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DOI： 10.1021/acsami.9b13763

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Language：English   Publishing type：Research paper (scientific journal)  

© 2019 The Japan Society of Applied Physics. Thicknesses of hexagonal boron nitride (hBN) on SiO2 were identified by the colors in the wide range from a few to hundreds of nanometers. We theoretically determined the colors of hBN on SiO2 as a function of the thickness in the standard red green blue (sRGB) color space. Theoretical RGB values of hBN with a given thickness were found to be in good agreement with those extracted from optical images, suggesting that the colors can be a standard for the thickness identification. This thickness identification method is facile, rapid, and versatile, and, therefore, would spur further researches on hBN.

DOI： 10.7567/1882-0786/ab0e45

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Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)  

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DOI： 10.1021/acs.jpcc.8b05857

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

The photogating effect is a photocurrent generation mechanism that leads to marked responsivity in two-dimensional (2D) semiconductor-based devices. A key step to promote the photogating effect in a 2D semiconductor is to integrate it with a high density of charge traps. Here, we show that self-limiting surface oxides on atomically thin WSe2 can serve as effective electron traps to facilitate p-type photogating. By examining the gate-bias-induced threshold voltage shift of a p-type transistor based on single-layer WSe2 with surface oxide, the electron trap density and the trap rate of the oxide are determined to be &gt
1012 cm-2 and &gt
1010 cm-2 s-1, respectively. White-light illumination on an oxide-covered 4-layer WSe2 transistor leads to the generation of photocurrent, the magnitude of which increases with the hole mobility. During illumination, the photocurrent evolves on a timescale of seconds, and a portion of the current persists even after illumination. These observations indicate that the photogenerated electrons are trapped deeply in the surface oxide and effectively gate the underlying WSe2. Owing to the pronounced photogating effect, the responsivity of the oxide-covered WSe2 transistor is observed to exceed 3000 A/W at an incident optical power of 1.1 nW, suggesting the effectiveness of surface oxidation in facilitating the photogating effect in 2D semiconductors.

DOI： 10.1063/1.5030525

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Characterization techniques available for bulk or thin-film solid-state materials have been extended to substrate-supported nanomaterials, but generally non-quantitatively. This is because the nanomaterial signals are inevitably buried in the signals from the underlying substrate in common reflection-configuration techniques. Here, we propose a virtual substrate method, inspired by the four-point probe technique for resistance measurement as well as the chop-nod method in infrared astronomy, to characterize nanomaterials without the influence of underlying substrate signals from four interrelated measurements. By implementing this method in secondary electron (SE) microscopy, a SE spectrum (white electrons) associated with the reflectivity difference between two different substrates can be tracked and controlled. The SE spectrum is used to quantitatively investigate the covering nanomaterial based on subtle changes in the transmission of the nanomaterial with high efficiency rivalling that of conventional core-level electrons. The virtual substrate method represents a benchmark for surface analysis to provide 'free-standing' information about supported nanomaterials.

DOI： 10.1038/ncomms15629

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

We have experimentally studied the dispersion of optical conductivity in few-layer graphene through reflection spectroscopy at visible wavelengths. A laser scanning microscope (LSM) with a supercontinuum laser source measured the frequency dependence of the reflectance of exfoliated graphene flakes, including monolayer, bilayer and trilayer graphene, loaded on a Si/SiO2 Fabry-Perot resonator in the 545-700 nm range. The complex refractive index of few-layer graphene, n - ik, was extracted from the reflectance contrast to the bare substrate. It was found that each few-layer graphene possesses a unique dispersionless optical index. This feature indicates that the optical conductivity does not simply scale with the number of layers, and that inter-layer electrodynamics are significant at visible energies.

DOI： 10.1038/srep34166

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The polarity of the charge carriers injected through Schottky junctions of a-phase molybdenum ditelluride (alpha-MoTe2) and various metals was characterized. We found that the Fermi-level pinning in the metal/alpha-MoTe2 Schottky junction is so weak that the polarity of the carriers (electron or hole) injected from the junction can be controlled by the work function of the metals, in contrast to other transition metal dichalcogenides such as MoS2. From the estimation of the Schottky barrier heights, we obtained p-type carrier (hole) injection from a Pt/alpha-MoTe2 junction with a Schottky barrier height of 40 meV at the valence band edge. n-Type carrier (electron) injection from Ti/alpha-MoTe2 and Ni/alpha-MoTe2 junctions was also observed with Schottky barrier heights of 50 and 100 meV, respectively, at the conduction band edge. In addition, enhanced ambipolarity was demonstrated in a Pt-Ti hybrid contact with a unique structure specially designed for polarity-reversible transistors, in which Pt and Ti electrodes were placed in parallel for injecting both electrons and holes.

