CHAPTER‐5 

Synthesis and characterization of CdSe and CdS nanocrystals and thin films 

 

 

5A. Introduction

Nanocrystalline semiconductor materials in recent times have attracted many

research workers for their intriguing physical properties which arise due to the

spatial confinement of carriers and increase in the number of surface atoms.1-9

Among them group II-VI binary semiconductors in nanocrystalline form occupy a

prominent place in the semiconductor physics as they show wide range of

applications in optoelectronic devices, solar energy conversions etc. These binary

semiconductor nanocrystals or quantum dots with dimensions smaller than the

bulk exciton Bohr radius exhibit unique quantum size effects and strongly size

dependant electronic, magnetic, optical and electrochemical properties which is

due to quantum confinement effect.1-19 Tremendous interest in nanocrystals of

these binary semiconductors have generated over the past decade and are often

considered for different applications, namely light emitting diodes,20-23 biological

applications,21,24-28 optoelectronic photovoltaic cells etc.29-34

The possibility of tuning the optical properties of semiconductor nanocrystals

by simply varying their size owing to the quantum confinement has gained

increasing attention for use in light emitting devices. The emission properties of

semiconductor nanocrystals can be characterized by four fundamental parameters,

which are the brightness, the emission color, the color purity, and the stability of

the emission. Its size dependant character is probably the most attractive property

of semiconductor nanocrystals which can be used for many purposes, such as light

emitting diodes and optoelectronic devices. The high surface to volume ratio of

small nanocrystals suggests that the surface properties should have significant

effects on their structural and optical properties. Thus, II-VI semiconducting

nanocrystals of Cadmium sulphide (CdS) and Cadmium selenide (CdSe) have

recently emerged as better phosphors compared to conventional phosphors in

particular.35-42 These phosphors have broader and stronger absorption and higher

resistance to photooxidation compared to the common emissive materials, such as

organic dyes and other inorganic phosphors: also in nanocrystals of size less than

10 nm, energy loss due to scattering is reduced. In addition to this, their

possessibility by functionalizing their surface using various organic molecules and

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makes them to be soluble in different polar and non-polar solvents.

In spite of such tunable photoluminescence emission properties with variety of

promising applications, the efficiencies of nanocrystals are known to be sensitive

to the nature of the particle surface because of the large surface area and possible

presence of surface states caused by uncoordinated atoms.9,43,44 Such surface

states act as quenchers of luminescence and competes with the band edge radiative

emissions. Therefore for improving the luminescence efficiency there is need for

passivating such surface state quenchers so as to minimize the non- radiative

emissions originating from surface states. In view of this there are many reports of

surface passivation by coating the surface of nanocrystals using suitable organic

ligands.45-49 Recently, it is however found much effective passivation of the

surface states by so called core-shell forming shell over the core particles.50-62

This has reported for improved luminescence efficiency. This is because of the

reduction in non-radiative recombination confining wavefunction of electron hole

pairs to the interior of the crystal which is achieved by passivating the traps and

surface states/defects with long chain organic surfactants or epitaxially growing an

inorganic shell of material with larger bandgap.60-62 Along with success in tuning

the optical properties of absorption and emission by tailoring the crystallite size of

these semiconducting nanoparticles, doping has also proven to be another

effective approach to tuning their properties.63-70 In such systems, the excitation

takes place in the host semiconducting material, whereas the deexcitation occurs

in the energy levels of the dopant ions. During the more recent times, an

alternative route of the tuning the optical properties of II-VI compound

semiconductors has come up. This is due to fact that the tuning of physical and

chemical properties by changing the particle size could cause in many

applications, in particular, if unstable particles (size less than ~2 nm) are used.71-80

