Contrast stability and "stripe" formation in Scanning Tunnelling Microscopy imaging of highly oriented pyrolytic graphite: The role of STM-tip orientations

TL;DR

This study uses first-principles STM simulations to show how the orientation of the STM tip influences contrast and stripe formation in HOPG imaging, revealing the role of electronic states in contrast variations.

Contribution

It demonstrates the significant impact of STM tip orientation and electronic state contributions on HOPG contrast and stripe formation, supported by simulation and experimental comparisons.

Findings

Tip orientation affects secondary contrast features and stripe formation.

Major $d_{3z^2-r^2}$ state contribution preserves primary contrast.

Enhanced $m e 0$ states alter primary contrast, changing surface appearance.

Abstract

Highly oriented pyrolytic graphite (HOPG) is an important substrate in many technological applications and is routinely used as a standard in Scanning Tunnelling Microscopy (STM) calibration, which makes the accurate interpretation of the HOPG STM contrast of great fundamental and applicative importance. We demonstrate by STM simulations based on electronic structure obtained from first principles that the relative local orientation of the STM-tip apex with respect to the HOPG substrate has a considerable effect on the HOPG STM contrast. Importantly for experimental STM analysis of HOPG, the simulations indicate that local tip-rotations maintaining a major contribution of the $d_{3z^2-r^2}$ tip-apex state to the STM current affect only the secondary features of the HOPG STM contrast resulting in "stripe" formation and leaving the primary contrast unaltered. Conversely, tip-rotations leading to enhanced contributions from $m\ne 0$ tip-apex electronic states can cause a triangular-hexagonal change in the primary contrast. We also report a comparison of two STM simulation models with experiments in terms of bias-voltage-dependent STM topography brightness correlations, and discuss our findings for the HOPG(0001) surface in combination with tungsten tip models of different sharpnesses and terminations.