DOI： 10.1021/acsami.6b02036

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Transition metal oxides show much promise as effective p-type contacts and dopants in electronics based on transition metal dichalcogenides. Here we report that atomically thin films of under-stoichiometric tungsten oxides (WOx with x < 3) grown on tungsten diselenide (WSe2) can be used as both controlled charge transfer dopants and low barrier contacts for p-type WSe2 transistors. Exposure of atomically thin WSe2 transistors to ozone (O-3) at 100 degrees C results in self-limiting oxidation of the WSe2 surfaces to conducting WOx films. WOx-covered WSe2 is highly hole-doped due to surface electron transfer from the underlying WSe2 to the high electron affinity WOx. The dopant concentration can be reduced by suppressing the electron affinity of WOx by air exposure, but exposure to O-3 at room temperature leads to the recovery of the electron affinity. Hence, surface transfer doping with WOx is virtually controllable. Transistors based on WSe2 covered with WOx show only p-type conductions with orders of magnitude better on-current, on/off current ratio, and carrier mobility than without WOx, suggesting that the surface WOx, serves as a p-type contact with a low hole Schottky barrier. Our findings point to a simple and effective strategy for creating p-type devices based on two-dimensional transition metal dichalcogenides with controlled dopant concentrations.

DOI： 10.1021/acs.nanolett.6b00390

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Low-frequency current fluctuations are monitored and the mechanism of electric noise investigated in layered 2H-type a-molybdenum ditelluride transistors. The charge transport mechanism of electric noise in atomically thin transitionmetal dichalcogenides is studied under different environments; the development of a new sensing functionality may be stimulated.

DOI： 10.1002/adma.201502677

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A doping-free transistor made of ambipolar cc-phase molybdenum ditelluride (alpha-MoTe2) is proposed in which the transistor polarity (p-type and n-type) is electrostatically controlled by dual top gates. The voltage signal in one of the gates determines the transistor polarity, while the other gate modulates the drain current. We demonstrate the transistor operation experimentally, with electrostatically controlled polarity of both p- and n-type in a single transistor.

DOI： 10.1021/acsnano.5b00736

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the second-order Raman process of mono- and few-layer MoTe2, by combining ab initio density functional perturbation calculations with experimental Raman spectroscopy using 532, 633, and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the resonance Raman process occurs at the M point in the Brillouin zone, where a strong optical absorption occurs due to a logarithmic Van Hove singularity of the electronic density of states. The double resonance Raman process with intervalley electron-phonon coupling connects two of the three inequivalent M points in the Brillouin zone, giving rise to second-order Raman peaks due to the M-point phonons. The calculated vibrational frequencies of the second-order Raman spectra agree with the observed laser-energy-dependent Raman shifts in the experiment.

DOI： 10.1103/PhysRevB.91.205415

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Growth of a uniform oxide film with a tunable thickness on two-dimensional transition Metal dichalcogenides is of great importance for electronic and optoelectronic applications. Here we demonstrate homogeneous surface oxidation of atomically thin WSe2 with a self-limiting thickness from single- to trilayers. Exposure to ozone (O-3) below 100 degrees C leads to the lateral growth of tungsten oxide selectively along selenium zigzag-edge orientations on WSe2. With further O-3 exposure, the oxide regions coalesce and oxidation terminates leaving a uniform thickness oxide film on top of unoxidized WSe2. At higher temperatures, oxidation evolves in the layer-by-layer regime up to trilayers. The oxide films formed on WSe2 are nearly atomically flat. Using photoluminescence and Raman spectroscopy, we find that the underlying single-layer WSe2 is decoupled from the top oxide but hole-doped. Our findings offer a new strategy for creating atomically thin heterostructures of semiconductors and insulating oxides with potential for applications in electronic devices.

DOI： 10.1021/nl5049753

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Understanding the interfacial electrical properties between metallic electrodes and low-dimensional semiconductors is essential for both fundamental science and practical applications. Here we report the observation of thickness reduction induced crossover of electrical contact at Au/MoS2 interfaces. For MoS2 thicker than 5 layers, the contact resistivity slightly decreases with reducing MoS2 thickness. By contrast, the contact resistivity sharply increases with reducing MoS2 thickness below 5 layers, mainly governed by the quantum confinement effect. We find that the interfacial potential barrier can be finely tailored from 0.3 to 0.6 eV by merely varying MoS2 thickness. A full evolution diagram of energy level alignment is also drawn to elucidate the thickness scaling effect. The finding of tailoring interfacial properties with channel thickness represents a useful approach controlling the metal/semiconductor interfaces which may result in conceptually innovative functionalities.

