(648c) Structure of Carbon Nanocrystals Nucleated by Hydrogen-Induced Intershell C-C Bonding in Multi-Walled Carbon Nanotubes

                It has been observed that the
exposure of multi-walled carbon nanotubes
(MWCNTs) to atomic hydrogen from a hydrogen plasma results in the formation of nanocrystalline
carbon phases (e.g., cubic diamond nanocrystals) embedded into the nanotubes.
Motivated by these experimental findings, in this presentation, we investigate
the formation of inter-shell sp³ C?C bonds in MWCNTs induced
by atomic hydrogen and carry out a comprehensive theoretical analysis of the
crystalline structures that can be nucleated through such C-C bonding.

In
this context, we report (i) pathways of H interactions with the graphene walls
of the MWCNTs that induce inter-shell C-C bonding, (ii) a detailed analysis of
the atomic and electronic structure of the resulting nanocrystal nuclei, and
(iii) a systematic study of the nucleated nanocrystal structure as a function
of the chiralities and relative alignment of the adjacent graphene walls of MWCNTs
that are bridged by the inter-shell C-C bonding. The analysis is based on a
synergistic combination of classical molecular-dynamics (MD) simulations with
first-principles density functional theory (DFT) calculations. The MD
simulations are used for large-scale structural relaxation 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 performed
within the generalized gradient approximation (GGA) and employ plane-wave basis
sets, ultrasoft pseudopotentials, and supercell models. They are used for
atomic and electronic structure computations, as well as for analysis of
optimal reaction pathways for the interactions of H with the MWCNT graphene
walls in conjunction with the climbing-image nudged elastic band method.

We show that the
resulting MWCNT structures with inter-shell C-C bonds are stable and
demonstrate that the atomic clusters in the vicinity of this inter-shell C-C
bonding provide seeds for the nucleation of crystalline carbon phases embedded
into the MWCNTs. We demonstrate that various such crystalline phases can be
generated, including cubic and hexagonal diamond, as well as new tetragonal and
monoclinic carbon phases. The structure of these crystalline seeds depends on the chiralities and on the relative alignment between
the adjacent concentric walls of the MWCNTs; this relationship is explored
systematically and quantified.