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Layer number-dependent band structures and symmetry are vital for the electrical and optical characteristics of two-dimensional (2D) transition metal dichalcogenides (TMDCs). Harvesting 2D TMDCs with tunable thickness and properties can be achieved through top-down etching and bottom-up growth strategies. In this study, we report a pioneering technique that utilizes the migration of in-situ generated Na-W-S-O droplets to etch out one-dimensional (1D) nanotrenches in few-layer WS$_2$. 1D WS$_2$ nanotrenches were successfully fabricated on the optically inert bilayer WS$_2$, showing pronounced photoluminescence and second harmonic generation signals. Additionally, we demonstrate the modulation of inkjet-printed Na$_2$WO$_4$-Na$_2$SO$_4$ particles to switch between the etching and growth modes by manipulating the sulfur supply. This versatile approach enables the creation of 1D nanochannels on 2D TMDCs. Our research presents exciting prospects for the top-down and bottom-up fabrication of 1D-2D mixed-dimensional TMDC nanostructures, expanding their use for photonic and optoelectronic applications., Comment: 37 pages, 16 figures

The capability to finely tailor material thickness with simultaneous atomic precision and non-invasivity would be useful for constructing quantum platforms and post-Moore microelectronics. However, it remains challenging to attain synchronized controls over tailoring selectivity and precision. Here we report a protocol that allows for non-invasive and atomically digital etching of van der Waals transition-metal dichalcogenides through selective alloying via low-temperature thermal diffusion and subsequent wet etching. The mechanism of selective alloying between sacrifice metal atoms and defective or pristine dichalcogenides is analyzed with high-resolution scanning transmission electron microscopy. Also, the non-invasive nature and atomic level precision of our etching technique are corroborated by consistent spectral, crystallographic and electrical characterization measurements. The low-temperature charge mobility of as-etched MoS$_2$ reaches up to $1200\,$cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$, comparable to that of exfoliated pristine counterparts. The entire protocol represents a highly precise and non-invasive tailoring route for material manipulation., Comment: 46 pages, 4 figures, with SI

The usage of molten salts, e.g., Na2MoO4 and Na2WO4, has shown great success in the growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). In comparison with the halide salt (i.e., NaCl, NaBr, KI)-assisted growth (Salt 1.0), the molten salt-assisted vapor-liquid-solid (VLS) growth technique (Salt 2.0) has improved the reproducibility, efficiency and scalability of synthesizing 2D TMDCs. However, the growth of large-area MoSe2 and WTe2 is still quite challenging with the use Salt 2.0 technique. In this study, a renewed Salt 2.0 technique using mixed salts (e.g., Na2MoO4-Na2SeO3 and Na2WO4-Na2TeO3) is developed for the enhanced CVD growth of 2D MoSe2 and WTe2 crystals with large grain size and yield. Continuous monolayer MoSe2 film with grain size of 100-250 {\mu}m or isolated flakes up to ~ 450 {\mu}m is grown on a halved 2-inch SiO2/Si wafer. Our study further confirms the synergistic effect of Na+ and SeO32- in the enhanced CVD growth of wafer-scale monolayer MoSe2 film. And thus, the addition of Na2SeO3 and Na2TeO3 into the transition metal salts could be a general strategy for the enhanced CVD growth of many other 2D selenides and tellurides., Comment: 35 pages, 11 figures and 1 table

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) with unique electrical properties are fascinating materials used for future electronics. However, the strong Fermi level pinning effect at the interface of TMDCs and metal electrodes always leads to high contact resistance, which seriously hinders their application in 2D electronics. One effective way to overcome this is to use metallic TMDCs or transferred metal electrodes as van der Waals (vdW) contacts. Alternatively, using highly conductive doped TMDCs will have a profound impact on the contact engineering of 2D electronics. Here, a novel chemical vapor deposition using mixed molten salts is established for vapor-liquid-solid growth of high-quality rhenium (Re) and vanadium (V)-doped TMDC monolayers with high controllability and reproducibility. A tunable semiconductor to metal transition is observed in the Re and V-doped TMDCs. Electrical conductivity increases up to a factor of 108 in the degenerate V-doped WS2 and WSe2. Using V-doped WSe2 as vdW contact, the on-state current and on/off ratio of WSe2-based field-effect transistors have been substantially improved (from ~10-8 to 10-5 A; ~104 to 108), compared to metal contacts. Future studies on lateral contacts and interconnects using doped TMDCs will pave the way for 2D integrated circuits and flexible electronics., Comment: 29 pages, 12 figures

