Optical band gap energy and urbach tail of CdS:Pb2+ thin films

Abstract

PbS-doped CdS nanomaterials were successfully synthetized using chemical bath. Transmittance measurements were used to estimate the optical band gap energy. Tailing in the band gap was observed and found to obey Urbach rule. The diffraction X-ray (XRD) show that the size of cristallites is in the $\sim$33 nm to 12 nm range. The peaks belonging to primary phase are identified at $2\theta = 26.5^{\circ}$ and $2\theta = 26.00^{\circ}$ corresponding to CdS and PbS respectively. Thus, a shift in maximum intensity peak from $2\theta = 26.4^{\circ}$ to $28.2^{\circ}$ is clear indication of possible transformation of cubic to hexagonal phase. Also peaks at $2\theta = 13.57^{\circ}$, 15.9$^{\circ}$ correspond to lead perchlorate thiourea. The effects on films thickness and substrate doping on the band gap energy and the width on tail were investigated. Increasing doping give rise to a shift in optical absorption edge $\sim$0.4 eV