Stability of sp³ Hybridized Amorphous Carbon and its Transformation to Nanodiamond

A newly synthesized carbon allotrope, fully sp³-hybridized amorphous carbon (sp³-AC), has generated widespread research interest, especially in terms of structural stability and evolution. Here, the molecular simulations with machine learning force field to explore the origin of its stability and the structural transformation from sp³-AC to diamond is performed. It is found that the diffusion of carbon in the covalently bonded sp³-AC is extremely slow. Although the sp³-AC exhibits a higher energy than diamond, the nucleation of diamond within it remains thermodynamically unfavorable. The transformation from sp³-AC to diamond is impeded by an exceptionally high nucleation energy barrier of ≈17 eV, contributing to its kinetic stability. Analysis indicates that diamond nucleation within sp³-AC induces a localized density reduction, which may serve as the underlying mechanism that inhibits nucleation and suppresses continuous growth. As a result, annealing the sp³-AC at high temperature and high pressure leads to a specific type of composite, where randomly oriented diamond nanocrystals are evenly distributed in sp³-AC. The specific composite of sp³-AC-nanodiamond serves as an intermediate between sp³-AC and diamond, which reveals the underlying mechanism of recent experimental observation of sp³-AC-nanodiamond composite.