Indian Journal of Pure & Applied Physics
Vol. 49, March 2011, pp. 186-189
ZnS nanocomposite formation: Effect of ZnS source concentration ratio
U Baishya & D Sarkar*
Department of Physics, Gauhati University, Guwahati 781 014, Assam, India
*E-mail: [email protected]
Received 18 March 2010; revised 23 December 2010; accepted 6 January 2011
Thin films of ZnS-PVA nanocomposites have been prepared by chemical bath deposition (CBD) method by varying the
concentration ratio of sulphur to zinc sources. The obtained films are characterized by XRD, TEM, UV-visible spectra,
photoluminescence (PL) spectra and current-voltage (I-V) characteristics. The results show strong dependence on source
concentration ratio. There is an increase in particle size and decrease in band gap with increase of S to Zn source ratio. PL
spectra show decrease in intensity and blue shift with the increase in this ratio.
Keywords: Thin films, Chemical synthesis, X-ray diffraction, Luminescence
1 Introduction
There has been considerable interest in the study of
semiconductor nanocrystals over last three decades1.
These materials exhibit unique chemical and physical
properties which are being applied to diverse fields,
such as nonlinear optics2, electronic devices3 and
biotechnology4. Also, considerable progress has been
made towards the synthesis of these materials to suit
particular needs. It is a well known fact that many of
the properties of these materials depend largely on the
size and shape of the nanomaterials. Hu et al.5 have
shown that the size and shape of these nanocrystals
can be tuned by controlling the reaction conditions
such as reaction temperature and concentration of the
precursor materials.
Wide band gap II-VI semiconductors are expected
to be novel materials for optoelectronic devices. Zinc
sulphide (ZnS), which is an important member of this
family, has been extensively investigated6.
Considerable efforts have been made towards the
preparation of ZnS nano in various ways. Most of the
researchers have used equal concentration of Zn2+ and
S2− precursors in the synthesis7. Though there have
been some reports on the variation of reactant
constituent ratio, the works are not substantial.
However, variations of band gap on stoichiometry in
semiconducting nanocrystals have been elaborately
given theoretically8. In the present paper, synthesis of
ZnS nanoparticles dispersed in PVA by the simplest
method of chemical bath deposition (CBD) using
polyvinyl alcohol as capping agent at different molar
ratios of zinc and sulphur sources has been studied.
We report here results of various studies, viz X-ray
diffraction (XRD), transmission electron microscope
(TEM), UV-visible, photoluminescent(PL) and I-V
characteristics. From XRD, TEM and UV-visible
investigations, particle size and band gap of the
nanodespersed particles have been studied. A possible
correlation between these parameters and PL
properties with reactant ratio is sought for.
2 Experimental Details
Materials used in the present synthesis are Zn
(CH3COO)2 (ZnAc) (as zinc source), Na2S (as sulphur
source), polyvinyl alcohol (PVA) (as capping agent),
NH3 (as reducing agent) and deionized water. ZnAc,
Na2S, NH3 and PVA are obtained from E Merck,
Ranbaxy Fine Chemicals Ltd., Merck and LOBA
Chemie, respectively and used without any further
purification. Deposition of ZnS-PVA composite on
glass substrates is done by chemical bath deposition
method at deposition temperature of 363 K as the film
quality and adherence of the film to the substrate are
reported to be the best at this temperature9,10. Using
the same method, we have repeated the procedure for
various S2-/ Zn2+ ratios. This was attained by keeping
the ZnAc concentration constant at 0.005 mol, and
varying the Na2S concentration for four different
values so that their concentration ratio comes out to
be 2.0, 1.5, 1.0 and 0.5 mol/mol, respectively. The
deposited films are dried and set for various
characterizations, viz XRD for crystallinity and
particle size, TEM for morphology and particle size,
UV- visible spectroscopy and PL for optical
properties and I-V characteristics for electrical
transport studies.
BAISHYA & SARKAR: ZnS NANOCOMPOSITE FORMATION
XRD spectra are recorded by Philips X-pert
prodiffraction (Model-PW 1830) with CuKα (0.154
nm) radiation. TEM pictures are taken by JEM
100CXII, JEOL Japan at accelerating voltage of 100
KV, UV-visible spectra are recorded by Hitachi U3210 spectrophotometer, photoluminescence spectra
are recorded by Hitachi F-2500 fluorescence
spectrometer and I-V characteristics are taken for
films deposited on ITO coated glass with the help of a
CV meter (Keithley Model 595) using its in-built
voltage source.
3 Results and Discussion
3.1 XRD study
Figure 1 shows the XRD pattern of the ZnS-PVA
composite thin films deposited on glass substrates.
These patterns show diffraction peaks at 2θ = 28.5°,
47.58° and 56.39° corresponding to the (002), (110)
and (112) planes, respectively for ZnS (JCPDS No.
80-007). This indicates that in the ZnS-PVA
composite films we have basically dispersion of ZnS
nanocrystalline particles in PVA matrix. The particle
sizes of nanocrystalline ZnS is estimated by using
Debye-Sherrer formula11:
T = Kλ/βcosθ
... (1)
where, K is a constant of value 0.9, λ is wavelength of
X-ray used, which is CuKα radiation and is equal to
0.154 nm and β is full width at half maximum
(FWHM) of the diffraction peak corresponding to a
particular crystal plane.
Fig. 1 — XRD patterns of ZnS nanocomposite films for
different Na2S/ZnAc concentrations: (A) 2.0 (B) 1.5 (C) 1.0 and
(D) 0.5 mol/mol
187
The calculated particle size from XRD spectra is
found to decrease gradually (from 4.4 to 1.8 nm) with
the decrease of Na2S/ Zn Ac, molar ratio (from 2.0 to
0.5). Particle sizes obtained by XRD spectra for
various films are given in Table 1.
