyte solution.

Such systems can reach solar to electric conversion efficiencies of about 10%, but are

still not produced on a large scale mainly because of technical problems such as sealing.

In 1994, a paper first reported the development of light-emitting diodes using a

hybrid organic/inorganic electroluminescent device.69

The light emission originates from

the recombination of holes injected into a layer of semiconducting p-paraphenylene

vinylene (PPV) with electrons injected into a multilayer film of the CdS nanocrystals.

The close matching of the energy level of the emitting layer of nanocrystals with the

work function of the metal contact can lead to an operating voltage of only 4V while the

operating voltage for PPV alone is 7V. At low voltages, the emission from the CdSe layer

occurs. The color of this emission varies from red to yellow due to the size of the

nanocrystals. At higher voltages the green emission from the polymer layer predominates.

A bilayer light-emitting diod made with organically capped CdSe/CdS core/shell

nanocrystals and PPV has been characterized by the same group subsequently. The

epitaxial growth of the shell on the surface of CdSe can improve the quantum yields, the

photo-oxidative stability and the electronic accessibility. These devices show significant

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improvements in quantum efficiency and lifetime as compared with the first CdSe

device.70

1.3.2 Biological labeling

There has been some exciting work in which biological molecules are attached or

adsorbed to inorganic nanoparticles. Two different groups have simultaneously reported

the use of CdSe nanoparticles with high photoluminescence as fluorescent dyes for

biological molecules.71, 72

In both cases, highly luminescent CdSe nanocrystals are

surface-passivated with a thin layer of a larger bandgap material and then a conjugatable

group is attached to the outer surface. One key in the synthesis is the production of water-

soluble nanoparticles. Subsequently, the biological molecules are attached to the surface

of nanoparticles and the localization of the molecules can then be imaged. The color of

the fluorescent dye is a function of the nanoparticle size; thus different colors of dyes can

be attached to a receptor of interest just by tuning the size of the nanoparticles. Adapting

the complementary base pairing as the means to organize the nanoparticles, it is now

possible for researchers to develop new diagnostic techniques in DNA quantitation and

detection of diseases.

For these biological applications, the alloyed quantum dots also can be made water-

soluble and biocompatible by using a similar surface-modification and cross-linking

procedures as reported for those binary quantum dots. If solubilized with mercaptoacetic

acid and coated with a biopolymer or a synthetic polymer, the ternary dots are found to

exhibit excellent optical properties such as a narrow spectral width, excellent

photostability and high quantum yields, compared with those of high-quality binary

nanocrystals reported before.

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1.3.3 Catalysis

The large surface-to-volume ratio, along with the ability to tune the band gap, by

varying the particle size, indicate that monodisperse semiconductors can be used as

sensitizers and catalysts in photochemical reactions. The redox levels of the conduction

and valence bands are especially sensitive to size quantization effects. Charge carriers

generated after the light absorption migrate to the surface of the particles where they can

reduce or oxidize surface-bound chemical species. One of the most studied oxidation

reactions on semiconductor particles is the photoinduced dehydrogeneration of alcohol to

aldehydes or ketones.73, 74 Photogeneration of hydrogen is carried out by irradiating the

micellar solution containing ZnS, CdS and the co-precipitated nanoparticles.75

Currently,

other catalytic applications of nano-sized semiconductor in condensation, isomerization,

substitution and polymerization are also under intensive investigation.

Although a wide range of synthetic methods is available for producing

semiconductor nanocrystals, it seems that there is still a major problem associated with

the reproducible preparation of robust materials of the kind that would be needed for

technological applications. All of the above-mentioned methods have their own

advantages. Colloidal method uses lower temperature and is easier to be manipulated, but

it has problems with instability and more surface defects. Microemulsion methods can

produce moderately monodisperse nanoparticles, but the product yield is very low. The

interplay between polymer chemistry and quantum dots synthesis will undoubtedly

become more important in the future. However, it is still a challenge to get monodisperse

nanocomposites in a polymer matrix. The organometallic approach can produce well-

defined nanocrystals with few defects and a high monodispersity of the size distribution.

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One of the drawbacks of this method is the use of toxic and volatile compounds during

the experimental process.

The use of single molecular precursor is a powerful method for the synthesis, shape

control and engineering of nano-sized semiconductors with superior optoelectronic and

physiochemical properties. The development and knowledge of this field are still limited

now. In addition, in all of the reports on preparing nanoparticles from single source

precursor method, high temperatures and the rapid injection are two indispensable

conditions for the experiment. However, few studies report the synthesis of nanoparticles

from single source precursor at moderate conditions, even at room temperature, which are

much more desirable for future industrial applications.

1.4 Objectives of the project

Our project mainly focuses on the synthesis, characterization and formation mechanism

of several different types of semiconductor nanocrystals at elevated temperatures.

The main objectives in this thesis are listed as follows:

1.4.1 Synthesis of semiconducting metal sulfide nanocrystals of CdS, ZnS and

ZnxCd1-xS with various sizes and shapes by the thermal reaction of metal salts and

sulfur in long-chain amine

In this part, a green chemical route to high-quality CdS, ZnS and ZnxCd1-xS

nanocrystals is reported. It is a one-pot method using stable, commonly available

precursors (metal salts such as metal stearate, acetate, chloride, and sulfate etc. as the

cadmium source and elemental sulfur as the sulfide source). The coordinating solvents

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used in this study include a series of convenient high-boiling long-chain amines such as

hexadecylamine, oleylamine, etc. Our research focuses mainly on the formation of cubic

zinc-blende CdS and ZnxCd1-xS nanocrystals at elevated temperature in this synthetic

approach, where rapid nucleation and instant termination of the crystal growth process

are observed. This nucleation and growth feature makes the reproducible and controllable

synthesis of nanocrystals with specific emission wavelength feasible. For the synthesis of

CdS, ZnS and ZnxCd1-xS nanocrystals, the metal salt is dissolved in amine to form a

metal-amine complex at an appropriate high temperature. To the resulting solution,

elemental sulfur dissolved in amine is swiftly injected. The reaction mixture is heated at a

certain temperature for some period of time to generate the desired metal sulfide

nanocrystals.

