Yasusei Yamada

Optical switching of Mg-rich Mg-Ni alloy thin films

Mg–Ni alloy thin films were prepared by dc magnetron sputtering using Mg and Ni targets, and their optical switching property for hydrogen exposure has been investigated. Pd-capped MgNix (0.1<x<0.3) films show a good switchable mirror property. Optical transmittance of these Mg-rich Mg–Ni thin films in the hydride state is much improved compared with that of Mg2Ni thin film. We speculate that the variation of the ratio of Mg2NiH4 to MgH2 formed in the hydride state may cause this difference. Such a wide modulation range of optical switching is important for the application of this material to energy efficient smart windows.

Fabrication of multifunctional coating which combines low-e property and visible-light-responsive photocatalytic activity

A novel multifunctional glazing which combines low-emissivity (low-e) properties and visible-light-responsive photocatalytic activity is proposed. One characteristic point is that it is composed of a simple two-layer coating of F-doped SnO2 (SnO2:F) as an inner layer and titanium dioxide (TiO2) as an outer layer. We confirm that SnO2:F and TiO2 coatings add the low-e property and the photocatalytic activity, respectively. Another characteristic point is that the outer TiO2 layer has a visible-light-responsive photocatalytic activity. A simultaneous irradiation processing of H and N ions onto the TiO2 films is presented to dope with N atoms substitutionally and modify its photocatalytic activity. We find that the co-irradiation with H and N ions of 0.2 keV onto the TiO2 surface improves the photocatalytic activity under visible-light (λ⩾370 nm) illumination by more than three times compared to that of non-irradiated samples. However, the photocatalytic activity under ultraviolet-light (λ⩾200 nm) illumination becomes worse by increasing the ion-acceleration energy. Very low ion-acceleration is the key to dope N atoms into TiO2 films substitutionally by reduction and diffusion processes as well as to suppress a preferential sputtering of oxygen.

Toward Solid-State Switchable Mirror Devices Using Magnesium-Rich Magnesium–Nickel Alloy Thin Films

All-solid-state devices with switchable mirror properties based on magnesium-rich magnesium?nickel alloy thin films have been prepared using magnetron sputtering. The structure of the devices was a Mg?Ni alloy (40 nm)/Pd (4 nm)/Ta2O5 (200 nm)/HxWO3 (500 nm)/ITO on a glass. The composition of the alloy was Mg5Ni. In the initial state, the devices showed a shiny metallic front side and a deep blue back side, namely, in a reflective state. The optical transmittance of the devices at a wavelength of 670 nm was modulated between ?0.3% (the reflective state) and ?40% (the transparent state) by applying a voltage of ?5 V between the ITO and the Mg?Ni alloy layer. The color of the portion changed to the transparent state was relatively neutral. The transition from the reflective state to the transparent state started from the fringe of the electric contact on the Mg?Ni alloy side, and the transparent region spread gradually.

Aluminum buffer layer for high durability of all-solid-state switchable mirror based on magnesium-nickel thin film

Aluminum buffer layer was inserted in all-solid-state switchable mirror based on Mg–Ni thin film to improve its durability. Optical switching speed of the device has been improved because the buffer layer might work to transport electrons and protons promptly as an electrode. Durability of the device was evaluated by cyclic switching test. The device with buffer layer showed three times higher stable durability than that of the device without buffer layer. The authors found that the aluminum buffer layer effectively works to suppress the diffusion of nickel and palladium atoms into the Ta2O5 thin film during cyclic switching test, resulted in high durability.

Flexible all-solid-state switchable mirror on plastic sheet

We developed all-solid-state switchable mirror on flexible plastic [polyethylene terephthalate (PET)] sheet in the view point of low process cost and easy handling. Its optical switching property was investigated. Though the device on PET sheet showed over 4000cycles of switching in durability test, it was lower than that on glass. The reasons seemed to be related with heterogeneous structure, oxidation of surface layer and adhesion of the film. However, it remarkably results that the device on plastic sheet showed such a high durability nonetheless because the sheet is weak to mechanical stress.

