Atomic and Electronic Structures of a Fullerene Molecule on a Ag/Si(111)√3x√3 Surface

We present first-principles total-energy calculations of a single fullerene (C-60) molecule on a Ag-terminated Si(111)root 3x root 3 surface (root 3-Ag), which can provide a deep understanding to the interesting atomic and electronic structures of C-60 overlayers on root 3-Ag. C-60 stably occupies the centers of the substrate triangles, and the on-top sites above Si and Ag atoms. In the most stable adsorption configuration with an adsorption energy of 0.87 eV, C-60 occupies a large Ag triangle site with a hexagon of C-60 facing the substrate. On a Si triangle site, unlike on the large Ag triangle site, a C-C bond that is shared by two adjacent hexagons takes the bottom position. Simulated scanning-tunneling microscopy (STM) images distinguish between the large Ag triangle and Si triangle configurations and are consistent with low-temperature STM observations [K. Tsuchie et al.: Phys. Rev. B 60 (1999) 11131]. Investigation of the energy band of the lowest-energy configuration reveals that C-60 weakly interacts with the substrate because the highest occupied molecular orbital and lowest unoccupied molecular orbital states of C-60 remain almost unchanged upon adsorption of C-60. However, slight upward shift of the S-1 band, responsible for the metallicity in root 3-Ag, is in contrast to the downward shifts for monovalent metal adatoms and gives rise to the decrease in the surface conductivity for C-60 adsorption.