Special issue: emerging leaders

Recent experiment has uncovered semimetal bismuth (Bi) as an excellent electrical contact to monolayer MoS₂ with ultralow contact resistance. The contact physics of the broader semimetal/monolayer-semiconductor family beyond Bi/MoS₂, however, remains largely unexplored thus far. Here we perform a comprehensive first-principle density functional theory investigation on the electrical contact properties between six archetypal two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors, i.e. MoS₂, WS₂, MoSe₂, WSe₂, MoTe₂ and WTe₂, and two representative types of semimetals, Bi and antimony (Sb). As Bi and Sb work functions energetically aligns well with the TMDC conduction band edge, Ohmic or nearly-Ohmic n-type contacts are prevalent. The interlayer distance of semimetal/TMDC contacts are significantly larger than that of the metal/TMDC counterparts, which results in only weak metalization of TMDC upon contact formation. Intriguingly, such weak metalization generates semimetal-induced gap states (SMIGSs) that extends below the conduction band minimum, thus offering an effective mechanism to reduce or eliminate the n-type Schottky barrier height (SBH) while still preserving the electronic structures of 2D TMDC. A modified Schottky–Mott rule that takes into account SMIGS, interface dipole potential, and Fermi level shifting is proposed, which provides an improved agreement with the density functional theory-simulated SBH. We further show that the tunneling-specific resistivity of Sb/TMDC contacts are generally lower than the Bi counterparts, thus indicating a better charge injection efficiency can be achieved through Sb contacts. Our findings reveal the promising potential of Bi and Sb as excellent companion electrode materials for advancing 2D semiconductor device technology.

Experimentally monitoring the dynamics of a physical system, one cannot possibly resolve all the microstates or all the transitions between them. Theoretically, these partially observed systems are modeled by considering only the observed states and transitions while the rest are hidden, by merging microstates into a single mesostate, or by decimating unobserved states. The deviation of a system from thermal equilibrium can be characterized by a non-zero value of the entropy production rate (EPR). Based on the partially observed information of the states or transitions, one can only infer a lower bound on the total EPR. Previous studies focused on several approaches to optimize the lower bounds on the EPR, fluctuation theorems associated with the apparent EPR, information regarding the network topology inferred from partial information, etc. Here, we calculate partial EPR values of Markov chains driven by external forces from different notions of partial information. We calculate partial EPR from state-based coarse-graining, namely decimation and two lumping protocols with different constraints, either preserving transition flux, or the occupancy number correlation function. Finally, we compare these partial EPR values with the EPR inferred from the observed cycle affinity. Our results can further be extended to other networks and various external driving forces.

A study on the scalability of discharge characteristics of a single-filament dielectric barrier discharge (DBD) to a spatially one-dimensional multi-filament arrangement driven by the same high-voltage (HV) pulses was performed for a gas mixture of 0.1 vol% O₂ in N₂ at 1 bar. Both arrangements feature a 1 mm gap with dielectric-covered electrodes featuring two hemispherical alumina caps for the single-filament and two parallel alumina-tubes for the multi-filament arrangement. The DBDs were characterised by electrical measurements (for peak current, energy, and power) accompanied by iCCD and streak imaging to determine the filament number and the discharge development in the gas gap and on the surfaces. It was found that the electrical quantities scale with a constant factor between the single- and multi-filament arrangement, which is expected to be related to the filament number. In the multi-filament arrangement, the pulsed operation leads to filament formation in the entire gap in lateral direction within less than 2 ns. Furthermore, particular breakdown or discharge inception regimes were identified for the multi-filament DBDs. These regimes could be generated at the falling slope of asymmetrical HV pulses featuring e.g. a double-streamer propagation, which was previously reported for single-filament DBDs. Consequently, it was proven that the discharge manipulation by varying the HV pulse widths obtained for single-filament DBDs can also be applied in a one-dimensional multi-filament arrangement, i.e. an upscaling based on the knowledge for single-filament DBDs seems to be generally possible.

A magnon and a phonon are the quanta of spin wave and lattice wave, respectively, and they can hybridize into a magnon polaron when their frequencies and wavenumbers match close enough the values at the exceptional point. Guided by an analytically calculated magnon polaron dispersion, dynamical phase-field simulations are performed to investigate the effects of magnon polaron formation on the attenuation of a bulk acoustic wave in a magnetic insulator film. It is shown that a stronger magnon–phonon coupling leads to a larger attenuation. The simulations also demonstrate the existence of a minimum magnon–phonon interaction time required for the magnon polaron formation, which is found to decrease with the magnetoelastic coupling coefficient but increase with the magnetic damping coefficient. These results deepen the understanding of the mechanisms of acoustic attenuation in magnetic crystals and provide insights into the design of new-concept spin interconnects that operate based on acoustically driven magnon propagation.

Live-cell imaging with fluorescence microscopy is a powerful tool, especially in cancer research, widely-used for capturing dynamic cellular processes over time. However, light-induced toxicity (phototoxicity) can be incurred from this method, via disruption of intracellular redox balance and an overload of reactive oxygen species (ROS). This can introduce confounding effects in an experiment, especially in the context of evaluating and screening novel therapies. Here, we aimed to unravel whether phototoxicity can impact cellular homeostasis and response to non-thermal plasma (NTP), a therapeutic strategy which specifically targets the intracellular redox balance. We demonstrate that cells incorporated with a fluorescent reporter for live-cell imaging have increased sensitivity to NTP, when exposed to ambient light or fluorescence excitation, likely through altered proliferation rates and baseline intracellular ROS levels. These changes became even more pronounced the longer the cells stayed in culture. Therefore, our results have important implications for research implementing this analysis technique and are particularly important for designing experiments and evaluating redox-based therapies like NTP.

Physical reservoir computing (RC) represents a computational framework that exploits information-processing capabilities of programmable matter, allowing the realization of energy-efficient neuromorphic hardware with fast learning and low training cost. Despite self-organized memristive networks have been demonstrated as physical reservoir able to extract relevant features from spatiotemporal input signals, multiterminal nanonetworks open the possibility for novel strategies of computing implementation. In this work, we report on implementation strategies of in materia RC with self-assembled memristive networks. Besides showing the spatiotemporal information processing capabilities of self-organized nanowire networks, we show through simulations that the emergent collective dynamics allows unconventional implementations of RC where the same electrodes can be used as both reservoir inputs and outputs. By comparing different implementation strategies on a digit recognition task, simulations show that the unconventional implementation allows a reduction of the hardware complexity without limiting computing capabilities, thus providing new insights for taking full advantage of in materia computing toward a rational design of neuromorphic systems.

Scandium nitride (ScN) is an emerging group III-B transition metal pnictide and has been studied extensively for its thermoelectric properties, as interlayers for defect-free GaN growth, in epitaxial metal/semiconductor superlattices, and recently for its polaritonic and optoelectronic synaptic functionalities. However, to realize the full potential of its semiconducting properties in electronic, thermionic, and optoelectronic device applications, it is necessary to develop Schottky diodes of ScN that are missing thus far. Here we show Schottky diodes of ScN with elemental metals such as silver (Ag) and gold (Au). As-deposited ScN thin films exhibit a high electron concentration in the (1–4) × 10²⁰ cm⁻³ range due to unintentional oxygen doping. These excess electrons are compensated by Mg hole doping, leading to a wider depletion region at the metal/ScN interface for activated electronic transport. Current–voltage (I–V) characteristics show the rectification nature in ScN/Ag and ScN/Au diodes, and the barrier heights of 0.55 ± 0.05 eV and 0.53 ± 0.06 eV, respectively, are obtained. Interface annealing with time and temperature results in a slight increase in the forward junction potential. The capacitance–voltage (C–V) measurements also revealed the presence of interface trap states. The demonstration of Schottky diodes marks an important step in realizing the full potential of ScN in electronic, thermionic, and optoelectronic devices.

Anomalous and topological Hall effect (THE) are the fascinating electronic transport properties in condensed matter physics and received tremendous interest in the field of spintronics. Here, we report the intrinsic anomalous Hall conductivity (AHC) and THE in the bulk Ni₂MnGa magnetic shape memory alloy. The magnetization measurement reveals the premartensite, martensite and magnetic phase transitions. A detailed analysis of AHC reveals that the intrinsic Berry phase mechanism dominates over skew scattering and side jump in all the structural phases of Ni₂MnGa. Further, an additional contribution in the Hall resistivity is observed as THE. The magnitude of the THE and its temperature independent behavior indicates that the THE arises due to the real space Berry curvature induced by topologically protected magnetic skyrmion textures in the martensite and premartensite phases of Ni₂MnGa. The larger magnetic field is required to vanish the topological Hall resistivity in the martensite phase in comparison to the premartensite phase, which manifests the more stable skyrmion textures in the martensite phase. The present findings open a new direction in the field of functional materials, which hosts skyrmion, exhibits anomalous transport and magnetic shape memory effect.

