Faculty Profiles - SAITO, Nagahiro

Papers - SAITO, Nagahiro

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Facile fabrication of PtAu alloy clusters using solution plasma sputtering and their electrocatalytic activity

Hu X., Shen X., Takai O., Saito N.

Journal of Alloys and Compounds Vol. 552 page： 351 - 355 2013.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Alloys and Compounds

Well-crystallized PtAu alloy clusters of about 2.0 nm in mean diameter are successfully fabricated via a one-step route of solution plasma sputtering at atmospheric pressure. The solution plasma provides a novel reaction field with highly energetic state for the formation of PtAu alloy clusters. The synergetic effects of rapid energetic radicals' bombardment, atom vapor diffusion, plasma expansion and solution medium condensation resulted in the formation of PtAu alloy clusters with various relative atom amount ratios. PtAu alloy clusters are highly dispersive in water for a long time without any polymeric stabilizer. As-fabricated PtAu alloy clusters show well electrocatalytic activity. PtAu alloy clusters are easily collected and well re-dispersed in various organic solvents for various potential applications. And PtAu alloy clusters supported on carbon were successfully fabricated by a one-step route. These results imply that this novel process will have potential application in future in the design and mass production of various multifunctional metal and/or alloy clusters. © 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jallcom.2012.08.033

Fabrication and characterization of epitaxial SrTiO <inf>3</inf>/Nb-doped SrTiO <inf>3</inf> superlattices by double ECR ion beam sputter deposition

Panomsuwan G., Takai O., Saito N.

Vacuum Vol. 89 ( 1 ) page： 35 - 39 2013.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Vacuum

Epitaxial oxide superlattices (SLs) of SrTiO 3 (STO) and Nb-doped SrTiO 3(STNO) were fabricated on LaAlO 3 (LAO) (001) substrates by an ion beam sputter deposition (IBSD) system having double electron cyclotron resonance (ECR) ion guns. The [STO x/STNO y] 10 SLs were epitaxially grown at different stacking sequences (x = 6 nm, y = 1-6 nm) and maintained the periodicity z of 10. Structural properties and surface morphology are found to be strongly dependent on the STNO sublayer thickness (y). Highly strained SLs with two-dimensional growth mode is observed at smaller STNO sublayer thickness (y = 1 nm). With increasing to a critical thickness (y = 4 nm), the SLs are freely strained and transformed to three-dimensional growth mode. The results demonstrate that the double ECR-IBSD is a versatile technique for the growth of high-quality oxide SLs. © 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.vacuum.2012.08.011

Growth of highly (110)- and (111)-textured SrTiO<inf>3</inf> thin films on Pt(111)/α-Al<inf>2</inf>O<inf>3</inf>(0001) substrates by ECR ion beam sputter deposition

Panomsuwan G., Takai O., Saito N.

Solid State Communications Vol. 158 page： 65 - 69 2013.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Solid State Communications

SrTiO3 (STO) thin films were directly grown on Pt(111)/α-Al2O3(0001) substrates without Ti buffer layer using electron cyclotron resonance ion beam sputter deposition. Highly (110)- and (111)-textured STO films were obtained at 600 and 750 °C, respectively. The structural, morphological and electrical properties of both textured films were studied and compared. From high-resolution X-ray diffraction data and in-plane pole figure analysis, highly (110)-textured STO film was found to contain three orientation variants related by a 120°in-plane rotation, while highly (111)-textured STO film had two orientation variants related by a 180°in-plane rotation. The presence of rod- and triangular-shaped grains was observed for the (110)- and (111)-textured film surfaces, respectively. Their grain orientations were also consistent with the structural analysis. The leakage current characteristics of Pt/STO/Pt capacitors were found to follow the Schottky emission and Poole-Frenkel conduction mechanisms. A larger dielectric constant of (111)-textured film (320 at 100 kHz) was obtained in comparison with that of (110)-textured film (238 at 100 kHz) due to the effect of biaxial in-plane tensile strain and larger grain size. © 2013 Elsevier Ltd.

