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Besides developing a large number of catalysts for hydrogen evolution reaction (HER) in alkaline electrolytes, its conversion efficiency remained low. Herein, we have developed mixed-dimensional heterostructures of niobium disulfide (NbS 2) with graphene foam grown on nickel foam (NbS 2 -Gr-NF). The strong lateral fusion results in activating the catalytic sites of NbS 2 , the three-dimensional substrate provides easy access of electrolyte to active sites and increased electrochemically active surface area, while enhanced conductivity provides faster transfer of electrons to and from active sites. Therefore, NbS 2 -Gr-NF heterostructures resulted in an exceptionally high current density of 500 mA cm−2 at a very low overpotential of 306 mV in 1 M KOH solution and even can achieve the current density values of 914 mAcm−2 at 338 mV only at a slight increase in overpotential (32 mV). Moreover, a Tafel value of ~72 mV dec−1 confirms that as-developed heterostructure provides fast reaction kinetics where the reaction is mainly controlled by the Volmer step. Achieving such high current density at a faster rate with high stability makes NbS 2 -Gr-NF heterostructures a potential candidate for water-splitting, especially in alkaline electrolytes. [Display omitted] • Synthesis of heterostructures of NbS 2 -graphene as a working electrocatalyst. • An exceptionally high current density ~500 mAcm−2 at a low overpotential ~306 mV. • A current density 914 mAcm−2 at 338 mV only at a slight increase in overpotential. [ABSTRACT FROM AUTHOR]

The scalable and conformal synthesis of two-dimensional (2D) transition metal dichalcogenide (TMDC) heterostructures is a persisting challenge for their implementation in next-generation devices. In this work, we report the synthesis of nanometer-thick 2D TMDC heterostructures consisting of TiSx-NbSx on both planar and 3D structures using atomic layer deposition (ALD) at low temperatures (200–300 °C). To this end, a process was developed for the growth of 2D NbSx by thermal ALD using (tert-butylimido)-tris-(diethylamino)-niobium (TBTDEN) and H2S gas. This process complemented the TiSx thermal ALD process for the growth of 2D TiSx-NbSx heterostructures. Precise thickness control of the individual TMDC material layers was demonstrated by fabricating multilayer (5-layer) TiSx-NbSx heterostructures with independently varied layer thicknesses. The heterostructures were successfully deposited on large-area planar substrates as well as over a 3D nanowire array for demonstrating the scalability and conformality of the heterostructure growth process. The current study demonstrates the advantages of ALD for the scalable synthesis of 2D heterostructures conformally over a 3D substrate with precise thickness control of the individual material layers at low temperatures. This makes the application of 2D TMDC heterostructures for nanoelectronics promising in both BEOL and FEOL containing high-aspect-ratio 3D structures. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) heterostructures based on the combination of transition metal dichalcogenides (TMDs) and transition metal oxides (TMOs) have aroused growing attention due to their integrated merits of both components and multiple functionalities. However, nondestructive approaches of constructing TMD-TMO heterostructures are still very limited. Here, we develop a novel type of lateral TMD-TMO heterostructure (NbS2-Nb2O5-NbS2) using a simple lithography-free, direct laser-patterning technique. The perfect contact of an ultrathin TMO channel (Nb2O5) with two metallic TMDs (NbS2) electrodes guarantee strong electrical signals in a two-terminal sensor. Distinct from sensing mechanisms in separate TMOs or TMDs, this sensor works based on the modulation of surface conduction of the ultrathin TMO (Nb2O5) channel through an adsorbed layer of water molecules. The sensor thus exhibits high selectivity and ultrahigh sensitivity for room-temperature detection of NH3 (ΔR/R = 80% at 50 ppm), superior to the reported NH3 sensors based on 2D materials, and a positive temperature coefficient of resistance as high as 15%–20%/°C. Bending-invariant performance and high reliability are also demonstrated in flexible versions of sensors. Our work provides a new strategy of lithography-free processing of novel TMD-TMO heterostructures towards high-performance sensors, showing great potential in the applications of future portable and wearable electronics. [ABSTRACT FROM AUTHOR]

