Takanori Wakita

SiC islands grown on Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces by C60 precursor

Abstract We have investigated the formation processes and characteristics of SiC islands grown on the Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces by C 60 precursor using high resolution electron energy loss spectroscopy (HREELS). The SiC islands are prepared by heating the C 60 adsorbed Si surfaces and confirmed by the observation of the optical surface phonon, Fucks-Kliewer mode. We found that SiC islands are formed at 1170 K and 1120 K on the Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces, respectively. We attribute the difference in the formation temperature to the different amount of charge which is transferred from Si surface to C 60 molecule at 500 K. The difference in the scattering cross-section of the Fucks-Kliewer mode indicates the different shape of SiC islands on each surface.

Interaction of C60 with Si(111)7×7 and Si(100)2×1 surfaces studied by STM, PES and HREELS: annealing effect

We report here measurements of the valence band spectra, the C 1s core level spectra and the vibrational excitation spectra of a C 60 monolayer (ML) film on Si(111)7 × 7 and Si(100)2 × 1 surfaces, using photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS) in combination with scanning tunneling microscopy. 1 ML films are formed after annealing the 5 ML films at 670 K. The bonding states between C 60 molecules and Si substrates are clearly observed at 2.1 eV on both surfaces in the valence band spectra. The shift of binding energies of the molecular orbitals and the C Is core level spectra indicate the charge transfer. The softening of several vibrational modes is observed on the Si(111)7 × 7 and Si(100)2 × 1 surfaces with HREELS. The charge transfer scheme explains the softening well. We discuss the bonding nature in terms of hybridization of C 60 molecular orbitals with the surface states.

Bonding nature of C60 adsorbed on Si(111)7×7 and Si(100)2×1 surfaces studied by HREELS and PES

We report here the measurements of vibrational excitation spectra, valence band spectra and C Is core level spectra of C 60 molecules adsorbed on Si(111)7 × 7 and Si(100)2 × 1 surfaces, using high-resolution electron energy loss spectroscopy (HREELS) and photoelectron spectroscopy (PES). At a coverage between 1 and 0.25 ML, HREELS and PES measurements show that the interaction is as weak as the van der Waals interaction on both surfaces. At a coverage lower than 0.25 ML, some contradictory results are obtained by HREELS and PES. With HREELS, the softening of several vibrational modes is observed on the Si(111)7 × 7 surface, but no indication of softening is observed on the Si(100)2 × 1 surface. The charge transfer scheme explains the softening well. These results by HREELS indicate that the nature of the bonding is ionic on the Si( 111)7 x 7 surface and of the van der Waals type on the Si(100)2 × 1 surface. In contrast, the bonding states between C 60 molecules and Si substrates are clearly observed at 2.4 eV on both surfaces with PES. The PES measurements indicate that the interaction has a covalent character on the Si(111)7 × 7 surface, and that it is in between covalent and ionic, i.e. not purely ionic or at the covalent limit, on the Si(100)2 x 1 surface. We will discuss the nature of the interaction manifested by HREELS and PES measurements in comparison with the spectra of C 60 molecules adsorbed on metal surfaces.

SiC film formation from C60 monolayer on Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces studied by HREELS-STM

Abstract We have investigated the thermal reaction of C 60 molecules and the formation of SiC films on Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces by the combined measurements of high-resolution electron-energy-loss spectroscopy (HREELS) and scanning tunnelling microscopy (STM), HREELS-STM. The surface phonon energy of SiC film, formed by heating the 1 ML film of C 60 adsorbed on Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces up to 900°C, is observed at 114 meV. Moreover, new peaks are measured at 91 and 102 meV in high resolution measurements. Taking into account the cross sections and the angular profiles of the scattered electrons, we attribute the energy loss peaks at 91, 102 and 114 meV to the SiC vibration at the SiC surfaces, the low frequency Fuchs-Kliewer phonon mode and the high-frequency one, respectively. STM images of SiC films on both surfaces show the presence of many islands which surface areas are 10 × 10–35 nm 2 . The well characterized SiC films are formed at approximately 900°C on both surfaces.

Adsorption and thermal reaction of C70 on Si(111)-(7 × 7) and Si(100)-(2 × 1) surfaces: Comparison with C60

Abstract We report here the measurements of vibrational excitation spectra and the temperature dependence of C 70 molecules adsorbed on Si(111)-(7×7) and Si(100)-(2×1) surfaces using high-resolution electron-energy-loss spectroscopy. It is found that a strong interaction of the C 70 molecule with the Si(100) surface is induced after annealing at 873 K and that the C 70 cage is easier to decompose on that surface. SiC films are formed at 1273 K and 1073 K on the Si(111)-(7×7) and Si(100)-(2×1) surfaces, respectively. Moreover, the formation temperature of 1073 K is lower by 50 K than that of the SiC film from C 60 on a Si(100)-(2×1) surface. We discuss the reactivity and the formation temperature of SiC films in terms of the interaction of C 70 with the substrates.

Local structure and chemical reaction of C60 films on Si(111)7 × 7 studied by HREELS-STM

Abstract We have investigated the thermal stability and the chemical reactions of C 60 thin films on Si(111)7 × 7 surfaces by the combined measurements of the high-resolution electron-energy-loss spectroscopy (HREELS) and the scanning tunneling microscopy (STM), HREELS-STM. After heating the Si up to 400 °C, the molecules did not align perfectly but made local arrangements. The energy shifts of the inelastic electrons indicate that the electrons in the Si dangling bond transfer to the C 60 molecules. The value of the charge transfer is estimated to be 1 ± 1 electron(s). After heating the Si up to 800 °C, the smooth C 60 monolayer film aggregates and forms islands. The nearest neighbouring distance between the C 60 molecules is shortened from 10 A to 9.3 A. The intensity of the 92 and 101 meV peaks drastically increase. These results indicate the formation of a covalent bond between the C 60 molecules. After heating the Si up to 1100 °C, an SiC film grows.

Interaction of C60 with silicon dangling bonds on the Si(111)-(7×7) surface

We report here the measurements of vibrational excitation spectra of C 60 molecules adsorbed on Si(111 )-(7 x 7) and H-terminated Si(11 1)-δ(7 x7) surfaces using high-resolution electron energy loss spectroscopy. Upon adsorption of C 60 on Si( 111)-(7 x7), a shift to a lower energy is measured for the v 1 and v 4 vibrational modes, but no indication of softening is observed on the H-terminated Si(111)-δ(7 x7) surface. These results show that the C 60 molecules interact strongly with the dangling bonds of adatom on the (7 x 7) structure. From the softening of the vibrational modes, the amount of charge transfer is estimated to be (3±1) electrons per C 60 molecule on the Si(111)-(7 x 7).