Charge transfer and screening in individualC60molecules on metal substrates: A scanning tunneling spectroscopy and theoretical study

We have used scanning tunneling microscopy and spectroscopy to study the electronic structure of individual ${\mathrm{C}}_{60}$ molecules adsorbed onto the Au(111) and Ag(100) surfaces. ${\mathrm{C}}_{60}$ molecules on Au(111) show an increase in the HOMO-LUMO gap of 0.6 eV compared to ${\mathrm{C}}_{60}$ on Ag(100). Splitting of the ${\mathrm{C}}_{60}$ LUMO manifold is suppressed for ${\mathrm{C}}_{60}$ on Au(111), in contrast to the strong splitting observed for ${\mathrm{C}}_{60}$ on Ag(100). Our data implies a 0.6 eV increase in intramolecular Coulomb energy for ${\mathrm{C}}_{60}$ on Au(111) as compared to ${\mathrm{C}}_{60}$ 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. ${\mathrm{C}}_{60}$-substrate bonding and charge transfer is further investigated by calculating ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}$, while about 0.2 electron is transferred to ${\mathrm{C}}_{60}$ resting on Ag(100), although the precise amount depends on the definition used. This charge transfer likely changes the electronic screening properties of ${\mathrm{C}}_{60}$, providing an explanation for observed spectroscopic differences on these two substrates.