International Journal of Advanced Engineering Research and Science (IJAERS) [Vol-1, Issue-5, Oct.- 2014]
ISSN: 2349-6495
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108
Absorption Spectra and Energy Band Gapof
Multilayer ZnS Films
Vishal Kumar Sharma, Mahendra Kumar, Kapil Sirohi
Dept.of Physics, University of Lucknow,Lucknow (U.P.), INDIA
Abstract—In the present investigation, the films of Zinc
Sulphide and Polyaniline have been prepared by vacuum
evaporation methods.The growth and characterization of
single layer and multilayer films have been done. Results
are reported in terms of absorption spectra and energy
band gap of Pani and Pani/ZnS thin films.
Keywords:-ZnS
thin
films,vaccum
evaporation
technique.
I.
INTRODUCTION
The present era of microminiaturization of electronic and
photonic devices to be used in civil and commercial
applications is the center of attention of present day
research. Fast and efficient electronic devices and
photovoltaic devices has been fabricated using elements
alloys compound, in various forms. These forms consists
of single crystal, poly crystal, amorphous bulk, the other
forms are there films prepared by various methods. In this
paper we shall study the multilayer ZnS thin films with
the help of vacuum evaporation technique.
ZnS is the II–VI family semiconductor, has wide band
gap (3.65 eV) at room temperature and large excitation
binding energy 60 meV, ZnS is an attractive
semiconductor material especially in electronic and
optoelectronic application. The dielectric constant of ZnS
(wurtzite structure) is 8.75 at lower frequencies and 3.8 at
higher frequencies. The molecular mass is 81.389 and the
melting temperature is 1450 K.
ZnS was used by Ernest Rutherford and others in the early
years of nuclear physics as a scintillation detector,
because it emits light on excitation by x-rays or electron
beam, making it useful for x-ray screens and cathode ray
tubes.It also exhibits phosphorescence due to impurities
on illumination with blue or ultraviolet light.
Zinc sulfide, with addition of few ppm of suitable
activator, is used as phosphor in many applications, from
cathode ray tubes through x-ray screens to glow in the
dark products.When silver is used as activator, the
resulting color is bright blue, with maximum at 450 nm.
Manganese yields an orange-red color at around 590 nm.
Copper provides long glow time and the familiar glow-in-
the-dark greenish color. Copper doped zinc sulfide
(ZnS+Cu) is also used in electroluminescent panels .
Zinc sulfide is also used as an infrared optical material,
transmitting from visible wavelengths to over 12
micrometres. It can be used planar as an optical window
or shaped into a lens. It is made as microcrystalline sheets
by the synthesis from H
2
S gas and zinc vapor and sold as
FLIR (Forward Looking IR) grade ZnS in a pale milky
yellow visibly opaque form.This material when hot
isostatically pressed (HIPed) can be converted to a water-
clear form known as Cleartran (trademark). Early
commercial forms were marketed as Irtran-2 but this
designation is now obsolete.
II.
SAMPLE PREPARATION OF ZnS
Thin films of ZnS have been prepared by vacuum
deposition technique. For sample preparation Zinc
Sulphide powder of 99.99%. purity was evaporated at
about 115°C from a deep narrow mouthed molybdenum
boat. Deposition was made on to highly cleaned glass
substrate held at 200°C in a vacuum of 10
-5
torr. The
substrate was cleaned in aquaregia washed in distilled
water and isopropyl alcohol (IPA). We have used glass
substrate for the preparation of Zinc Sulphide.
III.
SAMPLE PREPARATION OF POLY
ANILINE
Thin film of polyaniline have been prepared by vacuum
evaporation technique, polyaniline is usually prepared by
redox polymerization of aniline using ammonium
perdisulphate, (NH4)
2
S
2
O
2
as on oxidant. Distilled
aniline (0.02 M) is dissolved in 300 ml of pre-cooled HC1
(l.0M) solution, maintained at 0-50°C. A calculated
amount of ammonium perdisulphate, (0.05M) dissolved
in 200 ml of HCl (1M), pre-coated to 0-50° C, is added to
the above solution. The dark green precipitate (ppt)
resulting from this reaction is washed with HC1 (l.OM)
uptil the green colour disappears. This ppt is further
extracted with terta-hydofuran and NMP (N-Methyl
Pyrolidinone) solution by soxhelf extraction and dried to
yield the emeraldine salt. Emeraldine base can be
obtained by heating the emeraldine salt with ammonia
solution. Simultaneously, separate salt solution is
prepared by dissolving the MX (M=Metal and X=Halide)
in distilled water. The solution is then slowly added to the
International Journal of Advanced Engineering Research and Science (IJAERS) [Vol-1, Issue-5, Oct.- 2014]
ISSN: 2349-6495
Page
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109
precooled polymer solution with constant stirring. The
composite is then dried in an oven, at high temperature, to
get the conducting polymer in the powder form. This
powder is vacuum evaporated on to highly cleaned glass
substrate as well as metallic substrate.
