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.
137
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).
138
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).
139
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.
140
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.
141
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
145
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
147
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
148
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.
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