AB initio studies of the adsorption of atoms and molecules on graphene

Full text:In this talk, I will give an overview of the ab initio studies that we have performed on the adsorption of atoms and molecules on graphene. Examples of physisorption as well as chemisorption are discussed. The calculations show that physisorption of molecules leads to charge transfers to graphene, in agreement with experimental measurements. We found that adsorption of nonmagnetic molecules causes small charge transfers, while paramagnetic molecules may cause large charge transfers. An explanation based on molecular orbital theory will be discussed. In chemisorption, the adsorbates form covalent bonds with the graphene layers. Two graphene derivatives, namely graphane and fluorographene, were investigated. Fluorographene was found to be much more stable than graphane which is mainly due to a much higher desorption energy for F2 as compared to H2. It was also demonstrated that there are structural and electronic differences that are caused by the charged state of the F atoms in fluorographene. Comparison of the results to available experimental data for fluorographene revealed some marked discrepancies: for all the configurations studied, much larger band gaps in the electronic band structure are found and also the Young's modules is calculated to be much larger. In our opinion, this indicates that the experimental samples still contain appreciable amounts of defects. Next we discuss the electronic and magnetic properties of superlattices of graphene/graphane nanoribbons. We found that these properties strongly depend on the degree of hydrogenation at the interfaces between the two materials: ferromagnetic, anti-ferromagnetic as well as a half metal can be realized depending on the edge hydrogenation. Finally, we predict the existence of a new graphene derivative, namely the hydrogenated graphene bilayer. We show that 1) it is possible to hydrogenate the bilayer to a limit of 50 at. % through the adsorption of hydrogen atoms on both sides of the bilayer and on a single sublattice per layer, 2) in this case interlayer chemical bonds are formed which stabilize the structure, and 3) when the hydrogenation reaches its limit of 50 at. %, a bilayer analogue of graphane is formed which has qualitatively similar electronic properties as graphane. (authors)