Single-layer Transition Metal Dichalcogenides Research Articles

Stability and magnetism of strongly correlated single-layerVS2

Single-layer transition metal dichalcogenides exhibit a variety of atomic structures and associated exotic electronic and magnetic properties. Density-functional calculations using the LDA+$U$ approximation show that single-layer ${\mathrm{VS}}_{2}$ is a strongly correlated material, where the stability, phonon spectra, and magnetic moments of the octahedral ($1T$) and the trigonal prismatic ($2H$) structures significantly depend on the effective Hubbard $U$ parameter, ${U}_{\mathrm{eff}}$. Comparison with the HSE06 hybrid density functional used as a benchmark indicates that ${U}_{\mathrm{eff}}=2.5$ eV, which consistently shows that the $2H$ structure is more stable than the $1T$ structure and a ferromagnetic semiconductor. The magnetic moments are localized on the V atoms and coupled ferromagnetically due to the superexchange interactions mediated by the S atoms. Calculations of the magnetic anisotropy show an easy plane for the magnetic moment. Assuming a classical $\mathit{XY}$ model with nearest neighbor coupling, we determine the critical temperature, ${T}_{\mathrm{c}}$, for the Berezinsky-Kosterlitz-Thouless transition of $2H$ single-layer ${\mathrm{VS}}_{2}$ to be about 90 K. Applying biaxial tensile strains can increase ${T}_{\mathrm{c}}$. Using Wannier interpolation, we evaluate the Berry curvature and anomalous Hall conductivity of $2H$ single-layer ${\mathrm{VS}}_{2}$. The coexistence of quasi-long-range ferromagnetic ordering and semiconducting behavior enables $2H$ single-layer ${\mathrm{VS}}_{2}$ to be a promising candidate for spintronics applications.

Preparation of Single-Layer MoS(2x)Se2(1-x) and Mo(x)W(1-x)S2 Nanosheets with High-Concentration Metallic 1T Phase.

The high-yield and scalable production of single-layer ternary transition metal dichalcogenide nanosheets with ≈66% of metallic 1T phase, including MoS(2x)Se2(1-x) and Mo(x)W(1-x)S2 is achieved via electrochemical Li-intercalation and the exfoliation method. Thin film MoS(2x)Se2(1- x) nanosheets drop-cast on a fluorine-doped tin oxide substrate are used as an efficient electrocatalyst on the counter electrode for the tri-iodide reduction in a dye-sensitized solar cell.

The Ising on the monolayer

Single-layer transition metal dichalcogenides have already made their mark in the world of device physics. Twin studies have now found that they exhibit unconventional Ising pair superconductivity.

Determination of the thickness of two-dimensional transition-metal dichalcogenide by the Raman intensity of the substrate

Two-dimensional (2D) transition-metal dichalcogenide (TMD) exhibits thickness-dependent optical, electronic, mechanical, chemical and vibrational properties, and a fast, non-destructive thickness characterization technique is very important for fully understanding the thickness-dependent properties of TMD nanosheets. MoS2 and WSe2 nanosheets with different layer numbers are fabricated by a mechanical exfoliation method and transferred onto SiO2 (300 nm)/Si substrate. In this work, it is found that the Raman area (i.e. the integrated intensity) ratio between the Si peak from SiO2/Si substrate underneath TMD nanosheets (ASi) and that from bare SiO2/Si substrate (ASi(0)), ASi/ASi(0), can be used to accurately identify the layer number of those TMD nanosheets. ASi/ASi(0) is then calculated using the Fresnel equations, the absorption of TMD nanosheets is included in the calculation, which has not been considered in previous work. The consideration of absorption is especially necessary for TMD which has a quite large extinction coefficient. Identification of single-layer TMD with different complex refractive indices on SiO2/Si substrate and on Si substrate using this method is given. The effects of thickness of SiO2 and excitation wavelength on the layer number identification using this method are also discussed.

Predicting a new phase (T'') of two-dimensional transition metal di-chalcogenides and strain-controlled topological phase transition.

