Many fundamental properties of colloidal semiconductors quantum dots, particularly the photoluminescence quantum yield and related phenomena such as blinking, are highly sensitive to the surface structure. Previous studies of quantum dot photophysics suggest a variety of surface traps with different chemical origins lead to distinct dynamic behavior, affecting the excited state lifetime, blinking, or both. Moreover, fluorescence quantum yield is not always correlated to the measured total lifetime, due to the presence of an “OFF” (or “dark”) state that is not reflected in the recombination dynamics of the “ON” (or “bright”) state. The origins of nanocrystal blinking remain uncertain and, while the correct model is still under debate, distinct types of trap states may warrant different models of blinking. Here, we examine a particular type of surface trap by quantifying the effect of metal enrichment (relative fraction of surface Cd to surface Se) on the trapping of excitons in CdSe quantum dots. Displacement of surface bound cadmium carboxylate exposes selenium atoms and dramatically reduces the overall photoluminescence quantum yield of the quantum dot. Transient absorption and time resolved photoluminescence decay kinetics demonstrate that the rate of hole trapping increases as surface selenium atoms are exposed. A nonlinear relationship between the exciton decay rate and the fraction of surface selenium atoms is observed that demonstrates a heterogenous dependence of the hole trapping kinetics on the ligand coverage. The observed decrease in the PLQY cannot be accounted for by the change in photoluminescence decay kinetics alone and indicates that the fraction of nanocrystals in the “OFF” state increases in the stoichiometric nanocrystals.

Photophysics in metal enriched and stoichiometric quantum dots

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.