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We have performed low-temperature magnetotransport measurements on epitaxial graphene composed of domains of single-layer and bilayer areas simultaneously present on SiC(0 0 0 1). Positive magnetoresistance that tends to saturate at high magnetic fields is observed for longitudinal component , while the Hall resistance exhibits sublinear behaviour. The lineshapes for both and can be accounted for extremely well by the semiclassical magnetotransport model incorporating two types of carriers conducting in parallel. Two sets of mobilities and densities, corresponding to single-layer and bilayer regions, respectively, can be obtained by fitting the experimental traces to the two-carrier model formulae. From the carrier densities, in turn, the ratio of the single-layer to bilayer areas can be deduced, assuming the alignment of the Fermi levels in the two types of areas with differing dispersion relations and Dirac point energies. The ratio of areas thus obtained is consistent with the ratio directly ob...

The electrical contact properties of silicon carbide (SiC) and carbon nanotubes (CNTs) were measured by conductive atomic force microscopy (C-AFM). A CNT forest was synthesized by SiC surface decomposition. Trenches, which electrically separate the conduction area, were fabricated using a focused ion beam (FIB) without a cover layer, and the resistance of each island was measured by C-AFM. From the dependence of the resistance on the CNT forest island size, the contact resistance between the CNTs and the SiC substrate was measured. By varying the dopant density in the SiC substrate, the Schottky barrier height was evaluated to be ~0.5 eV. This is slightly higher than a previously reported result obtained from a similar setup with a metal covering the CNT forest. We assumed that the damaged region existed in the islands, which is due to the trench formation by the FIB. The commensurate barrier height was obtained with the length of the damaged region assumed to be ~3 μm. Here, we could estimate the resistivity of a CNT/SiC interface without a cover layer. This indicates that a CNT forest on SiC is useful as a brief contact electrode.

The feasibility of using vertically aligned carbon nanotubes (CNTs) on a burnishing slider in the cleaning process of a magnetic disk surface was studied. Vertically aligned CNTs on SiC flat sliders were grown by the SiC surface decomposition method. Burnishing sliders with different CNT lengths were prepared for friction measurement on magnetic disks and for the burnishing test. Non-lubricated vertically aligned CNTs sliding on non-lubricated magnetic disks resulted in large friction; however, coating the CNTs and the disks with a perfluoropolyether lubricant film dramatically decreased the friction force. In addition, it was found that a specific amount of lubricant was needed on the CNTs to decrease the friction force and that the friction force depended on the CNT length. The particle burnishing test indicated that the 34-nm-long CNT burnishing slider with a rinsed lubricant film showed good burnishing performance.

The misfit layer compounds (BiS)1.2(TiS2)2, with a natural superlattice structures, are of substantial interest as thermoelectric materials. In this work we doped the Ti sites of (BiS)1.2(TiS2)2 with a series of transition metal (TM) elements (V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), to optimize its transport properties and thermoelectric performance. X-ray diffraction confirmed all the resulting compositions were single-phase. X-ray photoelectron spectroscopy revealed the valence states of the doping elements, indicating they behave as acceptors and reduce the carrier concentration; this was also apparent from Hall measurements. However, because of the non-parabolic band structure of (BiS)1.2(TiS2)2, reduction of carrier concentration by doping with most of the TM elements did not improve the Seebeck coefficient. Exceptions were V and Cr. For these elements, the effective mass of electrons was maintained, or even enhanced, resulting in improvement of the Seebeck coefficient. Furthermore, the stacking disorder present in undoped (BiS)1.2(TiS2)2 was not observed for the TM element-doped samples, resulting in increased lattice thermal conductivity. Although the power factor of these materials was not optimized, because of the large reduction in electronic thermal conductivity upon doping, the Cr-doped sample had a higher figure of merit than the undoped material.

