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Structural, morphological and optical studies of
nanocrystalline CuO thin films by solution processed
Spin Coating technique
1Mahipal Singh Deora and 2S. K. Sharma
1Research Scholar and 2Professor
Department of Physics, Jai Narain Vyas University, Jodhpur,
India
Abstract: The nanocrystalline Copper oxide (CuO) thin films have
been synthesized on glass substrate by a facile solution processed
spin coating technique and as deposited CuO thin films were
annealed at 150 °C and 500 °C temperature for 1 hour in air.
X-ray
diffraction pattern shows that the films are nanocrystalline
with the monoclinic structure and also present the random
orientation of
crystallites. The crystallite size increases with increasing in
the annealing temperature of CuO thin films. The crystallite
size
corresponding to preferred diffraction peaks (111) were found to
be 30 nm and 62 nm for thin films annealed at 150 °C and 500 °C
temperature. The SEM studies exhibited homogeneous surface
morphological and uniform deposition of thin film on the glass
substrate. The EDAX spectra of thin film annealed at 150 °C
confirmed the presence of copper, oxygen elements in the film.
The
FTIR spectra displayed the formation of Cu-O bonding in thin
films. The optical transmission of CuO thin films in the visible
and
near infrared spectral region was found to be above 80%. The
optical band gap energy decreases with increasing the annealing
temperature. The estimated energy band gap of CuO thin film
annealed at temperature 150 °C and 500 °C were found to be
1.62eV
and 1.43 eV. The influence of annealing temperature on the
physical properties of CuO thin films were examined.
Keywords: Nanocrystalline, Solution-processed Spin coating
technique, Visible and near infrared spectral region.
1. Introduction
In recent years, the direct conversion of solar energy into
electrical energy has become a highly attractive and economical
process for providing clean and renewable energy in worldwide.
Inorganic semiconductor are promising nanomaterials with
different nanostructures such as nanoparticles, nanowires and
nanotubes etc. which display novel electronic and optical
properties
due to quantum confinement effects and unique structural
properties at nanoscale level. Currently the industry standard
transparent
semiconducting oxides (TSO) are ZnO, SnO2, ITO, FTO, Cu2O, CuO
and NiO [1-4]. Among the various metal oxide materials the
cupric oxide (CuO) are promising p-type semiconductors material
for solar cell and photodetector applications.
Copper oxide has many prospective applications in various fields
such as catalysis [5], electrode material in batteries [6],
gas sensors [7], biosensors [8], magnetic storage devices [9],
capacitors [10], optical switch [11] and active material for solar
cell
[12]. It exists in two different crystalline phases, cupric
oxide (CuO) and cuprous oxide (Cu2O). CuO has a monoclinic
structure,
p-type semiconductor and nontoxic important low cost material
with band gap between 1.4-1.9 eV [13]. It has high optical
absorption coefficient in visible and near infrared region due
to its direct band gap [14]. The different morphology of CuO
nanostructure thin films have been prepared by various method
thermal evaporation [15], RF-sputtering [16], pulsed laser
deposition
(PLD) [17], spray pyrolysis, solvothermal [18], hydrothermal
[19], sol gel spin coating [20] and chemical bath deposition
(CBD)
methods [21,22].
Many authors have reported the deposition of CuO thin films by
sol-gel spin coating method. Prabhu et al have reported
the deposition of CuO/ZnO thin films via sol-gel spin coating
technique at 60 °C temperature [20]. Lim et al. have reported
the
deposition of Cu2O and CuO thin films by sol-gel spin coating
for photocatalytic water splitting and solar cell application
[23].
Jundale et al. has demonstrated the preparation of CuO thin
films by facile sol-gel method and its subsequent
optoelectronic
properties [24].
The spin coating deposition technique is sol-gel solution
processed method to deposit thin films in the form of
nanomaterials and nanostructures. It is simple, fast, convenient
technique to produce uniform deposition over large areas at
relatively lower temperature and low cost. In present study, the
nanocrystalline CuO thin films have been deposited on glass
substrates by using a new facile solution processed spin coating
technique and their structural, morphological and optical
properties
were studied.
2. Experimental details
Materials
The chemicals used for the thin film preparation were analytical
grade cupric acetate monohydrate [Cu (CH3COO)2·H2O]
(99.99%) purchased from Alfa Asear. Solvent 2-methoxyethanol
anhydrous [CH3OCH2CH2OH] (99.8%) purchased from Sigma-
Aldrich. Mono-ethanolamine (MEA) [C2H7NO] (98%) and Polyethylene
Glycol 200 (PEG) (98%) purchased from Loba chemie.
