Abstract: CdS semiconductor nanofilms were grown on ITO glass substrates by means of chemical bath deposition (CBD) technique with Cd(NO_{3)2 as the Cd ions and (NH2)2CS as the S ions source. The concentration of Cd ions, deposition time and posttreatment temperature have impact on CdS nanofilms formation.UVVis absorption spectrum and Atomic Force Microscope (AFM) image indicate that the change of concen tration of Cd ions and posttreatment temperature may adjust the bandgap of CdS to obtain stable uniform and compact hard films. Formation mechanism of the crystal nucleus and CdS film was also discussed. Active sites on the surface of ITO are critical to the formation of crystal nucleus and uniform compact CdS nanofilm. The active site and crystal nucleus are formed due to the comprehensive effect of electricity, thermodynamics and chemistry.}

Key words: CdS nanofilm, chemicl bath deposition, uniform compact, active site, crystal nucleus