Previous high-pressure studies about WTe2 have reported divergent critical pressures for structural and electronic phase transitions, obscuring a comprehensive understanding about the complex quantum phases. In this work, we precisely assign the structural phase evolutions and relevant electronic changes of type-II Weyl semimetals, WTe2 and MoTe2, using various optical methods. We confirm that the Td to 1T′ structural phase transition occurs at about 2.5 GPa concomitantly with the previously reported decrease of magnetoresistance and emergence of superconductivity. Notably, electron-phonon coupling remains intact despite pressure-dependent variations in structural symmetry and atomic bond strengths, providing crucial insights into the origin of superconductivity in transition metal ditellurides. We also demonstrate an additional structural transition at about 10 GPa, possibly to a triclinic 1T″ structure, which has a significant influence on the electronic structure due to intra-layer distortion of atomic positions. Our findings on distinct evolutions of inter-layer and intra-layer structural parameters offer a generic understanding of the relationship between anisotropic bond strength in the van der Waals materials and pressure-dependent structural changes. We assign the high pressure structural phase of transition metal ditelluride (MTe2, M = W or Mo), one of the most extensively investigated material groups. From the accurate assignment of structural phases, we offer comprehensive understanding of the electronic and topological phase evolutions. We can also clearly demonstrate anisotropic pressure-dependent structural evolutions for transition metal ditellurides, which can be generic behaviors of two-dimensional van der Waals materials under pressure.

Interfaces with surrounding materials, where charged impurities and surface roughness are present, have a significant impact on the electrical and optical properties of 2D materials. In the change of the phonon modes of MoS 2 accompanied by thickness variation, the portion caused by intrinsic factors and the portion caused by the interface effect are separated by examining the result of encapsulation with hexagonal boron nitride (hBN). For instance, the frequency of the A 1g peak of MoS 2 supported by SiO 2 decreases by ≈4 cm −1 in air for a thickness reduction from ten layers to monolayer. Of this decrease, roughly 2 cm −1 is attributable to the weakening of the van der Waals interlayer interaction, while the remaining 2 cm −1 is due to the interface effect. The interface state, that is, the types and concentrations of impurities at the interface, between MoS 2 and SiO 2 is estimated to be similar to that between MoS 2 and air because the Raman properties when one surface of MoS 2 is in contact with SiO 2 and with air are identical within the measurement error. When entirely encapsulated with hBN, the width of the A 1g peak of few‐layer MoS 2 is significantly reduced, becoming comparable or equal to that of bulk MoS 2 .

We investigate magnetic properties of graphene oxide flake that have been locally oxidized using atomic force microscope (AFM) lithography. This approach reduces the possibility of magnetic contamination. Our Raman spectroscopy analysis reveals that the graphene oxide contains crystalline defects or disorders and differs from the pristine graphene in terms of its atomic structure. Using magnetic force microscopy measurements, we observe that the graphene oxide has a net magnetization pointing out of the surface plane. Furthermore, our magneto-optical Kerr effect data show small but clear hysteresis loops with non-zero remanent magnetization. We also conduct x-ray magnetic circular dichroism (XMCD) photoemission electron microscope measurements and identify remarkable asymmetry in carbon K edge spectra, which strongly suggests that the observed ferromagnetic order in the graphene oxide layer is intrinsic. A careful analysis of XMCD signals depending on the oxidized condition reveals the effects of chemical states of carbon atoms on the formation of ferromagnetic order in the graphene oxide.

Weak interlayer couplings at 2D van der Waals (vdW) interfaces fundamentally distinguish out‐of‐plane charge flow, the information carrier in vdW‐assembled vertical electronic and optical devices, from the in‐plane band transport processes. Here, the out‐of‐plane charge transport behavior in 2D vdW semiconducting transition metal dichalcogenides (SCTMD) is reported. The measurements demonstrate that, in the high electric field regime, especially at low temperatures, either electron or hole carrier Fowler–Nordheim (FN) tunneling becomes the dominant quantum transport process in ultrathin SCTMDs, down to monolayers. For few‐layer SCTMDs, sequential layer‐by‐layer FN tunneling is observed to dominate the charge flow, thus serving as a material characterization probe for addressing the Fermi level positions and the layer numbers of the SCTMD films. Furthermore, it is shown that the physical confinement of the electron or hole carrier wave packets inside the sub‐nm thick semiconducting layers reduces the vertical quantum tunneling probability, leading to an enhanced effective mass of tunneling carriers.

