Graphene on Insulating Oxide Substrates: Role of Surface Dangling States

TL;DR

This study investigates how different insulating oxide substrates and their surface dangling states affect the electronic properties of monolayer and bilayer graphene, revealing the importance of surface passivation and atomic relaxations.

Contribution

It provides a detailed first-principles analysis of substrate surface effects on graphene's band structure, highlighting the roles of dangling states and surface relaxations.

Findings

Si-terminated quartz retains graphene's linear bands with surface relaxation.

Surface dangling states influence the distortion of graphene's electronic spectrum.

Graphene on substrates exhibits ripples similar to suspended graphene, but weaker.

Abstract

We study the effect of insulating oxide substrates on the energy band structure of monolayer and bilayer graphene using a first principles density functional based electronic structure method and a local exchange correlation approximation. We consider two crystalline substrates, SiO2 (or alpha-quartz) and Al2O3 (alpha-alumina or sapphire), each with two surface terminations. We focus on the role of substrate surface dangling states and their passivation in perturbing the linear energy spectrum of graphene. On non-passivated surface terminations, with the relaxation of top surface layers, only Si-terminated quartz retains the linear band structure of graphene due to relatively large equilibrium separation from the graphene layer whereas the other three surface terminations considerably distort it. However, without relaxations of the top surface layer atoms, linear bands appear in the electronic spectrum but with the Dirac point shifted away from the Fermi level. Interestingly, with a second carbon layer on non-passivated oxygen terminated Quartz, with top surface layers relaxation, graphene features appear in the spectrum but sapphire with both surface terminations shows perturbed features even with two carbon layers. By passivating the surface dangling states with hydrogen atoms and without top layer atomic relaxations, the electron-hole symmetry occurs exactly at the Fermi level. This suggests that surface dangling states play a less important role than the atomic relaxations of the top surface layers in distorting the linear spectrum. In all cases we find that the first layer of graphene forms ripples, much like in suspended graphene, but the strength of rippling is found to be weaker probably due to the presence of the substrate.