DOI： 10.1016/j.ssc.2012.12.026

Nanomechanical properties of amorphous and polycrystalline SrTiO <inf>3</inf> transparent thin films prepared by ion beam sputtering

Panomsuwan G., Takai O., Saito N.

Journal of Materials Engineering and Performance Vol. 22 ( 3 ) page： 863 - 868 2013.3

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Materials Engineering and Performance

Transparent SrTiO3 thin films were deposited on glass substrates by ion beam sputtering at various substrate temperatures. The structural characteristic of the deposited films shows the transition from amorphous phase to polycrystalline phase at 600 C. The films exhibit a good transparency in the visible region (∼80-85%) and a relatively smooth surface. The calculated band gaps for amorphous and polycrystalline films were ∼4.20 and ∼3.84 eV, respectively. The refractive index of the films increases with increasing the substrate temperature, and the extinction coefficient of the films in the visible region is in the order of 10-3, indicating low optical loss. The nanomechanical properties of the films were investigated using nanoindentation technique. Young's modulus and hardness for all films are found to be increased with the increase of substrate temperature. © 2012 ASM International.

DOI： 10.1007/s11665-012-0320-2

Discharge time dependence of a solution plasma process for colloidal copper nanoparticle synthesis and particle characteristics

Pootawang P., Saito N., Lee S.Y.

Nanotechnology Vol. 24 ( 5 ) 2013.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Nanotechnology

In this study, we investigate a new synthetic route, termed the solution plasma process, for the synthesis of colloidal copper nanoparticles (CuNPs) in the presence of an amide and acid capping agent. Gelatin and ascorbic acid were selected as the capping agents to protect the particles against coalescence and oxidation side reaction. Using a high voltage power supply, CuNPs were rapidly formed by 1 min after the discharge. The size and shape of the CuNPs were dependent on the discharge time and were clearly influenced by the effect of the capping agents under two characteristics of the discharge medium (pH and temperature). With a long discharge time, the CuNP size tended to decrease with the formation of anisotropic particle morphologies: spherical, cubic, hexagonal, triangular and rod-like shapes. The decrease in CuNP size as a function of discharge time could be explained by the dissolution of CuNPs in a lower pH solution. After 5 min discharge the capping agent evidently allowed the protection of the synthesized CuNPs against oxidation with the presence of anisotropic CuNP shapes. It is demonstrated that the CuNP shape could be tuned from spherical to anisotropic shapes without the undesirable oxidation by adjusting the discharge time of the solution plasma. These advantages are valuable for material engineering to design the properties of Cu-based nanoparticles for the desired applications. © 2013 IOP Publishing Ltd.

DOI： 10.1088/0957-4484/24/5/055604

Optical and mechanical properties of transparent SrTiO<inf>3</inf> thin films deposited by ECR ion beam sputter deposition

Panomsuwan G., Takai O., Saito N.

Physica Status Solidi (A) Applications and Materials Science Vol. 210 ( 2 ) page： 311 - 319 2013.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Physica Status Solidi (A) Applications and Materials Science

SrTiO3 (STO) thin films have been deposited on glass substrates by electron cyclotron resonance (ECR) ion beam sputter deposition at different substrate temperatures (100-600 °C). The structural characteristics of the deposited films show a transition from amorphous phase to polycrystalline phase at 600 °C. The films exhibit good transparency with an interference pattern in the visible region and a very smooth surface without cracks. The refractive index of the films increases with an increase of the substrate temperature, and their dispersion was analyzed by a single-oscillator model. The extinction coefficient of the films in the visible region is of the order of 10 -3-10-2. The estimated values of the direct and indirect bandgap energies for the polycrystalline STO film are 3.80 and 3.32 eV, respectively. Its density is almost identical to the bulk STO single crystal, which is relatively high in comparison with those prepared by other techniques. The mechanical stability of the films is also improved with an increase of substrate temperature. Young's modulus and hardness of the polycrystalline STO film are 194.1 ± 15.5 and 22.5 ± 2.0 GPa, respectively. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssa.201228077

Simple synthesis of platinum nanoparticles by plasma sputtering in water

Hu X., Takai O., Saito N.

