Electronic energy alignment at the PbSe quantum dots/ZnO(101̅0) interface

A number of solar energy conversion strategies depend on exciton dissociation across interfaces between semiconductor quantum dots (QDs) and other electron or hole conducting materials. A critical factor governing exciton dissociation and charge transfer in these systems is the alignment of electronic energy levels across the interface. We probe interfacial electronic energy alignment in a model system, sub-monolayer films of PbSe QDs adsorbed on single crystal ZnO(101̅0) surfaces using ultraviolet photoemission spectroscopy. We establish electronic energy alignment as a function of quantum dot size and surface chemistry. We find that replacing insulating oleic-acid capping molecules on the QDs by the short hydrazine or ethanedithiol molecules results in pinning of the valence band maximum (VBM) of QDs to ZnO substrate states, independent of QD size. This is in contrast to similar measurements on TiO2(110) where the alignment of the PbSe QD VBM to that of the TiO2 substrate depends on QD size. We interpret these findings as indicative of strong electronic coupling of QDs with the ZnO surface but less with the TiO2 surface. Based on the measured energy alignment, we predict that electron injection from the 1se level in photo-excited PbSe QDs to ZnO can occur with small QDs (diameter ϕ = 3.4 nm), but energetically unfavorably for larger dots (ϕ = 6.7 nm). In the latter, hot electrons above the 1se level are necessary for interfacial electron injection.