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We investigate charge fractionalizations in artificial Tomonaga-Luttinger liquids (TLLs) composed of two capacitively coupled quantum Hall edge channels (ECs) in graphene. The interaction strength of the artificial TLLs can be controlled through distance W between the ECs. We show that the fractionalization ratio r and the TLL mode velocity v vary with W. The experimentally obtained relation between v and r follows a unique function predicted by the TLL theory. We also show that charged wavepackets are reflected back and forth multiple times at both ends of the TLL region., to be published in Phys. Rev. B Rapid Communication

A thorough understanding of the stability of graphene under ambient environmental conditions is essential for future graphene-based applications. In this paper, we study the effects of ambient temperature on the properties of monolayer graphene using terahertz time-domain spectroscopy as well as time-resolved terahertz spectroscopy enabled by an optical-pump/terahertz-probe technique. The observations show that graphene is extremely sensitive to the ambient environmental conditions and behaves differently depending on the sample preparation technique and the initial Fermi level. The analysis of the spectroscopic data is supported by van der Pauw and Hall effect measurements of the carrier mobility and carrier density at temperatures comparable to those tested in our THz spectroscopic experiments.

We report thermal effects on carrier dynamics in graphene revealed by terahertz spectroscopy. We attribute our observations to extreme sensitivity of carrier density and mobility to environmental changes.

We report a novel etchant-free transfer method of graphene using the intercalation of alkanethiol self-assembled monolayers (SAMs) at the graphene/Cu interfaces. The early stage of intercalation proceeds through graphene grain boundaries or defects within a few seconds at room temperature until stable SAMs are formed after a few hours. The formation of SAMs releases the compressive strain of graphene induced by Cu substrates and make graphene slightly n-doped due to the formation of interface dipoles of the SAMs on metal surfaces. After SAM formation, the graphene is easily delaminated off from the metal substrates and transferred onto insulating substrates. The etchant-free process enables us to decrease the density of charged impurities and the magnitude of potential fluctuation in the transferred graphene, which suppress scattering of carriers. We also demonstrate the removal of alkanethiol SAMs and reuse the substrate. This method will dramatically reduce the cost of graphene transfer, which will benefit industrial applications such as of graphene transparent electrodes.

We have observed the well-kown quantum Hall effect (QHE) in epitaxial graphene grown on silicon carbide (SiC) by using, for the first time, only commercial NdFeB permanent magnets at low temperature. The relatively large and homogeneous magnetic field generated by the magnets, together with the high quality of the epitaxial graphene films, enables the formation of well-developed quantum Hall states at Landau level filling factors $\nu=\pm 2$, commonly observed with superconducting electro-magnets. Furthermore, the chirality of the QHE edge channels can be changed by a top gate. These results demonstrate that basic QHE physics are experimentally accessible in graphene for a fraction of the price of conventional setups using superconducting magnets, which greatly increases the potential of the QHE in graphene for research and applications., Comment: 9 pages, 3 figures

We report etching-free and iodine-free transfer of highly aligned array of armchair-edge graphene nanoribbons (ACGNRs) and their field-effect transistor (FET) characteristics. They were prepared by on-surface polymerization on Au(788) templates. The ACGNRs were mechanically delaminated and transferred onto insulating substrates with the aid of a nano-porous support layer composed of hydrogen silsesquioxane (HSQ). The key process in the mechanical delamination is the intercalation of octanethiol self-assembled monolayers (SAMs), which penetrate the HSQ layer and intercalate between the ACGNRs and Au(788). After the transfer, the octanethiol SAMs were removed with Piranha solution, enabling the reuse of the Au single crystals. The FETs fabricated with the transferred ACGNR array showed ambipolar behavior when the channel length was as long as 60 nm. Quasi-one-dimensional conductivity was observed, which implies a good alignment of GNRs after the transfer. In contrast, short-channel ACGNR FETs (channel length ∼20 nm) suffer from a geometry-dependent short-channel effect. This effect is more severe in the FETs with ACGNRs parallel to the channel, which is an ideal geometry, than in ones perpendicular to the channel. Since the ID-VD curve is well fitted by the power-law model, the short-channel effect likely stems from the space-charge limited current effect, while the wide charge-transfer region in the GNR channel can be another possible cause for the short-channel effect. These results provide us with important insights into the designing short-channel GNR-FETs with improved performance.

Polarized transmission and reflection spectra in the terahertz region were obtained from a graphene complementary split ring resonator device. The complementary structure combined with an ion gel gate electrode rendered the optical properties of the device tuneable. The oscillator strength at the intraband plasmon resonance was largely enhanced with the gate-voltage-induced doping, and absorption exceeded 2.3%/layer of the interband transition. The resonance frequency could also be largely increased with the gate voltage. These results suggest the possibility of graphene-based metamaterials with tuneable permeability or permittivity and tuneable resonance frequencies.

ently, many studies on the application of Er: YAG laser in the dental field have been reported. Er: YAG laser can ablate dental hard tissues with small thermal damages, and is clinically applied for removal of dental caries. However, there are few studies on apicoectomy using Er: YAG laser. The purpose of this study was to compare apicoectomy using Er: YAG laser and the conventional cutting method in vitro.Extracted single-rooted human teeth were used in this experiment. Root resection was performed with Er: YAG laser (output energy: 70mJ10pps) or a fissure bur mounted to a micro-motor. Resected root surfaces were evaluated with scanning electron microscopy.In addition, cutting efficiency of both methods and dye penetration of the cut surface were compared. After removing the tooth crown of 10 extracted single-rooted human teeth, root canals were cleaned, shaped and obturated. Then, the samples were randomly divided into two groups. Root resection by Er: YAG laser or micro-motor was perfOrtned, and working time was counted and the cut surface area measured. Then, all the root surfaces were sealed with nail polish except the resected surface. All roots were placed into 0.1% methylene blue dye for 7 days, rinsed and air-dried. Finally, they were split and the dye penetration from the resected surface was measured and statistically analyzed.The results were as follows:1. There were no smear layer or debris left on the resected surface cut by Er: YAG laser, and the shape of the cut surface was irregular. On the other hand, both smear layer and debris were observed on the surface cut by the conventional method, and the cut surface was smooth.2. The cutting efficiency of Er: YAG laser and micro-motor was 14.8 (±7.3) sec./mm2 and 4.4 (±1.7) sec./ram2, respectively. Cutting by Er: YAG laser significantly needed more time than that by micro-motor (t-test, P