Cadmium Selenide (CdSe) and Cadmium sulphide (CdS) nanocrystals among

II-VI semiconductors are of great interest for their potential applications owing to

excellent optical conductivity, such as non linear optical properties, luminescent

properties and quantum size effect. In thin film form they are used as thin film

transistors, gas sensors,81 gamma ray detectors and large screen liquid crystal

display82 and window layers in solar cells.83-92 Further, deliberate doping of

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impurity can influence the electrical properties in particular also other optical,

electronic, structural and other characteristic properties.83,93-100 Quantum dots of

these semiconductors in thin film form have also shown the sensitive dependence

of their optical and electrical properties on size, shape, size distribution and

morphologies. Such properties attract research workers for fundamental and

technological interest.82,90-95,98-103

Among a number of methods for the preparation of the II-VI binary

semiconductors, colloidal wet chemical method offers an inexpensive and simple

means to synthesize such particles with good control over size and size

distribution by optimizing various parameters. Many workers use ammonia for its

dual role of forming complex metal ion and varying the pH of the reaction bath

and thereby slowing down the reaction rate.81,95,96,103 Among the thin film

preparation techniques/methods such as physical vapour deposition,30

sputtering,104 spray pyrolysis,105 pulse laser deposition,106 chemical bath

deposition (CBD),27,82,87,88,91, 101-103,113 etc.CBD is one of the widely adopted

methods of thin film preparation of these two mentioned binary semiconductors in

particular. Because, CBD is simple, inexpensive and preparation can be carried

out at moderately low temperature. Remarkably, this method can produce large

area deposition and also can yield stable and uniform adherent film with excellent

reproducibility.

Preparations of CdSe in various shapes, namely, nanorods, nanocables,

nanoballs, hollow nanospheres have been investigated by a number of workers.107-

110 R. Maity and K. K. Chattopadhyay reported the preparation of nanocrystalline

ZnS and ZnS:Mn by chemical synthesis process without using any capping

agent.111 Ghosh et al.112 have also reported the chemical bath deposition of

transparent polycrystalline ZnS nonobelts within the pores of polyvinyl alcohol on

glass and Si substrates. Bawendi and co-workers first time in 1993 reported the

preparation high quality quantum dots of II-VI binary semiconductors using

organometallic route.6 Since then this method has been widely adopted by many

research workers for preparation of highly monodispersed quantum dots.

Preparation using well known organometallic route requires standard airless

condition at relatively high temperature (220-240 °C) as well as chemicals use are

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less stable. Moreover, nanoparticles are soluble only in non-polar solvents limiting

their biological and environmental applications. Therefore there is necessary for

further surface modification of nanoparticles for bio compatible applications. In

the present thesis, synthesis and characterisation of nanostructured CdSe and CdS

crystals and thin films using wet chemical method without using capping agent

have been investigated. And nanocrystals were prepared using less toxic

chemicals in aqueous medium which will be soluble in polar solvents.

Part-I Nanocrystals and thin films of CdSe

5B. Experimental details

5B.1 Synthesis without capping agent

All reagents used are of analytical grade. Cadmium Acetate Cd(CH3COO)2 and

freshly prepared sodium selenosulphite Na2SeSO3 are used as the sources of Cd

and Se ions respectively. To prepare fresh Na2SeSO3 solution 3 gm of Na2SO3 is

dissolved in 250 ml of distilled water. Then 0.5 gm of Se metal powder is added to

this solution and heated at 90 °C under constant stirring for 8 hours and cooled to

room temperature and filtered to obtain fresh Na2SeSO3 solution. To 250 ml of

0.005M Cd(CH3COO)2 is added 0.05M NH4COOCH3 buffer solution in which

25% liquor ammonia is then added to this until the pH of the bath becomes 9.6.

Glass substrates are cleaned following the steps reported by Oladeji and Chow.113

The cleaned glass substrates are held vertically in the solution with the help of

teflon holders and tapes. The solution is now heated upto 65 °C and 30 ml of the

freshly prepared Na2SeSO3 is then added slowly at the rate of 1 ml per minute.

The reaction continues for 4 hours and red CdSe thin film gets deposited on the

glass substrate. The glass substrate with the thin film of CdSe is taken out of the

bath, cleaned with distilled water in an ultrasonic bath and air dried at room

temperature for its optical absorbance measurement. The temperature of the bath

is now increased to 70 °C for the homogeneous reaction to start and precipitation

of CdSe starts taking place in the solution. The CdSe colloids in the bath is

centrifuged and extracted with methanol. The CdSe precipitate is washed with

several times with distilled water and air dried at room temperature and kept in

desiccators for characterization.