Pvt Ltd. Commercially available micro glass slides with
dimension of 25mm x 25mm x 2mm were used as glass substrates.
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Preparation of cupric oxide (CuO) thin films
Copper oxide thin films were prepared by dissolving 0.2 M cupric
acetate in 20 ml of 2-methoxyethanol solvent then 1.0
ml of mono-ethanolamine was added drop wise to achieve a
homogeneous solution. The role of the mono-ethanolamine was to
stabilize the solution and prevent formation of aggregates.
After 15 minutes of magnetic stirring a light blue solution was
formed.
Finally 0.50 ml Polyethylene Glycol 200 (PEG) was added to
enhance film forming properties. The above solution was
magnetic
stirred at 60 °C for an hour after then this solution was aged
for 24 hours. The colour of the solution became dark blue. The
precursor
solution was uniformly deposited on cleaned glass substrates by
spin coating technique at a spin speed of 3000 rpm for 30 sec
and
then dried immediately on hot plate at 150 °C for 10 minutes
after each layer deposition. This coating process was repeated
5-6
times to attain the desirable thickness. Finally samples were
annealed in air at 150 °C and 500 °C temperature for 1hour.
The flow chat of procedure for preparing nanocrystalline CuO
thin film is shown in fig. 1. The cupric oxide thin films
were characterized for structural, morphological and optical
analysis. The crystalline structure of thin film has been studied
by using
X- ray diffractometer ( Bruker AXS, D8 Advanced) with a Cu Kα
(λ=1.5405Å) monochromatic incident radiation source, operating
at 40.0KV. XRD data of all thin film samples were recorded in
detector scan mode with incident X-ray source fixed at the 3°
glazing
incident angle and detector has moved to collect the 2θ
diffraction ranging from 20° to 80° at a step size of 0.02° per
second. The
surface morphology measurements along with elemental composition
analysis were carried out using EVO 18, especial edition,
Carl Zeiss scanning electron microscopy coupled with energy
dispersive X-Ray spectroscopy. The FTIR studies were done by a
BRUKER Vertex 70V Fourier transform infrared spectrometer in the
range of wavelength (400-4000 cm-1). The optical
transmission spectra of the samples were obtained by carry 4000
UV-VIS-NIR Spectrophotometer in the range of 300-1500nm
wavelength at room temperature.
3. Results and discussion
3.1 Structural analysis
The structural determination of all the thin films were carried
out by X- ray diffraction technique and diffraction patterns
were recorded by varying diffraction angle (2θ). Fig. 2 shows X-
ray diffraction patterns of CuO thin films deposited on glass
substrate with annealed at 150 °C and 500 °C temperature. The
major diffracted planes (002), (111) and (220) are located at
35.475°,
39.440° and 68.3° for the CuO thin films. The observed XRD
patterns are matched with standard JCPDS (Joint Committee on
Powder Diffraction Standards) data card 45-0937 for the CuO and
which shows the monoclinic phase of CuO thin film [25]. The
crystallite size increases with increasing in the annealing
temperature of CuO thin films in air. The average crystallite size
for CuO
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thin films were calculated by using Debye Scherer’s formula D
=0.94λ
BCosθ where B is full width half maxima (FWHM) of
diffraction
peak, λ is wavelength of characteristic Cu Kα X-ray incident
radiation. The calculated value of average crystallite size
corresponding
to preferred diffraction peaks (111) were found to be 30nm and
62nm for CuO thin films annealed at 150 °C and 500 °C
temperature
respectively.
3.2 Surface morphology and compositional analysis
The SEM images for the sample of CuO thin films annealed at 150
°C with different magnification are shown in fig. 3 (a-
b). The surface morphology of samples have homogeneous and
uniform deposition of thin film on the glass substrate. It can be
seen
that small particles were grown over the surface of glass
substrate with spherical shaped. The overgrowth could be explained
on the
basis of nucleation and coalescence. The average diameter of
grain about 40 nm. It was also observed that the nanoparticles
may
be converted into bigger clusters due to agglomeration of large
number of nanoparticles. The fig. 3(c) shows EDAX spectra of
nanocrystalline CuO thin film annealed at 150 °C. It confirmed
the presence of copper, oxygen and other elements from the
glass
substrates. The atomic percentage of Cu and O elements were
about 40% and 60% respectively.