IV.
ABSORPTION SPECTRA AND ENERGY
BAND GAP OF ZNS AND PANI ON ZNS
FILMS
The absorption spectra of these films have been recorded
in the wavelength range of 400 to 1000 nm, at room
temperature. The energy band gap of films was
determined by absorption spectra. To measure the energy
band gap of ZnS we use the Tauc relation for direct band
gap semiconductors, in which a graph between (αhν)
2
vs.
hν is plotted, where α is the absorption coefficient and hν
is the photon energy. The absorption coefficient α is also
proportional to Ln [(R
max
-R
min
) / (R-R
min
)], where
reflectance falls from R
max
to R
min
due to absorption of
photons. Hence we have α in terms of reflectance as Ln
[(R
max
-R
min
) / (R-R
min
)].When we plot a graph between
(αhν)
2
vs. hν, a straight line is obtained. The extrapolation
of this straight line to (αhν)
2
= 0, gives the value of band
gapof the film material. The band gap measurement of
ZnS on glass substrate comes out to be 3.65 eV which is
shown in fig.4.
400
600
800
1000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
(PANI)
A
b
s
o
rb
a
n
c
e
Wavelenth (nm)
Fig.1: UV-VIS-NIR Absorbance spectra of Polyaniline
400
600
800
1000
0.00
0.05
0.10
0.15
0.20
0.25
0.30
(PANI on ZnS)
A
b
s
o
rb
a
n
c
e
Wavelenth (nm)
Fig.2: UV-VIS-NIR Absorbance spectra of PANI on ZnS
Fig. 3: Absorbance spectra of ZnS
Fig.4: Band gap measurement of ZnS thin
film
International Journal of Advanced Engineering Research and Science (IJAERS) [Vol
Fig.5: Band gap measurement of PANI on ZnS
The band gap energy of PANI on ZnS as observed is 2.86
eV which is shown in fig.5. The absorption spectrum of
this sample is given by fig.2. It is observed that band gap
decreases when PANI is deposited on to it. Fig.
shows UV-VIS-NIR absorption spectra for both samples.
For Polyaniline on ZnS, the peak at 304 nm due to the
-
* transition of benzene amine structure. The
absorbance spectrum of pure polyaniline shows two peaks
at 329 nm and 633 nm. The peak at 329 nm due to the
* transition of benzene amine structure. And additional
peak at 633 nm due to a charge- transfer
transition from the highest occupied energy level to the
lowest unoccupied energy level.
V.
RESULTS AND DISCUSSIONS
The absorption spectra analyzed within the range of 300
to 1000 nm region at room temperature to determine the
energy band gaps for single layer and multilayer thin
films. It is observed that absorption spectra
and accurate information. Large variations in band gap
energy of thin films have been observed. So the band gap
technology can be used with modifying the structure of
the Polyaniline thin film by introducing the addition of
new material like CdS and ZnS to reduce the band gap
and different substrate can also be a part in s
phenomena. The band gap measurement of ZnS on glass
substrate comes out to be 3.65 eV while it reduces to 2.86
eV when pani is deposited onto it.
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Journal of Advanced Engineering Research and Science (IJAERS) [Vol
: Band gap measurement of PANI on ZnS
The band gap energy of PANI on ZnS as observed is 2.86
. The absorption spectrum of
. It is observed that band gap
decreases when PANI is deposited on to it. Fig.1. & 3,
NIR absorption spectra for both samples.
on ZnS, the peak at 304 nm due to the
* transition of benzene amine structure. The
absorbance spectrum of pure polyaniline shows two peaks
at 329 nm and 633 nm. The peak at 329 nm due to the
-
* transition of benzene amine structure. And additional
r excitation-like
transition from the highest occupied energy level to the
RESULTS AND DISCUSSIONS
the range of 300
to 1000 nm region at room temperature to determine the
energy band gaps for single layer and multilayer thin
absorption spectra gives the fast
and accurate information. Large variations in band gap
n films have been observed. So the band gap
technology can be used with modifying the structure of
the Polyaniline thin film by introducing the addition of
new material like CdS and ZnS to reduce the band gap
and different substrate can also be a part in such
phenomena. The band gap measurement of ZnS on glass
substrate comes out to be 3.65 eV while it reduces to 2.86
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Journal of Advanced Engineering Research and Science (IJAERS) [Vol-1, Issue-5, Oct.- 2014]
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