The substitutional doping of magnesium into the host layers (TiS2), calcium and strontium into phonon barrier layers (BiS) was attempted to improve the thermoelectric performance of (BiS)1.2(TiS2)2 misfit layer sulfides by decreasing the carrier concentration. The substitutional doping did reduce the carrier concentration; however, the Seebeck coefficient decreased slightly due to the decrease in the effective mass of carrier electrons. Meanwhile, the lattice thermal conductivity was increased by the formation of an ordered or commensurate structure.

A natural superlattice with composition (SnS)1.2(TiS2)2, built by intercalating a SnS layer into the van der Waals gap of layered TiS2, has been directly observed by high-resolution transmission electron microscopy (HRTEM). The thermoelectric performance is improved in the direction parallel to the layers because the electron mobility is maintained while simultaneously suppressing phonon transport, which is attributed to softening of the transverse sound velocities due to weakened interlayer bonding. In the direction perpendicular to the layers, the lattice thermal conductivity of (SnS)1.2(TiS2)2 is even lower than the predicted minimum thermal conductivity, which may be caused by phonon localization due to the translational disorder of the SnS layers parallel to the layers. Moreover, we propose a large family of misfit-layer compounds (MX)1+x(TX2)n (M = Pb, Bi, Sn, Sb, rare-earth elements; T = Ti, V, Cr, Nb, Ta; X = S, Se; n = 1, 2, 3) with a natural superlattice structure as possible candidate high-performance thermoelectric materials.

The carbon nanotubes (CNTs) on 6H-SiC and Si-doped CNTs on 6H-SiC were analyzed by hard x-ray photoelectron spectroscopy to identify the chemical bonding character of the doped Si in the CNT layer. We performed the depth profiling of the sample by changing photoelectron's take-off-angle (TOA). The spectral component associated with the doped Si is clearly seen in the Si 1s photoelectron spectra of Si-doped CNTs on 6H-SiC. The Si 1s peak shifts toward lower kinetic energy side from TOA = 80° (bulk-sensitive) to 8° (surface-sensitive), which implies the formation of the sp2-like structure in Si-doped CNTs. [DOI: 10.1380/ejssnt.2011.54]

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.

We have measured low-temperature ac impedance of films of closely-packed, highly-aligned carbon nanotubes prepared by thermal decomposition of silicon carbide wafers. The measurement was performed on films with the thickness (the length of the nanotubes) ranging from 6.5 to 65 nm. We found that the impedance rapidly decreases with the frequency. This can be interpreted as resulting from the electric transport via capacitive coupling between adjacent nanotubes. We also found numbers of sharp spikes superposed on frequency vs. impedance curves, which presumably represent resonant frequencies seen in the calculated conductivity of random capacitance networks. Capacitive coupling between the nanotubes was reduced by the magnetic field perpendicular to the films at 8.2 mK, resulting in the transition from negative to positive magnetoresistance with an increase of the frequency.

Graphene is a single-layer carbon sheet with a honeycomb structure, and bilayer graphene consists of two graphene sheets with AB stacking. In trilayer graphene, the third graphene sheet has two possible stacking sequences, A or C, when it is overlaid on bilayer graphene. It has been theoretically predicted that trilayer graphene exhibits a variety of novel electronic properties with/without a Dirac-cone band, depending on the stacking sequence. In this regard, trilayer graphene has a high potential for widening the capability of graphene-based electronic devices. However, the difficulty of selective fabrication has hindered the progress of research. Here, we report the first success in the selective fabrication of quasi-free-standing trilayer graphene with ABA or ABC stacking grown epitaxially on hydrogen-terminated silicon carbide. Angle-resolved photoemission spectroscopy (ARPES) clearly demonstrated that our trilayer graphene with ABA stacking has a massless Dirac-like band near the Fermi level, while that with ABC stacking shows a parabolic non-Dirac-like band dispersion. Strict control of temperature and pressure during annealing is shown to be crucial for controllably stacking graphene by a team in Japan. Graphene is a single layer of carbon atoms in a hexagonal arrangement. When two graphene layers are stacked, some carbon atoms in the top layer sit above the center of each hexagon of the lower layer. A third layer either sits directly aligned with the bottom layer (ABA stacking) or is again displaced from the previous two (ABC stacking). These two arrangements exhibit very different properties. Takashi Takahashi from Tohoku University and colleagues deterministically created either ABA or ABC trilayer graphene by first annealing bilayer graphene on a hydrogen-terminated silicon carbide substrate. They found that the alignment of the third layer is determined by the temperature and pressure of this anneal. Quasi-free-standing trilayer graphene with ABA or ABC stacking was selectively synthesized on hydrogen-terminated silicon carbide. The electronic structure was investigated by angle-resolved photoemission spectroscopy. While ABA graphene exhibits a massless Dirac-cone-like band at the K point in the Brillouin zone, ABC graphene was found to show a parabolic non-Dirac-like band. The present success in selective fabrication of ABA and ABC graphene would open a pathway toward graphene-based nano electronic devices with variable layer number and stacking sequence such as high-speed transistor and photodetector.

