Table of contents

Fuel cell and water electrolyzer technology have been intensively investigated in the last decades toward sustainable and renewable energy conversion systems. For improved device performance and service life, nanostructured electrocatalysts on electrode have been extensively developed based on the principle of structure-activity-stability correlation. However, overall device efficiency is seriously hindered by sluggish oxygen electrocatalysis, including oxygen reduction reaction and oxygen evolution reaction. As a result, tremendous efforts have been made to construct the most active surfaces with robust durability. For knowledge-based approaches toward systematic development of highly functional nanostructures, fundamental principles within oxygen electrocatalysis should be uncovered including reaction intermediate, active site structures, and atomic dissolution from surface. However, conventional ex situ characterizations only provide a static picture of electrode surfaces without electrocatalysis. On the other hand, in situ/operando analyses allow us to directly monitor dynamics on electrode under operating conditions. In this review, we will introduce a set of in situ/operando analytical tools and summarize their contribution to fundamental researches on oxygen electrocatalysis. Taking both precious and non-precious electrocatalyst materials as examples, the most impending issues in oxygen electrocatalysis are covered with in situ/operando studies to highlight the power of in situ/operando techniques and encourage further efforts on advanced analytic techniques.

Transition metal dichalcogenides (TMDs) attract research interest owing to their unique physical and chemical properties. Among the family of TMDs, tungsten disulfide (WS₂) has a unique band structure due to its semiconductor characteristics; namely, its broadband spectral response characteristics, ultra-fast bleach recovery time and excellent saturable light absorption. This article is a review of the current application of WS₂ in catalysts, lasers, batteries, photodetectors and lubricants. The review begins with a brief overview of the structure, properties and growth of WS₂ and describes the existing preparation methods for this material. Finally, methods for improving the performance of WS₂ in its current applications are presented. This review is limited to the most recent reports on this topic.

We propose a convertible metamaterial device with triple-band and broad-band characteristics based on bulk Dirac semimetal (BDS) and vanadium dioxide (VO₂). When VO₂ is in the fully insulating state, the proposed convertible device presents three distinctive absorption peaks in terahertz (THz) range with absorptance >98%. Absorptance spectra analysis shows a clear independence on the conductivity of VO₂ when the device act as a triple-band absorber. When VO₂ is in the fully metallic state, the convertible device expresses a broad-band absorption. In addition, this broad-band absorptivity can be continuously adjusted by changing the conductivity of VO₂. Importantly, without making any changes to the structure parameters, the system exhibits unique convertible mechanism from triple-band to broad-band absorption. Electric field distributions are further discussed to explore the physical origin of this convertible absorber. Benefitting from the variable Fermi level of BDS, resonance frequency can be dynamically tuned. This design approach combined the use of BDS and VO₂ not only paves a new way to realize a convertible absorber from triple-band to broad-band absorption, but also enables us to control the resonance frequency and absorption intensity in THz range. It is believed that the tunable converter provides plentiful applications such as modulator, energy harvesting and optic-electro switches.

Dielectric barrier discharges (DBDs) typically operate in the filamentary regime and thus exhibit great spatial and temporal non-uniformity. In order to optimize DBDs for various applications, such as in plasma catalysis, more fundamental insight is needed. Here, we consider how the millions of microdischarges, characteristic for a DBD, influence individual gas molecules. We use a Monte Carlo approach to determine the number of microdischarges to which a single molecule would be exposed, by means of particle tracing simulations through a full-scale packed bed DBD reactor, as well as an empty DBD reactor. We find that the fraction of microdischarges to which the molecules are exposed can be approximated as the microdischarge volume over the entire reactor gas volume. The use of this concept provides good agreement between a plasma-catalytic kinetics model and experiments for plasma-catalytic NH₃ synthesis. We also show that the concept of the fraction of microdischarges indicates the efficiency by which the plasma power is transferred to the gas molecules. This generalised concept is also applicable for other spatially and temporally non-uniform plasmas.

