Scanning tunneling microscopy and spectroscopy of the electronic local density of states of graphite surfaces near monoatomic step edges

TL;DR

This study uses STM and STS to investigate the local electronic density of states at graphite edges, revealing edge-specific states and coexistence of structural superstructures, supported by theoretical calculations.

Contribution

It provides direct experimental evidence of edge-specific electronic states at graphite edges, confirming theoretical predictions and analyzing structural coexistence at the atomic scale.

Findings

Edge states are observed only at zigzag edges.

Coexistence of superstructures extends 3-4 nm from edges.

Theoretical calculations support experimental edge state observations.

Abstract

We measured the electronic local density of states (LDOS) of graphite surfaces near monoatomic step edges, which consist of either the zigzag or armchair edge, with the scanning tunneling microscopy (STM) and spectroscopy (STS) techniques. The STM data reveal that the $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ and honeycomb superstructures coexist over a length scale of 3-4 nm from both the edges. By comparing with density-functional derived nonorthogonal tight-binding calculations, we show that the coexistence is due to a slight admixing of the two types of edges at the graphite surfaces. In the STS measurements, a clear peak in the LDOS at negative bias voltages from -100 to -20 mV was observed near the zigzag edges, while such a peak was not observed near the armchair edges. We concluded that this peak corresponds to the graphite "edge state" theoretically predicted by Fujita \textit{et al.} [J. Phys. Soc. Jpn. {\bf 65}, 1920 (1996)] with a tight-binding model for graphene ribbons. The existence of the edge state only at the zigzag type edge was also confirmed by our first-principles calculations with different edge terminations.