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Hexagonal boron nitride (h-BN) is a layered material that is generating interest in various fields owing to its fascinating properties. We present a multi-stranded analysis of carbon defects in h-BN using intentionally carbon (C)-doped h-BN crystals. Carbon defects were introduced into h-BN crystals synthesized under high-pressure and high-temperature (HPHT) conditions by a process of carbon annealing. We employ an innovative approach to explore impurity states in our C-doped h-BN by assembling it into van der Waals heterostructures with other 2D materials. Attaching graphene as a probe material to determine the impurity states in the h-BN bandgap, we locate an acceptor state above the Dirac point of graphene on C-doped h-BN, which causes anomalous bending in the Landau fan diagram. Furthermore, we adopt tungsten disulfide (WS2) as another probe material and assess the influence of C-doped h-BN on the emission spectrum of the WS2 monolayers. Our work reveals the nature of the carbon impurities in h-BN and also uncovers their effects on adjacent 2D materials.

We report the first cyclotron resonance study of monolayer graphene under double-moire potentials in which the crystal axis of graphene is nearly aligned to those of both the top and bottom hexagonal boron nitride (h-BN) layers. Under mid-infrared light irradiation, we observe cyclotron resonance absorption with the following unique features: (1) cyclotron resonance magnetic field BCR is entirely different from that of nonaligned monolayer graphene, (2) BCR exhibits strong electron-hole asymmetry, and (3) splitting of BCR is observed for |ν| < 1, with the split maximum at |ν| = 1, resulting in eyeglass-shaped trajectories. These features are well explained by considering the large bandgap induced by the double moire potentials, the electron-hole asymmetry in the Fermi velocity, and the Fermi-level-dependent enhancement of spin gaps, which suggests a large electron-electron correlation contribution in this system.

Landau levels (LLs) of ABA-stacked trilayer graphene (TLG) are described as the combination of monolayer graphene-like LLs and bilayer graphene-like LLs. They are extremely sensitive to the applied perpendicular electric displacement field D. Here, we demonstrate the electrical control of cyclotron resonance (CR) in a dual-gated ABA-stacked TLG. Under the irradiation of mid-infrared light, we observed the photovoltage induced by the CR absorption through the photothermoelectric effect. The resonant magnetic field in CR is changed by applying D while keeping the carrier density constant. Numerical simulations based on the tight-binding model complement the experimental observations. We believe that the present study provides a boost to graphene-based photodetectors and photoemitters with an electrically tunable wavelength in mid-infrared to terahertz spectral ranges.

Hexagonal boron nitride (h-BN) crystals grown under ultrahigh pressures and ultrahigh temperatures exhibit a high crystallinity and are used throughout the world as ideal substrates and insulating layers in van der Waals heterostructures. However, in their central region, these crystals have domains which contain a significant density of carbon impurities. In this study, we utilized cathodoluminescence and far-ultraviolet photoluminescence to reveal that the carbon (C)-rich domain can exist even after exfoliation. Then, we studied the carrier transport of graphene in h-BN/graphene/h-BN van der Waals heterostructures, precisely arranging the graphene to straddle the border of the C-rich domain in h-BN. We found that the carrier mobility of graphene on the C-rich h-BN domain was significantly suppressed. In addition, characteristic bending of the Landau fan diagram was observed on the electron-doped side. These results suggest that the C-rich domain in h-BN forms an impurity level and induces extrinsic carrier scattering into adjacent graphene.

We demonstrate the dry release transfer of single-layer and bi-layer graphene and few-layer h-BN by utilizing poly(propylene) carbonate (PPC) films. Because of the strong adhesion between PPC and 2D materials around room temperature, we demonstrate that single-layer to few-layer graphene, as well as few-layer h-BN can be fabricated on a spin-coated PPC film/SiO2/Si substrate via the mechanical exfoliation method. In addition, we show that these single-layer to few-layer crystals are clearly distinguishable using an optical microscope with the help of optical interference. Because of the thermoplastic properties of PPC film, the adhesion between the 2D materials and PPC significantly decreases at about 70 °C. Therefore, by tuning the temperature, the graphene and h-BN flakes on PPC can be easily dry-transferred onto another h-BN substrate. We demonstrate the fabrication of encapsulated h-BN/graphene/h-BN devices and graphene/few-layer h-BN/graphene vertical-tunnel-junction devices using this method. Our finding provides a simple method for constructing high-quality graphene and h-BN-based van der Waals heterostructures with a minimum amount of polymer contamination.

