A. Kasuya

Blue shift in ultraviolet absorption spectra of monodisperse CeO2−x nanoparticles

Blue shifts corresponding to the absorption edge in the UV-A (310–400 nm) range are obtained from ultraviolet (UV) absorption spectra of monodisperse CeO2−x (0<x<0.5) nanoparticles, which are produced as toluene suspensions by a successive microemulsification method. The blue shift of the nanoparticles is inversely proportional to the 2.2 power of the particle size for a direct transition, but no relation is found for an indirect transition. In contrast, the shift for thin films of 100–200 nm thickness exhibits a relation nearly inverse proportional to the square root of the thickness for the direct transition. These relations are different from that for the quantum size effect, though the difference is small for the nanoparticles. A possible explanation for the blue shift is a valence change of the Ce ions.

Structural study on monosize CeO2-x nano-particles

Electron diffraction patterns are obtained for monosize ceria nano-particles produced by a size-fractionation method with surfactants. A dependency of lattice parameters estimated by the least squares method on crystalline particle sizes strongly suggests that a C-type cerium sesquioxide, which has never been reported in the bulk crystal, exists in the particle 1.5 nm in diameter. Chemical reactions responsible for the valence change of cerium ions near the surface are proposed for the existence of the sesquioxide in a strong acid sol.

X-ray photoelectron spectroscopy of monodisperse CeO2−x nanoparticles

Abstract Cexa03d core-level spectra are measured by X-ray photoelectron spectroscopy for monodisperse CeO 2−x nanoparticles, 2.2±0.3, 3.0±0.4, and 3.8±0.6xa0nm in diameter, which are produced by a size-fractionation method using surfactants. It is shown for the first time that the intensities of two characteristic peaks of Ce 3+ ions, Ce 3+ xa03d 3/2 and Ce 3+ xa03d 5/2 , increase with decreasing particle size. Intensity ratios between the characteristic peaks and the two main peaks of Ce 4+ ions show a significant change of the ratio of Ce 3+ ions depending on particle size. The size dependence is discussed in comparison with that obtained by previous models which supposed that surface oxygens on the particle form peroxides.

Resonance Raman scattering and diameter-dependent electronic states in single-wall carbon nanotubes

Abstract Resonance Raman scattering has been measured with laser wavelengths of 450–800 nm on single-wall carbon nanotubes with diameters of 1.1–1.4 nm. The observed intensity enhancements of the Raman peaks reveal resonance electronic transitions at 700, 750 and 780 nm for nanotubes with energies of the breathing mode at 184, 171 and 162 cm −1 , respectively. Calculations show that these nanotubes have diameters of 1.21, 1.29 and 1.37 nm, respectively, and exhibit metallic electronic structures produced by the zone-folding effect. It is shown that metallic and semiconducting nanotubes can be distinguished by Raman scattering.

Observation of C60 film formation on a highly oriented pyrolitic graphite substrate via scanning tunnelling microscopy

We have investigated the early stages in the adsorption process of C60 molecules on a highly oriented pyrolitic graphite (HOPG) substrate. C60 powder was thermally evaporated in UHV of 10−8 Pa conditions onto a freshly cleaved HOPG surface. We did not observe individual fullerenes on the substrate for the case of short deposition times and low evaporation rates. However, small islands of C60 molecules with an fcc structure could be observed when the deposition rate was about 0.2 nm/min and the total thickness was above 1 nm. The islands did not grow in the vicinity of the HOPG steps. The typical lateral dimensions of these islands were of the order of a few hundred square nanometers, having thickness of up to five monolayers. We modified the shapes and positions of these islands by the STM tip, using a small (less than 1 V) bias voltage.

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.

Size and time dependences of the valence states of Sn ions in amphoteric tin oxide nanoparticles

Abstract Sn 3d core-level spectra are measured by X-ray photoelectron spectroscopy (XPS) for nearly monodisperse acidic and alkaline SnO2−x nanoparticles in the size range 3–4xa0nm (original) and 1.5–2.5xa0nm (ultrafiltrated) in diameter. The comparison between the spectra obtained from the acidic and alkaline filtrated samples reveals that the difference is small after a few days but very large after 3 months. The acidic samples show little change in the peak positions and the Sn valence remains around 2.9. The alkaline samples exhibit an increase in the binding energy of more than 0.1xa0eV and the Sn valence increases up to 3.60 after 3 months. The fact that the lattice strain and the valence change of the Sn ions in the alkaline samples are larger than those in the acidic samples is well explained by surface coating and structure, in which the tin oxalate in the acidic samples is much more stable than the ammonium tin oxide in the alkaline samples.

STM induced photon emission at the liquid-solid interface

Abstract We have observed STM-induced photon emission at the liquid–solid interface. The luminescence spectra as well as the photon intensity are recorded simultaneously with the measurement of the scanning tunneling microscopy (STM) topography image using an intensified charge coupled device detector and a spectrograph for the spectrum measurements, and a photomultiplier for the photon intensity measurements. The measurements STM induced luminescence are made in non-aqueous liquid which has a wide potential window. The luminescence could only be recorded for 1,4-dioxane at the STM bias voltage at 5xa0V, but other liquids were also tried. The reason behind this result is discussed in relation to the dielectric constant of the liquid.

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 Siue5f8C 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.