Preparation of an Inorganic All-Solid-State Electrochromic Device with Excellent Open-Circuit Memory.

Due to the spontaneous transport of small-sized cations and redox reactions under open circuit conditions, the currently reported coloring electrochromic devices (ECDs) may self-bleach easily. The resulting ECDs exhibit poor open-circuit memory, which limits their applications in static display advertisement. By constructing energy barriers to effectively control small-sized cation transport, the redox reaction could be suppressed, thereby inhibiting the self-bleaching of ECDs. In this study, phosphate glass is used as an electrolyte to construct high-energy barriers. Sodium ions in phosphate glass absorb external heat to cross energy barriers and become conductive charge carriers. In this case, the electrochromism of ECDs is allowed. On the contrary, after the absorbed heat energy is released, sodium ions are immediately trapped by oxygen ions in the PO4 unit, becoming frozen ions. At this point, the electrochromization of ECDs is prohibited. Based on the ionic conductive feature of phosphate glass, ECDs absorb heat and are colored by applying an electric field first. Then, ECDs release the thermal energy and the sodium ions transport in the electrolyte is blocked to cut off the self-bleaching pathway. The prepared inorganic all-solid-state ECDs maintained the colored state for several months using the method mentioned above, which solved the problem of the poor open-circuit memory of ECDs.

Room-temperature magnetoresistance in Ni78Fe22/C8-BTBT/Ni78Fe22 nanojunctions fabricated from magnetic thin-film edges using a novel technique.

Molecular spintronic devices are gaining popularity because the organic semiconductors with long spin relaxation times are expected to have long spin diffusion lengths. A typical molecular spintronic device consists of organic molecules sandwiched between two magnetic layers, which exhibits magnetoresistance (MR) effect. Nanosized devices are also expected to have a high spin polarization, leading to a large MR effect owing to effective orbital hybridization. However, most studies on nanosized molecular spintronic devices have investigated the MR effect at low temperatures because of the difficulty in observing the MR effect at room temperature. Here we focus on high-mobility molecules expected to show long spin diffusion lengths, which lead to the observation of the MR effect in nanoscale junctions at room temperature. In this study, we fabricate magnetic nanojunctions consisting of high-mobility molecules, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), sandwiched between two Ni78Fe22 thin films with crossed edges. Transmission electron microscopy (TEM) images reveal that C8-BTBT molecular layers with smooth and clear interfaces can be deposited on the Ni78Fe22 thin-film edges. Consequently, we observe a clear positive MR effect, that is, R P < R AP, where R P and R AP are the resistances in the parallel (P) and antiparallel (AP) configurations, respectively, of two magnetic electrodes in the Ni78Fe22/C8-BTBT/Ni78Fe22 nanojunctions at room temperature. The obtained results indicate that the spin signal through the C8-BTBT molecules can be successfully observed. The study presented herein provides a novel nanofabrication technique and opens up new opportunities for research in high-mobility molecular nano-spintronics.

Electric Transport Properties of NaAlB14 with Covalent Frameworks.

A synthetic protocol was developed for obtaining a single phase of polycrystalline NaAlB14 with strongly connected intergrain boundaries. NaAlB14 has a unique crystal structure with a tunnel-like covalent framework of B that traps monovalent Na and trivalent Al ions. Owing to the atmospheric instability and volatility of Na, the synthesis of polycrystalline NaAlB14 and its physical properties have not been reported yet. This study employed a two-step process to achieve single-phase polycrystalline NaAlB14. As a first step, a mixture of Al and B with excess Al was sintered in the Na vapor atmosphere followed by HCl treatment to remove excess Al as a second step. For obtaining bulk samples with strong grain connection, vacuum or high-pressure (HP) annealing was employed. HP annealing promoted bandgap shrinkage due to the crystal strain and defect levels and suppressed intergranular resistance. As a result, the HP-annealed sample achieved superior transport properties (0.1 kΩ cm at 300 K) to the vacuum-annealed sample (260 kΩ cm). Furthermore, from the viewpoint of its crystal structure and DFT calculations, the most probable site for the defect was suggested to be the Na site.

Anhydrous Silicophosphoric Acid Glass: Thermal Properties and Proton Conductivity.

