Transition Metal Dichalcogenides: Making Atomic-Level Magnetism Tunable with Light at Room Temperature.

The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D-DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M-doped TX , where M = V, Fe, and Cr; T = W, Mo; X = S, Se, and Te), may lead to innovative applications in spintronics, spin-caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D-TMD DMSs and heterostructures. By combining magneto-LC resonance (MLCR) experiments with density functional theory (DFT) calculations, we show that the magnetization can be enhanced using light in V-doped TMD monolayers (e.

Incommensurate Antiferromagnetic Order in Weakly Frustrated Two-Dimensional van der Waals Insulator CrPSe.

Although magnetic order is suppressed by a strong frustration, it appears in complex forms such as a cycloid or spin density wave in weakly frustrated systems. Herein, we report a weakly magnetically frustrated two-dimensional (2D) van der Waals material CrPSe. Polycrystalline CrPSe was synthesized at an optimized temperature of 700 °C to avoid the formation of any secondary phases (e.

High Pressure-Driven Magnetic Disorder and Structural Transformation in Fe GeTe : Emergence of a Magnetic Quantum Critical Point.

Among the recently discovered 2D intrinsic van der Waals (vdW) magnets, Fe GeTe (FGT) has emerged as a strong candidate for spintronics applications, due to its high Curie temperature (130 - 220 K) and magnetic tunability in response to external stimuli (electrical field, light, strain). Theory predicts that the magnetism of FGT can be significantly modulated by an external strain. However, experimental evidence is needed to validate this prediction and understand the underlying mechanism of strain-mediated vdW magnetism in this system.

CNT-molecule-CNT (1D-0D-1D) van der Waals integration ferroelectric memory with 1-nm junction area.

The device's integration of molecular electronics is limited regarding the large-scale fabrication of gap electrodes on a molecular scale. The van der Waals integration (vdWI) of a vertically aligned molecular layer (0D) with 2D or 3D electrodes indicates the possibility of device's integration; however, the active junction area of 0D-2D and 0D-3D vdWIs remains at a microscale size. Here, we introduce the robust fabrication of a vertical 1D-0D-1D vdWI device with the ultra-small junction area of 1 nm achieved by cross-stacking top carbon nanotubes (CNTs) on molecularly assembled bottom CNTs.

Escalating Ferromagnetic Order via Se-Vacancies Near Vanadium in WSe Monolayers.

Magnetic order has been proposed to arise from a variety of defects, including vacancies, antisites, and grain boundaries, which are relevant in numerous electronics and spintronics applications. Nevertheless, its magnetism remains controversial due to the lack of structural analysis. The escalation of ferromagnetism in vanadium-doped WSe monolayer is herein demonstrated by tailoring complex configurations of Se vacancies (Se ) via post heat-treatment.

Spin-Selective Hole-Exciton Coupling in a V-Doped WSe Ferromagnetic Semiconductor at Room Temperature.

While valley polarization with strong Zeeman splitting is the most prominent characteristic of two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors under magnetic fields, enhancement of the Zeeman splitting has been demonstrated by incorporating magnetic dopants into the host materials. Unlike Fe, Mn, and Co, V is a distinctive dopant for ferromagnetic semiconducting properties at room temperature with large Zeeman shifting of band edges. Nevertheless, little known is the excitons interacting with spin-polarized carriers in V-doped TMDs.

Gate-Tunable Magnetism via Resonant Se-Vacancy Levels in WSe.

The confined defects in 2D van der Waals (vdW)-layered semiconductors can be easily tailored using charge doping, strain, or an electric field. Nevertheless, gate-tunable magnetic order via intrinsic defects has been rarely observed to date. Herein, a gate-tunable magnetic order via resonant Se vacancies in WSe is demonstrated.

Doping-Mediated Lattice Engineering of Monolayer ReS for Modulating In-Plane Anisotropy of Optical and Transport Properties.

ReS exhibits strong anisotropic optical and electrical responses originating from the asymmetric lattice. Here, we show that the anisotropy of monolayer (1L) ReS in optical scattering and electrical transport can be practically erased by lattice engineering lithium (Li) treatment. Scanning transmission electron microscopy revealed that significant strain is induced in the lattice of Li-treated 1L-ReS, due to high-density electron doping and the resultant formation of continuous tiling of nanodomains with randomly rotating orientations of 60°, which produced a nearly isotropic response of polarized Raman scattering and absorption of Li-treated 1L-ReS.

Layer-controlled single-crystalline graphene film with stacking order via Cu-Si alloy formation.

Multilayer graphene and its stacking order provide both fundamentally intriguing properties and technological engineering applications. Several approaches to control the stacking order have been demonstrated, but a method of precisely controlling the number of layers with desired stacking sequences is still lacking. Here, we propose an approach for controlling the layer thickness and crystallographic stacking sequence of multilayer graphene films at the wafer scale via Cu-Si alloy formation using direct chemical vapour deposition.

Ferromagnetic Order at Room Temperature in Monolayer WSe Semiconductor via Vanadium Dopant.

