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.

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.

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.

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.

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.

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.

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.

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.

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.

Design and fabrication of an achromatic infrared wave plate with Sb-Ge-Sn-S system chalcogenide glass.

We designed and fabricated an achromatic infrared wave plate. To examine its phase retardation characteristics, the birefringence was calculated using the effective medium theory. A wave plate with a subwavelength grating was fabricated by direct imprint lithography on a low toxic chalcogenide glass (Sb-Ge-Sn-S system) based on calculated results. As a result of imprinting onto chalcogenide glass by a glassy carbon mold, a grating with 1.63 µm depth, a fill factor of 0.7, and a 3 µm period was obtained. The phase retardation of the elements reached around 30° in the 8.5-10.5 µm wavelength range. The fabrication of the infrared wave plate is less costly compared with conventional crystalline wave plates.

Cavity-resonator-integrated guided-mode resonance filter for aperture miniaturization.

A guided-mode resonance filter integrated in a waveguide cavity resonator constructed by two distributed Bragg reflectors is designed and fabricated for miniaturization of aperture size. Reflection efficiency of >90% and wavelength selectivity of 0.4 nm are predicted in the designed SiO(2)-based filter with 50-µm aperture by a numerical calculation using the finite-difference time-domain method. A maximum reflectance of 67% with 0.5-nm bandwidth is experimentally demonstrated by the fabricated device at around 850-nm wavelength.

Spontaneous emission control of CdSe/ZnS nanoparticle monolayer in polymer nanosheet waveguide assembled on a one-dimensional silver grating surface.

We present spontaneous emission control of a core-shell CdSe/ZnS nanoparticle array assembled with polymer ultrathin films consisting of polymer nanosheets on a silver grating substrate, which served as a unique and simple photonic cavity. The grating-coupled waveguide modes enabled 10(3) order luminescence enhancement and one-fourth spectral narrowing. The light emission from a CdSe/ZnS nanoparticle array can be controlled by tuning the film thickness of hybrid polymer nanoassemblies, which provides multiple emission performance with good tuning ability from red to green at low-power continuous wave laser excitation (∼µW).

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.

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.

Large refractive index changes of a chemically amplified photoresist in femtosecond laser nonlinear lithography.

We found that marked increases in refractive index of chemically amplified photoresists induced by highly repetitive femtosecond laser irradiation without post-exposure baking treatment. For laser writing speed less than 30 µm/s, the refractive index change of the nonlinear absorption region was as large as 8 × 10(-3). Moreover, cross-linking reactions of the resists were induced. The refractive index changes can generate optical confinement and subsequent channel propagations of femtosecond laser pulses. The coupling efficiency was estimated as high as 87% using a low numerical aperture objective lens. The peak intensities of the guiding modes exceeded the polymerization threshold of the resist.

Fabrication of a mid-IR wire-grid polarizer by direct imprinting on chalcogenide glass.

A mid-IR wire-grid polarizer with a 500 nm pitch was fabricated on a low toxic chalcogenide glass (Sb-Ge-Sn-S system) by the thermal imprinting of periodic grating followed by the thermal evaporation of Al metal. After imprinting, deposition of Al on the grating at an oblique angle produced a wire-grid polarizer. The fabricated polarizer showed polarization with TM transmittance greater than 60% at 5-9 µm wavelengths and an extinction ratio greater than 20 dB at 3.5-11 µm wavelengths. This polarizer with a high extinction ratio can be fabricated more simply and less expensively than conventional IR polarizers.

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.

Potential characterization of free-space-wave drop demultiplexer using cavity-resonator-integrated grating input/output coupler.

A prototype free-space-wave drop demultiplexer consisting of a cavity-resonator-integrated grating input/output coupler (CRIGIC) and a different-guided-mode-coupling distributed Bragg reflector (DGM-DBR) was designed for constructing a high-density wavelength-division-multiplexing intra-board chip-to-chip optical interconnection. The CRIGIC consists of one grating coupler and two DBRs, and can vertically couple a guided wave and a free-space wave with high efficiency. A two-channel drop demultiplexer operating at around 850-nm wavelength with 5-nm channel spacing in wavelength was fabricated in a thin-film SiO2-based waveguide. The device performance was predicted theoretically, characterized experimentally, and discussed.

Infrared wire-grid polarizer with Y2O3 ceramic substrate.

Using two-beam interference lithography and dry etching, we fabricated a mid-IR wire-grid polarizer consisting of a 350 nm pitch WSi grating on an Y(2)O(3) ceramic substrate, which has wider transparency than sapphire. The transmittance of TM polarization was greater than 70% in the 3-7 µm wavelength range without antireflection films, and the extinction ratio was over 20 dB in the 2.5-5 µm wavelength range. The wire-grid polarizer with the Y(2)O(3) ceramic substrate provides high durability and good IR transparency.