Yuichiro Hayasaka

Monolayer-to-bilayer transformation of silicenes and their structural analysis

Silicene, a two-dimensional honeycomb network of silicon atoms like graphene, holds great potential as a key material in the next generation of electronics; however, its use in more demanding applications is prevented because of its instability under ambient conditions. Here we report three types of bilayer silicenes that form after treating calcium-intercalated monolayer silicene (CaSi2) with a BF4− -based ionic liquid. The bilayer silicenes that are obtained are sandwiched between planar crystals of CaF2 and/or CaSi2, with one of the bilayer silicenes being a new allotrope of silicon, containing four-, five- and six-membered sp3 silicon rings. The number of unsaturated silicon bonds in the structure is reduced compared with monolayer silicene. Additionally, the bandgap opens to 1.08 eV and is indirect; this is in contrast to monolayer silicene which is a zero-gap semiconductor.

Coating and photochemical properties of calcia-doped ceria with amorphous silica by a seeded polymerization technique

Calcia doped ceria is of potential interest as an ultraviolet (UV) radiation blocking material in personal care products because of its excellent UV light absorption properties and its low catalytic ability for the oxidation of organic materials is superior to undoped ceria. In order to reduce the oxidation catalytic activity further, calcia doped ceria was coated with amorphous silica by means of a seeded polymerization technique. The silica shell was confirmed using scanning electron microscopy, TEM, XPS and FT-IR. Silica coating by seeded polymerization was much more efficient in the reduction of the oxidation catalytic activity without loss of the UV-shielding ability, than coating by acid hydrolysis of a sodium silicate solution.

Synthesis of ordered carbonaceous frameworks from organic crystals

Despite recent advances in the carbonization of organic crystalline solids like metal-organic frameworks or supramolecular frameworks, it has been challenging to convert crystalline organic solids into ordered carbonaceous frameworks. Herein, we report a route to attaining such ordered frameworks via the carbonization of an organic crystal of a Ni-containing cyclic porphyrin dimer (Ni2-CPDPy). This dimer comprises two Ni–porphyrins linked by two butadiyne (diacetylene) moieties through phenyl groups. The Ni2-CPDPy crystal is thermally converted into a crystalline covalent-organic framework at 581 K and is further converted into ordered carbonaceous frameworks equipped with electrical conductivity by subsequent carbonization at 873–1073 K. In addition, the porphyrin’s Ni–N4 unit is also well retained and embedded in the final framework. The resulting ordered carbonaceous frameworks exhibit an intermediate structure, between organic-based frameworks and carbon materials, with advantageous electrocatalysis. This principle enables the chemical molecular-level structural design of three-dimensional carbonaceous frameworks.Carbon-based materials are promising alternatives to noble metal catalysts, but their structures are typically disordered and difficult to control. Here, the authors obtain ordered carbonaceous frameworks with advantageous electrocatalytic properties via the carbonization of nickel-containing porphyrin dimer networks.

Structure of Graphite Nanosheets Formed by Plasma Discharge in Liquid Ethanol

Graphite nanosheets (GNSs) were synthesized by low-current plasma discharge in ultrasonically cavitated liquid ethanol. Effective elimination of the byproducts was confirmed by different techniques. The results indicated that there were two types of ordering along the crystallographic caxis in the GNSs, which were ABAB- and turbostratic stacking of successive graphene layers. Most of the GNSs were estimated to be between 8.8 and 30 nm thick, and their lateral sizes ranged from several hundred nanometers up to 11 μm with an average size of 4–5 μm. A simple mechanism for GNS formation by exfoliation of graphene layers covering the Fe–Pt alloy nanoparticles is proposed. The measured conductivity of GNSs was comparable to that of the reference graphene nanopowder and reference single graphene sheets.

Bi-2212 and Y123 highly curved single-crystal-like objects: whiskers, bows and ring-like structures

High-temperature superconducting objects of Bi2Sr2CaCu2O8 and Y Ba2Cu3O7 highly curved in the ab-plane, such as curved/kinked whiskers, bows and ring-like structures, were obtained within a solid?liquid?solid (SLS) grass-like growth mechanism. As-grown objects are crystals with three-dimensional epitaxy similar to conventional single crystals: they can be viewed as crystal parts ?cut? from a conventional rectangular crystal. Between our curved objects and conventional crystals, whiskers or thin films there are some differences in the superconducting properties induced only by the shape factors and no new physics is observed. Some details of the growth mechanism are discussed, emphasizing curved-line formation.

Preferentially Oriented Cu[111] Layer Formed on Thin Nb Barrier on SiO2

The texture of preferentially oriented Cu[111] deposited on a thin Nb layer is characterized in a thin-film stacked structure of Cu/Nb/SiO2/Si in an attempt to prepare a Cu[111] seed layer of interconnects on an extremely thin diffusion barrier. The Cu[111] layer is obtained on Nb films of [110] orientation at various thicknesses; however, the mosaic spread of Cu[111] texture depends on the thickness of Nb film underneath. The full width at half maximum of the ω-rocking curve measurement is ~3° for the Cu[111] layer on a 100-nm-thick Nb layer, which increases to ~4 and ~5° for that on 20- and 10-nm-thick Nb layers, respectively. Transmission electron microscopy reveals the 10-nm-thick Nb layer consisting of fine relatively mosaic-spread [110] grains, which is a result of the initial stage of the nucleation type growth of the Nb film in a columnar structure on SiO2. It is revealed that the Cu[111] texture of relatively good mosaicity in a columnar structure is obtained on an extremely thin Nb layer of 10 nm thickness.

Atomic and nanoscale imaging of a cellulose nanofiber and Pd nanoparticles composite using lower-voltage high-resolution TEM

We have examined the advanced application of transmission electron microscopy (TEM) for the structural characterization of a composite of cellulose nanofiber (CNF) and palladium (Pd) nanoparticles. In the present study, we focused on electron-irradiation damage and optimization of high-resolution TEM imaging of the composite. The investigation indicates that the CNF breaks even under low-electron-dose conditions at an acceleration voltage of 200 kV. We then applied lower-voltage TEM at 60 kV using a spherical aberration corrector and a monochromator, in order to reduce electron-irradiation damage and improve the spatial resolution. The TEM observation achieved high-resolution imaging and revealed the existence of small Pd nanoparticles, around 2 nm in diameter, supported on the CNF. It is considered that the use of a monochromator in combination with spherical aberration correction contributed to the atomic and nanoscale imaging of the composite, owing to the improvement of the information limit under a lower-acceleration voltage.

Formation of Preferentially Oriented Cu [111] Layer on Nb [110] Barrier on SiO2

A preferentially oriented Cu [111] layer is obtained on an oriented Nb [110] barrier formed on thermally grown SiO2. We examine the characteristics of the orientation and textures in the Cu/Nb/SiO2/Si model system by X-ray diffraction analysis and cross-sectional transmission electron microscopy (TEM). It was confirmed by X-ray pole figures and ω-rocking curve measurements that the preferentially oriented Cu [111] layer in the fiber structure showing a good mosaicity with a full width at half maximum of ~3° in the ω-rocking curve was grown on an oriented Nb [110] barrier. The TEM observation shows a pronounced columnar structure in the oriented Cu and Nb layers on amorphous SiO2 and the textures are stable due to annealing at 500°C.