In this work, an enhancement-mode (E-mode) AlGaN/GaN-based high-electron-mobility transistor (HEMT) with a graded AlGaN cap layer (GACL) is proposed and numerically studied by Silvaco technology computer-aided design. The GACL is designed with a decreasingly graded Al composition x along [0001] direction and the initial x is smaller than the Al composition of the Al_0.2Ga_0.8N barrier layer (BL). This GACL scheme can simultaneously produce high-concentration polarization-induced holes and negative net polarization charges at the GACL/BL interface. This can facilitate the separation of the conduction band (E_C) and Fermi level (E_F) at the 2DEG channel and therefore benefit the normally-OFF operation of the device. The optimized graded-AlGaN-gated metal-semiconductor HEMT can achieve a large threshold voltage of 4 V. Furthermore, we demonstrated that shortening the gate length on the GACL and inserting an oxide layer between the gate and GACL can be both effective to suppress gate leakage current, enhance gate voltage swing, and improve on-state drain current of the device. These numerical investigations can provide insights into the physical mechanisms and structural innovations of the E-mode GaN-based HEMTs in the future.

Active metasurfaces have attracted increased attention due to their capabilities in function switching and wavefront shaping. Here we develop a new paradigm for active control of metasurfaces via integrating a tunable and programmable spintronic terahertz emitter (STE). While compatible with almost all conventional materials for metasurfaces, the STE can empower the passive metasurfaces to be active with increased flexibility. For the sake of illustration, a STE integrated metasurface quarter-wave plate is demonstrated, which enables broadband full polarization control over the entire Poincaré sphere. We also share a future perspective that the STE integrated metasurface can be readily programmed by using a commercial spatial light modulator. This work bridges the studies of metasurfaces and spintronic THz emitters, and may inspire more fruitful active metasurface designs and applications.

In this work, the anisotropic deformation and anisotropic mechanical properties of 4H silicon carbide (4H-SiC) single crystal wafers are proposed by using nanoindentation. The C face of a 4H-SiC wafer has higher hardness and lower fracture toughness than those of the Si face. Because the deformation of 4H-SiC is assisted by the nucleation and slip of basal plane dislocations (BPDs), especially the slip of Si-core partial dislocations (PDs) of the BPDs, the nucleation and slip of the Si-core PDs in the Si face of 4H-SiC is easier than those in the C face, which releases the nanoindentation-induced stress and results in the decrease of the hardness and increase of the fracture toughness of the Si face of 4H-SiC wafers. Due to the hexagonal lattice of 4H-SiC, the hardness along $<1\bar 100>$ of 4H-SiC is higher than that along $<11\bar 20>$, but the fracture toughness along the $<1\bar 100>$ is lower than that along the $<11\bar 20>$, as a result of the enhanced glide of dislocations along the most closely-packed direction. The insights gained in this work are expected to shed light on the optimization of the mechanical processing of 4H-SiC wafers.

The rate, selectivity and efficiency of plasma-based conversion processes is strongly affected by nonequilibrium phenomena. High concentrations of vibrationally excited molecules are such a plasma-induced effect. It is frequently assumed that vibrationally excited molecules are important in plasma catalysis because their presence lowers the apparent activation energy of dissociative chemisorption reactions and thus increases the conversion rate. A detailed atomic-level understanding of vibrationally stimulated catalytic reactions in the context of plasma catalysis is however lacking. Here, we couple a recently developed statistical model of a plasma-induced vibrational nonequilibrium to molecular dynamics simulations, enhanced sampling methods, and machine learning techniques. We quantify the impact of a vibrational nonequilibrium on the dissociative chemisorption barrier of H₂ and CH₄ on nickel catalysts over a wide range of vibrational temperatures. We investigate the effect of surface structure and compare the role of different vibrational modes of methane in the dissociation process. For low vibrational temperatures, very high vibrational efficacies are found, and energy in bend vibrations appears to dominate the dissociation of methane. The relative impact of vibrational nonequilibrium is much higher on terrace sites than on surface steps. We then show how our simulations can help to interpret recent experimental results, and suggest new paths to a better understanding of plasma catalysis.

The paper is devoted to the study of the initiation and formation of a negative streamer in a sharply inhomogeneous electric field and the generation of runaway electrons (REs) in air and helium at atmospheric pressure and below, as well as in sulfur hexafluoride at low pressure. Nanosecond voltage pulses of negative polarity with an amplitude of 18 kV were applied across a point-to-plane gap 8.5 mm long. The studies were carried out using broadband measuring sensors and equipment with picosecond time resolution, as well as using a four-channel ICCD camera. Using a special method for measuring the dynamic displacement current caused by the redistribution of the electric field during streamer formation, the waveforms of voltage, discharge current, RE current, and dynamic displacement current were synchronized to each other, as well as to ICCD images. Data on the generation of REs with respect to the dynamics of streamer formation were obtained. It was found that REs are generated not only during the breakdown of the gap, but also after that. It has been found that the formation time of explosive emission centers affects the generation of REs after breakdown. Based on the measurement data of the voltage, discharge current, and dynamic displacement current, the electron concentration in the plasma channel after breakdown and the electric field strength near the surface of the grounded electrode were calculated.

Antiferromagnets (AFs) have recently surged as a prominent material platform for next-generation spintronic devices. Here we focus on the dynamics of the domain walls in AFs in the presence of magnetoelasticity. Based on a macroscopic phenomenological model, we demonstrate that magnetoelasticity defines both the equilibrium magnetic structure and dynamical magnetic properties of easy-plane AFs in linear and nonlinear regimes. We employ our model to treat non-homogeneous magnetic textures, namely an AF in a multi-domain state. Calculations of the eigen-modes of collective spin excitations and of the domain walls themselves are reported, even considering different kinds of domains. We also compare the output of our model with experimental results, substantiating the empirical observation, and showing that domain walls majorly affect the optically driven ultrafast nonlinear spin dynamics. Our model and its potential developments can become a general platform to handle a wide set of key concepts and physical regimes pivotal for further bolstering the research area of spintronics.

Semiconductor mode-locked lasers (MLLs) with extremely high repetition rates are promising optical frequency comb (OFC) sources for their usage as compact, high-efficiency, and low-cost light sources in high-speed dense wavelength-division multiplexing transmissions. The fully exploited conventional C- and L- bands require the research on O-band to fulfil the transmission capacity of the current photonic networks. In this work, we present a passive two-section InAs/InGaAs quantum-dot (QD) MLL-based OFC with a fundamental repetition rate of ∼100 GHz operating at O-band wavelength range. The specially designed device favours the generation of nearly Fourier-transform-limited pulses in the entire test range by only pumping the gain section while with the absorber unbiased. The typical integrated relative intensity noise of the whole spectrum and a single tone are −152 and −137 dB Hz⁻¹ in the range of 100 MHz–10 GHz, respectively. Back-to-back data transmissions for seven selected tones have been realised by employing a 64 Gbaud four-level pulse amplitude modulation format. The demonstrated performance shows the feasibility of the InAs QD MLLs as a simple structure, easy operation, and low power consumption OFC sources for high-speed fibre-optic communications.

Inevitable for the basic principles of skyrmion racetrack-like applications is not only their confined motion along one-dimensional channels but also their controlled creation and annihilation. Helical magnets have been suggested to naturally confine the motion of skyrmions along the tracks formed by the helices, which also allow for high-speed skyrmion motion. We propose a protocol to create topological magnetic structures in a helical background. We furthermore analyse the stability and current-driven motion of the skyrmions in a helical background with in-plane uniaxial anisotropy fixing the orientation of the helices.

We devised a proof-of-concept materials design that addresses the necessary requirements for magnetocaloric materials to have a constant magnetocaloric effect (MCE) over a large temperature range. For this purpose, we have fabricated epitaxial Co_1−x(z)Ru_x(z) films engineered to have a triangular gradient in exchange strength J along the thickness. Different from homogeneous Co_1−xRu_x layers, where the maximum value of magnetic entropy change ΔS_m falls rapidly with temperature away from the ferromagnetic (FM)–paramagnetic (PM) phase transition, the Co_1−x(z)Ru_x(z) graded structures exhibit high MCE over a large temperature range, leading to an improved cooling capacity. Theoretical modeling results confirm the enhanced temperature range and highlight a core aspect of our exchange graded materials approach, namely the ability to control and manipulate magnetism at nanoscale dimensions. As we demonstrate, this control is reliant on the fact that the temperature driven PM–FM phase transition does not occur in the entirety of the material system but only in well-defined nanoscopic regions of our samples at any given temperature, enabling us to significantly extend the useful temperature range for magneto-caloric utilization.