Japanese Journal of Applied Physics Vol. 52 ( 1 PART2 ) 2013.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

We applied a novel method of plasma sputtering in water for simple and fast preparation of carbon-supported platinum nanoparticles at atmospheric pressure from platinum wire electrodes without using any stabilizers, chemical reductant, plasma-generating gas, or salt solution in an open system. Upon plasma sputtering, special waveforms of voltage and current were obtained using a pulsed power supply. The obtained emission spectra clarified that platinum atoms were formed in the plasma region from the electrode surface owing to bombardment of the surface by highly energetic electrons and radicals derived from plasma. Thus, carbon-supported 2 nm and highly dispersed platinum nanoparticles were successfully synthesized by this fast and simple plasma sputtering method. Highly crystallized platinum nanoparticles show good electrocatalytic activity. This plasma sputtering technique has shown a potential application to the synthesis of other metal nanoparticles. © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.52.01AN05

Gold nanoparticles supported on SrTiO<inf>3</inf> by solution plasma sputter deposition for enhancing UV- and visible-light photocatalytic efficiency

Panomsuwan G., Zettsu N., Saito N.

Materials Research Society Symposium Proceedings Vol. 1509 page： 80 - 85 2013

Language：English Publishing type：Research paper (scientific journal) Publisher：Materials Research Society Symposium Proceedings

Gold (Au) nanoparticles were synthesized and deposited on the perovskite SrTiO3 (STO) via a one-step solution plasma sputter deposition (SPSD) without any reducing reagents at ambient condition. Good dispersion of the Au nanoparticles deposited on the STO surface was clearly observed. The synthesized Au nanoparticles were well-crystallized with a spherical shape and preferably exhibited multiply twinned structure. An average diameter of Au nanoparicles was estimated to be 6.1 ± 1.4 nm by transmission electron microscopy. Enhanced photocatalytic activity was found for the Au-STO when compared to the pure STO, as investigated from the degradation of methylene blue solution under ultraviolet and visible light irradiation. The SPSD seems to be a rapid and facile approach to prepare the Au nanoparticles supported on the metal oxide for photocatalytic applications. © 2013 Materials Research Society.

DOI： 10.1557/opl.2013.729

The role of activated nitrogen species on double-folded screen nitriding process

Kim S., Lee J., Saito N., Takai O.

Journal of Physics: Conference Series Vol. 417 ( 1 ) 2013

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Physics: Conference Series

As clean and energy saving surface hardening technology, plasma nitriding techniques have been evolved with object of higher performance in the last decades. Even though the diffusion of nitrogen inward to steel is occurred at the final step, solid diffusion from surface, energy transition from gas molecule of nitrogen to atomic or an activated state have many different steps depending on the plasma conditions, parameters and the design of each equipment. And this study made comparative on nitrogen sources transfer with conventional DC plasma nitriding and novel nitriding process using plasma diagnosis and metallurgical observation. With different vacuum pressure, gas ratio and new designed electrode (double-folded screen cathode electrode), it showed a different behavior of DC plasma nitriding including the nano-sized nitride on the outer surface of specimen due to nitrogen source of determining plasma species. In this study, plasma species was able to identify with optical emission spectroscopy (OES) studies. From these observations, we could understand better role of ions or neutral nitrogen species, like neutral nitrogen (N), N2+ and NHx radicals in plasma nitriding process with different parameters. And cutting layers of nitride specimen were showed the results due to a different species gas flow ratio or plasma conditions.

DOI： 10.1088/1742-6596/417/1/012023

Synthesis of gold nanoparticles by solution plasma sputtering in various solvents

Hu X., Takai O., Saito N.

