Variable optical properties and effective porosity of CdSe nanocrystalline films electrodeposited from selenosulfate solutions

CdSe films electrodeposited from a selenosulfate bath are normally made up of dense (nonporous) assemblies of nanocrystals (4-5 nm). At cathodic deposition currents higher than a certain value, the properties of the films change. While the CdSe crystal size remains the same, the apparent bandgap of the films increases by 0.2-0.3 eV, a change visually observable by a change in film color from brown to orange. In contrast to the low-current films, there is considerable strain in the high-current films, as evidenced by the tendency of the films to crack with increasing thickness and either peel off the substrate or roughen. This variation in morphology from smooth to rough films allows control over the effective porosity of the films. Several explanations for the different optical behaviors of the two types of films are suggested, including size quantization modified by overlap interaction between the tightly connected crystals and in situ hydrogen absorption on the crystal surfaces due to simultaneous hydrogen evolution at higher current densities. (c) 2005 The Electrochemical Society. All rights reserved.