Hexagonal boron nitride (h-BN) synthesized under high pressure and high temperature (HPHT) has been used worldwide in two-dimensional (2D) materials research as an essential material for constructing van der Waals heterostructures. Here, we study h-BN synthesized with another method, i.e., via synthesis at atmospheric pressure and high temperature (APHT) using a metal alloy solvent. First, we examine the APHT h-BN in a bulk crystal form using cathodoluminescence and find that it does not have carbon-rich domains that inevitably exist in a core region of all the HPHT h-BN crystals. Next, we statistically compare the size of the crystal flakes exfoliated on a SiO2/Si substrate from APHT and HPHT h-BN crystals by employing our automated 2D material searching system. Finally, we provide direct evidence that APHT h-BN can serve as a high-quality substrate for 2D materials by demonstrating high carrier mobility, ballistic transport, and Hofstadter butterfly in graphene and photoluminescence in WS2.

Landau Levels (LLs) of ABA-stacked trilayer graphene (TLG) is highly sensitive to the applied transverse electric field ( $E_{\perp}$ ). The energy shifts of LLs can be probed by studying cyclotron resonance (CR) in graphene. Here we demonstrate electrical gate control of CR signals in TLG using dual-gated TLG device. Tuning $E_{\perp}$ while keeping carrier density $n_{\mathrm{e}}$ constant, we observed that the resonance magnetic field of CR signals in TLG were greatly dependent on $E_{\perp}$ . In addition, the experimental data shows transitions between monolayer graphene (MLG)-like LLs and a bilayer graphene (BLG)-like LLs originate from a mixing of those LLs, which is allowed only in the presence of $E_{\perp}$ . The position and its shift of each transition is well reproduced by numerical calculation using tight-binding model, indicating that our data directly reflects the $E_{\perp}$ tunability of the band structure of TLG. ABA-stacked TLG has a great potential for future application as photovoltaic devices capable of external controlling of wavelength.

We demonstrate the detection of cyclotron resonance in graphene by using a photo-induced thermionic emission mechanism at the graphene/MoS2 van der Waals (vdW) Schottky junction. At cyclotron resonance in Landau-quantized graphene, the infrared light is absorbed and an electron-hole pair is generated. When the energy of a photoexcited electron exceeds the band offset energy at the graphene/MoS2 interface, the electron transfer occurs from graphene to the conduction band of MoS2, and the hole remains in graphene. This creates an electron-hole separation at the graphene/MoS2 interface at cyclotron resonance and a photovoltage is generated. The proposed method is an infrared photodetection technique through out-of-plane transport at the vdW junction, which is distinct from the previously reported methods that use in-plane transport in graphene for electronic detection of the cyclotron resonance. Despite the simple structure of our device with a single-vdW junction, our method exhibits a very high sensitivity of about 10^6 V/W, which shows an improvement of three orders of magnitude over the previously reported values. Therefore, the proposed method displays a high potential for cyclotron resonance-based infrared photodetector applications., Comment: This article has been published in Applied Physics Letters

We describe a technique for making one-dimensional ohmic contacts to individual graphene layers encapsulated in hexagonal boron nitride (h-BN) using CF4 and O2 plasmas. The high etch selectivity of h-BN against graphene (>1000) is achieved by increasing the plasma pressure, which enables etching of h-BN, while graphene acts as an etch stop to protect underlying h-BN. A low-pressure O2 plasma anisotropically etches graphene in the vertical direction, which exposes graphene edges at h-BN sidewalls. Despite the O2 plasma bombardment, the lower h-BN layer functions as an insulating layer. Thus, this method allows us to pattern metal electrodes on h-BN over a second graphene layer. Subsequent electron-beam lithography and evaporation fabricate metal contacts at the graphene edges that are active down to cryogenic temperatures. This fabrication method is demonstrated by the preparation of a graphene Hall bar with a graphite back-gate and double bilayer-graphene Hall bar devices. The high flexibility of the device geometries enabled by this method creates access to a variety of experiments on electrostatically coupled graphene layers., 4 figures, accepted for publication in Applied Physics Letters

