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Interlayer electronic coupling in two-dimensional materials enables tunable and emergent properties by stacking engineering. However, it also results in significant evolution of electronic structures and attenuation of excitonic effects in two-dimensional semiconductors as exemplified by quickly degrading excitonic photoluminescence and optical nonlinearities in transition metal dichalcogenides when monolayers are stacked into van der Waals structures. Here we report a van der Waals crystal, niobium oxide dichloride (NbOCl 2 ), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS 2 . Notably, the strong second-order nonlinearity enables correlated parametric photon pair generation, through a spontaneous parametric down-conversion (SPDC) process, in flakes as thin as about 46 nm. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported. Our work opens an avenue towards developing van der Waals material-based ultracompact on-chip SPDC sources as well as high-performance photon modulators in both classical and quantum optical technologies 1 – 4 . A van der Waals crystal, niobium oxide dichloride, with vanishing interlayer electronic coupling and considerable monolayer-like excitonic behaviour in the bulk, as well as strong and scalable second-order optical nonlinearity, is discovered, which enables a high-performance quantum light source.

Hydrogen embrittlement of high-strength steel is an obstacle for using these steels in sustainable energy production. Hydrogen embrittlement involves hydrogen-defect interactions at multiple-length scales. However, the challenge of measuring the precise location of hydrogen atoms limits our understanding. Thermal desorption spectroscopy can identify hydrogen retention or trapping, but data cannot be easily linked to the relative contributions of different microstructural features. We used cryo-transfer atom probe tomography to observe hydrogen at specific microstructural features in steels. Direct observation of hydrogen at carbon-rich dislocations and grain boundaries provides validation for embrittlement models. Hydrogen observed at an incoherent interface between niobium carbides and the surrounding steel provides direct evidence that these incoherent boundaries can act as trapping sites. This information is vital for designing embrittlement-resistant steels.

Improvements in temporal resolution of single-photon detectors enable increased data rates and transmission distances for both classical and quantum optical communication systems, higher spatial resolution in laser ranging, and observation of shorter-lived fluorophores in biomedical imaging. In recent years, superconducting nanowire single-photon detectors (SNSPDs) have emerged as the most efficient time-resolving single-photon-counting detectors available in the near-infrared, but understanding of the fundamental limits of timing resolution in these devices has been limited due to a lack of investigations into the timescales involved in the detection process. We introduce an experimental technique to probe the detection latency in SNSPDs and show that the key to achieving low timing jitter is the use of materials with low latency. By using a specialized niobium nitride SNSPD we demonstrate that the system temporal resolution can be as good as 2.6 ± 0.2 ps for visible wavelengths and 4.3 ± 0.2 ps at 1,550 nm.Knowledge about detection latency provides a guideline to reduce the timing jitter of niobium nitride superconducting nanowire single-photon detectors. A timing jitter of 2.6 ps at visible wavelength and 4.3 ps at 1,550 nm is achieved.

Laser fluence-dependent laser-induced periodic surface structures (LIPSS) on niobium alloys was analysed. Additionally, we explored the shift from LIPSS to self-organized, periodic microstructures resembling cones. The findings shed light on how surface structures in niobium evolve depending on laser fluence. Significantly influence of laser fluence to the gradual transition of low-spatial frequency LIPSS to highfrequency spatial LIPSS was demonstrated. Highly organized LIPSS on the surface of targets was obtained at the higher accumulated fluence of laser pulses.

Intercalation-type metal oxides are promising negative electrode materials for safe rechargeable lithium-ion batteries due to the reduced risk of Li plating at low voltages. Nevertheless, their lower energy and power density along with cycling instability remain bottlenecks for their implementation, especially for fast-charging applications. Here, we report a nanostructured rock-salt Nb 2 O 5 electrode formed through an amorphous-to-crystalline transformation during repeated electrochemical cycling with Li + . This electrode can reversibly cycle three lithiums per Nb 2 O 5 , corresponding to a capacity of 269 mAh g −1 at 20 mA g −1 , and retains a capacity of 191 mAh g −1 at a high rate of 1 A g −1 . It exhibits superb cycling stability with a capacity of 225 mAh g −1 at 200 mA g −1 for 400 cycles, and a Coulombic efficiency of 99.93%. We attribute the enhanced performance to the cubic rock-salt framework, which promotes low-energy migration paths. Our work suggests that inducing crystallization of amorphous nanomaterials through electrochemical cycling is a promising avenue for creating unconventional high-performance metal oxide electrode materials. Intercalation-type metal oxides are promising anodes for Li-ion batteries but suffer from low energy and power density together with cycling instability. A nanostructured rock-salt Nb 2 O 5 formed via amorphous-to-crystalline transformation during cycling with Li + is shown to exhibit enhanced performance.

The maximum power output and minimum charging time of a lithium-ion battery depend on both ionic and electronic transport. Ionic diffusion within the electrochemically active particles generally represents a fundamental limitation to the rate at which a battery can be charged and discharged. To compensate for the relatively slow solid-state ionic diffusion and to enable high power and rapid charging, the active particles are frequently reduced to nanometre dimensions, to the detriment of volumetric packing density, cost, stability and sustainability. As an alternative to nanoscaling, here we show that two complex niobium tungsten oxides—Nb 16 W 5 O 55 and Nb 18 W 16 O 93 , which adopt crystallographic shear and bronze-like structures, respectively—can intercalate large quantities of lithium at high rates, even when the sizes of the niobium tungsten oxide particles are of the order of micrometres. Measurements of lithium-ion diffusion coefficients in both structures reveal room-temperature values that are several orders of magnitude higher than those in typical electrode materials such as Li 4 Ti 5 O 12 and LiMn 2 O 4 . Multielectron redox, buffered volume expansion, topologically frustrated niobium/tungsten polyhedral arrangements and rapid solid-state lithium transport lead to extremely high volumetric capacities and rate performance. Unconventional materials and mechanisms that enable lithiation of micrometre-sized particles in minutes have implications for high-power applications, fast-charging devices, all-solid-state energy storage systems, electrode design and material discovery. Micrometre-sized particles of two niobium tungsten oxides have high volumetric capacities and rate performances, enabled by very high lithium-ion diffusion coefficients.

Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. In this article, we reveal that the large second-harmonic generation in NbO X 2 ( X  = Cl, Br, and I) may be partially contributed by the large band nesting effect in different Brillouin zone. Interestingly, the NbOCl 2 can exhibit dramatically different strain-dependent bulk photovoltaic effect under different polarized light, originating from the light-polarization-dependent orbital transitions. Importantly, we achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl 2 and a reversible ferroelectric-to-paraelectric phase transition in NbOI 2 under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. Our study establishes the interesting external-field tunability of NbO X 2 for nonlinear optical device applications. This paper reports the intralayer ferroelectric-to-antiferroelectric and ferroelectric-toparaelectric phase transitions in layered NbOCl 2 and NbOI 2 under a small pressure, respectively, along with the strong manipulations of nonlinear optics.

The solvent-free selective hydrogenation of nitroaromatics to azoxy compounds is highly important, yet challenging. Herein, we report an efficient strategy to construct individually dispersed Co atoms decorated on niobium pentaoxide nanomeshes with unique geometric and electronic properties. The use of this supported Co single atom catalysts in the selective hydrogenation of nitrobenzene to azoxybenzene results in high catalytic activity and selectivity, with 99% selectivity and 99% conversion within 0.5 h. Remarkably, it delivers an exceptionally high turnover frequency of 40377 h –1 , which is amongst similar state-of-the-art catalysts. In addition, it demonstrates remarkable recyclability, reaction scalability, and wide substrate scope. Density functional theory calculations reveal that the catalytic activity and selectivity are significantly promoted by the unique electronic properties and strong electronic metal-support interaction in Co 1 /Nb 2 O 5 . The absence of precious metals, toxic solvents, and reagents makes this catalyst more appealing for synthesizing azoxy compounds from nitroaromatics. Our findings suggest the great potential of this strategy to access single atom catalysts with boosted activity and selectivity, thus offering blueprints for the design of nanomaterials for organocatalysis. Single atom catalysts can endow exceptional activity and selectively. Here, the authors report a single atom Co catalyst and reveal the importance of geometric and electronic properties for synthesizing azoxy compounds under solvent-free conditions.

Thermodynamic models were established based on our previous studies to describe the equilibrium between the nonstoichiometric compound Nb(C,N) and the Fe-base solid solution (austenite and ferrite). With the assumption of equilibrium between fcc Nb and the Fe-based solid solution, the solubility of fcc Nb in the Fe-based solid solution was developed. Thus, the solubility product and equilibrium equations of NbCxNy in the Fe-base solid solution were further deduced. Two other thermodynamic interaction parameters for NbCxNy were considered in this study. The deduced solubility product of NbCxNy was in accordance with previous studies. The calculation results on compound NbCxNy in this study were in good agreement with both the measured data from the references and the calculation results from ThermoCalc. The deduced solubility expressions of fcc Nb provided references for developing solubility products of nonstoichiometric fcc Nb compounds in Fe-based solid solutions. This study offers guidance on the equilibrium of nonstoichiometric compounds in solid solutions. The solubility product of NbCxNy is (the solubility products of NbC and NbN are referenced from our previous work)logα/γKNbCxNy≅xlogα/γKNbC+ylogα/γKNbN+1-x-ylogα/γKNb+xy98T-x1-x-y4439T-y1-x-y3413T+xlogx+ylogy+1-x-ylog1-x-ylogαKNb=1.92+1227T+0.051-46.8Twt pct Mn+0.013-6.0Twt pct NilogγKNb=1.65+1365T+0.008-20.9Twt pct Mn+0.012+23.3Twt pct Ni+0.003-9.7Twt pct Cr-0.004wt pct Mo

In this study, we introduced a modified cathodic cage plasma deposition (CCPD) system to synthesize a Ti–Nb–C–N-based composite coating on AISI-4340 steel. This composite coating is widely applied in orthopedic joint implants due to its effective bioactivity and wear resistance. In the existing CCPD system, a metallic cathodic cage of a specific material is used to deposit the coating of such material. Thus, it cannot be used to deposit the coating of a composite material. Here, we inserted composite material rings (80% TiO 2  + 10% Nb 2 O 5  + 10% graphite) on the lid of the cathodic cage, where the cathodic cage holds these rings and works as a counter electrode for plasma generation. The composite coating is synthesized in nitrogen plasma at various temperatures (300–400 °C) and various hydrogen contents (20, 50, and 80%). It is observed that the hardness of AISI-4340 steel (280 HV 0.25 ) can be increased up to 1430 HV 0.25 for coating deposited at 400 °C and 50% hydrogen, which is 5 times higher than the untreated sample. The XRD pattern shows that composite coating consists of TiN, Ti 2 N, NbN, NbC, and TiO 2 . The contents of oxide phase TiO 2 are reduced by adding hydrogen gas to the processing environment; thus, the nitride-based composite coating can be synthesized. The wear volume is significantly reduced by composite coating, specifically while using higher hydrogen contents and higher temperatures. This study shows that this modified CCPD system can improve the hardness and wear resistance by synthesizing this composite coating. In contrast with conventional techniques, separate metallic targets for each element in composite coating (titanium, niobium, and carbon targets) are not required, simple equipment, rough vacuum level are required, and high-processing efficiency are advantages of this system. Graphical abstract