Finite-temperature stability of hydrocarbons: fullerenes versus flakes

TL;DR

This study uses computational simulations to investigate how temperature and hydrogen content influence the stability and phase transitions of hydrocarbon structures, revealing new insights into their equilibrium forms.

Contribution

It introduces a comprehensive phase diagram for hydrocarbons showing temperature and composition effects on structure stability, including the preference for planar forms with hydrogen.

Findings

Temperature and hydrogen addition favor planar nanoribbons.

Multiple phase changes occur at finite sizes and compositions.

Hydrocarbon structures exhibit diverse phases beyond known carbon cages.

Abstract

The effects of a finite temperature on the equilibrium structures of hydrocarbon molecules are computationally explored as a function of size and relative chemical composition in hydrogen and carbon. Using parallel tempering Monte Carlo simulations employing a reactive force field, we find that in addition to the phases already known for pure carbon, namely cages, flakes, rings and branched structures, strong effects due to temperature and the addition of little amounts of hydrogen are reported. Both entropy and the addition of moderate amounts of hydrogen favor planar structures such as nanoribbons over fullerenes. Accurate phase diagrams are proposed, highlighting the possible presence of multiple phase changes at finite size and composition. Astrophysical implications are also discussed.