Atomic and electronic structures of a fullerene molecule on a Ag/Si(111)√3×√3 surface

We present first-principles total-energy calculations of a single fullerene (C₆₀) molecule on a Agterminated Si(111)√3×√3 surface (√3-Ag), which can provide a deep understanding to the interesting atomic and electronic structures of Cgo overlayers on √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, C₆₀ occupies a large Ag triangle site with a hexagon of C₆₀ 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 C₆₀ weakly interacts with the substrate because the highest occupied molecular orbital and lowest unoccupied molecular orbital states of C₆₀ remain almost unchanged upon adsorption of C₆₀. However, slight upward shift of the S₁ band, responsible for the metallicity in √3-Ag, is in contrast to the downward shifts for monovalent metal adatoms and gives rise to the decrease in the surface conductivity for C₆₀ adsorption.