EFFECT OF THIOUREA CONCENTRATION ON THE STRUCTURAL AND OPTICAL PROPERTIES OF PbS THIN FILMS SYNTHESIZED BY CHEMICAL BATH DEPOSITION METHOD

Abstract

In the present work lead sulfide (PbS) thin films were synthesized on glass substrate by chemical bath deposition method as material employing lead acetate as a source of Pb2+ and thiourea as a source of precursor S2- and sodium hydroxide as complexing agent in the solution of distilled water. The thin film have been deposited at constant bath temperature of 800c and the deposition time was 20 minute for different samples in the various 0.2M thiourea concentrations of 8, 9, 10, 11 and 12 mL respectively. The thin films were characterized by using X – ray diffraction and UV – Vis spectrophotometer. X-ray diffraction analyses revealed that all the films were polycrystalline in nature with the diffraction peaks indices to the face centred cubic structure. For as the deposited thin films, it revealed that PbS nanocrystalline with preferred orientation along the (111) for 8 mL which changes to (200) for 9 – 12 mL in per set of 1 mL for various thiourea concentration due to the change of the total system free energy during the film growth. The structural parameters, such as the lattice constant (a), crystallite size (D), dislocation density (ρ) and micro strain (ε) were evaluated from the XRD spectra. The average crystalline size was calculated using the Debye – Scherer’s formula and found to decrease from 35.71 to 29.64 nm with an increase thiourea concentration in the case of increasing the growth rate, at fast deposition rate and growing species has no enough time to be organized. The dislocation density and micro strain were found to vary inversely with the crystallite size, whereas the lattice constant was found to decrease with a decrease of in crystallite size. The UV – Vis spectroscopy characterization of the sample revealed that the optical band gap of energy was increased from 1.55 to 1.61 eV with increasing thiouera concentration due to the quantum size effect of polycrystalline PbS thin film. Keywords: Chemical bath deposition, Lead sulphide, thin films