INVESTIGATION OF OPTICAL PROPERTIES OF TIO2/CDS/ PBS MULTILAYERED THIN FILM

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Volume-3 | Issue-12 | Jan-2014Available online at www.isrj.net

INVESTIGATION OF OPTICAL PROPERTIES OF TIO /CDS/2

PBS MULTILAYERED THIN FILM

Abstract:-TiO /CdS/PbS multilayer films have been prepared on glass substrates. SILAR method was 2

used to deposit TiO on glass substrate. Chemical Bath Deposition (CBD) method was employed to 2

deposit two successive layers of CdS and PbS on pre deposited TiO film. The optical properties of 2

TiO /CdS/PbS multilayered thin films have been investigated. The optical constants such as absorption 2

coefficient á, refractive index n, extinction coefficient k and optical conductivity ó of the deposited films were obtained from the analysis of experimentally recorded absorption spectral data. The optical constants of TiO , TiO /CdS and TiO /CdS/PbS thin films were calculated and their comparisons are 2 2 2

2presented. The optical band gap of films is calculated from (áhõ) vs photon energy curve. The band gap varies from 3.37 eV to 1.66 eV. From calculated constant it can be seen that there remarkable change in optical properties. The X ray diffraction pattern of the films was taken to study the crystallinity. The surface morphology was studied by scanning electron microscopy.

Keywords: Chemical bath deposition, Optical absorption, Absorption coefficient, extinction coefficient, refractive index, optical conductivity.

INTRODUCTION

TiO is one of the most extensively studied oxides because of its remarkable optical and electronic properties. CdS 2

and PbS are semiconductors with good optoelectronic properties, which make them suitable for the fabrication of a great variety of devices such as solar cells, infrared detectors, gas-sensors, etc. Cadmium Sulfide (CdS) is a wide gap semiconductor with bulk bandgap energy of 2.41 eV [1,2]. Lead Sulphide (PbS) is another interesting material with a bulk bandgap of 0.41 eV [3,4]. The purpose of deposition of CdS and PbS thin layer on TiO is to enhance the optical absorption of the film in visible 2

light region.In order to design optoelectronic devices based on a specific film, it is necessary to calculate and analyze the optical

constants like refractive index n, absorption coefficient á, optical band gap Eg, etc. All these optical parameters along with some others can be calculated solely from the absorption spectra of films.

EXPERIMENTAL

1. Material

Titanium tetrachloride was purchased from Sigma Aldrich. Thiourea, NaOH, CdCl , NH and Pb(NO) were 2 3 3

purchased from Loba chemicals. All the chemicals were of analytical grade. They were used as received.

2. Method

TiO /CdS/PbS coated thin film was prepared by chemical bath deposition method. Initially TiO was coated on glass 2 2

substrate by SILAR method. The successive layer of CdS and PbS were deposited on it by Chemical bath deposition method. The deposition of TiO thin film was carried out using SILAR method at room temperature. The 0.1M TiCl :15%HCl 2 3

solution was used as a cationic and 0.01M NaOH solution was used as an anionic precursor. One SILAR cycle consists of four

Laxmi J. Tomar , Rahul K. Desai , Piyush J. Bhatt and B. S. Chakrabarty , “INVESTIGATION OF OPTICAL PROPERTIES OF

TIO /CDS/PBS MULTILAYERED THIN FILM ” Indian Streams Research Journal | Volume 3 | Issue 12 | Jan 2014 | Online & Print2

Laxmi J. Tomar , Rahul K. Desai , Piyush J. Bhatt and B. S. Chakrabarty

Applied physics Department, Faculty of Technology and Engineering. M. S. University of Baroda, Vadodara, India

1

.

steps: (1) adsorption of titanium ions (2) rinsing with distilled water to remove excess adsorbed or loosely bound titanium ions (3) reaction with NaOH precursor solution to form stable TiO , and (4) rinsing with purified water to remove excess or un 2

reacted ions. In order to remove hydroxide from deposited film and to improve the crystallinity, the films were heat treated at 450°C for 2h.

The deposition of CdS was carried out by chemical bath deposition. A mixture of 0.12 M CdCl and 0.3 M thiourea 2

solutions was made and 2 M ammonia solution was slowly added to it. The deposited TiO2 film was immersed vertically in the solution. The solution was stirred for 1h at 75°C.

