Valence band alignment at cadmium selenide quantum dot and zinc oxide (101̄0) interfaces

A key issue governing efficient electron transfer between two semiconductors is interfacial electronic energy alignment. We address this issue in a model system relevant to quantum-dot-sensitized solar cells, cadmium selenide (CdSe) quantum dots adsorbed on a single crystal zinc oxide (ZnO) (101̄0) surface via 3-mercaptopropionic acid linkers, using ultraviolet photoelectron spectroscopy. The valence band maximum (VBM) of the CdSe quantum dots is found to be located at 1.1 ± 0.1 eV above the VBM of ZnO, nearly independent of the size of the quantum dots (2.1-4.2 nm). This finding suggests that, upon adsorption, there is direct electronic interaction between CdSe quantum dots and the ZnO surface involving CdSe valance bands. Such electronic interaction pins the CdSe valence band to the Fermi level. As a result, varying the quantum dot size mainly tunes the alignment of the conduction band minimum of CdSe with respect to that of the ZnO surface.