SIZE EFFECTS ON LUMINESCENCE DYNAMICS OF CDS MICROCRYSTALLITES EMBEDDED IN POLYMER-FILMS

We investigated the size dependence of picosecond dynamics of the band-edge emission from CdS microcrystallites. Excitonic superradiance was observed from CdS microcrystallites with larger diameters than 50 angstrom, while microcrystallites smaller than 35 angstrom yield only shallow-trap emission from surface-trapping states. The superradiant decay rate increases with particle diameter, is nearly constant below a threshold temperature, and decreases above that temperature. The threshold temperatures of microcrystallites of 25 and 29 angstrom in radius are 45 and 30 K, respectively, in good agreement with the calculated radial-compression-mode frequencies of 43.2 and 37.2 K, using the elastic-sphere model.