Higher-order Lamb waves with quasi-zero surface displacement components are reported on (100)-cut GaAs propagating along the <110> direction where the total displacement at the surface of the plate is less than 10% of the maximum total displacement. The dispersion curves and the displacement component profiles show the reduction of total displacement at the surface of the plate starting when the phase velocities of the higher-order modes are crossing the shear bulk acoustic wave velocity to the value as low as 5%. Due to the concentration of acoustic energy inside the plate, the reported quasi-zero plate acoustic waves (QZ-PAW) further reduce the radiation of acoustic energy when the plate surface is in contact with liquid. The experimental results validate the occurrence of QZ-PAW with a reduction of viscous damping insertion loss compared to previously reported quasi-longitudinal Lamb waves. The results demonstrate the potential QZ-PAW mode for emerging applications such as dual-mode PAW sensors, PAW devices with integrated sensor and actuator, thin-film and ultra-high frequency PAW sensors in highly viscous liquid media.

We examine the atomic intermixing phenomenon in three distinct amorphous CoB-based multilayer thin film platforms — Pt/CoB/Ir, Ir/CoB/Pt and Pt/CoB/MgO — which are shown to stabilise room-temperature chiral magnetic textures. Intermixing occurs predominantly between adjacent metallic layers. Notably, it is stack-order dependent, and particularly extensive when Ir sits atop CoB. Intermixing induced variations in magnetic properties are ascribed to the formation of magnetic dead layer arising from CoIr alloying in the metallic stacks. It also produces systematic variations in saturation magnetization, by as much as 30%, across stacks. Crucially, the resulting crossover CoB thickness for the transition from perpendicular to in-plane magnetic anisotropy differs by more than 2$\times$ across the stacks. Finally, with thermal annealing treatment over moderate temperatures of 150–300 °C, the magnetic anisotropy increases monotonically across all stacks, coupled with discernibly larger H_c for the metallic stacks. These are attributed to thermally induced CoPt alloying and MgO crystallization in the metallic and oxide stacks, respectively. Remarkably, the CoB in the Pt/CoB/MgO stacks retains its amorphous nature after annealing. Our results set the stage for harnessing the collective attributes of amorphous CoB-based material platforms and associated annealing processes for modulating magnetic interactions, enabling the tuning of chiral magnetic texture properties in ambient conditions.

Perfect optical vortices (POVs) arevortex beams with infinitely narrow rings and fixed radii independent of their topological charges. Here we propose the concept of generalized POVs (GPOVs) along arbitrary curves beyond the regular shapes of circles and ellipses. GPOVs share similar properties to POVs, such as defined only along infinitely narrow curves and owning topological charges independent of scale. Using a rigorous mathematical derivation in a curvilinear coordinate, we reveal theoretically that the GPOVs have a topological charge proportional to the area of the swept sector in tracing the curve, suggesting a unique mode for optical vortex beams. Experimentally, the complex-amplitude masks to generate the GPOVs are realized by using a pure-amplitude digital micro-mirror device with the super-pixel encoding technique. The phase profiles of the generated GPOVs are retrieved experimentally through self-built interferometry and exhibit good agreement with the simulations. We also derive a properly modified formula to yield the intensity-uniform GPOVs along predesigned curves, which might find applications in optical tweezers and communications.

In this paper we present our solid-state nanosecond pulse source (the solid-state impedance-matched Marx generator) which can generate arbitrary waveforms and which can be used for pulsed discharge generation. The purpose of the development of such a generator is twofold: by being able to change the waveform at will, we aim to control the discharge generated by such pulses very precisely which can be very useful for plasma applications, but also for more fundamental studies. In the presented study, we applied the arbitrary-waveform pulse source for streamer discharge generation in a cylinder-wire-like arrangement and used the arbitrary-waveform capability to change the rise time (in our experiments we used 6.8–26.2 ns) of unipolar positive pulses with 6-10 kV amplitude and 80 ns duration. Additionally, we introduced variations of a step in the rising edge of the waveform. We performed measurements both in air and nitrogen to electrically characterize the discharge while analyzing the streamer propagation in the plasma reactor with intensified charge-coupled device imaging and measured ozone generation (in air). The results show that we can indeed control the propagation of the streamer discharge with the stepped waveform, but that the rise-time variation has little effect on the streamer propagation in our system. However, the streamer velocity and structure differs significantly comparing discharges in nitrogen and air for the same applied voltage waveform. Additionally, for some of the stepped waveforms we found a slight increase of the ozone yield for air at low overall energy densities.

This study investigates the effects of voltage rise rates from 0.1 kV ns⁻¹ to 100 kV ns⁻¹ on streamer discharge in air at atmospheric pressure using numerical simulation. The curvature of the needle electrode is also used as an input parameter to change the streamer inception voltage and the average voltage, V_ave, during streamer propagation. The results show that the streamer propagation velocity, v_pri, the electric field strength at the streamer head, E_h, the diameter of the streamer channel, and the O radical production characteristics during the primary streamer propagation are dependent on V_ave. v_pri and E_h exhibit different effects on V_ave/d, where d is the gap distance of the electrode, which indicates that the streamer remains in the transient state from the filament-type discharge to the diffuse-type discharge. The comparison between the simulation and experimental results shows that the simulated v_pri characteristics are in good agreement with the experimental results, regardless of the voltage waveform and electrode configuration, which indicates that V_ave/d has a strong correlation with the primary streamer characteristics.

Understanding the role of physical processes contributing to breakdown is critical for many applications in which breakdown is undesirable, such as capacitors, and applications in which controlled breakdown is intended, such as plasma medicine, lightning protection, and materials processing. The electron emission from the cathode is a critical source of electrons which then undergo impact ionization to produce electrical breakdown. In this study, the role of secondary electron yields due to photons (γ_ph) and ions (γ_i) in direct current breakdown is investigated using a particle-in-cell direct simulation Monte Carlo model. The plasma studied is a one-dimensional discharge in 50 Torr of pure helium with a platinum cathode, gap size of 1.15 cm, and voltages of 1.2–1.8 kV. The current traces are compared with experimental measurements. Larger values of γ_ph generally result in a faster breakdown, while larger values of γ_i result in a larger maximum current. The 58.4 nm photons emitted from He(2¹P) are the primary source of electrons at the cathode before the cathode fall is developed. Of the values of γ_ph and γ_i investigated, those which provide the best agreement with the experimental current measurements are γ_ph = 0.005 and γ_i = 0.01. These values are significantly lower than those in the literature for pristine platinum or for a graphitic carbon film which we speculate may cover the platinum. This difference is in part due to the limitations of a one-dimensional model but may also indicate surface conditions and exposure to a plasma can have a significant effect on the secondary electron yields. The effects of applied voltage and the current produced by a UV diode which was used to initiate the discharge, are also discussed.

The details of the charged particle separation by mass in the configuration with axial magnetic and radial electric fields are studied. The radial electric field, oriented to the discharge axis, is induced in a background reflex discharge with a hot cathode (−550 V, 8–14 A). The plasma source is based on a hot cathode arc discharge with independent metal vapor injection (18–21 V, 30 A) was situated at 18 cm from the axis. It was shown that the separated Ag + Pb mixture is transported across the magnetic field under the background discharge electric field. Effective separation is possible in such a system, while the separation coefficient increases from 4.9 to 6.2–8.4 when the mixture injection point is moved away from the background discharge axis from 18 to 23 cm. The effect of mixture injection on the plasma potential distribution is examined. It was shown that the presence of a plasma source of separated substances can cause a local (1–2 cm) distortion of the background plasma potential profile. Such distortion, as well as fluctuations of the background plasma potential, can significantly affect the width of the deposited spots of separated substances.

We present a study of the electrical, structural and chemical properties of Ti contacts on atomic layer deposited α-Ga₂O₃ film. Ti forms an ohmic contact with α-Ga₂O₃. The contact performance is highly dependent on the post-evaporation annealing temperature, where an improved conductivity is obtained when annealing at 450 °C, and a strong degradation when annealing at higher temperatures. Structural and chemical characterisation by transmission electron microscopy techniques reveal that the electrical improvement or degradation of the contact upon annealing can be attributed to oxidation of the Ti metallic layer by the Ga₂O₃ film in combination with the possibility for Ti diffusion into the Au layer. The results highlight that the grain boundaries and inclusions in the Ga₂O₃ film provide fast diffusion pathways for this reaction, leaving the α-Ga₂O₃ crystallites relatively unaffected—this result differs from previous reports conducted on β-Ga₂O₃. This study underlines the necessity for a phase-specific and growth method-specific study of contacts on Ga₂O₃ devices.

