Abstract:
We investigate all hydrogen configurations that exist in a 1 1 unit cell of bilayer graphene at 100% coverage
to find the low energy competing configurations using density functional theory (DFT). Other
unique configurations, obtained from a 2 1 supercell, are also investigated. The GGA-PBE functional
and four variants of non-local van der Waals density functionals namely, vdW-DF, vdW-DF2, vdW-DFC09x,
and vdW-DF2-C09x are used to account for the exchange correlation effects. Ten unique hydrogen
configurations are identified for 1 1 unit cell bilayer graphene, and nine of these structures are found to
be energetically stable with three low energy competing configurations. One arrangement found to exist
in both 1 1 and 2 1 cell sizes is the most energetically stable configuration of all considered. For some
of the configurations identified from the 2 1 supercell, it is found that the effect of hydrogenation
results in greatly distorted hexagonal layers resulting in unequal bond distances between the carbon
atoms. Also, interaction between the hydrogen-decorated planes greatly affects the energetics of the
structures. The vdW-DF-C09x functional is found to predict the shortest interlayer distances for all the
configurations, whereas the GGA-PBE functional predicts the largest. For the most energetically favorable
configuration, hydrogenation is found to reduce the elastic properties compared with pristine bilayer
graphene.