Using scanning tunneling microscopy and spectroscopy, for a monolayer of transition metal dichalcogenide H-NbS 2 grown by molecular beam epitaxy on graphene, we provide unambiguous evidence for a charge density wave (CDW) with a 3 × 3 superstructure, which is not present in bulk NbS 2 . Local spectroscopy displays a pronounced gap on the order of 20 meV at the Fermi level. Within the gap, low-energy features are present. The gap structure with its low-energy features is at variance with the expectation for a gap opening in the electronic band structure due to a CDW. Instead, comparison with ab initio calculations indicates that the observed gap structure must be attributed to combined electron-phonon quasiparticles. The phonons in question are the elusive amplitude and phase collective modes of the CDW transition. Our findings advance the understanding of CDW mechanisms in 2D materials and their spectroscopic signatures.

Atomically thin layers of group VB transition metal dichalcogenides (TMDs) provide a unique platform for studying two-dimensional (2D) superconductivity and charge density waves. Thus far, the bottom-up synthesis of these 2D TMDs has often involved precursors that are corrosive or toxic, and their lateral sizes are typically only a few micrometers. In this paper, we report the growth of NbS2 nanoflakes with a thickness down to bilayers and a lateral dimension up to tens of micrometers without using harsh chemical species. NbS2 nanoflakes either standing or lying with respect to the sapphire substrate were obtained by sulfurization of niobium oxide films that were prepared via pulsed laser deposition. Standing nanoflakes are considered to grow epitaxially on the sapphire substrate according to their ordered orientation, while lying nanoflakes with random orientations were grown directly on top of the niobium oxide films. The Raman spectra of the 3R-phase exhibit strong dependence on the layer thickness, where the A1 mode softens as the layer number decreases. In contrast to the stable bulk NbS2, the ultra-thin nanoflakes were oxidized on their top surfaces after prolonged exposure to air, as revealed by X-ray photoelectron spectroscopy. Our work explores an important route to synthesize large-size NbS2 nanoflakes and studies the oxidation process, which is a critical factor to consider if practical applications should be realized in the future. [ABSTRACT FROM AUTHOR]

Niobium disulfide is a kind of transition metal dichalcogenides which has the advantages of large modulation depth, short excited carrier recovery time and high carrier mobility, theoretically being very suitable for the applications in ultrafast photonics. Up to now, there are few reports on the application of NbS 2 in ultrafast optical devices, especially in the devices with long working wavelength, such as 2 µm. In this work, few-layer NbS 2 nanosheets were prepared by liquid exfoliation method and the NbS 2 -SA was prepared by covering the nanosheets on tapered fiber. The device NbS 2 -SA exhibits good nonlinear saturable absorption property with a modulation depth of 10.2 % at 1550 nm. Based on the NbS 2 -SA, both 1.5 µm and 2 µm ultrafast fiber lasers are realized. The 1.5 µm mode-locked fiber laser has a pulse width of 753 fs and the 2 µm mode-locked fiber laser has an estimated pulse width of 5.77 ps and a maximum output power of 5.56 mW. All the results prove that NbS 2 is an excellent broadband saturable absorption material. • Few-layer NbS 2 nanosheets are prepared by liquid phase exfoliation. • A NbS 2 nanosheets-based tapered fiber saturable absorber with large modulation depth is fabricated. • Mode-locked pulses are obtained in Er-doped and Tm-doped fiber lasers using the same NbS 2 saturable absorber. [ABSTRACT FROM AUTHOR]