Single layered transition metal dichalcogenides have attracted tremendous research interest due to their structural phase diversities. By using a global optimization approach, we have discovered a new phase of transition metal dichalcogenides (labelled as T''), which is confirmed to be energetically, dynamically and kinetically stable by our first-principles calculations. The new T'' MoS2 phase exhibits an intrinsic quantum spin Hall (QSH) effect with a nontrivial gap as large as 0.42 eV, suggesting that a two-dimensional (2D) topological insulator can be achieved at room temperature. Most interestingly, there is a topological phase transition simply driven by a small tensile strain of up to 2%. Furthermore, all the known MX2 (M = Mo or W; X = S, Se or Te) monolayers in the new T'' phase unambiguously display similar band topologies and strain controlled topological phase transitions. Our findings greatly enrich the 2D families of transition metal dichalcogenides and offer a feasible way to control the electronic states of 2D topological insulators for the fabrication of high-speed spintronics devices.

Nonlinear Rashba spin splitting in transition metal dichalcogenide monolayers.

Single-layer transition-metal dichalcogenides (TMDs) such as MoS2 and MoSe2 exhibit unique electronic band structures ideal for hosting many exotic spin-orbital orderings. It has been widely accepted that Rashba spin splitting (RSS) is linearly proportional to the external field in heterostructure interfaces or to the potential gradient in polar materials. Surprisingly, an extraordinary nonlinear dependence of RSS is found in semiconducting TMD monolayers under a gate field. In contrast to small and constant RSS in polar materials, the potential gradient in non-polar TMDs gradually increases with the gate bias, resulting in nonlinear RSS with a Rashba coefficient an order-of-magnitude larger than the linear one. Most strikingly, under a large gate field MoSe2 demonstrates the largest anisotropic spin splitting among all known semiconductors to our knowledge. Based on the k·p model via symmetry analysis, we identify that the third-order contributions are responsible for the large nonlinear Rashba splitting. The gate tunable spin splitting found in semiconducting pristine TMD monolayers promises future spintronics applications in that spin polarized electrons can be generated by external gating in an experimentally accessible way.

Nonlinear optical selection rule based on valley-exciton locking in monolayer ws2

Optical selection rule fundamentally determines the optical transition between energy states in a variety of physical systems from hydrogen atoms to bulk crystals such as GaAs. It is important for optoelectronic applications such as lasers, energy-dispersive X-ray spectroscopy and quantum computation. Recently, single layer transition metal dichalcogenide (TMDC) exhibits valleys in momentum space with nontrivial Berry curvature and excitons with large binding energy. However, it is unclear how the unique valley degree of freedom combined with the strong excitonic effect influences the optical excitation. Here we discover a new set of optical selection rules in monolayer WS2,imposed by valley and exciton angular momentum. We experimentally demonstrated such a principle for second harmonic generation (SHG) and two-photon luminescence (TPL). Moreover, the two-photon induced valley populations yield net circular polarized photoluminescence after a sub-ps interexciton relaxation (2p->1s) and last for 8 ps. The discovery of this new optical selection rule in valleytronic 2D system not only largely extend information degrees but sets a foundation in control of optical transitions that is crucial to valley optoeletronic device applications such as 2D valley-polarized light emitting diodes (LED), optical switches and coherent control for quantum computing.

Strain-Induced Modulation of Electron Mobility in Single-Layer Transition Metal Dichalcogenides MX2 (<inline-formula> <tex-math notation="LaTeX">$M = {\rm Mo}$ </tex-math> </inline-formula>, W; <inline-formula> <tex-math notation="LaTeX">$X = {\rm S}$ </tex-math> </inline-formula>, Se)

In this paper, the effect of biaxial strain on the mobility of single-layer transition metal dichalcogenides (MoS2, MoSe2, WS2, and WSe2) is investigated by accounting for the scattering from intrinsic phonon modes, remote phonons, and charged impurities. $Ab~initio$ simulations are employed to study a strain-induced effect on the electronic bandstructure, and the linearized Boltzmann transport equation is used to evaluate the low-field mobility. The results indicate that tensile strain increases the mobility. In particular, a significant increase in the mobility of single-layer MoSe2 andWSe2 with a relatively small tensile strain is observed. Under a compressive strain, however, the mobility exhibits a nonmonotonic behavior. With a relatively small compressive strain, the mobility decreases and then it partially recovers with a further increase in the compressive strain.

