Nb Atoms Research Articles

Metal-to-semiconductor transitions in constituent-tunable layered two-dimensional Nb W1-Se2 based on first principles calculations

Using the first-principles calculation method, we have studied the electronic structure characteristics of monolayer, bilayer and trilayer NbxW1-xSe2 alloy structures with different concentrations of Nb at different supercell sizes. These alloy materials are formed by replacing one W atom with a Nb atom in supercells of layered two-dimensional (2D) WSe2 structure. Critical values of x in NbxW1-xSe2 were obtained to realize the transition from metal to semiconductor in monolayer, bilayer and trilayer alloy structures. Our results indicate that the transition metal dichalcogenides (TMDs), represented by WSe2, can retain semiconducting feature with increasing number of layers by means of concentration regulation under much lower doping of group-VB metal atoms represented by Nb. This doping approach opens up more possibilities for studying doping effects of layered materials with adjustable structure and properties.

Development of hydrophilic NbCuSi(N) &TiAlNb(N) coatings as a new strategy for medical implants modification

Herein we report the results of multi-element nitride and intermetallic coatings obtained by the arc-PVD method based on non-trivial elements combinations (Nb - 75.5, Si - 9.65, Cu - 14.85 at% and Ti - 81.8, Nb - 11.8, Al - 6.4 at%). Our experiments examine the development and evolution of biocompatible coatings for metal implants and medical devices. The elemental composition of the surface, microstructure, contact angle and biocompatibility of the coatings were studied. XRD results demonstrated that in the intermetallic TiAlNb samples, a multiphase state is formed between γ-TiAl, TiAl2, and α-Nb5Si3, since only a small part of Nb atoms occupy the same lattice sites as Ti atoms, which leads to the formation of a competing phase α-Nb5Si3. The NbCuSi coatings predominantly consist of Nbss with inclusions of silicon silicide. The human umbilical cord mesenchymal stem cells (MSC) were used for biocompatibility assessment using resazurin reduction assay and fluorescent DAPI staining. We show the high biocompatibility and ability to stimulate cell proliferation and distribution over the intermetallic coatings, especially when compared to nitrides ones. Although intermetallic coatings exhibit a higher surface contact angle due to uneven morphology, all samples exhibit hydrophilic properties, which favourably affect cell adhesion and proliferation.

Heterogeneous segregation behavior of Nb at the stepped migrating interface during phase transformation

Not as generally-assumed ‘smooth’, the actual interface should consist of ledge structure with riser and terrace due to crystallographic constraint. However, the combined information of structure and chemistry for such stepped interface has not yet been sufficiently explored. Herein the segregation behaviors of Nb at the migrating ferrite/austenite interface with ledge structure but totally different character are investigated. For the incoherent interface, a typical heterogeneous feature that the Nb atoms segregate at the less mobile terrace rather than at the mobile riser is captured for the first time, while those segregation are absent at the semicoherent interface. By fitting the Nb trans-interface diffusivity, the theoretical calculation based on solute drag theory can well explain the experimental measurements for both types of interfaces. The present results may shed new light on the nanoscaled segregation behaviors of alloying element and stimulate more reliable simulations on the diffusional transformation with ledgewise growth mode.

Full Composition Tuning of W1-xNbxSe2 Alloy Nanosheets to Promote the Electrocatalytic Hydrogen Evolution Reaction.

Composition modulation of transition metal dichalcogenides is an effective way to engineer their crystal/electronic structures for expanded applications. Here, fully composition-tuned W1-xNbxSe2 alloy nanosheets were produced via colloidal synthesis. These nanosheets ultimately exhibited a notable transition between WSe2 and NbSe2 hexagonal phases at x = 0.6. As x approaches 0.6, point doping is converted into cluster doping and eventually separated domains of WSe2 and NbSe2. Extensive density functional theory calculations predicted the composition-dependent crystal structures and phase transitions, consistently with the experiments. The electrocatalytic activity for the hydrogen evolution reaction (HER) in acidic electrolyte was significantly enhanced at x = 0.2, which was linked with the d-band center. The Gibbs free energy for the H adsorption at various basal and edge sites supported the enhanced HER performance of the metallic alloy nanosheets. We suggested that the dispersed doping structures of Nb atoms resulted in the best HER performance. Our findings highlight the significance of composition tuning in enhancing the catalytic activity of alloys.

