We present a newly developed multiplex sum-frequency generation (SFG) spectrometer that has wavelength tunability of the visible probe. The tunable wavelength range of the visible probe was 470∼800, and 401 nm. The frequency width of the visible probe were 7∼10 cm−1. An SFG spectrum over a 400 cm−1 spectral region was obtainable by a single measurement. We measured SFG spectra of a self-assembled monolayer of octadecanethiol on a gold substrate with the visible probes laser of various wavelengths (401, 477, 535, 633 nm). The observed spectra were drastically changed with the wavelength of the visible probe. We ascribed this spectral change to the change in the phase and magnitude of the contribution from the gold substrate because the interband transition threshold of gold is located in the wavelength range of the visible probe.
A comparative study of the growth processes of self-assembled monolayers (SAMs) of thiophene and alkylthiophene on Au(111) was made by Fourier-transform infrared reflection absorption spectroscopy (FTIR-RAS). Measurements of temporal peak intensity and the time-dependent Langmuir adsorption isotherm revealed two phases with different molecular orientations during SAM growth of thiophene. In the primary stage, thiophene in the SAM orients parallel to the gold surface. In the second stage, the molecule reorients to a standing-up configuration with respect to the surface. Introduction of alkyl chain to thiophene ring does not essentially change the growth process, but induces a faster growth rate than that of the thiophene SAM. The intermolecular forces of adsorbate, e.g., van der Waals force, play a role as a rate-limiting factor for the self-assembly of adsorbates on the solid surface.
We investigate adsorption of Si atoms on the Si(111)7×7 surface by STM experiments and by molecular orbital calculations. In experiments where Si atoms are deposited from an STM tip, not only “staying” Si adsorbates but also noise-like “diffusing” Si ones are observed. To identify their geometries, stable positions, transition states and electron density iso-surfaces for extra Si atoms on cluster models are obtained using molecular orbital calculations. Calculated stable geometries for both one and two Si atoms agree well with the observed STM images of the staying Si adsorbates. For one Si atom, however, calculated energy barrier around a rest atom is only several tenth eV, while it is more than 1 eV for two Si atoms. Therefore, we propose that the staying Si and diffusing Si adsorbates correspond to two Si atoms and one Si atom, respectively.
A ZnO epitaxial thin film could be formed by oxidation of a ZnS epitaxial thin film on a Si substrate. The orientation relation of the ZnO film was (0002), [11-20] ZnO//(111), [1-10] Si. ZnS films was oxidized from its surface to the Si substrate and gradually changed to ZnO film. ZnS film was completely changed to ZnO by annealed at 720oC for 10 min. By excess annealing above 30 min, an intermediate layer was formed at the interface between the ZnO layer and Si substrate. Exciton emission become dominant and visible emission disappeared by annealing the film at 800oC over 5 h, because of compensation of oxygen by the annealing.
Reflection high-energy electron diffraction (RHEED) is one of the most powerful techniques for surface investigation. The diffraction pattern is usually used as information source on the surface structure. On the other hand, much attention has not been paid to electron wave field (electron density distribution) of RHEED, which is constructed near the crystal surface by the incident and diffracted electron beams. In order to inspect the existence of the wave filed, Auger electron intensities excited by RHEED have been measured for Si(111)√3×√3-Al surfaces. Auger electron intensities of Si(LVV) and Al(LMM) were measured by a cylindrical mirror analyzer (CMA) and by a retarding field analyzer (RFA) by changing the glancing angle of the incident electron beam. Intensity distributions of the electron wave field were calculated on the basis of the dynamical diffraction theory. It has been found that the experimentally obtained Auger intensity anomalies and calculated wave fields are correlated to each other.
A model catalyst method has been adopted to clarify the effect of particle size of the Pt metals supported on carbon materials that are used for the electrocatalytic oxidation of methanol in an acidic solution at 60oC. The model catalyst electrodes, Pt/GC, were prepared by vacuum deposition of Pt on a mirror-polished surface of non-porous glassy carbon (GC) to avoid the influence of the carbon substrates pores on the catalytic properties. The electrocatalytic activity of the electrodes evaluated both by cyclic voltammetry and chronoamperometry in 0.5 M H2SO4 showed that the specific activity of the methanol oxidation increased with an increase in the Pt particle size. The oxidation of CO on the Pt/GC electrodes, which was examined to elucidate the size effect, indicated that smaller platinum particles need higher energy for the oxidation of carbon monoxide. The size effect on the specific activity was discussed in terms of the exposed crystal planes and characteristic valence band structure of the Pt particles supported on carbon.
The growth morphology and the electronic structures of thin metastable Ag films grown on Si(001)2×1 surfaces at low temperatures are investigated by scanning tunneling microscopy and angle-resolved photoemission spectroscopy using synchrotron radiation. The morphology of Ag films exhibits a strong thickness and substrate temperature dependence indicating an intriguing growth mechanism. At a nominal coverage larger than 5 ML, the as-deposited film is composed of homogeneous clusters having 3-dimensional character at the substrate temperatures of ∼100 K and of flat epitaxial Ag(111) films by a subsequent annealing at 300∼450 K. Discrete Ag 5 s states are observed at binding energies of 0.3∼3 eV together with the surface state. The discrete electronic states can be interpreted in terms of the quantum-well states (QWS) based on the phase-shift quantization. The phase shift, the energy dispersion and the thickness-versus-energy relation (Structure Plot) of the QWS are consistently derived. On the other hand, for the in-plane dispersion, in contrast to the free-electron-like behavior expected, these QWS show (i) a significant enhancement of the in-plane effective mass with decreasing binding energy and (ii) a splitting of a QWS into two electronic states with different dispersions at off-Γ− point. Such unexpected electronic properties of QWS are investigated in detail and found obviously related to the semiconductor substrate band structure. Furthermore, the QWS splitting is explained in terms of the energy dependence of phase shift at the film-substrate interface occurring at the substrate band edge.
Relative initial adsorption probabilities of O2 molecules on Si(001) surfaces at room temperature and Si18O desorption yield at a surface temperature region from 900 K to 1300 K have been measured as a function of O2 incident energy up to 3.3 eV. Although the adsorption probability showed the minimum at 0.3 eV and to be constant at incident energy over 1 eV, the SiO desorption yield at surface temperatures over 1000 K increased with increasing the incident energy due to the induced-oxidation at silicon dimer bridge sites, their backbond sites and subsurface backbond sites, and the variation of angular distribution of SiO desorption caused by the induced-oxidation.