Keigo Kasuya

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.

Stabilization of a tungsten ⟨310⟩ cold field emitter

Cold-field-emission current from a tungsten ⟨310⟩ emitter in a scanning electron microscope (SEM) gun, evacuated by an ion pump and a supplementary nonevaporative getter pump, was stabilized. It was verified that the probe current from a local (310) crystal plane exhibits different time variations in comparison to that of total current. As for the probe current under a pressure of 2×10−9 Pa, a stable plateau region—which lasted about 4 h—appeared just after flashing of the emitter. By observing emission patterns, it was verified that these different emission characteristics are originated from the anisotropy of current decay in accordance with crystal planes. With low-temperature “mild flashings” at 700 °C, the plateau region was extended to 12 h, which is long enough for practical SEM application. The superior properties of the plateau region, namely, high current, low noise, and small current variation, will enhance the performance and usability of electron microscopes.

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.

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.

Magnetic field superimposed cold field emission gun under extreme-high vacuum

A magnetic-field superimposed cold field emission gun (M-FEG) was developed for a 1.2-MV holography transmission electron microscope (TEM). Differential pumping using three nonevaporative getter pumps reduced the pressure of the gun to an extreme-high vacuum of 3 × 10−10 Pa and provided a stable probe current. The 90% decrease time, at which the current falls to 90% of the initial value, was 900 min. The current variation over the course of 8 h was 5.2%. The superimposed magnetic field provided large probe currents ranging from 1 to 170 nA for total currents ranging from 1 to 300 μA. The reduced pressure caused the angular current density to be twice that of a conventional field emission gun operating at 10−8 Pa. This increase can be explained by the difference between current densities emitted from the clean and adsorbed emitter surfaces. These results show that the developed M-FEG can provide the 1.2-MV TEM with a bright and stable emission current.

Real-Time Observation of Growth of Tungsten Oxide Nanowires with a Scanning Electron Microscope

We observed the growth process of tungsten oxide nanowires in real-time with a field emission scanning electron microscope (SEM). The observation was performed by a new in-situ observation system designed to perform two functions: heating of tungsten film and local supply of O2 gas. The nanowires grew after the nucleation on the surface. Their length extended with time keeping the direction constant. The growth rate decreased with exponential.

High emission current of 1000 μA at 4 × 10−10 Pa from W⟨310⟩ cold field emitter in electron gun

Total cold field emission (CFE) current has generally been limited to 30 μA because a higher current may initiate a vacuum arc that destroys the emitter. However, high emission current is needed to reduce the operation time in analytical electron microscopy and to further enhance the brightness of CFE. The authors achieved a total emission current of 1000 μA by reducing the pressure around the electron gun to 4 × 10−10 Pa. This high emission current was achieved without initiating a vacuum arc apparently because the number of ions striking the emitter surface was reduced. While a high angular current density of 1670 μA/sr was obtained at 1000 μA, current fluctuation occurred during the initial “plateau region” period of the probe current, and this needs to be investigated. This fluctuation is attributed to surface adsorption of gases emitted by electron stimulated desorption (ESD). Evaluation of the effect of introducing dominant ESD gases, namely, H2 or CO, on the current fluctuation revealed that the ad...

Development of an Aberration Corrected 1.2-MV Field Emission Transmission Electron Microscope

A practical holography electron microscope was first developed in 1978 by the late Dr. Tonomura. After that, we (Tonomura’s group) developed bright and monochromatic field-emission electron beams over 35 years for observing quantum phenomena by utilizing the wave nature of electrons. As it turns out, every time we developed a brighter electron beam, electron interference experiments became easier to perform, and the precision in the phase measurements increased, thereby opening up new application fields.