Amorphous oxide semiconductors, especially indium oxide-based (InOx) thin-films, have been major candidates for high mobility with easy-to-use device processability. As one of the dopants in InOx semiconductors, we proposed Si to design a thin-film transistor (TFT) channel. Because the suppression of unstable oxygen vacancies in InOx is crucial to maintaining the semiconducting behavior, Si was selected as a strong oxygen binder that is reasonably available for large production. In this review, we focus on the overall properties observed in Si-incorporated amorphous InOx TFTs in terms of bond-dissociation energy, Gibbs free energy, Si-concentration dependence of TFT properties, carrier transport mechanism, and bias stress instability. In comparing low and high doping densities, we found that the activation energy and density of states decreased at a high Si concentration in InOx TFTs, implying that the trap density was reduced. As a result, stable operation under bias stresses could be realized. Furthermore, the inverse Meyer-Neldel rule was observed in the highly Si-doped InOx TFT, indicating reasonable ohmic contact. Based on our fundamental knowledge of the Si-doped InOx film, we developed a high-mobility bilayer TFT with a homogeneous stacked channel that was different from a TFT with an etch stop layer structure. The TFT showed remarkably stable operation. With simple element components based on InOx, it is possible to systematically discuss vacancy engineering in terms of conduction properties.

Vapor transportation is the core process in growing transition-metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). One inevitable problem is the spatial inhomogeneity of the vapors. The non-stoichiometric supply of transition-metal precursors and chalcogen leads to poor control in products' location, morphology, crystallinity, uniformity and batch to batch reproducibility. While vapor-liquid-solid (VLS) growth involves molten precursors at the growth temperatures higher than their melting points. The liquid sodium molybdate can precipitate solid MoS2 monolayers when saturated with sulfur vapor. Taking advantage of the VLS growth, we achieved three kinds of important achievements: (a) 4-inch-wafer-scale uniform growth of MoS2 flakes on SiO2/Si substrates, (b) 2-inch-wafer-scale growth of continuous MoS2 film with a grain size exceeding 100 um on sapphire substrates, and (c) pattern (site-controlled) growth of MoS2 flakes and film. We clarified that the VLS growth thus pave the new way for the high-efficient, scalable synthesis of two-dimensional TMDC monolayers., Comment: 30 pages, 10 figures

The thermoelectric voltage developed across an atomic metal junction (i.e., a nanostructure in which one or a few atoms connect two metal electrodes) in response to a temperature difference between the electrodes, results from the quantum interference of electrons that pass through the junction multiple times after being scattered by the surrounding defects. Here we report successfully tuning this quantum interference and thus controlling the magnitude and sign of the thermoelectric voltage by applying a mechanical force that deforms the junction. The observed switching of the thermoelectric voltage is reversible and can be cycled many times. Our ab initio and semi-empirical calculations elucidate the detailed mechanism by which the quantum interference is tuned. We show that the applied strain alters the quantum phases of electrons passing through the narrowest part of the junction and hence modifies the electronic quantum interference in the device. Tuning the quantum interference causes the energies of electronic transport resonances to shift, which affects the thermoelectric voltage. These experimental and theoretical studies reveal that Au atomic junctions can be made to exhibit both positive and negative thermoelectric voltages on demand, and demonstrate the importance and tunability of the quantum interference effect in the atomic-scale metal nanostructures., Comment: 25 pages, 5 figures

We investigate the noise in single layer graphene devices from equilibrium to far from equilibrium and found that the 1/f noise shows an anomalous dependence on the source-drain bias voltage (VSD). While the Hooge relation is not the case around the charge neutrality point, we found that it is recovered at very low VSD region. We propose that the depinning of the electron-hole puddles is induced at finite VSD, which may explain this anomalous noise behavior.