1.4.2 Synthesis of CdS, ZnS and ZnxCd1-xS composites using bis(diethyldithio-

carbamate)-cadmium(II)/zinc(II) compounds as single-source precursors

CdS, ZnS and ZnxCd1-xS alloyed nanocrystals are obtained by the thermolysis

method. The nanocrystal composition is expected to remain constant during the whole

reaction process, yielding a homogeneous alloy that is uniform from the particle core to

the surface. The alloyed nanocrystals will give a blue shift in the emission and absorption

edge as compared to the binary CdS nanocrystals. The observed continuous shift of the

absorption and PL spectra of the obtained nanocrystals with different compositions is the

most direct evidence for the alloying process.

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Y.; Risbud, S. H. J. Phys. Chem. B 2000,104,11598.

(34) Hirai, T.; Watanabe, T.; Komasawa, I.J. Phys. Chem. B 2000, 104,8962.

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Chem. Mater. 2001, 13, 2201.

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(41) Murray, C. B.; Norris, D. J.; Bawendi, M. G.J. Am. Chem. Soc. 1993, 115, 8706.

(42) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P.J. Am. Chem.

Soc. 1997, 119, 7019.

(43) Hines, M. A.; Guyot-Sionnest, P.J. Phys. Chem. 1996, 100, 468.

(44) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V.L. J. Am. Chem.

Soc. 1999, 121,1613.

(45) Battaglia, D.; Peng, X. G.Nano Lett.2002, 2, 1027.(46) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc.2000, 122, 12700.(47) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.;

Alivisatos, A. P.Nature 2000, 404, 59.

(48) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc.2001, 123, 183.

(49) Qu, L. H.; Peng, Z. A.; Peng, X. G.Nano Lett. 2001, 1, 333.

(50) Yu, M. W.; Peng, X. G.Angew. Chem. Int. Ed. 2002, 41, 2368.

(51) Trindade, T.; OBrien, P.; Zhang, X. Chem. Mater.1997, 9, 523.

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(52) Malik, M. A.; Revaprasadu, N.; OBrien, P. Chem. Mater.2001, 13, 913.

(53) Ludolph, B.; Malik, M. A.; OBrien, P.; Revaprasadu, N.Chem. Commun.1998,1849.

(54) Malik, M. A.; OBrien, P.; Revaprasadu, N.Adv. Mater.1999, 11, 1441.

(55) Trindade, T.; OBrien, P.; Zhang, X.; Motevalli, M.J. Mater. Chem. 1997, 7, 1011.

(56) Lazell, M.; OBrien, P.Chem. Commun. 1999, 2041.(57) Jun, Y. W.; Koo, J. E.; Cheon, J. Chem. Commun.2000, 1243.

(58) Jun, Y. W.; Choi, C. S.; Cheon, J. Chem. Commun.2001, 101.

(59) Jun, Y. W.; Lee, S. M.; Kang, N. J.; Cheon, J.J. Am. Chem. Soc. 2001,123,5150.

(60) Yan, P.; Xie, Y.; Qian, Y. T.; Liu, X. M. Chem. Commun. 1999, 1293.

(61) Rockenberger, J.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 1999,121,

11595.

(62) Green, M.; OBrien, P. Adv. Mater.1998, 10, 527.(63) He, J.; Ji, W.; Ma, G. H.; Tang, S. H.; Elim, H. I.; Sun, W. X.; Zhang, Z. H.;

Chin, W. S.J. Appl. Phys.2004, 95, 6381.

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(65) Sooklal, K.; Cullum, B. S.; Angel, S. M.; Murphy, C. J.J. Phys. Chem. 1996, 100,

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http://adsabs.harvard.edu/cgi-bin/author_form?author=He,+J&fullauthor=He,%20J.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Ji,+W&fullauthor=Ji,%20W.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Ma,+G&fullauthor=Ma,%20G.%20H.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Tang,+S&fullauthor=Tang,%20S.%20H.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Elim,+H&fullauthor=Elim,%20H.%20I.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Sun,+W&fullauthor=Sun,%20W.%20X.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Zhang,+Z&fullauthor=Zhang,%20Z.%20H.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Chin,+W&fullauthor=Chin,%20W.%20S.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Chin,+W&fullauthor=Chin,%20W.%20S.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Zhang,+Z&fullauthor=Zhang,%20Z.%20H.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Sun,+W&fullauthor=Sun,%20W.%20X.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Elim,+H&fullauthor=Elim,%20H.%20I.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Tang,+S&fullauthor=Tang,%20S.%20H.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Ma,+G&fullauthor=Ma,%20G.%20H.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=Ji,+W&fullauthor=Ji,%20W.&charset=ISO-8859-1&db_key=PHYhttp://adsabs.harvard.edu/cgi-bin/author_form?author=He,+J&fullauthor=He,%20J.&charset=ISO-8859-1&db_key=PHY

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(75) Hirai, T.; Shiojiri, S.; Komasawa, I.J. Chem. Eng. Jpn.1994, 27, 590.

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Chapter 2

Experimental section

2.1 Chemical reagents

Cadmium acetate (98%), cadmium sulfate (99%), cadmium chloride (99.99%), zinc

stearate (90%), 1-hexadecylamine (90%), dodecylamine (98%), sulfur powder (99.98%),

zinc oxide (99%), cadmium oxide (99.99%), cadmium hydroxide (98%), diethylamine

(99.5%) and carbon disulfide (99.9%) were purchased from Aldrich.

Cadmium stearate (90%) was purchased from Sterm Chemicals.

Methanol (AR), toluene (AR and HPLC), chloroform (HPLC) and acetone (AR and

HPLC) were purchased from Merck.