Fig. 2 XRD patterns of CuO thin films deposited on glass
substrate
for (a) annealed at 150 °C and (b) annealed at 500 °C
(b)
(c) (c)
Fig. 3(a-b) SEM images and Fig. 3(c) typical EDAX spectra of CuO
thin film
annealed at 150°C
El AN Series C Atom. [at.%]
--------------------- O 8 K-series 60.24
Cu 29 K-series 39.76 --------------------- Total: 100.00
(a)
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3.3 Optical analysis
3.3.1 FTIR Studies
Fourier transform infrared (FTIR) spectroscopy was performed by
studying the adsorption of mono-ethanolamine (TEA)
and Polyethylene Glycol (PEG) as capping agent on the surface of
CuO thin film. Fig. 4 shows the FTIR spectrum of
nanocrystalline
CuO thin film annealed at 150 °C. All the data regarding to the
spectrum is summarized in the Table 1, from which it was
observed
that the most prominent peak at 3744 cm-1 is due to free O–H
stretching of water molecule. The peaks at 3617 & 3676 cm-1 are
due
to free O–H stretching of alcohol. The peaks at 1648 and 1695
cm-1 are due to C=O stretching vibration of carboxylic group.
The
N-H vibration of secondary amine absorbs near 1519 cm-1. The
peaks at 1441, 1423 cm-1 can be explained as –CH2 bending and –
CH2 wagging. There is a small band at 646 cm-1 is due to
stretching of Cu-O bond.
Table 1
FTIR Spectra band position of nanocrystalline CuO thin film with
their assignments
Assignments
Band position of CuO thin film
(in cm-1)
Free O–H stretching of water 3744
Free O–H stretching of alcohol 3676, 3617
C=O stretching vibration of Carboxylic group 1695, 1648
N-H vibration of secondary amine 1519
–CH2 bending 1441
–CH2 wagging 1423
Stretching of Cu–O bond 646
3.3.2 UV-VIS-NIR Spectrophotometer
The transmission measurements of all samples were carried out
from the 300–1500 nm wavelength range at room
temperature. Fig 5 (a) shows the transmission spectra of CuO
thin films annealed at temperature 150 °C and 500 °C. The
transmission of thin film shift towards the longer wavelength as
the annealing temperature increases.
Fig. 4 FTIR spectra of CuO thin film annealed at 150 °C
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The absorption coefficient (α) is calculated from transmission
spectra by using the relation oatI I e . The calculated
values have been used to understand the optical energy bandgap
of CuO thin films by using the Tauc relation1
( ) ( )n
Eh A h g , where α is absorption coefficient, hυ is the photon
energy, A is proportionality constant and Eg is the band gap of the
material. The
value of exponent ‘n’ can take 1/2, 3/2, 2 and 3 depending on
the nature of optical transition corresponding to direct allowed,
direct
forbidden, indirect allowed and indirect forbidden transitions
respectively [26].
The absorbance band of the films were found around the
wavelength 700–800 nm and there was also shifting in absorption
edge towards the shorter wavelength. Fig. 5 (b) shows the plot
between (αhυ)2 versus photon energy (hυ) (Tauc plot) for CuO
thin
films annealed at temperature 150 °C and 500 °C. The intercept
of extrapolated linear region in these plots on energy axis gives
the
values of direct band gap (Eg) of material. The estimated energy
band gap of CuO thin film annealed at temperature 150 °C and
500
°C were found to be 1.62 eV and 1.43 eV. The direct band gap of
CuO thin film was found from Tauc plot. It is observed that the
optical band gap energy decreases with increasing the annealing
temperature of CuO thin film increase. The value of band gap
energy is attributed to the increase in crystallinity of the CuO
phase in thin films.
4. Conclusions
In the present work, the nanocrystalline p-CuO thin films were
prepared successfully by cupric acetate precursor solution
processed Spin Coating technique. The XRD measurement confirms
the nanocrystalline nature and the monoclinic phase of CuO
thin film. SEM studies exhibited uniform surface morphological
observations in all thin films. EDAX spectra confirmed the
presence of copper, oxygen elements in the CuO thin film. The
FTIR studies displayed the formation of Cu-O bonding in thin
films.
The optical studies revealed that films have direct energy band
gap and the estimated energy band gap of CuO thin film annealed
at temperature 150 °C and 500 °C were found to be 1.62 eV and
1.43 eV. The CuO thin films have higher absorbance within the
visible and near infrared range of solar spectrum so it can be
used as an absorber layer in heterojunction solar cell device.
Acknowledgements
The author Mahipal Singh Deora, acknowledges University Grants
Commission (UGC), New Delhi for financial support
under Minor Research project for carrying out this experimental
work. We are thankful to IIT, Jodhpur for providing us
experimental
facilities in the campus. We also wish to thank to Defence Lab,
Jodhpur and USIC, University of Rajasthan, Jaipur for providing
us XRD and SEM facilities respectively.
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