Role of Graphitic Bowls in Temperature Dependent Fullerene Formation

Fullerenes are used extensively in organic electronics
as electron
acceptors among other uses; however, there are still several key mysteries
regarding their formation such as the importance of graphitic intermediates
and the thermokinetics of initial cage formation. To this end, we
have conducted density functional tight binding molecular dynamics
(DFTB-MD) calculations on disintegrated Ih-C60 to investigate the formation mechanisms
of fullerenes at high temperature conditions. From the results of
these DFTB-MD calculations we were able to develop a thermokinetic
model to describe the free energies and kinetics of fullerene formation
at a range of temperatures. Direct observation of the mechanism revealed
fullerenes readily forming in nanosecond times between 2000 and 3000
K but were hindered above this temperature window. Analysis revealed
temperature dependent formation mechanisms where at low temperatures
(<2750K) flat graphitic bowls play an important part as metastable
intermediates while highly curved bowls follow a direct fast transformation.
Meanwhile at higher temperatures (>2750 K), flat bowls become the
transitory structure between chains and fullerene. Free energy analysis
from our thermokinetic model shows this change in graphitic bowls
to being transitory hinders fullerene formation at high temperatures
compared to lower temperatures, essentially kinetically trapping C60 as chain networks. This investigation gives new key insights
into the formation mechanisms of C60 fullerenes and highlights
important intermediates while also illuminating the temperature window
for fullerene formation, facilitating better optimization of experimental
methods.