Anhydrous silicophosphoric acid glass with an approximate composition of H5 Si2 P9 O29 was synthesized and its thermal and proton-conducting properties were characterized. Despite exhibiting a glass transition at 192 °C, the supercooled liquid could be handled as a solid up to 280 °C owing to its high viscosity. The glass and its melt exhibited proton conduction with a proton transport number of ∼1. Although covalent O-H bonds were weakened by relatively strong hydrogen bonding, the proton conductivity (4×10-4  S cm-1 at 276 °C) was considerably lower than that of phosphoric acid. The high viscosity of the melt was due to the tight cross-linking of phosphate ion chains by six-fold-coordinated Si atoms. The low proton conductivity was attributed to the trapping of positively charged proton carriers around anionic SiO6 units (expressed as (SiO6/2 )2- ) to compensate for the negative charges.

A Novel Technique for Controlling Anisotropic Ion Diffusion: Bulk Single-Crystalline Metallic Silicon Clathrate.

Na-free Si clathrates consisting only of Si cages are an allotrope of diamond-structured Si. This material is promising for various device applications, such as next-generation photovoltaics. The probable technique for synthesizing Na-free Si clathrates is to extract Na+ from the Si cages of Na24 Si136 . Vacuum annealing is presently a well-known conventional and effective approach for extracting Na. However, this study demonstrates that Na+ cannot be extracted from the surface of a single-crystalline type-II metallic Si clathrate (Na24 Si136 ) in areas deeper than 150 µm. Therefore, a novel method is developed to control anisotropic ion diffusion: this is effective for various compounds with a large difference in the bonding strength between their constituent elements, such as Na24 Si136 composed of covalent Si cages and weakly trapped Na+ . By skillfully exploiting the difference in the chemical potentials as a driving force, Na+ is homogeneously extracted regardless of the size of the single crystal while maintaining high crystallinity. Additionally, the proposed point defect model is evaluated via density functional theory, and the migration of Na+ between the Si cages is explained. It is expected that the developed experimental and computational techniques would significantly advance material design for synthesizing thermodynamically metastable materials.

Understanding the effect of oxide components on proton mobility in phosphate glasses using a statical analysis approach.

The models to describe the proton mobility (µ H) together with the glass transition temperature (T g) of proton conducting phosphate glasses employing the glass composition as descriptors have been developed using a statical analysis approach. According to the models, the effects of additional HO1/2, MgO, BaO, LaO3/2, WO3, NbO5/2, BO3/2 and GeO2 as alternative to PO5/2 were found as following. µ H at T g is determined first by concentrations of HO1/2 and PO5/2, and µ H at T g increases with increasing HO1/2 concentration and decreasing PO5/2. The component oxides are categorized into three groups according to the effects on µ H at T g and T g. The group 1 oxides increase µ H at T g and decrease T g, and HO1/2, MgO, BaO and LaO3/2 and BO3/2 are involved in this group. The group 2 oxides increase both µ H at T g and T g, and WO3 and GeO2 are involved in this group. The group 3 oxides increase T g but do not vary µ H at T g. Only NbO5/2 falls into the group 3 among the oxides examined in this study. The origin of the effect of respective oxide groups on µ H at T g and T g were discussed.

Proton transport properties of proton-conducting phosphate glasses at their glass transition temperatures.

The proton transport properties of 32 kinds of proton-conducting phosphate glasses with broad ranges of glass transition temperature, proton conductivity, and the proton carrier concentration were studied. Almost constant proton mobility of around 2 × 10-8 cm2 V-1 s-1 at the glass transition temperature, corresponding to a diffusion coefficient of approximately 4 × 10-10 cm2 s-1, was found for the glasses. The reason why the diffusion coefficient of protons is almost constant in various proton-conducting phosphate glasses was discussed based on the role of the protons as a cross-linker within the phosphate framework via hydrogen bonding. We evaluated the highest proton conductivity of the phosphate glasses and melts based on the almost constant mobility at their glass transition temperatures and obtained a highest expected proton conductivity of 7.5 × 10-3 S cm-1 at 300 °C. The potential of proton-conducting phosphate glasses as electrolytes in intermediate temperature fuel cells was also discussed.

Surface relief hologram formed by selective SiO2 deposition on soda-lime silicate glass.