Preparation, uniaxial negative thermal expansion and Raman study in semiconductor oxide Nb2WO8 ceramic

Negative thermal expansion materials exhibit important promising to tailor thermal expansion due to their abnormal thermal shrinkage characteristics. Here, the Nb2WO8 ceramic has been synthesized, which exhibits axial negative thermal expansion (αa = −2.05 × 10−6 K−1) from 300 to 1000 K without water absorption or phase transformation. Along a axis, the one O atom is located between two identical Nb atoms and the Nb–O–Nb atomic linkages are formed, in which the Nb2–O2–Nb2 bond angle decreases upon heating, resulting in the negative thermal expansion of a axis. Although the b axis and c axis show positive thermal expansion, the low thermal expansion (αl = 2.30 × 10−6 K−1, 173–1073 K) occurs in unit cell volume due to the negative thermal expansion of a axis. The variable pressure Raman shows that the translational and rotational modes of the polyhedron play a key role in axial negative thermal expansion. This work shows potential application prospects of Nb2WO8 as a low thermal expansion semiconductor ceramic.

Effect of Deformation Parameters of an Initial Aged GH4169 Superalloy on Its Microstructural Evolution during a New Two-Stage Annealing.

This study aims to explore the effect of deformation parameters on microstructure evolution during the new two-stage annealing method composed of an aging treatment (AT) and a cooling recrystallization annealing treatment (CRT). Firstly, the hot compressive tests with diverse deformation parameters were finished for an initial aged deformed GH4169 superalloy. Then, the same two-stage annealing method was designed and carried out for the deformed samples. The results show that the deformation parameters mainly affect the grain microstructure during CRT by influencing the content, distribution and morphology of the δ phase after deformation. The reason for this is that there is an equilibrium of the content of the δ phase and Nb atom. When the deformation temperature is high, the complete dissolution behavior of the δ phase nuclei promotes the dispersion distribution of the δ phase with rodlike and needle-like shapes during AT. Thus, the fine and heterogeneous microstructure is obtained after annealing because the recrystallization nucleation is enhanced in those dispersed δ phases during CRT. However, when the retained content of δ phase nuclei is high after deformation, the clusters of intragranular δ phases will form during AT, resulting in the pinning of the motion for dislocation. The elimination of the mixed grain microstructure is slowed down due to the low static recrystallization (SRX) nucleation rate within the deformed grain.

Synthesis of a Selectively Nb-Doped WS2-MoS2 Lateral Heterostructure for a High-Detectivity PN Photodiode.

In this study, selective Nb doping (P-type) at the WS2 layer in a WS2-MoS2 lateral heterostructure via a chemical vapor deposition (CVD) method using a solution-phase precursor containing W, Mo, and Nb atoms is proposed. The different chemical activity reactivity (MoO3 > WO3 > Nb2O5) enable the separation of the growth temperature of intrinsic MoS2 to 700 °C (first grown inner layer) and Nb-doped WS2 to 800 °C (second grown outer layer). By controlling the Nb/(W+Nb) molar ratio in the solution precursor, the hole carrier density in the p-type WS2 layer is selectively controlled from approximately 1.87 × 107/cm2 at 1.5 at.% Nb to approximately 1.16 × 1013/cm2 at 8.1 at.% Nb, while the electron carrier density in n-type MoS2 shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS2-MoS2 heterostructure transforms from the N-N junction (0 at.% Nb) to the P-N junction (4.5 at.% Nb) and the P-N tunnel junction (8.1 at.% Nb). The band-to-band tunneling at the P-N tunnel junction (8.1 at.% Nb) is eliminated by applying negative gate bias, resulting in a maximum rectification ratio (105) and a minimum channel resistance (108 Ω). With this optimized photodiode (8.1 at.% Nb at Vg = -30 V), an Iphoto/Idark ratio of 6000 and a detectivity of 1.1 × 1014 Jones are achieved, which are approximately 20 and 3 times higher, respectively, than the previously reported highest values for CVD-grown transition-metal dichalcogenide P-N junctions.

