Research Journal of Recent Sciences ______ ______________________________ ______ ____ ___ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 322 - 325 (201 3 ) Res. J. Recent . Sci. International Science Congress Association 322 Effect of Doping Pb on the Structural and Optical Properties of Nanostructured CdS films Banerjee M. 1 Sharma A. 1,2 and Chongad L. 1,3 1 Nanoscience and Nanotechnology laboratory, School of Physics, Devi Ahilya University, Vigyan Bhawan, Khandwa Road, Indor e, INDIA 2 Shri Dadaji Institute of Technology and Science, Indore Road, Khandwa, INDIA 3 Government Arts and Science College, Ratlam, INDIA Available online at: www.isca.in Received 26 th November 2012, revised 29 th December 201 2 , accepted 23 rd January 201 3 Abstract Pure and Pb doped CdS films were prepared by chemical bath deposition method onto glass substrates at room temperature. The structural and optical properties of the prepared films were characterized using XRD and UV - VIS spectroscopy. X - ray diffraction patterns revealed polycrystalline nature of the films. The particle size of the crystallites was determined from the XRD patterns using Scherrer formula and was found to be 27.31 nm for CdS film. The particle size shows change in case of the Pb doped film. The bandgap for CdS film as determined from the absorption spectrum is found to be 3.82 eV for CdS film, which decreases on doping with Pb. Keywords: Nanoparticles, cadmium sulfide , UV - VIS spectroscopy, XRD . Introduction Thin films of polycrystalline semiconductors are of great scientific interest owing to their applications in various electronic and optoelectronic devices. The technological interest in the devices based on polycrystalline semiconductors is m ainly caused by their low production cost 1 . Cadmimium sulfide (CdS) polycrystalline thin films is a representative of II - VI semiconductor materials with many applications such as large area electronic devices and solar cells. It is a wide direct band gap (2.42 eV) material and has been used as a window material together with several semiconductors such as CdTe, Cu 2 S and CuInSe 2 . The interest in CdS thin films can be attributed to its piezoelectric properties and potential laser applications 2 - 4 . Many techn iques have been used for depositing CdS thin films. These include evaporation, sputtering, photochemical deposition, molecular beam epitaxy (MBE) technique, spray pyrolysis, successive ionic layer adsorption and reaction (SILAR) method 5 - 12 . The chemical ba th deposition (CBD) method appears to be a relatively simple method to prepare a homogenous films with controlled composition 13 . In particular CBD is widely used for a achieving good - quality CdS thin films 14 . The films deposited by this method better photo conductivity and improved morphological properties such as roughness and pinhole density as compared with films processed other techniques 15 . In this paper, chemical bath deposition has been carried out to prepare CdS thin films by using cadmium chloride as a source of cadmium ions (Cd +2 ) and thiourea as a source of sulfur ions (S - 2 ) and ammonia sol ution acts as a buffer solution . The structural and optical properties of CdS thin films as well as Pb doped CdS films were studied and the results described in this paper. Experimental details The CdS films were prepared using solutions of cadmium chloride [CdCl 2 ] thiourea ((NH 2 ) 2 C S ) and ammonia solution. For preparing Pb doped CdS films lead nitrate PbNO 3 was also added to CdCl 2 solution in appropriate amount . The pH of the solutions play major role in reaction. The pH of the bath solution during deposition was maitianed 9 - 10. CdS films have been prepared by chemical bath deposition (CBD) method on glass substrates at room temperature. Cadmium chloride was us ed as a source of Cd +2 and thiourea as a source S - 2 and lead nitrate as a source of Pb +2 . Clean glass substrates were mounted vertically in the solution at room temperature for 10 min and two samples – pure CdS and Pb doped CdS thin films were prepared on glass substrate. After the formations of the films the substrates were removed from the bath and rinsed several times in distilled water to remove any adherent particles and unreacted material. Samples were dried at room temperature. Adequate precaution w as taken to protect the films from dust. The XRD patterns were recorded using D8 Advanced X - ray diffractometer from Bruker. UV - Vis absorption spectra were recorded using Shimadzu 2450 double beams spectrophotometer . All the experiments were performed at room temperature. Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 20 12 ), 322 - 325 (201 3 ) Res.J.Recent.Sci. International Science Congress Association 323 Results and Discussion 3.1 XRD studies: 3.1 XRD studies of pure and Pb doped CdS films : The X - ray diffraction patterns of the CBD - CdS thin films are reproduced in Fig ure 1a. It can be se en that CdS nanostructures are polycrystalline in nature and possess hexagonal structure. CdS can exist in three crystalline phases, hexagonal, cubic (zincblend) and cubic (rock salt). The first two phases have been reported for CdS thin films prepared by CBD method. The crystal structure of CdS films in the present study are in agreement with the observation of earlier researchers. There are many variables which affect the crystal structure of CdS films including the nature of the starting material, the substrate etc. Lee 16 studied the effect of substrat es on the structure and phase of CBD CdS thin films and observed that different substrates led to hexagonal or cubic phase which agrees with our result. The interplanar distance d were calculated using Bragg’saw n  = 2d Sin  where n is the order of di ffraction ,  is the wavelength of the incident x - ray , d is the interplanar distance ,  is the Bragg angle The d - values corresponding to prominent peaks are reproduced in table 1. The interplanar spacing d of the CdS film under study match well with those reported by JCPDS files (JCPDF#892944, 890440 ) . The XRD pattern of Pb doped CdS film. The interplanar distance of the prominent peaks are reproduced in table 1. The shift in the peak positions can be attributed to doping of Pb in the CdS structure. X - ra y diffraction peak profiles as seen in fig ure 1a and 1b are broad which can be attributed to small crystallite size . The crystallite size of the nanoparticles is estimated by using Scherrer’sforma 16 D=k  /  Cos  Where k is constant.  is the wavelengt h of X - Ray used,  is the full width at half maximum (FWHM) . Figure - 1(a) XRD pattern of Pure CdS Thin Film Figure - 1(b) XRD Pattern of Pb doped CdS Thin film Table - 1 XRD data and crystallite size of pure and Pb doped CdS films For Pure CdS S.No . 2  ( degree) d - (nm) present study d - values (nm) Reported a,b Particle size (in nm) 01 27.21 0.327 0.335 (cubic) 02 31.43 0.285 0.291 (cubic) 27.31 03 34.36 0.261 0.245 (hexagonal) For Pb - doped CdS 01 27.60 0.324 0.316 (cubic) 02 28.47 0.315 0.316 (c ubic) 03 29.90 0.298 0.291 (cubic) 29.52 04 30.94 0.29 0.291 (cubic) 05 31.48 0.285 0.291 (cubic) 06 31.76 0.283 0.291 (cubic) 07 32.70 0.282 0.291 (cubic) 08 33.25 0.257 0.291 (cubic) 09 33.89 0.253 0.245 (hexagonal) a : JCPDF#892944 (Hexagonal) b : JCPDF#890440 (Cubic) Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 20 12 ), 322 - 325 (201 3 ) Res.J.Recent.Sci. International Science Congress Association 324 Absorption studies : The absorption spectra of pure CdS and Pb doped CdS thin films were recorded at room temperature and are reproduced in figures 2a and 2b respectively. Absorption edge of CdS thin film shows a clear s hift to the lower wavelength as compared to its bulk counterpart. This blue shift of the absorption edge indicates decrease in the crystallite sizes and confirms the formation of nanostructured CdS film. Similarly, the Pb doped CdS film is also nanostructu red in nature. Figure - 2(a) Absorption spectrum of Pure CdS Thin film Figure - 2(b) Absorption spectrum of Pb doped CdS Thin film Agraphetweenhνvs.(αhν) 2 is plotted and is reproduced in figure 3a. for CdS film and in figure 3b for Pb doped CdS film. Theextrapoationofstraightineportionofthegraphto(αhν) 2 = 0 axis gives the value of the energy band gap of film material s. The band gap of the films has been determined from the plots which are found to be 3.82 eV for pure CdS and 3.5 eV for Pb doped CdS. This is in confirmation with the XRD results, which shows that doping increases the particle size. The decrease in the b andgap as a consequence of the increase in particle size is in conformation with the earlier studies. Figure - 3(a) ( αh  ) 2 vs. h  for pure CdS Thin film Figure - 3(b) (αh  ) 2 vs. h  for Pb doped CdS Thin film Conclusion CdS films and Pb doped CdS films were prepared using Chemical Bath Deposition technique onto glass substrates. The energy gap, and particle size have bee n evaluated, which shows that the particle size of CdS increases in Pb doped film , while the bandgap decreases as a result of doping Pb in the CdS film. Acknowledgement The authors thank Dr. A. Gupta, Director UGC - DAE - CSR, Indore, for giving permission for using XRD facility and Dr. Mukul Gupta for helping us record the XRD patterns. 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