(152d) Opening and Tuning of Band Gap by Formation of Nanodiamond Superlattices in Twisted Bilayer Graphene

We report results of first-principles density functional theory (DFT) calculations, which introduce a novel class of carbon nanostructures formed due to the creation of interlayer covalent C-C bonds in twisted bilayer graphene (TBG).  In TBG, the two graphene planes of the bilayer are rotated with respect to each other by a specified twist angle over the range from 0 to 30 degrees.  Interlayer C-C bonding in TBG is the result of hydrogenation of the graphene layers according to certain hydrogenation patterns that generate stable interlayer-bonded configurations.  The fully relaxed hydrogenated interlayer-bonded structures consist of two-dimensional (2D) superlattices of diamond-like nanocrystals embedded within the graphene layers; the superlattices have the same periodicity as that of the Moiré pattern corresponding to the rotational layer stacking in TBG and the 2D diamond nanodomains may resemble the cubic or the hexagonal diamond phase.  The detailed structure of these superlattice configurations is determined by the twist angle, the effects of which we have examined over the range from 0 to about 15 degrees; the twist angle is an important parameter in this study.  Other important parameters in the analysis include the hydrogen coverage and hydrogenation pattern, which determine the number of interlayer C-C bonds formed and, consequently, the size of the embedded diamond nanodomains.

We have calculated the electronic band structure of all of the relaxed atomic configurations we generated and we demonstrate that the formation of interlayer-bonded finite domains causes the opening of a band gap, which depends on the density and spatial distribution of the interlayer C-C bonds.  We have found that the band gap increases monotonically with increasing size of the embedded diamond nanodomain in the unit cell of the superlattice and predicted band gaps as wide as 1.2 eV.  These results suggest that this approach is very promising for opening a tunable band gap in bilayer graphene, which can be controlled precisely in order to tailor graphene’s electronic properties for applications in nanoelectronics.