Abstract Cold-cathode ultrafast electron source is a key component for probing ultrafast dynamics behavior in materials, as well as developing high-frequency and high-power electromagnetic radiation devices. Developing large-current, high-brightness and tunable ultrafast electron sources by leveraging the intrinsic properties of nanomaterials is significant. In this study, we report the in-situ assembly of a double-walled carbon nanotube (DWCNT) cold-cathode based on a tungsten (W) tip via nanotransfer manipulation within a SEM chamber, enabled by electron-beam-induced carbon deposition. The resulting ultrafast electron emission exhibits excellent performance under dual-regime modulation by multiphoton photoemission (MPP) and optical field emission (OFE). Under co-excitation by 800 nm femtosecond laser pulses and a static electric field, the DWCNT cold-cathode demonstrates significantly enhanced emission in both regimes with a maximum peak current of ~65 A and corresponding brightness of 4.98 × 1018 A m-2 sr-1 V-1, and its optical excitation threshold reduces by an order of magnitude compared to the conventional metallic W tip cathode at equivalent emission current levels. Comprehensive material characterizations combined with density functional theory (DFT) calculations reveal that the semiconducting nature of the DWCNT emitter, along with its favorable electronic density of states and correspondingly lower effective work function, provides distinct advantages over metallic CNT and W for ultrafast electron emission. Furthermore, quantitative models are developed for both MPP- and OFE-dominated regimes, which elucidate polarization-dependent electron emission behavior and its underlying physical mechanisms. This work presents a promising semiconducting DWCNT cold-cathode for high-performance ultrafast electron sources, and provides a path for investigating ultrafast electron emission dynamics from multiple perspectives.

Two-dimensional (2D) graphene-like metal oxides are promising for gas sensing due to their high surface area to volume ratio. This study presents an optimized van der Waals stripping method for fabricating ultrathin In2O3 and SnO2 films and the In2O3/ SnO2 heterostructure. The heterojunction sensor demonstrated high performance ethanol detection at 100 ppm, with a response value (Ra/Rg) of 13.93, rapid response/recovery times (7 s/29 s), and excellent selectivity. Density functional theory (DFT) simulations revealed that the enhanced performance originates from synergistic interfacial effects that promote electron transfer and selective ethanol adsorption. This work provides a scalable strategy for synthesizing 2D heterostructures and demonstrates their potential for developing advanced gas sensors.

Dual-transition metal MXenes (DTM MXenes) demonstrate significant advantages in gas sensing, energy storage, and catalysis due to their unique structure and tunable composition. However, they suffer from issues such as low room-temperature response and long response/recovery times. In this work, Ti2TaC2Tx was combined with WS2 to construct a composite sensing material for room-temperature ammonia detection. Monolayer Ti2TaC2Tx nanosheets were prepared via HF etching, while WS2 nanosheets were obtained via liquid-phase exfoliation. Ti2TaC2Tx/WS2 composites were fabricated by controlling the mass ratio. Systematic characterization and gas sensing tests revealed that the Ti2TaC2Tx/WS2 composite exhibits high response (71% to 50 ppm NH3), excellent selectivity, and relatively fast response/recovery speeds (46.5s / 202.6s) at room temperature. Density functional theory (DFT) calculations further revealed that the heterojunction structure possesses a stronger adsorption energy for ammonia (-0.5357eV) compared to pure MXene (-0.4402eV), confirming that constructing the heterojunction effectively enhances ammonia adsorption capacity and sensing performance. This work expands the application scope of DTM MXenes and holds significant importance for promoting their development in the field of gas detection.

Spin-polarized field emission electron sources (SP-FEES) have significant applications in high-energy physics and surface analysis. Traditional approaches for calculating electron spin polarization (ESP), which rely on multidimensional wavefunction expansions, tend to be computationally intensive and struggle with convergence issues. This paper proposes a novel method for calculating ESP by incorporating the density of states as a global parameter into the Fowler-Nordheim (FN) theory formula; it enables efficient computation of electron emission current density and ESP. The ESP of the Fe/W structure calculated using this novel method is in good agreement with the experimental results, proving the feasibility of the method. The composite structures of low work function lanthanum hexaboride (LaB6) materials with atomically thin Fe magnetic layers on their surfaces were designed for SP-FEES. The influence of the overall structural evolution of Fe/LaB6 composite structures with a 1 : 1 Fe/LaB6 layer ratio on spin polarization and electron emission properties was investigated using the proposed method. Even-layered structures demonstrate higher ESP than odd-layered ones due to quantum size effects. 1- and 2-layer Fe/LaB6 structures exhibit further enhanced ESP influenced by the superposition of quantum confinement effects at the limiting thickness. Notably, the 2-layer Fe/LaB6 structure exhibits the optimal combination of performance parameters, achieving the highest ESP of -50.40%, the lowest energy spread of 0.136 eV, and a high reduced brightness of 1.71 × 1013 A (m2 sr V)-1. This study presents a computationally straightforward and physically transparent method for calculating polarizations and demonstrates the material advantages of LaB6 in realizing low energy spread and high ESP, which facilitate the design of spin-polarized electron sources.