Graphene is known to show a significant photo-thermoelectric effect that can exceed its photovoltaic contribution. Here, by utilizing this effect, we demonstrate a photovoltage measurement of cyclotron resonance in a double-back-gated h-BN/graphene/h-BN two-terminal device. A graphite local bottom gate was fabricated in addition to a p-doped Si global back gate. By tuning the two gate voltages, an in-plane graphene junction having an asymmetric carrier-doping profile was created. With the help of this asymmetric structure, the photo-thermoelectric voltage generated in the vicinity of the metal-electrode/graphene junction was detected. At a low temperature and in the presence of a magnetic field, a photo-induced voltage was measured under the irradiation of an infrared laser (Wavelength= 9.28 to 10.61 micron). We observed a strong enhancement of the photovoltage signal under the cyclotron resonance condition, at which the energy of excitation coincides with a transition between Landau levels. These results highlight the possibility of using the photo-thermoelectric effect in graphene for THz photo-detection., 18 pages, 4 Figures, Supplementary information is not included

We observed broken-symmetry quantum Hall effects and level crossings between spin- and valley- resolved Landau levels (LLs) in Bernal stacked trilayer graphene. When the magnetic field was tilted with respect to sample normal from $0^{\circ}$ to $66^\circ$, the LL crossings formed at intersections of zeroth and second LLs from monolayer-graphene-like and bilayer-graphene-like subbands, respectively, exhibited a sequence of transitions. The results indicate the LLs from different subbands are coupled by in-plane magnetic fields ($B_{\parallel}$), which was explained by developing the tight-binding model Hamiltonian of trilayer graphene under $B_{\parallel}$., Comment: 4 figures, accepted for publication in Physical Review Letters

Owing to the absence of a band gap, the direct applicability of graphene for transistors and logic devices is still limited. Recent studies have shown that the van der Waals heterostructure of graphene and other two-dimensional (2D) materials can be a great candidate to overcome these issues. The layered structure of these materials exhibits no direct chemical bonding between the different layers. Thus the vdW junction between these materials is, in principle, free from lattice mismatch issues and interface reactions. With these advantages, these vdW heterostructures has been demonstrated vertical field effect transistor (FET) operations as well as large current ON-OFF ratio [1]. However, realization of a large ON-OFF ratio simultaneously with a large ON current density has been still challenging. Here we fabricated exfoliated-graphene/MoS2/Ti vertical heterostructure including vdW interface built from freshly cleaved surface of graphene and MoS2 by using mechanical exfoliation and dry transfer method (Fig. 1(a)). The vertical transport of the device measured under the modulation of back gate voltage V G exhibits large modulation of its conductance (Fig. 1(b)). We obtained large current ON-OFF ratio of 105 as well as large ON current density of 104 A/cm2 [2]. Such large current modulation is attributed to the modulation of the Schottky barrier at graphene/MoS2 interface as schematically depicted in the right panel of Fig. 1(b). At the V G = +50 V, the Fermi level of the graphene is highest; therefore the Schottky barrier height at graphene/MoS2 junction is lowest. The reduction of Schottky barrier height makes device to be low resistance ON state. On the contrary, at the V G= -50 V, the Fermi level of the graphene is lowest; therefore the Schottky barrier height is highest. Here current flow in the device is greatly suppressed and works as OFF state. The importance of Schottky-barrier modulation on the performance of the transistor could be confirmed by the plot between the current ON-OFF ratio ION/IOFF vs. modulation of barrier height Δφ=φ max-φ min; these data are obtained from the series of devices with different thickness of MoS2 (Fig. 1(c)); results reveal correlation between two values [3]. These results suggest that even larger current modulation can be possible if one could have larger modulation of Schottky barrier height. For this purpose, we compared MoS2, MoSe2, and α-MoTe2 as transition metal dichalcogenide (TMD) materials in graphene/TMD/Ti heterostructure; these materials have different electron affinity and thus provide systematic change of band alignment to graphene (Fig. 1(d)). We show that the transport in the graphene/TMD junction is sensitive to the band alignment [4]. The V G dependence of the current revealed systematic change from MoS2 to MoTe2 (Fig. 1(e)). We demonstrated large current ON-OFF ratio and ON current density on MoSe2-based heterostructure; the value is comparable to that of MoS2-based device. These comparisons suggest that further improvement may be possible by optimizing device structure. We believe that our fabricated exfoliated-graphene/TMD/metal vertical heterostructure reveal superior performance to other existing graphene-based vertical transistors and present an important advance toward electronics applications. References [1] T. Georgiou et al., Nature Nanotechnol. 8(2013) 100. [2] R. Moriya et al., Appl. Phys. Lett. 105(2014) 083119. [3] Y. Sata, R. Moriya et al., Jpn. J. Appl. Phys. 54(2015) 04DJ04. [4] T. Yamaguchi, R. Moriya, et al., Appl. Phys. Lett. 105 (2014) 223109. Figure 1

