Proceedings of MATSUS Spring 2025 Conference (MATSUSSpring25)
Publication date: 16th December 2024
Colloidal quantum dots (QDs) find use in many optoelectronic applications due to their solution processability and size-tuneable properties. A large fraction of these materials has been made using RoHS restricted elements such as Pb, Cd and Hg. The materials in the III-V family, in particular indium-based materials, have been studied as a possible alternative. However, these QDs do not reach similar performance metrics yet in e.g., infrared photodetection or light amplification applications, even after applying advanced surface passivating techniques2,3. Density functional theory calculations on InP and InAs model QDs that feature Cl-passivated (100) and In(111) facets, and unpassivated and unreconstructed P(-111) or As(-111) facets predict the presence of a broad band of occupied orbitals within the energy gap separating the first occupied and first unoccupied delocalized orbital. These orbitals are related to the P(-111) or As(-111) facets, and could act as hole trap states if present in real InP and InAs QDs.
In light of these theoretical predictions, we report on the experimental detection of such in-gap states by means of fs pump-probe optical spectroscopy. The method is based on a sweep of the photon energy of the pump light through the band-edge transition. In the case of a polydisperse ensemble of QDs that do not feature any in-gap orbitals, the resulting bleach of the band-edge transition should simply follow the sweep of the pump photon energy. On the other hand, if the pump photons can excite electrons from occupied surface orbitals to delocalized conduction-band states, a band-edge bleach at photon energies representative for the average band-edge transition in the ensemble will be measured, irrespective of the pump photon energy. While we find that a benchmark dispersion of PbS QDs shows the behavior expected for an ensemble of QDs free of in-gap states, sub-bandgap excitation of InP QDs indeed leads to similar band-edge bleach as obtained through supra-bandgap excitation. This result indicates that the electronic structure of real InP QDs aligns with the predictions of DFT calculations on model QDs. The impact of this observation on next steps in InP QD surface termination are discussed.
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