A reconstructed Fowler-Nordheim (FN) model is developed to evaluate spin-polarized field-emission electron sources by introducing the spin-resolved density of states directly into the supply function. This approach enables efficient calculation of spin-resolved current density, electron spin polarization (ESP), brightness, and energy spread from first-principles electronic-structure data. Using this method, we further investigated Fe/LaB6 emitters with a revised geometry in which 2 layers of Fe cover a 4 layers LaB6 slab. The calculated ESP of the 2-layer-Fe/4-layer-LaB6 structure is -32.36%, corresponding to a considerable spin polarization in magnitude. Meanwhile, the structure also exhibits a high reduced brightness of 1.42 × 1013 A/(m2•sr•V) and a relatively small energy spread of 0.214 eV, demonstrating favorable overall beam quality. The results show that Fe/LaB6 remains a promising candidate for practical spin-polarized field emission electronic source (SP-FEES) design and that the reconstructed FN framework is suitable for efficient screening of spin-polarized cold cathodes.

Graphyne-family materials, deriving from the insertion of acetylenic groups into graphene, are graphene allotropes and have been predicted to exhibit unique physicochemical properties that can be utilized for various applications. In this study, we used density functional theory to systematically investigate the structural, electronic, and Raman properties of the graphyne-family monolayers: graphyne (GY), graphdiyne (GDY), and graphtriyne (GTY). All the considered materials are confirmed to be dynamically stable and possess the same point group symmetry, resulting in similar Raman active phonons. The band gap, the number of Raman active modes with the same irreducible representation, and thus the corresponding Raman peaks increase with acetylenic units. Isotropic Raman intensities were observed under backscattering conditions, while distinct angular dependencies were noted for parallel incident light. In particular, unlike GY, the nonpolarized Raman spectra of GDY and GTY exhibit a similar pattern in which more pronounced Raman intensity is excited by a 633 nm laser than a 532 nm laser, which is understood to be an effect of their different light absorbance properties.

The interaction between terahertz (THz) photons and phonons of materials is crucial for the development of THz photonics. In this work, typical two-dimensional (2D) van der Waals (vdW) transition metal chalcogenide (TMD) layers and heterostructures are used in THz time-domain spectroscopy (TDS) measurements, low-wavenumber Raman spectroscopy measurements, calculation of 2D materials’ phonon spectra, and theoretical analysis of thermal responses. The TDS results reveal strong absorption of THz photons in the frequency range of 2.5–10 THz. The low-wavenumber Raman spectra show the phonon vibration characteristics and are used to establish phonon energy bands. We also set up a computational simulation model for thermal responses. The temperature increases and distributions in the individual layers and their heterostructures are calculated, showing that THz photon absorption results in significant increases in temperature and differences in the heterostructures. These give rise to interesting photothermal effects, including the Seebeck effect, resulting in voltages across the heterostructures. These findings provide valuable guidance for the potential optoelectronic application of the 2D vdW heterostructures.

BiOBr quantum dots (QDs) were synthesized by aqueous phase method for room temperature ammonia detection. It was found that the BiOBr QDs exhibited a 65 % response to 100 ppm NH3 with a low detection limit of 1.54 ppm. DFT calculations further demonstrated the excellent selectivity and reproducibility of the BiOBr QDs. The ultra-small size, high specific surface area, and abundant unsaturated edge sites of the BiOBr QDs facilitated the oxygen adsorption and charge transfer, enabling efficient room-temperature chemo- resistive sensing. These findings emphasize the importance of BiOBr QDs as high-performance materials for ammonia detection under ambient conditions.

