The tunnelling spectra of quasi-free-standing graphene monolayer

TL;DR

This study resolves conflicting reports on the tunnelling spectra of quasi-free-standing graphene monolayers by demonstrating the role of out-of-plane phonons and substrate interactions, and shows how these effects can be controlled at the nanoscale.

Contribution

It provides a unified interpretation of the tunnelling spectra of graphene, highlighting the influence of out-of-plane phonons and substrate effects, and demonstrates nanoscale switching of phonon effects.

Findings

Out-of-plane phonons cause a spectral gap in graphene.

Substrate interactions suppress out-of-plane phonons.

Voltage pulses can switch phonon effects on and off.

Abstract

With considering the great success of scanning tunnelling microscopy (STM) studies of graphene in the past few years, it is quite surprising to notice that there is still a fundamental contradiction about the reported tunnelling spectra of quasi-free-standing graphene monolayer. Many groups observed V-shape spectra with linearly vanishing density-of-state (DOS) at the Dirac point, whereas, the others reported spectra with a gap of 60 meV pinned to the Fermi level in the quasi-free-standing graphene monolayer. Here we systematically studied the two contradicted tunnelling spectra of the quasi-free-standing graphene monolayer on several different substrates and provided a consistent interpretation about the result. The gap in the spectra arises from the out-of-plane phonons in graphene, which mix the Dirac electrons at the Brillouin zone corners with the nearly free-electron states at the zone center. Our experiment indicated that interactions with substrates could effectively suppress effects of the out-of-plane phonons in graphene and enable us to detect only the DOS of the Dirac electrons in the spectra. We also show that it is possible to switch on and off the out-of-plane phonons of graphene at the nanoscale, i.e., the tunnelling spectra show switching between the two distinct features, through voltage pulses applied to the STM tip.