Reversible Interfacial Charge Transfer and Delayed Emission in InP/ZnSe/ZnS Quantum Dots with Hexadecanethiol

Abstract

The results in this paper show that holes are rapidly and reversibly transferred from red-emitting InP/ZnSe/ZnS quantum dots (QDs) to adsorbed hexadecanethiol (HDT) forming an equilibrium between the thiols and the QD valence band. Photoexcitation results in populations of holes in the valence band and in slightly higher-energy shell-localized traps. Trap to valence band hole tunneling results in a photoluminescence risetime having time constants varying from 300 ps to 2 ns. The presence of adsorbed HDT eliminates the slower risetime component, indicating that hole transfer from the shell-localized traps that are closest to the particle surface efficiently competes with tunneling to the QD core. This shows that the interfacial charge transfer equilibrium is established in less than 2 ns. The population of the shell-localized traps corresponds to a reservoir of hole states that eventually tunnel to the core-localized valence band, resulting in delayed emission. The amount of delayed emission increases rapidly with ZnSe shell thickness and is slightly blue-shifted from the prompt photoluminescence. We propose an energetic model in which the HDT/valence band equilibrium is affected by the extent of valence band quantum confinement and an electric field produced by core−shell interfacial dipoles. This model explains the core size, shell thickness, and photoluminescence (PL) wavelength dependence of this equilibrium.

Angular Momentum Fine Structure in InP/ZnSe Quantum Dots

“…The magnitude of Δ is typically discussed in terms of the shape of the particle, oblate or prolate. Although TEM images indicate that these particles are close to spherical, 18 the large Δ values obtained here indicate that the potential in which the electron and hole move is far from spherical. The large deviation from spherical symmetry probably results not from the morphology of the particles, but from electric fields that are due to different types of core−shell interfaces, as suggested in ref 4.…”