Controlled growth of boron-doped epitaxial graphene by thermal decomposition of a B4C thin film.

We show that boron-doped epitaxial graphene can be successfully grown by thermal decomposition of a boron carbide thin film, which can also be epitaxially grown on a silicon carbide substrate. The interfaces of B4C on SiC and graphene on B4C had a fixed orientation relation, having a local stable structure with no dangling bonds. The first carbon layer on B4C acts as a buffer layer, and the overlaying carbon layers are graphene. Graphene on B4C was highly boron doped, and the hole concentration could be controlled over a wide range of 2 × 1013 to 2 × 1015 cm-2. Highly boron-doped graphene exhibited a spin-glass behavior, which suggests the presence of local antiferromagnetic ordering in the spin-frustration system. Thermal decomposition of carbides holds the promise of being a technique to obtain a new class of wafer-scale functional epitaxial graphene for various applications.

Preparation of Stable Silver Nanoparticles Having Wide Red-To-Near-Infrared Extinction.

The synthesis of silver nanoparticles (AgNPs) within the interlayer space of transparent layered titania nanosheet (TNS) films is investigated. A considerable number of silver ions (≈70% against the cation exchange capacity of the TNS) are intercalated in the TNS films using methyl-viologen-containing TNSs as a precursor. The silver ion (Ag+)-containing TNS films are treated with aqueous sodium tetrahydroborate (NaBH4), resulting in a gradual color change to bright blue. Various structural analyses clearly show that crystalline AgNPs are generated within the interlayer space of the TNSs. The NaBH4-treated films show intense and characteristic near-infrared (NIR) extinction spectra up to 1800 nm. The stability of the AgNPs within the TNS against oxygen and moisture is also investigated, and 96% and 82% of the AgNPs remain after standing in air for 1 month and 1 year, respectively. The NIR extinctions of the AgNP-containing TNS films are further extended by employing different preparation procedures, for example, using sintered TNS films as starting materials and irradiating the Ag+-containing TNSs with ultraviolet (UV) light. The obtained AgNP-containing TNS films exhibit photochemical activities in the production of hydrogen from ammonia borane under visible-light irradiation and the decomposition of nitrogen monoxide under UV-light irradiation.

Synthesis of Freestanding Graphene on SiC by a Rapid-Cooling Technique.

Graphene has a negative thermal expansion coefficient; that is, when heated, the graphene lattice shrinks. On the other hand, the substrates typically used for graphene growth, such as silicon carbide, have a positive thermal expansion coefficient. Hence, on cooling graphene on SiC, graphene expands but SiC shrinks. This mismatch will physically break the atomic bonds between graphene and SiC. We have demonstrated that a graphenelike buffer layer on SiC can be converted to a quasifreestanding monolayer graphene by a rapid-cooling treatment. The decoupling of graphene from the SiC substrate was actually effective for reducing the electric carrier scattering due to interfacial phonons. In addition, the rapidly cooled graphene obtained in this way was of high-quality, strain-free, thermally stable, and strongly hole doped. This simple, classical, but quite novel technique for obtaining quasifreestanding graphene could open a new path towards a viable graphene-based semiconductor industry.

Epitaxial graphene on SiC{0001}: advances and perspectives.

We review here recent progress on epitaxial graphene grown on a SiC substrate. Epitaxial graphene can be easily grown by heating the SiC single crystal in a high vacuum or in an inert gas atmosphere. The SiC surfaces used for graphene growth contain Si- and C-terminated faces. On the Si-face, homogeneous and clean graphene can be grown with a controlled number of layers, and the carrier mobility reaches as high as several m(2) V s(-1), although this is reduced by the presence of the substrate steps. On the C-face, although the number of layers is not homogeneous, twisted bilayer graphene can be grown, which is expected to be the technique of choice to modify the electronic structure of graphene. From the application point of view, graphene on SiC will be the platform used to fabricate high-speed electronic devices and dense graphene nanoribbon arrays, which will be used to introduce a bandgap.

Enhanced thermoelectric performance of Nb-doped SrTiO3 by nano-inclusion with low thermal conductivity.

Authors reported an effective path to increase the electrical conductivity while to decrease the thermal conductivity, and thus to enhance the ZT value by nano-inclusions. By this method, the ZT value of Nb-doped SrTiO3 was enhanced 9-fold by yttria stabilized zirconia (YSZ) nano-inclusions. YSZ inclusions, located inside grain and in triple junction, can reduce the thermal conductivity by effective interface phonon scattering, enhance the electrical conductivity by promoting the abnormal grain growth, and thus lead to the obvious enhancement of ZT value, which strongly suggests that, it is possible to not only reduce the thermal conductivity, but also increase the electrical conductivity by nano-inclusions with low thermal conductivity. This study will give some useful enlightenment to the preparation of high-performance oxide thermoelectric materials.