Van der Waals heterostructures with two-dimensional magnets offer a magnetic junction with an atomically sharp and clean interface. This attribute ensures that the magnetic layers maintain their intrinsic spin-polarized electronic states and spin-flipping scattering processes at a minimum level, a trait that can expand spintronic device functionalities. Here, using a van der Waals assembly of ferromagnetic Fe3GeTe2 with non-magnetic hexagonal boron nitride and WSe2 layers, we demonstrate electrically tunable, highly transparent spin injection and detection across the van der Waals interfaces. By varying an electrical bias, the net spin polarization of the injected carriers can be modulated and reversed in polarity, which leads to sign changes of the tunnelling magnetoresistance. We attribute the spin polarization reversals to sizable contributions from high-energy localized spin states in the metallic ferromagnet, so far inaccessible in conventional magnetic junctions. Such tunability of the spin-valve operations opens a promising route for the electronic control of next-generation low-dimensional spintronic device applications.

Electron–phonon scatterings in solid-state systems are pivotal processes in determining many key physical quantities such as charge carrier mobilities and thermal conductivities. Here, we report direct probing of phonon mode specific electron–phonon scatterings in layered semiconducting transition metal dichalcogenides WSe 2 , MoSe 2 , WS 2 , and MoS 2 through inelastic electron tunneling spectroscopy measurements, quantum transport simulations, and density functional calculation. We experimentally and theoretically characterize momentum-conserving single- and two-phonon electron–phonon scatterings involving up to as many as eight individual phonon modes in mono- and bilayer films, among which transverse, longitudinal acoustic and optical, and flexural optical phonons play significant roles in quantum charge flows. Moreover, the layer-number sensitive higher-order inelastic electron–phonon scatterings, which are confirmed to be generic in all four semiconducting layers, can be attributed to differing electronic structures, symmetry, and quantum interference effects during the scattering processes in the ultrathin semiconducting films.

Temperature-independent magnetoresistance (TIMR) has been studied for applications in magnetic field sensors operating in wide temperature ranges. Graphene is considered as one of the best candidates for achieving nonsaturating and large TIMR through engineering disorders. Nevertheless, large TIMR has not been achieved in disordered graphene with intrinsic defects, such as chemical doping and atomic dislocations. In this work, by introducing extrinsic defects, we realize nonsaturating and large TIMR in monolayer graphene transferred on a BiFeO3 nanoisland array (G/BFO-NIA). Furthermore, the G/BFO-NIA device exhibits a significantly larger MR (∼250% under 9 T) than other materials without gating operation, demonstrating its application feasibility. It is shown that the large MR is a result of the coexistence of electrons and holes with almost the same density, and the observed TIMR originates from the temperature dependence of carrier transport in graphene and of the dielectric property of BFO-NIA.

The effects of Fe content on the thermal, magnetic, and mechanical properties of a Co72B19.2Si4.8Mo4 alloy were studied. Amorphous soft magnetic alloys of (Co1−xFex)72 B19.2Si4.8Mo4 (0 ≤ x ≤ 1) with 3-mm widths and 27 - 30 μm thicknesses were obtained by using the melt spinning technique. The crystallization temperature (Tx) for each alloy was measured by using differential scanning calorimetry (DSC). All ribbons were identified as fully amorphous by using X-ray diffraction (XRD). The magnetic properties of the amorphous ribbons were measured by using a vibrating sample magnetometer (VSM). The Fe-only ribbon (x = 1) showed the highest saturation magnetization of 1.15 T in these alloys. Also, the nano-hardness (Hnanoindentation), and Young’s modulus (E) were evaluated by using nano-indentation experiments, and Co21.6Fe50.4B19.2Si4.8Mo4 (x = 0.7) show a nano-hardness (Hnanoindentation) of 18.92 GPa.

Highly nonlinear bistable current-voltage ( I–V ) characteristics are necessary in order to realize high density resistive random access memory (ReRAM) devices that are compatible with cross-point stack structures. Up to now, such I–V characteristics have been achieved by introducing complex device structures consisting of selection elements (selectors) and memory elements which are connected in series. In this study, we report bipolar resistive switching (RS) behaviours of nano-crystalline BiFeO 3 (BFO) nano-islands grown on Nb-doped SrTiO 3 substrates, with large ON/OFF ratio of 4,420. In addition, the BFO nano-islands exhibit asymmetric I–V characteristics with high nonlinearity factor of 1,100 in a low resistance state. Such selector-free RS behaviours are enabled by the mosaic structures and pinned downward ferroelectric polarization in the BFO nano-islands. The high resistance ratio and nonlinearity factor suggest that our BFO nano-islands can be extended to an N × N array of N = 3,740 corresponding to ~10 7 bits. Therefore, our BFO nano-island showing both high resistance ratio and nonlinearity factor offers a simple and promising building block of high density ReRAM.

