Scanning tunneling spectroscopy of hybrid semiconductor nanocrystals: Level structure, band offsets and localized states

The ability to tailor the properties of semiconductor nanocrystals through creating core/shell heterostructures is the cornerstone for their diverse application in nanotechnology. The band-offsets between the heterostructure components are determining parameters for their optoelectronic properties, dictating for example the degree of charge-carrier separation and localization. So far, however, direct measurement of these factors in colloidal nanocrystals were not reported and only indirect information could be derived from optical measurements. Here we demonstrate that scanning tunneling spectroscopy (STS) along with theoretical modeling can be used to determine band-offsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type-I band structure where both the hole and electron ground-state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization. We applied our approach also for ZnSe/CdS nanocrystals, portraying their type-II band alignment, leading to electron-hole separation.