Pseudo-Hydrodynamic Flow of Quasiparticles in Semimetal WTe2 at Room Temperature.

Recently, much interest has emerged in fluid-like electric charge transport in various solid-state systems. The hydrodynamic behavior of the electronic fluid reveals itself as a decrease of the electrical resistance with increasing temperature (the Gurzhi effect) in narrow channels, polynomial scaling of the resistance as a function of the channel width, violation of the Wiedemann-Franz law supported by the emergence of the Poiseuille flow. Similar to whirlpools in flowing water, the viscous electronic flow generates vortices, resulting in abnormal sign-changing electrical response driven by backflow. However, the question of whether the long-ranged sign-changing electrical response can be produced by a mechanism other than hydrodynamics has not been addressed so far. Here polarization-sensitive laser microscopy is used to demonstrate the emergence of visually similar abnormal sign-alternating patterns in semi-metallic tungsten ditelluride at room temperature where this material does not exhibit true hydrodynamics. It is found that the neutral quasiparticle current consisting of electrons and holes obeys an equation remarkably similar to the Navier-Stokes equation. In particular, the momentum relaxation is replaced by the much slower process of quasiparticle recombination. This pseudo-hydrodynamic flow of quasiparticles leads to a sign-changing charge accumulation pattern via different diffusivities of electrons and holes.

Super-Resolution Nanolithography of Two-Dimensional Materials by Anisotropic Etching.

Nanostructuring allows altering of the electronic and photonic properties of two-dimensional (2D) materials. The efficiency, flexibility, and convenience of top-down lithography processes are, however, compromised by nanometer-scale edge roughness and resolution variability issues, which especially affect the performance of 2D materials. Here, we study how dry anisotropic etching of multilayer 2D materials with sulfur hexafluoride (SF6) may overcome some of these issues, showing results for hexagonal boron nitride (hBN), tungsten disulfide (WS2), tungsten diselenide (WSe2), molybdenum disulfide (MoS2), and molybdenum ditelluride (MoTe2). Scanning electron microscopy and transmission electron microscopy reveal that etching leads to anisotropic hexagonal features in the studied transition metal dichalcogenides, with the relative degree of anisotropy ranked as: WS2 > WSe2 > MoTe2 ∼ MoS2. Etched holes are terminated by zigzag edges while etched dots (protrusions) are terminated by armchair edges. This can be explained by Wulff constructions, taking the relative stabilities of the edges and the AA' stacking order into account. Patterns in WS2 are transferred to an underlying graphite layer, demonstrating a possible use for creating sub-10 nm features. In contrast, multilayer hBN exhibits no lateral anisotropy but shows consistent vertical etch angles, independent of crystal orientation. Using an hBN crystal as the base, ultrasharp corners can be created in lithographic patterns, which are then transferred to a graphite crystal underneath. We find that the anisotropic SF6 reactive ion etching process makes it possible to downsize nanostructures and obtain smooth edges, sharp corners, and feature sizes significantly below the resolution limit of electron beam lithography. The nanostructured 2D materials can be used themselves or as etch masks to pattern other nanomaterials.

Ultrashort Vertical-Channel van der Waals Semiconductor Transistors.

Atomically thin 2D van der Waals semiconductors are promising candidates for next-generation nanoscale field-effect transistors (FETs). Although large-area 2D van der Waals materials have been successfully synthesized, such nanometer-length-scale devices have not been well demonstrated in 2D van der Waals semiconductors. Here, controllable nanometer-scale transistors with a channel length of ≈10 nm are fabricated via vertical channels by squeezing an ultrathin insulating spacer between the out-of-plane source and drain electrodes, and the feasibility of high-density and large-scale fabrication is demonstrated. A large on-current density of ≈70 µA µm-1 nm-1 at a source-drain voltage of 0.5 V and a high on/off ratio of ≈107-109 are obtained in ultrashort 2D vertical channel FETs with monolayer MoS2 synthesized through chemical vapor deposition. The work provides a promising route toward the complementary metal-oxide-semiconductor-compatible fabrication of wafer-scale 2D van der Waals transistors with high-density integration.

Room-Temperature Mesoscopic Fluctuations and Coulomb Drag in Multilayer WSe2.

Mesoscopic fluctuations, manifesting the quantum interference (QI) of electrons, have been theoretically proposed in bilayer Coulomb drag systems. Unfortunately, these phenomena are usually observed at cryogenic temperatures, which severely limits their novel physics for pragmatic applications. In this paper, observation of room-temperature QI and Coulomb drag in a multilayer WSe2 transistor is reported via graphene contacts separately at its top and bottom layers. The central layers of WSe2 act as an insulating region with a width of few nanometers, which spatially separates the top and bottom conducting channels and provides a strong Coulomb interaction between them, leading to large conductance oscillations at room temperature. The gradual suppression of the oscillations with the increase in the applied magnetic field and/or injected current further confirms the QI phenomenon. With the decrease in temperature, the Coulomb drag effect is exhibited in the system owing to the increased thickness of the insulating region. This study reveals a novel approach for realization of advanced quantum electronics operating at high temperatures.

Two-Terminal Multibit Optical Memory via van der Waals Heterostructure.