Journal of Physics: Conference Series Vol. 417 ( 1 ) 2013

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Physics: Conference Series

Gold clusters (average particle size is 1.5 nm) and nanoparticles (average particle size is 3.5 nm) with spherical morphology were fabricated by solution plasma sputtering in solution medium. This process was carried out in an open system under atmospheric pressure. During plasma region, rapid energetic radicals' bombardment, atom vapour diffusion, plasma expansion and medium condensation resulted in the formation of gold nanoparticles from its electrode wires. As-fabricated gold particles have well crystalline even for less than 1 nm cluster. Gold nanoparticles with average size about 3.5 nm gave obviously surface plasmon resonance band at near 511 nm. While surface plasmon resonance band was disappearing for 1.5 nm-cluster formed in liquid nitrogen.

DOI： 10.1088/1742-6596/417/1/012030

In situ preparation of gold nanoparticles in alginate gel matrix by solution plasma sputtering process

Watthanaphanit A., Panomsuwan G., Saito N.

Materials Research Society Symposium Proceedings Vol. 1569 2013

Language：English Publishing type：Research paper (scientific journal) Publisher：Materials Research Society Symposium Proceedings

The rapid synthesis of gold nanoparticles (AuNPs) was done by solution plasma sputtering (SPS) process in the presence of a biopolymer, sodium alginate. We utilized the alginate polymer in order to meet three important requirements: (1) to promote the generation of plasma in liquid environment, (2) to provide colloidal stability, and (3) to render biocompatibility to the AuNPs. The effect of sodium alginate concentration (varied as 0.2, 0.5, and 0.9 %w/v) and plasma sputtering time on the particle size and the physical absorption property of the AuNPs were studied. The results indicated that preparation of AuNPs in alginate gel matrix was successful by the SPS process in one step without any reducing agents. This technique has high potential as a novel strategy to produce AuNPs suspended in alginate aqueous solution which is suitable for biomedical application. Copyright © Materials Research Society 2013.

DOI： 10.1557/opl.2013.766

Synthesis of Nanomaterials by Solution Plasma Processing

Takai, O; Bratescu, MA; Ueno, T; Saito, N

MICRO-NANO MECHATRONICS - NEW TRENDS IN MATERIAL, MEASUREMENT, CONTROL, MANUFACTURING AND THEIR APPLICATIONS IN BIOMEDICAL ENGINEERING page： 109 - 121 2013

Orientation control of textured SrTiO<inf>3</inf> thin films on platinized α-Al<inf>2</inf>O<inf>3</inf> (0 0 0 1) by an ion beam sputter deposition method

Panomsuwan G., Takai O., Saito N.

Journal of Physics D: Applied Physics Vol. 45 ( 49 ) 2012.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Journal of Physics D: Applied Physics

SrTiO3 (STO) thin films were grown on Pt/Al2O 3 substrates without a Ti adhesion layer by an ion beam sputter deposition method in a range of growth temperatures between 600 and 750 °C. The effect of growth temperature on the film orientation was investigated by high-resolution x-ray diffraction, pole figure measurements and in-plane grazing incidence x-ray diffraction. The film orientation showed a strong dependence on the growth temperature. The films exhibited a predominant (1 1 0) orientation at a low growth temperature of 600 °C. With an increase in growth temperature to 750 °C, a highly (1 1 1)-textured STO film with two different orientation variants was achieved on the Pt/Al2O3 substrate. A narrow full-width at half-maximum of 0.12° for the rocking curve measured on 2 2 2 STO reflection and a six-fold symmetry from {1 0 0} STO and {1 1 0} STO pole figures were observed. Three-dimensional island growth mode was observed on the surfaces of all films, as investigated with an atomic force microscope. The evolution of grain shape and size was apparently found with an increase in growth temperature. The root-mean-square roughness of the STO film grown at 750 °C was raised to be about 4 nm due to the surface faceting of (1 1 1)-oriented grains. The growth mechanism of the (1 1 0)- and (1 1 1)-textured STO films on the Pt/Al2O3 substrates was also explained and discussed in detail. © 2012 IOP Publishing Ltd.

DOI： 10.1088/0022-3727/45/49/494003

Rapid sterilization of Escherichia coli by solution plasma process

Andreeva N., Ishizaki T., Baroch P., Saito N.

