Atomic and Electronic Structures of a Fullerene Molecule on a Ag/Si(111)$\sqrt{3}{\times}\sqrt{3}$ Surface

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We present first-principles total-energy calculations of a single fullerene (C60) molecule on a Ag-terminated Si(111)$\sqrt{3}{\times}\sqrt{3}$ surface ($\sqrt{3}$-Ag), which can provide a deep understanding to the interesting atomic and electronic structures of C60 overlayers on $\sqrt{3}$-Ag. C60 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 $S_{1}$ band, responsible for the metallicity in $\sqrt{3}$-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.
2010-07-15