Carbon Nanostructures Generated by Inter-Layer Bonding in Multilayer Graphene and Inter-Shell Bonding in Multi-Walled Carbon Nanotubes

In multi-layered carbon structures, such as graphite and multi-walled carbon nanotubes (MWCNTs), inter-layer or inter-shell C–C bonds can be formed under high temperature and pressure, or due to exposure to ion irradiation or to a hydrogen plasma. In this presentation, we report a comprehensive computational analysis of different nanostructures that can be generated upon formation of inter-shell and inter-layer sp³ C–C bonds in MWCNTs and multilayer graphene (MLG), respectively. The analysis is based on a combination of classical molecular-dynamics (MD) simulations with first-principles density functional theory (DFT) calculations. The MD simulations are used for large-scale structural relaxations in the isothermal-isobaric ensemble. In the MD simulations, the interatomic interactions are described according to the Adaptive Interatomic Reactive Empirical Bond Order (AIREBO) potential. The DFT calculations are used for atomic and electronic structure computations and for determining the materials physical properties. In the DFT calculations, we implement the generalized gradient approximation and employ plane-wave basis sets, ultrasoft pseudopotentials, and supercell models.

We demonstrate that the resulting structures with inter-shell or inter-layer C-C bonds are stable and provide seeds for the nucleation of crystalline carbon phases embedded into the layers of the MWCNT and MLG structures, respectively. Various crystalline phases can be generated, including the well-known cubic and hexagonal diamond phases, as well as new stable phases of carbon. The resulting structures are determined by the relative alignment of adjacent graphene layers/walls in the original multi-layered/walled material. The key parameter that determines the type and size of the generated nanocrystals is the chiral-angle difference between adjacent graphene layers/walls in the original structure. The results of our analysis generate experimentally testable hypotheses regarding different routes for the synthesis of nanostructured carbon materials.  The results also provide possible explanations for the initial steps in the process of formation of diamond nanocrystals upon exposure of MWCNTs to hydrogen plasmas that has been reported in the literature.