Chemical imaging of insulators by STM

Nanostructures of ${\mathrm{CaF}}_2$ and ${\mathrm{CaF}}_1$ on Si(111) are used to demonstrate a chemical imaging method for insulators. Chemical sensitivity is achieved in scanning tunneling microscopy via a sharp drop of the tunneling current for bias voltages below the conduction-band minimum. This imaging method has a spatial resolution of better than 1 nm and distinguishes different oxidation states. A resonance is found in $\mathrm{}(dI/dV)/(I/V)\mathrm{}$ at the conduction-band minimum that enables an accurate determination of its position. We observe enhancements by up to a factor of 5 and absolute values in the range of 20–50, compared to 1 for an Ohmic metal. A minimal model is given, explaining the resonance in terms of tunneling across a thin insulator film. These methods should be generally applicable for determining local Schottky barriers and band offsets in nanostructures and for chemically selective imaging of insulators and wide-gap semiconductors.