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The reaction mechanism involved in the formation of CdSe thin film and

nanocrystals are as follows.114,115

OHNHOHNH 234 +↔+ −+ (1)

( ) ++ ↔+ 2433

2 NHCd4NHCd (2)

In an alkaline solution, the inorganic sodium selenosulphite hydrolyses to give

Se2- ions and reacts with Cd2+ to form the CdSe.

−− ++↔+ 224232 SeOHSONa2OHSeSONa (3)

and CdSeSeCd 22 ↔+ −+ (4)

If the ionic product [Cd2+][Se2-] exceeds the solubility product, Ksp, of

CdSe (4.0×10-35), then CdSe will form as solid phase.82,115

5B.2 Synthesis with capping agent

Nanocrystals of CdSe were prepared with capping agent following the method

adopted by Zheng et al.75 All chemicals, Cd(CH3COO)2, Se metal powder,

L-Glutathione (GSH) as capping agent, Sodium borohydride (NaBH4) and Sodium

hydroxide (NaOH) are of analytical grade. All the preparations were carried out

under nitrogen (99.999% purity) atmosphere. In a typical synthesis, firstly the

required amount of Se (metal) (5 mMol) was dissolved in NaBH4 solution in

under vigorous stirring and obtained clear solution of NaHSe. It is kept in ice bath

for further use as Se ion source. The required concentrations of Cd(CH3COO)2 (10

mMol) and GSH (20 mMol) in 50 ml deionized water. The final pH of the

solution was made to 11.6 using NaOH solution. The reaction was carried out at

different temperatures for 30 min. The colloid formed precipitated using 2-

propanol and washed it in deionized water again precipitated. The process is

repeated for 4-5 times and dried in air. In these cases and remaining preparations,

the entire reaction was carried out in the nitrogen (purity 99.99%) atmosphere.

The particle size and crystal structure of the samples (prepared using both

methods) are determined from XRD data using a Rigaku 18 kW Rotating X-ray

generator equipped with Rigaku D-Max and PANalytical X’Pert PRO with CuKα

X-radiation. The ionic composition is determined using energy dispersive X-rays

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(EDX) data. The micrographs of the sample dispersed on the glass substrate are

studied using Scanning electron microscopy (SEM), JSM-5600 and Atomic force

microscopy (AFM), Nanoscope E Verson-245. Transmission electron microscopy

(TEM) studies were carried out using JEOL-100CXII and Philips CM200. The

optical absorbance was recorded with UV-Visible Spectrometers, Systronics-2202

and Perkin Elmer (Lambda-35). Photoluminescence (PL) was recorded using

Perkin Elmer LS-55.

5C. Results and discussion

Figure 1A shows the XRD pattern of the nanostructured CdSe crystals. The

diffraction peaks are observed at the 2θ values of 25.665°, 42.650°, 49.650°,

67.709° and 78.021° corresponding to the crystal planes of (111), (220), (311),

(331) and (422) respectively, showing cubic zinc blende structure of CdSe

(JCPDF -19-0191). Same crystal structure is also shown by the thin film of the

CdSe (Figure 1B). CdSe has been reported to exist both in cubic phase81,94,103,108

and in hexagonal phase.113 Different workers have revealed the co-existence of

hexagonal and cubic CdSe crystallites with preferential orientation along c-axis

and (111) direction respectively.82,117 In this work, CdSe nanocrystals are found to

exist only in cubic phase. The prominent peaks have been utilised to estimate the

crystallite size of the samples using Scherrer formula, θβ

λcosKD = , where D is the

average crystallite size, K is a constant (~1), β is the full width at half maximum

Figure 1 XRD patterns of the (A) powder and (B) thin film of nanostructured CdSe.

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and θ is the Bragg’s angle. The crystallite size estimated using the (111) peak is

found to be ~4 nm.

Figure 2 EDX spectrum of CdSe.

The ionic concentrations of cadmium and selenium of the prepared CdSe

sample are determined using EDX (Figure 2). Cadmium ion concentration

exceeds slightly that of the selenium ion by about 9% with a small trace of sulphur

ion of about 6% which may be due to presence of remains of unreacted Na2SeSO3.