Ba(Y0.25Zr0.5Nb0.25)O3: A novel three B-sites perovskite synthesis and structural characterization

In this study, a new perovskite compound Ba(Y0.25Zr0.5Nb0.25)O3 with three B-site elements is reported. This was successfully synthesized by solid-state reaction method at 1300 °C for 6 h using BaCO3, Y2O3, Nb2O5 and ZrO2 as raw materials. The structural properties of Ba(Y0.25Zr0.5Nb0.25)O3 were studied by X-ray diffraction (XRD) technique and further refined by Rietveld method as well as the transmission electron microscopy (TEM). The result shows that the space group of Ba(Y0.25Zr0.5Nb0.25)O3 is Pm-3 m with lattice constant a= 4.2081 Å. By using the empirical formula with the ionic radius and tolerance factor, the calculated lattice constant (a=4.2079 Å) is similar to the measured value, which also proved that the lattice constant prediction model of ideal perovskites is still applicable to multi-B sites perovskites. Compared with BaZrO3, the partly substitution of Zr atoms by Y and Nb atoms has an improvement on increasement of the lattice constant. This study provides insights into the process of designing new perovskites as buffer materials to solve the lattice mismatch problem for large lattice constant perovskites thin film.

Magnetism between magnetic adatoms on monolayer NbSe2

In this work, we report on an ab-initio computational study of the electronic and magnetic properties of transition metal adatoms on a monolayer of NbSe2. We demonstrate that Cr, Mn, Fe and Co prefer all to sit above the Nb atom, where the d states experience a substantial hybridization. The inter-atomic exchange coupling is shown to have an oscillatory nature accompanied by an exponential decay, in accordance with what theory predicts for a damped Ruderman–Kittel–Kasuya–Yosida interaction. Our results indicate that the qualitative features of the magnetic coupling for the four investigated adatoms can be connected to the fine details of their Fermi surface. In particular, the oscillations of the exchange in Fe and Co are found to be related to a single nesting vector, connecting large electrons and hole pockets. Most interestingly, this behavior is found to be unaffected by changes induced on the height of the impurity, which makes the magnetism robust to external perturbations. Considering that NbSe2 is a superconductor down to a single layer, our research might open the path for further research into the interplay between magnetic and superconducting characteristics, which could lead to novel superconductivity engineering.

The dissolution behavior of σ-phase and the plasticity recovery of precipitation-embrittlement super-ferritic stainless steel

In this study, a dissolution heat treatment was applied planning to recover the plasticity of an embrittled super-ferritic stainless steel caused by the precipitation of σ-phase, χ-phase and Laves phase. The dissolution behaviors of the brittle precipitation phase, and the concerned grain coarsening phenomena, as well as their effects on the mechanical properties were investigated. It was observed that the χ-phase dissolved first then did the σ-phase and therefore the Laves phase. The σ-phase dissolution kinetics showed a strong dependence on the reheating temperature. A weak grain growth phenomenon was found during the reheating process at 1100 °C. The Laves phase re-formation was observed during reheating at 1000 °C–1050 °C, which was generated by the Nb and Mo atoms diffusion. The plasticity of the tested steels was recovered after dissolution treatment at the temperature range of 1050 °C–1100 °C, during which the fracture mode was remodeled from brittle fracture to toughness fracture. The dissolution treatment at the temperature of >1050 °C was effective to recover the plasticity of precipitation- embrittlement SFSSs.

Demonstration of no catalytical activity of Fe‐N‐C and Nb‐N‐C electrocatalysts toward nitrogen reduction using in‐line quantification

AbstractAmmonia (NH3) production via the electrochemical nitrogen reduction reaction (NRR) is a promising method for sustainable generation of this important chemical. Efforts are ongoing in finding an efficient, stable, and selective catalyst that will enable the reaction. However, progress is hindered in the field due to lack of reproducibility, most likely a consequence of reports of false‐positive results due to improper measurement control and methods. In this study, we explore the NRR activity of a promising class of single atom catalysts, transition metal‐nitrogen‐carbon (M‐N‐C) electrocatalysts. Using a state‐of‐the‐art in‐line ammonia quantification methodology, with detection limit as low as 1 ppb for ammonia, we show that single atom Nb and Fe embedded in a stable carbon and nitrogen framework do not electrochemically reduce N2 to NH3. Critically, this demonstrates that our experimental setup with in‐line sequential injection analysis successfully excludes ammonia contamination from the gas supply and atmospheric sources, allowing for thorough and high‐throughput examination of potential NRR catalysts.