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2.2 General synthetic methods

2.2.1 Synthesis of CdS nanocrystals by the thermal reaction of metal salts and sulfur

in long-chain amine

Cd(ST)2 was dissolved in

hexadecylamine

The mixture was heated to 295oC under argon

flow

Sulfur in dodecylamine was swiftly injected into

this solution

The reaction mixture was then kept at 280oC

for the growth/annealing of the nanocrystals

A mixed solvent of methanol and acetone was

used to precipitate the resulting nanocrystalsfrom the solution, which were then isolated by

centrifugation and decantation

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2.2.2 Synthesis of ZnS nanocrystals by the thermal reaction of metal salts and sulfur

in long-chain amine

Zn(ST)2 was dissolved in

hexadecylamine


flow


this solution




used to precipitate the resulting nanocrystals

from the solution, which were then isolated bycentrifugation and decantation

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2.2.3 Synthesis of ZnxCd1-xS nanocrystals by the thermal reaction of metal salts and

sulfur in long-chain amine

Cd(ST)2 and Zn(ST)2 were mixed and

dissolved in hexadecylamine


flow


this solution and the injected amount of sulfur

was equal to the total molar amount of Zn andCd precursors.




used to precipitate the resulting nanocrystalsfrom the solution, which were then isolated by

centrifugation and decantation

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2.2.4 Synthesis of CdS nanocrystals using bis(diethyldithiocarbamate)-cadmium(II)

compounds as single-source precursor

Cd(S2CNEt2)2 was fully dissolved in

tri-n-octylphosphine

This solution was then injected into a hot

(300 C) solution of tri-n-octylphosphine oxide

and hexadecylamine

The reaction mixture was then kept at this

temperature for 30 minutes to produceCdS nanocrystals




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2.2.5 Synthesis of ZnS nanocrystals using bis(diethyldithiocarbamate)-zinc(II)

compounds as single-source precursor

Zn(S2CNEt2)2 was fully dissolved in



(300 C) solution of tri-n-octylphosphine oxide

and hexadecylamine

The reaction mixture was then kept at this

temperature for 30 minutes to produceZnS nanocrystals




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2.2.6 Synthesis of ZnxCd1-xS nanocrystals using bis(diethyldithiocarbamate)-

cadmium(II)/zinc(II) compounds as single-source precursors

Cd(S2CNEt2)2 and Zn(S2CNEt2)2were both dissolved in



(300 C) solution of tri-n-octylphosphine oxideand hexadecylamine

The reaction mixture was then kept at thistemperature for 30 minutes to produce

ZnxCd1-xS nanocrystals




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2.3 General characterization methods

2.3.1 Spectroscopic techniques

(1) Photoluminescence spectroscopy Shimadzu RF-5301 PC spectrometer

(2) Ultraviolet spectroscopy Shimadzu UV-1601 spectrophotometer

(3) X-ray powder diffraction Siemens D5005 X-ray powder diffractometer operate at

40 kV and 40 mA

(4) Inductively coupled plasma spectroscopy Thermal Jarrell As Duo Iris Inductively

coupled-optical emission spectrometer

2.3.2 Microscopic techniques

(1) Transmission electron microscopy JEOL JEM3010 transmission microscope

(operate at an accelerating voltage of 300 kV)

2.4 Principles of characterization methods

2.4.1 Principles of photoluminescence spectroscopy (PL)

Upon excitation, an electron will excite from valence band to conduction band, and

a hole will form in the valence band accordingly, thus electron-hole pairs are created. The

electron can subsequently recombine with the hole, either radiatively or nonradiatively.

Radiative recombination can give rise to a relatively sharp emission band centered at

approximately the band gap energy (exciton recombination band) and/or a relatively

broad emission band centered at lower energies when deep traps are involved in the

recombination process. For semiconductor nanoparticles, there is an increase in the band

edge energy with decreasing particle size due to the quantum size effect. Therefore,

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different emissions wavelength can be obtained by changing the particles size. This

photoluminescence is very sensitive to the presence of absorbates at the nanoparticles

surface. If the surface is well passivated, the photoluminescence wavelength maximum

will be close to the absorbance edge with a little red-shift due to stokes shift. More

surface traps will cause red-shift luminescence or even surface defects emission, and low

quantum yield. In addition, the surface ion can also affect the luminescent properties of

nanomaterials. In the case of nanocrystalline CdS particles, sulfur-rich and cadmium rich

nanoparticles are prepared respectively and optical properties are studied. It is found that

the sulfur can quench the luminescence while cadmium can improve the luminescence

efficiency.

excitation photoluminescence

hole traps

electron traps

CBe

-

+

VB

Figure 2.1Schematic generation of photoluminescence emission of semiconductor upon

excitation. CB: conduction band, VB: valence band, hole (h+

) and electron (e-

).

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2.4.2 Principles of ultraviolet and visible spectroscopy (UV-vis)

The wavelengths of UV-vis radiation range from 800 nm to 200 nm and they are

involved in excitation of valence electrons that commonly known as electronic excitation.

Electronic excitation refers to moving electrons in ground state (lowest energy state) to

excited state (higher energy state). The energy involved in the excitation is expressed as

E1 Eo = h

where E1 is the excited state and Eo is the ground state. There are basically three types of

electrons in organic compounds that are important to UV-vis.1

The first type of electrons

is found in saturated bond ( bond) and the amount of energy need to excite electrons in

bond exceeds what the UV photons possess. Hence, this type of electrons is not useful

in UV-vis. The second type of electrons is found in unsaturated bond ( bond). Electrons

in bond can be easily excited and absorbed in the UV. The last type of electrons is

electrons that are not found in any bond (n electrons). Like electrons, n electrons can

also be excited by UV radiation and most compounds that contain n electrons absorb UV

radiation. On extrapolating to molecules, UV-vis involves the excitation of electron from

a bonding orbital to an anti-bonding orbital. Hence, electrons in bond are excited to

antibonding orbital. Electrons in bonds are excited to antibonding orbital and n

electrons are excited to either antibonding orbital or antibonding orbital.

In this study that involves metal nanoparticles, UV-vis spectroscopy involves more

than just simple transition from bonding orbital to antibonding orbital. It is produced by a

collective excitation of electrons in the particles. This absorption band is often known as

surface plasmon band.1

Basically, a dipole excitation across the particle sphere is

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produced when the electrons move under the influence of electric field vector of the

incoming light. This results in the production of the positive polarization charge that acts

as a restoring force and makes the electrons to oscillate. Only the electron density on the

surface of the particles can oscillate whereas the electron density in the interior of the

particles remains constant.

UV-vis has many applications in providing the electronic structure for aromatic and

transition metal compounds.1

Compared to other characterization methods, the operation

is fairly easy, sensitive and inexpensive. Quartz or glass can be used as the window

material over the entire range thus allowing greater flexibility. On the other hand, surface

plasmon band allows one to calculate the size of the particles. Nonetheless, most of the

UV-vis absorption bands are generally broad and featureless.1This would have limited its

capability in the determination of the molecular structures. Although UV-vis is capable to

determine the plasmon band for some transition metals, there are some important metal

compounds notably surface-protected copper nanoparticles2

that have no feature in UV-

vis at all.

