Opening and tuning of band gap by the formation of diamond superlattices in twisted bilayer graphene

Abstract

We report results of first-principles density functional theory calculations, which introduce a new class of carbon nanostructures formed due to creation of covalent interlayer C-C bonds in twisted bilayer graphene (TBG). This interlayer bonding becomes possible by hydrogenation of the graphene layers according to certain hydrogenation patterns. The resulting relaxed configurations consist of two-dimensional (2D) superlattices of diamond-like nanocrystals embedded within the graphene layers, with the same periodicity as that of the Moiré pattern corresponding to the rotational layer stacking in TBG. The 2D diamond nanodomains resemble the cubic or the hexagonal diamond phase. The detailed structure of these superlattice configurations is determined by parameters that include the twist angle, ranging from 0 to ~15 o , and the number of interlayer C-C bonds formed per unit cell of the superlattice. We demonstrate that formation of such interlayer-bonded finite domains causes the opening of a band gap in the electronic band structure of TBG, which depends on the density and spatial 2 distribution of interlayer C-C bonds. We have predicted band gaps as wide as 1.2 eV and found that the band gap increases monotonically with increasing size of the embedded diamond nanodomain in the unit cell of the superlattice. Such nanostructure formation constitutes a promising approach for opening a precisely tunable band gap in bilayer graphene.

Observation of the intrinsic bandgap behaviour in as-grown epitaxial twisted graphene

“…Therefore, we conclude that the sp 3 bonds result from interlayer C-C bonding. With this increase in the concentration of sp 3 C-C bonding, we observed the bandgap increased from 35 to 80 meV, which agrees with the monotonic increase of bandgap size with increasing sp 3 C-C bonds density in the predication of Muniz and Maroudas 4,5 . The bandgap size was estimated by the fitting of the temperature-dependent resistivity data with thermal activation and NNH conduction model.…”

“…The even state under sublattice exchange provides the bandgap in TG, while the odd symmetry has two massive graphene band structures. The bandgap opening in TG was also predicted by Muniz and Maroudas 4,5 . They proposed that fullerene superlattices are formed due to interlayer covalent C-C sp 3 bonding and cause the bandgap opening in TG.…”

“…Here, we present evidence that direct growth by carbon source molecular beam epitaxy (CMBE) with C 60 molecular on pre-grown template graphene results in the twisting in graphene layers and the asgrown epitaxial twist graphene exhibits the tunable bandgap opening through the concentration variation of sp 3 interlayer C-C bonds. We previously reported the formation of an intrinsic gap in this stacked graphene 15 , but here we show that the bandgap is most likely due to fullerene superlattice and interlayer sp 3 C-C bonding in TG suggested by Muniz and Maroudas 4,5 .…”

“…T he rotational stacking of graphene layers alters their electronic characteristics resulting in novel and rich physics [1][2][3][4][5][6][7][8] . Bernal (AB)-stacked graphene rotated by 60°b etween adjacent layers is well understood and its electronic properties are radically different from single-layer graphene (SLG).…”

DFT simulations of inter-graphene-layer coupling with rotationally misaligned hBN tunnel barriers in graphene/hBN/graphene tunnel FETs

“…Prior theoretical studies have calculated the electronic structure of rotationally misaligned graphene bilayers using a continuum approximation 16 and a tight-binding method 17 . The opening of a band gap in the electronic structure of rotationally misaligned bilayer graphene has been demonstrated due to the formation of interlayer-bonded diamond nanodomains 18 . Variations in the electronic structure of graphene deposited on rotationally misaligned hBN even have been considered in a study of hydrogen desorption 19 .…”