We have investigated the length-dependence of the in-plane electrical resistivity of vertically aligned and highly dense carbon nanotube (CNT) films that were dense enough to conduct electrons. The in-plane conductivity is well accounted for by a combination of inter-tube hopping (variable range hopping, VRH) and graphitic conduction. VRH conduction was dominant in the thinner CNT films, and the films showed negative temperature dependence of resistivity. The dimension of the VRH component varied depending on the CNT length. In the thicker CNT films, the graphitic conduction appeared, and then, the localization length spread, leading to the positive temperature dependence of resistivity. This behavior can be explained by the presence of a labyrinthine arrangement of graphene walls among aligned CNTs, which was confirmed by transmission electron microscopy observations.

Direct cross-sectional observations of graphene-on-SiC (0 0 0 1) by high-resolution transmission electron microscopy have revealed the presence of a metastable transitional structure formed by decomposition of a single SiC bilayer as well as fully-packed honeycomb graphene as an interface structure between graphene and SiC. A first principles calculation studied in parallel has clarified that the metastable transitional state exhibits the metallic behavior, while the fully-packed honeycomb graphene exhibits the semiconducting behavior.

To encourage the recycling of spent SiC abrasive powders, we investigated the synthesis of CNTs from several types of powders. The surface decomposition method was applied to as-received powders, and graphite was observed on the SiC surface but CNTs were not. This could be caused by the turbulence of the crystal structure and Fe contaminants on the SiC surface. On the other hand, from TEM observations we found that particle covered of CNTs could be synthesized by applying the surface decomposition method to powders pretreated by acid and heat, because the turbulence of the crystal structure on the surface could be improved and Fe contaminants could be removed. Consequently, the spent SiC abrasive powders pretreated by acid and heat could be used as a resource for synthesizing CNT particles.

The electronic system in the layered perovskite Sr2−xNdxMnO4 is characterized as a two-dimensional eg-electron system. In order to understand the stability of electronic states in the Nd-content region of 0 ⩽ x ⩽ 0.50, the crystallographic features of this manganite have been investigated by transmission electron microscopy. The increase in the Nd-content led to the orbital ordered state in 0.10 ⩽ x ⩽ 0.25 and the charge and orbital ordered state in 0.25 ⩽ x 0.35.

We report on a nitride interface structure formed between epitaxial graphene and SiC (0001). The nitride interface can be obtained by the pretreatment of SiC in an $\mathrm{Ar}/{\mathrm{N}}_{2}$ atmosphere at 1600 \ifmmode^\circ\else\textdegree\fi{}C, followed by graphene growth in Ar at 1700 \ifmmode^\circ\else\textdegree\fi{}C. Our detailed high-resolution transmission electron microscopy revealed a nitride atomic layer between the 0th carbon layer and the SiC substrate. Due to the nitride interface, the interface carrier scattering was reduced, which resulted in an improvement of the room temperature mobility of graphene, indicating that this is another technique to modify the electronic properties of graphene.