We propose the fabrication of surface relief holograms via selective SiO2 deposition on soda-lime silicate glass substrates. Initially, the original hologram was recorded on an azobenzene photosensitive polymer film coated on the soda-lime silicate glass by irradiation with a conventional continuous wave Ar+ laser with a wavelength of 514.5 nm. The hologram was transferred to the soda-lime silicate glass surface via a corona discharge treatment as an index modulation hologram, which was created by partial substitution of protons for sodium ions during the corona discharge treatment in air. After the corona discharge treatment, the polymer film was removed from the substrate. The diffraction efficiency of the index hologram on the soda-lime silicate glass was estimated to be 5.8 × 10-2% at a wavelength of 532 nm. Finally, the glass substrate was subjected to corona discharge treatment in air with vaporized cyclic siloxane. A surface relief hologram with the diffraction efficiency of 2.3% was successfully fabricated on the soda-lime silicate glass.

Pressure-Induced Transition of Bisamide-Substituted Diacetylene Crystals from Nonphotopolymerizable to Photopolymerizable State.

Mechanoresponsive diacetylenes (DAs) exhibiting a transition of crystalline orientation from light-inert to light-active state upon applied force are reported. Amide units are introduced to DAs where hydrogen bonding is utilized to control intermolecular interactions. Application of external pressure (2-150 MPa) to DAs results in an emergence of new crystal phases with changing the d-spacing which possibly reduces the reaction barrier. Accordingly, the dramatic crystalline transition from "perfectly off" to "on" state to undergo the light-induced topochemical polymerization of bulk DA crystals is obtained. Subsequent UV irradiation at a wavelength of 254 nm enables the polymerization of the pressed region, changing its color from white to blue which suggests the selective formation of polydiacetylene (PDA) polymorphs. In addition, by utilizing the mechanoresponsive crystallinity with low-enough activation pressure, a new strategy for PDA patterning is demonstrated based on the selective transfer of information by means of force to a DA film. This phenomenon can be applicable to a new nanoimprinting technique where no mechanical deformation of resist materials but phase transition is induced by the mold.

Robustness of Voltage-induced Magnetocapacitance.

One of the most important achievements in the field of spintronics is the development of magnetic tunnel junctions (MTJs). MTJs exhibit a large tunneling magnetoresistance (TMR). However, TMR is strongly dependent on biasing voltage, generally, decreasing with applying bias. The rapid decay of TMR was a major deficiency of MTJs. Here we report a new phenomenon at room temperature, in which the tunneling magnetocapacitance (TMC) increases with biasing voltage in an MTJ system based on Co40Fe40B20/MgO/Co40Fe40B20. We have observed a maximum TMC value of 102% under appropriate biasing, which is the largest voltage-induced TMC effect ever reported for MTJs. We have found excellent agreement between theory and experiment for the bipolar biasing regions using Debye-Fröhlich model combined with quartic barrier approximation and spin-dependent drift-diffusion model. Based on our calculation, we predict that the voltage-induced TMC ratio could reach 1100% in MTJs with a corresponding TMR value of 604%. Our work has provided a new understanding on the voltage-induced AC spin-dependent transport in MTJs. The results reported here may open a novel pathway for spintronics applications, e.g., non-volatile memories and spin logic circuits.

Proton-Driven Intercalation and Ion Substitution Utilizing Solid-State Electrochemical Reaction.

The development of an unconventional synthesis method has a large potential to drastically advance materials science. In this research, a new synthesis method based on a solid-state electrochemical reaction was demonstrated, which can be made available for intercalation and ion substitution. It was referred to as proton-driven ion introduction (PDII). The protons generated by the electrolytic dissociation of hydrogen drive other monovalent cations along a high electric field in the solid state. Utilizing this mechanism, Li+, Na+, K+, Cu+, and Ag+ were intercalated into a layered TaS2 single crystal while maintaining high crystallinity. This liquid-free process of ion introduction allows the application of high voltage around several kilovolts to the sample. Such a high electric field strongly accelerates ion substitution. Actually, compared to conventional solid-state reaction, PDII introduced 15 times the amount of K into Na super ionic conductor (NASICON)-structured Na3-xKxV2(PO4)3. The obtained materials exhibited a thermodynamically metastable phase, which has not been reported so far. This concept and idea for ion introduction is expected to form new functional compounds and/or phases.

The mobility of proton carriers in phosphate glasses depends on polymerization of the phosphate framework.

