Tohoku University · Physics and Astronomy
Professor Michio Niwano's research lab specializes in surface and interface science of semiconductors and oxides, with a focus on the atomic-scale mechanisms of oxidation, passivation, and surface reactions. The lab employs advanced spectroscopic techniques—such as infrared spectroscopy in multiple internal reflection geometry, high-resolution photoemission spectroscopy with synchrotron radiation, and in situ real-time analysis—to investigate the chemical and electronic structures of silicon and titanium dioxide surfaces under various conditions. Key research directions include the initial oxidation of hydrogen-terminated silicon, the formation and stability of interfacial suboxides, and the generation and characterization of nanobubbles and photocatalytic nanostructures. The lab also explores surface modification processes using UV-ozone, HF treatment, and anodization for applications in nanoelectronics and environmental technologies.
Figures are computed from collected data and may differ slightly.
We have studied the initial stages of oxidation of the hydrogen-terminated Si(111) and (100) surfaces stored in air, using infrared spectroscopy in the multiple internal reflection geometry. We investigate the effect of surface roughness and humidity of air on the oxidation of the hydrogen-terminated Si surfaces. We suggest that surface roughness on a microscopic scale does not significantly affect the oxidation of the hydrogen-terminated Si surface and the oxidation occurs on the entire surface
The SiO2/Si interface structure of thin oxide films thermally grown on Si(100), (111), and (110) surfaces under device processing conditions has been investigated using high-resolution photoemission spectroscopy with synchrotron radiation. The intensity distribution of the so-called suboxides, Si1+, Si2+, Si3+, displays a strong dependence on the crystallographic orientation of the substrate over the oxidation temperature range from 600 to 900 °C; Si1+ is enhanced in intensity on Si(111) and (11
We have investigated the initial stages of UV ozone oxidation of hydrogen-terminated Si(100) and (111) surfaces using infrared spectroscopy in the multiple internal reflection geometry. Spectral features due to intermediate oxidation species such as SiH2(O2) and SiH(O3), which are generated by the attack of the back bonds of a surface Si atom by oxygen, are clearly observed. Upon UV ozone oxidation the concentration of the intermediate oxidation species initially increases and then drops, while
The chemical nature of Si(100) and (111) surfaces during immersion in dilute hydrofluoric acid (HF) solution was investigated ‘‘in situ’’ and in real time using infrared absorption spectroscopy in the multiple internal reflection geometry. In dilute HF solution, the Si surface is not completely terminated with hydrogen, but may be covered in part with hydrogen-associated Si fluorides, such as SiH2(SiF) and SiH2F2. It is found that the hydrogen coverage of the surface depends on the HF concentrat
The photocatalytic bactericidal activity of titanium dioxide (TiO<sub>2</sub>) thin films has been extensively studied. In this study, we investigated the bactericidal activities of TiO<sub>2</sub> nanotube (NT) thin films using <i>Escherichia coli</i> and <i>Staphylococcus aureus</i> cells as the model bacteria. Metallic titanium (Ti) thin films were anodized on a silicon (Si) wafer substrate to form TiO<sub>2</sub> NT thin films. To evaluate the bactericidal activity of the TiO<sub>2</sub> NT
Morphologies of Si surfaces treated with aqueous solutions of hydrofluoric acid (HF) and ammonium fluoride (NH4F) have been investigated using surface infrared spectroscopy. We confirm that HF-treated Si(111) surfaces are terminated with a monohydride (Si—H), dihydride (Si—H2), and trihydride (Si–H3), whereas NH4F-treated Si(111) surfaces are dominantly terminated with Si—H. For Si(100), treatment in NH4F produces a surface for which the dihydride mode is enhanced compared to HF treatment, sugge
Nanobubbles (NBs), with their unique physicochemical properties and promising applications, have become an important research topic. Generation of monodispersed bulk NBs with specified gas content remains a challenge. We developed a simple method for generating bulk NBs, using porous alumina films with ordered straight nanoscaled holes. Different techniques, such as nanoparticle tracking analysis (NTA), atomic force microscopy (AFM), and infrared absorption spectroscopy (IRAS), are used to confi
We investigated the bactericidal activity of bulk nanobubbles (NBs) using <i>E. coli</i>, a model bacterium. Bulk NBs were produced by forcing gas through a porous alumina membrane with an ordered arrangement of nanoscale straight holes in contact with water. NBs with different gas contents, including CO<sub>2</sub>, O<sub>2</sub>, and N<sub>2</sub>, were generated and evaluated for their bactericidal effects. The survival rate of <i>E. coli</i> was significantly reduced in a suspension of CO<su
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