2.4.3 Principles of x-ray powder diffraction (XRD)

XRD is used to obtain information about the structure, composition as well as state

of polycrystalline materials.3

A few typical applications include identification of

unknown based on the crystalline peaks, variable temperature studies, precise

measurement of lattice constants and residual strains as well as refinement of atomic

coordinates.3

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In order to understand how the interaction of X-ray with crystals yields a diffraction

pattern and ultimately (in most cases) a three-dimensional crystal structure, it is necessary

to know the basic diffraction physics. Imagine the ions or molecules are arranged in well-

defined position to make up the crystal as shown in Figure 2.2. We can clearly observe

that the ions form planes in three dimensions. Hence, when a monochromatic x-ray beam

falls on the crystal, the beam will be reflected by each of the crystal plane. Each reflected

beam will interact with each other and if they are not in phase, they will destroy each

other and no beam will emerge. Conversely, when they are in phase, reinforced beams

will emerge to produce a diffraction pattern.

Figure 2.2 Crystal structure of sodium chloride: (+) Na ions and (-) Cl ions.

The mathematical expression for this kind of interaction is given by Braggs law:

n= 2dsin

where n is an integer, is the wavelength of the radiation, d is the perpendicular spacing

between adjacent planes in the crystal lattice and is the angle of incident and

reflection of the X-ray beam. This equation states that constructive interference can

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only occur when n = 2dsin as shown in Figure 2.3. Hence, XRD can be used to

measure d that is the basis of crystallography, an important field of science.1

A

B

C

O

d

Top plane

Second plane

Incident

radiation

Light

defracted

in phase

Figure 2.3 Reflection of X-ray from crystal lattice planes.

Earlier on, we have been mentioning about the diffraction of X-rays from different

planes, it is necessary to designate these planes in a consistent manner. This is done by

assigning Miller indices to these lattice planes. Miller indices are represented by (hkl)

family of planes. Each reflection of an X-ray from a crystal is assigned a unique hkl

value.

It is interesting to note that the diffraction peaks are usually broadened when the

crystallite is small. Scherrer first treated this particle size broadening and the Scherrer

equation4

is given by

B(2) =

cos

94.0

L

where B(2 ) is the full width at half maximum (FWHM) in radians, L is the edge

dimension of the crystals and is the wavelength of the radiation. Hence, we can easily

obtain the crystallite size from this equation once the FWHM of the most intense peak is

known.

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Although XRD is proved to be very useful in characterizing the crystal structure, it

has its own limitations. For the study of materials with preferred orientation, film

methods are employed to complement XRD. In addition, more detailed characterization

may require specialty instruments such as thin-film camera, texture goniometer or double

crystal diffractometer.

2.4.4 Principles of transmission electron microscopy (TEM)

The transmission electron microscopy was first developed in the 1930s after

wavelength was apparent to play an important role on theoretical resolution. Green light

that is used for light microscopy has a wavelength of 0.5 m and therefore a theoretical

resolution of about 0.2 m. As we move towards the shorter wavelength of the

electromagnetic spectrum, it will pass blue and violet into the range of ultraviolet (< 0.4

m). This means that an ultraviolet microscopy should have a theoretical resolution of

0.05 m and could be used in TEM. However, ultraviolet light can absorb by glass, and

the lenses have to be made from quartz. This certainly makes the microscopy extremely

expensive and fragile for only a moderate increase in resolution. X-ray microscopy is

better in resolution but x-ray cannot be easily refracted to form an image. Electron waves

offer the best alternative. The electron being a charged particle can be easily refracted in

a magnetic field. Moreover, it can be accelerated by an electric potential. The stronger the

potential the faster the electrons will move, and as described by the de Broglie

relationship, the shorter the wavelength therefore the better resolution:

=m

h

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In a typical transmission electron microscopy, the electrons that are high in energy

are generated at the gun chamber. These electrons are produced by a tungsten filament, a

LaB6 crystal or a field emission gun. The generated beam of electrons is then collimated

by magnetic lenses and allowed to pass through the specimen under high vacuum.5

This

results in the diffraction pattern that contains transmitted beam and a number of

diffracted beams, which will then form images on a fluorescent screen below the

specimen. In this case, one can obtain the lattice spacing and symmetry information for

the structure under consideration.5

On the other hand, the transmitted beam or one of the diffracted beams can also

form magnetic image of the sample on the viewing screen. These bright- and dark-field

imaging modes can give information about the size and shape of the microstructural or

nanostructural constituents of the materials. One can obtain the high-resolution image

that contains information on the atomic structural of the materials if the transmitted beam

and one or more diffracted beams are to recombine.

It seems that TEM offers much help in the study of local structure, morphology and

chemistry of material in extremely high resolution.5

Nonetheless, TEM requires the

preparation of samples that could be time consuming. In addition, there are some

materials especially polymers that lose their crystallinity and mass upon interaction with

strong electron beam. Imaging resolution is limited to about 0.2 nm because of the great

difficulties in manipulating the magnetic field to get a higher resolution.

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2.4.5 Principles of inductively coupled plasma-atomic emission spectroscopy (ICP-

AES)

ICP-AES is commonly employed to determine the concentration of elements in

solution. The main advantage of ICP-AES is the ability to analyze many elements, either

simultaneously or in a rapid sequential manner. This depends a lot on the type of

instrument being used.

The principle of ICP-AES is based on the different emission energy that is specific

for different element. As shown in Figure 2.4, the liquid sample is first atomized by a

nebulizer into a stream of argon gas that carried the gaseous atoms into the plasma. The

plasma will then thermally excite the elements in the solution. Upon relaxing back to

their ground state, these excited elements will release photons that can be detected by

photomultiplier tubes. The number of photomultiplier tubes used depends on the type of

instrument. The instrument meant for simultaneous system utilizes photomultipliers tubes

positioned at predetermined wavelengths on a focal curve with one photomultiplier tube

for each element. On the other hand, the instrument for sequential system has only one

photomultiplier tube with a computer controlled grating that rotates to focus pre-

programmed regions of the spectrum on the exit slit. The intensity of the signal obtained

at the end corresponds to the concentration of the elements.

