Geometrical indications of adsorbed hydrogen atoms on graphite producing starlike and ellipsoidal features in scanning tunneling microscopy images

TL;DR

This study uses first-principles simulations to explain star and ellipsoidal features in STM images caused by specific arrangements of hydrogen atoms on graphite, revealing stable configurations.

Contribution

It introduces a new geometrical model for hydrogen adsorption on graphite that explains observed STM patterns and identifies stable hydrogen pair orientations.

Findings

Star-shaped STM patterns are reproduced by the model.

Ellipsoidal images correspond to ortho-hydrogen pairs.

Same-orientation hydrogen pairs are energetically favored.

Abstract

Recent scanning tunneling spectroscopy (STM) experiments display images with star and ellipsoidal like features resulting from unique geometrical arrangements of a few adsorbed hydrogen atoms on graphite. Based on first-principles STM simulations, we propose a new model with three hydrogen atoms adsorbed on the graphene sheet in the shape of an equilateral triangle with a hexagon ring surrounded inside. The model reproduces the experimentally observed starlike STM patterns. Additionally, we confirm that an ortho-hydrogen pair is the configuration corresponding to the ellipsoidal images. These calculations reveal that when the hydrogen pairs are in the same orientation, they are energetically more stable.