Fujio Wakaya

Development of planar x-ray source using gated carbon nanotube emitter

A planar x-ray source using gated carbon nanotube (CNT) emitters was developed. In the diode structure measurement, the turn-on electric field of the CNT cathode was approximately 1.2 V/μm. In the triode structure measurement, characteristic x-ray peaks of Cu and bremsstrahlung x-rays were observed. The maximum energies of the bremsstrahlung x-rays were in agreement with anode voltages. Pulse x-rays synchronized with pulsed gate voltages. Clear x-ray transmission images with high spatial resolution ≤165 μm were obtained.

Maskless laser processing of graphene

Display Omitted Graphene on a SiO2/Si substrate was removed by UV pulsed laser irradiation.Threshold laser power density to remove graphene depended on the graphene thickness.The mechanism is discussed using kinetic energy of thermal expansion.Thickness (or layer-number) selective process for graphene is demonstrated.Maskless patterning of graphene using laser irradiation in the air is demonstrated. Graphene on a SiO2/Si substrate was removed by ultraviolet pulsed laser irradiation. Threshold laser power density to remove graphene depended on the graphene thickness. The mechanism is discussed using kinetic energy of thermal expansion of the substrate surface. Utilizing the thickness dependence, thickness (or layer-number) selective process for graphene is demonstrated. Maskless patterning of graphene using laser irradiation in the air is also demonstrated.

X-ray source using carbon-nanotube field emitter with side-gate electrode

A carbon-nanotube (CNT) field emitter with side-gate electrodes, which are placed in-plane with the CNT emitters, is one of the most promising structures to realize a large-area X-ray charge neutralizer with a cost-effective and easy process. An X-ray source using the screen-printed CNT field emitter with side-gate electrodes is developed. The field-emission and X-ray properties are evaluated with anode voltages of 5 and 10 kV. It is shown that the emission current can be controlled by the side-gate voltage even with such high anode voltages as 5 and 10 kV. It is reported that the highest anode current is approximately 1 µA, which is high enough for an X-ray charge neutralizer. The X-ray radiation is successfully controlled by the pulsed side-gate voltage with subsecond length.

Characterization of X-ray charge neutralizer using carbon-nanotube field emitter

An X-ray charge neutralizer using a screen-printed carbon-nanotube field emitter is demonstrated to show the possibility of a large-area flat-panel charge neutralizer, although the device dimensions in the present work are not very large. The X-ray yields and spectra are characterized to estimate the ion generation rate as one of the figures of merit of neutralizers. Charge neutralization characteristics are measured and show good performance.

Observation of fringelike electron emission pattern from triode Pt nano electron source fabricated by electron-beam-induced deposition

A triode Pt nano electron source with a gate electrode to control the intensity of fringelike emission pattern with low gate voltage has been fabricated. Characteristics of the triode Pt nano electron source were measured at room temperature in a vacuum of less than 10−7 Pa. The emission current was observed when the gate voltage was higher than 30 V and showed a linear dependence in the Fowler–Nordheim plot. Electron wave interference patterns from the triode Pt nano electron source were first observed. The intensity of the electron wave interference pattern was successfully controlled by the gate voltage up to 50 V.

Electron emission from pyroelectric crystal excited using high power infra-red laser light and its x-ray source application

The characteristics of the x-ray source with the LiTaO3 crystal excited using 0.89-W Nd:YLF and 5.0-W fiber laser lights were measured and compared to each other for the development of the high energy and high dose x-ray source. The maximum x-ray energy from the LiTaO3 x-ray source excited using a 5.0-W fiber laser light was higher than that using a 0.89-W Nd:YLF laser light. The average radioactivity of the LiTaO3 x-ray source excited using a fiber laser light with a power of 5.0 W for 90 s was 3.2 times higher than that using a Nd:YLF laser light with a power of 0.89 W. The peak radioactivity of the x-ray source excited using a 5.0-W fiber laser light was 9.8 times higher than that using a 0.89-W Nd:YLF laser light. These results were due to the faster and higher temperature rising using the high power fiber laser light than the low power Nd:YLF laser light.

CNT cold cathode with side-gate electrode for flat panel X-ray source

A carbon nanotube (CNT) cold cathode with a side-gate electrode for a flat panel X-ray source was developed and basic characteristics of the CNT cathode with surface treatments were evaluated. The CNT cathode treated by KrF excimer laser irradiation with a high electric field pulse aging showed the best performance concerning long lifetime, while the CNT cathode treated by tape peeling method without a high electric field pulse aging showed the best performance concerning high anode current. Observed anode current at 4 kV is comparable with some of the commercially available X-ray tubes, suggesting that the CNT cathode with side-gate electrodes can be used for an electron source for a flat panel X-ray source.

Photoassisted electron emission from metal-oxide-semiconductor cathodes based on nanocrystalline silicon

This paper investigates the effect of optical pulses on the electron emission properties of metal-oxide-semiconductor (MOS) cathodes based on nanocrystalline silicon (nc-Si). The emission current is enhanced by about two orders of magnitude by the irradiation of 405 nm laser light. The increase of the emission current under irradiation was proportional to incident laser power. The differential quantum efficiency of the nc-Si based MOS diode itself was estimated to be 3 × 10−2. However, the value of the photoemission current was only 3 × 10−7 due to the short mean free path of hot electron for Pt used as the gate electrode. We obtained a pulsed electron beam from the cathode device by a pulsed laser. The result shows that MOS type cathodes have a suitable structure for optically generating a train of short electron bunches.