PbS was also coated on this film by chemical bath deposition. A mixture of 0.01 M Pb(NO) and 0.146 M NaOH 3

solution was made and 0.057 M thiourea was added to it. TiO /CdS coated film was immersed in this solution. The solution was 2

stirred for 30 min at 40 °C.

RESULTS AND DISCUSSIONS

The as prepared films were analyzed by a Bruker D8 Advance X- ray Diffractometer to examine its composition. The morphologies of the sample were analyzed by scanning electron microscopy on a machine of JEOL make MODEL JSM 5810 LV. Optical properties of thin films were studied with the help of absorption spectra in the UV–visible region. The spectra were recorded using a double beam UV-visible spectrophotometer - Evolution 600 of Thermofisher make in the wavelength range 200–900 nm.

XRD Analysis

Figure 1 shows the XRD pattern of TiO , TiO /CdS, and TiO /CdS/PbS multilayered thin films. The broad X-ray 2 2 2

diffraction pattern of TiO coated film indicates that the film is amorphous in nature. Two humps at 2è values of 22.97° and 2

42.43° are observed. From the figure it can be observed that successive deposition of CdS and PbS layers increase the

crystallinity. After deposition of CdS on TiO , few peaks appear. The presence of TiO and CdS phases has been confirmed from 2 2

the XRD data. Peaks located at 2è values of 24.72° and 47.96° are the characteristic peaks of Anatase TiO . Peaks at 26.36°, 2

43.62° and 51.75° correspond to CdS. These peaks become sharper and intense after deposition of PbS on the previous layer.

After deposition of PbS, strong peaks at 25.66°, 29.75° and 42.73° are observed which correspond to the cubic phase of PbS.

The average crystallite sizes of TiO /CdS and TiO /CdS/PbS films were calculated by Scherrer equation and were 2 2

found to be 14.54nm and 38.69nm respectively. The lattice strain produced in the film may be estimated approximately using equation [1] and is found to be 0.037 & 0.013 for TiO /CdS and TiO /CdS/PbS films respectively. For TiO /CdS film one 2 2 2

unknown peak at 28.01° has been observed. This peak may be attributed to some intermediate unstable phase. The strain

produced by this phase is highest compared to the strain produced by others phases present in the film which is around 0.059. This strain has broadened the peak and which lead to the smallest crystallite size of 10.55 nm.

Figure 1: XRD pattern of as prepared thin films

Investigation Of Optical Properties Of Tio /cds/Pbs Multilayered Thin Film2

2Indian Streams Research Journal | Volume 3 | Issue 12 | Jan 2014

.

SEM Analysis

Figure 2 : SEM images of (a) TiO , (b) TiO /CdS, (c) TiO /CdS/PbS multilayered thin films2 2 2

Figure 2 shows the scanning electron micrographs of thin films. Figure 2(a) shows the SEM image of TiO coated thin 2

film. From the image it is very clear that the film is not uniform. A bunch of mass can be seen at some places. Figure 2(b) shows the SEM image of deposition of CdS on TiO film. Dark sections of CdS can be seen deposited on the white mass of TiO in the 2 2

background. Figure 2(c) shows the SEM images of TiO /CdS/PbS coated thin film. A smooth and continuous layer can be seen 2

above which thin rod like structure is grown. Bunch of thin rods can be seen at different places on the film. Since the rods are seen only after the deposition of PbS, the rod structures in the morphology can be attributed to PbS.

UV-Visible Spectroscopy

Figure 3 shows the absorption spectra of the as-deposited thin films recorded in the range 200–900 nm. Broad absorbance in the visible range is observed for CdS and PbS coated films. TiO coated film shows the cutoff wavelength of 2

absorbance at 409 nm. Good absorption up to 500 nm is observed for CdS coated thin film. PbS coated thin film shows high optical absorption up to 700 nm. The deposition of CdS and PbS on TiO increases absorbance of TiO in visible region. A clear 2 2

shift of cutoff wavelength towards longer wavelength region can be observed from figure 1. To study the optical properties of prepared films different optical constants were evaluated using absorption spectra.