Conductivity is a fundamental property of materials; in particular the precise quantification of electrical sheet resistance is essential for the development of electronic thin-film devices. Conventionally, resistive probes are used to perform the corresponding measurements. However, non-invasive methods are more desirable as they minimize the required sample preparation, as well as geometrical influences. Existing non-contact conductivity measurements mostly rely on the transmission of electromagnetic waves through conductive thin-films. Hence, they only characterize translucent samples at high frequencies. We present an alternative technique based on the attenuation of low frequency 10 kHz electric fields. The approach is used to quantify the field-effect induced conductivity variation of a flexible indium-gallium-zinc-oxide semiconductor film. A custom-built high-impedance electrometer is used to capacitively measure the attenuation of an alternating electric field passing through the film. The obtained data is discussed and related to the absolute conductivity with the aid of two bespoke models describing the impedance mismatch between the sample and its surroundings. The sheet conductivity is modulated and measured from 660 nS$_{\Box}$ to 116 µS$_{\Box}$ while conventional DC current/voltage measurements serve as a reference. Both methods show a high degree of correlation to the reference measurements. Unlike techniques based on light, the low frequency signal used here resembles quasi-static characteristics, and enables the direct measurement of DC parameters.

Misfit dislocations (MDs) classically form at interfaces when an epitaxially strained film exceeds a critical thickness. We show that metastable MDs also form between layers nominally below critical thickness with respect to each other when externally driven threading dislocations (TDs) have significant dissimilarities in dislocation mobility in these layers controlled by glide kinetics. The InAs quantum dot laser on silicon presents a technologically important case for this phenomenon where TDs are pinned by indium-containing regions but glide in GaAs or AlGaAs cladding regions driven by thermal expansion mismatch strain with silicon during sample cool down following growth. This generates long MDs adjacent to the active region that is responsible for gradual degradation in performance. We calculate the driving force for MD formation and its dynamics in model structures, building up to full lasers, and describe the design of intentionally introduced indium-containing trapping layers that displace the MDs away from the active region, which is key to long laser lifetime. We show that factors controlling dislocation glide kinetics: doping, indium alloying, and dislocation core character have a strong influence on the final structure of defects. Yet, the introduction of indium must be done with care, illustrated using two cases where indium is not useful to overall device defect engineering.

Hydrocarbons are of great importance in carbon-bearing fluids in deep Earth and in ice giant planets at extreme pressure (P)–temperature (T) conditions. Raman spectroscopy is a powerful tool to study the chemical speciation of hydrocarbons. However, it is challenging interpreting Raman data at extreme conditions. Here, we performed ab initio molecular dynamics simulations coupled with the modern theory of polarization to calculate Raman spectra of methane, ethane and propane up to 48 GPa and 2000 K. Our method includes anharmonic temperature effects. We studied the pressure and temperature effects on the Raman bands and identified the characteristic Raman modes for the C–C and C–C–C bonds. To the best of our knowledge, this is the first time that the Raman spectra of hydrocarbons have been calculated at extreme P–T conditions. Our results may help with the interpretation of in situ Raman data of hydrocarbons at extreme P–T conditions, with important implications for studying hydrocarbon reactions in the deep carbon cycle inside Earth and the composition of ice giant planets.

Facilitating the separate production of ozone (O₃) and nitrogen oxides (NO_x) in air discharges without a thermal process is of most merit in diversifying plasma technology; in particular, it is a primary requirement in certain cold, heat-sensitive plasma applications. Here, we propose a new method of nonheating ozone suppression in air discharges. The present work demonstrates that controlling the plasma chemical kinetics by adjusting the duration (width) and/or repetition frequency of the high-voltage DC pulse is effective in suppressing ozone formation in a surface dielectric barrier discharge in static ambient air. The temporal development of each oxygen- and nitrogen-related species in air discharge is complicated and shows different trends in the time range <10 µs; relatively long-lived O₃ and NO_x are strongly governed by the temporal behavior of short-lived reactive species, such as excited N₂(A) and N₂(v). To quantify time-varying O₃ and NO_x, an in situ UV absorption spectroscopy is applied to our gas-tight plasma reactor, which is operated in air at 21 °C. With a fixed frequency at 10 kHz and decreasing pulse duration from 10 μs to 0.18 μs, ozone is quenched faster in the plasma reactor, resulting in an irreversible chemical mode transition from an O₃- to NO-rich environment. From a different set of experiment (with a 200 ns pulse duration and a frequency range of 1–10 kHz), we can conclude that the off-pulse period also plays a crucial role in the temporal evolution of O₃ and NO_x; the larger the applied driving frequency is, the earlier the ozone-free phenomenon appears over the discharge time. Our findings represent a breakthrough in expanding the usage of air discharges and their application in various fields of interest.

The molecular beam epitaxy-grown epitaxial, partially relaxed, GaAs$_{1-x}$Bi_x bismide layers of ${\sim}1\ \mu\textrm{m}$ thickness and x ≈ 0.04 composition are examined. The atomic-structure analysis by x-ray diffraction and transmission electron microscopy shows the bismides to be CuPt$_\mathrm{B}$-type atomic-ordered in both $\mathrm{B}_+$ and $\mathrm{B}_-$ subvariants. The ordering induces an optical anisotropy, which manifests at normal incidence light-beam propagation. The anisotropy is revealed by various optical spectroscopy techniques—polarized photoluminescence, photo-modulated transmittance and reflectance, polarized transmittance, and spectroscopic ellipsometry. The ordering-induced valence band splitting, determined from modulation spectroscopy measurements, is of about $60\ \textrm{meV}$. The splitting is comparable to that in CuPt-ordered conventional III–V semiconductor alloys even though the maximal ordering parameter in investigated bismides is one order of magnitude lower.

Carbon monoxide flame band emission (CO + O → CO₂ + hν) in CO₂ microwave plasma is quantified by obtaining absolute calibrated emission spectra at various locations in the plasma afterglow while simultaneously measuring gas temperatures using rotational Raman scattering. Comparison of our results to literature reveals a contribution of O₂ Schumann–Runge UV emission at T > 1500 K. This UV component likely results from the collisional exchange of energy between CO₂(¹B) and O₂. Limiting further analysis to T < 1500 K, we demonstrate the utility of CO flame band emission by analyzing afterglows at different plasma conditions. We show that the highest energy efficiency for CO production coincides with an operating condition where very little heat has been lost to the environment prior to ∼3 cm downstream, while simultaneously, T ends up below the level required to effectively freeze in CO. This observation demonstrates that, in CO₂ plasma conversion, optimizing for energy efficiency does not require a sophisticated downstream cooling method.

Single-charge pumps are the main candidates for quantum-based standards of the unit ampere because they can generate accurate and quantized electric currents. In order to approach the metrological requirements in terms of both accuracy and speed of operation, in the past decade there has been a focus on semiconductor-based devices. The use of a variety of semiconductor materials enables the universality of charge pump devices to be tested, a highly desirable demonstration for metrology, with GaAs and Si pumps at the forefront of these tests. Here, we show that pumping can be achieved in a yet unexplored semiconductor, i.e. germanium. We realise a single-hole pump with a tunable-barrier quantum dot electrostatically defined at a Ge/SiGe heterostructure interface. We observe quantized current plateaux by driving the system with a single sinusoidal drive up to a frequency of 100 MHz. The operation of the prototype was affected by accidental formation of multiple dots, probably due to disorder potential, and random charge fluctuations. We suggest straightforward refinements of the fabrication process to improve pump characteristics in future experiments.

Applied cold atmospheric plasma allows for the controlled delivery of reactive oxygen and nitrogen species tailored for specific applications. Through the manipulation of the plasma parameters, feed gases, and careful consideration of the environment surrounding the treatment target, selective chemistries that preferentially influence the target can be produced and delivered. To demonstrate this, the COST reference microscale atmospheric pressure plasma jet is used to study the generation and transport of O and $^\cdot$OH from the gas phase through the liquid to the biological model target cysteine. Relative and absolute species densities of $^\cdot$OH and O are measured in the gas phase through laser induced fluorescence (LIF) and two-photon absorption LIF respectively. The transport of these species is followed into the liquid phase by hydrogen peroxide quantification and visualized by a fluorescence assay. Modifications to the model biological sample cysteine exposed to $^\cdot$OH and H₂O₂ dominated chemistry (He/H₂O (0.25%)) and O dominated chemistry (He/O₂ (0.6%)) is measured by FTIR spectroscopy. The origin of these species that modify cysteine is considered through the use of heavy water (H$_2^{18}$O) and mass spectrometry. It is found that the reaction pathways differ significantly for He/O₂ and He/H₂O. Hydrogen peroxide is formed mainly in the liquid phase in the presence of a substrate for He/O₂ whereas for He/H₂O it forms in the gas phase. The liquid chemistry resulting from the He/O₂ admixture mainly targets the sulfur moiety of cysteine for oxidation up to irreversible oxidation states, while He/H₂O treatment leads preferentially to reversible oxidation products. The more O or OH/H₂O₂ dominated chemistry produced by the two gas admixtures studied offers the possibility to select species for target modification.

