Transition metal substitution for H in graphane: a high-throughput study.

This systematic study of transition metal (TM) substitution for H on graphane TM H C, (TM  =  Sc, Ti, V, Cr, Mn) combines ab initio calculations and cluster expansion to explore a huge variety of structures in more than 20 supercells over the full concentration range from x  =  0 to 1. We find energetically favorable structures at each concentration in supercells not studied before. At low x the lowest-energy structures contain lines and bands of TM atoms. For the larger atoms (Sc, Ti, V) the ordering becomes complex at higher concentrations, and their increased interaction in graphene causes H atoms to detach from the graphene to positions above the TMs. The smaller atoms (Cr, Mn) have much simpler ordering that favors TM atoms all on one side before filling the other side. At full coverage (x  =  1), the TM atoms remain well bound to the graphene, the structure being more stable than a free monolayer by 0.5 to 0.8 eV. The binding energies of TM atoms are strongly enhanced by the binding of H to graphene, with strengths similar to the bulk cohesive energy of Ti.