Monolayer PtSe2, a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt

Single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. A combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrast to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.

ChemInform Abstract: Two‐Dimensional Architecture at Nanojunctions for Photocatalytic Hydrogen Generation

AbstractReview: [10 refs.

Three-fold rotational defects in two-dimensional transition metal dichalcogenides.

As defects frequently govern the properties of crystalline solids, the precise microscopic knowledge of defect atomic structure is of fundamental importance. We report a new class of point defects in single-layer transition metal dichalcogenides that can be created through 60° rotations of metal–chalcogen bonds in the trigonal prismatic lattice, with the simplest among them being a three-fold symmetric trefoil-like defect. The defects, which are inherently related to the crystal symmetry of transition metal dichalcogenides, can expand through sequential bond rotations, as evident from in situ scanning transmission electron microscopy experiments, and eventually form larger linear defects consisting of aligned 8–5–5–8 membered rings. First-principles calculations provide insights into the evolution of rotational defects and show that they give rise to p-type doping and local magnetic moments, but weakly affect mechanical characteristics of transition metal dichalcogenides. Thus, controllable introduction of rotational defects can be used to engineer the properties of these materials.

High performance pseudocapacitor based on 2D layered metal chalcogenide nanocrystals.

Single-layer and few-layer transition metal dichalcogenides have been extensively studied for their electronic properties, but their energy-storage potential has not been well explored. This paper describes the structural and electrochemical properties of few-layer TiS2 nanocrystals. The two-dimensional morphology leads to very different behavior, compared to corresponding bulk materials. Only small structural changes occur during lithiation/delithiation and charge storage characteristics are consistent with intercalation pseudocapacitance, leading to materials that exhibit both high energy and power density.

Two‐Dimensional Architecture at Nanojunctions for Photocatalytic Hydrogen Generation

A new junction: CdS nanocrystals combined with single-layer transition-metal dichalcogenide nanosheets with two-dimensional nanojuctions are reported to possess excellent photocatalytic activity and stability under visible-light irradiation. Two-dimensional architecture at nanojunctions offers a promising pathway to obtain high performance photocatalyst systems by using single-layer nanosheets as building blocks.

Electronic Structure of Epitaxial Single-LayerMoS2

The electronic structure of epitaxial single-layer MoS2 on Au(111) is investigated by angle-resolved photoemission spectroscopy. Pristine and potassium-doped layers are studied in order to gain access to the conduction band. The potassium-doped layer is found to have a (1.39±0.05) eV direct band gap at K[over ¯] with the valence band top at Γ[over ¯] having a significantly higher binding energy than at K[over ¯]. The moiré superstructure of the epitaxial system does not lead to the presence of observable replica bands or minigaps. The degeneracy of the upper valence band at K[over ¯] is found to be lifted by the spin-orbit interaction, leading to a splitting of (145±4) meV. This splitting is anisotropic and in excellent agreement with recent calculations. Finally, it is shown that the potassium doping does not only give rise to a rigid shift of the band structure but also to a distortion, leading to the possibility of band structure engineering in single-layers of transition metal dichalcogenides.

Phase engineering of transition metal dichalcogenides.

Transition metal dichalcogenides (TMDs) represent a family of materials with versatile electronic, optical, and chemical properties. Most TMD bulk crystals are van der Waals solids with strong bonding within the plane but weak interlayer bonding. The individual layers can be readily isolated. Single layer TMDs possess intriguing properties that are ideal for both fundamental and technologically relevant research studies. We review the structure and phases of single and few layered TMDs. We also describe recent progress in phase engineering in TMDs. The ability to tune the chemistry by choosing a unique combination of transition metals and chalcogen atoms along with controlling their properties by phase engineering allows new functionalities to be realized with TMDs.