We report lock-in thermoreflectance (LITR) measurements of spin-caloritronic phenomena in magnetic hybrid structures. In the LITR measurements, the temperature modulation signals due to the spin Peltier effect (SPE) and anomalous Ettingshausen effect (AEE) were detected through the temperature dependence of the optical reflectivity of the sample surface. By using the lock-in technique, the reflectivity modulation in response to an alternating charge current applied to the samples can be detected sensitively, which allows us to clarify the transient responses of the SPE and AEE and gives a clue to separate these phenomena. We applied this method to a junction system comprising a ferromagnetic metal film formed on a magnetic insulator substrate and found that the transient response unique to the SPE can be observed even when the SPE and AEE coexist. We also checked that the lock-in signals of the reflected light intensity exhibit no light polarization rotation; the observed temperature modulation is free from parasitic magneto-optical effects. The SPE and AEE can thus be measured not only by the conventional laser-based LITR but also by light-emitting-diode-based LITR, which enables versatile and cost-effective measurements.

Exciton related processes in two-dimensional (2D) transition metal dichalcogenides (TMDCs) play important roles in their optoelectronic applications. In this work, through broadband transient absorption spectroscopy, the electronic band structure evolution, exciton energy shifting dynamics and power-dependence spectral characteristics of WSe₂ layers, including monolayer, bilayer, tri-layer and bulk WSe₂ under 400 nm and 800 nm excitations are investigated. Particularly, under 400 nm excitation, due to the hot-exciton effect, the A-exciton energy shifting dynamics in WSe₂ layers have been analysized in detail, where thicker WSe₂ samples possess slower hot-exciton cooling lifetimes, and the exciton recombination approaches are affected by the band structure and interlayer interactions, in comparison with that under 800 nm excitation. The power-dependence spectral evolution in WSe₂ layers suggests that the charged states like trions could be facilitated in tri-layer WSe₂ (or thicker samples) at the same experimantal conditions. These findings in WSe₂ layers could provide a deep insight into the hot-exciton related processes in 2D TMDCs from transient experiments ponit of view.

In this work, we experimentally demonstrate comprehensively optimized anti-ferroelectric HfAlO_x films, achieving high saturated polarization charge density and doping concentration in doped-HfO₂ films. This allowed us to produce an ultrathin anti-ferroelectric energy storage device with high energy storage density (ESD). With the optimized deposition temperature of 300 °C, Hf:Al ratio of 18:1 and an electrode of tungsten, a 6.5 nm thick anti-ferroelectric HfAlO_x film is realized with a high ESD of 63.7 J cm⁻³, which is the thinnest anti-ferroelectric film among all the reported works, associated with such a high ESD. This not only provides an effective way to improve the scaling ability of anti-ferroelectric HfAlO_x films, but also demonstrates a new approach to strengthen the control of the phase transition.

Recently, all-inorganic cesium lead bromide (CsPbBr₃) perovskite has attracted tremendous interest due to its promise for green light photonic and optoelectronic devices. The growth of high-quality planar micro-/nano-structures of CsPbBr₃ on the technologically important silicon substrate is an important issue for these applications, which has not yet been well explored. Herein we fabricate millimeter-scale single-crystalline CsPbBr₃ microplatelets (MPs) on SiO₂/Si substrate by chemical vapor deposition. A preferred growth direction is demonstrated with the [110] direction parallel to the flux direction of carrier gas. Strong green emission is observed at room temperature with superior photo-stability, long lifetime, and excitonic features. Furthermore, the room temperature whispering-gallery-mode lasing is achieved from the focused ion beam fabricated microdisks with the thresholds comparable to those of pristine square MPs. These results suggest the great potentials of CsPbBr₃ MPs for the Si-based photonic devices.

Here we investigate the possibility of gas sensing with ultrahigh sensitivity by exploiting the exceptional point (EP) in a non-Hermitian system. The underlying mechanism is that the transverse displacement induced by the photonic spin Hall effect (PSHE), which is significantly enhanced near EPs, is ultra-sensitive to the variation of refractive index in gas media. With the application of the weak measurement technique, we find that the sensitivity of the gas sensing can reach ${10^{ - 6}}{\text{RIU }}\mu {{\text{m}}^{ - 1}}$. Moreover, if we further reduce the in-plane wavevector component of probe Gaussian light, the sensitivity can be readily improved, up to ${10^{ - 8}}{\text{RIU }}\mu {{\text{m}}^{ - 1}}$, which is much better than that of previous PSHE-based gas sensors operating away from the EP. Therefore, our proposed scheme may be favorable for the development of high-performance sensors.

Layered transition metal dichalcogenide materials grown over a conventional 3D semiconductor substrate have ignited a spark of interest in the electronics industry. The integration of these 2D layered materials extensively addresses the formidable challenges faced by a new generation of opto-electronic and photovoltaic devices. Herein, we have demonstrated a 2D/3D heterojunction type photodetector by depositing MoS₂ on a GaN substrate with a mass-scalable sputtering method. Spectroscopic and microscopic characterizations expose the signature of the highly crystalline, homogeneous and controlled growth of a deposited few-layer MoS₂ film. The greater light absorption of few-layer MoS₂ results in the high performance of the MoS₂/GaN photodetector. Our device shows high external spectral responsivity (~10³ A W⁻¹) and detectivity (~10¹¹ Jones) with a very fast response time (~5 ms). Our obtained results are significantly better than previous MoS₂- and GaN-based photodetectors. This work unveils a new perspective in MoS₂/GaN heterojunctions for high-performance optoelectronic applications.

Recently, 2D layered transition metal dichalcogenides (TMDs) have attracted great scientific interest in electrochemical energy storage research. Vanadium disulfide (VS₂) as an important family member of TMDs, is a promising electrode material for lithium-ion cells because of its remarkable electrical conductivity and high Li⁺ diffusion rate, but its electrochemical reaction mechanism is still poorly understood. Herein, we have prepared VS₂ nanosheets as the electrode and systematically investigated its structural and chemical evolution during the electrochemical processes by employing both in situ and ex situ x-ray spectroscopy. The VS₂ undergoes intercalation and conversion reactions in sequence during discharge and this process is found to be partially reversible during the subsequent charge. The decreased reversibility of the conversion reaction over extended cycles could be mainly responsible for the capacity fading of the VS₂ electrode. In addition, the hybridization strength between S and V shows a strong dependence on the states of charge, as directly illustrated by the intensity change of the V–S hybridized states and pure V states. We have also found that the solid electrolyte interphase on the electrode surface is dynamically evolved during cycling, which may be a universal phenomenon for the conversion-based electrodes. This study is expected to be beneficial for the further development of high-performance VS₂-based electrodes.

Electromagnetically induced transparency (EIT) and Autler–Townes splitting (ATS) originating from multilevel atomic systems have similar transparency windows in transmission spectra which causes confusion when discriminating between them, despite the difference in their physical mechanisms. Indeed, Fano interference is involved in EIT but not in ATS. There has been significant interest in the classic analogues of EIT and ATS in recent years, such as in photonics, plasmonics, optomechanics; however, the acoustic analogue of ATS has been rarely studied. In this work, we propose to investigate these phenomena in a pillared metasurface consisting of two lines of pillars on top of a thin plate. The existence of Fabry–Pérot resonance and the intrinsic resonances of the two lines of pillars act as a three-level atomic system that gives rise to the acoustic analogue of EIT and ATS. Since the frequency of Fabry–Pérot resonance can be tuned by controlling the distance between the two lines, the underlying physics, whether Fano interference is involved or not, is quite clear in order to discriminate between them. The realizations of EIT and ATS are put forward to control elastic waves for potential applications such as sensing, imaging, filtering.

