Atom-by-atom growth of CdS thin films by an electrochemical co-deposition method:: Effects of pH on the growth mechanism and structure

Thin films of cadmium sulfide (CdS) were prepared on Au(111) by a new electrochemical co-deposition method. The appropriate electrodeposition potentials based on the underpotential deposition potentials (upd) of Cd and S have been determined by the cyclic voltammetric measurements. The films were grown at room temperature from an aqueous solution of USO4, ethylenediaminetetraacetic acid (EDTA), and Na2S at various pHs. The thin films were characterized by atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray diffraction (XRD), and UV-visible spectroscopy techniques. The Cd/S ratio of electrodeposited thin films was found to be 1: 1 determined by electron dispersive spectroscopy (EDS). XRD, STM, and AFM results revealed that the electrosynthesized thin films deposited at -0.520 V (vs Ag/AgCl) have a highly preferential orientation along (110) directions for hexagonal crystal and (111) directions for cubic crystal in the solutions with pH = 4 and pH = 5, respectively. The phase transition from cubic to hexagonal structure was observed with decreasing pH below 5. UV-visible absorption measurements as a function of deposition time indicated that the band gap of the CdS film increases as the deposition time decreases.