Charge transfer and screening in individual C60 molecules on metal substrates:: A scanning tunneling spectroscopy and theoretical study -: art. no. 115418

We have used scanning tunneling microscopy and spectroscopy to study the electronic structure of individual C-60 molecules adsorbed onto the Au(111) and Ag(100) surfaces. C-60 molecules on Au(111) show an increase in the HOMO-LUMO gap of 0.6 eV compared to C-60 on Ag(100). Splitting of the C-60 LUMO manifold is suppressed for C-60 on Au(111), in contrast to the strong splitting observed for C-60 on Ag(100). Our data implies a 0.6 eV increase in intramolecular Coulomb energy for C-60 on Au(111) as compared to C-60 on Ag(100). Topographs and energy-resolved spectral maps, however, show nearly identical features and indicate a similar influence of the two substrates on molecular-orbital geometry. C-60-substrate bonding and charge transfer is further investigated by calculating C-60 charge redistribution using ab initio pseudopotential density-functional theory methods. These calculations indicate that a negligible amount of charge is transferred from Au(111) to adsorbed C-60, while about 0.2 electron is transferred to C-60 resting on Ag(100), although the precise amount depends on the definition used. This charge transfer likely changes the electronic screening properties of C-60, providing an explanation for observed spectroscopic differences on these two substrates.