Azusa N. Hattori

Osaka University, Suika, Ōsaka, Japan

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Publications (36)70.67 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report a strategy for controlling nanoscopic electronic domains to produce gigantic Mott metal–insulator transition phenomena in strongly correlated oxides by fabricating oxide micro-nano-wires, nanowalls, nanoboxes. We investigated the dependence of spatial dimensionality on wire width for a disordered configurations of metallic domains in VO2 microwires to nanowires on TiO2(001) and Al2O3(0001) substrates with well-positioned alignment by a nanoimprint (NIL) technique. We observed a temperature-induced steep multistep metal–insulator transition in artificial VO2 micro/nano-wires. With further development, we report a new bottom-up fabrication method for the formation of extremely small transition-metal oxide nanostructures employing a combination of NIL and pulsed laser deposition (PLD) techniques, called the three-dimensional nanotemplate-PLD method, to demonstrate functional oxide nanowall wires, nanoboxes, and hetero-nanowall oxide devices with widths of 20–120 nm and excellent size controllability.
    Japanese Journal of Applied Physics 04/2014; 53(5S1):05FA10. · 1.07 Impact Factor
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    ABSTRACT: We have established a unique technique to fabricate three-dimensional (3D) well-defined transition-metal oxide epitaxial nanostructures. Fabrication of epitaxial spinel ferrite Fe2.2Zn0.8O4 (FZO) nanowall wires with a tunable width down to 20 nm was achieved. Cross-sectional transmission electron microscopy revealed the existence of an epitaxially matched lateral interface between the FZO nanowall wire and the side surface of 3D-MgO substrate. Magnetoresistance measurements showed ferromagnetic properties of the FZO nanowall wire at 300 K. The role of antiphase boundaries on the functionalities of the FZO nanoconfined wire is discussed.
    Applied Physics Express 03/2014; 7(4):045201. · 2.73 Impact Factor
  • Azusa N. Hattori, Hidekazu Tanaka
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    ABSTRACT: By combining inclined pulsed-laser deposition (PLD) with nanoimprint lithography, an original three-dimensional (3D) nanofabrication technique, namely "3D nanotemplate PLD technique E has been established. 3D nanotemplate PLD enables to fabricate the large arrays of programmable ZnO nanostructures: nanoboxes and nanowires with a width of 20nm. Cathodoluminescence (CL) measurements at 300K showed an intense luminescence peak around 380 nm corresponding to near-band-edge (NBE) emission from even a single ZnO nanobox. The CL intensity mapping also showed the brilliant NBE luminescence from the entire single ZnO nanobox. The architecturally designed ZnO nanostructures with an excellent wide-gap luminescent semiconductor character should be good candidates for optoelectronic materials for nanoscale device applications.
    02/2014;
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    ABSTRACT: Core-shell PMMA-Au nanocube structures made by a combination of nano-imprint lithography and sidewall deposition were used as template for electrodeposition of MoS2, Ni and Pt. Linear sweep voltammetry experiments obtained in an aqueous solution containing 0.29 M H2SO4 (pH 0.24) showed that the onset potential of the core-shell-shell PMMA-Au-MoS2 nanocube electrode for the hydrogen evolution reaction (HER) was shifted to the positive direction (i.e. requiring a lower overpotential) by 20-40 mV compared to planar MoS2 films. This indicates that the nanocube electrodes have a significantly increased HER activity, which is probably because of a higher density of catalytically active edge sites available at the nanocube surface. It was also found that the HER activity initially increased with increasing MoS2 deposition time, but decreased after deposition for 60 min because the edges of the nanocubes became rounded, thereby decreasing the number of active edge sites. By depositing Ni and Pt on top of PMMA-Au nanocubes, it was shown that this method can also be used for the synthesis of nanocube structures with varying compositions.
    ACS Applied Materials & Interfaces 01/2014; · 5.01 Impact Factor
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    ABSTRACT: The magnetic and crystallographic microstructures in Fe2.5Zn0.5O4 (FZO) wires fabricated using nano-imprint lithography, pulsed laser deposition and a molybdenum lift-off mask technique were studied by transmission electron microscopy (TEM). A process using a focused ion beam completely separated the FZO wires from the insulating MgO substrate, and accordingly allowed in-depth TEM studies of the domain structures. Observations using energy-filtered TEM demonstrated good crystallinity of the FZO wires. Both Lorentz microscopy and electron holography studies revealed unexpectedly small magnetic domains (∼100 nm or smaller) due to a significant interaction with antiphase boundaries. The role of antiphase boundaries on the functionalities observed in the constrained wires (e.g., nonlinear I–V characteristics and large magnetoresistance) is discussed on the basis of these microscopic observations.
