  Fullerenes, including \ce{C60} and \ce{C70}, have been detected in various
astronomical environments. Understanding how their structures evolve over time
is essential for gaining insights into their life cycle and making further
observations. To address this, we conducted reactive molecular dynamics
simulations to investigate the evolution of fullerenes in the circumstellar
envelopes surrounding carbon-rich asymptotic giant branch stars. Our
simulations employed a bottom-up chemistry scheme, wherein fullerenes grow by
absorbing and condensing small carbon-based molecules. The results revealed the
formation of different structures through heterogeneous reactions based on
hydrogen concentration, leading to the emergence of onion-like nanostructures
or single-layer fullerenes. To examine the impact of these structural changes
on the infrared emission characteristics of fullerenes, we performed quantum
chemical calculations. The results indicate that as fullerenes grow larger,
additional emission features are introduced in the infrared spectrum. Moreover,
two-layered fullerenes show noticeable blue-shift or weakening effects on the
bands associated with out-of-plane vibration modes.
