Toshimasa Wadayama

In situ photoluminescence spectral study of porous Si in HF aqueous solution

Time‐dependent change in the visible luminescence of porous Si immersed in HF aqueous solution was measured in a short acquisition time (5 s) with a charge coupled device detector. Under Ar+ laser light (514.5 nm) irradiation, the photoluminescence band of the porous Si shifted to shorter wavelength accompanying a reduction of the peak intensity. The result suggests that the size of the Si nanostructure is closely related to the gap energy of the porous Si.

Change in luminescence properties of porous Si by F2 and D2O exposure: In situ photoluminescence, Raman, and Fourier-transform infrared spectral study

In situ photoluminescence (PL), Raman, and infrared (IR) spectra of porous Si (PS) during F2/D2O exposures were investigated. F2 exposure at 298 K resulted in a peak shift of PL band from 750 to 670 nm with an intensity reduction. IR spectra revealed that the surface hydrogenated Si of the PS was displaced by fluorinated one. By subsequent D2O exposure, the PL band further shifted to a shorter wavelength with a significant intensity increase: IR bands due to surface oxides as well as SiD and SiOD bonds were observed after the exposure. On the contrary, the average size of the Si crystallites in the PS evaluated from Raman spectra remained almost unchanged throughout the exposures. These results suggest that surface chemistry plays a crucial role in the PL of the PS.

Metastable surface species on MoO3 observed in methanol vapor at an elevated temperature with polarization modulation infrared spectroscopy

Abstract Thin films of MoO 3 heated in methanol vapor show new infrared absorption bands, which can be assigned to a methoxy intermediate of the oxidation reaction of methanol and to a Mo(V)=O group. All these bands disappear upon evacuation, showing the species are metastable and persist only in the presence of gas phase molecules.

Carbon monoxide adsorption on ultra-thin Fe film deposited on Cu(100)

This paper is mainly concerned with vibrational properties of CO adsorbed on fcc Fe(100) observed using IR reflection absorption spectroscopy. An epitaxial Fe film of eight monolayers has been grown at 300 K on Cu(100). At 0.1 L exposure only a single C-O stretch band appears at 1920 cm -1 which shifts to 1977 cm -1 at 0.55 L. This band further shifts to 1998 cm -1 and saturates in intensity at 0.88 L. In contrast, at 0.25 L a second C-O stretch band appears at 2020 cm - 1 which increases in intensity and shifts to 2048 cm -1 at 1.2 L. The initially observed band can be ascribed to the bridging CO and the second band to the on-top CO. Although the bridging CO was stable up to ca. 380 K, the band due to the on-top CO was less stable and disappeared above 313 K. The observed CO adsorption/desorption behavior on the epitaxial fcc Fe(100) surface is compared with that on a vacuum-evaporated bcc Fe polycrystalline film.

In situ FT-IR and photoluminescence study of porous silicon during exposure to F2, H2O, and D2O

Abstract In situ FT-IR, photoluminescence (PL), and Raman spectral changes of porous silicon (PS) during F 2 and subsequent D 2 O (H 2 O) exposures were investigated at room temperature. The transmission FT-IR spectra revealed that the initial F 2 exposure etched the H-terminated PS surface to form SiF x species. After D 2 O exposure in succession, the IR bands due to the SiF x stretching modes decreased in intensity whereas the bands due to the Si–O–Si (around 1100 cm −1 ), SiD (1540 cm −1 ) and SiOD (2700 cm −1 ) stretching modes were observed. Significant changes in PL features of the PS were observed during these gaseous treatments. The average size of the silicon nano-particles estimated from the LO phonon band of the PS remained almost unchanged during the treatments. These results clearly demonstrate that the surface chemistry of PS is crucial for a better understanding of its optical properties.