Recently, various electronic components including transistor, barristor, memory, and transparent electrode were implemented using graphene. While integrated circuits were demonstrated by combining graphene transistors and passive components, system on chip (SoC) platform, state-of-the-art semiconductor technology, by combining transistors and memories on the same chip, has not yet been demonstrated. The main obstacle of the realization of SoC is the complexity of fabrication processes originated from the process differences between the transistors and the memories. In this study, using simple and clean atomic force microscope lithography, we fabricated both the switching devices and the memories by forming very thin graphene oxide (GO) barriers in mono-layer graphene at the controlled oxidation voltages. Formed with 7 V and 9 V, the lateral graphene/GO/graphene junction devices exhibit switching of Fowler-Nordheim tunneling current and resistive memory behavior, respectively. The combination of high on/off current ratio (similar to 1000) of the switching device and nonvolatility of the memory device fabricated by the same process demonstrates the possibility of graphene SoC platform. (C) 2015 Elsevier Ltd. All rights reserved.

High-quality channel layer is required for next-generation flexible electronic devices. Graphene is a good candidate due to its high carrier mobility and unique ambipolar transport characteristics but typically shows a low on/off ratio caused by gapless band structure. Popularly investigated organic semiconductors, such as pentacene, suffer from poor carrier mobility. Here, we propose a graphene/pentacene channel layer with high-k ion-gel gate dielectric. The graphene/pentacene device shows both high on/off ratio and carrier mobility as well as excellent mechanical flexibility. Most importantly, it reveals ambipolar behaviors and related negative differential resistance, which are controlled by external bias. Therefore, our graphene/pentacene barristor with ion-gel gate dielectric can offer various flexible device applications with high performances.

Resistive random access memory (ReRAM) devices have been extensively investigated resulting in significant enhancement of switching properties. However fluctuations in switching parameters are still critical weak points which cause serious failures during ‘reading’ and ‘writing’ operations of ReRAM devices. It is believed that such fluctuations may be originated by random creation and rupture of conducting filaments inside ReRAM oxides. Here, we introduce defective monolayer graphene between an oxide film and an electrode to induce confined current path distribution inside the oxide film and thus control the creation and rupture of conducting filaments. The ReRAM device with an atomically thin interlayer of defective monolayer graphene reveals much reduced fluctuations in switching parameters compared to a conventional one. Our results demonstrate that defective monolayer graphene paves the way to reliable ReRAM devices operating under confined current path distribution.

A precise measurement of graphene thickness is required for the design and development of nano-devices based on the material. Many factors affect this measurement when using scanning tunneling microscope (STM) and atomic force microscope (AFM), including the interaction between the scanning tip and ripples on graphene; such effects have not previously been explored. To investigate this, we measure the sample rotation angle dependence of graphene thickness as determined by contact mode and tapping mode AFM. The graphene thickness as determined by contact mode AFM follows a cosine modulus function of sample rotation angle, while tapping mode AFM reveals a constant graphene thickness, independent of sample rotation angle. For comparison, the AFM torsion signal is measured and follows a sine function of the sample rotation angle. All the measured sample rotation angle dependences can be explained by the interaction between linearly aligned ripples on graphene and the AFM tip in contact with the graphene.

Recently, resistance random access memory (ReRAM) has attracted much attention in transparent electronics devices due to its simple structure and stable memory switching characteristics. This switching characteristics of transition metal oxide (TMO) thin films demonstrate high potential for applications in high operation speed and high-density of next-generation nonvolatile memory devices. It is also generally accepted that indium tin oxide (ITO) thin film is one of strong candidates for transparent conductive oxide (TCO) and n-type semiconductor oxide electronics. In this study, we fabricated NiO devices which show the bipolar resistive switching behaviors. By using the ITO as top and bottom electrodes, transparent and flexible device of ITO/NiO/ITO structure was prepared. The migration of oxygen ions in the thin film can be controlled for the negative bias applied to the top electrode, which is testified by transmission electron microscopy and Auger electron microscopy. The resistive switching properties according to the distribution of oxygen ions at the NiO interfaces are analyzed.

