Evidence for the role of holes in blinking: negative and oxidized CdSe/CdS dots

Thin shell CdSe/CdS colloidal quantum dots with a small 3 nm core diameter exhibit typical blinking and a binary PL intensity distribution. Electrochemical charging with one electron suppresses the blinking. With a larger core of 5 nm, the blinking statistics of on and off states is identical to that of a smaller core but the dots also display a grey state with a finite duration time (~6 ms) on glass. However, the grey state disappears on the electron-accepting ZnO nanocrystals film. In addition, the grey state PL lifetime on glass is similar to the trion lifetime measured from electrochemically charged dots. Therefore, the grey state is assigned to the photocharged negative dots. It is concluded that a grey state is always present as the dots get negatively photocharged even though it might not be observed due to the brightness of the trion and/or the duration time of the negative charge. With thick shell CdSe/CdS dots under electrochemical control, multiple charging, up to four electrons per dot, is observed as sequential changes in the photoluminescence lifetime which can be described by the Nernst equation. The small potential increment confirms the weak electron confinement with the thick CdS shell. Finally, the mechanism of hole-trapping and surface oxidation by the hole is proposed to account for the grey state and off state in the blinking.