The formation of large-area homogeneous graphene on a C-terminated SiC (000$\overline{1}$) surface was achieved via decomposition of the SiC (000$\overline{1}$) surface covered with an ultrathin but much more stable TiC layer than the reactive SiC(000$\overline{1}$). By heating the SiC (000$\overline{1}$) surface with a mixed powder of TiO${}_{2}$ and carbon at 1500--1550 \ifmmode^\circ\else\textdegree\fi{}C in vacuum, an extremely homogeneous, epitaxial 0.75-nm-thick TiC(111) layer was grown on the SiC(000$\overline{1}$) surface over a millimeter-scale area. Graphitization of the TiC-masked SiC surface led to the growth of high-quality graphene layers, which consist of TiC- and SiC-derived carbon. High-resolution transmission electron microscopy revealed the presence of disordered stacking of graphene layers on SiC through an amorphous layer at the interface. This unique method will promise further progress of relatively high carrier mobility of graphene formed on the SiC (000$\overline{1}$) surface.

Graphene layers were grown on a C-terminated SiC (000$\overline{1}$) surface in a high-pressure Ar atmosphere. Their growth mechanism was investigated using high-resolution transmission electron microscopy (TEM). First at a low temperature, local areas of SiC surface are decomposed, and several layers of graphene nucleus are formed in the resulting craters. Then graphene layers grow in all directions laterally, keeping their number of layers invariant. These results indicate that control of the number of graphene layers require precise control of the first stage of decomposition. After the graphene layers cover the surface completely, SiC decomposition occurs at a higher temperature along the [0001]${}_{\mathit{SiC}}$ direction, and the number of graphene layers increase. The formation of local wrinkles accompanies the increase of the number of layers. In addition we propose that the formation mechanism strongly affects the rotational stacking, which is characteristic of multilayer graphene on the C-face of SiC.

An investigation using high-resolution transmission electron microscopy of the stacking sequence of several layers of graphene formed on SiC(0001) shows that graphene layers selectively exhibit an ABC-type stacking. Using the well-known Slonczewski-Weiss-McClure model based on the tight-binding method, we suggest that a ${\ensuremath{\gamma}}_{5}$-like interatomic interaction, which corresponds to the formation of a linear trimer of ABA type stacking, is spontaneously weakened by the interaction between graphene and SiC. This can lead to the destabilization of the ABA stacking and to the formation of the ABC stacking, indicating the possibility of bandgap tuning by an electric field in more than three layers of graphene on SiC.

The crystallographic features of the orbital-ordered and charge-and-orbital-ordered states in ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Nd}}_{x}\mathrm{Mn}{\mathrm{O}}_{4}$ have been investigated by in situ observations, using a transmission electron microscope, in an attempt to understand the distinct characteristics of their electronic states. In the orbital-ordered state with orthorhombic symmetry, there exist four banded-domain-structure states, that is, two orthorhombic variant states $({O}_{I}+{O}_{II})$ accompanying the $C$-type antiferromagnetic ordering at lower temperatures and $(\mathrm{DT}+{O}_{I})$ coexistence and $({O}_{I}+{O}_{II})$ states lacking magnetic ordering at higher temperatures, where DT represents the disordered tetragonal state and ${O}_{I}$ and ${O}_{II}$ the two orthorhombic variants. On the other hand, the stability of the charge-and-orbital-ordered state present for $0.25\ensuremath{\leqslant}x\ensuremath{\leqslant}0.43$ is strongly suppressed when $xg0.38$. As a result of the strong suppression, the charge-exchange-type charge-and-orbital-ordered state is absent for $x=0.5$. The important features of the charge-and-orbital-ordered state are that it is basically characterized by incommensurate structural modulations and that round-shaped domains separated by disordered regions appear for $0.38lx\ensuremath{\leqslant}0.43$ near the DT state. On the basis of these data, the origin of the appearance of the four banded-structure states in the orbital-ordered state was attributed to a coupling between the local Jahn-Teller distortion and the long-range distortions, including a dilational one. In addition, we propose a model for the ground-state change between the orbital-ordered and charge-and-orbital-ordered states in terms of orbital degree of freedom.

