Evidence for a thermal contribution to emission intermittency in single CdSe/CdS core/shell nanocrystals

The on–off intermittent behavior of emission from single CdSe/CdS core/shell nanocrystals was investigated as a function of temperature and excitation intensity. Off times were found to be independent of excitation power and the temperature dependence reveals substantial reduction in the number of on–off cycles prior to final particle darkening at low temperatures. On times are found to vary linearly with excitation intensity over a broad range and the turn off rate shows activated Arrhenius behavior down to $\text{T=50\hspace{0.17em}}\mathrm{K}.$ These observations are consistent with a darkening mechanism that is a combination of Auger photoionization and thermal trapping of charge. The inhomogeneity of various possible trap sites is discussed. A thermally activated neutralization process is required for the particle to return to the on state. The influence of shell composition on intermittency is compared for CdS and ZnS [M. Nirmal et al., Nature 383, 802 (1996)].