Raman spectroscopy has been used in chemistry and physics to investigate the fundamental process involving light and phonons (quantum of lattice vibration). The carbon nanohorn introduces a new subject to Raman spectroscopy, namely topology. We show theoretically that a photo-excited carrier with a non-zero 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., 12 pages, 3 figures

We have fabricated a planar cold cathode based on a multilayer (ML)-graphene/SiO2/Si heterostructure. When voltage is applied between the ML graphene and Si layer, electrons tunnel from the Si layer to the ML-graphene through the SiO2. During this tunneling event, electrons repeatedly gain and lose energy in the SiO2 owing to the electric field and scattering, respectively. Electrons whose energy is larger than the work function of the ML-graphene are emitted from its surface to a vacuum. The thinness of the ML-graphene reduces the energy loss of electrons in it and thus improves electron emission characteristics.

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.

Carrier density and temperature-dependent transport properties of monolayer graphene grown on SiC(0001) were systematically studied. With increasing temperature, resistivity at the charge neutrality point decreases, while resistivity away from the charge neutrality point, where the carrier density is above 1 \ifmmode\times\else\texttimes\fi{} 10${}^{11}$ cm${}^{\ensuremath{-}2}$, increases. From Hall measurements and the temperature dependence of conductivity, carrier density at the charge neutrality point at 2 K is estimated to be 1 \ifmmode\times\else\texttimes\fi{} 10${}^{10}$ cm${}^{\ensuremath{-}2}$ or below. Such low carrier density implies a small potential fluctuation in graphene on SiC(0001). The carrier density at the charge neutrality point increases with increasing temperature and thus the resistivity decreases. At carrier densities away from the charge neutrality point, the mobility decreases with increasing temperature and the resistivity increases. We analyzed the decrease of mobility, assuming phonon scatterings that originate from the substrate.

A back-gate graphene p-n junction was achieved by selective interfacial modification of a chemical vapor deposition (CVD)-grown graphene field effect transistor (FET). Silane self-assembled monolayer (SAM) patterns were used to fabricate uniform p- and n-doped regions and a sharp p-n junction in the graphene FET channel. A gate-dependent photocurrent response was observed at the graphene p-n junction, and exhibited a maximum signal between two Dirac point voltages of SAM-doped graphene regions. A spatial photocurrent map shows that the photocurrent generated at the junction region was much larger than that from graphene/electrode junctions under the same incident laser power. This single-peak characteristic photocurrent in CVD graphene is dominated by the photothermoelectric contribution, and is highly sensitive to the power of incident laser. The SAM interfacial modification method provides a feasible route for the fabrication of efficient graphene-based photodetectors.

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. In particular, we observe anomalous values of the quantized resistance and a peculiar asymmetry with magnetic field which was not observed before for graphene on SiC. A quantitative model involving enhanced inter-channel scattering mediated by the presence of bilayer inclusions is presented that successfully explains the observed symmetry properties.

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.

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, ranging from 0.005 to 0.01 cm2 V−1s−1. Our synthesis method is simple and could also be used to synthesize other TMDs.

A histopathological study was performed on the tooth pulp of mongrel dogs after cavity preparation using the conventional high speed method (control group) and the newly developed Er:YAG laser system (laser group, output energy: 100 mJ, 150 mJ, and 200 mJ/pulse). All samples were divided into two groups (deep cavity and shallow cavity) according to the remaining dentin thickness (RDT) and histopathologically evaluated. After 1, 2, 4, 7 and 28 days postoperatively, there was no histopathological difference between the control group and the laser group. Er:YAG laser irradiation with three different output energies made little difference in the degree of pulpal damage. In the deep cavities, damage of the pulp was more remarkable than the shallow cavities in all groups. The efficacy of cutting rate in the laser group was not very different from the control group.

Quasi-free-standing bilayer graphene was grown by hydrogen intercalation of epitaxial monolayer graphene on SiC(0001). A larger size of stacking domains compared to that of epitaxial bilayer graphene grown on SiC(0001) was observed in a low-energy electron microscopy analysis of its morphology. By evaluating its electronic transport characteristics in top-gated devices, we found that the quasi-free-standing bilayer graphene is p-doped at zero-gate voltage. Further, an increase in mobility was found compared to that of epitaxial bilayer graphene. As a result of the higher mobility, Shubnikov–de Hass oscillations were observed. We attribute the improved quality of the quasi-free-standing bilayer graphene to its structural properties.

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.

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.

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.

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.

Terahertz field-induced transmission enhancement in monolayer epitaxial graphene is observed with increasing terahertz field. Photoexcitation leads to further transmission enhancement that is found to be less for the higher terahertz field amplitudes.

We report temperature dependence and thermal hysteresis behavior of terahertz transmission through photoexcited graphene. We vary the temperature between room temperature and 180° C, and use the optical-pump/terahertz-probe differential transmission technique.