Despite their high specific energy and great promise for next-generation energy storage, lithium-sulfur (Li-S) batteries suffer from polysulfide shuttling, slow redox kinetics, and poor cyclability. Catalysts are needed to accelerate polysulfide conversion and suppress the shuttling effect. However, a lack of structure-activity relationships hinders the rational development of efficient catalysts. Herein, we studied the Nb-V-S system and proposed a V-intercalated NbS 2 (Nb 3 VS 6 ) catalyst for high-efficiency Li-S batteries. Structural analysis and modeling revealed that undercoordinated sulfur anions of [VS 6 ] octahedra on the surface of Nb 3 VS 6 may break the catalytic inertness of the basal planes, which are usually the primary exposed surfaces of many 2D layered disulfides. Using Nb 3 VS 6 as the catalyst, the resultant Li-S batteries delivered high capacities of 1541 mAh g -1 at 0.1 C and 1037 mAh g -1 at 2 C and could retain 73.2% of the initial capacity after 1000 cycles. Such an intercalation-induced high activity offers an alternative approach to building better Li-S catalysts.

Here we report the facile synthesis of alkali metal intercalation compounds A x NbS 2 (A = Li, Na) through the reaction of bulk 3 R phase of NbS 2 with organometallic compounds in solution. Especially, the Na insertion compound of various compositions was obtained as a new polytype of A x NbS 2 compared with the 2 H phase reported previously. Upon intercalation the structure of the host was preserved, which was confirmed by XRD and HRTEM analysis. The interlayer distances for Li 0.44 NbS 2 and Na 0.87 NbS 2 were expanded by 0.187 Å and 0.938 Å as a result of Li and Na intercalation, respectively. The electrical transport properties were studied in the temperature range 4–298 K, exhibiting metallic conductivity and slight variation in the resistivity value in general. Interestingly, a minimum resistivity which was ascribed to the localized electrons in the crystal lattice emerged for all the investigated samples. [ABSTRACT FROM AUTHOR]

Highlights: [•] At 26GPa, NbS2 transits from the 2H structure to the novel structure. [•] The Nb and S atoms forms a new [NbS8] hexahedron unit in the I4/mmm-NbS2. [•] The -NbS2 exhibits a higher than 2H-NbS2. [•] The higher is resulted from the stronger electron–phonon coupling coefficients. [ABSTRACT FROM AUTHOR]

Highly efficient and durable flexible solid-state supercapacitors (FSSSCs) are emerging as low-cost devices for portable and wearable electronics due to the elimination of leakage of toxic/corrosive liquid electrolytes and their capability to withstand elevated mechanical stresses. Nevertheless, the spread of FSSSCs requires the development of durable and highly conductive solid-state electrolytes, whose electrochemical characteristics must be competitive with those of traditional liquid electrolytes. Here, we propose an innovative composite solid-state electrolyte prepared by incorporating metallic two-dimensional group-5 transition metal dichalcogenides, namely, liquid-phase exfoliated functionalized niobium disulfide (f-NbS 2 ) nanoflakes, into a sulfonated poly(ether ether ketone) (SPEEK) polymeric matrix. The terminal sulfonate groups in f-NbS 2 nanoflakes interact with the sulfonic acid groups of SPEEK by forming a robust hydrogen bonding network. Consequently, the composite solid-state electrolyte is mechanically/dimensionally stable even at a degree of sulfonation of SPEEK as high as 70.2%. At this degree of sulfonation, the mechanical strength is 38.3 MPa, and thanks to an efficient proton transport through the Grotthuss mechanism, the proton conductivity is as high as 94.4 mS cm -1 at room temperature. To elucidate the importance of the interaction between the electrode materials (including active materials and binders) and the solid-state electrolyte, solid-state supercapacitors were produced using SPEEK and poly(vinylidene fluoride) as proton conducting and nonconducting binders, respectively. The use of our solid-state electrolyte in combination with proton-conducting SPEEK binder and carbonaceous electrode materials (mixture of activated carbon, single/few-layer graphene, and carbon black) results in a solid-state supercapacitor with a specific capacitance of 116 F g -1 at 0.02 A g -1 , optimal rate capability (76 F g -1 at 10 A g -1 ), and electrochemical stability during galvanostatic charge/discharge cycling and folding/bending stresses.

