Monolayer VSe2 Research Articles

Coexisting Charge-Ordered States with Distinct Driving Mechanisms in Monolayer VSe2.

Thinning crystalline materials to two dimensions (2D) creates a rich playground for electronic phases, including charge, spin, superconducting, and topological order. Bulk materials hosting charge density waves (CDWs), when reduced to ultrathin films, have shown CDW enhancement and tunability. However, charge order confined to only 2D remains elusive. Here we report a distinct charge ordered state emerging in the monolayer limit of 1T-VSe2. Systematic scanning tunneling microscopy experiments reveal that bilayer VSe2 largely retains the bulk electronic structure, hosting a tridirectional CDW. However, monolayer VSe2 ─consistently across distinct substrates─exhibits a dimensional crossover, hosting two CDWs with distinct wavelengths and transition temperatures. Electronic structure calculations reveal that while one CDW is bulk-like and arises from the well-known Peierls mechanism, the other is decidedly unconventional. The observed CDW-lattice decoupling and the emergence of a flat band suggest that the second CDW could arise from enhanced electron-electron interactions in the 2D limit. These findings establish monolayer-VSe2 as a host of coexisting charge orders with distinct origins, and enable the tailoring of electronic phenomena via emergent interactions in 2D materials.

Single transition metal atom anchored on VSe2 as electrocatalyst for nitrogen reduction reaction

There has been an increasing interest in converting nitrogen (N2) to ammonia (NH3) via electrochemical nitrogen reduction reaction (NRR). In this study, the NRR catalytic performance of a series of single transition metal (TM) atoms anchored on two dimensional VSe2 (TM-VSe2) was studied via performing the first-principles calculations. Among the investigated single-atom catalyst of TM-VSe2, W-VSe2 demonstrates the best NRR catalytic performance with a limiting potential of −0.36 V and effective inhibition of the competitive hydrogen evolution reaction. Meanwhile, our calculations demonstrate that W-VSe2 is highly stable as single-atom anchored on the surface of VSe2 monolayer, whose improved electrical conductivity ensures the electron transform during NRR process. Moreover, we found that the limiting potential of NRR on TM-VSe2 was negatively correlated with the adsorption energy of reaction intermediate *N-NH, and related with the total magnetic moment of TM-VSe2. Importantly, we identified the origin of the NRR catalytic activity from electronic aspect. Our results provide a new way for measuring the NRR catalytic activity of single-atom catalysts.

Controllable phase transitions between multiple charge density waves in monolayer 1T-VSe2 via charge doping

Two-dimensional materials are known to possess emergent properties that are not found in their bulk counterparts. Recent experiments have shown a 7×3 charge density wave (CDW) in monolayer 1 T-VSe2, in contrast to the 4×4×3 phase in bulk. Here, via first-principles calculations, we show that multiple CDW phases compete in monolayer VSe2, the ground state of which can be tuned by charge doping. With doping, the 7×3 CDW of the pristine VSe2 transfers to a 4×4 or 3×3 phase, at critical doping concentrations of around 0.2 electrons per formula unit and 0.2 holes per formula unit, respectively. These transitions are further understood by analyzing the role of Fermi surface nesting and momentum-dependent electron–phonon coupling in monolayer 1T-VSe2. These results make VSe2 an appealing material for electronic devices based on controllable CDW phase transitions.

Topological Phase and Quantum Anomalous Hall Effect in Ferromagnetic Transition-Metal Dichalcogenides Monolayer 1T-VSe2.

Magnetic two-dimensional (2D) van der Waals materials have attracted tremendous attention because of their high potential in spintronics. In particular, the quantum anomalous Hall (QAH) effect in magnetic 2D layers shows a very promising prospect for hosting Majorana zero modes at the topologically protected edge states in proximity to superconductors. However, the QAH effect has not yet been experimentally realized in monolayer systems to date. In this work, we study the electronic structures and topological properties of the 2D ferromagnetic transition-metal dichalcogenides (TMD) monolayer by first-principles calculations with the Heyd–Scuseria–Ernzerhof (HSE) functional. We find that the spin-orbit coupling (SOC) opens a continuous band gap at the magnetic Weyl-like crossing point hosting the quantum anomalous Hall effect with Chern number . Moreover, we demonstrate the topologically protected edge states and intrinsic (spin) Hall conductivity in this magnetic 2D TMD system. Our results indicate that monolayer serves as a stoichiometric quantum anomalous Hall material.

