Gray State Dynamics in the Blinking of Single Type I Colloidal Quantum Dots

We analyze the photoluminescence intermittency (blinking) of single colloidal CdSe/ZnS quantum dots (QDs). Two distinct emission levels, a bright on-state and a low-intensity gray state, correspond to monoexponential decay times of 58 ns and 4 ns, respectively. The ratio gray/on states increases upon increasing excitation intensity. Conversely, the gray/on level intensity ratio increases upon coupling to a plasmonic nanostructure, while the states maintain their mono-exponential character. Corroborated by data from a CdSeTe/ZnS QD, our results demonstrate that type I QDs can indeed show a gray (rather than completely dark) emission level with a distinct monoexponential decay, a point that is discussed controversially in the literature.