(454b) 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.