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/shel l 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 o n-off cycles prior to final particle darkening at low temperatures. On time s are found to vary linearly with excitation intensity over a broad range a nd the turn off rate shows activated Arrhenius behavior down to T=50 K. The se observations are consistent with a darkening mechanism that is a combina tion of Auger photoionization and thermal trapping of charge. The inhomogen eity of various possible tray sites is discussed. A thermally activated neu tralization process is required for the particle to return to the on state. The influence of shell composition on intermittency is compared for CdS an d ZnS [M. Nirmal et al., Nature 383, 802 (1996)]. (C) 1999 American Institu te of Physics. [S0021-9606(99)705101-5].