Surface Molecular Rearrangements on the (0001) Face of C70 Single Crystals

Abstract

Surface molecular rearrangements on the (0001) face of C70 single crystals were investigated using atomic force microscopy (AFM). On the surface of a freshly prepared crystal, the molecules are arranged in a slightly distorted hexagonal close-packed (hcp) structure, with an average nearest-neighbor distance of 10.5±0.3 Å. After the samples were stored for two weeks in dry air, the surfaces of the crystals became relatively rough. A quasi-two-dimensional molecular rearrangement from a purely hexagonal structure to a mixture of hexagonal, cubic, and rhombic structures was observed over a several-nanometer region. For different crystal growth conditions, a superstructure resulting from a quasi-three-dimensional surface molecular rearrangement was grown and observed on the surface of the crystals. The superstructure appears as an ordered array of domain boundaries between surfaces regions with face-centered cubic (fcc)-type stacking (CBA) and hcp-type stacking (ABA) regions. The coexistence of the different phases in a nano-scale area is probably due to the similarity of the cohesive energies of the phases. The AFM images represent the direct observation of a transient state in a surface phase transformation on the C70 single crystals.