Transformation of amorphous carbon clusters to fullerenes

TL;DR

This study uses molecular dynamics simulations to investigate how amorphous carbon clusters transform into fullerenes, revealing a two-stage process involving shell formation and defect analysis, with some structures forming defect-free cages.

Contribution

It provides detailed insight into the step-by-step transformation process and identifies conditions under which defect-free fullerenes can form directly from amorphous carbon.

Findings

Fullerene formation occurs in two stages: shell formation and chain insertion.

Defects such as 7-membered rings and single-coordinated atoms are observed.

Some fullerenes are formed without any defects, indicating defect-free cages can form directly.

Abstract

Transformation of amorphous carbon clusters into fullerenes under high temperature is studied using molecular dynamics simulations at microsecond times. Based on the analysis of both structure and energy of the system, it is found that fullerene formation occurs in two stages. Firstly, fast transformation of the initial amorphous structure into a hollow sp$^2$ shell with a few chains attached occurs with a considerable decrease of the potential energy and the number of atoms belonging to chains and to the amorphous domain. Then, insertion of remaining carbon chains into the sp$^2$ network takes place at the same time with the fullerene shell formation. Two types of defects remaining after the formation of the fullerene shell are revealed: 7-membered rings and single one-coordinated atoms. One of the fullerene structures obtained contains no defects at all, which demonstrates that defect-free carbon cages can be occasionally formed from amorphous precursors directly without defect healing. No structural changes are observed after the fullerene formation, suggesting that defect healing is a slow process in comparison with the fullerene shell formation. The schemes of the revealed reactions of chain atoms insertion into the fullerene shell just before its completion are presented. The results of the performed simulations are summarized within the paradigm of fullerene formation due to selforganization of the carbon system.