Van der Waals heterostructures are comprised of stacked atomically thin two-dimensional crystals and serve as novel materials providing unprecedented properties. However, the random natures in positions and shapes of exfoliated two-dimensional crystals have required the repetitive manual tasks of optical microscopy-based searching and mechanical transferring, thereby severely limiting the complexity of heterostructures. To solve the problem, here we develop a robotic system that searches exfoliated two-dimensional crystals and assembles them into superlattices inside the glovebox. The system can autonomously detect 400 monolayer graphene flakes per hour with a small error rate (, The fabrication of van der Waals heterostructures of atomically thin materials often relies on the search, manual transferring, and alignment of suitable flakes. Here, the authors develop a robotic system capable of identifying exfoliated 2D crystals and assembling them in complex heterostructures.

We have realized a Dirac fermion reflector in graphene by controlling the ballistic carrier trajectory in a sawtooth-shaped npn junction. When the carrier density in the inner p-region is much larger than that in the outer n-regions, the first straight np interface works as a collimator and the collimated ballistic carriers can be totally reflected at the second zigzag pn interface. We observed clear resistance enhancement around the np+n regime, which is in good agreement with the numerical simulation. The tunable reflectance of ballistic carriers could be an elementary and important step for realizing ultrahigh-mobility graphene field effect transistors utilizing Dirac fermion optics in the near future.

Cyclotron resonance of a Landau-quantized graphene can absorb significant amount of infrared light. However, application of this phenomenon to the photodetector had been limited due to the lack of efficient photon to charge conversion scheme. Here, we demonstrate the detection of cyclotron resonance in a partially metal-masked monolayer graphene two-terminal device using photo-Nernst effect. Due to the presence of the mask, incident infrared light is irradiated on only one-half of the graphene channel. This partial irradiation creates a temperature gradient perpendicular to the graphene channel. In the presence of an external magnetic field, thermopower is generated perpendicular to the temperature gradient due to the Nernst effect. Consequently, photo-Nernst voltage is generated along the graphene channel, which can be detected from the contacts on both ends of the channel. We demonstrate selective detection of the photo-Nernst effect while minimizing the other photovoltaic contributions, such as the photo-Seebeck effect. We investigate the dependence of the photo-Nernst effect on magnetic field and excitation wavelength, which reveals a significant enhancement of photo-Nernst signal at the cyclotron resonance conditions in graphene. Our finding could facilitate the realization of far-infrared light detector using cyclotron resonance of graphene., This paper has been published in Applied Physics Letters

We demonstrated exfoliated-graphene/transition metal dichalcogenide (TMD)/metal vertical field-effect transistor (vFET). The van der Waals junction between graphene and TMD exhibit Schottky barrier. Owing to the low density of state of graphene, the position of graphene's Fermi level can be modulated by the external electric field, thus enable us electric field modulation of Schottky barrier height at graphene/TMD interface. Based on this mechanism, a vertical transport in the heterostructure exhibits large current modulation of 105, and at the same time the heterostructure reveal operation current density of 104 A/cm2. These results reveal potential high performance of vFET for electronics applications.