DOI： 10.1021/nn506138y

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We demonstrate charge and spin current transport in a graphene-based lateral spin valve using yttrium oxide (Y-O) as a tunneling barrier between graphene and a ferromagnetic electrode. A Y-O layer grown on graphene is flat, with a root-mean-square roughness of 0.17 nm, which is much lower than that of conventional barrier materials. This flatness allows the utilization of a very thin but well-defined tunneling barrier, leading to a large spin signal of similar to 20 Omega and a high spin injection efficiency of 15% with a low contact resistance of similar to 1 k Omega. These findings represent important progress toward the realization of graphene-based spintronics applications. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.085101

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

We report ambipolar charge transport in a-molybdenum ditelluride (MoTe2) flakes, whereby the temperature dependence of the electrical characteristics was systematically analyzed. The ambipolarity of the charge transport originated from the formation of Schottky barriers at the metal/MoTe2 contacts. The Schottky barrier heights as well as the current on/off ratio could be modified by modulating the electrostatic fields of the back-gate voltage (V-bg) and drain-source voltage (V-ds). Using these ambipolar MoTe2 transistors we fabricated complementary inverters and amplifiers, demonstrating their feasibility for future digital and analog circuit applications.

DOI： 10.1002/adma.201305845

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Two-dimensional layered crystals could show phonon properties that are markedly distinct from those of their bulk counterparts, because of the loss of periodicities along the c-axis directions. Here we investigate the phonon properties of bulk and atomically thin alpha-MoTe2 using Raman spectroscopy. The Raman spectrum of alpha-MoTe2 shows a prominent peak of the in-plane E-2g(1) mode, with its frequency upshifting with decreasing thickness down to the atomic scale, similar to other dichalcogenides. Furthermore, we find large enhancement of the Raman scattering from the out-of-plane B-2g(1) mode in the atomically thin layers. The B-2g(1) models Raman inactive in the bulk, but is observed to become active in the few-layer films. The intensity ratio of the B-2g(1) to E-2g(1) peaks evolves significantly with decreasing thickness, in contrast with other dichalcogenides. Our observations point to strong effects of dimensionality on the phonon properties of MoTe2.

DOI： 10.1021/nn5007607

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Exposure to oxygen at 300-340 degrees C results in triangular etch pits with uniform orientation on the surfaces of atomically thin molybdenum disulfide (MoS2), indicating anisotropic etching terminating on lattice planes. The triangular pits grow laterally with oxidation time. The density of pits scarcely depends on oxidation time, temperature, and MoS2 thickness but varies significantly from sample to sample, indicating that etching is initiated at native defect sites on the basal plane surface rather than activated by substrate effects such as charged impurities or surface roughness. Raman spectroscopy confirms that oxygen treatment produces no molybdenum oxide (MoO3) below 340 degrees C. However, upon oxidation above 200 degrees C, the Raman A(1g) mode upshifts and the linewidth decreases, indicating p-type doping of MoS2. Oxidation at 400 degrees C results in complete conversion to MoO3.

DOI： 10.1021/jp410893e

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Thin membranes exhibit complex responses to external forces or geometrical constraints. A familiar example is the wrinkling, exhibited by human skin, plant leaves, and fabrics, that results from the relative ease of bending versus stretching. Here, we study the wrinkling of graphene, the thinnest and stiffest known membrane, deposited on a silica substrate decorated with silica nanoparticles. At small nanoparticle density, monolayer graphene adheres to the substrate, detached only in small regions around the nanoparticles. With increasing nanoparticle density, we observe the formation of wrinkles which connect nanoparticles. Above a critical nanoparticle density, the wrinkles form a percolating network through the sample. As the graphene membrane is made thicker, global delamination from the substrate is observed. The observations can be well understood within a continuum-elastic model and have important implications for strain-engineering the electronic properties of graphene.

DOI： 10.1103/PhysRevX.2.041018

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Single-layer graphene (SLG) supported on SiO2 shows anomalously large chemical reactivity compared to thicker graphene, with charge inhomogeneity-induced potential fluctuations or topographic corrugations proposed as the cause. Here we systematically probe the oxidative reactivity of graphene supported on substrates with different surface roughnesses and charged impurity densities: hexagonal boron nitride (hBN), mica, thermally grown SiO2 on Si, and SiO2 nanoparticle thin films. SLG on low charge trap density hBN is not etched and shows little doping after oxygen treatment at temperatures up to 550 degrees C, in sharp contrast with oxidative etching under similar conditions of graphene on high charge trap density SiO2 and mica. Furthermore, bilayer graphene shows reduced reactivity compared to SLG regardless of its substrate-induced roughness. Together the observations indicate that graphene's reactivity is predominantly controlled by charge inhomogeneity-induced potential fluctuations rather than surface roughness.

