Size-dependent melting of onion-like fullerenic carbons: a molecular dynamics and machine learning study

The melting thermodynamic characteristics of 2- to 20-layered onion-like fullerenes (OLF) (C₆₀@C₂₄₀to C₆₀@···@C₆₀₀₀···@C₂₄₀₀₀) are comprehensively explored using first-principles-based ReaxFF atomistic simulations and random forest machine learning (RF ML). It is revealed that OLFshows lower thermal stability than the counterparts of single-walled fullerenes (SWF). The melting point of SWFincreases monotonically with increasing size, whereas for OLF, an unusual size-dependent melting point is observed; OLFwith intermediate size shows the highest melting point. For small OLF, the melting occurs from the inner to the outer, whereas for large OLF, it nucleates from the inner to the outer and to intermediate fullerenes. The melting and erosion behaviors of both SWFand OLFare mainly characterized by the nucleation of non-hexagons, nanovoids, carbon chains and emission of C₂. RF ML model is developed to predict the melting points of both SWFand OLF. Moreover, the analysis of the feature importance reveals that the Stone-Wales transformation is a critical pathway in the melting of SWFand OLF. This study provides new insights and perspectives into the thermodynamics and pyrolysis chemistry of fullerenic carbons, and also may shed some lights onto the understanding of thermally-induced erosion of carbon-based resources and spacecraft materials.