Akihisa Ogino

Plasma treatment of multiwall carbon nanotubes for dispersion improvement in water

Microwave excited Ar/H2O surface-wave plasma was used to treat multiwall carbon nanotubes (MWCNTs) to modify their surface characteristics and thus improve their dispersion capability in water. Changes in the atom composition and structure properties of MWCNTs were analyzed using x-ray photoelectron spectroscopy and Raman spectroscopy, and the surface morphology of MWCNTs was observed by field emission scanning electron microscopy and scanning transmission electron microscopy. The results indicated that Ar/H2O plasma treatment greatly enhanced the content of oxygen, and modified surface microstructure properties. The integrity of nanotube patterns, however, was not damaged.

Low-temperature sterilization of wrapped materials using flexible sheet-type dielectric barrier discharge

A flexible sheet-type dielectric barrier discharge (DBD) was studied for the low-temperature sterilization of medical instruments wrapped with Tyvek packaging. Sterilization experiments using Geobacillus stearothermophilus spores with a population of 106 were carried out with various mixtures of nitrogen and oxygen. We confirmed the inactivation of spores after 4.5 min of DBD irradiation at a temperature of 28.4 °C and relative humidity of 64.4%. The main sterilizing factors of this method are the ozone and UV emissions generated by DBD in dry air and synergistic OH radicals generated by DBD in moist air.

Inactivation factors of spore-forming bacteria using low-pressure microwave plasmas in an N2and O2gas mixture

In this study, we investigated the inactivation characteristics of Geobacillus stearothermophilus spores under different plasma exposure conditions using low-pressure microwave plasma in nitrogen, oxygen and an air-simulated (N2 : O2 = 4 : 1) gas mixture. The microwave-excited surface- wave plasma discharges were produced at low pressure by a large volume device. The directly plasma-exposed spores, up to 10 6 populations, were successfully inactivated within 15, 10 and 5min of surface-wave plasma treatment using nitrogen, oxygen and an air-simulated gas mixture, respectively, as working gases within the temperature of 75 C. The contribution of different inactivation factors was evaluated by placing different filters (e.g. a LiF plate, a quartz plate and a Tyvek ® sheet) as indirect exposure of spores to the plasma. It was observed that optical emissions (including vacuum UV (VUV)/UV) play an important role in the inactivation process. To further evaluate the effect of VUV/UV photons, we placed an evacuated isolated chamber, inside which spores were set, into the main plasma chamber. The experimental results show that the inactivation time by VUV/UV photons alone, without working gas in the immediate vicinity of the spores, is longer than that with working gas. This suggests that the VUV/UV emission is responsible not only for direct UV inactivation of spores but also for generation of reactive neutral species by photoexcitation. The scanning electron microscopy images revealed significant changes in the morphology of directly plasma-exposed spores but no change in the spores irradiated by VUV/UV photons only.

Effect of hydrogen on amino group introduction onto the polyethylene surface by surface-wave plasma chemical modification

This work reports the effect of hydrogen concentration on the amino group introduction by the plasma treatment of the polyethylene surface. The amino group selectivity on the polyethylene sheet treated by the surface-wave plasma in various N2/H2 gas mixtures was examined by chemical derivatization combined with x-ray photoelectron spectroscopy (XPS). Amino derivatization along with XPS was employed in order to evaluate the amount of the introduced amino group. The highest concentration of the amino group, about 2.0 in amino-N/100 C-atoms, was obtained in the case of ammonia plasma treatment as well as the highest amino selectivity of about 100% in the amino-N/N ratio.

Cathodoluminescence Property of ZnO Nanophosphors Prepared by Laser Ablation

ZnO nanophosphors with a diameter of 7?50 nm have been fabricated under an oxygen gas atmosphere at room temperature by evaporating ZnO powder or Zn targets using pulsed laser ablation. The size and uniformity of ZnO nanophosphors strongly depend on oxygen gas pressure. Results of cathodoluminescence analysis show strong ultraviolet, blue, green, and green-yellow emissions from ZnO nanophosphors excited by a ?150 eV low-energy electron beam emitted from carbon nanotubes, depending upon the target material and oxygen gas pressure. Ultraviolet, blue, green, and green-yellow emissions can be attributed to the transitions from the conduction band to the valence band, the Zni level to the VZn level or the valence band, the VO level to the valence band, and the Zni level to the Oi level, respectively.

