(172g) Formation of Fullerene Superlattices by Interlayer Bonding In Twisted Bilayer Graphene

Graphene, a two-dimensional material consisting of one-atom-thick sheets of sp²-hybridized carbon atoms arranged in a honeycomb lattice, possesses an outstanding and unique set of electronic and mechanical properties, which has potential to enable a broad range of technological applications. Chemical modification of graphene, such as hydrogenation, has been used to properly tailor graphene properties aiming at specific applications.  Exposure of graphitic single- and multi-layered carbon nanomaterials to atomic hydrogen leads to formation of sp³-hybridized C-H and C-C bonds, which alter the materials structure remarkably. In this presentation, based on a systematic computational analysis, we report a class of novel carbon nanostructures formed from rotationally misoriented graphene bilayers (twisted bilayer graphene), upon creation of interlayer covalent C-C bonds. These interlayer C-C bonds are formed due to chemisorption of H atoms onto the outer surfaces of the graphene layers for certain H coverages and hydrogenation patterns. The analysis is based on a combination of first-principles density functional theory (DFT) calculations and classical molecular-dynamics (MD) simulations. The DFT calculations are used for relaxing the atomic structure and determining the electronic band structure of the relaxed atomic configurations, while the MD simulations are based on a realistic reactive bond order potential and are used to relax nanostructures that require computational supercells much larger than those employed in the DFT calculations.

The carbon nanostructures that we have discovered consist of superlattices of caged fullerene-like configurations embedded within the graphene bilayer and are generated from pairs of graphene layers rotated by angles q ~ 30° with respect to each other; in this limit, a zigzag- and an armchair-oriented pair of planes are aligned. We have relaxed the atomic structures and computed the corresponding electronic structures of these materials for both non-commensurate bilayers at q = 30^o and commensurate bilayers at an angle q deviating slightly from the perfect value of 30^o. For certain such structures, we show that the linear dispersion around the K point in the Brillouin zone (Dirac cones), characteristic of single-layer graphene and twisted bilayer graphene, is preserved in spite of the introduction of sp³ bonds due to hydrogenation and interlayer C-C bonding. We also demonstrate that an entire class of similar caged structures of various sizes can be generated in an analogous manner.

Our findings have far reaching implications: 2-dimensional carbon nanostructures can be functionalized by formation of superstructures through embedding of 0-dimensional carbon structures to tailor their electronic properties without deviating from those of single-layer graphene. This embedding of fullerene-like configurations can be accomplished by chemical modification through hydrogenation to form certain patterns of chemisorbed H. We have found these structures to be less stable than their hydrogenated non-bonded counterparts at low pressure; under higher pressure, however, these structures are expected to become thermodynamically favorable. The structures proposed in this study may be related to the intermediate carbon phases observed in laser-induced shock wave loading and high-temperature shock compression experiments on graphitic samples, which were likely to exhibit a turbostratic stacking of planes.