Optical properties of CdS nanoclusters: effects of size, stoichiometry and alloying with Ag2S

The optical absorption and photoluminescence (PL) properties of nanosize CdS clusters synthesized by chemical bath deposition technique using precursor chemicals of high purity CdCl2, thiourea and NH4Cl are presented. The crystallite sizes were controlled by the reaction rate, concentration of the reactants of the chemical bath and thickness of the film. Relative to bulk crystals, the band gap (Eg∼2.5 eV) of CdS clusters is significantly blue-shifted with decreasing cluster size. CdS nanoclusters present a mixed hexagonal/cubic structure, which indicates that CdS formation occurs primarily via “ion-to-ion” process. Scanning electron microscopic studies of CdS films revealed that the films are composed of domains, which are formed from the coalescence of smaller crystallites. The PL excitation band is interpreted as an excitation of CdS molecular levels in the interior of the cluster. Results on Ag2S alloying with the CdS nanoclusters show that surface related states of one material can be excited through states of a different interior material.