Density functional study of the adsorption of aC60monolayer on Ag(111) and Au(111) surfaces

Abstract

Large-scale density functional calculations have been applied to investigate thoroughly the energetics and electronic structure of a ${\mathrm{C}}_{60}$ monolayer adsorbed on Ag(111) and Au(111) surfaces in the $(2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ phase. Various molecular orientations and adsorption sites have been examined. We find that the most energetically preferred adsorption configuration corresponds to a hexagon of ${\mathrm{C}}_{60}$ aligned parallel to the surface and centered on an hcp site, which gives an adsorption energy of -1.5 eV and -1.2 eV on Ag(111) and Au(111) surfaces, respectively. Analysis of the electron density difference and density of states indicates that the interaction between ${\mathrm{C}}_{60}$ and the noble metal surfaces is primarily covalent, with some ionic character [a small amount of electronic charge transfer of 0.5 and 0.2 electrons per molecule from the Ag(111) and Au(111) surfaces to ${\mathrm{C}}_{60},$ respectively]. This picture is in contrast with the common notion developed from experiments that the interaction between ${\mathrm{C}}_{60}$ and noble metal surfaces is mostly ionic. We also find that the work function increases by 0.1 eV on the Ag(111) surface and decreases by 0.6 eV on the Au(111) surface upon adsorption of a ${\mathrm{C}}_{60}$ monolayer, in good agreement with the experimental data. The opposite change of the work function on the two noble metal surfaces is explained well by examining multiple surface dipole formation in the interface region.