Multiple charge density wave phases of monolayer VSe2 manifested by graphene substrates

A combined study of scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) is conducted to understand the multiple charge density wave (CDW) phases of monolayer (ML) VSe2 films manifested by graphene substrates. Submonolayer (∼0.8 ML) VSe2 films are prepared on two different substrates of single-layer graphene (SLG) and bi-layer graphene (BLG) on a 6H-SiC(0001). We find that ML VSe2 films are less coupled to the SLG substrate compared to that of ML VSe2/BLG. Then, ML VSe2 grown on SLG and BLG substrates reveals a very different topography in STM. While ML VSe2/BLG shows one unidirectional modulation of √3 × 2 and √3 × √7 CDW in topography, ML VSe2/SLG presents a clear modulation of 4 × 1 CDW interfering with √3 × 2 and √3 × √7 CDW which has not been previously observed. We explicitly show that the reciprocal vector of 4 × 1 CDW fits perfectly into the long parallel sections of cigar-shaped Fermi surfaces near the M point in ML VSe2, satisfying Fermi surface nesting. Since bulk VSe2 is also well-known for the 4 × 4 × 3 CDW formed by Fermi surface nesting, the 4 × 1 CDW in ML VSe2/SLG is attributed to the planar projection of 4 × 4 × 3 CDW in bulk. Our result clarifies the nature of the 4 × 1 CDW in ML VSe2 system and is a good example demonstrating the essential role of substrates in two-dimensional transition metal dichalcogenides.

Two‐dimensional magnetic transition metal chalcogenides

AbstractThe field of two‐dimensional (2D) magnets has expanded rapidly during the past several years since the first demonstration of intrinsic 2D magnetism in atomically thin CrI3 and Cr2Ge2Te6 in 2017. 2D transition metal chalcogenides (TMCs), a class of strongly correlated materials, have exhibited a wide variety of novel electronic and optical properties, and more recently magnetism. Here, we review recent experimental progress achieved in the growth of 2D magnetic TMC materials using chemical vapor deposition and molecular beam epitaxy methods. Outstanding examples include the demonstration of room temperature intrinsic and extrinsic ferromagnetism in monolayer VSe2, MnSe2, Cr3Te4, V‐doped WSe2, and so on. A brief discussion on the origin of the exotic magnetic properties and emergent phenomena is also presented. Finally, we summarize the remaining challenges and future perspective on the development of 2D magnetic materials for next‐generation spintronic devices.

Role of charge doping and strain in the stabilization of in-plane ferromagnetism in monolayer at room temperature

We study the origin of in-plane ferromagnetism in monolayer VSe2 focusing on the effect of charge doping and mechanical strain. We start from an anisotropic spin Hamiltonian, estimate its parameters from density functional calculations, and determine the spectrum of spin-wave excitations. We show that 1T-VSe2 is characterized by relatively strong on-site Coulomb repulsion (U ≃ 5 eV), favoring an antiferromagnetic ground state, which contradicts experimental observations. We calculate the magnetic phase diagram as a function of charge doping and strain, and find a transition to the ferromagnetic state with in-plane easy axis under moderate hole doping (∼1014 cm−2). Analysis of spin-wave excitations in doped monolayer VSe2 reveals a gap due to the in-plane anisotropy, giving rise to long-range magnetic order well above 300 K, in agreement with recent experiments. Our findings suggest that experimentally available 1T-VSe2 monolayer samples might be intrinsically or extrinsically doped, which opens up the possibility for a controllable manipulation of their magnetic properties.

Electronic and magnetic characterization of epitaxial VSe2 monolayers on superconducting NbSe2

There has been enormous recent interest in heterostructures of two-dimensional van der Waals materials. Integrating materials with different quantum ground states in vertical heterostructures is predicted to lead to novel electronic properties that are not found in the constituent layers. Here, we present direct synthesis of a superconductor-magnet hybrid heterostructure by combining superconducting niobium diselenide (NbSe2) with the monolayer vanadium diselenide (VSe2). Molecular-beam epitaxy growth in ultra-high vacuum yields clean and atomically sharp interfaces. Combining different characterization techniques and density-functional theory calculations, we investigate the electronic and magnetic properties of VSe2 on NbSe2. Low temperature scanning tunneling microscopy measurements show an absence of the typical charge density wave on VSe2 and demonstrate a reduction of the superconducting gap of NbSe2 on the VSe2 layer. This suggests magnetization of the VSe2 sheet, at least on the local scale. Our work demonstrates superconducting-magnetic hybrid materials with potential applications in future electronics devices.

