Vanadium Dichalcogenides Research Articles

Effect of substrate temperature on the morphology of chemical vapor deposition grown two-dimensional metallic vanadium diselenide

Two-dimensional (2D) metallic transition metal dichalcogenides (TMDs) have emerged as a promising class of materials for engineering novel electronic phases, 2D magnetism, superconductivity, and unique electronic applications. Vanadium dichalcogenides, in particular, exhibit a wide range of unusual physical and chemical properties, making them suitable for novel applications in condensed matter physics, materials science, and device physics. Here, we investigate the effect of substrate temperature on the morphology of 2D vanadium diselenide (VSe2) nanosheets synthesized by the atmospheric pressure chemical vapor deposition (APCVD) method. The growth is carried out on SiO2/Si, fluorphlogopite mica [KMg3(AlSi3O10)F2], and h-BN/SiO2/Si substrates. Our findings reveal that continuous thick VSe2 films are obtained at a substrate temperature of 650 °C, while large-sized VSe2 nanosheets with lower thickness are formed at intermediate substrate temperatures of 550–500 °C. This study provides insights into the substrate temperature-controlled growth of 2D VSe2 nanosheets for various potential applications.

Doped 2D VX2 (X = S, Se, Te) monolayers as electrocatalysts for ammonia production: A DFT based study

Electrocatalytic nitrogen fixation under ambient conditions on vanadium dichalcogenides (VX2) with non-metal dopants has been explored herein. Understanding the interface chemistry, inherent electronic and acute synergistic nature of non-metal dopants on two unique phases of VX2 has been meticulously explored through a scrutiny of several non-metal atoms as catalytic centers. The efficacity of N2 chemisorption and NN bond activation has been implemented as crucial parameters to realize boron and carbon doped VX2 monolayers to be electrocatalytically active for nitrogen reduction reaction (NRR). Detailed investigation on the NRR mechanism brings out the pivotal role of thermodynamic favourability for product formation obtained from Gibbs free energy differences. The charge transfer on N and π-π* orbital hybridization and electron “donor–acceptor” mechanism between the non-metal and N2 has been found to modulate the electrocatalytic barrier for NRR on VX2 monolayers. This study proposes boron doped VS2 as an efficient chemically feasible, earth abundant sustainable electrocatalyst for NRR with an overpotential as low as 0.06 eV.

Boosting the zinc ion storage capacity and cycling stability of interlayer-expanded vanadium disulfide through in-situ electrochemical oxidation strategy

Metallic vanadium dichalcogenides with high conductivity and large layer spacing are fantastically potential to be cathode candidates for aqueous zinc ion batteries. However, simply reliance on the reversible Zn2+ intercalation/deintercalation process in the layer structure of vanadium dichalcogenides makes it suffer from low specific capacity and limited cycling number. Here we report a facile in-situ electrochemical oxidation strategy to boost the zinc ion storage capacity of interlayer-expanded vanadium disulfide (VS2·NH3) hollow spheres with satisfying cyclic stability. The hydrated vanadium oxide (V2O5·nH2O) generated from oxidized VS2·NH3, are endowed with reduced nanosheet size and subordinated porous structure, which provides abundant accessible sites and accelerates the zinc ion diffusion process. As a result, the VS2·NH3 derived cathode after the electrochemical oxidation process delivers a high reversible capacity of 392 mA h g−1 at 0.1 A g−1 and long cyclic stability (110% capacity retention at 3 A g−1 after 2000 cycles). The efficient oxidation process of VS2·NH3 cathode and the storage mechanism in the subsequent cycles are schematically investigated. This work not only reveals the zinc ion storage mechanism of the oxidized VS2·NH3 but also sheds light on advanced design for high-performance Zn ion cathode materials.

