▎ 摘　　要

We investigate all hydrogen configurations that exist in a 1 x 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 x 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-DF-C09(x), and vdW-DF2-C09(x) are used to account for the exchange correlation effects. Ten unique hydrogen configurations are identified for 1 x 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 x 1 and 2 x 1 cell sizes is the most energetically stable configuration of all considered. For some of the configurations identified from the 2 x 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-C09(x) 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. (c) 2013 Elsevier B.V. All rights reserved.