Spatial Mapping of Sub-Bandgap States Induced by Local Nonstoichiometry in Individual Lead Sulfide Nanocrystals

The properties of photovoltaic devices based on colloidal nanocrystals are strongly affected by localized sub-bandgap states associated with surface imperfections. A correlation between their properties and the atomic-scale structure of chemical imperfections responsible for their appearance must be established to understand the nature of such surface states. Scanning tunneling spectroscopy is used to visualize the manifold of electronic states in annealed ligand-free lead sulfide nanocrystals supported on the Au(111) surface. Delocalized quantum-confined states and localized sub-bandgap states are identified, for the first time, via spatial mapping. Maps of the sub-bandgap states show localization on nonstoichiometric adatoms self-assembled on the nanocrystal surfaces. The present model study sheds light onto the mechanisms of surface state formation that, in a modified form, may be relevant to the more general case of ligand-passivated nanocrystals, where under-coordinated surface atoms exist due to the steric hindrance between passivating ligands attached to the nanocrystal surface.