2D van der Waals (vdWs) heterostructures exhibit intriguing optoelectronic properties in photodetectors, solar cells, and light-emitting diodes. In addition, these materials have the potential to be further extended to optical memories with promising broadband applications for image sensing, logic gates, and synaptic devices for neuromorphic computing. In particular, high programming voltage, high off-power consumption, and circuital complexity in integration are primary concerns in the development of three-terminal optical memory devices. This study describes a multilevel nonvolatile optical memory device with a two-terminal floating-gate field-effect transistor with a MoS2 /hexagonal boron nitride/graphene heterostructure. The device exhibits an extremely low off-current of ≈10-14 A and high optical switching on/off current ratio of over ≈106 , allowing 18 distinct current levels corresponding to more than four-bit information storage. Furthermore, it demonstrates an extended endurance of over ≈104 program-erase cycles and a long retention time exceeding 3.6 × 104 s with a low programming voltage of -10 V. This device paves the way for miniaturization and high-density integration of future optical memories with vdWs heterostructures.

Minimizing Trap Charge Density towards an Ideal Diode in Graphene-Silicon Schottky Solar Cell.

Photovoltaic device performance of graphene/n-Si Schottky diodes is largely affected by inhomogeneous oxide formation at the interface that suppresses the tunneling current of injected and photoexcited charges. The accumulated trap charges at low current induce charge recombination at the interface and degrade the ideality factor of the diode and the fill factor (FF) of the solar cell. This consequently gives rise to a nonlinear current-voltage ( I- V) feature in solar cells, commonly known as an S-shaped kink, which can be engineered by optimizing the interface barrier thickness or by increasing the carrier mobility. Here, we present chemical and electrochemical doping methods to increase the conductivity of graphene that transforms nonlinear kink photodiodes with a low FF and solar cell efficiency towards trap-free linear photovoltaic I- V. Space-charge-limited-current manifested Ohmic I- V diode behavior with enhanced conductance in graphene by injecting homogeneous ionic liquid; confirming the significant reduction of trap charge density. This was further congruent with the disappearance of the nonlinear kink in photodiodes with a high FF and nearly ideal diodes. The solar cell efficiency obtained with our strategy is around 13.6% and suggests possibilities to reach the theoretical limit of 19% by tailoring parameters such as conductance of graphene, carrier density of Si, and oxidation of the interfaces.

Facile Doping in Two-Dimensional Transition-Metal Dichalcogenides by UV Light.

Two-dimensional (2D) materials have been emerging as potential candidates for the next-generation materials in various technology fields. The performance of the devices based on these 2D materials depends on their intrinsic band structures as well as the extrinsic (doping) effects such as surrounding chemicals and environmental oxygen/moisture, which strongly determines their Fermi energy level. Herein, we report the UV treatments on the 2D transition-metal dichalcogenides, to controllably dope the samples without damaging the crystal structures or quenching the luminescence properties. More surprisingly, both n-type and p-type doping can be achieved depending on the initial status of the sample and the UV treatment conditions. The doping mechanisms were elaborated on the atomic scale with transmission electron microscopy and ab initio calculations. The facile doping by UV light has potential to be integrated with photolithography processes, aiming for the large-scale integrated device/circuits design and fabrications.

Role of Hole Trap Sites in MoS2 for Inconsistency in Optical and Electrical Phenomena.

Because of strong Coulomb interaction in two-dimensional van der Waals-layered materials, the trap charges at the interface strongly influence the scattering of the majority carriers and thus often degrade their electrical properties. However, the photogenerated minority carriers can be trapped at the interface, modulate the electron-hole recombination, and eventually influence the optical properties. In this study, we report the role of the hole trap sites on the inconsistency in the electrical and optical phenomena between two systems with different interfacial trap densities, which are monolayer MoS2-based field-effect transistors (FETs) on hexagonal boron nitride (h-BN) and SiO2 substrates. Electronic transport measurements indicate that the use of h-BN as a gate insulator can induce a higher n-doping concentration of the monolayer MoS2 by suppressing the free-electron transfer from the intrinsically n-doped MoS2 to the SiO2 gate insulator. Nevertheless, optical measurements show that the electron concentration in MoS2/SiO2 is heavier than that in MoS2/h-BN, manifested by the relative red shift of the A1g Raman peak. The inconsistency in the evaluation of the electron concentration in MoS2 by electrical and optical measurements is explained by the trapping of the photogenerated holes in the spatially modulated valence band edge of the monolayer MoS2 caused by the local strain from the SiO2/Si substrate. This photoinduced electron doping in MoS2/SiO2 is further confirmed by the development of the trion component in the power-dependent photoluminescence spectra and negative shift of the threshold voltage of the FET after illumination.

Charge Transport in MoS2/WSe2 van der Waals Heterostructure with Tunable Inversion Layer.

Despite numerous studies on two-dimensional van der Waals heterostructures, a full understanding of the charge transport and photoinduced current mechanisms in these structures, in particular, associated with charge depletion/inversion layers at the interface remains elusive. Here, we investigate transport properties of a prototype multilayer MoS2/WSe2 heterojunction via a tunable charge inversion/depletion layer. A charge inversion layer was constructed at the surface of WSe2 due to its relatively low doping concentration compared to that of MoS2, which can be tuned by the back-gate bias. The depletion region was limited within a few nanometers in the MoS2 side, while charges are fully depleted on the whole WSe2 side, which are determined by Raman spectroscopy and transport measurements. Charge transport through the heterojunction was influenced by the presence of the inversion layer and involves two regimes of tunneling and recombination. Furthermore, photocurrent measurements clearly revealed recombination and space-charge-limited behaviors, similar to those of the heterostructures built from organic semiconductors. This contributes to research of various other types of heterostructures and can be further applied for electronic and optoelectronic devices.