Two-dimensional NbSe2 Research Articles

Scalable Production of Highly Conductive 2D NbSe2 Monolayers with Superior Electromagnetic Interference Shielding Performance.

Thin, flexible, and electrically conductive films are in demand for electromagnetic interference (EMI) shielding. Two-dimensional NbSe2 monolayers have an electrical conductivity comparable to those of metals (106-107 S m-1) but are challenging for high-quality and scalable production. Here, we show that electrochemical exfoliation of flake NbSe2 powder produces monolayers on a large scale (tens of grams), at a high yield (>75%, monolayer), and with a large average lateral size (>20 μm). The as-exfoliated NbSe2 monolayer flakes are easily dispersed in diverse organic solvents and solution-processed into various macroscopic structures (e.g., free-standing films, coatings, patterns, etc.). Thermal annealing of the free-standing NbSe2 films reduces the interlayer distance of restacked NbSe2 from 1.18 to 0.65 nm and consequently enhances the electrical conductivity to 1.16 × 106 S m-1, which is superior to those of MXenes and reduced graphene oxide. The optimized NbSe2 film shows an EMI shielding effectiveness (SE) of 65 dB at a thickness of 5 μm (>110 dB for a 48-μm-thick film), among the highest in materials of similar thicknesses. Moreover, a laminate of two layers of the NbSe2 film (2 μm thick) with an insulating interlayer shows a high SE of 85 dB, surpassing that of the 20-μm-thick NbSe2 film (83 dB). A two-layer theoretical model is proposed, and it agrees with the experimental EMI SE of the laminated NbSe2 films. The ability to produce NbSe2 monolayers on a tens of grams scale will enable their diverse applications beyond EMI shielding.

Controllable growth of two-dimensional NbSe2 flakes with irregular geometries under ion etching

Nanostructured NbSe2 has gained enormous interest in recent decades owning to its exotic properties and potential applications. Although two-dimensional (2D) NbSe2 structures have been successfully synthesized on various substrates, it remains a great challenge to prepare NbSe2 nanostructures with high edge-to-surface ratio, which is highly desirable for potential applications in catalysis. Here, we demonstrate that 2D NbSe2 with irregular geometries can be controllably grown on highly oriented pyrolytic graphite substrates via tuning the subtle balance between growth and ion etching. Atomic force microscopy, transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy were employed to characterize the morphology and composition of the as-grown NbSe2 flakes. Our work highlights the important role of ion etching and paves the way for the growth of 2D materials with various geometries.

Tribological characteristics of atomic-scale niobium diselenide grown via chemical vapor deposition

The nanoscale tribological characteristics of atomic-layered niobium diselenide (NbSe2) were investigated using atomic force microscopy (AFM). Two-dimensional NbSe2 atomic layers were produced on a weakly-adherent silicon wafer using two different methods: (1) mechanical exfoliation and (2) controlled synthesis via chemical vapor deposition. Using an AFM cantilever tip, a normal force was applied on the NbSe2 atomic-scale thin films in order to measure friction force. The as-synthesized NbSe2 thin films exhibited atomic lattice stick-slip friction, with the thinnest sheets showing a sliding length-dependent increase in static friction. This tendency is attributed to the increased susceptibility of the thinner chemically-synthesized NbSe2 sheets toward out-of-plane elastic deformation.

Metal to semiconductor transition of two-dimensional NbSe2 through hydrogen adsorption: A first-principles study

Based on first-principles calculations, we predict that the recently synthesized two-dimensional (2D) NbSe2 can be changed from the metallic to the semiconducting phase upon the adsorption of H with an indirect bandgap of 2.99 eV. The bandgap opening of the 2D NbSe2 only occurs when the hydrogen coverage is high, and it is sensitive to mechanical strain. The hydrogenated 2D NbSe2 is dynamically stable under a tensile strain of up to 9%, whereas a compressive strain leads to instability of the system. The optical spectra obtained from the GW-Bethe–Salpeter equation calculations suggest that 2D NbSe2 is highly isotropic, and it will not affect the polarization of light along the x- or y-direction. The optical bandgap, describing the transition energy of the exciton, is sensitive to the mechanical strain with the calculated exciton binding energy of ∼0.42 eV. These intriguing properties suggest that H functionalized 2D NbSe2, grown on a substrate with a larger lattice parameter, can be used to modulate the bandgap of NbSe2. This is beneficial in developing a nanoscale field effect and optoelectronic devices.

Restoring Superconductivity in the Quantum Metal Phase of NbSe2 Using Dissipative Coupling.

Localization arguments forbid the appearance of a metallic ground state in two dimensions. Yet, a large variety of disordered superconductors are known to manifest an anomalous metal phase in the zero temperature limit. While previous observations were confined to noncrystalline "dirty" superconductors, the recent observation of the so-called Bose metal phase in crystalline thin flakes of NbSe2 has sparked off intense debate. While the exact nature of this phase remains unknown, it is thought that quantum fluctuations play a decisive role in Bose metal physics. In this work, we study the response of the anomalous metal phase in thin flakes of NbSe2 to dissipative coupling. We evince a dramatic quenching of the Bose metal phase when dissipative coupling is strong, fully restoring a zero resistance superconducting state in the entire region of the magnetic field (H)-temperature (T) phase diagram where the Bose metal phase is otherwise observed. The suppression of the Bose metal phase by dissipative coupling is possible only in a quantum system where dissipation can directly affect system thermodynamics. Our observation of a dissipative phase transition in two-dimensional NbSe2 firmly establishes the quantum nature of the anomalous metal phase in this class of "clean" superconductors.