[Display omitted] • Synthesis of niobium disulfide 3D nanaosheets and nanoflake in liquid phase method. • P and Se co-doping strategy is proposed to enhance the HER activity of NbS 2 3D nanosheets. • P doping can activate the adjacent original inert Nb sites and Se doping can stabilize the catalytic activity. • P and Se co-doped NbS 2 3D nanosheets exhibit significantly enhanced HER activity and stability. Promoting the intrinsic activity and accessibility of basal plane sites in transition metal dichalcogenides (TMDs) is an important way to optimize their catalytic performance for energy conversion and storage. Here, a simple liquid-phase phosphating selenization pyrolysis method is developed to prepare P and Se co-doped niobium disulfide (NbS 2) 3D nanosheets with active basal plane for efficient catalyzing hydrogen evolution reaction (HER). Experimental results indicate that the P and Se co-doped NbS 2 3D nanosheets exhibit significantly enhanced HER activity and stability compared to their undoped counterparts. It realizes 165 mV decreased overpotential at 10 mA cm−2 and a stable durability over 40 h continuous operation at 14 mA cm−2 without observable fading. Density functional theory (DFT) studies suggest that the P dopant is to activate the adjacent original inert Nb sites, where both S and Nb sites in P, Se co-doped NbS 2 monolayer can together act as the active sites for HER. Furthermore, the addition of selenium can stabilize the catalytic activity. Our work provides a promising approach and theoretical framework for improving the electrocatalytic HER performance of NbS 2 , and may have general guidelines for other transition metal disulfides. [ABSTRACT FROM AUTHOR]

• We have developed a novel sensor based on niobium disulfide high perfect performance. • The maximum value of SPR sensitivities approached to 193°∕RIU achieved by coating the NbS 2 nanosheets in simulation calculation. • These sensors were used to detect mercy and the value of sensitivity is 13.55°∕μM, and detection limit approach 1 pM. Mercury ion (Hg2+) can affect the quality of water and cause damage to human health. Therefore, the detection of Hg2+ in water, especially trace Hg2+, is very important. However, the detection limit (approximately 200 pM) of conventional Hg2+ sensors is far lower than the detection limit (7.5 pM) proposed by the US Environmental Protection Agency for measuring Hg2+. Here, we have developed a novel sensor based on niobium disulfide (NbS 2)'s high perfect performance. The maximum value of SPR sensitivities approached to 193°∕RIU achieved by coating the NbS 2 nanosheets in simulation calculation, which had increased by 38.1% compared with the case without NbS 2 overlayer. More importantly, this work also detected the ultra-low concentration of Hg2+ and describes the sensing mechanism of NbS 2 based SPR sensors. Surprisingly, these sensors were used to detect mercury and the value of sensitivity is 13.55°∕μM, and detection limit approach 1 pM. NbS 2 nanosheets based SPR sensors provided innovative design ideas for the development of ultra-sensitive detection technology. [ABSTRACT FROM AUTHOR]

Metallic two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attracting great attention because of their interesting low-temperature properties such as superconductivity, magnetism, and charge density waves (CDW). However, further studies and practical applications are being slowed down by difficulties in synthesizing high-quality materials with a large grain size and well-determined thickness. In this work, we demonstrate epitaxial chemical vapor deposition (CVD) growth of 2D NbS 2 crystals on a sapphire substrate, with a thickness-dependent structural phase transition. NbS 2 crystals are epitaxially aligned by the underlying c-plane sapphire resulting in high-quality growth. The thickness of NbS 2 is well controlled by growth parameters to be between 1.5 and 10 nm with a large grain size of up to 500 μm. As the thickness increases, we observe in our NbS 2 a transition from a metallic 3R-polytype to a superconducting 2H-polytype, confirmed by Raman spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electrical transport measurements. A Berezinskii-Kosterlitz-Thouless (BKT) superconducting transition occurs in the CVD-grown 2H-phase NbS 2 below the transition temperature ( T c ) of 3 K. Our work demonstrates thickness and phase-controllable synthesis of high-quality superconducting 2D NbS 2 , which is imperative for its practical applications in next-generation TMDC-based electrical devices.