Two-dimensional van der Waals MnBi_2nTe_3n+1 (n = 1, 2, 3, 4) compounds have been recently found to be intrinsic magnetic topological insulators rendering quantum anomalous Hall effect and diverse topological states. Here, we summarize and compare the crystal and magnetic structures of this family, and discuss the effects of chemical composition on their magnetism. We found that a considerable fraction of Bi occupies at the Mn sites in MnBi_2nTe_3n+1 (n = 1, 2, 3, 4) while there is no detectable Mn at the non-magnetic atomic sites within the resolution of neutron diffraction experiments. The occupancy of Mn monotonically decreases with the increase of n. The polarized neutron diffraction on the representative MnBi₄Te₇ reveals that its magnetization density is exclusively accumulated at the Mn site, in good agreement with the results from the unpolarized neutron diffraction. The defects of Bi at the Mn site naturally explain the continuously reduced saturated magnetic moments from n = 1 to n = 4. The experimentally estimated critical exponents of all the compounds generally suggest a three-dimensional character of magnetism. Our work provides material-specified structural parameters that may be useful for band structure calculations to understand the observed topological surface states and for designing quantum magnetic materials through chemical doping.

In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, blue, and green emission bands from n-type (010) Ga₂O₃ films grown by metalorganic vapor phase epitaxy induced by annealing at different temperatures under O₂ ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in the formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O₂ annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find activation energy for the generation of non-radiative defects of 1.34 eV in the bulk but 2.53 eV near the surface. We also deduce activation energies for the green emission-related defects of 1.20 eV near the surface and 2.21 eV at low temperatures and 0.74 eV at high temperatures through the films, whereas the blue-related defects have activation energy in the range 0.72–0.77 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 °C. These observations are consistent with the formation and diffusion of V_Ga and its complexes as a dominant process during O₂ annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations.

Na–O₂ batteries have been attracting attention owing to their intrinsically high theoretical energy density. Several Na–O₂ systems can produce various discharge products with different electrochemical performances. For example, sodium superoxide (NaO₂) batteries have a low overpotential, and sodium peroxide (Na₂O₂) batteries have a high capacity. Studies of Na₂O₂ batteries are relatively scarce, owing to the difficulty of forming pure Na₂O₂ discharge products. A pure Na₂O₂ battery system is highly desirable for fully exploring the formation and decomposition of Na₂O₂ in Na–O₂ batteries and evaluating their potential. This model of a Na₂O₂ battery should also be compatible with in situ characterization. To this end, we constructed a simple rechargeable all-solid-state Na₂O₂ battery. Using a nanoporous gold film as the cathode and Na–β''-Al₂O₃ as a solid electrolyte, we assembled a Na–O₂ battery that can produce and decompose Na₂O₂. The all-solid-state Na–O₂ battery is a simple model for conducting in situ ambient-pressure x-ray photoelectron spectroscopy (APXPS) investigations. The battery can be cycled at a low overpotential (≈450 mV). Qualitative and quantitative analyses of the APXPS and Raman results demonstrated that Na₂O₂ was the main discharge product and its transformation occurred during the charge and discharge periods. The operando investigation of this type of all-solid-state Na₂O₂ battery can help in the comprehensive exploration of the potential of Na–O₂ batteries.

Near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) was used to study the chemical states of a range of alumina-supported monometallic Pd and bimetallic Pd–Pt nanocatalysts, under methane oxidation conditions. It has been suggested before that for optimal complete methane oxidation, palladium needs to be in an oxidised state. These experiments, combining NAP-XPS with a broad range of characterisation techniques, demonstrate a clear link between Pt presence, Pd oxidation, and catalyst activity under stoichiometric reaction conditions. Under oxygen-rich conditions this behaviour is less clear, as all of the palladium tends to be oxidised, but there are still benefits to the addition of Pt in place of Pd for complete oxidation of methane.

