Monolayer VS2 Research Articles

MoS2@VS2 Nanocomposite as a Superior Hybrid Anode Material.

Using density functional theory, MoS2@VS2 nanocomposite is reported as a hybrid anode with upgraded electronic conductivity and Li/Na storage capacity. The chemically active monolayer VS2 can be stabilized in energy and phonon vibrations by using the monolayer MoS2 as a substrate. The stability of the chemically active monolayer VS2 is attributed to the interfacial charge accumulation between the monolayer MoS2 and VS2. The maximum specific capacity of the nanocomposite has been enhanced to 584 mAh/g both for Li and for Na storage. We attribute the high enhancement in the Li/Na storage capacity of MoS2@VS2 nanocomposite to the charge redistribution in the formation of the nanocomposite. The lithiation/sodiation open-circuit voltage range of the nanocomposite is quite feasible to be used as anode. Diffusion barriers of Li/Na ions on the surfaces of the nanocomposite are comparable to the barriers on corresponding monolayers, while at the interface the barriers are lower than that for bulk MoS2. This study utilizes different aspects of the two different materials in a hybrid anode with highly enhanced electrochemical performance.

Optical characteristic study of monolayer VS2 based on first-principles calculations

Monolayer VS2 has two common stable phases, 2H-phase and 1T-phase. Based on first-principles calculations, we studied the optical properties of 2D-VS2, including their dielectric functions, absorption coefficients, refraction indexes and energy loss functions. Also, we fully analyzed the relationship between the electronic structure and the optical characteristics of 2D-VS2. We found that the differences of density of states are the fundamental reasons for the differences of the two phase optical absorption behavior.

Thermal spin transport properties based on VS2 monolayer: A first-principles study

Two-dimensional transition-metal dichalcogenides have attracted increasing research interest due to their potential applications in electronics, thermoelectrics, and spintronics. Inspired by the recent experimental realization of ultrathin ferromagnetic pristine VS2 nanosheets, we use the first-principles combined with nonequibrium Green's function method to investigate the thermal spin transport properties of the devices based on the ferromagnetic VS2 monolayer. The excellent spin Seebeck effect, thermal spin filtering effect and negative differential thermal resistance effect are found, which are explained by the calculated spin-dependent band structure and transmission spectrum. We also show that the thermal magnetoresistance is as high as 1.1 × 105%. These findings suggest that ferromagnetic VS2 monolayer would be a promising candidate for spin caloritronic applications.

The Role of Intrinsic Defects in Electrocatalytic Activity of Monolayer VS2 Basal Planes for the Hydrogen Evolution Reaction

Two-dimensional VS2 nanomaterials have emerged as highly efficient and inexpensive electrocatalysts for the hydrogen evolution reaction (HER), and the further improvement of their HER performance depends on the understanding of the catalytic mechanism and activity in various pristine and defective structures. Here, structural stability, electronic properties, and HER activity of monolayer VS2 nanosheets with various intrinsic point defects are studied by using first-principles calculations. Compared to the most-studied 2H-phase MoS2 basal plane, both 2H- and 1T-phase VS2 basal planes exhibit superior catalytic activity due to their metallic properties. With the introduction of intrinsic point defects onto VS2 basal planes, we find that there are four types of stable defects in the 2H phase (i.e., Sad, Svac, Vad, and VS) and three types of stable defects in the 1T phase (i.e., Sad, Svac, and Vad). Moreover, the formation of Svac, Vad, and VS structures in the 2H phase and Vad in the 1T phase can enhance th...

Two-dimensional VS2 monolayers as potential anode materials for lithium-ion batteries and beyond: first-principles calculations

VS2 monolayers exhibit promising electrochemical properties for Li-, K-, and Mg-ion batteries.

Negative differential resistance and spin filter effects in VS2 monolayers

We have calculated the spin current based on the bias voltage, band structure and transmission spectrum for VS2 monolayers along the zigzag and armchair orientations by first-principles calculations combined with the non-equilibrium Green's function method.

First-Principles Studies of the Magnetic Anisotropy of Monolayer VS2

First-principles calculations are performed to study the magnetic anisotropy of monolayer VS2. The magnetic anisotropy energy (MAE) for H-VS2 is −0.213 meV, and the magnetic preferential direction is in the monolayer plane, while the corresponding value for T-VS2 is only 0.004 meV, which can be ignored for two-dimensional materials. According to the second-order perturbation of the spin–orbit coupling (SOC) interactions, the physical origin of magnetic anisotropy for H-VS2 is derived from the occupied and unoccupied dxy/dx 2-y 2 states with different spin channels in light of the electronic structure. In the reciprocal space, the negative contributions mainly stem from the corners of hexagonal Brillouin zone. Interestingly, there are non-equivalent K and K′ for the MAEs, which are observed for the first time for MAE in the reciprocal space. We predict that the lack of inversion symmetry results in the different signs of MAEs in the K and K′. Our studies open up broad prospects to trace the physical origin of magnetic anisotropy in the reciprocal space.