    Acta Materialia. 01/2014; 64:144–153.
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    ABSTRACT: Temperature-dependent conductivities at dc and terahertz (THz) frequency region (σTHz(ω,T)) were obtained for a strongly correlated (La0.275Pr0.35Ca0.375)MnO3 (LPCMO) film using THz time domain spectroscopy. A composite model that describes σTHz(ω,T) for LPCMO through the insulator-metal transition (IMT) was established by incorporating Austin-Mott model characterizing the hopping of localized electrons and Drude model explaining the behavior of free electrons. This model enables us to reliably investigate the dc transport dynamics from THz conductivity measurement, i.e., simultaneously evaluate the dc conductivity and the competing composition of metal and insulator phases through the IMT, reflecting the changes in microscopic conductivity of these phases.
    Applied Physics Letters 01/2014; 105(2):023502-023502-4. · 3.79 Impact Factor
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    ABSTRACT: We have studied in situ structures and magnetic properties for several iron silicides grown on Si(111)7×7 clean surfaces by the solid phase epitaxy (SPE) method: deposition of Fe at 40 K and subsequently annealing, and summarized almost all silicide phases depending on the deposition thickness and the annealing temperature as a “schematic magnetic phase diagram”. In the SPE growth, bcc-Fe(111)1×1, c-FeSi(111)1×1, 2×2, and c(4×8), α-FeSi2(112)2×2, ∊-FeSi(111)√3 ×√ 3-R30°, β-FeSi2(101)/(110) and polycrystalline phases are formed on Si(111)7×7 surfaces depending on Fe coverage (0.15-10.0 nm) and annealing temperature (470-1070 K). The structures and magnetic properties of all the above Fe silicide phases have been characterized by using in situ reflection high-energy electron diffraction, scanning tunneling microscopy, and surface magneto-optical Kerr effect. Bulk-unstable c-FeSi phases showed ferromagnetic property at 40 K and this ferromagnetism remained even after air exposure. For the c-FeSi(111)/Si(111) system, the large spin polarization was proposed at the Fe interface atoms in the B5-type interface structure between the c-FeSi(111) ultra-thin film and the Si(111) substrate by first-principle calculations.
    Journal of Magnetism and Magnetic Materials 01/2014; 363:158–165. · 1.83 Impact Factor
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    ABSTRACT: A method for simple, cost-effective and reproducible formation of Ni and Cu2O nanocubes with dimensions of 200-500 nm and a small size distribution is introduced. For this, the well-known templated electrodeposition technique was extended to cubic PMMA templates made by nanoimprint lithography. This method was successfully used for the formation of Ni and p-Cu2O nanocubes, and for the formation of segmented nanobars containing both phases. The lateral dimensions of the nanocubes exactly resembled the dimensions of the template, and the height could be varied by adjustment of deposition time. Nanocubes formed via this method can either remain attached to the substrate or can be dispersed in solution. p-Cu2O is considered to be one of the most promising photocathode materials for solar water splitting. It is demonstrated that the activity of the p-Cu2O nanocubes for photocatalytic water splitting can be measured, and it was found that the nanocube morphology enhances the photocatalytic activity compared to thin films.
    ACS Applied Materials & Interfaces 10/2013; · 5.01 Impact Factor
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    ABSTRACT: We propose a novel approach to fabricate metal oxide nanostructures with locally modified thicknesses. Utilizing a nanoscale shadowing effect induced by a three-dimensional nanotemplate patterned on a substrate, we demonstrate that pulsed-laser deposition in an in-plane oblique configuration enables us to locally grow an oxide. The analysis on the growth position reveals that it can be precisely defined by adjusting only a few geometrical parameters. A ferromagnetic oxide Fe2.5Zn0.5O4 nanoconstriction with a bottleneck width of 65 nm and a length of 53 nm is fabricated by controlling the incident azimuthal angle and growth duration in sequential deposition processes.