Figure 3A shows the SEM picture of the powder sample of CdSe. The picture

shows that the 3.4 nm nanograins have aggregated to form bigger particle of

nearly 1 micron size with sharp boundaries. The SEM picture shows that bigger

aggregated particles are homogeneous in size. In thin film form, surface of the

Figure 3 SEM images of nanostructured CdSe (A) powder and (B) thin film.

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film is well continuous without breaks with average grain size about 200 nm

(Figure 3B). Figures 4(A and B) show two dimensional (2D) and three

dimensional (3D) AFM images of CdSe powder samples dispersed in acetone

over a glass substrate. The particles are found to form small spherical islands of an

average diameter of 150 nm. However, section analysis of two spherical islands

shows that the islands have vertical distances of 5.991 nm and 11.701 nm. This

shows that although a single particle could not be selected for the AFM

observation the particles lie in the range of 6-12 nm. Kale et al.103 attributed the

formation of small islands of ZnSe to the indication of three dimensional growth

of the film of ZnSe deposited using modified chemical bath deposition method.

The 2D and 3D AFM images of CdSe thin films prepared using CBD method are

Figure 4 AFM images of CdSe powder samples (A) 2 dimension (2D) and (B) 3 dimension (3D).

Figure 5 AFM images of CdSe thin film (A) 2 dimension (2D) and (B) 3 dimension (3D).

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shown in Figures 5(A and B). From the 2D image, it is seen that the grains of

CdSe particles are found to exist in spindle shape. The highly elliptical structure

of the grains have size distributions having major axis ranging between 60-120 nm

and minor axis between 30-90 nm. Figure 6 shows the TEM images of the (A)

powder and (B) thin film CdSe samples. For the powder samples the TEM sample

Figure 6 TEM images of the (A) powder and (B) thin film CdSe.

was prepared by dispersing in the methanol and put over 200 mesh amorphous

carbon coated copper grids while the thin film, the film was etched out using

diluted hydrofluoric acid (1:99) and carefully put over the grid. From the TEM

image, the particle size of the powder sample is found to be ~10-20 nm. Though

there is variation of particle size, the agglomeration among them is hardly

observed. TEM image of the thin film shows presence of grains formed by

nanostructured particles. These grains are of almost similar in size of ~200 nm.

The absorption spectrum of the CdSe thin film deposited on the glass substrate

is shown in Figure 7. The band gap of the sample is calculated from the absorption

Figure 7 Absorption spectrum of the CdSe thin film and optical bandgap (inset).

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spectrum shown in Figure 7 using the relation118 (αhν)1/n = A(hν-Eg), where α is

the absorption co-efficient, hν is the photon energy, A is a constant and Eg is the

optical band gap of the material. The value of n in the exponent is 1/2 for direct,

allowed transitions. Inset of Figure 7 shows the plots of (αhν)2 vs hν.

The linear portion of the plot of (αhν)2 vs hν in the region of strong absorption has

been extrapolated to obtain the intercept on the hν axis. The intercept gives the

value of the band gap Eg. The value of Eg is found to be 1.82 eV, which exceeds

the band gap 1.74 eV of bulk CdSe. This is attributed to the quantum confinement

effect as the particle size becomes smaller.2 The grain size of 3.4 nm obtained

from the XRD data shows that it is appreciably smaller than the Bohr exciton

radius 5.6 nm of CdSe. The exciton energy sE obtained using effective mass

approximation (EMA) for strong confinement is given by2

*Ry

0

2

2

22248.0

4786.1

2E

Re

REE gs −−+=

επεμπ , (5)

where μ is the reduced effective mass, ε is the dielectric constant of CdSe and ε0

is the permittivity in vacuum, E*Ry is the effective Rydberg energy. Using this

relation the grain size of the prepared CdSe is found to be 3.8 nm, which is quite

near to the size ~3.4 nm obtained from XRD data.

The size of the nanoparticles has considerably reduced when the preparation

was carried out using organic capping agents. This is due to the fact that the

capping agent hinders the growth of the particles compared to that of without

capping agent as well as the preparation was carried out at relatively higher

Figure 8 XRD pattern of CdSe nanoparticles.

