High-temperature quantum chemical molecular dynamics simulations of carbon nanostructure self-assembly processes

This chapter presents an analysis of a recent fully quantum chemical high-temperature molecular dynamics simulations for (i) self-assembly capping processes of open-ended single-walled CNT models of different diameter, chirality, and lengths, and (ii) self-assembly formation of fullerene molecules from ensembles of C₂ without imposing a designed reaction pathway. Density functional tight binding is used to compute the quantum chemical potential energy surfaces in direct trajectory calculations, and its accuracy is estimated in benchmark calculations. Capping of open-ended CNTs is observed to be a rapid process at temperatures of 2000 and 3000 K involving long-lived "wobbling C₂" and longer chains, typically within 14 ps simulation time. The self-assembly formation mechanism of fullerenes from ensembles of randomly oriented C₂ molecules was discovered by periodically adding batches of more C₂ molecules to the simulations, modeling an open environment. Three distinct steps of fullerene formation can be identified: nucleation of polycyclic structures by entangled polyyne chains, growth by ring condensation of attached chains, and cage closure. In this "size-up" roadmap, giant fullerenes C_n with n > 120 appear to be the dominant species, and a subsequent "size-down" roadmap leading to smaller fullerene cages by C₂ elimination is suggested based on prolonged heating of these large carbon cages. The combined two-stage "size-up/size-down" mechanism explains readily the abundance of buckminsterfullerene C₆₀ in experiment as well as the distribution of larger fullerenes obtained by typical combustion processes.