Thermal Quenching Mechanisms in II-VI Semiconductor Nanocrystals

The mechanisms of temperature-dependent nonradiative processes, often referred to as thermal quenching, are studied in CdSe, CdSe/ZnSe, and CdTe nanoparticles. These particles exhibit reversible thermal quenching, the extent of which is strongly dependent on the composition of the surface and nature of the surface ligands. Thermal quenching has dynamic (affecting the luminescence lifetimes) and static (affecting the fraction of particles that are bright versus dark) components. The temperature dependence of quantum yields and time-resolved luminescence decays as well as room temperature transient absorption spectroscopy are used to elucidate the thermal quenching mechanisms. Dynamic thermal quenching is due to thermally activated trapping dynamics that occur on the same time scale as the radiative lifetime. This paper focuses on static thermal quenching and several different mechanisms are considered. It is concluded that the dominant mechanism involves thermal promotion of valence band electrons to empty chalcogenide P orbitals on the particle surfaces. This leaves a hole in the valence band, and subsequent photoexcitation produces a positive trion. The trion undergoes relatively rapid nonradiative Auger relaxation, rendering the particle dark. The differences in the extents of thermal quenching between different surface compositions, different types of particles, and different surface ligands can be understood in terms of the density of empty surface chalcogenide orbitals and the valence band energies.