Manganites La_0.93K_0.07Mn_1−xCu_xO₃$(0.0\,\leqslant\,x\,\leqslant\,0.09),$ prepared by the solid state reaction method at high temperature, were studied structurally and magnetically. The unit cell parameters, as well as bond length ${{\text{d}}_{({\text{Mn,Cu)-O}}}}{\text{ }}\left({{{\unicode{x00C5}}}} \right)$ and the bond angle ${\theta _{({\text{Mn,Cu)-}}{{\text{O}}_{\text{1}}}{\text{-Mn}}}}$, were determined from the Rietveld refinement of the x-ray diffraction patterns. The Fourier-transform infrared spectroscopy analysis shows that Cu²⁺ substitution induces variations in the vibration modes of the MnO₆ octahedra. Magnetization vs. temperature ${\text{M}}\left( T \right)$ at low magnetic field $H = 0.01{ }T$ were performed in the range $5 < T < 300{{\,K}}$ under field coolingand zero field cooling conditions. All the samples exhibited a second-order paramagnetic–ferromagnetic (FM) transition at Curie temperature, ${T_{\text{C}}}$, in the range between 199 and 285 K. The inverse susceptibility, ${\chi ^{ - 1}}\left( T \right){ }$ exhibits a linear Curie-Weiss (C-W) behavior for $T > T_{{\text{CW}}}^{\text{*}}$, while for ${T_{\text{C}}} < T < T_{{\text{CW}}}^{\text{*}}$, it shows a deviation from the linear behavior predicted by the Heisenberg model. The mentioned deviation of ${\chi ^{ - 1}}\left( T \right)$ means that a short ferromagnetic state formation is present even for $T > {T_{\text{C}}}$, which were characterized by the experimental effective magnetic moment, $\mu _{{\text{eff}}}^{{\text{exp}}}.$ A null spontaneous magnetization, ${{\text{m}}_{\text{S}}},$ above ${T_{\text{C}}}$ was evaluated for all samples by using the Kouvel-Fisher method. This work evaluates the short-range FM clusters by means of an extension of the C-W approach to the ${T_{\text{C}}} < T < T_{{\text{CW}}}^{\text{*}}$ region, i.e., ${ }\mu _{{\text{eff}}}^{{\text{exp}}}\left( T \right) = 2.3\sqrt {{\text{C}}\left( T \right)} { }{\mu _{\text{B}}}$. Finally, the critical coefficient values, $\beta$ and $\gamma$, showed that the 3D Heisenberg model fits adequately the $x = 0.0{ }$ and $x = 0.03$ samples, while the 3D Ising model fits the $x = 0.09$ sample.

Magnetic damping of the free layer of CoFeB in the spin valve IrMn/CoFe/Cu/CoFeB with large exchange bias has been characterized by frequency-swept ferromagnetic resonance under a series of fixed magnetic fields. The damping constant shows little difference between the parallel and antiparallel magnetization configurations, consistent with the theoretical prediction. Remarkably, in the intermediate states of the pinned CoFe layer under reversal, the effective damping constant of the CoFeB layer is significantly enhanced from 0.0119 up to 0.0292. This enhancement, exceeding the effect of the pumped spin current appreciably, is mainly due to the inhomogeneous broadening and/or two-magnon scattering caused by the stray field emerging from the domain walls (DW) of the pinned CoFe layer when its magnetization is partially reversed. Meanwhile, a resonance frequency shift is also observed in the presence of DW. Our result confirms the strong influence of the pinned layer DW on the magnetic damping in spin valves, which should be properly excluded while dealing with the nonlocal spin-transport-induced damping in heterostructures.

The effect of Co addition on magnetic hysteresis, martensitic transformation temperature, and magnetic entropy change of rapidly-quenched Mn_50−xCo_xNi₄₀In₁₀ alloy nanomaterials has been investigated. The melt-spun Mn₅₀Ni₄₀In₁₀ sample exhibits a small magnetic hysteresis which is further reduced by Co doping as measured between 0 and 2 T. The martensitic transformation temperature increases linearly with the electron concentration in the alloy from 195 K for Mn₅₀Ni₄₀In₁₀ to 378 K for Mn₄₅Co₅Ni₄₀In₁₀. The Mn₄₇Co₃Ni₄₀In₁₀ alloy, which has phase-transition temperature close to room temperature, exhibits a substantial peak entropy change of 29.7 J kg⁻¹ K⁻¹ at magnetic field change of 2 T. Our results demonstrate that Mn₄₇Co₃Ni₄₀In₁₀ nanomaterial exhibits promising magnetocaloric properties for near-room-temperature magnetic refrigeration.

In this paper, the supporting capacity of a ferrofluids (FFs) ring bearing is investigated experimentally and numerically. The bearing consists of a floating plate, a substrate, and a FFs ring sandwiched between them. The FFs ring is formed and restrained by applying a ring magnet below the substrate. For a FFs ring with a thickness of 0.25 mm, the maximum supporting force is about 1.78 N. Numerical analysis reveals that, compared with the Laplace force, the magnetic force plays a dominating role in the total supporting force. Such a liquid ring support may give underlying applications for frictionless bearings or precision positioning systems.