Characteristics of surface-wave plasma with air-simulated N2?O2 gas mixture for low-temperature sterilization

Sterilization experiments using low-pressure air discharge plasma sustained by the 2.45?GHz surface-wave have been carried out. Geobacillus stearothermoplilus spores having a population of 3.0 ? 106 were sterilized for only 3?min using air-simulated N2?O2 mixture gas discharge plasma, faster than the cases of pure O2 or pure N2 discharge plasmas. From the SEM analysis of plasma-irradiated spores and optical emission spectroscopy measurements of the plasmas, it has been found that the possible sterilization mechanisms of air-simulated plasma are the chemical etching effect due to the oxygen radicals and UV emission from the N2 molecules and NO radicals in the wavelength range 200?400?nm. Experiment suggested that UV emission in the wavelength range less than 200?nm might not be significant in the sterilization. The UV intensity at 237.0?nm originated from the NO ? system (A?2?+ ? X?2?) in N2?O2 plasma as a function of the O2 percentage added to N2?O2 mixture gas has been investigated. It achieved its maximum value when the O2 percentage was roughly 10?20%. This result suggests that air can be used as a discharge gas for sterilization, and indeed we have confirmed a rapid sterilization with the actual air discharge at a sample temperature of less than 65??C.

Surface Amination of Biopolymer Using Surface-Wave Excited Ammonia Plasma

Surface-wave plasma has been used to modify polymer surfaces. The density and selectivity of the functionalizations are very important for biomaterial applications. Low-temperature plasma treatment has been successfully applied to many types of polymers to generate functional surfaces. In this study, we focus on the surface modification of amino groups on chitosan surfaces by ammonia plasma treatment. Chitosan is a nontoxic, biocompatible and biodegradable polymer, which promotes the scarless healing of skin through embolization. The experimental results of X-ray photoelectron spectroscopy measurements showed that the ammonia plasma exposure to the chitosan surface increased the nitrogen incorporation on the surface from 4.67 to 9.92%. The selectivity of amino group functionalizations (–NH2/N) increased from 53.7 to 78.4% after the plasma irradiation.

Enhanced photoluminescence of nitrogen-doped ZnO nanoparticles fabricated by Nd : YAG laser ablation

Undoped and nitrogen-doped ZnO (ZnO : N) with a typical size of about 60 nm were fabricated by Nd : YAG laser ablation of a Zn target under mixed O2/N2 gas. Their photoluminescence (PL) was studied. Compared with undoped ZnO, approximately a 500% PL enhancement was observed when nitrogen gas at 150 kPa was added to a fixed 100 kPa background oxygen atmosphere. The PL evolution of ZnO : N nanoparticles with temperature increasing from 20 to 290 K indicates that the dominant emission at room temperature is mainly composed of near band-edge(NBE) emissions. Furthermore, it is possible that NO–Zni complexes, formed by nitrogen substitution of oxygen (NO) and interstitial zinc (Zni), are responsible for the observed PL enhancement, which was centred at the ~3.26 eV NBE emission.

Effect of Hydrogen Reduction on Characteristics of Cu Thin-Films Deposited by RF-Driven Ar/H2 Atmospheric Pressure Plasma Jet

The effect of hydrogen reduction on the characteristics of Cu films deposited using a 13.56 MHz RF Ar/H2 atmospheric pressure plasma jet with a Cu thin wire set inside a quartz tube was studied. By adding a small amount of H2 gas into Ar carrier gas, it was found that intense emissions of N2 molecule second positive systems, OH and O emissions were significantly suppressed, while Cu and Hα lines were intensified. It was confirmed by an X-ray photoelectron spectrometry that a high purity Cu film was synthesized via hydrogen reduction reaction, preventing the oxidization of Cu film.

Mass spectrometric study on inactivation mechanism of spore-forming bacteria by low-pressure surface-wave excited oxygen plasma

In this letter, the etching phenomena of the spore-forming bacteria by oxygen plasma were investigated by using quadrupole mass spectrometry. The etching by-products of H2O and CO2 were obviously detected during the oxygen plasma irradiation by the multiple ion detection measurement. Inactivation of roughly 106 spores population was achieved under almost the same reduced spore shapes for three different incident microwave powers. It is considered from the present results that the oxygen radical etching could cause damage to the germinant receptors located in the inner membrane inevitable for germination of spores, without any damage of the DNA in the cores.