On the STM imaging contrast of graphite:: towards a "true'' atomic resolution

Different phenomena observed in the high-resolution images of graphite by scanning tunneling microscopy (STM) or atomic force microscopy (AFM) such as the asymmetry in the charge density of neighboring carbon atoms in a hexagon, the high corrugation amplitudes and the apparent absence of point defects has led to a controversial discussion since the first published STM images of graphite. Different theoretical concepts and hypotheses have been developed to explain these phenomena. Despite these efforts a generally accepted interpretation is still lacking. In this paper we discuss a possible imaging mechanism based on mechanical considerations. Forces acting between tip and sample are taken into account to explain the image contrast. We present for the first time a direct atomic resolution of the graphite hexagonal structure by transmission electron microscopy (HRTEM), revealing the expected hexagonal array of atoms and the existence of several types of defects. We discuss the possibility that the STM image of graphite is a result of convolution of the electronic properties and the atomic hardness of graphite.