Luminescence and photophysics of CdS semiconductor clusters: The nature of the emitting electronic state.

The nature of the electronic states present in semiconductor clusters made by arrested precipitation is investigated via luminescence spectroscopy. This work reveals different aspects of these states, and their dynamics, than observed in prior optical absorption studies. The optical emission lineshape, and the temperature dependence of the lifetime, indicate the carriers are very strongly coupled to lattice phonons. The pronounced non-Arrhenius temperature dependence suggests that excited state decay occurs by multiphonon (i.e., radiationless) electron tunneling. The temperature dependence can be fit by theoretical multiphonon rate expressions. A range of lifetimes, almost independent of average cluster size, is observed. The shorter lifetimes occur at shorter emission wavelengths, as is characteristic of donor-acceptor pair emission in the bulk crystal, indicating a significant Coulomb interaction between the traps. Luminescence is attributed to a photogenerated, trapped electron tunneling to a pre-existing, trapped hole. The range of tunneling distances is almost independent of cluster size.