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Figure 2.4 Schematics of typical ICP instrument with scanning monochromator.

ICP-AES share many similar advantages as atomic absorption techniques such as

analysis of many types of samples and very good detection limit. The most important

advantage of ICP-AES is the ability to analyze many elements in one go. This certainly

helps to save a great deal of analysis time. Furthermore, ICP-AES also allows

quantitative analysis over a wide concentration range thus making the determination of

major and trace component possible without dilution. However, ICP-AES has it own

limitations such as spectral overlap and matrix effect. Spectral overlap is caused by the

direct overlap of the emission lines from the analyte and the interfering element. This can

be avoided by taking an alternate line only in a sequential instrument. Matrix effects, on

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the other hand, affect the sensitivity of this analysis method by changing the efficiency of

nebuliztion. Only though matching the matrix of the standard with that of the sample can

help to minimize some of these matrix effects.

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2.5 References

1. Robinson, J. W. Undergraduate Instrumental Analysis 5th Edition, Marcel Dekker,

Inc., New York, 1995.

2. Chen, W. S.; Sommers, J. M.J. Phys. Chem. B2001, 105, 8816.

3. Murphy, N. S.; Reidinger, F. A Guide to Materials Characterization and Chemical

Analysis 2nd

edition, VCH publishers, Inc., New York, 1996.

4. Bradley, J. S. Clusters and Colloids, VCH, Weinhein, 1994.

5. Macur, J. E.; Marti, J.; Lui, S. C. A Guide to Materials Characterization and

Chemical Analysis 2nd

edition, VCH publishers, Inc., New York, 1996.

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Chapter 3

Facile synthesis of high-quality CdS, ZnS and ZnxCd1-xS nanocrystals

using metal salts and elemental sulfur

3.1 Introduction

Semiconductor nanocrystals are of great interest for both fundamental research and

technical applications as a consequence of the large surface-to-volume ratio and the

three-dimensional quantum confinement of excitons.1-6

Among various semiconductor

materials, the binary cadmium chalcogenides have been the most frequently studied due

to their size-dependent photoluminescence (PL) tunable across the visible spectrum. Also,

the relevant ternary compounds ZnxCd1-xS are promising materials for high density

optical recording devices, blue or even ultraviolet laser diodes, and photovoltaic cells.7-8

The lack of adequate synthetic methods for producing the desired high-quality

nanocrystals is still a bottleneck in this field. Since the introduction of the high

temperature organometallic approach and various alternatives, the synthetic approaches

to CdSe nanocrystals have been well developed.9-12

Through these synthetic procedures,

CdSe nanocrystals are produced with a size distribution of 5-10% relative standard

deviation without any size selection and the room temperature quantum yields (QYs) of

30-80% for most of the visible spectral range. In comparison, the synthesis of CdS

nanocrystals is not as advanced. Conventionally, arrested precipitation from simple

inorganic ions in solutions has been utilized to prepare CdS.13-15

Recently, several groups

have already reported the synthesis of sulfide nanocrystals by thermolysis of single

source precursors.16-18

Qian and co-workers developed a solvothermal method to obtain

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many kinds of semiconductors including CdS.19-20

Although most of these developed

methods for synthesizing CdS are of widespread importance, there are still some

limitations to their utilities. For example, the resulting quantum yields are not very high,

or the particle size distributions are relatively broad. Moreover, toxic, unstable or

pyrophoric agents are employed such as H2S, bis(trimethylsilyl)sulfide (TMS)2S or

Cd(CH3)2. Recently, Peng et al. reported the synthesis of cadmium chalcogenide

nanocrystals through a chemical approach using CdO or Cd-phosphonic acid as Cd

precursor and using non-coordinating solvents.10, 21

In this chapter, a green chemical route to produce high-quality CdS, ZnS and

ZnxCd1-xS nanocrystals is reported. And this green chemical route means safe, simple,

inexpensive, reproducible, versatile and user friendly. The controllable and narrow size

distributions have a relative standard deviation of 7-11%. The obtained CdS and ZnxCd1-

xS nanocrystals possess quantum-confinement wavelength-tunable optical absorption and

band-edge emission with high quantum yields of 25-45%. The PL spectra are dominated

by the band-edge emission without the broad emission from deep traps. Furthermore, it is

a one-pot method using stable, commonly available precursors (metal salts such as metal

stearate, acetate, chloride, and sulfate etc. as the cadmium source and elemental sulfur as

the sulfide source). The coordinating solvents used in this study include a series of

convenient high-boiling point long-chain amines such as hexadecylamine, oleylamine,

etc. Our research focuses mainly on the formation of cubic zinc-blende CdS and ZnxCd1-

xS nanocrystals at elevated temperature in this synthesis approach, and the rapid

nucleation and instant termination of the crystal growth process is observed. This

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nucleation and growth feature makes the reproducible and controllable synthesis of

nanocrystals with specific emission wavelength feasible.

3.2 Experimental section

3.2.1 Materials

Cadmium acetate (Cd(Ac)2, 98%), cadmium sulfate (CdSO4, 99%), cadmium

chloride (Cd(Cl)2, 99.99%), zinc stearate (Zn(ST)2, 90%), 1-hexadecylamine (HDA,

90%), dodecylamine (98%) and sulfur powder (99.98%) were purchased from Aldrich.

Cadmium stearate (Cd(ST)2, 90%) was purchased from Strem Chemicals. Methanol,

toluene, chloroform and acetone were purchased from Merck. Carbon-coated copper

grids (200 mesh) for preparing TEM specimens were purchase from Ted Pella Inc.

3.2.2 Synthetic methods

Typically, a mixture of 68.1 mg Cd(ST)2 (0.1 mmol) and 3.5 g technical-grade HDA

is heated to 290 oC under argon flow. 1.0 ml solution of sulfur (1.6 mg, 0.05 mmol) in

dodecylamine is then swiftly injected into this solution. The reaction mixture is kept at

280oC for the growth/annealing of the nanocrystals while monitoring the evolution of the

absorption and PL emission spectra. During the growth stage, aliquots are taken at

different time intervals and UV-vis and PL spectra are recorded for each aliquot. The

insoluble dark solid, if existing, is separated by centrifugation and decantation prior to

any further measurements. XRD and TEM measurements are also performed in order to

characterize the crystallinity, size, and size distribution of the resulting particle samples.