The laser-produced plasma and laser-induced breakdown spectroscopy (LIBS) of two steels from a nuclear power plant, including the Z3CN20-09M steel used for the main reactor coolant pipes, and the 16MND5 steel used for nuclear reactor pressure vessels. These were investigated based on a fiber-optic LIBS system. Under the same input laser energy, the laser-produced plasma from the Z3CN20-09M steel had a weaker emission intensity compared to the 16MND5 steel, probably due to a greater content of Cr. Thus, plasma from the Z3CN20-09M steel had a larger kinetic energy and induced a faster expansion shockwave in air. Calibrations of the fiber-optic LIBS system by the means of internal standardization, support vector regression and random forest regression were set up. For the Cr element, as a major element in Z3CN20-09M (19.33wt.%), it was determined with the relative error controlled under 3.5%; as a trace element (0.11wt.%) in 16MND5 it was measured with the best root-mean-square error of prediction as 0.032wt.%, and the Mn content (1.19–1.51wt.%) in both steels was measured with the relative error around 10%.

Line-scanning temporal focusing microscopy (LTFM) is promising for various biomedical studies, due to its higher imaging speed compared to the gold-standard point-scanning two-photon microscopy while maintaining the same axial confinement. However, LTFM is susceptible to deteriorated performances in excitation efficiency, penetration depth, and resolutions, as a result of wavefront distortions in deep-tissue imaging. Here, we report the hybrid spatio-spectral coherent adaptive compensation (HSSCAC), the first reported technique to fully correct the wavefront distortions in LTFM. Compared with the conventional hill-climbing method, HSSCAC can fully compensate the aberrations. We demonstrate the performance of HSSCAC in deep imaging of neurons in cleared mouse brains and in vivo dynamic imaging of microglias in living mouse brains.

A thorough understanding of the skyrmion motion through nanotracks is a prerequisite to realize the full potential of spintronic applications like the skyrmion racetrack memory. One of the challenges is to place the data, i.e. skyrmions, on discrete fixed positions, e.g. below a read or write head. In the domain-wall racetrack memory, one proposed solution to this problem was patterning the nanotrack with notches. Following this approach, this paper reports on the skyrmion mobility through a nanotrack with periodic notches (constrictions) made using variations in the chiral Dzyaloshinskii–Moriya interaction. We observe that such notches induce a coupling between the mobility and the skyrmion breathing mode, which manifests itself as velocity-dependent oscillations of the skyrmion diameter and plateaus in which the velocity is independent of the driving force. Despite the fact that domain walls are far more rigid objects than skyrmions, we were able to perform an analogous study and, surprisingly, found even larger plateaus of constant velocity. For both systems it is straightforward to tune the velocity at these plateaus by changing the design of the notched nanotrack geometry, e.g. by varying the distance between the notches. Therefore, the notch-induced coupling between the excited modes and the mobility could offer a strategy to stabilize the velocity against unwanted perturbations in racetrack-like applications. In the last part of the paper we focus on the low-current mobility regimes, whose very rich dynamics at nonzero temperatures are very similar to the operating principle of recently developed probabilistic logic devices. This proves that the mobility of nanomagnetic structures through a periodically modulated track is not only interesting from a fundamental point of view, but has a future in many spintronic applications.

A new diagnostic approach using multi-mode microwave cavity resonance spectroscopy (MCRS) is introduced. This can be used to determine electron dynamics non-invasively in an absolute sense, as a function of time and spatially resolved. Using this approach, we have for the first time fully mapped electron dynamics specifically during the creation and decay of a highly transient pulsed plasma induced by irradiating a background gas with extreme ultraviolet (EUV) photons. In cylindrical geometry, electron densities as low as 10¹² m⁻³ could be detected with a spatial resolution of (sub)100 µm and a temporal resolution of (sub)100 ns. Our experiments clearly show production of electrons even after the in-band EUV irradiation fades out. This phenomenon can be explained by both photoionization by out-of-band EUV radiation emitted by the EUV source later in time and delayed electron impact ionization by electrons initially created by in-band EUV photoionization. From the analysis, the absolute width of the electron cloud in the probing volume could also be retrieved temporally resolved. This data clearly indicates cooling of electrons. From an application perspective, it is demonstrated that the method can be used as a non-invasive and in-line monitor for ionizing radiation in terms of beam power, profile and pointing stability.

FAPbI₃ perovskites are excellent candidates for fabrication of perovskite solar cells (PSCs) with high efficiency and stability. However, these perovskites exhibit phase instability problem at room temperature. In this work, to address this challenge we use methylammonium chloride (MACl) as an additive and employed a layer-by-layer thermal evaporation technique to fabricate high-quality perovskite films on a large scale of 25 cm². The optimized perovskite films show high crystallinity with large grains in the µm-range and reveals phase stability due to the presence of MACl after the annealing process. Finally, we achieved PSCs with 17.7% and 15.9% for active areas of 0.1 cm² and 0.8 cm², respectively.

Solar-powered vanadium redox-flow batteries (VRFB) have emerged as an attractive method for large-scale and efficient energy storage and conversion. However, due to the stringent charging voltage requirements of vanadium-based systems (1.4–1.7 V), common photobatteries, applying standard photovoltaics with nonoptimized photovoltages, cannot be completely charged bias-free, i.e. by only using bias-free solar energy, or if they can be, only at unpractical low current densities of just a few mA cm⁻². In response to this critical challenge, the present study aimed to design and test a compact device combining a high-photovoltage silicon multijunction solar cell with an all-vanadium continuous-flow battery. In particular, we applied a monolithic triple junction solar cell, which can provide photovoltage of up to 2.2 V. Additionally, we have introduced the concept of increased illumination intensity for the solar VRFB. As a first demonstration, a complete bias-free solar charging at 25 mA cm⁻² (300 mW cm⁻² illumination) is reported. Moreover, we investigated the influence of the operation parameters of the redox-flow battery itself: the membrane type and the vanadium concentration in the electrolyte (i.e. storage capacity). The presented results provide evidence that the low-cost thin-film silicon based solar VRFB can be considered as an outstanding alternative for practical energy storage and conversion usage. A maximum bias-free solar conversion efficiency of 12.3% was achieved during charging, combined with promising and competitive energy efficiencies for the complete charge–discharge process that can guarantee an overall solar-to-electricity conversion efficiency of  >10%.

This paper experimentally discusses roles of dinitrogen pentoxide (N₂O_5gas) in the air plasma effluent gas on the effluent gas exposed liquid water, through nitrate () generation and nitration processes in the liquid phase. The generation from N₂O_5gas is suppressed approximately up to 50% by a competing reaction with a p-hydroxyl-phenyl-acetate (p-HPA) at pH  =  11. This indicates a near-surface generation of an intermediate species derived from N₂O_5gas, which turns to . Also, it is found that the nitration reaction rate observed with NiSPY-3N can be of the order of the generation rate. From experimental results on the air plasma effluent gas and the liquid phase species, the role of the N₂O_5gas exposure to the liquid surface is interpreted as an intermediate species supply to the liquid surface, such as the solvated nitronium ion (). This deduced roles of N₂O_5gas can contribute to developments and understanding of the N₂O_5gas-involved plasma biological applications.

A double layer DBD plasma jet driven by a pulsed generator is used for SiO_x thin film deposition. The effects of dielectric and conductive targets on discharge behavior and film properties are analyzed. Different fraction of N₂ is added to the working gas (argon) to modulate film growth rate. The electrical property and optical emission of the plasma jet at different positions are characterized. Film surface morphology, composition and thickness are measured for different targets. The excitation temperature (T_exc), vibrational temperature (T_vib) and rotational temperature (T_rot) are calculated. The relationship between the discharge behavior and the film properties is studied. Results show that the relative optical emission intensities of Si, O, Ar and N₂, as well as T_exc, T_vib and T_rot are higher for the PMMA target between the high voltage electrode and the grounded electrode (referred to as the H–G region). However, T_vib and T_rot appear to be higher on the surface of the copper (conductive) target. The morphology of SiO_x film is different: nanoparticles are formed on the PMMA (dielectric) target while dense amorphous film is formed on the copper target. With the same depositing parameters, the thickness of the SiO_x film is higher on the copper target. The presence of PMMA at the downstream of the plasma can be considered as a small capacitor with a large resistance, which inhibits the current into the primary circuit passing through the H–G region. Although the presence of a dielectric target enhances the discharge in the H–G region, the charge accumulation on the PMMA surface limits the film growth. These results may contribute to the development of next-generation plasma surface modification technologies for diverse applications in electronics, material synthesis and other related areas.

A computationally assisted spectroscopic technique to measure secondary electron emission coefficients (γ-CAST) in capacitively-coupled radio-frequency plasmas is proposed. This non-intrusive, sensitive diagnostic is based on a combination of phase resolved optical emission spectroscopy and particle-based kinetic simulations. In such plasmas (under most conditions in electropositive gases) the spatio-temporally resolved electron-impact excitation/ionization rate features two distinct maxima adjacent to each electrode at different times within each RF period. While one maximum is the consequence of the energy gain of electrons due to sheath expansion, the second maximum is produced by secondary electrons accelerated towards the plasma bulk by the sheath electric field at the time of maximum voltage drop across the adjacent sheath. Due to these different excitation/ionization mechanisms, the ratio of the intensities of these maxima is very sensitive to the secondary electron emission coefficient γ. This sensitvity, in turn, allows γ to be determined by comparing experimental excitation profiles and simulation data obtained with various γ-coefficients. The diagnostic, tested here in a geometrically symmetric argon discharge, yields an effective secondary electron emission coefficient of $\gamma =0.066\pm 0.01$ for stainless steel electrodes.