3.2 TEM study
Figure 2 shows the morphology of the ZnS-PVA
composite thin films. From the micrographs it is clear
that ZnS grains are not uniformly distributed. Also,
the grain sizes are not same in the films. The average
grain size estimated from the histograms of frequency
of occurrence versus particles size is found to be 40
nm for the films with molar ratio 2.0 and it is found to
decrease gradually to 15 nm as this ratio decreases to
0.5 mol/mol. These values are also presented in Table
1. Particle size calculated from XRD and grain size
from TEM suggests smaller grain size formation at
smaller ratio Na2S/ZnAc. It is however to be noted
that the average grain size determined by TEM are
comparatively larger than the particle size measured
by XRD. Also with decrease in Na2S/ZnAc ratio the
TEM pictures show more clear spherical grains.
3.3 UV-visible spectra
The optical properties of the composite films are
investigated by recording the UV-visible transmission
spectra. In Fig. 3(a) we show these spectra.
Absorption co-efficient (α) can be calculated from the
optical transmittance (T) using the relation:
α = 1/d ln (1/T)
... (2)
where d is the thickness of the films which is 180-190
nm in our case. The fundamental absorption which
corresponds to the transition from valence band to
Fig. 2 — HRTEM micrographs of ZnS nanocomposite thin films
for different molar ratios: (A) 2.0, (B) 1.5, (C) 1.0, (D) 0.5
188
INDIAN J PURE & APPL PHYS, VOL 49, MARCH 2011
conduction band can be used to determine the band
gap of the material. The relation between absorption
co-efficient (α) and photon energy (hν) can be written
as 9:
α = A (hν/Eg)n / hν
... (3)
where A is a constant, Eg is the band gap of the
material and n is the exponent which depends on the
type of the transition9 that has occurred. The values of
n for direct allowed, indirect allowed and direct
forbidden transitions are n = 1/2, 2 and 3/2,
respectively. To determine possible transitions, (αhν)2
versus hν can be plotted to give corresponding band
gap by extrapolating the straight line portion of the
graph on the hν axis to α=0. Plots of (αhν)2 versus hν
for various films with varying ratios of Na2S/ ZnAc
are shown in Fig. 3(b). Band gap obtained from these
plots are obviously the direct band gap12. It is found
that the obtained direct band gap values are higher
than that of the bulk ZnS (3.6 eV). This again
indicates that particles present in the composite are
nanoparticles. The enhancement in band gap occurs
due to well established phenomena of quantum
confinement13 in nanocrystallites. Also the band gap
is observed to increase from 3.65 to 3.81eV as the
Na2S/ ZnAc ratio decreases, which leads to decrease
in particle size. Direct band gap values thus obtained
are presented in Table 1 for composite films of
various Na2S/ZnAc ratios.
3.4 Photoluminescence (PL) study
Photoluminescence (PL) spectra of the composite
films at room temperature (300 K), excited at
wavelength 280 nm are shown in Fig. 4. The spectrum
for the highest ratio of Na2S/ ZnAc, i.e. 2.0 mol/mol
shows a weak peak at 396 nm and a very intense peak
at 560 nm. The lower wavelength peak is due to
defect related emission of ZnS with short life time7,
whereas the higher wavelength peak is the
characteristic green emission of ZnS. With decrease
of the molar ratio, both these peaks show increase in
intensity. This may arise due to creation of S2vacancy in the lattice14. Also, the shorter wavelength
peaks show red-shift from 396 to 410 nm with
decrease in the concentration ratios which may be due
Table 1 — Particle size and bandgap with different ratios of Na2S
to Zn Ac
Fig. 3(a) — Transmittance versus wavelength plots of ZnS thin
films for molar ratios A, B, C and D
Fig. 3(b) — Plots of (αhν)2 versus hν ofn anocrystalline ZnS thin
films for ratios A, B, C and D
Na2S/Zn Ac
(mol/mol)
Particle size
from XRD
(nm)
Grain size
from TEM
(nm)
Bandgap
from UV-Visible
(eV)
A) 2.0
B) 1.5
C) 1.0
D) 0.5
4.4
2.3
2.0
1.8
40
33
30
15
3.65
3.70
3.78
3.81
Fig. 4 — PL Spectra ZnS-PVA thin films for four ratios A, B, C
and D
BAISHYA & SARKAR: ZnS NANOCOMPOSITE FORMATION
189
and red shift of one of the PL peaks. I-V
characteristics show non-linear diode like behaviour
which suggests possible application of these
nanocomposites in device fabrication.
Acknowledgement
The authors acknowledge Dr P K Baruah,
Deptartment of Chemistry, Guwahati University,
Guwahati, for UV-visible and PL measurements, Dr S
Karmakar, University Science Instrumentation
Centre, Guwahati University for XRD measurements
and Regional Sophisticated Instrument Centre, NEHU
Shillong for TEM measurements.
Fig. 5 — I-V characteristics for ZnS-PVA film for ratio A
to change in particle size or due to phase change of
ZnS nanocrystal14,15. However, the shift in the higher
wavelength peak is insignificant.
3.5 I-V characteristics
I-V characteristics of the composite film for the
ratio 2.0 are shown in Fig. 5. This shows non linear
behaviour similar somewhat to that of a diode. This
may indicate possibility of application in optical
devices. However, there is not much change in the
nature of I-V plots with decrease in constituent ratio.
4 Conclusions
The effect of change in molar ratio of constituent
concentration on ZnS-PVA nanocomposite formation
has been observed. With decrease in this value, the
particle size is found to decrease with more clear
spherical grains, whereas the band gap for direct
optical transition increases. With decrease in
concentration we observed increase in PL intensity
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