Polytypic omega/omega-like transformation in a refractory high-entropy alloy

Understanding the atomistic details on microstructures and phase transformation mechanisms is essential to tailor the performances of materials. Refractory body-centered cubic (bcc) high-entropy alloys (HEAs) are considered as promising candidates for future applications in the high-temperature, superelasticity, and superconductivity fields. The classical omega (ω) transformation occurs in such systems is hardly expected due to the high configurational entropy and sluggish diffusion. Here, we confirmed unusual ω and ω-like transformations occurred in a configurational entropy stabilized refractory bcc TiZrNbTa model HEA facilitating the formation of colony hierarchical microstructures. These chemically ordered ω and polytyped ω-like superstructures nucleate from one of two phase-separated bcc products. Self-adapted atomic shuffling can convert the simple bcc lattice to the non-close-packed hexagonal ω superstructure, and a certain strain induced collective displacement of the (0001) atomic-layers along one of the <211¯0>ω directions can transition the traditional orthogonal ω to the non-orthogonal polytyped ω-like superstructures. Two ordered ω and/or ω-like superstructures with Ti, Zr, Nb, and Ta atoms occupied specific sites have thus been confirmed. Moreover, the chemistry, orientations, and interface relationships of the ω and polytyped ω-like superstructures were fully revealed by using aberration-corrected scanning transmission electron microscopy combined with first-principles calculations. These atomistic insights into the ω and polytyped ω-like superstructures transformations would provide theoretical guidance for the design of advanced refractory HEAs, especially for designing alloys with specific properties based on the control of transformed microstructures and interfacial modification.

Transport properties of few-layer NbSe2: From electronic structure to thermoelectric properties

4-layer NbSe2 is grown on SiO2 by molecular beam epitaxy. The in-situ X-ray photoelectron spectroscopy measurements suggest an Nb-rich stoichiometry (Nb1+xSe2) likely due to the intercalation of Nb atoms in between the NbSe2 layers. The metallic nature of the samples is confirmed using scanning tunneling microscopy and local density of state measurements as well as band structure calculations. This metallic nature is consistent with the small measured Seebeck coefficient and large electrical conductivity values. A change of sign in the Seebeck coefficient is observed in the bulk single crystal sample at 50 K, and in the polycrystalline few-layer sample at 120 K. Since the samples are metallic, this change of sign is the result of a change in the density of state slope at the Fermi level. The temperature dependence of the measured Seebeck coefficient matches with theoretical calculations for 4-layer NbSe2. The room temperature Seebeck coefficient is negative, but when oxidized, that of the few-layer sample changed to positive. The in-plane thermal conductivity of the few-layer samples is measured using the heat diffusion imaging method at low temperatures and is (32 ± 10) W/m∙K at 200 K.

Single Nb atom modified anatase TiO2(110) for efficient electrocatalytic nitrogen reduction reaction