Pristine and Janus monolayers of vanadium dichalcogenides: potential materials for overall water splitting and solar energy conversion

Emerging two-dimensional (2D) transition metal dichalcogenides with admirable optoelectronic and electrochemical properties can be used in many practical applications like photocatalysis. In this study, through first-principles calculations, a zealous effort has been devoted to investigating the electronic, photocatalytic, and optical properties of pristine and Janus monolayers of vanadium dichalcogenides. Binding energies and elastic constants indicate the structural and mechanical stability of these layers. Pristine and Janus monolayers show band gaps in a range of 0.4 to 1.6 eV. Band gap alignments by using empirical and DFT approach indicate that VS2, VSe2, and VSeS have large enough kinetic overpotentials for photocatalytic overall water splitting. Additionally, biaxial strain displays that the band gap decreases with the variation of strain from -3% to + 3%. In the case of an electric field (Eext), band gap decreases linearly when varied from -0.8 VA−1 to + 0.8 VA−1. All pristine and Janus monolayers exhibit a high absorption coefficient ~ 105 cm−1. The results of this study suggest that pristine and Janus monolayers of vanadium dichalcogenides are potential candidates for photocatalytic water splitting and optoelectronic device applications.

Enhancement of Curie Temperature under Built-in Electric Field in Multi-Functional Janus Vanadium Dichalcogenides**Supported by the National Natural Science Foundation of China (Grant Nos. 61704083, 61605087 and 61874060), the Natural Science Foundation of Jiangsu Province (Grant Nos. BK20160881 and BK20181388), and the Foundation of Nanjing University of Posts and Telecommunications (Grant No. NY219030).

Functionalized two-dimensional materials with multiferroicity are highly desired to be next-generation electronic devices. Here we theoretically predict a family of Janus vanadium dichalcogenides VXX’ (X/X’ = S, Se, Te) monolayers with multiferroic properties, combing ferromagnetism, ferroelasticity and piezoelectricity. Due to the unpaired electrons on the V atom, the Janus VXX’ monolayers have intrinsic long-range ferromagnetic orders. Particularly, the Curie temperature of 1T-VSeTe monolayer is up to 100 K, which is greatly higher than 2D 1T-VSe2 and 1T-VTe2. Furthermore, the six Janus VXX’ monolayers have similar crater-like ferroelastic switching curves. Compared to black phosphorus, 2H-VSSe monolayer has the similar ferroelastic switching signal and 4 times lower energy barrier. In addition, the out-of-plane piezoelectricity induced by the structure asymmetry in the vertical direction gives the 2H-VXX’ monolayers the potential to be piezoelectric materials. It is found that a built-in electric field in the vertical direction due to the different electronegativity values of chalcogen atoms induces the changes of electronic structures, which leads to the appearance of three different types of band gaps in the three H-phase structures. Recently, the experimental growth of the Janus MoSSe monolayers and the electrochemical exfoliation of ferromagnetic monolayered VSe2 make the Janus VXX’ monolayers possibly fabricated in experiments.

Multimorphism and gap opening of charge-density-wave phases in monolayer VTe2

Vanadium dichalcogenides have attracted increasing interests for the charge density wave phenomena and possible ferromagnetism. Here, we report on the multiphase behavior and gap opening in monolayer VTe2 grown by molecular beam epitaxy. Scanning tunneling microscopy (STM) and spectroscopy study revealed the (4×4) metallic and gapped (2√3×2√3) charge-density wave (CDW) phases with an energy gap of ≈ 40 meV. Through the in-plane condensation of vanadium atoms, the typical star-of-David clusters and truncated triangle-shaped clusters are formed in the (4×4) and (2√3×2√3) phases respectively, resulting in different surface morphologies and electronic structures as confirmed by density functional theory (DFT) calculations with on-site Coulomb repulsion. The CDW-driven reorganization of the atomic structure weakens the ferromagnetic superexchange coupling and strengthens the antiferromagnetic exchange coupling on the contrary, suppressing the long-range magnetic order in monolayer VTe2. The electron correlation is found to be important to explain the gap opening in the (2√3×2√3) phase.