A gradient refractive index design strategy is proposed for a flat lens, which can transform a wavefront by rectifying the local transmission phase. The designed lens is composed of two types of low-loss dielectrics with subwavelength gradient periodic structure and manufactured by 3D printing and computer controlled machining. The measured results of the near and far field agree well with those of theoretical predictions and numerical simulations. It is demonstrated that this light-weight, low cost, compact lens antenna is highly directive (side lobes below −10 dB) and the incident plane waves are focused well with high focusing efficiency (above 80%) over an ultrabroadband frequency range with a bandwidth ratio of 138% (4–22 GHz). The demonstrated flat lens provides an alternative strategy for microwave communication, detection, and imaging applications.

Underwater images always suffer from low contrast and inaccurate colors due to scattering and absorption by particles when the target light propagates through turbid water. In this paper, we first found that a lot of intensity space is occupied by fewer pixels, called 'tails', on both sides of the histograms for the red, green and blue channels of the image. Based on this histogram attenuation prior and taking account of the advantage of a polarization filter we proposed an effective polarimetric recovery method to enhance the underwater image quality, which includes a specially designed histogram processing method, named 'cut-tail histogram stretching'. This processing overcomes the limitation of traditional histogram-based methods and can further improve the restoration performance. The experimental results corresponding to underwater scenes with different turbidities and colors show that the proposed method can simultaneously enhance the image contrast and reduce the color distortion to some extent, and thus realize clear underwater vision.

A multilayer graphene frequency doubler (GFD) with inductance–capacitor resonators (LCRs) and microstrip reflective stubs (MRS) is proposed in this paper. Graphene has strong nonlinear characteristics. Under the excitation of electromagnetic waves, the output power of odd harmonic of graphene is greater than that of even harmonic. Under the joint excitation of electromagnetic wave and bias voltage, the even harmonic output power of graphene is enhanced and the odd harmonic is suppressed, which is very suitable for making GFD. On the basis of analyzing the conductivity of graphene, the symbolically defined device model of multilayer graphene is established, and the model is applied to GFD circuit, the simulation results are basically consistent with the experimental data. The multiplier efficiency of graphene can be effectively improved by the bias voltage and LCR and the MRS. At an operating frequency of 0.65–1.15 GHz, the minimum conversion loss (CL) of the GFD is 20.57 dB when the input power is 16 dBm.

The undoped BAlN electron-blocking layer (EBL) is investigated to replace the conventional AlGaN EBL in light-emitting diodes (LEDs). Numerical studies of the impact of variously doped EBLs on the output characteristics of LEDs demonstrate that the LED performance shows heavy dependence on the p-doping level in the case of the AlGaN EBL, while it shows less dependence on the p-doping level for the BAlN EBL. As a result, we propose an undoped BAlN EBL for LEDs to avoid the p-doping issues, which a major technical challenge in the AlGaN EBL. Without doping, the proposed BAlN EBL structure still possesses a superior capacity in blocking electrons and improving hole injection compared with the AlGaN EBL having high doping. Compared with the Al_0.3Ga_0.7N EBL with a doping concentration of 1 × 10²⁰ cm⁻³, the undoped BAlN EBL LED still shows lower droop (only 5%), compatible internal quantum efficiency (2% enhancement), and optical output power (6% enhancement). This study provides a feasible route to addressing electron leakage and insufficient hole injection issues when designing ultraviolet LED structures.

Frequency-dispersive impedance analysis of CH₃NH₃PbI₃ perovskite is carried out under the external Direct current (DC) field to investigate the interplay of dielectric polarization and delocalized carrier transport. Switching of capacitance from positive to negative values is observed in the radio frequency range (42.1–42.5 MHz) for the external bias ranging from 0–4 V. The switching frequency outlined a decreasing trend with an increase in bias. Upon fitting the experimentally obtained dispersions, a bi-relaxation mechanism is unveiled. One of its constituents arises due to the typical Maxwell–Wagner interfacial polarization between the grain cores and boundaries and acts at the lower frequencies. The other one is manifested via hopping of delocalized carriers, resulting in a high frequency degenerative pseudo inductive response. The interference of these two mechanisms is manifested into an asymmetric Breit–Wigner–Fano profile of the dielectric susceptance spectra. The results are further elaborated from a theoretical point of view involving the energy band structure, electron localization function, and Mulliken charge distribution.