Adsorption sites of molecules critically determine the electric/photonic properties and its stability of heterogeneous molecule-metal interfaces. Then, selectivity of adsorption site is essential for development of the fields including organic electronics, catalysis, and biology. However, due to current technical limitations, site-selectivity remains a major challenge because of difficulty in precise selection of meaningful one among the sites. Here, we report the single site-selection at a single-molecule junction by performing newly developed hybrid technique: simultaneous characterization of surface enhanced Raman scattering (SERS) and current-voltage (I-V) measurements. The I-V response of 1,4-benzenedithiol junctions, reveals the existence of three meta-stable states arising from different adsorption sites. Notably, correlated SERS measurements show selectivity towards one of the adsorption sites, bridge sites. This site-selectivity represents an essential step towards the reliable integration of individual molecules on metallic surfaces. Furthermore, the hybrid spectro-electric technique reveals the dependence of the SERS intensity on the strength of the molecule-metal interaction, showing the interdependence between the optical and electronic properties in single-molecule junctions., Comment: 4 figures, J. Am. Chem. Soc., in press

Atomically thin two-dimensional chalcogenides such as MoS2 monolayers are structurally ideal channel materials for the ultimate atomic electronics. However, a heavy thickness dependence of electrical performance is shown in these ultrathin materials, and the device performance normally degrades while exhibiting a low carrier mobility as compared with corresponding bulks, constituting a main hurdle for application in electronics. In this brief review, we summarize our recent work on electrode/channel contacts and carrier scattering mechanisms to address the origins of this adverse thickness dependence. Extrinsically, the Schottky barrier height increases at the electrode/channel contact area in thin channels owing to bandgap expansion caused by quantum confinement, which hinders carrier injection and degrades device performance. Intrinsically, thin channels tend to suffer from intensified Coulomb impurity scattering, resulting from the reduced interaction distance between interfacial impurities and channel carriers. Both factors are responsible for the adverse dependence of carrier mobility on channel thickness in two-dimensional semiconductors., Comment: 18 pages, 8 figures. A brief review for the JPSJ SPECIAL TOPICS "Recent Progress in Science of Atomic Layers"

We discuss the environmental instability of amorphous indium oxide (InOx)-based thin-film transistors (TFTs) in terms of the excess oxygen in the semiconductor films. A comparison between amorphous InOx doped with low and high concentrations of oxygen binder (SiO2) showed that out-diffusion of oxygen molecules causes drastic changes in the film conductivity and TFT turn-on voltages. Incorporation of sufficient SiO2 could suppress fluctuations in excess oxygen because of the high oxygen bond-dissociation energy and low Gibbs free energy. Consequently, the TFT operation became rather stable. The results would be useful for the design of reliable oxide TFTs with stable electrical properties., Comment: 22 pages, 5 figures, Applied Physics Letters, in press

We study the second-order Raman process of mono- and few-layer MoTe$_2$, by combining {\em 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 electron-photon resonance process occurs at the high-symmetry M point in the Brillouin zone, where a strong optical absorption occurs by a logarithmic Van-Hove singularity. Double resonance Raman scattering with inter-valley 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 predicted frequencies of the second-order Raman peaks agree with the observed peak positions that cannot be assigned in terms of a first-order process. Our study attempts to supply a basic understanding of the second-order Raman process occurring in transition metal di-chalcogenides (TMDs) and may provide additional information both on the lattice dynamics and optical processes especially for TMDs with small energy band gaps such as MoTe$_2$ or at high laser excitation energy., Comment: 10 pages, 5 figures

Amorphous metal oxide thin-film transistors (TFT) are fabricated using InOx-based semiconductors doped with TiO2, WO3 or SiO2. Although density of dopant is low in the film, change in the electrical properties showed strong dependence on the dopant species. We found that the dependence could be reasonably explained by the bond-dissociation energy. By incorporating the dopant with higher bond-dissociation energy, the film becomes less sensitive to oxygen partial pressure used during sputtering deposition and remains electrically stable to thermal annealing treatment. The concept of bond-dissociation energy can contribute to the realization of more stable metal oxide TFTs for flat panel displays., Comment: 19 pages, 4 figures, Applied Physics Letters, in press