In-Depth Structural Characterization of 1T-VSe2 Single Crystals Grown by Chemical Vapor Transport

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been extensively studied. Among them, vanadium dichalcogenides, a unique metallic family of the TMDCs, have been considered as pot...

Recent Developments on Emerging Properties, Growth Approaches, and Advanced Applications of Metallic 2D Layered Vanadium Dichalcogenides

AbstractThe electronic structures of transition metal dichalcogenides (TMDs) has a strong dependency on the d‐band electrons of the central metal (M) atom. In particular, trigonal prismatic coordinated (2H) and octahedral coordinated (1T) phases with distinct crystal structure‐property relationships. Extraordinarily diverse electrical properties ranging from semiconducting, semimetallic to intrinsic metallic basically underlie from different Fermi level locations. Amid all TMDs, most specifically metallic VX2 possess plenty of interesting physical properties among them superconductivity, electrical conductivity, charge density wave, optical and magnetism etc. have offered intriguing applications in the fields of condensed matter physics, materials and device physics over the last few decades. Further modification of these materials by defect engineering, Li intercalation, electron beam/light irradiation, alloying and dimensional tuning can lead several tunable device applications. Due to their superior mechanical flexibility, controllable electrical properties, planar fabrication and high surface to volume ratio etc., 2D metallic TMDs materials emerge as active material for sensing, energy storage, conversion and field emission applications. This review article aims to provide the insights on the recent developments of different emerging properties, growth approaches and applications of 2D metallic VX2 (X = S, Se and Te) and its heterojunctions.

Real-space investigation of the charge density wave in VTe2 monolayer with broken rotational and mirror symmetries

Recently the charge density wave (CDW) in vanadium dichalcogenides have attracted increasing research interests, but a real-space investigation on the symmetry breaking of the CDW state in VTe2 monolayer is still lacking. We have investigated the CDW of VTe2 monolayer by low energy electron diffraction (LEED) and scanning tunneling microscope (STM). While the LEED experiments revealed a (4X4) CDW transition at 192+-2 K, our low-temperature STM experiments resolved the (4X4) lattice distortions and charge-density modulation in real space, and further unveiled a 1D modulation that breaks the three-fold rotational and mirror symmetries in the CDW state. In accordance with the CDW state at low temperature, a CDW gap of 12 meV was detected by scanning tunneling spectroscopy (STS) at 4.9 K. Our work provides real-space evidence on the symmetry breaking of the (4X4) CDW state in VTe2 monolayer, and implies there is a certain mechanism, beyond the conventional Fermi surface nesting or the q-dependent electron-phonon coupling, is responsible for the formation of CDW state in VTe2 monolayer.

Charge Density Wave State Suppresses Ferromagnetic Ordering in VSe2 Monolayers

Ferromagnetic ordering of monolayer vanadium dichalcogenides (VSe2 and VS2) has been predicted by density functional theory (DFT), and suggestive experimental evidence for magnetic ordering in VSe2 monolayers has been reported. However, such ferromagnetic ordering would be in stark contradiction to the known paramagnetic nature of the bulk VSe2. Herein, we investigate the electronic structure of VSe2 monolayers by angle-resolved photoemission spectroscopy (ARPES) and first-principles DFT. The ARPES measurements demonstrate the absence of spin-polarized bands for monolayers in close correspondence to nonmagnetic nature of the bulk VSe2. We demonstrate that the stabilization of the nonmagnetic state occurs due to the appearance of a charge density wave (CDW) state in VSe2 monolayers. In contrast to well-established 4 × 4 × 3 periodicity of the CDW in bulk VSe2, we identify a √3 × √7 unit cell for VSe2 monolayers from both scanning tunneling microscopy imaging and first-principles calculations. Moreover, DFT...

