Theoretical scanning tunneling microscopy and atomic force microscopy study of graphite including tip-surface interaction

Abstract

The graphite surface, due to its unusual electronic structure, offers challenges and opportunities for scanning tunneling microscopy (STM) and atomic force microscopy (AFM). To draw a measurable current at small voltages (1 nA at 50 mV) the tip has to approach close to the surface; estimated tip-to-surface distance is ∽ 2 A. At such distances repulsive interatomic forces between the tip and surface set in (the basis of AFM) and it is important to consider the tip-surface interaction. Therefore, we have calculated interatomic forces and charge densities including the tip (atom) as an integral part of the system rather than treating it perturbatively. Our calculations reveal that the tip at close proximity to the surface disturbs the states near the Fermi level, and induces localized states which in turn influence the STM images. The tunneling barrier appears to collapse at small tip-to-surface distances. Some experimental evidence for this effect is cited. The repulsive force on the tip has a value in the range of 10-9 N when the tip-surface separation is ~2.5 A. The identification of the two inequivalent atoms by AFM is examined. It is suggested that AFM and STM are sensitive to different types of sites on the graphite surface. The effect of multiatomic tip and the tip induced relaxation on the AFM images is briefly discussed. © 1988, American Vacuum Society. All rights reserved.