Chemical imaging of insulators by STM

Nanostructures of CaF2 and CaF1 on Si(111) are used to demonstrate a chemic al imaging method for insulators. Chemical sensitivity is achieved in scann ing tunneling microscopy via a sharp drop of the tunneling current for bias voltages below the conduction-band minimum. This imaging method has a spat ial resolution of better than 1 nm and distinguishes different oxidation st ates. A resonance is found in (dI/dV)/(I/V) at the conduction-band minimum that enables an accurate determination of its position. We observe enhancem ents by up to a factor of 5 and absolute values in the range of 20-50, comp ared to 1 for an Ohmic metal. A minimal model is given, explaining the reso nance in terms of tunneling across a thin insulator film. These methods sho uld be generally applicable for determining local Schottky barriers and ban d offsets in nanostructures and for chemically selective imaging of insulat ors and wide-gap semiconductors. [S0163-1829(99)07215-X].