SUPERRADIANCE QUENCHING BY CONFINED ACOUSTIC PHONONS IN CHEMICALLY PREPARED CDS MICROCRYSTALLITES

We report the luminescence spectrum and picosecond dynamics of excitons in CdS microcrystallites embedded in an acrylonitrile-styrene copolymer film by a new method. The intensity ratio of band-edge emission due to the direct recombination of excitons to red-shifted emission from the trapped states was much larger than any other samples previously reported. This indicates much higher quality of our microcrystallites. From the comparison of luminescence spectrum of samples prepared by different cadmium salts as raw materials, we propose a "site-substitution" model for the origin of the trapped states. According to the model, the trapped states yielding the red-shifted emission are impurities substituting the sulfur sites. The band-edge emission was found to be emitted mainly from the larger microcrystallites among the size distribution. The luminescence lifetime of the band-edge emission was almost constant at 60 ps below a threshold temperature (45 +/- 5 K) and increases nearly proportionally with temperature above this temperature. This temperature dependence of the lifetime suggests the quenching of superradiance of excitons by acoustic phonons confined in the microcrystallites. The threshold temperature corresponds to the frequency of radial compression mode (30.0 cm-1 = 43.2 K) of the spherical microcrystallites.