We report the theory-guided design of anatase-supported Nb catalysts for electrochemical N 2 reduction reaction (NRR). Theoretical calculations predict that Nb atoms deliver multi-functional enhancement toward the NRR when incorporated in an anatase TiO 2 (110) catalyst: (1) decreasing the band gap and inducing electrons to promote the conductivity of TiO 2 (110); (2) suppressing the undesired competitive hydrogen evolution reaction; (3) activating the inert Ti sites for N 2 adsorption; (4) enabling fast charge transfer between ∗NNH and the TiO 2 (110) surface; and (5) reducing the energy barrier of the potential-determining ∗N 2 → ∗NNH step, further facilitating NH 3 formation. As a result, our Nb-TiO 2 (110) catalyst exhibits superior activity and selectivity for the NRR, which affords an NH 3 production rate of about 21.3 μg h −1 mg cat −1 and NH 3 faradaic efficiency of ∼9.2% at −0.5 V (versus reversible hydrogen electrode). This study provides insights for the rational design of efficient electrocatalysts for the NRR. • Nb single-atom-decorated TiO 2 (110) facilitates nitrogen reduction reaction (NRR) • TiO 2 (110) is more active than TiO 2 (101) for NRR • Nb-TiO 2 (110) delivers an NH 3 production rate of ∼21.3 μg h −1 mg cat −1 • Single Nb atom modified anatase TiO 2 (110) for efficient electrocatalytic nitrogen reduction reaction An electrochemical N 2 reduction reaction (NRR) that can be powered by electricity generated from renewable sources has recently gained heightened research interest, providing a promising alternative to the conventional Haber-Bosch process. Current major research efforts are being devoted to the design and development of advanced electrocatalysts to enhance the efficiency of NRR. Herein, guided by density functional theory calculations that predict the cooperative effect of Nb and anatase TiO 2 (110) in promoting electrocatalytic NRR performance, we successfully synthesize TiO 2 single crystals with selectively exposed (110) facets, supplemented with Nb atomic loading, which exhibited remarkable performance for NRR, achieving an NH 3 production rate of ∼21.3 μg h −1 mg cat −1 at −0.5 V (versus reversible hydrogen electrode). Ammonia is extensively used in industry, agriculture, and energy storage. Currently, the industrial synthesis of ammonia predominantly still relies on the energy- and capital-intensive Haber-Bosch (HB) process. Electrochemical N 2 reduction reaction (NRR) provides a renewable and distributed route for ammonia production. To boost the NRR, the development of electrocatalysts is key. Herein, we design and synthesize Nb single-atom-decorated anatase TiO 2 (110) for enhanced NRR, affording an NH 3 production rate of ∼21.3 μg h −1 mg cat −1 .

Improving photoelectric properties by using Nb-doping on TiO2

We investigate the photoelectric properties of intrinsic TiO2 and different Nb concentrations doped TiO2 by using density functional theory. The density of state show that the Fermi level of all doping systems have entered the conduction band, which N degenerate semiconductor is formed. Differential charge density shows that after single doping of TiO2, Nb atoms can provide more free electrons than Ti atoms, which is beneficial to them as electron transport layer materials. Optical properties show that Nb-doped TiO2 has lower visible light absorption rate and higher transmittance, indicating that Nb single-doped TiO2 is suitable as a transparent electrode material.

Positive Seebeck coefficient of niobium-doped MoS2 film deposited by sputtering and activated by sulfur vapor annealing

Herein we report on the positive Seebeck coefficient S = 162 μV K−1 of niobium (Nb)-doped MoS2 films prepared by sputtering and activation of Nb atoms by sulfur vapor annealing. The p-type doping achieved via these processes is discussed based on changes in chemical bonding states and resistivity behavior in terms of annealing and measurement temperatures. The results of this study provide a new option for p-type doping of MoS2 films and are expected to contribute to the development of nanoelectronics and a smart society.

Probing the electronic and local structure of Sr2−xLaxCoNbO6 using near-edge and extended x-ray absorption fine structures

We report the electronic and local structural investigation of double pervoskites Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0--1) using x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structures (EXAFS) at the Nb, Co, and Sr $K$-edges. The $ab$ $initio$ simulations and detailed analysis of the Nb and Co $K$-edge XANES spectra demonstrate that the observed pre-edge features arise from the transition of 1$s$ electrons to the mixed $p-d$ hybridized states. We reveal a $z$-out Jahn-Teller (JT) distortion in the CoO$_6$ octahedra, which decreases monotonically due to an enhancement in the JT inactive Co$^{2+}$ ions with $x$ . On the other hand, the $z$-in distortion in NbO$_6$ octahedra remains unaltered up to $x=$ 0.4 and then decreases with further increase in $x$. This sudden change in the local coordination around Nb atoms is found to be responsible for the evolution of the antiferromagnetic interactions in $x \geqslant$ 0.6 samples. Also, we establish a correlation between the degree of octahedral distortion and intensity of the white line feature in the XANES spectra and possible reason for this are discussed. More interestingly, we observe the signature of KN$_1$ double electron excitation in the Sr $K$-edge EXAFS spectra for all the samples, which is found to be in good agreement with the $Z$+1 approximation. Further, the Co L$_{2,3}$ edge shows the reduction in the crystal field strength and hence an increase in the charge transfer energy ($\Delta_{ct}$) with the La substitution.