Proton conducting phosphate glasses were prepared by electrochemical substitution of sodium ions with protons applied to glasses with the compositions xNaO1/2-1WO3-8NbO5/2-5LaO3/2-(86 - x)PO5/2 (x = 28, 32, 35, 38, and 40). The mobilities of proton carriers in the glasses were studied in terms of the polymerization degree of the phosphate framework. The proton mobility at 200 °C increased as the depolymerization of the phosphate framework developed up to x = 38, and decreased at x = 40. On the basis of Raman and infrared spectra measurements of the O-H stretching vibration region, the decreasing mobility at x > 38 was attributed to the increasing concentration of protons trapped by non-bridging oxygen in P2O74- ions, owing to strong O-H bonding. We found that the highly polymerized phosphate framework decreased the mobility of proton carriers, not because of suppression of the proton dissociation from oxygen atoms but rather the suppression of the proton migration. The compositions at which the phosphate framework was sufficiently depolymerized and did not contain P2O74- ions as a main component, achieved high mobility of proton carriers in phosphate glasses.

Formation of Amorphous H3Zr2Si2PO12 by Electrochemical Substitution of Sodium Ions in Na3Zr2Si2PO12 with Protons.

The sodium ions in Na3Zr2Si2PO12 (NASICON) were substituted with protons using an electrochemical alkali-proton substitution (APS) technique at 400 °C under a 5% H2/95% N2 atmosphere. The sodium ions in NASICON were successfully substituted with protons to a depth of <400 µm from the anode. Completely protonated NASICON, i.e., H3Zr2Si2PO12, was obtained to a depth <40 µm from the anode, although complete protonation of NASICON cannot be achieved by ion exchange in aqueous acid. H3Zr2Si2PO12 was amorphous, whereas the partially protonated NASICON was crystalline, and its unit cell volume decreased with an increase in the extent of substitution. Amorphous H3Zr2Si2PO12 was prepared by pressure-induced amorphization of the NASICON framework, in which an internal pressure of ∼3.5 GPa was induced by the substitution of large sodium ions with small protons during APS at 400 °C.

Using Laser Interference Lithography in the Fabrication of a Simplified Micro- and Nanofluidic Device for Label-free Detection.

Recently, we developed a label-free detection method based on optical diffraction, and implemented it in on our fabricated micro- and nanofluidic device. This detection method is simple and useful for detecting biomolecules, but the device fabrication consists of complicated processes. In this paper, we propose a simple method for fabricating the micro- and nanofluidic device; the fabrication combines laser interference lithography with conventional photolithography. The performance of a device fabricated by the proposed method is comparable to the performance of the device in our previous study.

Inverse Tunnel Magnetocapacitance in Fe/Al-oxide/Fe3O4.

Magnetocapacitance (MC) effect, observed in a wide range of materials and devices, such as multiferroic materials and spintronic devices, has received considerable attention due to its interesting physical properties and practical applications. A normal MC effect exhibits a higher capacitance when spins in the electrodes are parallel to each other and a lower capacitance when spins are antiparallel. Here we report an inverse tunnel magnetocapacitance (TMC) effect for the first time in Fe/AlOx/Fe3O4 magnetic tunnel junctions (MTJs). The inverse TMC reaches up to 11.4% at room temperature and the robustness of spin polarization is revealed in the bias dependence of the inverse TMC. Excellent agreement between theory and experiment is achieved for the entire applied frequency range and the wide bipolar bias regions using Debye-Fröhlich model (combined with the Zhang formula and parabolic barrier approximation) and spin-dependent drift-diffusion model. Furthermore, our theoretical calculations predict that the inverse TMC effect could potentially reach 150% in MTJs with a positive and negative spin polarization of 65% and -42%, respectively. These theoretical and experimental findings provide a new insight into both static and dynamic spin-dependent transports. They will open up broader opportunities for device applications, such as magnetic logic circuits and multi-valued memory devices.

Discovery of the Pt-Based Superconductor LaPt5As.

A novel superconductor, LaPt5As, which exhibits a new crystal structure was discovered by high-pressure synthesis using a Kawai-type apparatus. A superconducting transition temperature was observed at 2.6 K. Depending on the sintering pressure, LaPt5As has superconducting and non-superconducting phases with different crystal structures. A sintering pressure of around 10 GPa is effective to form single-phase superconducting LaPt5As. This material has a very unique crystal structure with an extremely long c lattice parameter of over 60 Å and corner-sharing tetrahedrons composed of network-like Pt layers. Density functional theory calculations have suggested that the superconducting current flows through these Pt layers. Also, this unique layered structure characteristic of LaPt5As is thought to play a key role in the emergence of superconductivity. Furthermore, due to a stacking structure which makes up layers, various structural modifications for the LaPt5As family are conceivable. Since such a high-pressure synthesis using a Kawai-type apparatus is not common in the field of materials science, there is large room for further exploration of unknown phases which are induced by high pressure in various materials.

