First-principles calculation and scanning tunneling microscopy study of highly oriented pyrolytic graphite (0001)

Substrate Effects in the Supramolecular Assembly of 1,3,5-Benzene Tricarboxylic Acid on Graphite and Graphene

“…The two carbon atoms in the surface plane basis can be differentiated according to whether there is an atom or hole located directly beneath, with the former being denoted α atoms and the latter, β atoms. Atomic-resolution STM images of HOPG reveal different atomic features depending on the tunnelling conditions, with both tip sharpness 40 and tip-sample distance 41 being cited as important factors, in addition to an augmentation of the effect at low bias voltages. 40,42 In general, atomic-resolution images of HOPG show a concentration of contrast on one of the two atoms in the basis.…”

“…40,42 In general, atomic-resolution images of HOPG show a concentration of contrast on one of the two atoms in the basis. Both first-principles considerations of the localization of Bloch functions 40,42 and DFT 41 suggest that these are the β atoms. This electronic effect is not observed in atomic-resolution atomic force microscopy (AFM), which images both atoms in the basis as nearly equivalent.…”

Forces and Currents in Carbon Nanostructures: Are We Imaging Atoms?

“…The current on top of the carbon atoms deviates from the exponential behavior and saturates [8,32]. Therefore, at distances smaller than 4 Å , the hollow sites become the brighter spots, leading to a contrast inversion that cannot be captured by the standard perturbative approaches considered so far [5,12,13]. Atomic relaxations induced by the tip-sample interaction, not included in our STM calculations, could enhance the top-hollow STM corrugation in the attractive interaction regime favoring honeycomblike patterns [8].…”

“…C atoms, with a nearest neighbor right below in the second layer, do not contribute significantly to the density of states close to the Fermi level, and only the C atoms are imaged as bright spots at low bias voltages. Although a honeycomb pattern should be recovered for larger biases, the experimental evidence accumulated over the past 25 years [9][10][11][12][13] shows that STM images with a hexagonal pattern are overwhelmingly recorded over a broad range of bias voltages and distance operation conditions. Nevertheless, several groups have reported honeycomb patterns even for small bias voltage [13][14][15][16][17].…”

“…Although a honeycomb pattern should be recovered for larger biases, the experimental evidence accumulated over the past 25 years [9][10][11][12][13] shows that STM images with a hexagonal pattern are overwhelmingly recorded over a broad range of bias voltages and distance operation conditions. Nevertheless, several groups have reported honeycomb patterns even for small bias voltage [13][14][15][16][17].The FM-AFM, relying on the forces between the tip and surface, is expected not to be so sensitive to electronic effects and to reflect the real atomic structure of the surface. However, the first reported FM-AFM image on a highly oriented pyrolytic graphite(0001) surface [18] also showed a hexagonal array of bright spots.…”