On the inhomogeneous structure and charge transfer of C$_{60}$ monolayer on Ag(001)

We have studied the morphological and electronic structure of C$_{60}$ ML/Ag(001) by STM/STS and first principles calculation. The bright-dim contrast of adsorbed C$_{60}$ molecules is identified as originating from geometric effects. Among the dim C$_{60}$ molecules, there consist of dim monomers (DM) and dim dimers (DD), which display distinguished STS. The dI/dV spectra give the energy locations of HOMO, LUMO and LUMO+1-derivated energy bands and the different STS spectra for B, DM, and DD C$_{60}$ molecules indicate that the charge transfer from the substrate is strongly inhomogeneous. The charge transfer has been estimated as $\sim $0, $\sim $1, $\sim $2 electrons/molecule for B, DM and DD from the shift of their corresponding LUMO+1 energy bands, respectively. The DFT calculations give the consistent spectra for B, DM and DD C$_{60}$ molecules with experimental STS data, but with smaller charge transfer.