We demonstrate electrical spin injection into multilayer graphene (MLG) in a lateral spin valve device from a highly spin-polarized Co2FeSi (CFS) Huesler electrode. Exfoliated MLG was transferred onto pre-patterned epitaxial CFS wires grown on an Si(111) substrate by a polymer-based transfer method. This method enabled us to fabricate multiple single-crystal CFS electrodes in contact with MLG. Electrical spin injection from CFS to MLG was detected through non-local magnetoresistance (MR) measurement. A non-local spin signal of 430 ohm was observed; this is the largest value among all reported non-local MR values in graphene-based devices., 12 pages, 5 figures

We demonstrate electrical tunnel spin injection from a ferromagnet to graphene through a high-quality Al2O3 grown by atomic layer deposition (ALD). The graphene surface is functionalized with a self-assembled monolayer of 3,4,9,10-perylene tetracarboxylic acid (PTCA) to promote adhesion and growth of Al2O3 with a smooth surface. Using this composite tunnel barrier of ALD-Al2O3 and PTCA, a spin injection signal of 30 ohm has been observed from non-local magnetoresistance measurements at 45 K, revealing potentially high performance of ALD-Al2O3/PTCA tunnel barrier for spin injection into graphene., Comment: 14 pages, 3 figures, accepted to Journal of Magnetism and Magnetic Materials

We use scanning gate microscopy to map out the trajectories of ballistic carriers in high-mobility graphene encapsulated by hexagonal boron nitride and subject to a weak magnetic field. We employ a magnetic focusing geometry to image carriers that emerge ballistically from an injector, follow a cyclotron path due to the Lorentz force from an applied magnetic field, and land on an adjacent collector probe. The local electric field generated by the scanning tip in the vicinity of the carriers deflects their trajectories, modifying the proportion of carriers focused into the collector. By measuring the voltage at the collector while scanning the tip, we are able to obtain images with arcs that are consistent with the expected cyclotron motion. We also demonstrate that the tip can be used to redirect misaligned carriers back to the collector., 15 pages including 3 figures. Appl. Phys. Lett. 107, 243102 (2015)

We have fabricated graphene/graphene oxide/graphene (G/GO/G) junctions by local anodic oxidation lithography using atomic force microscopy (AFM). The conductance of the G/GO/G junction decreased with the bias voltage applied to the AFM cantilever V(tip). For G/GO/G junctions fabricated with large and small |V(tip)|. GO was semi-insulating and semiconducting, respectively. AFM-based LAO lithography can be used to locally oxidize graphene with various oxidation levels and achieve tunability from semiconducting to semi-insulating GO.

We review our recent experiments on the fabrication of nano-electronic devices in single-layer graphene. A graphene nanoribbon device was fabricated by conducting local anodic oxidation lithography using tapping-mode atomic force microcsope. The conductance of the graphene nanoribbon at the charge neutrality point was suppressed at low temperature, which suggests the opening of an energy gap due to the lateral confinement of charge carriers. A single quantum dot was fabricated by using the conventional electron beam lithography combined with plasma etching of the graphene layer. The quantum dot works as a single-electron transistor, which shows Coulomb blockade characteristics. These results suggest possibilities of the future application of the nano-electronic devices based on graphene.

We demonstrate a vertical field-effect transistor based on a graphene/MoSe2 van der Waals (vdW) heterostructure. The vdW interface between the graphene and MoSe2 exhibits a Schottky barrier with an ideality factor of around 1.3, suggesting a high-quality interface. Owing to the low density of states in graphene, the position of the Fermi level in the graphene can be strongly modulated by an external electric field. Therefore, the Schottky barrier height at the graphene/MoSe2 vdW interface is also modulated. We demonstrate a large current ON-OFF ratio of 10^5. These results point to the potential high performance of the graphene/MoSe2 vdW heterostructure for electronics applications., 17 pages, 4 figures

We demonstrated exfoliated-graphene/transition metal dichalcogenide (TMD)/metal vertical field-effect transistor (vFET). The van der Waals junction between graphene and TMD exhibit Schottky barrier. Owing to the low density of state of graphene, the position of graphene's Fermi level can be modulated by the external electric field, thus enable us electric field modulation of Schottky barrier height at graphene/TMD interface. Based on this mechanism, a vertical transport in the heterostructure exhibits large current modulation of 105, and at the same time the heterostructure reveal operation current density of 104 A/cm2. These results reveal potential high performance of vFET for flexible electronics applications.