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40 50 60 70 80

1.8

2.0

2.2

2.4

Opt

ical

ban

dgap

(eV

)

Reaction temperature (oC)

Bulk Eg = 1.74 eV

300 400 500 600 700

Reaction temperature (oC)

40 60 70 80

Wavelength (nm)

Inte

nsity

(arb

. uni

ts)

temperature thus nucleation becomes faster. Figure 8 shows the XRD pattern of

CdSe nanoparticles prepared using capping agent (L-Glutathione). The pattern

shows very broad nature of peak indicating nanocrystallinity of the sample. The

pattern shows cubic zinc blende structure of CdSe (JCPDF -19-0191). The particle

size calculated using Scherrer’s relation is found to be ~2 nm. From the TEM

image (Figure 9), it is observed that the nanoparticles are almost spherical in

shape with average size of ~4-5 nm.

Figure 9 TEM image of CdSe nanoparticles prepared at 60 °C.

Figure 10A shows the absorption spectra of CdSe nanoparticles prepared at

different temperatures. Clearly red shift in the absorption edge is observed in the

spectra with the increase of reaction temperature. Formation of exciton in the

absorption is observed in the case of nanoparticles prepared at 40 °C. In all the

Figure 10 (A) UV-visible absorption spectra and (B) optical bandgap of CdSe nanoparticles prepared at different reaction temperatures.

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cases the optical bandgap calculated is observed to be more than that of bulk value

(1.74 eV) (Figure 10B). The blue shift in the optical bandgap compared to bulk

value shows the quantum confinement effect of size. Figure 11A shows the

photoluminescence (PL) spectra of CdSe samples prepared at different reaction

Figure 11 Photoluminescence spectra of CdSe nanoparticles (A) prepared at different temperatures and (B) under different excitation of CdSe prepared at 70 °C.

temperatures. There is a considerable blue shift of ~30 nm of the emission peak

(512 nm) of sample prepared at 40 °C compared to others whose emission peaks

are at 540 nm. All the samples show green emission of the light which shows

considerable blue shift as compared to red emission of the bulk CdSe. Figure 11B

shows the normalized PL emission spectra CdSe nanoparticles prepared at 70 °C

under different excitation wavelengths. All the spectra show similar PL emission

peak at 540 nm.

Part-II Nanocrystals and thin films of CdS

5D. Synthesis

In all cases, the preparation was carried out in similar manner as mentioned in the

Part-I of this chapter. In this case the source of sulphur was used from thiourea for

preparation of CdS nanoparticles and thin films without capping agent. Sodium

sulphide (Na2S) was used for the preparation of CdS nanoparticles with capping

agent. The thin film of CdS was grown over the cleaned glass substrate by CBD

method. Analytical grade reagents of cadmium acetate Cd(CH3COO)2 and

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thiourea CS(NH2)2 were used as the Cd and S sources respectively. Concentrated

liquor ammonia solution (25%) was used as complexing agent of Cd and

NH4CH3COO as a buffer. Solutions of Cd(CH3COO)2 (0.005M), NH4CH3COO

(0.1M), and NH3 (0.6M) were dissolved in a reaction bath containing 250 ml of

deionised water. The resultant solution was then heated up to 85 °C. The glass

cleaned substrate (procedure mentioned above) was immersed properly in the

reaction bath. The 0.01M CS(NH2)2 is then added at the rate of 1 ml per minute

with constant stirring. The deposition time is varied from 11/2 to 4 hours. The

deposited films were taken out from the bath, washed with distilled water and

finally cleaned with ultrasonic cleaner. The film was then dried at room

temperature for characterization. Moreover, the films were annealed in ambient

atmosphere also at 200, 300 and 400 °C for optical absorption studies. The

nanoparticles of CdS using GSH as capping agent was prepared at 85 °C

following the procedure mentioned in the part-I of this chapter. Typically

Cd(CH3COO)2 (10 mMol) and GSH (15 mMol) in 50 ml deionized water. The

final pH of the solution was made to 11.6 using NaOH solution. Then the bath is

heated at 85 °C and Na2S (5mMol) solution was swiftly injected. The reaction was

carried out at different temperatures for different durations.

5E. Results and discussion

All the characterizations of samples are similar to above otherwise stated.

