Exafs Analysis of Size-constrained Semiconducting Materials

Semiconducting materials such as CdSe, CdS, PbS and GaP are included in crystalline zeolite Y and mordenite and structurally flexible ethylenemethacrylic acid copolymer solid matrices. EXAFS anal sis reveals formation of while the polymer matrices allow agglomeration of larger semiconducting particles. Zeolite anchored structures are distinctively different to small particles with bulk crystal structure as usually found in colloidal systems. species with dimensions of molecular size up to ca. 13 b: in the crystalline hosts, Small semiconducting clusters of nanometer dimensions represent a new class of materials with hybrid electronic and optical properties in comparison to their bulk and molecular counterparts. Widened electronic band gaps and the appearance of discrete absorption peaks in the electronic spectra of small semiconductor clusters have been interpreted as due to quantum size effects and exciton formation' 1213. Structurally defined semiconductors with homogeneous morphology and narrow size distribution are a prerequisite for understanding the physical origin of these effects. The majority of previous studies have been performed on colloids. For example, small CdS particles of 35 A diameter could be prepared in aqueous solution by reaction of cadmium salts with HzS, but the colloids were found to be unstable against aggregation to 125 A within a day4. Some stabilization is obtained by arrested precipitation in non-aqueous solvents at low temperatures or upon addition of complexing agents (e.g., sodium hexametaphosphate) and stabilizers (e.9. colloidal silicon d i ~ x i d e ) ~ . Thus, even smaller CdS particles of 15 to 25 A diameter could be prepared in acetonitrile. with band gaps between 2.5 and 2.3 eVs. Colorless "Q-CdS"