Mechanism of Hole Trap Passivation in CdSe Quantum Dots by Alkylamines

Abstract

The mechanism by which alkylamines brighten CdSe nanoparticles is investigated by transient absorption and time-resolved photoluminescence spectroscopy. The main effect of the amine is to reduce the extent of surface hole trapping, which cannot be rationalized by a model in which the amine directly binds to and thereby passivates a surface trap site. An alternative mechanism is proposed wherein the absolute energies of the valence and conduction band edges shift to more negative redox potentials upon binding of the amine. This decreases the thermodynamic driving force for hole trapping or hole transfer to an adsorbed acceptor, 4-methylbenzenethiol. Amine binding also increases the thermodynamic driving force for electron transfer to an adsorbed acceptor, methyl viologen. The rates of charge transfer to these acceptors have been measured, and the differences in the rates before and after treatment with alkylamines are consistent with the proposed mechanism.