Unveiling Charge-Density Wave, Superconductivity, and Their Competitive Nature in Two-Dimensional NbSe2.

Recently, charge-density wave (CDW) and superconductivity are observed to coexist in atomically thin metallic NbSe2. Lacking of knowledge on the structural details of CDW, however, prevents us to explore its interplay with superconductivity. Using first-principles calculations, we identify the ground state 3 × 3 CDW atomic structure of monolayer NbSe2, which is characterized by the formation of triangular Nb clusters and shows a scanning tunnelling microscopy (STM) image and Raman CDW modes in good agreement with experiments. We further demonstrate that from bulk to monolayer NbSe2, as the layer thickness decreases, the CDW order is gradually enhanced with rising energy gain and strengthened Fermi surface gapping, while superconductivity is weakened due to the increasingly reduced Fermi level density of states in the CDW state. These results well explain the observed opposite thickness dependencies of CDW and superconducting transition temperatures and uncover the nature of competitive interaction between the two collective orders in two-dimensional NbSe2.

Surface chemical-modification of inorganic two-dimensional nanomaterials for engineering the intrinsic magnetic properties and related applications

Recently, due to the novel spin structures and unique magnetic states, the magnetic properties of two-dimensional (2D) nanomaterials, especially monolayer or few-layer inorganic ultrathin nanosheets, exhibit great promise for the next-generation spintronics. As is known, practical application usually requires materials with magnetic properties which can be deliberately regulated by external control parameters, such as, applied electric fields, elastic stress or light. However, most of the 2D inorganic nanomaterials are intrinsically non-magnetic. To tackle this challenge, tremendous efforts have been dedicated to effectively tuning the magnetic properties of two-dimensional nanomaterials, such as, transitional metal atoms doping and point defects, which provide feasible ways of magnetic modulation to induce magnetism. Unfortunately, compared with the high demands of practical magnetic application, there is still way to inducing magnetism in two-dimensional nanomaterials in a precise and controlled manner. In this regard, how to effectively obtain magnetic two-nanomaterials remains a great but noteworthy challenge. Atomic-scale-thickness 2D nanomaterials possess extremely large specific surface area, making their surface phase as important as bulk counterparts. Compared to bulk counterparts, the ultrathin nanosheets with outmost atom layers exposed are more vulnerable to the surface modification, which has been theoretically and experimentally confirmed. Therefore, surface chemical modification of inorganic 2D nanomaterials offers a novel way to engineer intrinsic magnetic properties of 2D nanomaterials. Furthermore, the exposed high specific surface area and quantum confinement of electrons in the two-dimensional nanomaterials endow the surface chemical modification strategies to regulate the intrinsic magnetic properties in a controllable and precise manner. In this review, we focus on recent research about regulation strategies of magnetic behaviors, especially concerning surface chemical modification strategies, which bring about novel magnetic inorganic two-dimensional materials. For instance, introducing hydrogen dangling bonds into graphitic carbon nitride nanosheets successfully inducing ferromagnetism; surface adsorption of hydrazine molecules onto two-dimensional NbSe2 nanosheets triggering the distortion of Nb-Se covalent bond, which results in the spin polarization; surface molecules meditating synthesis of ultrathin VSe2 nanosheets which display ferromagnetism and charge density wave. Benefiting from regulated magnetic properties, inorganic two-dimensional nanomaterials deliver promising magnetic energy conversion and magnetic response applications, which ranges from the giant magnetoresistance, magnetocaloric effect to efficiently hydrogen evolution. Surface-chemical modification of two-dimensional nanomaterials has seen the advance for regulating their intrinsic magnetic properties.

Defect-induced magnetism in two-dimensional NbSe2

The energetics and magnetism of perfect and vacancy-containing two-dimensional NbSe2 monolayers is investigated by first-principle calculations. It has been found the single Se vacancy has the lowest formation energy. Perfect NbSe2 monolayers, as well as monolayers containing single-selenium and double-selenium vacancies, are nonmagnetic metallic. However, Nb vacancies create a magnetic moment of 1.5 μB per vacancy. The moment is highly localized, confined nearly exclusively on the Se atoms adjacent to the vacancy, and mainly originates from the Se 4p electrons. The moment distribution indicates strongly anisotropic exchange bonds between vacancy moments.