Correlated electronic structures of group-V transition metal dichalcogenide monolayers from hybrid density-functional calculations

We propose that a half semiconducting state can exist in trigonal-prismatic transition metal dichalcogenide (TMDC) monolayers of d1 configuration. In that state both electrons and holes are spin polarized and share the same spin channel. At the level of hybrid density functional theory, our results show in particular that VS2 monolayers are half semiconductors with a direct band gap. Moreover, we find that the conduction electron spin orientation of VS2 switches under moderate strain. Our analysis of trigonal-prismatic group-V MX2 (M = V, Nb, Ta; X = S, Se, Te) monolayers reveals a broad diversity of electronic states that can be understood qualitatively in terms of localization of d electrons.

Newtype single-layer magnetic semiconductor in transition-metal dichalcogenides VX2 (X = S, Se and Te).

We present a newtype 2-dimensional (2D) magnetic semiconductor based on transition-metal dichalcogenides VX2 (X = S, Se and Te) via first-principles calculations. The obtained indirect band gaps of monolayer VS2, VSe2, and VTe2 given from the generalized gradient approximation (GGA) are respectively 0.05, 0.22, and 0.20 eV, all with integer magnetic moments of 1.0 μB. The GGA plus on-site Coulomb interaction U (GGA + U) enhances the exchange splittings and raises the energy gap up to 0.38~0.65 eV. By adopting the GW approximation, we obtain converged G0W0 gaps of 1.3, 1.2, and 0.7 eV for VS2, VSe2, and VTe2 monolayers, respectively. They agree very well with our calculated HSE gaps of 1.1, 1.2, and 0.6 eV, respectively. The gap sizes as well as the metal-insulator transitions are tunable by applying the in-plane strain and/or changing the number of stacking layers. The Monte Carlo simulations illustrate very high Curie-temperatures of 292, 472, and 553 K for VS2, VSe2, and VTe2 monolayers, respectively. They are nearly or well beyond the room temperature. Combining the semiconducting energy gap, the 100% spin polarized valence and conduction bands, the room temperature TC, and the in-plane magnetic anisotropy together in a single layer VX2, this newtype 2D magnetic semiconductor shows great potential in future spintronics.

A first-principles study of magnetic variation via doping vacancy in monolayer VS2

The electronic structure and magnetic properties are investigated by first-principles calculations in monolayer VS2 by doping S vacancy. Three kinds of doping monolayer VS2 are investigated. The structural defects have significant influence on the electronic and magnetic properties of the material. The results unveil that the magnetic moment is determined by the strength of the V–S covalent bond and the charge distribution around the vacancies. We believe that the calculated results will be fit for experimental verification and implementation, opening an effective path for the exploration of two-dimensional spintronic devices.

Tension-Enhanced Hydrogen Evolution Reaction on Vanadium Disulfide Monolayer.

Water electrolysis is an efficient way for hydrogen production. Finding efficient, cheap, and eco-friendly electrocatalysts is essential to the development of this technology. In the work, we present a first-principles study on the effects of tension on the hydrogen evolution reaction of a novel electrocatalyst, vanadium disulfide (VS2) monolayer. Two electrocatalytic processes, individual and collective processes, are investigated. We show that the catalytic ability of VS2 monolayer at higher hydrogen coverage can be efficiently improved by escalating tension. We find that the individual process is easier to occur in a wide range of hydrogen coverage and the collective process is possible at a certain hydrogen coverage under the same tension. The best hydrogen evolution reaction with near-zero Gibbs free energy can be achieved by tuning tension. We further show that the change of catalytic activity with tension and hydrogen coverage is induced by the change of free carrier density around the Fermi level, that is, higher carrier density, better catalytic performance. It is expected that tension can be a simple way to improve the catalytic activity, leading to the design of novel electrocatalysts for efficient hydrogen production from water electrolysis.

Effect of Doping on Hydrogen Evolution Reaction of Vanadium Disulfide Monolayer.

As cheap and abundant materials, transitional metal dichalcogenide monolayers have attracted increasing interests for their application as catalysts in hydrogen production. In this work, the hydrogen evolution reduction of doped vanadium disulfide monolayers is investigated based on first-principles calculations. We find that the doping elements and concentration affect strongly the catalytic ability of the monolayer. We show that Ti-doping can efficiently reduce the Gibbs free energy of hydrogen adsorption in a wide range of hydrogen coverage. The catalytic ability of the monolayer at high hydrogen coverage can be improved by low Ti-density doping, while that at low hydrogen coverage is enhanced by moderate Ti-density doping. We further show that it is much easier to substitute the Ti atom to the V atom in the vanadium disulfide (VS2) monolayer than other transitional metal atoms considered here due to its lowest and negative formation energy. It is expected that the Ti-doped VS2 monolayer may be applicable in water electrolysis with improved efficiency.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1182-y) contains supplementary material, which is available to authorized users.