Diffraction peaks of CdS thin film of vacuum-annealed at 200 °C (Figure 12) are

observed at the 2θ values of 26.9, 44.4 and 52.4 (in degrees) corresponding to the

crystal planes of (111), (220) and (311) respectively, showing the cubic zinc

blende structure of CdS (JCPDF No. # 10-0454). Both the films (a) as-deposited

and (b) vacuum-annealed at 2000C show the same crystal structure without any

impurity phase. This result is in agreement to that of Kale et al103 who showed

identical crystal structure for as deposited and air-annealed at 200 °C films of

chemical bath deposited CdSe. Lee119 however reported the mixed phase of cubic

and hexagonal structures of the CBD grown CdS films. The crystallite size was

obtained taking into account the strain broadening. The crystallite size of the CdS

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Figure 12 XRD pattern of thin film CdS (A) as-deposited and (B) vacuum annealed at 200 °C.

was obtained from the Scherrer formula with an added term for strain

broadening10

θηελθβ sincos += (6)

where ε is the effective particle size, and η is the effective strain, β is the full

width at half maximum, θ is the Bragg’s angle and λ is the wavelength of Cu Kα

x-radiation. The crystallite size and the strain were obtained from the values of

β of the diffraction peaks. In Figure 13 the solid circles (•) represents the

variation of βCosθ versus θsin of the XRD peaks of the as-deposited thin film of

CdS and the (*) symbol represents the variation for the vacuum annealed film.

Figure 13 Plot of βCosθ versus Sinθ of as-deposited (o), and vacuum annealed at 200 °C (*), thin film CdS. The straight line represents the linear fit vacuum annealed data.

144

The as-deposited film (o) shows non-uniform strain and departure from the

uniform shape along different crystallographic orientations in agreement with the

reports given for ZnS crystals by Qadri et al.10 They have reported that there is no

significant strain for annealed samples. However, the variation of βCosθ versus

θsin for the 200 °C vacuum annealed sample (Figure 13) shows that there is a

uniform strain. The effective particle size and strain have been calculated for the

vacuum annealed sample. The effective particle size and strain obtained for the

vacuum annealed sample are 31 nm and 2.34×10-3 respectively.

The thickness of the deposited films for different durations is measured

with a XP-Stylus Profiler. The plot of the thickness with deposition time is shown

in Figure 14. The figure indicates clearly that the thickness increases with

deposition time but saturates at about 200 nm when deposition time is beyond

about 5 h.

Figure 14 Thickness of the film with the time of deposition.

Surface morphology and homogeneity of the CdS film deposited on the glass

substrate are studied using scanning electron microscope. Figure 15A shows the

SEM image of the CdS thin film showing uniform surface morphology with

intermittent gaps among the grains. The Cd and S stoichiometry of the CdS thin

films was determined by EDX to equal 1:1 suggesting good film composition

(Figure 15B).Figure 16 shows the AFM image of the thin film CdS in (A) 2 and

(B) 3-dimension. From Figure 16A, it is evident that the grain size distribution is

nearly monodispersed. The image shows that there is not much distribution in the

grain size indicating nearly monodispersed nature of the grains. The small amount

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Figure 15 (A) SEM image and (B) EDX of CdS thin film.

of dispersion in the grain size is understandable as Oswald ripening process can

also contribute during the crystal growth in the ion-by-ion deposition.114 The

average grain size of as-deposited film is ~ 180 nm. The roughness of the CdS

film scanned by AFM (Figure 16B) over an area of 2 × 2 μm2 is found to be 20

nm. Figure 17A shows the TEM micrograph of the as-deposited thin film of CdS.

Figure 16 AFM images of CdS thin film (A) 2-Dimensional and (B) 3-Dimansional.

The micrograph exhibits fairly uniform particle size. The histogram of grain size

distribution is shown in Figure 6b. It is observed that average grain size of the as-

deposited thin film is ~180 nm. This finding intuitively indicates that the grain

size determined from TEM data agrees well that found from AFM. Moreover, on

close observation in Figure 17A, it is apparent that these grains are agglomerated

consisting of still smaller grains than the nearly 31 nm size obtained from XRD

data. Figure 17B shows the distribution of the grain size fitted using log-normal

distribution. The median of the grain size is found to be ~170 nm.

