Takaaki Taniguchi

Super proton/electron mixed conduction in graphene oxide hybrids by intercalating sulfate ions

We successfully developed an efficient proton/electron mixed conductor composed of a single phase material that functions at room temperature by introducing sulfate ions into graphene oxide interlayers. The promising properties of this material would allow for its wide use in fuel cells, supercapacitors, and gas separation membranes.

Vapour–liquid–solid growth of monolayer MoS 2 nanoribbons

Chemical vapour deposition of two-dimensional materials typically involves the conversion of vapour precursors to solid products in a vapour–solid–solid mode. Here, we report the vapour–liquid–solid growth of monolayer MoS2, yielding highly crystalline ribbons with a width of few tens to thousands of nanometres. This vapour–liquid–solid growth is triggered by the reaction between MoO3 and NaCl, which results in the formation of molten Na–Mo–O droplets. These droplets mediate the growth of MoS2 ribbons in the ‘crawling mode’ when saturated with sulfur. The locally well-defined orientations of the ribbons reveal the regular horizontal motion of the droplets during growth. Using atomic-resolution scanning transmission electron microscopy and second harmonic generation microscopy, we show that the ribbons are grown homoepitaxially on monolayer MoS2 with predominantly 2H- or 3R-type stacking. Our findings highlight the prospects for the controlled growth of atomically thin nanostructure arrays for nanoelectronic devices and the development of unique mixed-dimensional structures.Monolayer MoS2 can be laterally grown in the vapour–liquid–solid mode, forming highly crystalline nano- and microribbon structures.

Coal Oxide as a Thermally Robust Carbon-Based Proton Conductor

Inexpensive solid proton conducting materials with high proton conductivity and thermal stability are necessary for practical solid state electrochemical devices. Here we report that coal oxide (CO) is a promising carbon-based proton conductor with remarkable thermal robustness. The CO produced by simple liquid-phase oxidation of coal demonstrates excellent dispersibility in water owing to the surface carboxyl groups. The proton conductivity of CO, 3.9 × 10(-3) S cm(-1) at 90% relative humidity, is as high as that of graphene oxide (GO). Remarkably, CO exhibits much higher thermal stability than GO, with CO retaining the excellent proton conductivity as well as the capacitance performance even after thermal annealing at 200 °C. Our study demonstrates that the chemical modification of the abundant coal provides proton conductors that can be used in practical applications for a wide range of energy devices.

Layer-by-layer engineering of two-dimensional perovskite nanosheets for tailored microwave dielectrics

We report an approach for constructing ultrathin microwave dielectrics using two-dimensional (2D) perovskite nanosheets (Ca2Nb3O10). The solution-based layer-by-layer assembly of Ca2Nb3O10 nanosheets enables the tailoring of dielectric responses and breakdown voltage in order to realize miniaturized capacitors with improved performances. Multilayer stacked nanosheet films showed a stable dielectric response of up to 10 GHz; the dielectric constant of ~200 is the highest among high-κ perovskites in the ultrathin region (<30 nm). Moreover, the nanosheet films realized a high capacitance density (~20 µF/cm2) with a robust thermal stability up to 250 °C. These superior dielectric properties of perovskite nanosheets offer a new solution for high-performance microwave devices.

Self-sensitization and photo-polymerization of diacetylene molecules self-assembled on a hexagonal-boron nitride nanosheet

Long poly-diacetylene chains are excellent candidates for planar, on-surface synthesized molecular electronic wires. Since hexagonal-Boron Nitride (h-BN) was identified as the best available atomically flat insulator for the deposition of poly-diacetylene precursors, we demonstrate the polymerization patterns and rate on it under UV-light irradiation, with subsequent polymer identification by atomic force microscopy. The results on h-BN indicate self-sensitization which yields blocks comprised of several polymers, unlike on the well-studied graphite/diacetylene system, where the polymers are always isolated. In addition, the photo-polymerization proceeds at least 170 times faster on h-BN, where it also results in longer polymers. Both effects are explained by the h-BN bandgap, which is larger than the diacetylene electronic excitation energy, thus allowing the transfer of excess energy absorbed by polymerized wires to adjacent monomers, triggering their polymerization. This work sets the stage for conductance measurements of single molecular poly-diacetylene wires on h-BN.