First-principles investigation of graphene-substrate interactions

Interactions of graphene with 6H-SiC{0001} surfaces are investigated via first principles calculations within the framework of Density Functional Theory. In order to describe the substrate correctly, bare and dipole-corrected atomic relaxations, as well as relaxations of the surfaces where the dangling bonds of the opposite unrelaxed termination are saturated by H are implemented. Convergence behavior of atomic relaxations with respect to slab thickness and the number of relaxed bilayers are investigated. Dipole-corrected computations reveal that 3 and 6-bilayer relaxations yield faster converging results in 6 and 12-bilayer thick slabs, respectively.All approaches regarding graphene-6H-SiC{0001} substrate interactions in the sqrt(3)xsqrt(3)R30-degrees model and taking van der Waals interactions into account lead to the fact that the first graphene layer on the (0001) face is covalently-bonded and exhibit significant buckling. While the Si-C bond length is close to the value in bulk SiC for this buffer layer, the second graphene layer preserves Bernal stacking bond lengths over the first one and exhibits significantly reduced height fluctuations. Free-standing graphene band structure on the substrate is observed in the second layer. Over the (000-1) face, on the other hand, omission of van der Waals inteactions lead to weakly-bonded first graphene layer exhibiting free-standing graphene electronic structure.