The structural response of graphene on copper to surface- and interfacial-oxygen

Graphene deposited by Chemical Vapour Deposition on different copper substrates was characterised using Near Edge X-ray Absorption Fine Structure and X-ray Photoelectron spectroscopies. Pronounced angular dependencies of the carbon π*- and σ*-resonances confirmed successful synthesis. Annealing removed surface adsorbates and enhanced the anisotropy of the carbon-bonds, consistent with an observed increase of the carbon 1s binding energy. Most oxygen intercalated at the substrate interface was thus removed, resulting in direct contact between graphene and copper. Oxygen remaining at the interface is bound to carbon. Such bonds are common if graphene on copper is exposed to air for long periods before annealing. The bonds give rise to an X-ray absorption peak at 288.3 eV. When annealed graphene rests in vacuum a deterioration of its structural anisotropy occurs. This is driven by the surface-adsorption of oxygen-containing groups. Renewed annealing reverses both effects, which subsequently reoccur. The correlation of anisotropy deterioration and adsorption suggests that by accommodating functional groups at the surface, the hybridization of carbon atoms and the graphene sheet topography are modified. These structural changes occur relatively quickly, so that even in vacuum the graphene sheet only briefly sustains a pristine structure.