    Applied Physics Express 03/2013; 6(3):5201-. · 2.73 Impact Factor
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    ABSTRACT: An architecturally designed nanowall-shaped MgO (nanowall MgO) was fabricated by the combination of nanoimprint lithography (NIL) and pulsed-laser deposition (PLD). The sidesurface on the nanowall MgO exhibited (111) facets with edge truncation instead of the most stable (100) face when the aspect ratio between the height and width of the nanowall MgO was lower than 0.7. By optimizing the surface crystallography, typically by designing the nanowall aspect ratio and controlling the postannealing treatment conditions, nanowall MgO with a single-crystal flat (100) sidesurface could be produced. Applying the nanowall MgO to a substrate, we demonstrated the formation of extremely small three-dimensional (3D) epitaxial metal oxide nanostructures with an arbitrarily controlled size. The nanofabrication technique utilizing the nanowall MgO substrate will open a new route to high-quality 3D epitaxial metal oxide nanostructures.
    Japanese Journal of Applied Physics 01/2013; 52(1):5001-. · 1.07 Impact Factor
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    ABSTRACT: We demonstrate an advanced fabrication method based on standard surface diffusion theory for preparation of core-shell nano-heterostructure arrays consisting of ferromagnetic (Fe,Zn)3O4 (FZO) and ferroelectric BiFeO3 (BFO) using a 3D nano-seeding-assembly technique. By adapting epitaxial spinel FZO nanodot arrays on a perovskite SrTiO3 substrate to templates, well-positioned selective epitaxial growth of FZO cores and BFO shells was spontaneously stimulated. This technique resolves the longstanding issues of the precise positional alignment and configuration inversion of materials that conventional self-assembly growth has faced.
    Journal of Applied Physics 01/2013; 113(10):104302. · 2.21 Impact Factor
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    ABSTRACT: Precisely size-controlled and crystalline (La0.275Pr0.35Ca0.375)MnO3 (LPCMO) nanobox array structures were fabricated down to 30 nm in wall-width by the three-dimensional nanotemplate pulsed laser deposition technique. The hard X-ray photoemission spectroscopy in LPCMO nanobox array structures showed the existence of the satellite on the Mn 2p3/2 peak at 203 K with a higher intensity than that in the LPCMO film even at 153 K. This result indicated that the insulator-metal transition (IMT) in the nanobox array structures occurred at the higher IMT temperature.
    Applied Physics Letters 01/2013; 103(22):223105-223105-4. · 3.79 Impact Factor
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    ABSTRACT: A novel nanofabrication technique for metal oxide has been developed by combining inclined pulsed laser deposition with a three-dimensional (3D) nanotemplate prepared by nanoimprint lithography. ZnO nanobox structures with a tunable wall width of 20 to 100 nm can be successfully fabricated by controlling their alignment on a large scale. Cathodoluminescence measurements at 300 K showed an intense luminescence peak at around 380 nm corresponding to near-band-edge emission from even a single ZnO nanobox. The architecturally designed 3D ZnO nanostructures with an excellent wide-gap luminescent character should be good candidates for nanoscale device applications as a luminescent source.
    Applied Physics Express 12/2012; 5(12):5203-. · 2.73 Impact Factor
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    ABSTRACT: We have developed a new method to fabricate extremely small transition-metal oxide nanowires. Using a combination of nanoimprint template patterning and inclined substrate pulsed laser deposition, we successfully fabricated magnetic oxide Fe2.5Mn0.5O4 nanowall-wires, and controlled the width in a range from 120 nm down to about 20 nm by varying deposition parameters. Magnetoresistance measurements revealed ferromagnetic properties of the Fe2.5Mn0.5O4 nanowall-wire. This method enables the study of mesoscopic transport properties of transition-metal oxides towards the development of oxide-based nanodevices.
    Nanotechnology 11/2012; 23(48):485308. · 3.84 Impact Factor
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    ABSTRACT: We demonstrate an advanced fabrication method for perfectly position-controlled ferromagnetic semiconductor (Fe,Zn)(3)O(4) nanodot arrays down to several hundred nanometers in size surrounded by a ferroelectric BiFeO(3) matrix. By performing position-selective crystal growth of perovskite BiFeO(3) on the position-controlled epitaxial spinel (Fe,Zn)(3)O(4) nanodot-seeding template, which is prepared using a hollow molybdenum mask lift-off nanoimprint lithography process on a perovskite La-doped SrTiO(3)(001) substrate, we produce functional oxide three-dimensional lateral heterojunctions. The position-selectivity can be explained based on standard surface diffusion theory with a critical nucleation point. Establishing this fabrication process could lead to innovative nanointegration techniques for spintronic oxide materials.
