Yoshiaki Sekine

Scalable synthesis of layer-controlled WS2 and MoS2 sheets by sulfurization of thin metal films

Transition metal dichalcogenides (TMDs) have emerged as exciting 2D materials beyond graphene due to their promising applications in the field of electronics and optoelectronics. Hence, the ability to produce controllable and uniformly thick TMD sheets over a large area is of utmost important for large-scale applications. Here, a facile method of synthesizing large-area, layer-controlled WS2, and MoS2 sheets by sulfurization of their corresponding thin metal films is reported. A metal film, which is deposited by magnetron sputtering method, can be adjusted to produce, with great control, the desired sheet thickness down to a monolayer. Various characterization techniques, such as Raman, photoluminescence, and transmission electron microscopy, were used to evaluate the grown films. The results confirmed some of the exotic properties of TMDs such as the thickness dependent band-gap transition (indirect to direct band gap) and Raman shift. Devices made directly on the as-grown film showed modest mobility, ra...

Half-Integer Quantum Hall Effect in Gate-Controlled Epitaxial Graphene Devices

High-quality monolayer graphene was grown on the Si face of SiC by thermal decomposition, and its electrical properties were investigated in top-gated devices. At 2 K, the carrier mobility of the graphene exceeded 10,000 cm2 V-1 s-1 and the half-integer quantum Hall effect was observed. The quantum Hall states were even observed at various carrier densities when top-gate bias was applied. These findings suggest high-quality epitaxial graphene possesses the unique nature of monolayer graphene and is robust against device fabrication, which holds potential for graphene-based electronics applications.

Experimental realization of a ballistic spin interferometer based on the Rashba effect using a nanolithographically defined square loop array

As quantum wells. Inthis experiment, we demonstrate electron spin precession in quasi-one-dimensional channels that iscaused by the Rashba effect. It turned out that the spin precession angle θ was gate-controllableby more than 0.75π for a sample with L = 1.5µm, where L is the side length of the SL. Largecontrollability of θ by the applied gate voltage as such is a necessary requirement for the realizationof the spin FET device proposed by Datta and Das [Datta et. al., Appl. Phys. Lett. 56, 665 (1990)]as well as for the manipulation of spin qubits using the Rashba effect.

Observation of Band Gap in Epitaxial Bilayer Graphene Field Effect Transistors

Bilayer graphene was grown on the Si-face of SiC by thermal decomposition. Its electronic properties were investigated in top-gate Hall bar devices. By controlling the carrier density using gate voltage, we were able to access the charge neutrality point. The conductance at the charge neutrality point showed a strong temperature dependence, and its temperature dependence was well fitted with thermal activation and variable-range hopping mechanisms. The electrical detection of a band gap opening in bilayer graphene grown on SiC is a promising step toward the realization of graphene-based electronics using epitaxial graphene.

Theoretical Study on Magnetoelectric and Thermoelectric Properties for Graphene Devices

Two of our recent theoretical efforts on elucidating the functions of graphene are reported. A first-principles calculation of the growth process of graphene islands on SiC(0001) shows that an embedded structure is energetically preferable. Island with this embedded structure do not have any broken dangling bonds at their edges. Their electronic states clearly show that they surely act as islands. Islands with zigzag edges have edge-localized states, which causes magnetoelectric effects. Graphene is also expected as a highly efficient material for thermoelectric elements according to a theoretical study. If the carrier scattering sources are adequately suppressed, the thermoelectric figure of merit greatly exceeds 1 at temperatures higher than 300 K with the Fermi energy fixed around the Dirac point. Since graphene is cheaper, resource abundant, more harmless, higher in melting temperature, and much lighter in density, than the present typical material, BiTe/Sb, many new applications could be considered.

Nonlinear transmission of an intense terahertz field through monolayer graphene

We report nonlinear terahertz (THz) effects in monolayer graphene, giving rise to transmission enhancement of a single-cycle THz pulse when the incident THz peak electric field is increased. This transmission enhancement is attributed to reduced photoconductivity, due to saturation effects in the field-induced current and increased intraband scattering rates arising from transient heating of electrons. We have developed a tight-binding model of the response using the length gauge interaction Hamiltonian that provides good qualitative agreement. The model fully accounts for the nonlinear response arising from the linear dispersion energy spectrum in graphene. The results reveal a strong dependence of the scattering time on the THz field, which is at the heart of the observed nonlinear response.

Bilayer-induced asymmetric quantum Hall effect in epitaxial graphene

The transport properties of epitaxial graphene on SiC(0001) at quantizing magnetic fields are investigated. Devices patterned perpendicularly to SiC terraces clearly exhibit bilayer inclusions distributed along the substrate step edges. We show that the transport properties in the quantum Hall regime are heavily affected by the presence of bilayer inclusions, and observe a significant departure from the conventional quantum Hall characteristics. A quantitative model involving enhanced inter-channel scattering mediated by the presence of bilayer inclusions is presented that successfully explains the observed symmetry properties.

Topological Raman band in the carbon nanohorn.

Raman spectroscopy has been used in chemistry and physics to investigate the fundamental process involving light and phonons. The carbon nanohorn introduces a new subject to Raman spectroscopy, namely topology. We show theoretically that a photoexcited carrier with a nonzero winding number activates a topological D Raman band through the Aharonov-Bohm effect. The topology-induced D Raman band can be distinguished from the ordinary D Raman band for a graphene edge by its peak position.

Atomic Structure and Physical Properties of Epitaxial Graphene Islands Embedded in SiC(0001) Surfaces

The atomic structures of graphene islands on SiC(0001) surfaces are studied theoretically together with their growth mechanism. Two types of embedded graphene island structure are proposed. It is shown that these structures actually act as the graphene island electronically, and that those with zigzag edges have the magnetoelectric effect.

Evaluation of Disorder Introduced by Electrolyte Gating through Transport Measurements in Graphene

We evaluate the degree of disorder in electrolyte-gating devices through transport measurements in graphene. By comparing the mobility in ion- and standard metal-gated devices, we show that the deposition of the ionic liquid introduces charged impurities that limit the mobility in graphene to 3 × 103 cm2 V−1 s−1. At higher temperatures (>50 K), phonons in the ionic liquid further reduce the mobility, making its upper limit 2 × 103 cm2 V−1 s−1 at room temperature. Since the degree of disorder is independent of the base material, these results are valuable for understanding disorder effects in general devices using electrolyte gating.