(Invited) Luminescent InP Quantum Dots: They're Not Just Like CdSe
Authors: Cavanaugh Paul , Sun Haochen , Jen-La Plante Ilan , Wang Xudong , Bautista Maria , Ippen Christian , Kelley Anne Myers , Kelley David 
Published: August 2024

Time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy have been used to elucidate the electron and hole dynamics in high quality InP/ZnSe/ZnS quantum dots (QDs) at room temperature. These dynamics depend critically on the overall In:P ratio. Stoichiometric QDs (those having an In:P ratio close to unity), have PL quantum yields of > 90% and exhibit dynamics that are very simple: both electron and hole relaxation to the bandedge is fast compared to the ~ 40 ps time-correlated photon-counting instrument response and the PL follows a single exponential decay having a time constant of about 26 ns. However, QDs having an In:P ratio greater than about 1.1 maintain PL quantum yields > 90%, but show much more complicated PL kinetics. In the non-stoichiometric QDs, a significant fraction of the PL exhibits a slower risetime. The PL rises over a range of timescales varying from close to the instrument response to approximately 2.0 ns. The time constant of the slowest component increases with increasing InP core size and the fraction of the PL that has the slow risetime increases with excess indium in the particle. The slow risetime is absent in the TA results, indicating that the slow dynamics may be assigned to relaxation in the valence band. The risetime increases with the thickness of the ZnSe shell, varying from < 40 ps for the thinnest (1.1 nm) to about 2.0 ns for the thickest (2.7 nm) ZnSe shells. In addition to the slow risetime, the QDs having excess indium show a significant long-lived (>100 ns) decay component. The magnitude and time constant of this slow decay also increases with ZnSe shell thickness.
These results suggest that holes are transiently trapped at sites associated with indium dopants in the ZnSe shell. The trapped hole has negligible overlap with the conduction band electron, making this an optically dark state. The holes are in an equilibrium between the shell traps and the valence band, giving rise to the delayed emission. Several possible assignments of the trapping species can be considered, and a substitutional indium adjacent to a zinc vacancy, In3+/VZn
2-, is the most likely. 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. Further experiments show that addition of alkyl carboxylic acids causes increased trapping, presumably by the formation of zinc oleate, thereby creating additional zinc vacancies. Chloride ion is a common impurity in the synthesis of InP/ZnSe/ZnS QDs and the possibility of In3+/Cl- impurities acting as the hole trap can be also considered. However, additional experimental data on InP/ZnSe/ZnS QDs synthesized in the absence of chloride show the same PL risetimes and delayed emission, eliminating this possibility. Density functional theory calculations further corroborate assignment of the trapping species to In3+/VZn
2-. These calculations show that either a single In2+ ion or an In2+-In3+ dimer is much too easily oxidized to form the reversible traps observed experimentally, while In3+ is far too difficult to oxidize. However, a zinc vacancy adjacent to a substitutional indium is calculated to have its highest occupied orbitals about 1 eV above the top of the valence band of bulk ZnSe, in the appropriate energy range to act as reversible traps for quantum confined holes in the InP valence band.