Azusa N. Hattori

MoS2 Nanocube structures as catalysts for electrochemical H2 evolution from acidic aqueous solutions

Core-shell PMMA-Au nanocube structures made by a combination of nanoimprint 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.

Position-, size-, and shape-controlled highly crystalline ZnO nanostructures.

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.

Formation of ferromagnetic interface between β-FeSi2 and Si(111) substrate

Epitaxial β-FeSi2 thin films were grown on Si(111)7×7 clean surfaces by solid phase epitaxy in ultrahigh vacuum: iron deposition at low temperature and subsequent annealing. We found that a ferromagnetic interface layer of iron-rich silicides forms between a β-FeSi2 surface layer and a Si(111) substrate spontaneously from transmission electron microscopy observations and magnetization measurements.

Selection of di(meth)acrylate monomers for low pollution of fluorinated mold surfaces in ultraviolet nanoimprint lithography.

We used fluorescence microscopy to show that low adsorption of resin components by a mold surface was necessary for continuous ultraviolet (UV) nanoimprinting, as well as generation of a low release energy on detachment of a cured resin from a template mold. This is because with low mold pollution, fracture on demolding occurred at the interface between the mold and cured resin surfaces rather than at the outermost part of the cured resin. To achieve low mold pollution, we investigated the radical photopolymerization behaviors of fluorescent UV-curable resins and the mechanical properties (fracture toughness, surface hardness, and release energy) of the cured resin films for six types of di(meth)acrylate-based monomers with similar chemical structures, in which polar hydroxy and aromatic bulky bisphenol moieties and methacryloyl or acryloyl reactive groups were present or absent. As a result, we selected bisphenol A glycerolate dimethacrylate (BPAGDM), which contains hydroxy, bisphenol, and methacryloyl moieties, which give good mechanical properties, monomer bulkiness, and mild reactivity, respectively, as a suitable base monomer for UV nanoimprinting under an easily condensable alternative chlorofluorocarbon (HFC-245fa) atmosphere. The fluorescent UV-curable BPAGDM resin was used for UV nanoimprinting and lithographic reactive ion etching of a silicon surface with 32 nm line-and-space patterns without a hard metal layer.

Epitaxial inversion on ferromagnetic (Fe,Zn)3O4 /ferroelectric BiFeO3 core-shell nanodot arrays using three dimensional nano-seeding assembly

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.

Controlled fabrication of artificial ferromagnetic (Fe,Mn)3O4 nanowall-wires by a three-dimensional nanotemplate pulsed laser deposition method

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 Fe(2.5)Mn(0.5)O(4) 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 Fe(2.5)Mn(0.5)O(4) nanowall-wire. This method enables the study of mesoscopic transport properties of transition-metal oxides towards the development of oxide-based nanodevices.

Three dimensional nano-seeding assembly of ferromagnetic Fe/LaSrFeO4 nano-hetero dot array

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.

Nanowall-Shaped MgO Substrate with Flat (100) Sidesurface : A New Route to Three-Dimensional Functional Oxide Nanostructured Electronics

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.

ZnO Nanobox Luminescent Source Fabricated by Three-Dimensional Nanotemplate Pulsed-Laser Deposition

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.

Enhancement of photoluminescence efficiency from GaN(0001) by surface treatments

We investigated the photoluminescence (PL) efficiency of GaN(0001) single crystals with clean and well-defined surfaces using the PL technique in ultrahigh vacuum in situ. We found typical degradation factors: native oxides at the top surface, damaged layers in the subsurface, and hydrogenated non-radiative states inside bulk GaN. By eliminating the degradation factors, a band-to-band PL intensity of approximately 120 times higher than that of the as-received samples was achieved. The PL efficiency enhancement mechanism is discussed, and the role of hydrogen in GaN crystals is proposed.