In Situ IR Spectroscopic Observation of a-Si:H(F) Films Growing under Spontaneous Chemical Deposition Method

Polarization modulation infrared spectroscopy has been applied to in situ observation of a-Si:H(F) films deposited by the spontaneous chemical deposition (SCD) method. The film showed multiple IR absorption bands due to the formation of various hydrogenated and fluorinated silicon species. These bands increased in intensity with deposition time, although the rates of intensity increase of the bands were different from each other. The absorption intensity change of these bands with the deposition time suggests that the predominant species in the initial stage of the deposition is fluorinated silicon. The exposure to air of the film deposited at a low substrate temperature resulted in the remarkable intensity decrease of the bands due to fluorinated silicon, accompanying the appearance of the Si–O stretching band.

Kretschmann ATRIR spectroscopy of investigating metal/liquid(or solid) interfaces

Abstract In the Kretschmann ATR configuration, a short-range strong electric field is generated on the surface of a thin metal film by the collective resonance of free electrons upon incidence of infrared light. This strong field enhances the intensity of infrared absorption from species present in the immediate neighborhood of the metal surface. In a few cases, infrared absorption of adsorbed species is enhanced through the coupling of vibration modes with charge-transfer excitation. Consequently, the Kretschmann ATR configuration is feasible for the in-situ observation of infrared spectra of species on metal surfaces without significant interference of signals from bulk species.

Infrared reflection absorption study of carbon monoxide adsorption on Fe-deposited Pt(111) surface

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on sub-monolayer (ML)-thick Fe deposited Pt(111). The CO exposure to a clean Pt(111) surface at room temperature yielded linear-bonded and bridge-bonded CO-Pt bands at 2090 and 1850 cm-1. The CO-Pt band intensities decreased steeply with increasing Fe thickness. For a 1-ML-thick-Fe/Pt surface, the bridge-bonded CO-Fe band at 1950 cm-1 dominated the spectra. Furthermore, the Fe depositions showed a new absorption band at around 2060 cm-1. The IRRAS spectrum for CO adsorption on the 3-A-thick-Pt-covered a 1-ML-Fe/Pt(111) surface showed a single absorption at 2070 cm-1. Based on the IRRAS results, we discuss CO adsorption behavior on sub-ML-thick Fe/Pt(111) surfaces.

Dynamic photoluminescence change of porous Si upon exposure to thermoelectrons/D atoms and D2O

In situ photoluminescence (PL) and transmission IR spectral measurements have been carried out for porous Si (PS) after exposure to thermoelectrons and subsequent exposure to D atoms or D2O. Upon exposure to thermoelectrons the PL band (765 nm) of the PS almost diminished accompanied by the intensity reduction of the IR bands due to Si-Hx (x=1–3) species. A subsequent D atom exposure resulted in a recovery of the PL band with the formation of Si-Dx bonds. In contrast, D2O exposure gave rise to a new PL band at 650 nm in addition to a 745 nm band accompanied by the emergence of IR bands due to Si-OD and Si-D bonds: the integrated PL intensity after the D2O exposure is 1.2 times larger than the PL intensity of the as-anodized PS. These results suggest that the PL of the PS contains an important contribution from the surface Si-O bonds.

Polarization modulation IR spectroscopic study of electrochemically generated species from TCNE at a gold electrode in the Kretschmann ATR geometry

Abstract Polarization modulation infrared spectroscopy using the Kretschmann ATR geometry has been applied to the in-situ observation of tetracyanoethylene (TCNE) anions generated electrochemically on a gold electrode in contact with an acetonitrile solution. Besides the electrode potential-independent band at 2252 cm −1 due to the CN stretching vibration of acetonitrile, a fairly strong band was observed at ca. 2170 cm −1 at a potential of −1.6 V (vs. a Ag/10 −2 M Ag + electrode). Upon a positive shift of potential, this band shifted to higher wavenumbers and decreased in intensity. At −1.2 V, a new band appeared at around 2110 cm −1 and shifted to higher wavenumbers when the potential was made more positive. On the basis of the potential dependence and peak frequencies, the 2170 and 2110 cm −1 bands are attributed to the CN stretching vibrations of TCNE 2− and TCNE .− adsorbed on the Au electrode, respectively. The orientations of these species with respect to the surface are discussed on the basis of the surface selection rule.