The Dynamics of Reversible Hole Trapping in InP/ZnSe/ZnS Quantum Dots

The presence of impurities or defects in most quantum dots leads to irreversible carrier trapping. However, high quality InP/ZnSe/ZnS quantum dots (QDs) behave very differently. In this case, excess indium in the ZnSe shell of gives rise to hole traps that are transiently populated following photoexcitation. This leads to a slow (hundreds of picoseconds to nanoseconds) photoluminescence (PL) risetime.^[1] Equilibrium between these traps, which are optically dark, and the emissive valence-band state also results in delayed emission and therefore longer PL lifetimes. The radiative lifetime can be varied from 32 to 48 ns, depending on the density of traps. Transiently trapped holes also affect the biexciton kinetics, allowing for variation of the electron versus hole Auger excitation ratios under high flux. In addition, the amplitude and time scale of the slow PL rise and the delayed emission time can be varied by different treatments of the InP cores prior to shell deposition, as indicated by resonance Raman spectroscopy. The Raman spectra are interpreted in terms of different radial distributions of the indium-based hole traps which can be related to differences in the interfacial lattice strain. Modulating the core/shell interface can also create interfacial dipoles varying the trap depth, in extreme cases leading to 10% of the photoluminescence coming from repopulation of the band edge by deep traps that occurs on the microsecond timescale.^[2]<br/>Several possible chemical and structural assignments of the traps are considered, and a substitutional indium adjacent to a zinc vacancy, In³⁺/VZn^2- , is found to be the most likely.^[3] This assignment is consistent with the observation that trapping occurs only when the QD has excess indium and is supported by experiments showing that the addition of zinc oleate or acetate decreases the extent of trapping, presumably by filling some of the vacancy traps. We also show that addition of alkyl carboxylic acids causes increased trapping, presumably by creation of additional zinc vacancies.<br/>This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Award Number DE-EE0009164 and NSF CHE-1506803.<br/><br/><br/>[1] P. Cavanaugh et al., Radiative Dynamics and Delayed Emission in Pure and Doped InP/ZnSe/ZnS Quantum Dots. J. Chem. Phys. 2021, 155, 224705 - 224715.<br/>[2] H. Sun et al. Reversible Interfacial Charge Transfer and Delayed Emission in InP/ZnSe/ZnS Quantum Dots with Hexadecanethiol. J. Phys. Chem. 2022, in press.<br/>[3] A.M. Kelley et al., Identity of the reversible hole traps in InP/ZnSe core/shell quantum dots. J. Chem. Phys. 2022, in press.