Figure 3 : Absorption Spectra (absorbance vs. wavelength (nm)) of thin films

The absorption coefficient á of the films was determined from absorbance measurements, using the expression á (õ) =

3Indian Streams Research Journal | Volume 3 | Issue 12 | Jan 2014

(c) (a) (b)

Investigation Of Optical Properties Of Tio /cds/Pbs Multilayered Thin Film2

.

2.303(A/t), where A is absorbance and t is the thickness of the film. The optical band gap of thin film is obtained by establishing 2the relation between (áhõ) and hõ (Tauc's Plot). Extrapolation of the linear part of the line to the hõ axis gives the band gap. The

band gap of TiO , CdS and PbS coated thin films were found to be 3.39 eV, 2. 17 eV and 1.47 eV respectively. The highest 2

bandgap is observed for the TiO coated film and lowest for the PbS coated thin film. 2

The extinction coefficient or absorption index is a measure of the fraction of light lost due to scattering and absorption per unit distance of the penetration medium[5]. It can be estimated from the values of á and ë using the relation K = áë/4ð.

The refractive index of thin films was evaluated by using the relation between refractive index (n) and energy gap (E ), gCn = KE proposed by V. Kumar and J.K Singh, where n = refractive index, Eg = bandgap, K and C are the constants having g

values K = 3.3668 and C = -0.032234.Optical conductivity is one of the powerful tools for studying the electronic states in materials. The optical

conductivity (ó) is given by the relation ó = ánc/4ð, where c is the velocity of light, á is the absorption coefficient and n is the refractive index [6]. The optical conductivity is in direct proportion to the absorption coefficient and the refractive index of the material.

The variation in various parameters of the films is shown in Table 1.

Table 1: Values of different optical constants

TiO has a high band gap. However the bandgap of a film decreases on successive coating of CdS and PbS. The values 2

of other optical constants also change with successive depositions. The values of refractive index, extinction co-efficient, absorption co-efficient and optical conductivity increases with the depositions. On the basis of enhanced optical absorption and optical conductivity, the material can be very useful for solar cell application.

CONCLUSION

The crystallinity of the prepared TiO /CdS/PbS multilayered thin film has been seen to have improved with 2

successive layers of deposition. The optical band gap decreased with increase in deposition layer. The band gap of TiO coated 2

film was found to be 3.39 eV where as the band gap of final TiO /CdS/PbS coated film was found to be 2.11 eV. The refractive 2

index and optical conductivity have increased compared to the individual material. Because of high optical conductivity and low band gap this material has potential for solar cell application.

ACKNOWLEDGEMENTS

The authors would like to acknowledge UGC for sponsoring RFSMS fellowship.

REFERENCE

1.K. Suresh Babu, T. Ranjith Kumar, P. Haridoss, C. Vijayan, Talanta 66, (2005)160-165. 2.D.Sreekantha Reddy, B. Kang , S. Cho Yu, K. R. Gunsekhar, K. Narasimha, A. Divya, P. Sreedhara, J. of Optoelectronics and Adv. Mat.l 9, (2007) 3747-3750.3.M.Navaneethan, K.D.Nisha, S.Ponnusamy and C.Muthamizhchelvan, Rev. Adv. Mater. Sci. 21, (2009) 217-224. 4.E. Pentia, L. Pintilie, I. Matei, T. Botila, E. Ozbaya, J. of Optoelectronics and Adv. Mat. 3, 525-530 (2001).5.A R Forouhi and I Bloomer, Phys. Rev. B38, 1865 (1988)6.P Sharma and S C Katyal, J. Phys. D: Appl. Phys. 40, 2115 (2007)

4Indian Streams Research Journal | Volume 3 | Issue 12 | Jan 2014

Sample Bandgap eV

Refractive Index

Absorption coefficient

cm-1

Extinction coefficient

Optical conductivity

(s-1) TiO2 3.39 2.27 58.90 1.72× 10-4 0.32 × 1012

TiO2/CdS 2.58 2.48 198.75 7.61 × 10-4 1.18 × 1012 TiO2/CdS/PbS 2.11 2.65 419.27 19.6 × 10-4 2.65 × 1012

Investigation Of Optical Properties Of Tio /cds/Pbs Multilayered Thin Film2

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