Michiko Kusunoki

A formation mechanism of carbon nanotube films on SiC(0001)

We report a remarkable difference of decomposed structures on the Si(0001) and C(0001) faces of a SiC single crystal observed by using a cross-sectional high-resolution electron microscopy. An aligned carbon nanotube (CNT) film was fabricated on the C face perpendicular to the surface after heating at 1700 °C for half an hour in a vacuum. On the contrary, a very thin layer of graphite sheets parallel to the surface was formed on the Si face under the same condition. It is proposed that the growth of CNTs is determined by the generation of nanocaps at the initial stage, by comparing the difference of the decomposition mechanisms on the both faces.

Epitaxial carbon nanotube film self-organized by sublimation decomposition of silicon carbide

A film of well-oriented carbon nanotubes was produced by sublimation decomposition of silicon carbide at 1700 °C by using YAG laser heating in a transmission electron microscope (TEM). The processes of SiC decomposition and the formation of carbon nanotubes were observed successively by high-resolution electron microscopy (HREM). Carbon nanotubes were mostly oriented along the [111] direction on the (111) surface plane of β-SiC single crystal. The interface between them was observed by HREM.

Development of novel thermoelectric materials by reduction of lattice thermal conductivity

Abstract 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.

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.

Aligned Carbon Nanotube Film Self-Organized on a SiC Wafer

An aligned carbon nanotube film was fabricated on the surface of an α-SiC wafer by heating at 1700°C for 30 min. in a vacuum electric furnace due to the decomposition of SiC by selected desorption. It was found to be easy to produce a large-area carbon nanotube film on the SiC substrate. The (0002) lattice distance of graphite constructing the CNTs was obtained to be 0.344 nm from the electron diffraction pattern.

Nanoscale stacking faults induced low thermal conductivity in thermoelectric layered metal sulfides

Layered metal sulfides (MS)1+x(TiS2)2 (M = Pb, Sn, Bi) with alternative stacking of MS layers and TiS2 layers (a natural superlattice) have been proposed as thermoelectric materials. In this paper, various nanoscale stacking faults have been found in these materials, including the translational disorder in (SnS)1.2(TiS2)2 and the staging disorder in (BiS)1.2(TiS2)2. The lattice thermal conductivities along the layers are systematically and significantly reduced by these stacking faults which are only a few unit cells apart, without deteriorating the electron mobility, demonstrating a “phonon-blocking, electron-transmitting” scenario.

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.

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.

Growth process of close-packed aligned carbon nanotubes on SiC

Densely aligned carbon nanotubes (CNTs) by surface decomposition of SiC (0001¯) C-face in vacuum were observed by plan-view transmission electron microscopy. It was found that the wall number of CNTs was directly proportional to the diameter of CNTs. Comparing with the theoretical calculation, it was revealed that all of the carbon atoms remained on the surface after the selective evaporation of Si atoms by decomposition of each monolayer of SiC (0001¯), and then constructed the CNT walls with the minimum diffusion distance at the interface.

Atom-by-atom simulations of graphene growth by decomposition of SiC (0001): Impact of the substrate steps

The atomic-scale carbon rearrangement into graphene by the thermal decomposition of SiC (0001) was simulated by the density-functional tight-binding technique. By decomposing the terrace of the SiC (0001) surface, the carbon chains formed a three-dimensional structure, because the carbon atoms are released by losing their original contacts to silicon atom. On the other hand, in the step model, the silicon atoms at the step-edge act as trapping sites for the released carbon atoms, and the carbon network effectively nucleated and expanded. After nucleation at the step, graphene can grow by the further decomposition together with retreat of the step.