Gallium oxide (Ga₂O₃) has become a viable candidate for certain types of high-power devices due to its large energy bandgap of 4.9 eV, which has attracted widespread attention. In particular, Ga₂O₃ nanowire structures have more unique properties due to its larger specific surface area for the high performance solar-blind ultraviolet (UV) photodetectors. In this work, the ultrafine Ga₂O₃ nanowire network structure is obtained on the sapphire substrate with an Au catalyst by chemical vapor deposition method at 960 °C for 10 min. We can confirm that the growth of the nanowire follows the vapor–liquid–solid growth mechanism and is a β-type Ga₂O₃ crystal through the performance test results. A solar-blind UV photodetector based on the nanowires network shows an apparent response to solar-blind UV light and almost no response to 365 nm wavelength. Furthermore, the on–off ratio, light responsivity, and response time are also measured under a 254 nm wavelength UV light irradiation, respectively. This work provides a new preparation method to improve the performance of solar-blind UV photodetector.

We investigate magneto-optical rotation (MOR) of surface plasmon polaritons (SPPs) at the interface of a metal and a four-level atomic system. The MOR of SPPs can be controlled and modified with the intensity and frequency of the applied fields. We show the birefringence enhancement of the weak probe fields propagating through the atomic medium in the presence of the static magnetic field. The external magnetic field has a vital role in the generation and control over the MOR of SPPs. The MOR completely stops when either the external magnetic field or the probe field or both are tuned to resonance. A pretty behavior of MOR of SPPs is observed for a specific set of control field frequency. The enhanced MOR of SPP has significant applications in atomic spectroscopy, optical communication, nano-photonics, optical switches, and precise measurement.

Operation of semiconductor lasers in the 20–50 µm wavelength range is hindered by strong non-radiative recombination in the interband laser diodes, and strong lattice absorption in GaAs-based quantum cascade structures. Here, we propose an electrically pumped laser diode based on multiple HgTe quantum wells with band structure engineered for Auger recombination suppression. Using a comprehensive model accounting for carrier drift and diffusion, electron and hole capture in quantum wells, Auger recombination, and heating effects, we show the feasibility of lasing at λ = 26, ..., 30 µm at temperatures up to 90 K. The output power in the pulse can reach up to 8 mW for microsecond-duration pulses.

Optical imaging systems play an extremely important role for humans in exploring the world, but the existence of chromatic aberration greatly reduces the imaging ability. Conventional optical systems require the combination of multiple lenses to reduce chromatic aberration, but such a solution is not conducive to the miniaturization and weight reduction of the optical system. In this paper, we design dual-wavelength multilevel diffractive lenses that focus pairs of wavelengths on the same focal plane, using a modified direct-binary-search algorithm to maximize the focusing efficiency. The simulated focusing efficiencies are 72% (92%) and 79% (92%) at the wavelength of 1.064 μm and 1.55 μm, respectively, for the two-dimensional (one-dimensional) ones. Through this approach, the results presented here suggest good focusing performance at two wavelengths, providing a new opportunity for various applications in dual-wavelength imaging systems and lightweight collimators.

Artificial second harmonic generation (SHG) based on magnetic Lorentz force has attracted abundant attention from researchers because of the initial breakthrough in physics. It is still a challenging task to boost this type of SHG emission due to the relative lower efficiency and the specific polarization of artificial SHG. Here, we demonstrate an effective way to enhance the magnetic Lorentz force-based SHG in a double-resonances plasmonic metasurface. The design of our method is twofold: firstly, a dark resonance at fundamental frequency and a bright resonance at second harmonic frequency (SHF); secondly, polarization consistency between the bright resonance and the SHF signal. The results demonstrate that the SHF conversion efficiency of this mode-matching plasmonic metasurface can reach 1.4 × 10⁻⁹, which is enhanced by a factor of 5.17 compared to the case without the mode-matching mechanism. This high efficiency and free design of a plasmonic metasurface offer a promising way for the applications of nonlinear optics.