Photocatalytic plastic degradation is significant for achieving sustainable waste conversion and reducing environmental burden. Single component photocatalysts face challenges such as narrow absorption range and low charge separation efficiency, making efficient photocatalytic degradation plastics and hydrogen production a challenging task. Constructing Z-scheme heterojunctions a promising approach to effectively broaden the range of photo-response and improve the separation and transfer rate of photo-generated charge carriers. Herein, ZnO/ ZnSe Z-scheme heterojunction photocatalyst was in situ synthesized via simple anion exchange. The ZnO/ZnSe-2 heterojunction exhibits excellent photocatalytic plastic degradation performance, with an organic production reaching as high as 52.8 mu mol, which is 6.1 times and 2.9 times higher than pure ZnO and ZnSe, respectively. In addition, the photocatalytic hydrogen production rate of ZnO/ZnSe-2 heterojunction is optimized to be 123.2 mu mol h-1, which is 5.4 times and 2.8 times compared to pure ZnO and ZnSe, respectively. Time of flight (TOF) testing results indicate that the mobility of ZnO/ZnSe heterojunction is 15.74 times that of ZnSe, indicating that the significant improvement in plastic degradation performance can be attributed to the close contact interface between ZnO and ZnSe, which facilitates the migration of charge carriers. In addition, the Z-scheme heterojunction maintains stronger oxidation and reduction potentials.

The original theory of phonon polariton is Huang's equation which is suitable for diatomic polar crystals only. We proposed a generalized Huang's equation without fitting parameters for phonon polariton in polyatomic polar crystals. We obtained the dispersions of phonon polariton in GaP (bulk), hBN (bulk and 2D), α-MoO3 (bulk and 2D) and ZnTeMoO6 (2D), which agree with the experimental results in the literature and of ourselves. We also obtained the eigenstates of the phonon polariton. We found that the circular polarization of the ion vibration component of these eigenstates is nonzero in hBN flakes. The result is different from that of the phonon in hBN.

Based on the density functional theory (DFT) computing method, a kind of bulk carbon allotrope consisting of non-coplanar pentagon carbon atom rings was predicted. The helical polarization Raman spectroscopies are got by numerical calculation. The physical properties, such as band structures, elastic tensors and thermal conductivity tensors, are calculated and compared with the diamond and the tetragonal crystal structure of carbon (T12C).

MXenes have aroused intensive enthusiasm because of their exotic properties and promising applications. However, to date, they are usually synthesized by etching technologies. Developing synthetic technologies provides more opportunities for innovation and may extend unexplored applications. Here, we report a bottom-up gas-phase synthesis of Cl-terminated MXene (Ti2CCl2). The gas-phase synthesis endows Ti2CCl2 with unique surface chemistry, high phase purity, and excellent metallic conductivity, which can be used to accelerate polysulfide conversion kinetics and dramatically prolong the cyclability of Li-S batteries. In-depth mechanistic analysis deciphers the origin of the formation of Ti2CCl2 and offers a paradigm for tuning MXene chemical vapor deposition. In brief, the gas-phase synthesis transforms the synthesis of MXenes and unlocks the hardly achieved potentials of MXenes.

The recent discovery of field emission devices based on one-dimensional nanostructures has attracted much interest in emerging applications on next-generation flat panel displays, molecule-based sensors, and so forth. To achieve a comprehensive understanding of surface potentials at the nano-emitters during the tunneling process, in this study we systematically investigated the image potentials of single-walled boron nitride nanotubes with different edges, diameters and lengths in the frame of a composite first-principles calculation. The image potentials of zigzag single-walled boron nitride nanotubes are found to be dependent on the non-equivalent sides. Only the image potentials of isolated armchair single-walled boron nitride nanotube can be well fitted with the image potential of an ideal metal sphere of a size comparable to the tube diameter. On the contrary, the image potentials of zigzag and grounded armchair single-walled boron nitride nanotubes exhibit a strong length-dependence characteristic and are significantly different from that of an ideal metal sphere, which originates from the significant axial symmetry breaking of induced charge at the tip for the long tube. The correlation between the testing electron and electronic structure of single-walled boron nitride nanotube has also been discussed.

Double-transition-metal (DTM) MXenes are potentially multifunctional materials with greater compositional and structural tunability than mono-transition-metal (MTM) MXenes, enabling controllable formation of defects and controllable tuning of material properties. Herein, we investigate Ti2VC2Tx's ammonia gas sensing via experiments and theoretical calculations. Ti2VC2Tx exhibited a notable response, exceeding 25 %, to 100 ppm ammonia at room temperature with rapid response/recovery times of 4 s/16.2 s. Furthermore, the Ti2VC2Tx sensor shows exceptional ammonia selectivity, attributed to closer adsorption distances and higher adsorption energies of ammonia molecules. This study broadens the application area of DTM MXenes and further demonstrates the potential of MXene materials for gas sensing.