Structural asymmetry of materials plays a crucial role in developing multipurpose devices. Layered metallic transition metal dichalcogenides (MTMDCs) have been proposed as promising materials in electronics. However, they are still subject to native surface oxidation, limiting their practical applications. Combination of surface protection and native surface oxidation of MTMDCs will create asymmetric structures for devices but has yet to be explored. Here, we report a bifunctional NbS 2 -based vertical heterostructure derived from epitaxially grown NbS 2 on MoS 2 followed by a natural oxidation (MoS 2 -NbS 2 -NbO x ), which simultaneously exhibits both high-efficiency tunneling conductive and memristive surfaces. With the tunneling conductive surface, the heterostructure functions as nearly ohmic contact electrodes with a two-dimensional (2D) channel in lateral transistors, delivering an enhanced mobility ∼140 times higher than that of the exfoliated NbS 2 -contacted device. With the memristive surface, the heterostructure can be used to build high-performance lateral or vertical memristors with low working voltages and synaptic functions. By combining both types of surfaces, a memristor array for nonvolatile memory is further developed. Moreover, the memristors show a good endurance for 2000 cycles of bending as flexible devices. The bifunctional heterostructure based on NbS 2 offers a strategy toward the future applications of layered metallic materials.

Transition metal dichalcogenides (TMDs) have attracted enormous attention in diverse research fields. Especially, gas sensors are considered in a promising application exploiting TMDs. However, the studies are confined to only major TMDs such as MoS 2 and WS 2 . Particularly, the chemoresistive sensing properties of two-dimensional (2D) NbS 2 have never been explored. For the first time, we report room temperature NO 2 sensing characteristics of 2D NbS 2 nanosheets and the sensing mechanisms using first-principles calculations based on density functional theory. The results demonstrate that the NbS 2 edges possessing different configurations depending on synthetic conditions differ in the sensing ability of the TMD nanosheets. This study not only broadens the potential of 2D NbS 2 for gas sensing applications, but also presents the important role of edge configuration of TMDs depending on synthetic conditions for further studies.

Two dimensional (2D) materials-based plasmon-free surface-enhanced Raman scattering (SERS) is an emerging field in nondestructive analysis. However, impeded by the low density of state (DOS), an inferior detection sensitivity is frequently encountered due to the low enhancement factor of most 2D materials. Metallic transition-metal dichalcogenides (TMDs) could be ideal plasmon-free SERS substrates because of their abundant DOS near the Fermi level. However, the absence of controllable synthesis of metallic 2D TMDs has hindered their study as SERS substrates. Here, we realize controllable synthesis of ultrathin metallic 2D niobium disulfide (NbS 2 ) (<2.5 nm) with large domain size (>160 μm). We have explored the SERS performance of as-obtained NbS 2 , which shows a detection limit down to 10 -14 mol·L -1 . The enhancement mechanism was studied in depth by density functional theory, which suggested a strong correlation between the SERS performance and DOS near the Fermi level. NbS 2 features the most abundant DOS and strongest binding energy with probe molecules as compared with other 2D materials such as graphene, 1T-phase MoS 2 , and 2H-phase MoS 2 . The large DOS increases the intermolecular charge transfer probability and thus induces prominent Raman enhancement. To extend the results to practical applications, the resulting NbS 2 -based plasmon-free SERS substrates were applied for distinguishing different types of red wines.