In₂S₃ (β-In₂S₃), semiconducting chalcogenide with desirable physicochemical properties, has fascinated researchers in photoelectrochemistry. Because of its wide band gap, In₂S₃ can utilize solar energy below 600 nm. However, rapid photogenerated electron–hole recombination and low quantum efficiency have limited the practical application of In₂S₃ in this field. In a two-step in situ hydrothermal process we introduced a narrow band gap semiconductor (ReS₂) below the In₂S₃ and constructed a direct Z-scheme heterostructure with nanoflower and honeycomb morphology. The formation of a direct Z-scheme heterostructure and coordination of the trap-like structure of the composite give a wider absorption range, higher migration and separation efficiency, and faster interfacial transfer speed than for pristine In₂S₃, and the photoelectrochemical performance is approximately three times better than that of pristine In₂S₃ at 1.23 V versus a reversible hydrogen electrode under sunlight. This method therefore provides a new prospect for optimizing the performance of In₂S₃ and applying the novel heterojunction.

This paper presents the results of studying dispersed media formation during the electrical explosion of thin metal wires in vacuum by using low-current generators (∼1–10 kA). Particular attention is paid to the analysis of the composition and structure of the corresponding explosion products as well as to the problem of their visualization using simultaneous laser interferometry and shadow imaging at two wavelengths (1.064 µm and 0.532 µm). Our findings indicate the important role in the visualization of the explosion products that belongs to multiple scattering by submicron droplets of dense condensed matter, which are mixed with metal vapor. The hypothesis on the existence of submicron droplets in the products of exploding metal wires correlates with the results obtained by soft x-ray radiography combined with a laser probing technique. Taking into account the multiple scattering by submicron droplets, it is possible to significantly clarify the parameters of the explosion products visualized via laser probing techniques as well as to gain a deeper insight into the physics behind the electrical wire explosion.

In computational models of atmospheric pressure surface barrier discharges (SBDs) the role of heating of the dielectric material and the quiescent gas is often neglected, impacting the accuracy of the calculated chemical kinetics. In this contribution, a two-dimensional fluid model of an SBD was developed and experimentally validated to determine the relative contribution of the dominant heat transfer mechanisms and to quantify the impact of discharge heating on the resultant chemistry. Three heating mechanisms were examined, including electron heating of the background gas due to inelastic collisions, ion bombardment of the dielectric surface and dielectric heating by the time-varying electric field. It was shown that electron heating of the background gas was not significant enough to account for the experimentally observed increase in temperature of the dielectric material, despite being the dominant heating mechanism of the gas close to the electrode. Dielectric heating was ruled out as the frequency response of typical dielectric materials used in SBD devices does not overlap with the experimentally observed power spectrum of an SBD excited at kHz frequencies. The ionic flux heating was found to be the dominant heating mechanism of the dielectric material and the downstream flow driven by the SBD. The largest impact of plasma heating on discharge chemistry was found in reactive nitrogen species (RNS) production, where the densities of RNSs increased when an appropriate treatment of heating was adopted. This had a marked effect on the discharge chemistry, with the concentration of NO₂ increasing by almost 50% compared to the idealized constant temperature case.

Pulsed laser ablation of compound materials often occurs with delayed evaporation of a less volatile component; however, the effect of the delay on ablation plume expansion remains virtually unexplored. Here, we have performed an experimental and theoretical study of the delayed evaporation effect using an example of a plume produced by nanosecond laser ablation of a gold–silver alloy in a vacuum and comparing it with ablation of pure gold and silver targets. The plume expansion dynamics are investigated by time-of-flight (TOF) mass spectrometry and direct simulation Monte Carlo (DSMC), while the laser-induced target evaporation is analyzed using a thermal model. A dramatic effect of the delay time on the average kinetic energy of the plume particles, especially for the less volatile gold, is demonstrated and the main collisional processes governing the two-component plume expansion under the conditions of delayed evaporation are revealed. Based on comparison of experimental and DSMC data, the delay of the gold evaporation onset is estimated as approximately 0.6 ns. The delayed evaporation is therefore an important factor for correct interpretation of TOF measurements in ablation plumes with components of different volatilities.

