  We present first-principles calculations of silicene/graphene and
germanene/graphene bilayers. Various supercell models are constructed in the
calculations in order to reduce the strain of the lattice-mismatched bilayer
systems. Our energetics analysis and electronic structure results suggest that
graphene can be used as a substrate to synthesize monolayer silicene and
germanene. Multiple phases of single crystalline silicene and germanene with
different orientations relative to the substrate could coexist at room
temperature. The weak interaction between the overlayer and the substrate
preserves the low-buckled structure of silicene and germanene, as well as their
linear energy bands. The gap induced by breaking the sublattice symmetry in
silicene on graphene can be up to 57 meV.
