Energy-Level Structure and Band Alignment in InP/ZnSe Core/Shell Quantum Dots

Quantum dots (QDs) feature a sequence of discrete electron and hole energy levels that are often characterized by the envelope symmetry of the electron orbitals. Furthermore, these orbitals exhibit a specific localization in the case of core/shell QDs, extended across the entire QD or restricted to either the core or the shell, depending on the band alignment. Here, we investigate the energy-level alignment in InP/ZnSe core/shell QDs. In agreement with predictions based on k · p calculations, we show that optical transitions in these QDs follow well-defined, mutually exclusive selection rules in 1-photon and 2-photon absorption that can be related to the envelope symmetry of the orbitals involved. In addition, we argue based on a combination of the transient absorption spectrum and the k · p calculations that the lower energy transitions in InP/ZnSe QDs excite electrons from different valence band (VB) levels to the lowest conduction band level. We use this insight to align the InP- and ZnSe-based energy levels and conclude that the VB offset exceeds the natural band offset between InP and ZnSe. Applying this procedure to QDs with decreasing InP core sizes shows a progressive reduction of the offset between the core and shell VB levels. The enhanced tailing of the hole wave function into the ZnSe shell for smaller InP cores can affect the stability of such QDs.