Aerosol particles play an important role in atmospheric physical or chemical reactions. Charging of aerosol particles is also widely used in various engineering applications, such as electrical low-pressure impactors and differential mobility analyzers. In this paper, the charging process of nanometer-sized liquid aerosol particles in an atmospheric environment is studied theoretically and experimentally. The traditional charging equation is modified taking the variation of carried charges and the number density of liquid aerosol particles into consideration, due to the coalescence of liquid aerosol particles that brings 100% charge conversion efficiency. By fitting the experimental data under a low discharge voltage, an appropriate combination (r, η) is selected, where r is a specific droplet radius and η is the corresponding equivalent conversion factor of charges. The results from the fitting combination (r, η) are in good agreement with the experimental data and it further demonstrates that the charging evolution of droplets with various radiuses under various voltages can be derived from the existing experimental data under a low voltage. In addition, the concept of a charging time constant τ_0.1 is introduced to describe the charging rate. This paper may provide a reference to reveal and optimize the charging process of liquid aerosol particles and broaden the engineering applications for the charging of aerosol particles.

Oxygen and oxygen-containing plasmas offer great potential for the surface functionalization of polymeric substrates: thermal reactive neutral species are combined with high energy ions to alter both the micro/nanomorphology and composition of polymeric surfaces in a dry process. Although plasma processing is an attractive option for polymer surface modification, plasma–surface interactions are complex and the process design is usually based on a trial-and-error procedure. Toward a comprehensive process design, a hybrid modeling framework, addressing both effects of plasmas on polymeric surfaces, is developed and applied to an investigation of the oxygen-plasma-induced surface roughening of poly(methyl methacrylate). A kinetic Monte Carlo surface model, considering the synergy of neutral species and ions, is used for the calculation of the local etching rate. The novel element of the model is that it takes into account the surface morphology through the calculation of the trajectories of the species joining the surface reactions. The local etching rate is utilized by a profile evolution module based on the level set method to predict the surface roughness evolution. A method for tracking the local variables of the evolving surface profile (e.g. surface coverage), treating a fundamental weakness of the level set method, is proposed and used to effectively reduce the computational time. The results of the framework are validated by comparison to a theoretical model. The prediction of roughness evolution is consistent with measurements vs time and at different operating conditions. The potential of the framework to additionally handle the chemical composition (oxidation) of the surface is demonstrated, enabling the study of the wetting behavior of plasma-etched polymeric surfaces.

Wave demultiplexers transporting desired wavelengths towards proper directions or ports are attracting numerous interests and applications in both physical and engineering areas. In acoustics, there is still a lack of compact and simple designs to achieve demultiplexers in three-port systems. In this work, we propose such a design using Helmholtz resonators where the frequency selection is based on the phenomenon of acoustically induced transparency (AIT). First, a modified transfer matrix method is derived to analytically describe and analyze the AIT effect with Helmholtz resonators. Then, the good performances of wave routing in these designs are further demonstrated by both simulation and experiment. These AIT based demultiplexers are subwavelength and simple in their designs. Therefore, they are promising for various potential applications such as signal processing, information communication and sensing.

Epitaxial GaN films were grown on c-sapphire by rf magnetron reactive sputtering of GaAs at different partial pressures of nitrogen in Ar–N₂ sputtering atmosphere. High-resolution x-ray diffraction and φ-scans reveal the mosaic growth of c-axis oriented, wurtzite GaN films. The c and a parameters were independently determined to obtain the corresponding in-plane and out-of-plane strain components. Raman measurements confirmed the in-plane strain behavior. The surface morphology and elemental composition of films were studied by atomic force microscopy and secondary ion mass spectroscopy, respectively. High-resolution ω-2θ, ω, and in-plane φ-rocking curve scans were used to obtain micro-strain, screw and edge dislocation densities, respectively. The films grown at 30%–100% N₂ reveal dominance of edge (∼10¹² cm⁻²) over screw (∼10¹⁰ cm⁻²) dislocations, with both approaching similar densities at lower N₂ percentages. The strain data has been analyzed to separate the hydrostatic and biaxial contributions and their dependences on N₂ percentage. The film grown at 100% N₂ displays large hydrostatic strain and micro-strain due to the presence of excess/interstitial nitrogen. The hydrostatic strain and micro-strain decrease substantially with initial decrease of N₂ percentage, but increase slightly in the films grown below 30% N₂, primarily due to the incorporation of Ar. The films grown below 75% N₂ display growth-related intrinsic tensile stress, originating from crystallite coalescence. The stress reversal from tensile to compressive, seen in the films grown at higher N₂ percentages is primarily attributed to the incorporation of excess/interstitial nitrogen into grain boundaries and the tensile side of edge dislocations. The decrease of intrinsic tensile stress in the films grown below 30% N₂ is attributed to the incorporation of Ar and their voided structure.