Metal chalcogenides are promising visible-light absorption materials; however, their application in overall water-splitting has long been hampered by the sluggish kinetics of oxygen evolution reaction (OER) and serious photocorrosion. Fundamentally, these critical issues are related to the behavior of photogenerated holes. Here, using ZnSe as a model catalyst, we achieve high-performance overall water-splitting in intrinsic activity and stability by facilitating the utilization of holes in the OER rather than self-corrosion. This is guided by our microkinetic analysis that the kinetic bottleneck of hole-mediated OER on ZnSe is the high reaction barrier and low concentration of holes reaching the photocatalyst surface. Accordingly, we radically modify the conduction characteristic of ZnSe surface layer into p-type to break the above OER bottleneck. The resulting ZnSe photocatalyst exhibits an impressive overall water-splitting performance in pure water with an ideal H-2/O-2 molar ratio of similar to 2 and a solar-to-hydrogen conversion efficiency of 0.1891% without the assistance of any cocatalyst, outperforming ever-reported overall water-splitting of state-of-the-art metal chalcogenides under identical conditions. In addition, due to the greatly promoted OER, the critical photocorrosion issue is successfully suppressed on the engineered ZnSe photocatalyst. This work breaks the long-standing limitations of metal chalcogenides for overall water-splitting.

Hyperbolic phonon polariton is important in precisely controlling photons at the nanoscale. It was common practice to calculate the dielectric function of the phonon polariton system with the Drude-Lorenz model. We considered the impact of LO-TO splitting while applying the Drude-Lorenz model. Then the dielectric functions become wave vector direction dependent besides electric polarization direction dependent. Our results show that considering LO-TO splitting can more accurately predict dielectric functions. Additionally, we discovered that, besides hexagonal BN, hexagonal AlN exhibits a wide hyperbolic frequency band range, while the other four materials display it scarcely. Furthermore, we found that the phonon frequency, lifetime, and the difference of infrared active transverse optical phonon frequencies with different wave vector directions are critical factors in determining the width of the hyperbolic frequency band range. We also found some dumbbell-shaped and butterfly-shaped isofrequency curves in h-AlN, h-GaP, and especially h-GaN. Our study provides a fresh perspective on understanding the dielectric properties of these materials and lays a theoretical foundation for further exploration and development of new hyperbolic phonon polariton materials.

One-dimensional nanoribbons have foreshadowed potential applications in nanoelectronics due to their fascinating quantum confinement effects. We present theoretical assessment from first-principles calculations to explore the work functions of Ti2B-based MBene nanoribbons, focusing on the size, edge, and functionalization dependencies. We found that the bare and halogen-functionalized Ti2B nanoribbons exhibit metallic properties and chemical stability. The work function of both bare and halogen-terminated armchair Ti2B nanoribbons tends to saturate as width. In contrast, the work function of zigzag Ti2B nanoribbons varies complexly with width and has been discussed in terms of the non-equivalent edges and halogen termination. The work functions of halogen-terminated Ti2B nanoribbons have been found to be significantly affected by the edge dipole moments, which are determined by three factors: functionalization-induced electron redistribution at the edge, the electronegativity of halogen functional groups, and the edge reconstruction. These findings provide valuable insights for designing, characterizing, and utilizing the proposed nanostructures.

Dielectric polymer materials are capable of storing energy stably and are utilized in numerous fields such as modern electronic devices and power systems. Thus far, the unappealable discharged energy density of the current dielectric polymer film has mainly been caused by insufficient breakdown strength and lower discharged efficiency. Herein, multilayer dielectric films consisting of polyethersulfone (PESU) as the outer layer and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) doped with Ni-MOF as the middle layer were constructed. The PESU layer could block charge injection and effectively disperse the electric field strength of the single-layer film; moreover, the PESU with multiple active sites facilitates restricting the motion of carriers. Consequently, an elevated breakdown strength of 824 MV m-1 is exhibited in multilayer nanocomposites, which is 164.8% that of PESU films, concomitant with a surged efficiency of 87.3%, which is 24.9% higher than that of pure P(VDF-HFP), and a superior discharged energy density of 20.8 J cm-3 is achieved. The multilayer dielectric material provides a feasible example for fabricating energy storage devices with the coupling of ultrahigh breakdown strength and efficiency. Nanocomposite films obtain superior energy storage density, which is by far the most energy storage performance reported to date.

In recent studies, it has been discovered that phonons can carry angular momentum, leading to a series of investigations into systems with three-fold rotation symmetry. However, for systems with two-fold screw rotational symmetry, such as α-MoO3, there has been no relevant discussion. In this paper, we investigated the pseudoangular momentum of phonons in crystals with two-fold screw rotational symmetry. Taking α-MoO3 as an example, we explain the selection rules in circularly polarized Raman experiments resulting from pseudoangular momentum conservation, providing important guidance for experiments. This study of pseudoangular momentum in α-MoO3 opens up a new degree of freedom for its potential applications, expanding into new application domains.