DOI： 10.1021/nn303082a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BEILSTEIN-INSTITUT  

Key developments in NC-AFM have generally involved atomically flat crystalline surfaces. However, many surfaces of technological interest are not atomically flat. We discuss the experimental difficulties in obtaining high-resolution images of rough surfaces, with amorphous SiO2 as a specific case. We develop a quasi-1-D minimal model for noncontact atomic force microscopy, based on van der Waals interactions between a spherical tip and the surface, explicitly accounting for the corrugated substrate (modeled as a sinusoid). The model results show an attenuation of the topographic contours by similar to 30% for tip distances within 5 angstrom of the surface. Results also indicate a deviation from the Hamaker force law for a sphere interacting with a flat surface.

DOI： 10.3762/bjnano.3.26

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER SOC MECHANICAL ENGINEERS  

Graphene has attracted great interest due to its exceptional electrical, mechanical, and chemical properties since its discovery in 2004. Since its first realization, the substrate of choice for graphene exfoliation has been Si wafer with approximately 300 nm thick SiO2 dielectric layer, because it allows 1) direct optical detection of monolayer flakes, and 2) a convenient back gate with dielectric for controlling carrier density in the graphene. However; the amorphous structure of SiO2 and its associated surface roughness has led to ongoing controversy in determining the structure of SiO2-supported graphene. The conductivity of graphene allows scanning tunneling microscopy (STM) to be used to measure its topography, generally allowing its structure to be atomically resolved. In contrast, the insulating SiO2 must be probed with atomic force microscopy (AFM), and this is often done using ambient tapping-mode AFM. STM measurements of graphene on SiO2 generally show greater roughness and finer corrugation than is seen in AFM measurements of SiO2, and this has been interpreted as evidence for "intrinsic" corrugation of the graphene. However, when the energetics of adhesion and elasticity are considered, the idea of intrinsic structure becomes quite controversial for graphene supported on a substrate. Here we show that UHV non-contact AFM (NC-AFM) measurement of SiO2 reveals structure unresolved in previous measurements, and shows both greater roughness and smaller lateral feature size than seen for graphene measured by STM. High-resolution measurement of the SiO2 topography enables an analysis based on the energetics of graphene bending and adhesion, showing that the graphene structure is highly conformal to the SiO2 beneath it. The topographies reported here contrast the atomically-flat crystalline surfaces used in benchmark NC-AFM measurements. They pose unique challenges for measurement resolution, and highlight the very different physical mechanisms which determine resolution in STM vs. NC-AFM. We discuss these issues and our recent efforts at quantitative modeling of the imaging process, with particular focus on the role of van der Waals forces and their contribution to the image signal.

DOI： 10.1115/DETC2011-48737

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

High-resolution noncontact atomic force microscopy of SiO2 reveals previously unresolved roughness at the few-nm length scale, and scanning tunneling microscopy of graphene on SiO2 shows graphene to be slightly smoother than the supporting SiO2 substrate. A quantitative energetic analysis explains the observed roughness of graphene on SiO2 as extrinsic, and a natural result of highly conformal adhesion. Graphene conforms to the substrate down to the smallest features with nearly 99% fidelity, indicating conformal adhesion can be highly effective for strain engineering of graphene.

DOI： 10.1103/PhysRevLett.105.215504

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The decay behavior of nanoscale multilayer holes on SrTiO3(001) has been studied using scanning tunneling microscopy. The multilayer hole shrinks rigidly keeping steps in bunches, which is followed by rapid decay of a bottom single or a few layers in the hole at a critical volume. Both the critical volume and the number of layers exhibiting the rapid decay increase with depth of the hole. We have found that the anomalous morphological evolution during decay is induced as a result of the competition between the curvature effect and the short-range attractive step interaction. The attractive interaction gives rise to metastable step configurations of the bunching in the hole. Then the metastable state vanishes at the critical volume due to the enhanced curvature effect with decrease in size, which causes the debunching of the steps. The presented experimental results combined with a numerical simulation based on a step-flow model allowing for attractive step interactions have revealed that the depth of the step interaction potential well on SrTiO3(001) is approximately 4.6 +/- 2.0 meV.

DOI： 10.1103/PhysRevB.82.115436

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have studied the decay kinetics of nanoscale multilayer holes on SrTiO3(001) surfaces, using variable temperature scanning tunneling microscopy. We have performed real time observation of the decay of multilayer holes with diameters of 10 nm order at 750 degrees C. We have found that the hole decays, filling layer by layer from the bottom while expanding the periphery of the hole. We have performed numerical simulations of hole decay based oil a step flow model. The observed decay kinetics is found to be diffusion limited with local mass conservation. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.susc.2006.12.036

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