Dynamical mean-field study of Vanadium diselenide monolayer ferromagnetism

To understand the magnetism of VSe2, whose monolayer form has recently been reported to be a room temperature ferromagnet, here, the combined method of conventional density functional theory with dynamical mean-field theory has been adopted. This higher-level computation method enables us to resolve many of existing controversies and contradictions in between theory and experiment. First of all, this new approach is shown to give the correct magnetic properties of both bulk and two-dimensional limit of VSe2 which demonstrates its superiority to the conventional methods. The results demonstrate that monolayer VSe2 without charge density waves is a ferromagnet with ordering temperature of 250 K. From the direct simulation of temperature-dependent magnetic susceptibility and ordered moment, it is shown that its ferromagnetism is clearly two-dimensional in nature. Further, it is shown that this ferromagnetic order is vulnerable to extra charge dopings which provides the important insight to elucidate recent experimental controversies.

Can Reconstructed Se-Deficient Line Defects in Monolayer VSe2 Induce Magnetism?

There have been several recent conflicting reports on the ferromagnetism of clean monolayer VSe2 . Herein, the controllable formation of 1D defect line patterns in vanadium diselenide (VSe2 ) monolayers initiated by thermal annealing is presented. Using scanning tunneling microscopy and q-plus atomic force microscopy techniques, the 1D line features are determined to be 8-member-ring arrays, formed via a Se deficient reconstruction process. The reconstructed VSe2 monolayer with Se-deficient line defects displays room-temperature ferromagnetism under X-ray magnetic circular dichroism and magnetic force microscopy, consistent with the density functional theory calculations. This study possibly resolves the controversy on whether ferromagnetism is intrinsic in monolayer VSe2 , and highlights the importance of controlling and understanding the atomic structures of surface defects in 2D crystals, which could play key roles in the material properties and hence potential device applications.

√3 × 2 and √3 × √7 Charge Density Wave Driven by Lattice Distortion in Monolayer VSe2

A charge density wave (CDW), an ordered modulation of electron distribution and lattice distortion, is one of the intriguing phenomena observed in transition metal dichalcogenides. Recent STM studies reported a new CDW phase with √3 × 2 and √3 × √7 periodicities in monolayer (ML) VSe2 grown on graphene, which is totally different from the 4 x 4 x 3 CDW periodicity in bulk. Although the emergence of new CDW phase is of great research interest, the origin of the new modulation in ML VSe2 has not been clearly investigated. In this report, we conduct a systematic study to understand the nature of the √3 × 2 and √3 × √7 CDW in ML VSe2 using scanning tunneling microscopy (STM). Bias dependent topography and differential conductance (dI/dV) mapping indicate that the √3 × 2 and √3 × √7 modulations are mostly driven by strong lattice distortions of the Se atoms rather than by charge orderings. In addition, STM topography reveals that the v3 × 2 modulation corresponds to a gap feature, and the v3 × v7 modulation corresponds to an isolated Se atom in the distorted lattice structures. Our work provides prerequisite information to understand the emergence of √3, × 2 and √3, × √7 CDW in ML VSe2.

Spintronic Transport Performances of VSe2 Nanoribbons

The spin-resolved electronic transport behaviors of VSe2 nanoribbons are theoretically investigated to explore their applications in spintronic devices, employing by the first-principles calculations combined with nonequilibrium Green's function scheme. The band structure, current varying curves with bias voltage between nanoribbon terminal electrodes and electronic transmission spectra of two representative VSe2 nanoribbons along zigzag and armchair crystallographic orientations are calculated respectively. The evident negative differential resistance under the bias voltage of ∼0.6 V for the armchair orientation has been found, implying the significant crystal anisotropy of VSe2 monolayer. Meanwhile, both the VSe2 nanoribbon devices along zigzag and armchair directions represent favorable spin filtering effect, suggesting they are prospective candidates as a multifunction material in spintronic and digital applications.

Unveiling the origin of room-temperature ferromagnetism in monolayer VSe2: the role of extrinsic effects.