We report on the detailed molecular beam epitaxial growth and characterization of Al(Ga)N nanowire heterostructures on Si and their applications for deep ultraviolet light emitting diodes and lasers. The nanowires are formed under nitrogen-rich conditions without using any metal catalyst. Compared to conventional epilayers, Mg-dopant incorporation is significantly enhanced in nearly strain- and defect-free Al(Ga)N nanowire structures, leading to efficient p-type conduction. The resulting Al(Ga)N nanowire LEDs exhibit excellent performance, including a turn-on voltage of ∼5.5 V for an AlN nanowire LED operating at 207 nm. The design, fabrication, and performance of an electrically injected AlGaN nanowire laser operating in the UV-B band is also presented.

Magnetic induction can be regarded as a negative feedback effect, where the motive-force opposes the change of magnetic flux that generates the motive-force. In artificial electromagnetics emerging from spintronics, however, this is not necessarily the case. By studying the current-induced domain wall dynamics in a cylindrical nanowire, we show that the spin motive-force exerting on electrons can either oppose or support the applied current that drives the domain wall. The switching into the anomalous feedback regime occurs when the strength of the dissipative torque β is about twice the value of the Gilbert damping constant α. The anomalous feedback manifests as a negative domain wall resistance, which has an analogy with the water turbine.

MnBi is unusual for having a magnetic anisotropy energy which increases with temperature. Recent theoretical works have studied how the lattice effects the anisotropy. However, the role of spin fluctuations has been hitherto overlooked, even though this is the primary mechanism for the temperature dependence of anisotropy in magnetic materials. We have created a model of MnBi including all anisotropy terms which are indicated from experiments and theory. Parameterizing based on experimental measurements we used the Callen–Callen theory to calculate the temperature dependence of the magnetic anisotropy due to spin fluctuations. An excellent agreement is found with experiments, across the entire temperature range. Our results indicate the driving force to be the competition between in-plane single ion and out of plane two-ion anisotropies.

To determine the types of applications for which plasma-based water treatment (PWT) is best suited, the treatability of 23 environmental contaminants was assessed through treatment in a gas discharge reactor with argon bubbling, termed the enhanced-contact reactor. The contaminants were treated in a mixture to normalize reaction conditions and convective transport limitations. Treatability was compared in terms of the observed removal rate constant (k_obs). To characterize the influence of interfacial processes on k_obs, a model was developed that accurately predicts k_obs for each compound, as well as the contributions to k_obs from each of the three general degradation mechanisms thought to occur at or near the gas–liquid interface: 'sub-surface', 'surface' and 'above-surface'. Sub-surface reactions occur just underneath the gas–liquid interface between the contaminants and dissolved plasma-generated radicals, contributing significantly to the removal of compounds that lack surfactant-like properties and so are not highly concentrated at the interface. Surface reactions occur at the interface between the contaminants and dissolved radicals, contributing significantly to the removal of surfactant-like compounds that have high interfacial concentrations. The contaminants' interfacial concentrations were calculated using surface-activity parameters determined through surface tension measurements. Above-surface reactions are proposed to take place in the plasma interior between highly energetic plasma species and exposed portions of compounds that extend out of the interface. This mechanism largely accounts for the degradation of surfactant-like contaminants that contain highly hydrophobic perfluorocarbon groups, which are most likely to protrude from the interface. For a few compounds, the degree of exposure to the plasma interior was supported by new and previously reported molecular dynamics simulations results. By reviewing the predicted contributions from the three general mechanisms, it was determined that surface concentration is the dominant factor determining a compound's treatability. These insights indicate that PWT would be most viable for the treatment of surfactant-like contaminants.

Current perpendicular-to-plane (CPP) giant magnetoresistance (GMR) effects in devices including Co₂Fe_0.4Mn_0.6Si (CFMS)/Ag_100−xMg_x/CFMS structures were investigated theoretically and experimentally.

First-principles transport calculation revealed that the Fermi surface matching between CFMS and L1₂ Ag₃Mg is better than that between CFMS and fcc-Ag. In the experiments the Mg composition, x was changed from 0 to 26 at.%, in which both face centered cubic phase and L1₂ phase of Ag–Mg alloys are included depending on the Mg composition. It was confirmed by a cross-sectional high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) image that the Ag–Mg spacer layer with L1₂ ordered phase was successfully fabricated for x  =  22 at.%. The maximum CPP–GMR ratio and the change of the areal resistance ($\Delta RA$) were 56% and 20 m$\Omega \cdot \mu$m², respectively, for x  =  22 at.% at room temperature, which is much higher than that of the conventionally used pure Ag spacer devices. It was suggested from the HAADF-STEM images and the results of the temperature dependence of CPP–GMR effects that the diffusion of Mn atoms occurred less at the CFMS/Ag–Mg interfaces for the L1₂ ordered Ag–Mg spacer devices than the Ag spacer devices, which might be a key factor for the enhancement of the $\Delta RA$ value. The newly developed L1₂ Ag–Mg spacer is a promising material for realizing large $\Delta RA$ of the CPP–GMR devices.

Recent developments in the fabrication of lithium niobate (LiNbO₃) structures down to the nanoscale opens up novel applications of this versatile material in nonlinear optics. Current nonlinear optical studies in sub-micron waveguides are mainly restricted to the generation of second and third harmonics. In this work, we demonstrate the generation and waveguiding of the sum-frequency generation (SFG) signal in a single LiNbO₃ nanowire with a cross-section of 517 nm  ×  654 nm. Furthermore, we enhance the guided SFG signal 17.9 times by means of modal phase matching. We also display tuning of the phase-matched wavelength by varying the nanowire cross-section and changing the polarization of the incident laser. The results prove that LiNbO₃ nanowires can be successfully used for nonlinear wave-mixing applications and assisting the miniaturization of optical devices.

This article presents a kinetic Monte-Carlo study of thermally activated magnetisation dynamics in clusters of statistically distributed magnetic nanoparticles. The structure of clusters is assumed to be of fractal nature, consistently with recent observations of magnetic particle aggregation in cellular environments. The computed magnetisation relaxation decay and frequency-dependent hysteresis loops are seen to significantly depend on the fractal dimension of aggregates, leading to accelerated magnetisation relaxation and reduction in the size of hysteresis loops as the fractal dimension increases from one-dimensional-like to three-dimensional-like clusters. Discussed are implications for applications in nanomedicine, such as magnetic hyperthermia or magnetic particle imaging.

Semiconductor nanowires (NWs) have been attracting an increasing interest in the scientific community. This is due to their peculiar filamentary shape and nanoscale diameter, which renders them versatile and cost-effective components of novel technological devices and also makes them an ideal platform for the investigation of a variety of fascinating physical effects. Absorption spectroscopy is a powerful and non-destructive technique able to provide information on the physical properties of the NWs. However, standard absorption spectroscopy is hard to perform in NWs, because of their small volume and the presence of opaque substrates. Here, we demonstrate that absorption can be successfully replaced by photoluminescence excitation (PLE). First, the use of polarization-resolved PLE to address the complex and highly-debated electronic band structure of wurtzite GaAs and InP NWs is shown. Then, PLE is used as a statistically-relevant method to localize the presence of separate wurtzite and zincblende NWs in the same InP sample. Finally, a variety of resonant exotic effects in the density of states of In_xGa_1−xAs/GaAs core/shell NWs are highlighted by high-resolution PLE.

Nanoscale spacing between the plasma membrane and the underlying cortical actin cytoskeleton profoundly modulates cellular morphology, mechanics, and function. Measuring this distance has been a key challenge in cell biology. Current methods for dissecting the nanoscale spacing either limit themselves to complex survey design using fixed samples or rely on diffraction-limited fluorescence imaging whose spatial resolution is insufficient to quantify distances on the nanoscale. Using dual-color super-resolution STED (stimulated-emission-depletion) microscopy, we here overcome this challenge and accurately measure the density distribution of the cortical actin cytoskeleton and the distance between the actin cortex and the membrane in live Jurkat T-cells. We found an asymmetric cortical actin density distribution with a mean width of 230 (+105/−125) nm. The spatial distances measured between the maximum density peaks of the cortex and the membrane were bi-modally distributed with mean values of 50  ±  15 nm and 120  ±  40 nm, respectively. Taken together with the finite width of the cortex, our results suggest that in some regions the cortical actin is closer than 10 nm to the membrane and a maximum of 20 nm in others.

