  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.
