Floating two-dimensional solid monolayer of C60 on graphite

Experiments on both single-crystal graphite and highly oriented pyrolytic graphite indicate that for 60 < T < 300 K, C-60 forms single-layer islands of close-packed molecules at low coverages. Low-energy electron-diffraction measurements on the single crystal indicate that there is almost no preferred orientation of the C-60 lattice relative to the graphite lattice, producing continuous diffraction rings. A slight preference for the C-60 lattice oriented at 30 degrees relative to the graphite lattice is explained as originating in the preference for the C-60 islands to nucleate and align at step edges, observed with scanning tunneling microscopy and low-energy electron microscopy. The energetics of this C-60 layer were investigated using the Novaco-McTague theory of epitaxial orientation, which found several minimum-energy angles near the experimental C-60-C-60 spacing, inconsistent with the experiment and suggesting an extremely small C-60-graphite corrugation. The thermal expansion of this "floating solid" C-60 lattice for 60 < T < 120 K was compared to theoretical models using previously formulated C-60-C-60 pair potentials. The calculated values, assuming perfect two-dimensional layers of spherical C-60, are significantly smaller than the measured values, suggesting that additional thermal excitations, such as those involving molecular orientations, are present in this temperature range.