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Figure 17 (A) TEM micrograph of as deposited CdS thin film and (B) grain size distribution.

Figure 18A shows the absorption spectrum of as-deposited CdS thin film.

Figure 18B shows the plot of (αhν)2 vs. hν. The optical band gap of the sample is

calculated using the relation mentioned above. α is the absorption co-efficient,

hν is the photon energy, A is a constant and Eg is the optical band gap of the

material. For direct, allowed transitions, n = ½. In the region of strong absorption

the curve has been extrapolated to obtain the intercept on the hν−axis.

Figure 18 (A) Absorbance spectrum and (B) Plot of (αhν)2 vs. hν of the CdS thin film.

Τhe intercept gives the value of the band gap Eg~2.29 eV. The absorbance

spectrum of the films annealed in ambient atmosphere has also been measured. A

red shift in optical band gap is observed as the annealing temperature increases

from 200 to 400 °C (Figure 19). The band gaps thus determined are 2.24, 2.20 and

2.12 respectively. There is gradual decrease in the values of band gap as the

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annealing temperature increases and it is to be noted that the band gap of these air

annealed films lies below the value of 2.29 eV.

Figure 19 Plot of (hνα)2 vs. hν of CdS thin films air annealed at 200, 300 and 400 °C.

Figure 20 shows the X-ray diffraction pattern of CdS nanoparticles prepared

by colloidal wet chemical method using GSH as capping agent at 80 °C. The

pattern shows cubic zinc blende structure of CdS (JCPDF No. # 10-0454). The

diffraction peak clearly shows broad nature of peak showing nanocrystalline

nature of CdS. The crystallite size of the nanoparticle calculated using Scherrer

relation is found to be 3 nm.

Figure 20 XRD pattern of CdS nanoparticles.

Figure 21 shows the TEM (A) image and (B) SAED pattern of CdS nanoparticles

prepared at 80 °C. From the TEM image, the presence of small nanoparticles

having size of ~5-10 nm is observed. Some of the small particles are seen to be

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aggregated among them with size bigger than 10 nm. SAED pattern does not show

well diffraction rings indicating presence of very small nanoparticles.

Figure 21 (A) TEM image of CdS nanoparticles prepared at 80 °C and (B) SAED pattern.

Figure 22A shows the absorption spectra of CdS nanoparticles prepared at 80

°C for different durations. From the spectra it is clearly observed that the

absorption onset shifts towards higher absorption wavelength with the increase of

reaction time. This indicates the ability to tune the particle size with the duration

of reaction time from same reaction medium. Figure 22B shows the corresponding

optical bandgap, it is clearly observed that the bandgap value exceeds correspond-

Figure 22 (A) UV-visible absorption spectra and (B) optical bandgap of CdS nanoparticles prepared at different duration of reaction.

ing bulk value. Figure 23A shows the corresponding photoluminescence (PL)

emission spectra of CdS nanoparticles prepared for different durations. There is

gradual peak shift towards red region (Figure 23B) though not prominent. But

quite appreciable change in PL peaks is observed when samples prepared in 5 and

60 or 140 min are compared. Figure 24 shows the normalized PL emissions of the

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Figure 23 Photoluminescence spectra of CdS nanoparticles (A) prepared at different durations and (B) its expanded PL spectra.

Figure 24 PL emission spectra of CdS prepared for 5 min under different excitation wavelengths.

sample prepared in 5 min under different excitation wavelengths. The emission

peak is observed at 505 nm. This indicates that any wavelength shorter than its

absorption peak can be utilized to get the emission of CdS nanoparticles.

5F. Conclusions

CdSe and CdS nanoparticles have been prepared successfully using wet chemical

method. And their thin films have been successfully deposited over the glass

substrate by chemical bath deposition method. The prepared samples have been

characterized. From the optical absorption study the shift of absorption edge

towards blue region is observed indicating nanocrystallinity. Strong green

150

emission from CdSe nanoparticles is observed. And blue-green emission is

observed from CdS nanoparticles. It is observed that CdSe and CdS nanoparticles

size can be tuned easily using organic capping agent by simple change of reaction

temperature and duration.