Application of highly stretchable and conductive two-dimensional 1T VS2 and VSe2 as anode materials for Li-, Na- and Ca-ion storage

The atomic structure of single layer transition metal-dichalcogenides (TMDs) like two-dimensional Vanadium dichalcogenides (VS2 and VSe2) with octahedral (1T) phase has attracted remarkable interest due to its excellent physical and chemical properties. In this work, we conduct density functional theory (DFT) simulations to study the applicability of 1T VS2 and VSe2 as anode materials for Al-, Mg-, Ca-, Na- or Li-ion batteries. To explore the application of 1T VS2 and VSe2 in rechargeable batteries, charge transfer efficiency, adsorption energy, open circuit voltage profile, stability, electronic conductivity and diffusion energy barrier were evaluated. The results revealed that, both 1T VS2 and VSe2 show highly promising performances as anode material for Li-, Ca- or Na-ions batteries, whereas theAl and Mg atoms are not favourable for the adsorption. Consequently, our first-principles analysis reveals that 1T VS2 and VSe2 have considerable charge capacities of 466 mAh/g and 257 mAh/g for Li-, Ca- or Na-ion storages, which is very promising for stretchable rechargeable batteries.

Coexistence of piezoelectricity and magnetism in two-dimensional vanadium dichalcogenides.

Akin to three dimensional (3D) multiferroics, two dimensional (2D) piezoelectric materials with intrinsic magnetic properties are promising applications in nanoscale spintronic devices. In this study, 2D magnetic transition metal dichalcogenides (VS2, VSe2, and Janus-VSSe) have been investigated by the first principles method for their structural, magnetic, electronic, and piezoelectric properties. H type Janus-VSSe has been shown to be more stable than the T type, and dynamically stable through phonon frequency analysis. Our calculations show that H type Janus-VSSe is not only a magnetic semiconductor but also exhibits appreciable in-plane and vertical piezoelectricity. Additionally, H type VS2 and VSe2 also show high in-plane piezoelectricity. The calculated values of in-plane piezoelectricity for these magnetic materials are higher than traditional 3D piezoelectric materials such as α-quartz. The coexistence of magnetism and piezoelectricity in H type VS2, Janus-VSSe, and VSe2 makes them promising piezoelectric materials in nanoscale spin devices.

Exfoliated Vanadium Dichalcogenides (VS2, VSe2, VTe2) By Lithium Intercalation Exhibit Dramatically Different Properties from Their Bulk Counterparts

Layered transition metal dichalcogenides (TMDs) have been attracting great interest due to their excellent electrical, optical, and electrochemical properties they exhibit when exfoliated down to mono- or a few-layer material. Driven by the unique properties, numerous methods have been developed to isolate few-layered TMDs. In this study, lithium intercalation route which is initially established for MoS2 and WS2 has been adopted to exfoliate the vanadium dichalcogenides (VS2, VSe2, and VTe2). Following the inherent electrochemical properties of these bulk and exfoliated vanadium dichalcogenides were studied along with their potential as electrocatalysts towards hydrogen evolution reaction (HER). This lithium intercalation method leads to the different degree of exfoliation following the VS2 > VSe2 > VTe2 order. Besides, upon exfoliation to single/few layer VX2 become partly converted to nonstoichiometric vanadium oxide and lithium vanadates, which is strikingly different when compared to group 6 TMDs, such as MoX2 and WX2. Inherent electrochemical results indicate that both bulk and exfoliated VS2, VSe2, and VTe2 possess inherent electrochemical activity, which can limit their application for electrocatalysts and electrode materials. HER performance study of bulk and exfoliated vanadium dichalcogenides shows that the presence of vanadium oxides and lithium vanadates formed during the exfoliation process can highly influence the HER performance.

Hysteresis features of the transition-metal dichalcogenides VX2 (X = S, Se, and Te)

Very recently, it has been shown that vanadium dichalcogenides (VX2, X = S, Se and Te) monolayers show intrinsic ferromagnetism, and their critical temperatures are nearly to or beyond room temperature. Hence, they would have wide potential applications in next-generation nanoelectronic and spintronic devices. In this work, being inspired by a recent study we systematically perform Monte Carlo simulations based on single-site update Metropolis algorithm to investigate the hysteresis features of VX2 monolayers for a wide range of temperatures up to 600 K. Our simulation results indicate that, both remanence and coercivity values tend to decrease with increasing temperature. Furthermore, it is found that hysteresis curves start to evolve from rectangular at the lower temperature regions to nearly S-shaped with increasing temperature.