Diluted magnetic semiconductors including Mn-doped GaAs are attractive for gate-controlled spintronics but Curie transition at room temperature with long-range ferromagnetic order is still debatable to date. Here, the room-temperature ferromagnetic domains with long-range order in semiconducting V-doped WSe monolayer synthesized by chemical vapor deposition are reported. Ferromagnetic order is manifested using magnetic force microscopy up to 360 K, while retaining high on/off current ratio of ≈10 at 0.

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.

Probing Multiphased Transition in Bulk MoS by Direct Electron Injection.

Structural phase transitions in layered two-dimensional (2D) materials are of significant interest owing to their ability to exist in multiple metastable states with distinctive properties. However, phase transition in bulk MoS by nondestructive electron infusion has not yet been realized. In this study, we report the 2H to 1T' phase transition and in-between intermediates in bulk MoS using MoS/[CaN]·e heterostructures, in which kinetic free electrons were directly injected into MoS.

Proximity Engineering of the van der Waals Interaction in Multilayered Graphene.

The van der Waals (vdW) interaction in two-dimensional (2D)-layered materials affects key characteristics of electronic devices, such as the contact resistance, with a vertical heterostructure geometry. While various functionalizations to manipulate the properties of 2D materials have shown issues such as defect generation or have a limited spatial range for the methods, engineering the vdW interaction in nondestructive ways for device applications has not been tried or properly achieved yet. Here, we introduce the proximity engineering of the vdW interaction in multilayered graphene, which is observed as modified interlayer distances and deviated stacking orders by Raman spectroscopy.

Ultrahigh Gauge Factor in Graphene/MoS Heterojunction Field Effect Transistor with Variable Schottky Barrier.

Piezoelectricity of transition metal dichalcogenides (TMDs) under mechanical strain has been theoretically and experimentally studied. Powerful strain sensors using Schottky barrier variation in TMD/metal junctions as a result of the strain-induced lattice distortion and associated ion-charge polarization were demonstrated. However, the nearly fixed work function of metal electrodes limits the variation range of a Schottky barrier.

VSe: a novel antifluorite-type cubic phase with a metal-metal bonding.

A new antifluorite-type (LiO-type) cubic compound, VSe, has been synthesized for the first time by changing the amount of selenium in chemical vapor transport. The vanadium-based cubic phase studied here reveals a metal-metal bonding feature in the electronic band structure. This compound is the first example of an antifluorite-type cubic structure in a V-Se system.

van der Waals Metallic Transition Metal Dichalcogenides.

Transition metal dichalcogenides are layered materials which are composed of transition metals and chalcogens of the group VIA in a 1:2 ratio. These layered materials have been extensively investigated over synthesis and optical and electrical properties for several decades. It can be insulators, semiconductors, or metals revealing all types of condensed matter properties from a magnetic lattice distorted to superconducting characteristics.

Role of Hole Trap Sites in MoS 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 MoS-based field-effect transistors (FETs) on hexagonal boron nitride (h-BN) and SiO substrates.

Telluriding monolayer MoS and WS via alkali metal scooter.

The conversion of chalcogen atoms to other types in transition metal dichalcogenides has significant advantages for tuning bandgaps and constructing in-plane heterojunctions; however, difficulty arises from the conversion of sulfur or selenium to tellurium atoms owing to the low decomposition temperature of tellurides. Here, we propose the use of sodium for converting monolayer molybdenum disulfide (MoS) to molybdenum ditelluride (MoTe) under Te-rich vapors. Sodium easily anchors tellurium and reduces the exchange barrier energy by scooting the tellurium to replace sulfur.

van der Waals Layered Materials: Opportunities and Challenges.

Since graphene became available by a scotch tape technique, a vast class of two-dimensional (2D) van der Waals (vdW) layered materials has been researched intensively. What is more intriguing is that the well-known physics and chemistry of three-dimensional (3D) bulk materials are often irrelevant, revealing exotic phenomena in 2D vdW materials. By further constructing heterostructures of these materials in the planar and vertical directions, which can be easily achieved via simple exfoliation techniques, numerous quantum mechanical devices have been demonstrated for fundamental research and technological applications.

Efficient Photothermoelectric Conversion in Lateral Topological Insulator Heterojunctions.

Tuning the electron and phonon transport properties of thermoelectric materials by nanostructuring has enabled improving their thermopower figure of merit. Three-dimensional topological insulators, including many bismuth chalcogenides, attract increasing attention for this purpose, as their topologically protected surface states are promising to further enhance the thermoelectric performance. While individual bismuth chalcogenide nanostructures have been studied with respect to their photothermoelectric properties, nanostructured p-n junctions of these compounds have not yet been explored.

Raman Characterization of the Charge Density Wave Phase of 1T-TiSe: From Bulk to Atomically Thin Layers.

Raman scattering is a powerful tool for investigating the vibrational properties of two-dimensional materials. Unlike the 2H phase of many transition metal dichalcogenides, the 1T phase of TiSe features a Raman-active shearing and breathing mode, both of which shift toward lower energy with increasing number of layers. By systematically studying the Raman signal of 1T-TiSe in dependence of the sheet thickness, we demonstrate that the charge density wave transition of this compound can be reliably determined from the temperature dependence of the peak position of the E mode near 136 cm.