We demonstrate a quantum Hall edge-channel interferometer in a high-quality graphene pn junction under a high magnetic field. The co-propagating p and n quantum Hall edge channels traveling along the pn interface functions as a built-in Aharanov-Bohm-type interferometer, the interferences in which are sensitive to both the external magnetic field and the carrier concentration. The trajectories of peak and dip in the observed resistance oscillation are well reproduced by our numerical calculation that assumes magnetic flux quantization in the area enclosed by the co-propagating edge channels. Coherent nature of the co-propagating edge channels are confirmed by the checkerboard-like pattern in the dc-bias and magnetic-field dependences of the resistance oscillations., Comment: 16 pages, 4 figures, supplementary material: 3 pages, 2 figures

Graphene-based vertical field effect transistors have attracted considerable attention in the light of realizing high-speed switching devices; however, the functionality of such devices has been limited by either their small ON-OFF current ratios or ON current densities. We fabricate a graphene/MoS2/metal vertical heterostructure by using mechanical exfoliation and dry transfer of graphene and MoS2 layers. The van der Waals interface between graphene and MoS2 exhibits a Schottky barrier, thus enabling the possibility of well-defined current rectification. The height of the Schottky barrier can be strongly modulated by an external gate electric field owing to the small density of states of graphene. We obtain large current modulation exceeding 10^5 simultaneously with a large current density of ~10^4 A/cm^2 , thereby demonstrating the superior performance of the exfoliated-graphene/MoS2/metal vertical field effect transistor

This paper demonstrates the high-quality tunnel barrier characteristics and layer number controlled tunnel resistance of a transition metal dichalcogenide (TMD) measuring just a few monolayers in thickness. Investigation of vertical transport in WS2 and MoS2 TMDs in graphene/TMD/metal heterostructures revealed that WS2 exhibits tunnel barrier characteristics when its thickness is between 2 to 5 monolayers, whereas MoS2 experiences a transition from tunneling to thermionic emission transport with increasing thickness within the same range. Tunnel resistance in a graphene/WS2/metal heterostructure therefore increases exponentially with the number of WS2 layers, revealing the tunnel barrier height of WS2 to be 0.37 eV., Comment: 15 pages, 3 figures

We demonstrate electrical spin injection from a ferromagnet to a bilayer graphene (BLG) through a monolayer (ML) of single-crystal hexagonal boron nitride (h-BN). A Ni81Fe19/ML h-BN/BLG/h-BN structure is fabricated using a micromechanical cleavage and dry transfer technique. The transport properties across the ML h-BN layer exhibit tunnel barrier characteristics. Spin injection into BLG has been detected through non local magnetoresistance measurements., 15 pages, 4 figures, accepted to Appl. Phys. Express

We report magnetotransport measurements in ballistic graphene/hexagonal boron nitride mesoscopic wires where the charge carrier mean free path is comparable to wire width $W$. Magnetoresistance curves show characteristic peak structures where the peak field scales with the ratio of cyclotron radius $R_\textrm{c}$ and wire width $W$ as $W/R_\textrm{c} = 0.9 \pm 0.1$, due to diffusive boundary scattering. The obtained proportionality constant between $R_\textrm{c}$ and $W$ differs from that of a classical semiconductor 2D electron system where $W/R_\textrm{c} = 0.55$., Comment: 14 pages, 4 figures

Dirac fermions exhibit various characteristic transport phenomena in graphene. Particularly in high magnetic fields, the electronic states of Dirac fermions are quantized into Landau levels, and graphene shows a half-integer quantum Hall effect. Here, we discuss the edge-channel picture in graphene quantum Hall junctions and review experiments on the quantum Hall effect in graphene in-plane unipolar and bipolar junctions.

We conducted local anodic oxidation (LAO) lithography in single-layer, bilayer, and multilayer graphene using tapping-mode atomic force microscope. The width of insulating oxidized area depends systematically on the number of graphene layers. An 800-nm-wide bar-shaped device fabricated in single-layer graphene exhibits the half-integer quantum Hall effect. We also fabricated a 55-nm-wide graphene nanoribbon (GNR). The conductance of the GNR at the charge neutrality point was suppressed at low temperature, which suggests the opening of an energy gap due to lateral confinement of charge carriers. These results show that LAO lithography is an effective technique for the fabrication of graphene nanodevices., 4 pages, 4 figures

We performed detailed studies of the current-voltage characteristics in graphene/MoS2/metal vertical field-effect transistors. Owing to its low density of states, the Fermi level in graphene is very sensitive to its carrier density and thus the external electric field. Under the application of a bias voltage VB between graphene and the metal layer in the graphene/MoS2/metal heterostructure for driving current through the van der Waals interface, the electric field across the MoS2 dielectric induces a shift in the Fermi level of graphene. When the Fermi level of graphene coincides with the Dirac point, a significant nonlinearity appears in the measured I-V curve, thus enabling us to perform spectroscopy of the Dirac point. By detecting the Dirac point for different back-gate voltages, we revealed that the capacitance of the nanometer-thick MoS2 layer can be determined from a simple DC transport measurement., Comment: 14 pages, 3 figures

