Photoemission and photoabsorption study ofC60adsorption on Cu(111) surfaces

Abstract

We have carried out an extensive study of C 60 adsorption on Cu͑111͒ surfaces using low-energy electron diffraction, photoemission, and x-ray-absorption spectroscopy. It is found that in valence-band photoemission a state forms right below the Fermi energy for an annealed, well-ordered monolayer, similar to the case of K-doped C 60 . This peak disperses across the Fermi energy at off normal emission geometry. The spectra of carbon core-level photoemission show that the line shape is highly asymmetric with a metalliclike tail. The carbon near-edge absorption spectra show that the lowest unoccupied molecular orbital ͑LUMO͒ is attenuated, and a clear Fermi edge jump appears at the absorption onset. This evidence indicates that charge transfers from the substrate to the C 60 molecular orbitals and the overlayer becomes metallic. The amount of charge transfer can be determined to be 1.5-2 electrons per molecule from both the area of the occupied LUMO in photoemission and the peak shift in near-edge absorption spectra. It has been reported that many metal surfaces with originally different work functions covered by a monolayer of C 60 have a similar work function of about 5 eV. We suggest that the measured work functions are due to the metallic C 60 overlayers and are similar regardless of the metal substrates. This is in line with the reported alignment of monolayer energy levels to substrate Fermi energy. Since the work functions are similar, the energy levels with respect to the vacuum level are also similar. Finally we compare near-edge x-ray-absorption with inverse photoemission spectroscopy to address the screening effects.

On interface dipole layers between C 60 and Ag or Au

“…3a, imply an electric field pointing from C 60 (δ+) towards the Au substrate (δ−). This seems to contradict the partial filling of the LUMO of the C 60 monolayer by electron transfer from the Au substrate to the organic layer, as was already reported for C 60 monolayers on several crystalline metal substrates [13,[15][16][17]. These authors argue that the C 60 overlayer acts as a metal and that the image plane moves from the interface to the outside of the C 60 overlayer.…”

“…1. Many metal/C 60 interfaces have been studied and substantial workfunction changes have been observed (see for example [13][14][15][16][17]). At these interfaces electron transfer from the metal substrate to the C 60 monolayer was observed but the charge transfer could not explain the observed workfunction change of the C 60 overlayer.…”

Geometric and electronic structure of aC60monolayer on Ag(100)

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Among these adsorption systems, C 60 on Ag͑100͒ has attracted particular attention. The widely quoted charge transfer of ϳ2.7 electrons from the metal substrate to each C 60 molecule as observed in an electron energy loss spectroscopy ͑EELS͒ study 8 has stimulated great interest in the possible two-dimensional superconducting states of this system.…”

Resonant core spectroscopies of the charge transfer interactions between C60 and the surfaces of Au(111), Ag(111), Cu(111) and Pt(111)

“…As superspectator and super-Auger processes rely on the transfer of electrons from 75 the surface into the LUMO of the molecule, by tuning the coupling strength of the molecule, in this case C 60 with the metal surface it is in principle possible to observe differences in the intensity of these channels in the RPES. Previous studies have found varying amounts of charge transfer between different metal surfaces and adsorbed C 60 molecules, [8,9,10,11,12,13,14,15] …”