Single-layer transition metal dichalcogenide nanosheet-based nanosensors for rapid, sensitive, and multiplexed detection of DNA.

Single-layer transition metal dichalcogenide nanosheets, including MoS2, TiS2, and TaS2, are used as novel sensing platforms for sensitive and selective detection of DNA, based on their high fluorescence-quenching ability and different affinities toward single-stranded DNA and double-stranded DNA. Importantly, for the first time, a single-layer TaS2 nanosheet-based multiplexed DNA sensor is also developed.

One-pot synthesis of CdS nanocrystals hybridized with single-layer transition-metal dichalcogenide nanosheets for efficient photocatalytic hydrogen evolution.

Exploration of low-cost and earth-abundant photocatalysts for highly efficient solar photocatalytic water splitting is of great importance. Although transition-metal dichalcogenides (TMDs) showed outstanding performance as co-catalysts for the hydrogen evolution reaction (HER), designing TMD-hybridized photocatalysts with abundant active sites for the HER still remains challenge. Here, a facile one-pot wet-chemical method is developed to prepare MS2-CdS (M=W or Mo) nanohybrids. Surprisedly, in the obtained nanohybrids, single-layer MS2 nanosheets with lateral size of 4-10 nm selectively grow on the Cd-rich (0001) surface of wurtzite CdS nanocrystals. These MS2-CdS nanohybrids possess a large number of edge sites in the MS2 layers, which are active sites for the HER. The photocatalytic performances of WS2-CdS and MoS2-CdS nanohybrids towards the HER under visible light irradiation (>420 nm) are about 16 and 12 times that of pure CdS, respectively. Importantly, the MS2-CdS nanohybrids showed enhanced stability after a long-time test (16 h), and 70% of catalytic activity still remained.

Large conduction band and Fermi velocity spin splitting due to Coulomb interactions in single-layerMoS2

We study the effect of Coulomb interactions on the low-energy band structure of single-layer transition metal dichalcogenide semiconductors using an effective low-energy model. We show how a large conduction band spin splitting and a spin dependent Fermi velocity are generated in ${\mathrm{MoS}}_{2}$, as a consequence of the difference between the gaps of the two spin projections induced by the spin-orbit interaction. The conduction band and Fermi velocity spin splitting found are in agreement with the optical absorption energies of the excitonic peaks A, B measured in the experiments.

Ultrafast electronic and structural response of monolayer MoS2 under intense photoexcitation conditions.

We report on the dynamical response of single layer transition metal dichalcogenide MoS2 to intense above-bandgap photoexcitation using the nonlinear-optical second order susceptibility as a direct probe of the electronic and structural dynamics. Excitation conditions corresponding to the order of one electron-hole pair per unit cell generate unexpected increases in the second harmonic from monolayer films, occurring on few picosecond time-scales. These large amplitude changes recover on tens of picosecond time-scales and are reversible at megahertz repetition rates with no photoinduced change in lattice symmetry observed despite the extreme excitation conditions.

Electron and hole mobilities in single-layer WSe2.

Single-layer transition metal dichalcogenide WSe2 has recently attracted a lot of attention because it is a 2D semiconductor with a direct band gap. Due to low doping levels, it is intrinsic and shows ambipolar transport. This opens up the possibility to realize devices with the Fermi level located in the valence band, where the spin/valley coupling is strong and leads to new and interesting physics. As a consequence of its intrinsically low doping, large Schottky barriers form between WSe2 and metal contacts, which impede the injection of charges at low temperatures. Here, we report on the study of single-layer WSe2 transistors with a polymer electrolyte gate (PEO:LiClO4). Polymer electrolytes allow the charge carrier densities to be modulated to very high values, allowing the observation of both the electron- and the hole-doped regimes. Moreover, our ohmic contacts formed at low temperatures allow us to study the temperature dependence of electron and hole mobilities. At high electron densities, a re-entrant insulating regime is also observed, a feature which is absent at high hole densities.