We performed local anodic oxidation (LAO) lithography on monolayer graphene and highly oriented pyrolytic graphite (HOPG) using atomic force microscope (AFM). Auger electron spectroscopic measurements in the oxidized area formed on the HOPG revealed that the number of oxygen atoms systematically increased with the bias voltage applied to the AFM cantilever |Vtip|, which demonstrates the tunability of the extent of oxidation with |Vtip|. By optimizing the LAO conditions, we fabricated monolayer graphene nanoconstrictions with a channel width as small as 10 nm, which is the smallest graphene nanoconstriction so far achieved by utilizing AFM lithography techniques.

Detail transport properties of graphene/MoS2/metal vertical heterostructure have been investigated. The van der Waals interface between the graphene and MoS2 exhibits Schottky barrier. The application of gate voltage to the graphene layer enables us to modulate the Schottky barrier height; thus gives rise to the control of the current flow across the interface. By analyzing the temperature dependence of the conductance, the modulation of Schottky barrier height {\Delta}{\phi} has been directly determined. We observed significant MoS2 layer number dependence of {\Delta}{\phi}. Moreover, we demonstrate that the device which shows larger {\Delta}{\phi} exhibits larger current modulation; this is consistent with the fact that the transport of these devices is dominated by graphene/MoS2 Schottky barrier., Comment: Proceeding for 2014 International Conference on Solid State Devices and Materials (SSDM2014)

We demonstrate mid-infrared (MIR) photodetection by using a graphene nanoribbon (GNR) bolometer. Graphene is patterned into a GNR structure to open the transport gap (TG). Within the TG, the GNR exhibits Coulomb oscillation (CO), and the conductance peak in the CO exhibits electron temperature dependence. MIR photoirradiation induces electron heating of the GNR and thus enables us to detect the MIR light through the conductance change of the GNR.

We report on the fabrication and characterization of high quality graphene p–n–p junctions encapsulated by hexagonal boron nitride. By tuning the back gate and top gate bias voltages, a graphene p–n–p junction with tunable polarity and doping levels was realized. The p–n–p junction displayed distinct resistance oscillations, which was attributed to the Fabry–Perot interference of charge carriers in the p–n–p cavity. When a small magnetic field was applied, the oscillation phase was shifted by π, indicating the observation of Klein tunneling of charge carriers in the p–n–p junctions. The observation of Fabry–Perot interference and Klein tunneling with a macroscopic cavity length of L c = 500 nm demonstrates the markedly high quality of our graphene p–n–p junction.

We have fabricated parallel-coupled quantum dots on single-layer graphene. The tunnel coupling between the quantum dots can be tuned by a graphene in-plane gate. Owing to the tunnel coupling, the Coulomb blockade oscillation peaks exhibit periodic shifts as the number of electron in the non-conducting side-coupled QD is changed. The result suggests the observation of the single electron switching effect, which is a prerequisite for a single photon detection scheme using parallel-coupled quantum dots.

We report on the magnetotransport measurements of mono- and bi-layer graphene in pulsed high magnetic fields up to B = 53 T. In a mono-layer graphene, the Hall resistance RH is quantized to RH = (h/e2)ν−1 with ν = 2, 6, and 10 for either hole or electron doping. In a bi-layer graphene, RH is quantized to RH = (h/e2)ν−1 with ν = 4, 8, and 12. These results indicate that the precise magnetotransport measurements of the graphene can be conducted in the environment of pulse magnetic fields.

We investigated the electronic band structure and interlayer interactions in graphene synthesized by alcohol-chemical vapor deposition (a-CVD) using microprobe Raman spectroscopy and tight-binding band-structure calculations. The number of graphene layers was determined from the spectrally integrated intensity ratios of the G phonon to 2D phonon peaks. We found that the value of the parameter determining interlayer interactions in a-CVD bilayer graphene was less than half that of exfoliated bilayer graphene. The weak interlayer interaction in a-CVD bilayer graphene was attributed to non-AB stacking order.