Two-dimensional semiconductors are structurally ideal channel materials for the ultimate atomic electronics after silicon era. A long-standing puzzle is the low carrier mobility ({\mu}) in them as compared with corresponding bulk structures, which constitutes the main hurdle for realizing high-performance devices. To address this issue, we perform combined experimental and theoretical study on atomically thin MoS2 field effect transistors with varying the number of MoS2 layers (NLs). Experimentally, an intimate relation is observed with a 10-fold degradation in {\mu} for extremely thinned monolayer channels. To accurately describe the carrier scattering process and shed light on the origin of the thinning-induced mobility degradation, a generalized Coulomb scattering model is developed with strictly considering device configurative conditions, i.e., asymmetric dielectric environments and lopsided carrier distribution. We reveal that the carrier scattering from interfacial Coulomb impurities (e.g., chemical residues, gaseous adsorbates and surface dangling bonds) is greatly intensified in extremely thinned channels, resulting from shortened interaction distance between impurities and carriers. Such a pronounced factor may surpass lattice phonons and serve as dominant scatterers. This understanding offers new insight into the thickness induced scattering intensity, highlights the critical role of surface quality in electrical transport and would lead to rational performance improvement strategies for future atomic electronics., Comment: With Supporting Information

We demonstrate the possibility in quantifying the Raman intensities for both specimen and substrate layers in a common stacked experimental configuration and, consequently, propose a general and rapid thickness identification technique for atomic-scale layers on dielectric substrates. Unprecedentedly wide-range Raman data for atomically flat MoS2 flakes are collected to compare with theoretical models. We reveal that all intensity features can be accurately captured when including optical interference effect. Surprisingly, we find that even freely suspended chalcogenide few-layer flakes have a stronger Raman response than that from the bulk phase. Importantly, despite the oscillating intensity of specimen spectrum versus thickness, the substrate weighted spectral intensity becomes monotonic. Combined with its sensitivity to specimen thickness, we suggest this quantity can be used to rapidly determine the accurate thickness for atomic layers.

Unipolar transport is demonstrated in a bilayer graphene with a series of p-n junctions and is controlled by electrostatic biasing by a comb-shaped top gate. The OFF state is induced by multiple barriers in the p-n junctions, where the band gap is generated by applying a perpendicular electric field to the bilayer graphene, and the ON state is induced by the p-p or n-n configurations of the junctions. As the number of the junction increases, current suppression in the OFF state is pronounced. The multiple p-n junctions also realize the saturation of the drain current under relatively high source-drain voltages., Comment: 18 pages, 4 figures, Applied Physics Letters, in printing

The concept of a novel graphene P-I-N junction switching device with a nanoribbon is proposed, and its basic operation is demonstrated in an experiment. The concept aims to optimize the operation scheme for graphene transistors toward a superior on-off property. The device has two bulk graphene regions where the carrier type is electrostatically controlled by a top gate, and these two regions are separated by a nanoribbon which works as an insulator. As a result, the device forms a (P or N)-I-(P or N) junction structure. The off state is obtained by lifting the band of the bulk graphene of the source (drain) side and lowering that of the drain (source) side, so that the device forms a P-I-N (N-I-P) junction. In this configuration, the leakage current can be reduced more effectively than the conventional single gate transistors with the same band gap size due to a high barrier height and a long tunneling length in the nanoribbon. The on state is obtained by flipping the polarity of the bias of either top gate to form a P-I-P or N-I-N junction. An experiment showed that the drain current was suppressed in the cases of P-I-N and N-I-P compared to the cases of P-I-P and N-I-N, and all of the behaviors were consistent with what was expected from the device operation model., Comment: 9 pages, 4 figures. To be published in Applied Physics Express

We demonstrate a tunneling and rectification behavior in bilayer graphene. A stepped dielectric top gate creates a spatially modulated electric field, which opens the band gap in the graphene and produces an insulating region at the p-n interface. A current-voltage relationship exhibiting differential resistance peak at forward bias stems from the tunneling current through the insulating region at the p-n interface. The tunneling current reflects singularities in the density of states modified by the electric field. This work suggests that the effect of carrier charge tuning by external electric field in 2D semiconductors is analogously to that by impurity doping in 3D semiconductors., Comment: 18 pages, 7 figures, to be published in Journal of the Physical Society of Japan

A method for inducing nonuniform strain in graphene films is developed. Pillars made of a dielectric material (electron beam resist) are placed between graphene and the substrate, and graphene sections between pillars are attached to the substrate. The strength and spatial pattern of the strain can be controlled by the size and separation of the pillars. Application of strain is confirmed by Raman spectroscopy as well as from scanning electron microscopy (SEM) images. From SEM images, the maximum stretch of the graphene film reaches about 20%. This technique can be applied to the formation of band gaps in graphene., Comment: Appl. Phys. Express, in press