Room-temperature ferromagnetism in monolayer vanadium diselenide (VSe2) on graphite is the object of a controversial debate. Herein, we unveil the contribution from extrinsic factors to the magnetic properties of monolayer VSe2 by means of density functional theory. Specifically, we demonstrate that either intrinsic defects or the adsorption of molecules enhances ferromagnetic interactions. The expansion of the VSe2 lattice increases the magnetic moment on vanadium ions, whereas both compression and out-of-plane distortion withdraw magnetic moments. The exchange interactions between vanadium ions and magnetic defects (vacancies and impurities) in the surface and subsurface layers of the substrate are able to turn the unstable two-dimensional (2D) ferromagnetism into stable three-dimensional (3D) ferromagnetism. Definitely, the combination of effects related to chemisorption, substrate-induced distortion and magnetic defects of the substrate could enhance or suppress ferromagnetism in monolayer VSe2.

Bi-stable electronic states of cobalt phthalocyanine molecules on two-dimensional vanadium diselenide

Hybrid organic/2D interfaces combine the wide spectrum of 2D material properties with the major advantages of organic materials, such as low cost, mechanical flexibility, and chemical tunability. Here, we report the electronic properties of cobalt phthalocyanine (CoPc) molecules adsorbed on molecular beam epitaxy-grown monolayer vanadium diselenide (VSe2). Using scanning tunneling microscopy/spectroscopy, we provide evidence of highly ordered molecular assembly on monolayer VSe2, with two distinctive bright and regular molecular contrasts, which are not observed on graphite. These contrasts also lead to a distinct difference in the electronic state of the molecule’s central Co atom, for which density functional theory calculations indicate the regular state as the ground state. A correlation between these two molecular states and the charged states of individual molecules is postulated, as demonstrated by the possibility of switching the bright state to the regular state using a negative tip voltage pulse.

First principle study of temperature-dependent spin transport in VSe2 monolayer

An applicable use of a linear combination of atomic orbitals (LCAO) based density functional theory (DFT) together with non-equilibrium Green’s function (NEGF) is performed for investigating the dependence of temperature on spin-resolved transport characteristics of electron in magnetic Vanadium Diselenide (VSe2) monolayer. To know the dependence of heat (temperature) on spin Seebeck effect and spin filtration, important factors like transmission spectrum and spin-resolved current are calculated. Approximately 100% spin injection efficiency is attained in the low-temperature gradient. Low temperature (TL) of cold (left) electrode with respect to high temperature (TR) of hot (right) electrode leads to a smaller spin injection efficiency. Giant thermal magnetoresistance around 3.936 × 103% is achieved and this giant thermal magnetoresistance is directly proportional to the difference between the temperature of right and left electrode (ΔT). This resultant thermal magnetoresistance is also dependent on TL. A large value of temperature gradient based magnetoresistance (MR) and magnificent temperature gradient based spin filtration have been achieved for VSe2 monolayer advocates the useful application of this compound in the area of spin caloritronic.

Designed Synthesis and Structure–Property Relationships of Kinetically Stable [(PbSe)1+δ]m(VSe2)1 (m = 1, 2, 3, 4) Heterostructures

Understanding structure–function relationships is essential to guide the designed synthesis of novel materials with emergent properties. In this work, we targeted the metastable heterostructures [(PbSe)1+δ]m(VSe2)1, where m = 1–4, to test if the charge density wave (CDW) transition temperature increases as the layer thickness separating the VSe2 monolayers increases, as was observed when SnSe was the separating layer. The modulated elemental reactant approach was used to make the targeted products. This approach involves depositing elemental layers in which the number of atoms of each element per square angstrom in Pb|Se and V|Se bilayers equals the number calculated for a rock salt-structured PbSe bilayer and a CdI2-structured VSe2 slab, respectively. Layered elemental precursors with the correct composition and nanoarchitecture for each of the targeted compounds were prepared by repeatedly depositing a single V|Se bilayer followed by m Pb|Se bilayers. Precursors close to the targeted number of atoms per unit area were determined via X-ray fluorescence and the correct nanoarchitecture self-assembled to the targeted compounds during a low-temperature anneal. Resistivity measurements show that the number of PbSe layers per repeat unit (m) does not change the charge density transition onset temperature as previously reported for the analogous [(SnSe)1+δ]m(VSe2)1 compounds. The temperature dependence and absolute values of the resistivity of the m = 3 and 4 heterostructures scale as expected for composite behavior. The difference in the thickness dependence of the CDW transition between the PbSe- and SnSe-containing compounds highlights that the identity of the intervening rock salt layer plays a more important role in modifying the CDW onset temperature than the separation of the VSe2 layers.