The electronic, magnetic and half-metallic predictions of Mx (M = Ag, Cd, Y, Zr, Nb, and x = 0, 0.125, 0.25, 1)W1-xSn alloys

ABSTRACT The changes in the magnetic and half-metallic characters of the structures formed by the doping of Ag, Cd, Y, Zr, and Nb atoms at the ratios of x = 0.25 and 0.125 to WSn alloy were investigated. While the spin-up states of all alloys showed metallic character, energy band gaps were observed in the spin-down states, except YSn alloy. Of the 10 structures, 84.10% spin polarization was obtained only in the Zr0.25W0.75Sn alloy, and the spin polarizations were 100% in all remaining alloys. Band calculations were made with both GGA and GGA + mBJ methods. When the GGA + mBJ method was used, an increase in band gaps was observed in proportion to the increase in Fermi energy levels. In addition, the obtained 3, 4, 5, 6, 7 μB/f.u. magnetic moments for M0.25(M = Ag, Cd, Y, Zr, Nb)W0.75Sn alloys, respectively, and the calculated magnetic moment per unit formula of 5.5, 6, 6.5, 7, 7.5 μB/f.u. for M0.125(M = Ag, Cd, Y, Zr)W0.875Sn alloys support their fields that can be used in spintronic applications. The magnetic moment values obtained with GGA-U (U = 1, 2, 3, 4 eV) calculations showed that while the magnetic moment values of Zr0.25W0.75Sn and Nb0.25W0.75Sn alloys were preserved, they directly increased the magnetic moments of some alloys. These high ratios of polarization, band gaps, and magnetic moments make all alloys ideal materials for use in spintronic applications.

Femtomolar‐Level Molecular Sensing of Monolayer Tungsten Diselenide Induced by Heteroatom Doping with Long‐Term Stability

AbstractSurface‐enhanced Raman scattering (SERS) is a sensitive, fast, and nondestructive technology to detect trace amounts of molecules. The development of ultrasensitive and environmentally stable noble‐metal‐free SERS substrates is crucial for practical applications but still very challenging. In this contribution, an in situ substitutional doping strategy to synthesize Re‐doped WSe2 (Re‐WSe2) with different doping levels is reported. By increasing the Re content to ≈50 at%, the Re‐WSe2 alloy inherits the 1T″ phase of the ReSe2 lattice. Furthermore, Nb atoms are doped into the 1T″ Re‐WSe2 alloy to further modulate its electronic structure. The as synthesized 1T″ Nb, Re‐WSe2 demonstrates a femtomolar‐level molecular sensing capability with a detectable concentration of 5 × 10–15 m and the corresponding enhancement factor is 2.0 × 109, which is superior to that of most non‐noble‐metal SERS substrates and comparable or even superior to that of noble‐metal substrates to the best of the authors’ knowledge. More importantly, the as‐synthesized 1T″ Nb, Re‐WSe2 exhibits excellent air‐stability over a long term (≈6 months) and selective detection capability in the mixed molecular solution, which are essential for their practical applications. The work provides a new strategy for the rational design of noble‐metal‐free SERS substrates to achieve ultrasensitive molecular sensing.

First-principles calculations of the cleavage energy in random solid solutions: A case study for TiZrNbHf high-entropy alloy

The {100} and (110) cleavage energies of body-centered cubic TiZrNbHf high-entropy alloy are calculated using two alloy models: special quasi-random structures (SQSs) and the coherent potential approximation (CPA). The projector augmented wave method, as implemented in the Vienna ab initio simulation package (VASP), in combination with SQSs is adopted to evaluate the impact of local lattice distortions, whereas the exact muffin-tin orbitals (EMTO) method is used in combination with both SQSs and CPA to study the effect of chemical disorder using rigid underlying lattices. The variations of the cleavage energy as a function of surface chemistry and structure from the EMTO and VASP calculations are consistent with each other. Furthermore, the cleavage energies from CPA are in good agreement with those from SQSs, confirming that an averaged supercell approach reproduces well the mean-field CPA results. The alloy’s cleavage energies estimated by the rule of mixtures compare well with those from the direct calculations, and the surface chemistry dependence of the cleavage energies is mainly controlled by the number of Nb atoms in the surface terminal layers owing to the large cleavage energy of Nb metal.