Realization of logic circuits from graphene is very attractive for high-speed nanoelectronics. However, the intrinsic ambipolar nature hinders the formation of graphene logic devices with the conventional complementary architecture. Using electrostatic doping modulation, we show here a facile method to control the charge neutrality points and form a complementary-like structure, in which the ambipolar conduction is used as a benefit rather than a drawback to construct logic devices. A band gap is also introduced in the channels to improve the switching ratio of the graphene transistors. For the first time, complementary-like NOR and NAND logic gates were demonstrated. This method provides a possible route for logic circuits from ambipolar graphene and, in principle, can be also extended to other ambipolar semiconductors, such as organic compounds and carbon nanotube thin films., Comment: 16 pages, 7 figures

Realization of logic circuits in graphene with an energy gap (EG) remains one of the main challenges for graphene electronics. We found that large transport EGs (>100 meV) can be fulfilled in dual-gated bilayer graphene underneath a simple alumina passivation top gate stack, which directly contacts the graphene channels without an inserted buffer layer. With the presence of EGs, the electrical properties of the graphene transistors are significantly enhanced, as manifested by enhanced on/off current ratio, subthreshold slope and current saturation. For the first time, complementary-like semiconducting logic graphene inverters are demonstrated that show a large improvement over their metallic counterparts. This result may open the way for logic applications of gap-engineered graphene., Comment: Accepted by ACS Nano

Electron transport in bilayer graphene placed under a perpendicular electric field is revealed experimentally. Steep increase of the resistance is observed under high electric field; however, the resistance does not diverge even at low temperatures. The observed temperature dependence of the conductance consists of two contributions: the thermally activated (TA) conduction and the variable range hopping (VRH) conduction. We find that for the measured electric field range (0 - 1.3 V/nm) the mobility gap extracted from the TA behavior agrees well with the theoretical prediction for the band gap opening in bilayer graphene, although the VRH conduction deteriorates the insulating state more seriously in bilayer graphene with smaller mobility. These results show that the improvement of the mobility is crucial for the successful operation of the bilayer graphene field effect transistor.

Using a scanning electron microscope, we observed a reproducible, discrete distribution of secondary electron intensity stemming from an atomically thick graphene film on a thick insulating substrate. The discrete distribution made it possible to uniquely relate the secondary electron intensity to the number of graphene layers. Furthermore, we found a distinct linear relationship between the relative secondary electron intensity from graphene and the number of layers, provided a low primary electron acceleration voltage was used. Based on these observations, we propose a practical method to determine the number of graphene layers in a sample. This method is superior to the conventional optical method in its capability to characterize graphene samples with sub-micrometer squares in area on various insulating substrates., Comment: 15 pages, 4 figures

We report the influence of the field effect on the dc resistance and Hall coefficient in the strain-induced Mott insulating state of an organic superconductor $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br. Conductivity obeys the formula for activated transport $\sigma_{\Box} = \sigma_{0}\exp(-W/k_{B}T)$, where $\sigma_{0}$ is a constant and $W$ depends on the gate voltage. The gate voltage dependence of the Hall coefficient shows that, unlike in conventional FETs, the effective mobility of dense hole carriers ($\sim1.6\times 10^{14}$ cm$^{-2}$) is enhanced by a positive gate voltage. This implies that carrier doping involves delocalization of intrinsic carriers that were initially localized due to electron correlation., Comment: 5 pages, 3 figures

Electric conduction in thin graphite film was tuned by two gate electrodes to clarify how the gate electric field induces electric carriers in thin graphite. The graphite was sandwiched between two gate electrodes arranged in a top and bottom gate configuration. A scan of the top gate voltage generates a resistance peak in ambiploar response. The ambipolar peak is shifted by the bottom gate voltage, where the shift rate depends on the graphite thickness. The thickness-dependent peak shift was clarified in terms of the inter-layer screening length to the electric field in the double-gated graphite film. The screening length of 1.2 nm was experimentally obtained., Comment: 5 pages, 4 figures. To be published in Applied Physics Express