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Earlier studies of C 60 adsorption on Au(111) reported many interesting and complex features. We have performed coordinated low-energy electron diffraction, scanning tunneling microscopy (STM), and density functional theory studies to elucidate some of the details of the monolayer commensurate (2Ý3 × 2Ý3)R30°p hase. We have identified the adsorption geometries of the two states that image as dim and bright in STM. These consist of a C 60 molecule with a hexagon side down in a vacancy (hex-vac) and a C 60 molecule with a carbon-carbon 6:6 bond down on a top site (6:6-top), respectively. We have studied the detailed geometries of these states and find that there is little distortion of the C 60 molecules, but there is a rearrangement of the substrate near the C 60 molecules. The two types of molecules differ in height, by about 0.7Å, which accounts for most of the difference in their contrast in the STM images. The monolayer displays dynamical behavior, in which the molecules flip from bright to dim, and vice versa. We interpret this flipping as the result of the diffusion of vacancies in the surface layers of the substrate. Our measurements of the dynamics of this flipping from one state to the other indicate that the activation energy is 0.66 ± 0.03 eV for flips that involve nearest-neighbor C 60 molecules, and 0.93 ± 0.03 for more distant flips. Based on calculated activation energies for vacancies diffusing in Au, we interpret these to be a result of surface vacancy diffusion and bulk vacancy diffusion. These results are compared to the similar system of Ag(111)-(2Ý3 × 2Ý3)R30°-C 60 . In both systems, the formation of the commensurate C 60 monolayer produces a large number of vacancies in the top substrate layer that are highly mobile, effectively melting the interfacial metal layer at temperatures well below their normal melting temperatures.

In a combined scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and density functional theory (DFT) study of the surface of Al 13 Co 4 (100), all techniques have found that after annealing to 1165 K, the surface structure is consistent with a dense Al-rich plane with surface Co atom depletion. Various structure models were considered, and in the LEED study, the best agreement was found with a model that consists of Al-rich terminating planes with no Co atoms, and otherwise a structure similar to the bulk puckered layers. This structure was also found to be stable in the DFT study. The best-fit structural parameters are presented for the two domains of this structure, which contain bipentagons that can be related to the pentagonal bipyramidal structures in the bulk, plus additional "glue" atoms between them.These domains are not strictly related to each other by symmetry as they have different surface relaxations. The STM study found significant differences in the surfaces of samples grown by different methods, and is able to explain a different interpretation made in an earlier study.

Experiments on both single-crystal graphite and highly oriented pyrolytic graphite indicate that for 60Ͻ T Ͻ 300 K, C 60 forms single-layer islands of close-packed molecules at low coverages. Low-energy electrondiffraction 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°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 60Ͻ T Ͻ 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.

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