The effect of interstitial clusters and vacancies on the STM image of graphite

TL;DR

This study uses a tight-binding Green's function approach to analyze how interstitial clusters and vacancies affect the STM images of graphite, revealing different mechanisms for hillock formation.

Contribution

It demonstrates that both interstitials and vacancies can create hillocks in STM images, with large hillocks only caused by interstitial clusters, advancing understanding of defect imaging in graphite.

Findings

Small hillocks can originate from both interstitials and vacancies.

Large hillocks (>10 Å) are caused only by interstitial clusters.

Surface buckling due to interstitials affects STM images.

Abstract

Making use of the tight-binding Green's function technique, we have calculated the STM images of graphite with surface and sub-surface defects, while taking into account the relaxation of the lattice due to defects. We have demonstrated that two different physical mechanisms may result in the formation of hillocks in the STM images: buckling of the graphite surface due to interstitials between the uppermost graphite layers and the enhancement of the electron density of states close to the Fermi energy on the carbon atoms in the vicinity of vacancies. Our results indicate that small hillocks may originate both from the interstitial clusters and from the vacancies. By contrast, however, large hillocks in excess of 10 \AA~ in diameter can be caused only by interstitial clusters.