Ortho and Para Hydrogen Dimers on G/SiC(0001): Combined STM and DFT Study

The hydrogen (H) dimer structures formed upon room-temperature H adsorption on single layer graphene (SLG) grown on SiC(0001) are addressed using a combined theoretical–experimental approach. Our study includes density functional theory (DFT) calculations for the full (6√3 × 6√3)R30° unit cell of the SLG/SiC(0001) substrate and atomically resolved scanning tunneling microscopy images determining simultaneously the graphene lattice and the internal structure of the H adsorbates. We show that H atoms normally group in chemisorbed coupled structures of different sizes and orientations. We make an atomic scale determination of the most stable experimental geometries, the small dimers and ellipsoid-shaped features, and we assign them to hydrogen adsorbed in para dimers and ortho dimers configuration, respectively, through comparison with the theory.P.M acknowledges financial support from a R. C. Rodes grant. The research leading to these results has received funding from the European Union Seventh Framework Program under grant agreement no. 604391 Graphene Flagship. We acknowledge financial support by the Spanish project MAT2011-26534. M.S., P.M., and P.J acknowledge the financial support of GACR project no. 14-02079S.Peer reviewe