All the measurements are performed on the original aliquots without any size-selection.

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A similar fashion is adopted for the synthesis of CdS nanocrystals using other Cd

precursors (such as Cd(Ac)2, CdSO4 or CdCl2). ZnS nanocrystals are also prepared with

this synthetic approach by using the corresponding Zn precursor, while an equimolar

amount of Zn and S precursors are used.

The preparation method for ternary ZnxCd1-xS alloyed nanocrystals is similar to that

for the binary CdS. Cd and Zn precursors are with different molar ratios and the injected

amount of sulfur is equal to the total molar amount of the Cd and Zn precursors.

3.2.3 Characterization

All the resulting samples are immediately cooled and diluted with chloroform. The

UV-vis and PL spectra of the nanocrystals are recorded promptly. The room-temperature

PL efficiencies are determined by comparing the integrated emission of the samples to

that of dyes in solutions with identical optical density at the excitation wavelength. The

excitation wavelengths are set at the first absorption peak of the measured QDs. The dyes

(such as rhodamine 6G, rhodamine 640 or coumarin 540) should have significant

overlaps in their PL spectra with those of the QDs to be measured. A quadratic refractive

index correction is done if two different solvents are used to dissolve QDs and dyes. Also

the known efficiencies of the QDs in chloroform can be used to measure the efficiencies

of other QDs by comparing their integrated emission or PL intensity in solution. A low

concentration of the solutions is used to avoid obvious reabsorption. The UV-vis

absorption spectra are recorded on a Shimadzu UV-1601 scanning spectrophotometer

operating at a slit width of 1.0 nm. Bandgap energy determinations are made by

analyzing the absorption data using the method outlined by Fendler and co-workers to

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extract the value for the absorption onset. Photoluminescence spectra are acquired on a

Shimadzu RF-5301PC spectrometer. Emission spectra are taken using an excitation

wavelength of= 300 nm with excitation and emission slit widths set at 2.0 nm.

A mixed solvent of methanol and acetone is used to precipitate the resulting

nanocrystals in chloroform solution, which are then isolated by centrifugation and

decantation. Samples are prepared by placing a diluted solution of nanocrystals in

chloroform onto carbon-coated copper grids and allowing them to dry in a vacuum

desiccator overnight. The excess ligands and reaction precursors are removed by

extensive purification prior to high solution transmission electron microscopy (HRTEM),

powder X-ray diffraction (XRD) and inductively coupled plasma atomic emission (ICP)

measurements. No further size-selective purification is done for the samples. A JEOL

JEM3010 transmission electron microscope (operated at an accelerating voltage of 300

kV) is used to analyze the size, size distribution, and structure of the resulting

nanocrystals, which are deposited on carbon-coated copper grids using the QD hexane

solutions. The size distribution histograms for all the samples are obtained by analyzing

over 200 crystallites in each sample. The XRD patterns of the final products are recorded

by a Siemens D5005 X-ray powder diffractometer. The composition of the obtained

ZnxCd1-xS nanocrystals is measured by means of ICP by a standard HCl/HNO3 digestion.

The mean particle sizes for some samples of CdS and ZnxCd1-xS nanocrystals are

determined by transmission electron microscopy, and found to be in good agreement with

the values estimated from the excition absorption peaks. Considering the aim of our work,

UV-vis absorption spectroscopy is a more convenient technique to estimate the mean

diameters of nanocrystal in colloidal suspensions, because it has the important advantages

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of probing the whole ensemble used for the quantum yield and lifetime measurements

and allowing for the analysis of the crude samples.

Photoluminescence quantum yields (QY) are obtained by comparing with a standard

dye (Coumarin 540) and using data collected from the luminescence and the absorption

spectra as follows:

ST

ST

X

x

ST qT

TQY

=

1

1

where TST and TX are the transmittances at = 400 nm for the standard and the sample,

respectively, and qST is the quantum yield of the standard (98% as indicated by the

supplier, Aldrich). The term X and ST give the integrated emitted photon fluxes

(photons s-1

) for the sample and the standard, respectively. Utmost care is taken to ensure

a constant and reproducible position for the sample/standard holder and unchanged

instrumental conditions throughout the whole measurement process. The values of X

and are determined from the photoluminescence spectra corrected for the

instrumental response, by integrating the emission intensity over the desired spectral

range. Only the band-edge luminescence peak is integrated (any other luminescence

bands are discarded as background). Correction for the luminescence reabsorption is

performed, based on the Lambert-Beer law, but is found to be unnecessary for optical

densities . The accuracy of the method is calculated to be , based on re-

measurements.

ST

1.0 %10

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3.2.4 Synthesis of high-quality CdS, ZnS and ZnxCd1-xS nanocrystals

3.2.4.1 Synthesis of CdS nanocrystals

0.1 mmol (68 mg) Cd(ST)2

is dissolved in 3.5 g technical-grade HDA. The mixture

is heated to 295oC under argon flow. Then 0.05 mmol sulfur is swiftly injected into this

solution. The reaction mixture is then kept at 280oC for the growth/annealing of the

nanocrystals.

350 400 450 500 550

PLIntensity(a.u.)

Wavelength / nm

10 sec

2 min

10 min30 min

300 350 400 450 500

Absorbance(a.u.)

Wavelength / nm

10 sec

2 min

10 min30 min

Figure 3.1 Photoluminescence (Left) and UV-vis spectra for CdS nanocrystals prepared

from Cd(ST)2.

The photoluminescence spectrum of the CdS nanocrystals (Figure 3.1) shows a very

narrow emission bandwidth (FWHM = 26 nm) with an emission maximum at = 450 nm.

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20 30 40 50 60

311220

111

Intensity(a.u

.)

2 Theta

Figure 3.2 Powder X-ray diffraction pattern of CdS nanocrystals prepared from Cd(ST)2.

As shown in Figure 3.2, the XRD pattern of the CdS nanocrystals reveals three

diffraction peaks which can be attribute to the (111), (220) and (311) lattice planes. These

peaks correspond to the signatures for the cubic (zincblende) structural form. And the

intensity of the (111) orientation is predominant.

