Oxidation of graphene on metals

TL;DR

This study investigates how graphene is oxidized and removed from Ru(0001) and Ir(111) surfaces using low-energy electron microscopy, revealing two distinct etching mechanisms involving carbon monomers and intercalated oxygen.

Contribution

It uncovers two different oxidation mechanisms of graphene on metals, detailing the roles of carbon monomers and intercalated oxygen in the etching process.

Findings

Graphene etching rate depends on carbon monomer concentration.

Intercalated oxygen accelerates etching and creates holes.

Bilayer graphene islands etch simultaneously on Ir.

Abstract

We use low-energy electron microscopy to investigate how graphene is removed from Ru(0001) and Ir(111) by reaction with oxygen. We find two mechanisms on Ru(0001). At short times, oxygen reacts with carbon monomers on the surrounding Ru surface, decreasing their concentration below the equilibrium value. This undersaturation causes a flux of carbon from graphene to the monomer gas. In this initial mechanism, graphene is etched at a rate that is given precisely by the same non-linear dependence on carbon monomer concentration that governs growth. Thus, during both growth and etching, carbon attaches and detaches to graphene as clusters of several carbon atoms. At later times, etching accelerates. We present evidence that this process involves intercalated oxygen, which destabilizes graphene. On Ir, this mechanism creates observable holes. It also occurs mostly quickly near wrinkles in the graphene islands, depends on the orientation of the graphene with respect to the Ir substrate, and, in contrast to the first mechanism, can increase the density of carbon monomers. We also observe that both layers of bilayer graphene islands on Ir etch together, not sequentially.