Japanese Journal of Applied Physics Vol. 51 ( 12 ) 2012.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

Solution plasma (SP), which is a discharge in the liquid phase, has the potential for rapid sterilization of water without chemical agents. The discharge showed a strong sterilization performance against Escherichia coli bacteria. The decimal value (D value) of the reduction time for E. coli by this system with an electrode distance of 1.0mm was estimated to be approximately 1.0 min. Our discharge system in the liquid phase caused no physical damage to the E. coli and only a small increase in the temperature of the aqueous solution. The UV light generated by the discharge was an important factor in the sterilization of E. coli. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.51.126201

15th International conference on thin films (ICTF-15)

Takai O., Saito N., Terashima C.

Journal of the Vacuum Society of Japan Vol. 55 ( 10 ) page： 464 - 468 2012.11

Synthesis and characteristics of Ag/Pt bimetallic nanocomposites by arc-discharge solution plasma processing

Pootawang P., Saito N., Takai O., Lee S.Y.

Nanotechnology Vol. 23 ( 39 ) 2012.10

Language：English Publishing type：Research paper (scientific journal) Publisher：Nanotechnology

Arc discharge in solution, generated by applying a high voltage of unipolar pulsed dc to electrodes of Ag and Pt, was used as a method to form Ag/Pt bimetallic nanocomposites via electrode erosion by the effects of the electric arc at the cathode (Ag rod) and the sputtering at the anode (Pt rod). Ag/Pt bimetallic nanocomposites were formed as colloidal particles dispersed in solution via the reduction of hydrogen radicals generated during discharge without the addition of chemical precursor or reducing agent. At a discharge time of 30s, the fine bimetallic nanoparticles with a mean particle size of approximately 5nm were observed by transmission electron microscopy (TEM). With increasing discharge time, the bimetallic nanoparticle size tended to increase by forming an agglomeration. The presence of the relatively small amount of Pt dispersed in the Ag matrix could be observed by the analytical mapping mode of energy-dispersive x-ray spectroscopy and high-resolution TEM. This demonstrated that the synthesized particle was in the form of a nanocomposite. No contamination of other chemical substances was detected by x-ray photoelectron spectroscopy. Hence, solution plasma could be a clean and simple process to effectively synthesize Ag/Pt bimetallic nanocomposites and it is expected to be widely applicable in the preparation of several types of nanoparticle. © 2012 IOP Publishing Ltd.

DOI： 10.1088/0957-4484/23/39/395602

Rapid synthesis of ordered hexagonal mesoporous silica and their incorporation with Ag nanoparticles by solution plasma

Pootawang P., Saito N., Takai O., Lee S.Y.

Materials Research Bulletin Vol. 47 ( 10 ) page： 2726 - 2729 2012.10

Language：English Publishing type：Research paper (scientific journal) Publisher：Materials Research Bulletin

Rapid synthesis of silica with ordered hexagonal mesopore arrangement was obtained using solution plasma process (SPP) by discharging the mixture of P123 triblock copolymer/TEOS in acid solution. SPP, moreover, was utilized for Ag nanoparticles (AgNPs) incorporation in silica framework as one-batch process using silver nitrate (AgNO 3) solution as precursor. The turbid silicate gel was clearly observed after discharge for 1 min and the white precipitate formed at 3 min. The mesopore with hexagonal arrangement and AgNPs were observed in mesoporous silica. Two regions of X-ray diffraction patterns (2θ < 2° and 2θ = 35-90°) corresponded to the mesoporous silica and Ag nanocrystal characteristics. Comparing with mesoporous silica prepared by a conventional sol-gel route, surface area and pore diameter of mesoporous silica prepared by solution plasma were observed to be larger. In addition, the increase in Ag loading resulted in the decrease in surface area with insignificant variation in the pore diameter of mesoporous silica. SPP could be successfully utilized not only to enhance gelation time but also to increase surface area and pore diameter of mesoporous silica. © 2012 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.materresbull.2012.04.040

Enhanced memory window of Au/BaTiO <inf>3</inf>/SrTiO <inf>3</inf>/Si(001) MFIS structure with high c-axis orientation for non-volatile memory applications

Panomsuwan G., Takai O., Saito N.