Recent developments in super-resolution microscopy make it possible to resolve structures in biological cells at a spatial resolution of a few nm and observe dynamical processes with a temporal resolution of ms to μs. However, the optimal structural resolution requires repeated illumination cycles and is thus limited to chemically fixed cells. For live cell applications substantial improvement over classical Abbe-limited imaging can already be obtained in adherent or slow moving cells. Nonetheless, a large group of cells are fast moving and thus could not yet be addressed with live cell super-resolution microscopy. These include flagellate pathogens like African trypanosomes, the causative agents of sleeping sickness in humans and nagana in livestock.

Here, we present an embedding method based on a in situ forming cytocompatible UV-crosslinked hydrogel. The fast cross-linking hydrogel immobilizes trypanosomes efficiently to allow microscopy on the nanoscale. We characterized both the trypanosomes and the hydrogel with respect to their autofluorescence properties and found them suitable for single-molecule fluorescence microscopy (SMFM). As a proof of principle, SMFM was applied to super-resolve a structure inside the living trypanosome. We present an image of a flagellar axoneme component recorded by using the intrinsic blinking behavior of eYFP.

Surface interactions involving biomembranes, such as cell–cell interactions or membrane contacts inside cells play important roles in numerous biological processes. Structural insight into the interacting surfaces is a prerequisite to understand the interaction characteristics as well as the underlying physical mechanisms. Here, we work with simplified planar experimental models of membrane surfaces, composed of lipids and lipopolymers. Their interaction is quantified in terms of pressure–distance curves using ellipsometry at controlled dehydrating (interaction) pressures. For selected pressures, their internal structure is investigated by standing-wave x-ray fluorescence (SWXF). This technique yields specific density profiles of the chemical elements P and S belonging to lipid headgroups and polymer chains, as well as counter-ion profiles for charged surfaces.

Lipid packing is a crucial feature of cellular membranes. Quantitative analysis of membrane lipid packing can be achieved using polarity sensitive probes whose emission spectrum depends on the lipid packing. However, detailed insights into the exact mechanisms that cause the changes in the spectra are necessary to interpret experimental fluorescence emission data correctly. Here, we analysed frequently used polarity sensitive probes, Laurdan and di-4-ANEPPDHQ, to test whether the underlying physical mechanisms of their spectral changes are the same and, thus, whether they report on the same physico-chemical properties of the cell membrane. Steady-state spectra as well as time-resolved emission spectra of the probes in solvents and model membranes revealed that they probe different properties of the lipid membrane. Our findings are important for the application of these dyes in cell biology.

The integration of increasingly complex functionalities within thermally drawn multi-material fibers is heralding a novel path towards advanced soft electronics and smart fabrics. Fibers capable of electronic, optoelectronic, piezoelectric or energy harvesting functions are created by assembling new materials in intimate contact within increasingly complex architectures. Thus far, however, the opportunities associated with the integration of cantilever-like structures with freely moving functional domains within multi-material fibers have not been explored. Used extensively in the micro-electromechanical system (MEMS) technology, electro-mechanical transductance from moving and bendable domains is used in a myriad of applications. In this article we demonstrate the thermal drawing of micro-electromechanical fibers (MEMF) that can detect and localize pressure with high accuracy along their entire length. This ability results from an original cantilever-like design where a freestanding electrically conductive polymer composite film bends under an applied pressure. As it comes into contact with another conducting domain, placed at a prescribed position in the fiber cross-section, an electrical signal is generated. We show that by a judicious choice of materials and electrical connectivity, this signal can be uniquely related to a position along the fiber axis. We establish a model that predicts the position of a local touch from the measurement of currents generated in the 1D MEMF device, and demonstrate an excellent agreement with the experimental data. This ability to detect and localize touch over large areas, curved surfaces and textiles holds significant opportunities in robotics and prosthetics, flexible electronic interfaces, and medical textiles.

Nano-contact spin-torque vortex oscillators (STVOs) are anticipated to find application as nanoscale sources of microwave emission in future technological applications. Presently the output power and phase stability of individual STVOs are not competitive with existing oscillator technologies. Synchronisation of multiple nano-contact STVOs via magnetisation dynamics has been proposed to enhance the microwave emission. The control of device-to-device variations, such as mode splitting of the microwave emission, is essential if multiple STVOs are to be successfully synchronised. In this work a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM) was used to demonstrate how mode splitting in the microwave emission of STVOs was related to the magnetisation dynamics that are generated. The free-running STVO response to a DC current only was used to identify devices and bias magnetic field configurations for which single and multiple modes of microwave emission were observed. Stroboscopic Kerr images were acquired by injecting a small amplitude RF current to phase lock the free-running STVO response. The images showed that the magnetisation dynamics of a multimode device with moderate splitting could be controlled by the injected RF current so that they exhibit similar spatial character to that of a single mode. Significant splitting was found to result from a complicated equilibrium magnetic state that was observed in Kerr images as irregular spatial characteristics of the magnetisation dynamics. Such dynamics were observed far from the nano-contact and so their presence cannot be detected in electrical measurements. This work demonstrates that TRSKM is a powerful tool for the direct observation of the magnetisation dynamics generated by STVOs that exhibit complicated microwave emission. Characterisation of such dynamics outside the nano-contact perimeter permits a deeper insight into the requirements for optimal phase-locking of multiple STVOs that share common magnetic layers.

Bare unpassivated GaAs nanowires feature relatively high electron mobilities (400–2100 cm² V⁻¹ s⁻¹) and ultrashort charge carrier lifetimes (1–5 ps) at room temperature. These two properties are highly desirable for high speed optoelectronic devices, including photoreceivers, modulators and switches operating at microwave and terahertz frequencies. When engineering these GaAs nanowire-based devices, it is important to have a quantitative understanding of how the charge carrier mobility and lifetime can be tuned. Here we use optical-pump–terahertz-probe spectroscopy to quantify how mobility and lifetime depend on the nanowire surfaces and on carrier density in unpassivated GaAs nanowires. We also present two alternative frameworks for the analysis of nanowire photoconductivity: one based on plasmon resonance and the other based on Maxwell–Garnett effective medium theory with the nanowires modelled as prolate ellipsoids. We find the electron mobility decreases significantly with decreasing nanowire diameter, as charge carriers experience increased scattering at nanowire surfaces. Reducing the diameter from 50 nm to 30 nm degrades the electron mobility by up to 47%. Photoconductivity dynamics were dominated by trapping at saturable states existing at the nanowire surface, and the trapping rate was highest for the nanowires of narrowest diameter. The maximum surface recombination velocity, which occurs in the limit of all traps being empty, was calculated as 1.3  ×  10⁶ cm s⁻¹. We note that when selecting the optimum nanowire diameter for an ultrafast device, there is a trade-off between achieving a short lifetime and a high carrier mobility. To achieve high speed GaAs nanowire devices featuring the highest charge carrier mobilities and shortest lifetimes, we recommend operating the devices at low charge carrier densities.

There is a growing literature database that demonstrates the therapeutic potential of cold atmospheric plasma (herein referred to as plasma). Given the breadth of proposed applications (e.g. from teeth whitening to cancer therapy) and vast gamut of plasma devices being researched, it is timely to consider plasma interactions with specific components of the cell in more detail. Plasma can produce highly reactive oxygen and nitrogen species (RONS) such as the hydroxyl radical (OH^•), peroxynitrite (ONOO⁻) and superoxide ($\text{O}_{2}^{-}$) that would readily modify essential biomolecules such as DNA. These modifications could in principle drive a wide range of biological processes. Against this possibility, the reported therapeutic action of plasmas are not underpinned by a particularly deep knowledge of the potential plasma-tissue, -cell or -biomolecule interactions.

In this study, we aim to partly address this issue by developing simple models to study plasma interactions with DNA, in the form of DNA-strand breaks. This is carried out using synthetic models of tissue fluid, tissue and cells. We argue that this approach makes experimentation simpler, more cost-effective and faster than compared to working with real biological materials and cells. Herein, a helium plasma jet source was utilised for these experiments. We show that the plasma jet readily induced DNA-strand breaks in the tissue fluid model and in the cell model, surprisingly without any significant poration or rupture of the phospholipid membrane. In the plasma jet treatment of the tissue model, DNA-strand breaks were detected in the tissue mass after pro-longed treatment (on the time-scale of minutes) with no DNA-strand breaks being detected in the tissue fluid model underneath the tissue model. These data are discussed in the context of the therapeutic potential of plasma.