The fullerene retains its structure upon sublimation. The microcristalline environment oft he film affects a change in local symmetry which can impact on some of the physical properties, however. 

Thanks for the information. Dr. Hugh J. However, in SEM, I obtained small circles. I don't understand the shape when annealed at the oxygen atmosphere. It looks like small flakes/flower petals

Annealing in oxygen might be different to vacuum sublimation!!!

What size are the flakes?

less than 500nm average width