Photoluminescence of single colloidal quantum dots.

摘要

The photoluminescence of single colloidal quantum dots is studied to understand the underlying mechanism for the universal "blinking" behavior and nonradiative Auger recombination. Particularly, served as the possible origin of "off" state and the simplest species for Auger process, the single negative trion is intensively investigated which is formed by electrochemistry and monitored by single dot spectroscopy at room temperature. In the first part, the photostable alloyed CdSeS/ZnS CQDs is firstly prepared and characterized to study the "blinking" behavior. Compared to the conventional CdSe/ZnS CQDs with the similar size, it showed lengthened negative trion lifetimes, giving rise to the observable single negative trion with no occurrences of "off" state. This directly excludes the validity of conventional charging model in the "blinking" hehavior and provides a possible way to suppress blinking. However, even though the corresponding biexciton lifetime is longer than that of CdSe/ZnS, the Auger process is also very efficient, pointing out the difficulty in controlling the alloyed structures to reduce Auger rate. To further study "blinking", we prepared three CdSe/CdS dots with different core and shell sizes, the "blinking" of which varies from "on/off" binary distribution to "on/grey/off" trimodal distribution, and finally to "grey/off" distribution. Particularly, the "grey" state is confirmed to come from a negative trion. Based on these observations, we put forward our phenomenological model, in which "off" state originates from the surface states oxidized by the photo-excited hole. This is consistent with the elimination of "off" state when the electrochemical potential is reduced to form the negative trion. In the second part, the type-II CdTe/CdSe CQDs are investigated to understand how the electron wavefunction affects the Auger process. By measuring the lifetimes of negative trions, they show a peaked value at an optimum shell thickness, which holds for two sets of samples from different synthetic methods. Combined with theoretical calculation, this phenomenon is further explained by the achievement of zero kinetic momentum in the electron at the particular shell thickness. Finally, some of the single dots within the sample of optimum shell thickness show the negative trion which is as bright as the neutral state. This shows that, for the first time, the complete elimination of Auger process is achieved in a small colloidal quantum dot. This will give us a clearer guidance to design photostable colloidal quantum dots, which can be used in high-efficiency LEDs and lasers. In the third part, the results for multiply-charged quantum dots at low temperature were showed, which would provide a possible way to obtain the pure charged dots under electrochemical control.