Keisuke Nagato

Effects of Oxidation during Synthesis on Structure and Field-Emission Property of Tungsten Oxide Nanowires

We investigated the structure and field-emission property of tungsten oxide nanowires synthesized under different oxidation degrees. We annealed a sputtered tungsten film at 800 °C, controlling the ratio of hydrogen to oxygen (RHO) using Ar/H2 (97/3%) gas in a vacuum furnace. The resulting differences in shape, number density, length, and width of the nanowires were observed by scanning electron microscopy. In the RHO range of 0 to 0.4, beltlike structures were synthesized. In the RHO range of 0.8 to 4, only thin nanowires were synthesized. In this range, length and width did not differ with RHO, but the number density decreased as RHO increased. The sample with a nanowire density of 2 µm-2, annealed with an RHO of 4, showed the highest field-emission property, i.e., a current of 1 mA/cm2 for an electric field of 22 V/µm. We demonstrated their emission by fluorescence imaging and showed that the nanowires are promising candidates for field emitters.

Rapid and Localized Synthesis of Single-Walled Carbon Nanotubes on Flat Surface by Laser-Assisted Chemical Vapor Deposition

The synthesis of single-walled carbon nanotubes (SWNTs) at a controlled position on a flat surface was demonstrated by laser-assisted chemical vapor deposition (CVD). The developed multilayer substrate including an energy-confining layer (ECL) enabled the efficient heating of catalysts on the surface, resulting in the rapid and localized syntheses of SWNTs. Using a Nd:YAG laser as a heat source, we achieved the rapid synthesis with laser irradiation for 1 s and the localized synthesis in an area of approximately 1 µm diameter. In addition, the scanning of the laser irradiation spot at a rate of 1 µm/s enabled the line-patterned synthesis of SWNTs at a linewidth of 2 µm. The resulting synthesis of SWNTs on a flat surface by laser-assisted CVD will lead to the easy and controllable fabrication of SWNT-based nanodevices.

Solid-phase growth mechanism of tungsten oxide nanowires synthesized on sputtered tungsten film

The authors have proposed a solid-phase growth mechanism of tungsten oxide nanowires synthesized on sputtered tungsten films. Transmission electron microscopy observation, x-ray diffraction analyses, and some ex situ experiments were performed to verify the growth model. The nanowire nuclei are generated at irregular points on the W surface. An amorphous oxidized layer is formed on the W surface by annealing, and tungsten oxide molecules migrate on the W surface into the nuclei, thus contributing to the nanowire growth.

Planarization of amorphous carbon films on patterned substrates using gas cluster ion beams

Surface planarization and modification of a patterned surface were demonstrated using gas cluster ion beam (GCIB). Grooves with 100–400 nm intervals were formed on amorphous carbon films using focused ion beams to study the special frequency dependence of the planarization. Also, line and space patterns were fabricated on Si substrates, and amorphous carbons were deposited as a model structure of discrete track media. Subsequently, surface planarization using Ar-GCIB was carried out. After GCIB irradiations, all of the grooves were completely removed, and a flat surface was realized. And it showed that GCIB irradiation planarized grooves without huge thickness loss. From the power spectrum density of an atomic force microscope, GCIB preferentially removed grooves with small intervals. It was found from energy dispersive x-ray spectroscopy that surface planarization without severe damage in the amorphous carbon and magnetic layers was carried out with GCIB.

Study of Gas Cluster Ion Beam Planarization for Discrete Track Magnetic Disks

We studied planarization of carbon overcoat by gas cluster ion beam (GCIB) for discrete track magnetic disks. We fabricated discrete tracks with variant pitches by focused ion beam on a 30-nm-thick carbon overcoat deposited on magnetic disks, and we planarized them with an Ar GCIB using lateral sputtering effect. It was found that planarization by GCIB was more effective for lower wavelengths than for those with a few hundreds nanometers. On the discrete tracks with 100-nm pitch and 20-nm depth, the initial peaks and valleys were removed by GCIB; however, on the ones with over 200-nm pitch, the initial peaks and valleys remained though the heights were drastically decreased. These results indicate the capability of GCIB planarization as a process for the discrete track media. Furthermore, cross sections were observed by transmission electron microscopy. The GCIB with relatively low dose could also planarize the 120-nm-pitch carbon tracks without etching the magnetic layer.

Electrodynamic trapping of spinless neutral atoms with an atom chip

Three-dimensional electrodynamic trapping of neutral atoms has been demonstrated. By applying time-varying inhomogeneous electric fields with micron-sized electrodes, nearly 10(2) strontium atoms in the 1S0 state have been trapped with a lifetime of 80 ms. In order to design the electrodes, we numerically analyzed the electric field and simulated atomic trajectories in the trap, which showed reasonable agreement with the experiment.

Local Synthesis of Tungsten Oxide Nanowires by Current Heating of Designed Micropatterned Wires

We locally synthesized tungsten oxide nanowires at predetermined positions by current heating of designed micropatterned wires. The current wires were fabricated from tungsten thin film and had two different widths in the same wire, and the narrower sections were heated more than the wider sections due to the difference in electric resistance. The temperature of the narrower sections was controlled to be optimal for nanowire synthesis in an O2 atmosphere in a vacuum chamber. We demonstrated the synthesis of nanowires over an area of approximately 1×1 µm2 and successfully synthesized nanowires on a regular 20 by 20 array with narrow sections with 10 µm pitch.

Injection compression molding of high-aspect-ratio nanostructures

High-aspect-ratio nanostructures were replicated by injection compression molding. The effects of the mold temperature and pattern shape on the degree of replication were investigated using a replication technique for optical disks (digital versatile disks) and a Ni stamper with patterns with a width and height of 200 and 400 nm. A higher mold temperature and a smaller area-duty ratio (hill area/unit area) of the Ni stamper, i.e., a larger area-duty ratio of the replica resulted in better replication because a Ni stamper with a smaller area-duty ratio suppresses the formation of a skin layer.

Fabrication of antireflection-structured surface using vertical nanowires as an initial structure

An antireflection structure was fabricated using vertically synthesized iron oxide nanowires as an initial structure. Iron oxide nanowires can be synthesized by simply annealing a thin Fe film in air. The authors propose a process for transforming these nanowires into an array of cone: An additional thin Cr film is deposited onto the nanowires and then etched by reactive ion etching. As a result of the redeposition of Cr, the vertical nanowires were successfully transformed into conical structures. This antireflection nanostructure is replicated onto a polymer surface by thermal nanoimprinting, and the replicated surface exhibited low reflectivity at all visible wavelengths.

Planarization of Bit-Patterned Surface Using Gas Cluster Ion Beams

The planarization of the bit-patterned surface using gas cluster ion beams (GCIBs) was studied. By applying the features of gas cluster ions, such as low-energy irradiation and surface smoothing effects, it is possible to carry out effective smoothing of a patterned surface. We fabricated a dot pattern on the Si substrate as a model structure for bit-patterned media by electron beam lithography and inductive coupled plasma etching. A carbon overcoat covered the Si dots. On Ar-GCIB irradiation at an acceleration voltage of 20 kV, the Si dots with a diameter of 150 nm were planarized. The required ion dose for planarization was 5 times 1014 ions/cm2. The cross-sectional transmission electron microscope images showed that only the carbon overcoat layer was planarized without a change in the structure of Si dots. Etching depth of the overcoat increased linearly with the ion dose, indicating good reproducibility of the method with precise control of residual thickness.