Analysis of diamond nanocrystal formation from multiwalled carbon nanotubes

A systematic analysis is presented of the nanocrystalline structures generated due to the intershell C-C bonding between adjacent concentric graphene walls of multiwalled carbon nanotubes (MWCNTs). The analysis combines a comprehensive exploration of the entire parameter space determined by the geometrical characteristics of the individual graphene walls comprising the MWCNT with first-principles density-functional theory calculations of intershell C-C bonding and structural relaxation by molecular-dynamics simulation of the resulting nanocrystalline structures. We find that these structures can provide seeds for the nucleation of the cubic-diamond and hexagonal-diamond phase in the form of nanocrystals embedded in the MWCNTs. The resulting lattice structure is determined by the chirality and relative alignment of adjacent graphene walls in the MWCNT. These crystalline phases are formed over the broadest range of nanotube diameters and for any possible combination of zigzag, armchair, or chiral configurations of graphene walls. The key parameter that determines the size of the generated nanocrystals is the chiral-angle difference between adjacent graphene walls in the MWCNT.