3.2.4.2 Synthesis of ZnS nanocrystals.

0.1 mmol (63 mg) Zn(ST)2 is dissolved in 3.5 g technical-grade HDA. The mixture

is heated to 288oC under argon flow. Then 0.1 mmol sulfur is swiftly injected into this

solution. The reaction mixture is then kept at 280oC for the growth/annealing of the

nanocrystals.

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350 400 450 500 550

PLIntens

ity(a.u.)

Wavelength / nm

10 sec

30 sec

2 min

5 min

10 min

300 350 400 450 500

Absorbanc

e(a.u.)

Wavelength / nm

10 sec

30 sec

2 min

5 min

10 min

Figure 3.3 Photoluminescence (Left) and UV-vis spectra for ZnS nanocrystals prepared

from Zn(ST)2.

The photoluminescence spectrum of the ZnS nanocrystals (Figure 3.3) shows an

emission maximum at = 347 nm.

20 30 40 50 60

311

220

111

Intensity(a.u.)

2 Theta

Figure 3.4 Powder X-ray diffraction pattern of ZnS nanocrystals prepared from Zn(ST)2.

As shown in Figure 3.4, the XRD pattern of the ZnS nanocrystals reveals three

diffraction peaks which can be attribute to the (111), (220) and (311) lattice planes. These

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peaks correspond to the signatures for the cubic (zincblende) structural form. And the

intensity of the (111) orientation is predominant.

3.2.4.3 Synthesis of ternary alloyed Zn0.17Cd0.83S nanocrystals

0.1 mmol (68 mg) Cd(ST)2 and 0.05 mmol (31.5 mg) Zn(ST)2 are mixed and

dissolved in 3.5 g technical-grade HDA. The mixture is heated to 267oC under argon

flow. Then 1.0 ml solution of sulfur in dodecylamine is swiftly injected into this solution.

The injected amount of sulfur (4.8 mg, 0.15 mmol) is equal to the total molar amount of

Zn and Cd precursors. The reaction mixture is then kept at 260oC for the

growth/annealing of the nanocrystals.

350 400 450 500 550

PLInten

sity(a.u.)

Wavelength / nm

30 sec

2 min

10 min

30 min

300 350 400 450 500 550

Absorbance(a.u.)

Wavelength / nm

30 sec

2 min

10 min

30 min

Figure 3.5 Photoluminescence (Left) and UV-vis spectra for Zn0.17Cd0.83S nanocrystals.

The photoluminescence spectrum of the Zn0.17Cd0.83S nanocrystals (Figure 3.5)

shows an emission bandwidth (FWHM = 31 nm) with an emission maximum at = 452

nm.

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20 30 40 50 60

311

220

111

Intensity(a.u

.)

2 Theta

Figure 3.6 Powder X-ray diffraction pattern of Zn0.17Cd0.83S nanocrystals.

As shown in Figure 3.6, the XRD pattern of the Zn0.17Cd0.83S nanocrystals reveals

three diffraction peaks which can be attribute to the (111), (220) and (311) lattice planes.

These peaks correspond to the signatures for the cubic (zincblende) structural form. And

the intensity of the (111) orientation is predominant.


0.1 mmol (68 mg) Cd(ST)2 and 0.1 mmol (63 mg) Zn(ST)2 are mixed and dissolved

in 3.5 g technical-grade HDA. The mixture is heated to 270oC under argon flow. Then

1.0 ml solution of sulfur in dodecylamine is swiftly injected into this solution. The

injected amount of sulfur (6.4 mg, 0.2 mmol) is equal to the total molar amount of Zn and

Cd precursors. The reaction mixture is kept at 260oC for the growth/annealing of the

nanocrystals.

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350 400 450 500 550

PLIntensity(a.u.)

Wavelength / nm

10 sec2 min

10 min30 min

300 350 400 450 500 550

Absorbanc

e(a.u.)

Wavelength / nm

10 sec

2 min

10 min

30 min



shows a narrow emission bandwidth (FWHM = 28 nm) with an emission maximum at

= 445 nm.

20 30 40 50 60

311220

111

Intensity(a.u.)

2 Theta


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These peaks correspond to the signatures for the cubic (zincblende) structural form.

3.2.4.5Synthesis of ternary alloyed Zn0.28Cd0.72S nanocrystals

0.05 mmol (34 mg) Cd(ST)2 and 0.1 mmol (63 mg) Zn(ST)2 are mixed and


flow. Then 1.0 ml solution of sulfur in dodecylamine is swiftly injected into this

solution. The injected amount of sulfur (4.8 mg, 0.15 mmol) is equal to the total molar

amount of Zn and Cd precursors. The reaction mixture is kept at 260oC for the


350 400 450 500 550

PLIntensity(a

.u.)

Wavelength / nm

10 sec

30 sec

2 min

10 min

30 min

300 350 400 450 500

Absorbance(a

.u.)

Wavelength / nm

10 sec

30 sec

2 min

10 min

30 min




nm.

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20 30 40 50 60

311

220

111

Intensity(a.u

.)

2 Theta




These peaks correspond to the signatures for the cubic (zincblende) structural form.

3.2.4.6Synthesis of ternary alloyed Zn0.32Cd0.68S nanocrystals

0.05 mmol (34 mg) Cd(ST)2 and 0.15 mmol (94.5 mg) Zn(ST)2 are mixed and




Zn and Cd precursors. The reaction mixture is then kept at 260oC for the


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350 400 450 500 550

PLIntens

ity(a.u.)

Wavelenght / nm

10 sec

2 min

10 min

30 min

300 350 400 450 500

Absorbance(a.u.)

Wavelength / nm

10 sec

2 min

10 min

30 min




nm.

20 30 40 50 60

311

220

111

Intensity(a.u.)

2 Theta




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0.05 mmol (34 mg) Cd(ST)2 and 0.2 mmol (126 mg) Zn(ST)2 are mixed and




Zn and Cd precursors. The reaction mixture is kept at 260oC for the growth/annealing of

the nanocrystals.

350 400 450 500 550

PLInte

nsity(a.u.)

Wavelength / nm

30 sec

2 min

10 min

30 min

300 350 400 450 500 550

Absorban

ce(a.u.)