Titanium-Buffer-Layer-Inserted Switchable Mirror Based on Mg–Ni Alloy Thin Film

To increase the switching stability of switchable mirrors based on Mg–Ni alloy thin films, we inserted a thin Ti buffer layer between a Mg–Ni alloy thin film and a Pd top layer, which were prepared by DC magnetron sputtering. The optical properties and switching durability of the Ti-inserted switchable mirrors were investigated by gasochromic switching. These samples showed no degradation up to 450 switching cycles and an improvement in their stability was apparent. Moreover, the responsivity in dehydrization was also improved by Ti insertion.

Durability of all-solid-state switchable mirror based on magnesium-nickel thin film

An all-solid-state switchable mirror with Mg-Ni thin film was prepared for basic research on a new type of smart window. The device has a multilayer of Mg-Ni/Pd/Ta 2 O5/WO 3 /indium tin oxide/glass and can switch reversibly from a reflective state to a transparent one. The durability of the device was investigated by a cyclic switching test. Though a transmittance change was slightly decreased, the device showed high durability up to 2000 switching cycles. We found that the degradation was related with the damaged surface of Mg-Ni thin film and the diffusion of Ni and Pd atoms into Ta 2 O 5 thin film during a cyclic switching test.

Effect of ZnO buffer layer on the quality of GaN films deposited by pulsed laser ablation

Abstract ZnO, GaN and ZnO/GaN films have been deposited on c-cut sapphire wafers by Nd:YAG pulsed laser deposition to develop preparation techniques of multi-layer devices in optoelectronics. The quality of the films was investigated as functions of deposition temperature and pressure by means of X-ray diffraction, reflection high energy electron diffraction, scanning electron microscopy and optical transmission spectroscopy. It was found that GaN films could be epitaxially grown on ZnO buffered sapphire substrates even at a low substrate temperature of 300 °C and inactivated N2 flow, confirming the significant role of ZnO as a buffer layer. The crystallinity, surface morphology and optical transmittance of these films could be improved with increasing deposition temperature and were optimized at 800 °C. The optical band gaps of ZnO and GaN were estimated from the optical transmission spectra to be 3.31 and 3.40 eV, respectively.

Near colorless all-solid-state switchable mirror based on magnesium-titanium thin film

Magnesium-titanium (Mg–Ti) thin film was applied to all-solid-state switchable mirror by magnetron sputtering. Mg–Ti thin film works as an anodic electrochromic material of optical switching layer of the device. For clear transparency and durability of the device, a composition of Mg–Ti thin film was optimized. Though a maximum transmittance at transparent state of the device decreased with increasing titanium content in the film, the device showed fast recovery speed to reflective state. The transmittance at a wavelength of 670nm of the device with Mg0.88Ti0.12 was reached from 0.1% (reflective state) to 36% (transparent state) within 15s by applying voltage. The reflectance was also reduced from 57% to 20% within 10s. The transparent state of the device with Mg0.88Ti0.12 thin film showed almost colorless state with a chromaticity coordinates of x=0.335 and y=0.336. In the viewpoint of transmittance and durability, the authors found that the device with Mg0.88Ti0.12 thin film was the most suitable for pr...

Degradation of Switchable Mirror Based on Mg–Ni Alloy Thin Film

Palladium-capped magnesium–nickel switchable mirror thin films were prepared by DC magnetron sputtering, and their optical switching property using a hydrogen-containing gas was investigated. Although these films show excellent switching between metallic and transparent states initially, optical modulation range decreases with increasing number of switching cycles; it is reduced to 10% after 170 cycles. To study the degradation mechanism, the depth profile, electronic structure and surface morphology of the samples were determined. Photoelectron spectroscopy measurements revealed that the interface structure between palladium and magnesium–nickel layers was modified owing to repeated switchings. Electron microscope observations showed that the surface structure was markedly changed for the degraded sample.