Cytotoxicity of Exfoliated Layered Vanadium Dichalcogenides.

Transition-metal Group 5 vanadium dichalcogenides have shown promising properties for many applications, such as batteries, capacitors, electrocatalysts for hydrogen production and many more. However, their toxicological effects have not yet been well understood. Here, we studied the cytotoxicity of exfoliated VS2 , VSe2 and VTe2 by incubating various concentrations of the materials with human lung carcinoma (A549) cells for 24 h and measuring the remaining cell viabilities after the treatment. We found that these vanadium dichalcogenides are relatively more toxic compared to Group 6 transition-metal dichalcogenides (TMDs), namely MoS2 , WS2 and WSe2 . This study is important for a better understanding of the toxicity of TMDs in preparation for their actual commercialisation in the future.

Lithium Exfoliated Vanadium Dichalcogenides (VS2, VSe2, VTe2) Exhibit Dramatically Different Properties from Their Bulk Counterparts

Ultrathin 2D transition metal dichalcogenides (TMDs) have attracted widespread attention due to their excellent electrical, optical, and electrochemical properties. Ultrathin vanadium disulfide is a metallic member of two dimensional materials aside graphene which does not possess a bandgap in the electronic structure. Research on vanadium disulfide reveals its versatile application, such as supercapacitor, battery material, electrocatalyst, and moisture sensor. Considering the superior performance of vanadium disulfide, it is necessary and essential to study vanadium diselenide and vanadium ditelluride. In this study, an effort has been devoted to study the inherent electrochemistry of three vanadium dichalcogenides (VX2: VS2, VSe2, and VTe2) and explore their potential as electrocatalysts for hydrogen evolution reaction. Following the exfoliation routes established for MoS2 and WS2, Li intercalation route, using n‐buthyllithium is utilized. Interestingly, upon exfoliation to single/few layer VX2 become partly converted to nonstoichiometric vanadium oxide and lithium vanadates, which is strikingly different when compared to group 6 TMDs, such as MoX2 and WX2.

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.

Electronic and Magnetic Properties of Vanadium Dichalcogenides Monolayers Tuned by Hydrogenation

First-principles calculations based on density-functional theory are carried out to systematically investigate the effects of hydrogenation on the electronic and magnetic properties of vanadium dic...

X-ray-absorption spectra and electronic structures of vanadium dichalcogenides and their first-row transition-metal intercalates

The x-ray $K$ absorption spectra of vanadium in vanadium dichalcogenides and in their first-row transition-metal intercalates ${M}_{\frac{1}{3}}\mathrm{V}{X}_{2}$ ($M=\mathrm{V},\mathrm{ }\mathrm{C}\mathrm{r},\mathrm{ }\mathrm{M}\mathrm{n},\mathrm{ }\mathrm{F}\mathrm{e},\mathrm{ }\mathrm{C}\mathrm{o},\mathrm{ }\mathrm{Ni}$; $X=\mathrm{S},\mathrm{ }\mathrm{Se}$) have been measured together with those in V, ${\mathrm{V}}_{2}$${\mathrm{O}}_{5}$, and VSe, using a two-crystal spectrometer. The spectra of vanadium dichalcogenides and their first-row transition-metal intercalates have strong resemblance to each other and exhibit a weak flat structure followed by strong absorption. They reflect the transitions to the unoccupied vanadium $d$ band with some $p$ character and to the unoccupied vanadium $4s\ensuremath{-}4p$ bands, respectively. The midpoint of the initial rise of absorption is shifted to higher energies by intercalating the first-row transition metals. This shift is explained in terms of the charge-transfer model. It is, consequently, concluded that the rigid-band-model approach is valid as a first approximation at least for the unoccupied levels of these intercalation compounds.