Wavelenght / nm

30 sec

2 min

10 min

30 min




nm.

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20 30 40 50 60

311

220

111

Intensity(a.

u.)

2 Theta






3.3 Results and discussion

For the synthesis of CdS, ZnS and ZnxCd1-xS nanocrystals, the metal salt is dissolved

in amine to form a metal-amine complex at an appropriate high temperature. To the

resulting solution, elemental sulfur dissolved in amine is swiftly injected. The reaction

mixture is heated at a certain temperature for some period of time to generate the desired

metal sulfide nanocrystals.

Figure 3.15 displays the temporal evolution of the absorption and the PL spectra of

the CdS nanocrystals, which are produced at 280oC through the reaction of cadmium

stearate in HDA with the injected sulfur dissolved in dodecylamine. The growth kinetics

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of CdS nanocrystals prepared by this new approach possess a similar pattern to that of the

CdS nanocrystals formed by the reaction of CdO-hexylphosphonic acid with (TMS)2S in

the TOPO/TOP system.10

After the growth/annealing for 1 h at the reaction temperature,

the sharp excitonic absorption peaks are still retained, with the narrow band-edge PL

emission peak located at 451 nm with a full width at half maximum (fwhm) of 24 nm.

The PL quantum yield gradually reaches its maximum value of30%, and the broad

deep-trap emission peaks disappear completely due to the removal of structural defects.

These sharp features of the absorption and the PL peaks demonstrate that the size

distribution of the resulting nanocrystals is nearly monodisperse. A certain control of the

particle size corresponding to the tunable absorption onset and emission wavelength is

possible by adjusting the nucleation and growth temperature or the concentration of the

reactant precursors.

300 350 400 450 500 550 600 650

350 400 450 500 550

wavelength / nm

UV-Vis

PL

60 min

30 min

5 min

2 min

1 min

30 sec

Absorbance(a.u.)

Wavelength / nm

Figure 3.15 Temporal evolution of UV-vis spectra of a growth reaction of CdS

nanocrystals at 280oC. Inset: PL and absorption of a CdS nanocrystal sample.

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Figure 3.16 TEM images of (a) CdS nanocrystals prepared from Cd(ST)2, (b) CdS

nanocrystals from Cd(Ac)2, (c) ZnS nanocrystals from ZnCl2, (d) ZnxCd1-xS nanocrystals.

Figure 3.16 shows a TEM micrograph of the as-prepared CdS nanocrystals without any

size selection after 1 h growth at the above-mentioned synthesis conditions. Nearly

monodisperse particles are observed with an average size of 4.5 0.4 nm in diameter. It

is surprising that the X-ray diffraction patterns in Figure 3.17 clearly show a cubic zinc-

blende crystal phase of the obtained CdS nanocrystals prepared at 280oC all of the

peaks match well with the Bragg reflections for the standard cubic CdS structure. As

expected, the width of the diffraction peaks is considerably broadened and decreases with

increasing particle size. The diameter of the crystallite is related to the width of the Bragg

reflection peak in terms of the Scherrer formula:

FWHM = )cos/()94.0( BL

where L is the crystal domain size. By using the Scherrer formula, we can calculate the

mean sizes of the nanocrystals from the peak width at half-maximum. Particles sizes

obtained from the width of the (111) reflection are depicted in Figure 3.17. And at the

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same time, we use direct TEM observations to confirm our calculation results. It is well-

known that CdS cluster molecules forming single crystals usually display a cubic

structure.22-24

and at room temperature the cubic CdS nanocrystals are obtained by using

polyphosphate or thioglycerol as the stabilizer.14, 25, 26

However, the hexagonal wurtzite

crystal phase of CdS is thermodynamically stable, thus it is the favorite phase at elevated

temperature.26

To our best knowledge, almost all of the CdS nanocrystals prepared at

such elevated temperature exhibit a hexagonal structure.

20 30 40 50 60

Intensity(a.u.)

CdS/Cd(ST)2, 4.1 nm

ZnS, 4.4 nm

Zn0.42

Cd0.58

S, 4.3 nm

Zn0.32

Cd0.68

S, 4.6 nm

Zn0.28

Cd0.72

S, 4.2 nm

Zn0.23Cd0.77S, 4.3 nm

Zn0.17

Cd0.83

S, 4.5 nm

CdS/Cd(Ac)2, 7.4 nm

2 Theta

Figure 3.17 Powder X-ray diffraction patterns of CdS nanocrystals prepared from

Cd(ST)2 or Cd(Ac)2, ZnxCd1-xS alloyed nanocrystals with Zn molar fractions ((a) 0.17, (b)0.23, (c) 0.28, (d) 0.32, and (e) 0.42), and ZnS nanocrystals from ZnCl2. The stated

particles sizes were calculated from the Scherrer equation. The line spectra indicate the

reflections of bulk cubic CdS (bottom) and bulk cubic ZnS (top).

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In addition to Cd(ST)2, the commonly available cadmium salts such as Cd(Ac)2,

CdCl2, CdSO4, etc. can also act as cadmium sources when reacting with elemental sulfur

to form uniform CdS nanocrystals, while their PL QY is nearly zero. It should be noted

that cubic CdS nanocrystals are made by using Cd(Ac)2 or Cd(ST)2, while hexagonal CdS

is made by using CdCl2 or CdSO4 as cadmium precursors. Moreover, other high-boiling

alkyl amines with different lengths of their alkyl chains such as tetradecylamine and

octadecylamine, or unsaturated amine such as oleylamine can also act as coordinating

solvents to obtain high-quality nanocrystals. It is based on the consideration of

operational convenience that liquid dodecylamine is chosen as the solvent to prepare the

sulfur stock solution. The corresponding zinc salts (Zn(ST)2, Zn(Ac)2, ZnCl2, ZnSO4, etc.)

and elemental sulfur can also be used to prepare ZnS nanocrystals. Figure 3.16 (b), (c)

show the TEM images of the CdS (average size of 7.5 0.5 nm) and ZnS (average size of

4.3 0.4 nm) using Cd(Ac)2 and ZnCl2 as the metal sources respectively. A cubic

structure is readily derived for both CdS and ZnS nanocrystals from their corresponding

diffractograms (Figure 3.17).

The synthetic approach to ternary alloye

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