Thermal induced transition in the bonding nature of C-60 molecules adsorbed on a Si(111)-(7 X 7) surface

We have investigated the electronic structure and the thermal dependence of C-60 molecules adsorbed on a Si(111)-(7 X 7) surface using photoelectron s pectroscopy. At room temperature, the binding energies of the molecular orb itals and the C 1s core level of the 1.0 monolayer (ML) film an the same as those of the 5.0 ML C-60 film. This result indicates that most of C-60 mol ecules interact with the Si surface by van der Waals force at a coverage of 1.0 ML. After annealing the 1.0 ML C-60 film at 500 K, the full-width at h alf maximum of the molecular orbitals becomes broader indicating the chemis orption at this temperature. Moreover, the highest occupied molecular orbit al (HOMO) of a C-60 molecule splits into two peaks that are assigned to be the shifted HOMO and the bonding state with the polarization-dependent meas urements and the Si 2p core level spectrum. The molecular orbitals and the C Is core level shift to lower binding energies. These results indicate tha t the chemisorption between C-60 molecules and the surfaces has both covale nt and ionic characters at 500 K. We attribute the transition in the bondin g nature of C-60 molecules to the thermal induced change in surface structu re during the annealing process. (C) 1999 Elsevier Science B.V. All rights reserved.