Large-scale synthesis of NbSe 2 nanosheets and their use as nanofillers for improving the tribological properties of epoxy coatings

Abstract The paper presents a facile and effective method to exfoliate bulk niobium diselenide (NbSe2) into nanosheets which were used as nanofillers to improve the tribological properties of epoxy coatings. X-ray diffraction (XRD) analysis indicates that the hexagonal crystal structure of NbSe2 nanosheet is preserved after ball milling. Transmission electron microscopy (TEM) and atomic force microscope (AFM) observations show that NbSe2 nanosheets were produced with sizes in hundreds of square nanometers and graphene-like structure. The tribological properties of NbSe2/epoxy composite coatings were systematically investigated and results show that they possess better friction-reducing and anti-wear performances due to the addition of NbSe2 nanosheets. It is demonstrated that the superior tribological properties is ascribed to a transfer film formed by NbSe2 nanosheets preventing a direct contact between steel ball and coating during the friction process. Furthermore, this work shows the potential applications of two-dimensional NbSe2 nanosheets as reinforcing nanofillers in polymer matrix.

Electrostatic and electrochemical tuning of superconductivity in two-dimensional NbSe2 crystals

We report modulation of the superconducting critical temperature (Tc) of ultrathin niobium diselenide (NbSe2) single crystals by gating an electric double-layer transistor. We realized reversible and irreversible changes of the Tc by adjusting the operating range of the voltage. The reversible and irreversible responses correspond to the electrostatic carrier doping and the electrochemical etching of the crystal, respectively. The results suggest that electric double-layer gating provides opportunities to control and functionalize collective electronic phenomena in two-dimensional crystals.

Signature of coexistence of superconductivity and ferromagnetism in two-dimensional NbSe2 triggered by surface molecular adsorption.

Ferromagnetism is usually deemed incompatible with superconductivity. Consequently, the coexistence of superconductivity and ferromagnetism is usually observed only in elegantly designed multi-ingredient structures in which the two competing electronic states originate from separate structural components. Here we report the use of surface molecular adsorption to induce ferromagnetism in two-dimensional superconducting NbSe2, representing the freestanding case of the coexistence of superconductivity and ferromagnetism in one two-dimensional nanomaterial. Surface-structural modulation of the ultrathin superconducting NbSe2 by polar reductive hydrazine molecules triggers a slight elongation of the covalent Nb–Se bond, which weakens the covalent interaction and enhances the ionicity of the tetravalent Nb with unpaired electrons, yielding ferromagnetic ordering. The induced ferromagnetic momentum couples with conduction electrons generating unique correlated effects of intrinsic negative magnetoresistance and the Kondo effect. We anticipate that the surface molecular adsorption will be a powerful tool to regulate spin ordering in the two-dimensional paradigm.

Strongly enhanced charge-density-wave order in monolayer NbSe2.

Two-dimensional materials possess very different properties from their bulk counterparts. While changes in single-particle electronic properties have been investigated extensively, modifications in the many-body collective phenomena in the exact two-dimensional limit remain relatively unexplored. Here, we report a combined optical and electrical transport study on the many-body collective-order phase diagram of NbSe2 down to a thickness of one monolayer. Both the charge density wave and the superconducting phase have been observed down to the monolayer limit. The superconducting transition temperature decreases on lowering the layer thickness, but the newly observed charge-density-wave transition temperature increases from 33 K in the bulk to 145 K in the monolayer. Such highly unusual enhancement of charge density waves in atomically thin samples can be understood to be a result of significantly enhanced electron-phonon interactions in two-dimensional NbSe2 (ref.4) and is supported by the large blueshift of the collective amplitude vibration observed in our experiment. Our results open up a new window for search and control of collective phases of two-dimensional matter, as well as expanding the functionalities of these materials for electronic applications.

Solution‐Processed 2D Niobium Diselenide Nanosheets as Efficient Hole‐Transport Layers in Organic Solar Cells

Thin-layer, two-dimensional NbSe2 nanosheets with lower trap density have been obtained and act as an alternative hole-transporting layer to replace MoO3 in organic solar cells. If poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}):[6,6]-phenyl-C71-butyric acid methyl ester acts as an active layer, a power conversion efficiency of 8.10 % has been achieved without any further thermal treatment. The properties of this hole-transporting layer were investigated and the improvements in the devices are discussed.

Superconductivity in two-dimensional NbSe2 field effect transistors

We describe investigations of superconductivity in few molecular layer NbSe2 field effect transistors. While devices fabricated from NbSe2 flakes less than eight molecular layers thick did not conduct, thicker flakes were superconducting with an onset Tc that was only slightly depressed from the bulk value for 2H-NbSe2 (7.2 K). The resistance typically showed a small, sharp high temperature transition followed by one or more broader transitions which usually ended in a wide tail to zero resistance at low temperatures. We speculate that these multiple resistive transitions are related to disorder in the layer stacking. The behavior of several flakes has been characterized as a function of temperature, applied field and back-gate voltage. We find that the conductance in the normal state and transition temperature depend weakly on the gate voltage, with both conductivity and Tc decreasing as the electron concentration is increased. The application of a perpendicular magnetic field allows the evolution of different resistive transitions to be tracked and values of the zero temperature upper critical field, Hc2(0), and coherence length, ξ(0), to be independently estimated. Our results are analyzed in terms of available theories for these phenomena.