Approaching ideal graphene: The structure of hydrogen-intercalated graphene on 6H-SiC(0001)

We measure the adsorption height of hydrogen-intercalated quasi-free-standing monolayer graphene on the (0001) face of 6H silicon carbide by the normal incidence x-ray standing wave technique. A density functional calculation for the full ($6\sqrt{3}\times 6\sqrt{3}$)-R30${}^{\circ }$ unit cell, based on a van der Waals corrected exchange correlation functional, finds a purely physisorptive adsorption height in excellent agreement with experiments, a very low buckling of the graphene layer, a very homogeneous electron density at the interface and the lowest known adsorption energy per atom for graphene on any substrate. A structural comparison to other graphenes suggests that hydrogen intercalated graphene on 6H-SiC(0001) approaches ideal graphene.