Development of an environmental high-voltage electron microscope for reaction science.

Environmental transmission electron microscopy and ultra-high resolution electron microscopic observation using aberration correctors have recently emerged as topics of great interest. The former method is an extension of the so-called in situ electron microscopy that has been performed since the 1970s. Current research in this area has been focusing on dynamic observation with atomic resolution under gaseous atmospheres and in liquids. Since 2007, Nagoya University has been developing a new 1-MV high voltage (scanning) transmission electron microscope that can be used to observe nanomaterials under conditions that include the presence of gases, liquids and illuminating lights, and it can be also used to perform mechanical operations to nanometre-sized areas as well as electron tomography and elemental analysis by electron energy loss spectroscopy. The new instrument has been used to image and analyse various types of samples including biological ones.

Epitaxial growth of boron-doped graphene by thermal decomposition of B4C.

We grew graphene by thermal decomposition of B(4)C and investigated its features by high-resolution transmission electron microscope observations. At temperatures higher than 1600 °C in a vacuum, B(4)C decomposes and graphene forms epitaxially on its surface. The number and the morphology of the graphene layers depend on the surface orientation. An electron diffraction technique revealed the presence of a superstructure with a two-times larger unit cell, which is consistent with the structure of BC(3). We have directly confirmed boron in the graphene layers by electron energy loss spectroscopy measurements and boron-mapping experiments.

Transmission electron microscope observation of interface structures of graphene on 6H-SiC.

High-resolution transmission electron microscopic cross-sectional observations of graphene-on-SiC(0001) were carried out to directly observe the interface structure. A first principles calculation allowed us to understand the interface structures and their electronic states. Our observations revealed a metastable transitional interface structure formed by decomposition of a single SiC bilayer as well as complete honeycomb graphene formed by the decomposition of three SiC bilayers. The calculations further showed that the differences in the interface structures should strongly influence the electronic states, producing either metallic or semiconducting behavior. These results may help to resolve the controversy over the electronic states of graphene-on-SiC, and promote more accurate band-gap engineering via surface decomposition.

Development of novel thermoelectric materials by reduction of lattice thermal conductivity.

Thermal conductivity is one of the key parameters in the figure of merit of thermoelectric materials. Over the past decade, most progress in thermoelectric materials has been made by reducing their thermal conductivity while preserving their electrical properties. The phonon scattering mechanisms involved in these strategies are reviewed here and divided into three groups, including (i) disorder or distortion of unit cells, (ii) resonant scattering by localized rattling atoms and (iii) interface scattering. In addition, we propose construction of a 'natural superlattice' in thermoelectric materials by intercalating an MX layer into the van der Waals gap of a layered TX2 structure which has a general formula of (MX)1+x (TX2) n (M=Pb, Bi, Sn, Sb or a rare earth element; T=Ti, V, Cr, Nb or Ta; X=S or Se and n=1, 2, 3). We demonstrate that one of the intercalation compounds (SnS)1.2(TiS2)2 has better thermoelectric properties compared with pure TiS2 in the direction parallel to the layers, as the electron mobility is maintained while the phonon transport is significantly suppressed owing to the reduction in the transverse phonon velocities.

Tribological properties of densely packed vertically aligned carbon nanotube film on SiC formed by surface decomposition.

Characteristic tribological properties, such as nonlinearity of the friction force-normal load curve, high coefficient of friction, and good wear-resistant performance were observed on densely packed, vertically aligned carbon nanotubes (CNTs) with different diameters and lengths using atomic force microscopy. Shorter and thicker CNTs were found to have higher coefficients of friction. The observed properties were attributed to the nonlinear elastic property of the CNTs caused by buckling.

Theory and experiment agree: single-walled carbon nanotube caps grow catalyst-free with chirality preference on a SiC surface.

High-temperature quantum chemical molecular dynamics simulations have been performed on model systems of thin SiC crystal surfaces with two graphene sheets placed on top of either C or Si face. In agreement with experiment, we find that (a) the C-face-attached graphene layer warps readily to form small diameter, stable nanocaps, suitable for further perpendicular growth of nanotubes, (b) the Si-face-attached graphene sheet does not readily wrap and forms more volatile Si-graphene bonds, and (c) C face nanocaps appear to anneal to dome-shape structures with zigzag chirality.

Electron holographic observation of micro-magnetic fields current-generated from single carbon coil.

A carbon coil was evaluated for use as a micro-solenoid in a small magnetic device. A single carbon coil was lifted out of the aggregate using a tungsten fine probe in a focused ion beam (FIB) system and was wired to two small electrodes in the specimen holder of a transmission electron microscope (TEM). A direct current was supplied to the single carbon coil. A micro/nano-magnetic field generated from the coil was directly observed by electron holography. A computer simulation of electron holography was also done to quantitatively analyze the magnetic field. Details on the FIB technique, the electron holographic observation and the simulation are described.