Atomic force microscope studies of fullerene films: Highly stable C₆₀ fcc (311) free surfaces

Atomic force microscopy and x-ray diffractometry were used to study 1500 Å-thick films of pure C₆₀ grown by sublimation in ultrahigh vacuum onto a CaF₂ (111) substrate. Topographs of the films did not reveal the expected close-packed structures, but they showed instead large regions that correspond to a face-centered cubic (311) surface and distortions of this surface. The open (311) structure may have a relatively low free energy because the low packing density contributes to a high entropy of the exposed surface.