Quantum size effects in optical properties of CdS-glass composites

The optical properties of extremely small isolated semiconductors provide a sensitive probe of the developing electronic structure in the materials. Cadmium sulfide and ${\mathrm{CdS}}_{\mathrm{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ crystallites are precipitated in an insulating glass matrix during a secondary heat-treatment procedure. Variation in the heat treatment significantly alters the final crystallite size, enabling the examination of a size-dependent change in the observed optical-absorption edge and exciton-related photoluminescence peak energies. A diffusion-limited coarsening behavior is exhibited by the CdS crystallites precipitated from a Zn-free base glass, indicating a high degree of purity in these crystals. Growth behavior of mixed crystallites, however, indicates some stoichiometric variation with heat-treatment time. Quantum size effects measured using the CdS precipitates in the size range from 40--400 A\r{} reflect confinement primarily in the translational motion of the Wannier exciton with some slight modification of its internal electron and hole orbits.