Different charge-storage mechanisms in disulfide vanadium and vanadium carbide monolayer

Associating the calculated PDOS and charge density with charge–discharge processes to understand the charge-storage mechanisms in VS2 and V2C monolayers.

Magnetic and electronic evolutions of hydrogenated VTe₂ monolayer under tension.

Two-dimensional nanostructures with controllable magnetic and electronic properties are desirable for their versatile applications in quantum devices. Here, we present a first-principles design on their magnetic and electronic switching controlled by tension. We find that hydrogenated VTe2 monolayer experiences a transfer from anti-ferromagnetism to ferromagnetism via a turning-point of paramagnetism, and switches from semiconductor, to metal, further to half-metal as tension increases. We show that its anti-ferromagnetism with semiconducting or metallic character under low tension is contributed to super-exchange or mobile-carrier enhanced super-exchange, while the ferromagnetism with half-metallic character under high tension is induced by carrier-mediated double exchange. We further show that the magnetic and electronic evolutions of hydrogenated VS2 and VSe2 monolayers under tension follow the same trend as those of hydrogenated VTe2 monolayer. We predict that tension is efficient and simple to control the magnetic and electronic properties of hydrogenated vanadium dichalcogenides monolayers. The monolayers with controllable magnetism and conductivity may find applications in multi-functional nanodevices.

A density functional theory study of the tunable structure, magnetism and metal-insulator phase transition in VS2 monolayers induced by in-plane biaxial strain

We report a density functional theory study of a phase transition of a VS2 monolayer that can be tuned by the in-plane biaxial strain. This results in both a metal-insulator transition and a low spin-high spin magnetic transition. At low temperature, the semiconducting H-phase is stable and large strain (>3%) is required to provoke the transition. On the other hand, at room temperature (300 K), only a small tensile strain of 2% is needed to induce the phase transition from the semiconducting H-phase to the metallic T-phase together with the magnetic transition from high spin to low spin. The phase diagram dependence on both strain and temperature is also discussed in order to provide a better understanding of the phase stability of VS2 monolayers.

Metallic VS2 Monolayer: A Promising 2D Anode Material for Lithium Ion Batteries

By means of density functional theory computations, we systematically investigated the adsorption and diffusion of lithium on the recently synthesized VS2 monolayer, in comparison with MoS2 monolayer and graphite. Intrinsically metallic, VS2 monolayer has a higher theoretical capacity (466 mAh/g), a lower or similar Li diffusion barrier as compared to MoS2 and graphite, and has a low average open-circuit voltage of 0.93 V (vs Li/Li+). Our results suggest that VS2 monolayer can be utilized as a promising anode material for Li ion batteries with high power density and fast charge/discharge rates.

Dimension-dependent phase transition and magnetic properties of VS2

Among dozens of layered transition metal dichalcogenides (TMDs), VS2 has attracted particular interest due to its intrinsic magnetism and potential applications as a high-performance functional nanomaterial. The phase stability and electronic properties of the typical crystal structures of both monolayer and bulk VS2 are carefully investigated based on first-principle calculations. The results reveal that the relative stability between different phases is greatly affected by the thickness of the layers and the temperature. Below room temperature, both bulk and monolayer VS2 prefer to exhibit the hexagonal (H) structure instead of the trigonal (T) structure. Interestingly, at room temperature, although the H monolayer VS2 remains more stable than the T-VS2, the bulk T-VS2 becomes more stable than H-VS2. These results reveal that a phase transition between H and T will occur on changing either the thickness of the slab or the temperature. Furthermore, the different crystal structures (H and T) exhibit significantly distinct magnetism: the bulk T-VS2 has the lowest magnetism (0.31 μB), while the monolayer H-VS2 has the largest magnetism (about 1.00 μB) among the structures. Most importantly, our results reveal that the magnetism will increase sharply on the exfoliation of monolayer VS2 from the bulk at room temperature because of the phase transition from T to H. The present results provide an efficient way to modulate the magnetic moment through controlling the crystal structure and the thickness of the VS2 nanosheets.

Ferromagnetism in ultrathin VS2 nanosheets

Two-dimensional nanosheet crystals have potential applications in nano-electronic devices. Prompted by the recently observed ferromagnetism in pristine VX2 (X = S and Se) monolayers by first-principle calculations, we provide the experimental evidence for room temperature ferromagnetism in ultrathin VS2 nanosheets by observing their clear hysteresis and ferromagnetic resonance. The first-principle calculation results indicate that unusual hybridization of the V 3d and S 3p orbitals occurs in the monolayer VS2, and the relative transfer of electrons between the V and S atoms plays an important role in the transition of the spin moments. According to the previous work, the stable ferromagnetism in ultrathin VS2 nanosheets is considered as a competitive effect between through-bond and through-space interactions, where the through-bond interactions dominate. This finding opens a new path to explore the spintronics in pristine two-dimensional nanostructures.