Change in electronic structure of C60 molecules adsorbed on a Si(001)-(2 x 1) surface by thermal effect

We report here the temperature-dependent electronic structure of C60 molecules adsorbed on a Si(001)-(2x1) surface, measured by photoelectron spectroscopy. At room temperature, the valence spectra reveal that the interaction between a 1 monolayer (ML) C60 film and a Si(001)-(2x1) surface is mainly a physisorption After annealing the 1 ML C60 film adsorbed on a Si(001)-(2x1) surface at 500 K in the valence spectra the highest occupied molecular orbital (HOMO) of a C60 molecule splits into two peaks. The binding energies of the two peaks are 1.6 and 2.1 eV. We assign the 1.6-eV peak to the shifted HOMO and the 2.1-eV peak to the bonding state between C60 molecules and a Si(001)-(2x1) surface from the polarization-dependent measurement and the Si 2p core level spectrum. Moreover, the molecular orbitals and the C 1s core level of a C60 molecule shift to the lower binding energy side. These results indicate that the interaction between C60 molecules and the Si(001)-(2x1) surface changes from physisorption to chemisorption, which has both covalent and ionic characters, at 500 K.