Applied Physics A: Materials Science and Processing Vol. 108 ( 2 ) page： 337 - 342 2012.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Physics A: Materials Science and Processing

Metal-ferroelectric-insulator-semiconductor (MFIS) structures with BaTiO 3 (BTO) as a ferroelectric film and SrTiO 3 (STO) as an insulating buffer layer were fabricated on p-type Si(001) substrates using an ion beam sputter deposition technique. The effect of out-of-plane orientation on the electrical properties of the MFIS structures, including leakage current density and memory window behavior, were studied using the growth of the BTO ferroelectric film on Si substrate buffered by highly c-axis-oriented and randomoriented STO buffer layers. The experimental results show that the out-of-plane orientations of the BTO films were almost identical to those of the STO buffer layers. The MFIS structure with a high c-axis orientation exhibited a maximum clockwise capacitance-voltage memory window of 1.17 V with a low leakage current of 1.05 × 10 -7 A/cm 2 at an applied voltage of 4 V, which is a significant improvement compared to the MFIS structure with a random orientation. The difference in the electrical properties of the MFIS structures with both types of orientation is discussed in detail. The results obtained from this study indicate that the Au/BTO/STO/p-Si MFIS structure with high c-axis orientation has good potential for use in non-volatile memory applications. © Springer-Verlag 2012.

DOI： 10.1007/s00339-012-7011-6

Treatment of non-point sources by a thermal plasma system under DC partial transferred mode

Li O.L., Guo Y., Chang J.S., Urashima K., Saito N.

International Journal of Plasma Environmental Science and Technology Vol. 6 ( 1 ) page： 63 - 67 2012.7

Language：English Publishing type：Research paper (scientific journal) Publisher：International Journal of Plasma Environmental Science and Technology

Stormwater from urban areas is often accompanied with a variety of chemicals such as heavy metals, phosphorus, trace organic and hydrocarbons. Thermal plasma under DC transferred mode was proposed for decontamination of sediment carried by urban stormwater. After dredging, stormwater sediment/sludge accumulated in a stormwater detention pond was spread on top of a filter and the residues were transferred to a thermal plasma reactor. The reduction of total organic carbon (TOC) is 22% on average. Sediment was melted into slag with thermal plasma treatment under DC partial transferred operating mode. After treatment, the major compositions of the original stormwater sludge (e.g. sand (SiO 2) and calcite (CaCO 3)) decreased while other compositions such as KAlSi 3O 8, Fe 3O 4, NaCl and CaSO 4 were formed. The accumulated concentrations of C xH y, CO, CO 2, NO, NO 2, H 2S and SO 2 after a twohour thermal plasma treatment were 6100, 10000, 6200, 9700, 140, 40 and 27 ppm, respectively.

Study on the combustion inhibition of poly phenylene ether alloy

Yamashita T., Nakashima E., Takeda K., Saito N., Takai O.

Kobunshi Ronbunshu Vol. 69 ( 6 ) page： 297 - 299 2012.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Kobunshi Ronbunshu

In this study, we evaluated the flame retardant properties of several chemical compounds to polyphenylene ether alloy. Some of them (metallic sulfide and a variety of carboxylate salts) are stronger than general flame retardants such as red phosphorus. Molybdenum disulfide is widely used as lubricant and sodium benzoate and disodium succinate are used as food additives. So it is a new insight that they have a significant effect on the flame retardancy of polyphenylene ether alloy. Molybdenum disulfide is a catalyst that degrades polyphenylene ether alloy. We also found a combinatorial effect when blending polyphenylene ether alloy with both molybdenum disulfide and sodium benzoate that show a stronger combustion inhibition than each single compound. ©2012, The Society of Polymer Science, Japan.

DOI： 10.1295/koron.69.297

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