The graphene/SiO2 system is a promising building block for next-generation electronic devices, integrating the high electromagnetic performance of graphene with the mature technology of Si-based electronic devices. It is well known that the electromagnetic performance of graphene/SiO2 is dramatically reduced by structural defects, such as wrinkles and folding, which are suspected to result from water droplets. Therefore, understanding water diffusion between graphene and SiO2 is required for controlling structural defects and thus improving the electromagnetic performance of this system. Although the behavior of water between graphene and atomically flat mica has been investigated, the characteristics and effects of diffused water between graphene and SiO2 remain unidentified. We have investigated water diffusion between monolayer graphene and SiO2 under high humidity conditions using atomic force microscopy. For a relative humidity of over 90%, water diffuses into graphene/SiO2 and forms an ice-like structure up to two layers thick. Liquid-like water can further diffuse in, stacking over the ice-like layer and evaporating relatively easily in the air causing graphene to wrinkle and fold. By similarly investigating water diffusion between graphene and mica, we argue that water-induced wrinkle formation depends on the hydrophilicity and roughness of the substrate.

Embryonic stem (ES) cells can undergo continual proliferation and differentiation into cells of all somatic cell lineages in vitro; they are an unlimited cell source for regenerative medicine. However, techniques for maintaining undifferentiated ES cells are often inefficient and result in heterogeneous cell populations. Here, we determined effects of nanopattern polydimethylsiloxane (PDMS) as a culture substrate in promoting the self-renewal of mouse ES (mES) cells, compared to commercial plastic culture dishes. After many passages, mES cells efficiently maintained their undifferentiated state on nanopattern PDMS, but randomly differentiated on commercial plastic culture dishes, as indicated by partially altered morphologies and decreases in alkaline phosphatase activity and stage-specific expression of embryonic antigen-1. Under nanopattern PDMS conditions, we found increased activities of STAT3 and Akt, important proteins involved in maintaining the self-renewal of mES cells. The substrate-cell interactions also enhanced leukemia inhibitory factor (LIF)-downstream signaling and inhibited spontaneous differentiation, concomitant with reduced focal adhesion kinase (FAK) signaling. This reduction in FAK signaling was shown to be important for promoting mES cell self-renewal. Thus, our data demonstrates that nanopattern PDMS contributes to maintaining the self-renewal of mES cells and may be applicable in the large-scale production of homogeneously undifferentiated mES cells.

Ripples in graphene monolayers deposited on SiO2/Si wafer substrates were recently shown to give rise to friction anisotropy. High friction appears when the AFM tip slides in a direction perpendicular to the ripple crests and low friction when parallel. The direction of the ripple crest is, however, hard to determine as it is not visible in topographic images and requires elaborate measurements of friction as a function of angle. Here we report a simple method to characterize ripple crests by measuring the cantilever torsion signal while scanning in the non-conventional longitudinal direction (i.e., along the cantilever axis, as opposed to the usual friction measurement). The longitudinal torsion signal provides a much clearer ripple domain contrast than the conventional friction signal, while both signals show respective rotation angle dependences that can be explained using the torsion component of the normal reaction force exerted by the graphene ripples. We can also determine the ripple direction by comparing the contrast in torsion images obtained in longitudinal and lateral scans without sample rotation or complicated normalization.

Monolayer graphene is one of the most interesting materials applicable to next-generation electronic devices due to Its transport properties. However, realization of graphene devices requires suitable nanoscale lithography as well as a method:to open a band gap in monolayer graphene. Nanoscale hydrogenation and oxidation are promising methods to open an energy band, gap by modification of surface structures and to fabricate nanostructures such as graphene nanoribbons (GNRs). Until now It has been difficult to fabricate nanoscale devices' consisting of both hydrogenated and oxidized graphene because the hydrogenation of graphene requires a complicated process composed of large-scale chemical modification, nanoscale patterning, and etching. We report on nanoscale hydrogenation and oxidation of graphene under normal atmospheric conditions and at room temperature without etching, wet process, or even any gas treatment by controlling just an external bias through atomic force Microscope lithography. Both the lithographically defined nanoscale hydrogenation and oxidation have been confirmed by micro-Raman spectroscopy measurements. Patterned hydrogenated. and oxidized graphene show insulating behaviors, and their friction values are several times larger than those of graphene. These differences can be used for fabricating electronic or electromechanical devices based on graphene.

Graphene produced by exfoliation has not been able to provide an ideal graphene with performance comparable to that predicted by theory, and structural and/or electronic defects have been proposed as one cause of reduced performance. We report the observation of domains on exfoliated monolayer graphene that differ by their friction characteristics, as measured by friction force microscopy. Angle-dependent scanning revealed friction anisotropy with a periodicity of 180° on each friction domain. The friction anisotropy decreased as the applied load increased. We propose that the domains arise from ripple distortions that give rise to anisotropic friction in each domain as a result of the anisotropic puckering of the graphene.