Yoshimitsu Ushimi

Thermal-dependent electronic structure at the interface of C60-adsorbed Si(111)-(7×7) surface

We report here the temperature-dependent measurements of the valence spectra, and the C Is and the Si 2p core-level spectra of the C 60 adsorbed Si(111)-(7 × 7) surface, using photoelectron spectroscopy. At 300 K the valence spectra show the physisorption of most C 60 molecules for a one monolayer C 60 film. After annealing the sample at 1120 K the molecular orbitals disappear due to the breakdown of the C 60 cage. The C Is and Si 2p core-level spectra obtained at the same temperature indicate the progress of SiC formation at the interface between C 60 and the Si(111) surface. After annealing at 1170 K, the formation of SiC islands is confirmed by the binding energies of the peaks observed in the valence and C Is core-level spectra.

Thermal induced transition in the bonding nature of C60 molecules adsorbed on a Si(111)–(7×7) surface

Abstract We have investigated the electronic structure and the thermal dependence of C 60 molecules adsorbed on a Si(111)–(7×7) surface using photoelectron spectroscopy. At room temperature, the binding energies of the molecular orbitals and the C 1 s core level of the 1.0 monolayer (ML) film are the same as those of the 5.0 ML C 60 film. This result indicates that most of C 60 molecules 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 half maximum of the molecular orbitals becomes broader indicating the chemisorption at this temperature. Moreover, the highest occupied molecular orbital (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 measurements and the Si 2 p core level spectrum. The molecular orbitals and the C 1 s core level shift to lower binding energies. These results indicate that the chemisorption between C 60 molecules and the surfaces has both covalent and ionic characters at 500 K. We attribute the transition in the bonding nature of C 60 molecules to the thermal induced change in surface structure during the annealing process.

Change in Electronic Structure of C60 Molecules Adsorbed on a Si(001)-(2×1) Surface by Thermal Effect

We report here the temperature-dependent electronic structure of C60 molecules adsorbed on a Si(001)-(2×1) 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)-(2×1) surface is mainly a physisorption. After annealing the 1 ML C60 film adsorbed on a Si(001)-(2×1) 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)-(2×1) 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)-(2×1) surface changes from physisorption to chemisorption, which has both covalent and ionic characters, at 500 K.