An in situ observation of the tetragonal-to-orthorhombic phase transition in Fesubstituted Sr1.8La0.2MnO4 by transmission electron microcopy revealed that the domainstructure changes, as a relaxation phenomenon, took place during the aging for the formation of the orbital ordered state. At each aging temperature, the final banded domain structure could be produced from any starting state. The characteristic features of the domain-structure relaxation found in this study are also discussed in terms of a coupling between the local JT and long-range dilational distortions.

When ${\mathrm{Sr}}^{2+}$ ions in ${\mathrm{Sr}}_{2}\mathrm{Mn}{\mathrm{O}}_{4}$ containing only ${\mathrm{Mn}}^{4+}$ ions were partially replaced by ${\mathrm{La}}^{3+}$, a new phase having orthorhombic symmetry appeared around an La content of $x=0.15$ between the tetragonal $I4∕mmm$ $(T)$ phase and the charge and orbital ordered (COO) phase, accompanying the introduction of ${\mathrm{Mn}}^{3+}$ ions. Our in situ observation using a transmission electron microscope revealed that the orthorhombic (O) phase could be identified as an orbital ordered state without charge ordering, and that its microstructure is characterized by an alternating array of two banded-shape variants with different orthorhombicities, ${\mathrm{O}}_{I}$ and ${\mathrm{O}}_{II}$. It was also found that the $T$-to-O phase transition exhibited a unique evolution of domain structures, which resulted in the above-mentioned banded microstructure. In particular, the domain-structure evolution consisted of three steps: the appearance of the $(T+{\mathrm{O}}_{I})$ and then the $({\mathrm{O}}_{I}+{\mathrm{O}}_{II})$ coexisting states, followed by the annihilation of the interface between the ${\mathrm{O}}_{I}$ and ${\mathrm{O}}_{II}$ variants. The evidence suggests that this unique pattern of evolution is due to coupling between the short-wavelength Jahn-Teller (JT) distortion, associated with the ${\mathrm{Mn}}^{3+}$ ion, and the long-wavelength O distortion.

Graphene, a one-atom-layer carbon material, can be grown by thermal decomposition of SiC. On Si-terminated SiC(0001), graphene nucleates at steps and grows layer-by-layer, and as a result a homogeneous monolayer or bilayer can be obtained. We demonstrate this mechanism both experimentally and theoretically. On the C-face (000), multilayer graphene nucleates not only at steps, but also on the terraces. These differences reflect the distinct differences in the reactivity of these faces. Due to its high quality and structural controllability, graphene on SiC{0001} surfaces will be a platform for high-speed graphene device applications.

We investigated the atomic-scale structural properties of graphene epitaxially grown on SiC {0?0?0?1} surfaces by high-resolution transmission electron microscope observations. In this review paper, we summarize our results about the interface structure, the growth mechanisms and the growth techniques. Graphene on the Si-terminated surface has a buffer layer that is strongly bonded to the substrate and has a low carbon atom density. Multilayer graphene on the Si-face exhibited an ABC-stacking selectively. Graphene on the Si-face nucleates at the step-edge, and grows layer-by-layer over the upper terrace. Based on the mechanism, we succeeded in growing high-quality and homogeneous monolayer and bilayer graphene on the Si-terminated surface with low-height and low-density steps using our original technique. Graphene on the C-face has weaker bonds with the substrate, and the resulting multilayer contains the rotational stacking disorder. The origin of the rotational stacking is closely related to the growth mechanism, where graphene layers nucleate on a terrace at a lower temperature and grow in all directions on the surface.

The crystallographic features of Sr2−xPrxMnO4 samples, prepared by a coprecipitation method, have been investigated by transmission electron microscopy to understand their electronic states for 0.10⩽x⩽0.30. The orbital-ordered (OO) state lacking charge ordering existed for 0.10⩽x⩽0.25, and accompanied C-type antiferromagnetic (CAF) ordering at lower temperatures. It was also found that the suppression of CAF ordering occurred in a variant with smaller orthorhombicity, which is one of two variants forming a banded domain structure in the OO state.

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.