structural, morphological and optical studies of ... · substrates by using a new facile solution...

© 2019 JETIR January 2019, Volume 6, Issue 1 www.jetir.org (ISSN-2349-5162)

JETIR1901993 Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org 669

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




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)




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




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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Fig. 5 (a) Plot between transmission with wavelength of

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Fig. 5 (b) Plot between (αhυ)2 vs. photon energy (hυ) of

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