    Nanotechnology 08/2012; 23(33):335302. · 3.84 Impact Factor
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    ABSTRACT: Well-ordered ferromagnetic Fe nanodots/LaSrFeO4 nanocomposites have been fabricated by self-assembled crystal growth on La-SrTiO3 substrates having Fe nanoseed array fabricated by nanoimprint lithography (NIL). The Fe nanoseeds with spacing of 200 nm make possible the formation of perfectly arranged Fe/LaSrFeO4 nanocomposites; phase-separated Fe nanodots and the LaSrFeO4 matrix grew only on the nanoseeds and on the area except nanoseeds, respectively. A calculation based on a surface diffusion model has indicated that the nanoseed spacing required for the formation of the perfectly arranged nanocomposite is less than 400 nm. Magnetic force microscopy revealed an arrangement of isolated ferromagnetic domain corresponding to Fe nanodots grown on the Fe nanoseeds. The combination of self-assembled growth and NIL gives a route of the rational formation of high-density ferromagnetic memory devices.
    Journal of Applied Physics 07/2012; 112(2). · 2.21 Impact Factor
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    ABSTRACT: This paper reports the structural and chemical characteristics of atomically flat gallium nitride (GaN) surfaces prepared by abrasive-free polishing with platinum (Pt) catalyst. Atomic force microscopy revealed regularly alternating wide and narrow terraces with a step height equivalent to that of a single bilayer on the flattened GaN surfaces, which originate from the differences in etching rate of two neighboring terraces. The material removal characteristics of the method for GaN surfaces were investigated in detail. We confirmed that an atomically smooth GaN surface with an extremely small number of surface defects, including pits and scratches, can be achieved, regardless of the growth method, surface polarity, and doping concentration. X-ray photoelectron spectroscopy showed that the flattening method produces clean GaN surfaces with only trace impurities such as Ga oxide and metallic Ga. Contamination with the Pt catalyst was also evaluated using total-reflection X-ray fluorescence analysis. A wet cleaning method with aqua regia is proposed, which markedly eliminates this Pt contamination without affecting the surface morphology.
    Journal of Crystal Growth 06/2012; 349(1):83–88. · 1.55 Impact Factor
  • Source
    Azusa N. Hattori, Katsuyoshi Endo
    04/2012; , ISBN: 978-953-51-0512-1
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nanocomposite thin films consisting of spinel-type magnetic semiconductor (Fe,Zn)3O4 and perovskite-type ferroelectric BiFeO3 were prepared by a self-assembled growth method from a composition-adjusted Fe--Zn--Bi--O single target using a pulsed laser deposition technique. It is found that BiFeO3 square patterns reflecting the in-plane lattice shape of the SrTiO3 substrate are grown directly from the substrate interface with inverse pyramid shapes embedded in a (Fe,Zn)3O4 matrix in the absence of other phases. This self-assembled growth should be a convenient method to fabricate a lot of nano-heterojunctions with complicated oxide structures, which is applicable to spintronic oxide materials.
    Japanese Journal of Applied Physics 03/2012; 51(3):5504-. · 1.07 Impact Factor
  • Azusa N Hattori, Atsushi Ono, Hidekazu Tanaka
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    ABSTRACT: Highly ordered ZnO nanoboxes and nanowire structures with a width of ∼ 20 nm have been successfully fabricated by the combination of nanoimprint lithography and pulsed laser deposition utilizing a glancing angle deposition (GLAD) technique. The periodicity, size, and shape of the ZnO nanoboxes and nanobelts can be easily controlled over a large area by changing the molds and deposition conditions. At the initial stage of growth by GLAD, nanonucleation led to nanopillar structures, which agglomerated to form nanobox and nanobelt structures at room temperature (RT). The ZnO nanostructures have a c-axis orientation along the nanopillar direction after postannealing and exhibit an intense cathodoluminescence peak around 380 nm at RT.
    Nanotechnology 10/2011; 22(41):415301. · 3.84 Impact Factor

Publication Stats

36 Citations
70.67 Total Impact Points

Institutions

  • 2010–2014
    • Osaka University
      • • Institute of Scientific and Industrial Research
      • • The Institute of Scientific and Industrial Research (ISIR)
      • • Department of Precision Science and Technology and Applied Physics
      • • Division of Electronic and Information Engineering
      Suika, Ōsaka, Japan
  • 2004–2014
    • Nara Institute of Science and Technology
      • Graduate School of Materials Science
      Ikuma, Nara, Japan