Chemically Exfoliated VSe2 Monolayers with Room-Temperature Ferromagnetism.

Among van der Waals layered ferromagnets, monolayer vanadium diselenide (VSe2 ) stands out due to its robust ferromagnetism. However, the exfoliation of monolayer VSe2 is challenging, not least because the monolayer flake is extremely unstable in air. Using an electrochemical exfoliation approach with organic cations as the intercalants, monolayer 1T-VSe2 flakes are successfully obtained from the bulk crystal at high yield. Thiol molecules are further introduced onto the VSe2 surface to passivate the exfoliated flakes, which improves the air stability of the flakes for subsequent characterizations. Room-temperature ferromagnetism is confirmed on the exfoliated 2D VSe2 flakes using a superconducting quantum interference device (SQUID), X-ray magnetic circular dichroism (XMCD), and magnetic force microscopy (MFM), where the monolayer flake displays the strongest ferromagnetic properties. Se vacancies, which can be ubiquitous in such materials, also contribute to the ferromagnetism of VSe2 , although density functional theory (DFT) calculations show that such effect can be minimized by physisorbed oxygen molecules or covalently bound thiol molecules.

Strain-enhanced electron mobility and mobility anisotropy in a two-dimensional vanadium diselenide monolayer

This paper investigates the impacts of compressive and tensile uniaxial strains on the lattice and electronic properties of the two-dimensional (2H-phase) vanadium diselenide (VSe2) monolayer, which hitherto has not yet been synthesized, by using first-principles density functional theory and deformation potential theory. Under equilibrium, it is found that the mobility in the “zigzag” direction (μzig≈307cm2/Vs) is about 3.20 times larger than the mobility in the “armchair” direction (μarm≈96cm2/Vs). It is revealed that by applying 1%, 2%, and 3% compressive uniaxial strains along the “zigzag” direction, μzig can be enhanced by 2.20, 5.25, and 10.55 times, respectively. The strain dependence of the bandgap, effective mass, elastic modulus, and deformation potential constant are analyzed quantitatively, in order to better understand the underlying mechanism. The strain-enhanced electron mobility and mobility anisotropy properties of the emerging two-dimensional VSe2 monolayer might be useful to implement novel devices and sensors.

Magnetic Transition in Monolayer VSe2 via Interface Hybridization.

Magnetism in monolayer (ML) VSe2 has attracted broad interest in spintronics, while existing reports have not reached consensus. Using element-specific X-ray magnetic circular dichroism, a magnetic transition in ML VSe2 has been demonstrated at the contamination-free interface between Co and VSe2. Through interfacial hybridization with a Co atomic overlayer, a magnetic moment of about 0.4 μB per V atom in ML VSe2 is revealed, approaching values predicted by previous theoretical calculations. Promotion of the ferromagnetism in ML VSe2 is accompanied by its antiferromagnetic coupling to Co and a reduction in the spin moment of Co. In comparison to the absence of this interface-induced ferromagnetism at the Fe/ML MoSe2 interface, these findings at the Co/ML VSe2 interface provide clear proof that the ML VSe2, initially with magnetic disorder, is on the verge of magnetic transition.

Monolayer VTe2 : Incommensurate Fermi surface nesting and suppression of charge density waves

We investigated the electronic structure of monolayer VTe2 grown on bilayer graphene by angle-resolved photoemission spectroscopy (ARPES). We found that monolayer VTe2 takes the octahedral 1T structure in contrast to the monoclinic one in the bulk, as evidenced by the good agreement in the Fermi-surface topology between ARPES results and first-principles band calculations for octahedral monolayer 1T-VTe2. We have revealed that monolayer 1T-VTe2 at low temperature is characterized by a metallic state whereas the nesting condition is better than that of isostructural monolayer VSe2 which undergoes a CDW transition to insulator at low temperature. The present result suggests an importance of Fermi-surface topology for characterizing the CDW properties of monolayer TMDs.