Selective Deposition of SiO2 on Ion Conductive Area of Soda-lime Glass Surface.

Selective deposition of SiO2 nanoparticles was demonstrated on a soda-lime glass surface with a periodic sodium deficient pattern formed using the electrical nanoimprint. Positively charged SiO2 particles generated using corona discharge in a cyclic siloxane vapor, were selectively deposited depending on the sodium pattern. For such phenomena to occur, the sodium ion migration to the cathode side was indispensable to the electrical charge compensation on the glass surface. Therefore, the deposition proceeded preferentially outside the alkali-deficient area. Periodic SiO2 structures with 424 nm and 180 nm heights were obtained using one-dimensional (6 µm period) and two-dimensional (500 nm period) imprinted patterns.

Phase separation and crystallization in sodium lanthanum phosphate glasses induced by electrochemical substitution of sodium ions with protons.

Electrochemical substitution of sodium ions with protons (alkali-proton substitution; APS), and the injection of proton carriers was applied to sodium lanthanum phosphate glasses. A clear and homogeneous material was obtained for a glass of composition 25NaO1/2-8LaO3/2-66PO5/2-1GeO2 following APS, with a resulting proton conductivity of 4 × 10(-6) S cm(-1) at 250 °C. The glass underwent phase separation and crystallization at temperatures >255 °C, forming a highly hygroscopic and proton conducting H3PO4 phase in addition to LaP5O14 and other unidentified phases. A glass of composition 25NaO1/2-8LaO3/2-67PO5/2 underwent phase separation and crystallization during APS, forming both H3PO4 and LaP5O14 phases. Sodium lanthanum phosphate glasses are prone to phase separation and crystallization during APS unlike the previously reported NaO1/2-WO3-NbO5/2-LaO3/2-PO5/2 glasses. The phase separation was explained by a reduction in viscosity following APS and the introduction of protons, which exhibit high field strength. Thus, phase separation and crystallization of glasses during APS was difficult to avoid. An approach to suppress phase separation is discussed.

Structural change of NaO1/2-WO3-NbO5/2-LaO3/2-PO5/2 glass induced by electrochemical substitution of sodium ions with protons.

Structural changes of 35NaO1/2-1WO3-8NbO5/2-5LaO3/2-51PO5/2 glass (1W-glass) before and after the electrochemical substitution of sodium ions with protons by alkali-proton substitution (APS) are studied by Raman and (31)P magic-angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopies. The glass before APS consists of (PO3(-))8.6(P2O7(4-)) chains on average and the terminal Q(1) units (-O-PO3(3-)) are bound to MO6 octahedra (M denotes niobium or tungsten) through P-O-M bonds. Some non-bridging oxygens (NBOs) in the MO6 octahedra are present in addition to the bridging oxygens (BOs) in P-O-M bonds. APS induces fragmentation of the phosphate chains because the average chain length decreases to (PO3(-))3.7(P2O7(4-)) after APS, despite the total number of modifier cations of sodium and lanthanum ions and protons being unaffected by APS. This fragmentation is induced by some of the NBOs in the MO6 octahedra before APS, changing to BOs of the newly formed M-O-P bonds after APS, because of the preferential formation of P-OH bonds over M-OH ones in the present glass. We show that APS under the conditions used here is not a simple substitution of sodium ions with protons, but it is accompanied by the structural relaxation of the glass to stabilize the injected protons.

In situ sensitive fluorescence imaging of neurons cultured on a plasmonic dish using fluorescence microscopy.

A plasmonic dish was fabricated as a novel cell-culture dish for in situ sensitive imaging applications, in which the cover glass of a glass-bottomed dish was replaced by a grating substrate coated with a film of silver. Neuronal cells were successfully cultured over a period of more than 2 weeks in the plasmonic dish. The fluorescence images of their cells including dendrites were simply observed in situ using a conventional fluorescence microscope. The fluorescence from neuronal cells growing along the dish surface was enhanced using the surface plasmon resonance field. Under an epi-fluorescence microscope and employing a donut-type pinhole, the fluorescence intensity of the neuron dendrites was found to be enhanced efficiently by an order of magnitude compared with that using a conventional glass-bottomed dish. In a transmitted-light fluorescence microscope, the surface-selective fluorescence image of a fine dendrite growing along the dish surface was observed; therefore, the spatial resolution was improved compared with the epi-fluorescence image of the identical dendrite.