Table of contents

Silicon-doped n-type (0 0 0 1) AlGaN materials with 60% and 85% AlN content were studied close to the doping condition that gives the lowest resistivity (Si/III ratios in the ranges 2.8–34  ×  10⁻⁵ and 1.3–6.6  ×  10⁻⁵, respectively). Temperature-dependent conductivity and Hall-effect measurements showed that, apart from the diffusion-like transport in the conduction band, a significant amount of the conductivity was due to phonon-assisted hopping among localized states in the impurity band, which became almost completely degenerate in the most doped sample of the Al_0.6Ga_0.4N series. In the doping range explored, impurity-band transport was not only dominant at low temperature, but also significant at room-temperature, with contributions to the total conductivity up to 46% for the most conductive sample. We show that, as a consequence of this fact, the measurements of Hall carrier concentration and Hall mobility using the usual single-channel approach are not reliable, even at high temperatures. We propose a simple method to separate the contributions of the two channels. Our model, although only approximate, can be used to gain insight into the doping mechanism: particularly it shows that the room-temperature free-electron concentration in the conduction band of the Al_0.6Ga_0.4N material reaches its maximum at about 1.6  ×  10¹⁸ cm⁻³, well below the value that would have been obtained with the standard single-channel analysis of the data. This maximum is already achieved at dopant concentrations lower than the one that gives the best conductivity. However, further increase of the doping levels are required to enhance the impurity-band channel, with concentrations of the carriers participating in this type of transport that increase from 2.1  ×  10¹⁸ cm⁻³ up to 4.3  ×  10¹⁸ cm⁻³. For the Al_0.85Ga_0.15N, even though it was not possible to estimate the actual carrier concentrations, our measurements suggest that a significant impurity-band channel is present also in this material.

A streamer is a non-linear and non-local gas breakdown mode. Its large-scale coherent structures, such as the ionization front, are the final results of a hierarchical cascade starting from the single particle dynamics. Therefore, this phenomenon covers, by definition, different space and time scales. In this study, we have reproduced the ionization front formation and development by means of a particle-based numerical methodology. The physical system investigated concerns of a high-voltage ns-pulsed surface dielectric barrier discharge. Different reduced electric field regimes ranging from 50 to 500 Td have been considered for two gases: pure atomic Ar and molecular N₂. Results have shown the detailed structure of the negative streamer: the leading edge, the head, the interior and the tail. Its dynamical evolution and the front propagation velocity have been calculated for the different cases. Finally, the deviation of the electron energy distribution function from equilibrium behavior has been pointed out as a result of a fast and very localized phenomenon.

Mn₂VAl Heusler alloy films were epitaxially grown on MgO(1 0 0) single-crystal substrates by ultra-high-vacuum magnetron sputtering. A2- and L2₁-type Mn₂VAl order was controlled by the deposition temperature. A2-type Mn₂VAl films showed no spontaneous magnetization, while L2₁-type Mn₂VAl films showed ferrimagnetic behaviour with a maximum saturation magnetization of 220 emu cm⁻³ at room temperature (RT). An antiferromagnetic reflection was observed with neutron diffraction at RT for an A2-type Mn₂VAl film deposited at 400 °C. A bilayer sample of the antiferromagnetic A2-type Mn₂VAl and Fe showed an exchange bias of 120 Oe at 10 K.

The magnetic and magnetocaloric properties of a self-doped MnNiGe alloy of nominal composition MnNi_0.9Ge_1.1 have been investigated in ambient as well as in high pressure conditions. It orders ferromagnetically below around 225 K and undergoes first order martensitic phase transition (MPT) to an antiferromagnetic (AFM) martensite phase below 147 K. This self-doping results in a significant decrease in the lattice volume and hence the Mn–Mn intra-layer distance which induces ferromagnetism (FM) in otherwise AFM alloys. MPT affects this FM ordering and the alloy becomes predominantly AFM in nature below the structural transition temperature. The observed values of the magnetocaloric effects (MCE) are reasonably large at the magnetic (−5.5 J kg⁻¹ K⁻¹ for magnetic field changing from 0 to 50 kOe around 210 K) and structural (8.3 J kg⁻¹ K⁻¹ for magnetic field changing from 0 to 50 kOe around 136 K) transition temperatures in ambient condition. MCE is found to decrease with increasing external hydrostatic pressure (P) at MPT region, whilst this external P has vanishingly small effect on MCE around the magnetic transition temperature.

This paper presents an experimental study of the inverse spin Hall effect (ISHE) in a bilayer consisting of a yttrium iron garnet (YIG) and platinum (Pt) loaded on a metamaterial split ring resonator (SRR). The system is excited by a microstrip feed line which generates both surface and bulk spin waves in the YIG. The spin waves subsequently undergo spin pumping from the YIG film to an adjacent Pt layer, and is converted into a charge current via the ISHE. It is found that the presence of the SRR causes a significant enhancement of the mangetic field near the resonance frequency of the SRR, resulting in a significant increase in the ISHE signal. Furthermore, the type of spin wave generated in the system can be controlled by changing the external applied magnetic field angle ($\theta_{\rm H}$). When the external applied magnetic field is near parallel to the microstrip line ($\theta_{\rm H} = 0$), magnetostatic surface spin waves are predominantly excited. On the other hand, when the external applied magnetic field is perpendicular to the microstrip line ($\theta_{\rm H} = \pi/2$), backward volume magnetostatic spin waves are predominantly excited. Hence, it can be seen that the SRR structure is a promising method of achieving spin-charge conversion, which has many advantages over a coaxial probe.

The scaled target measurement is an important method to get the target characteristic. Radar absorbing materials are widely used in the low detectable target, considering the absorbing material frequency dispersion characteristics, it makes designing and manufacturing scaled radar absorbing materials on the scaled target very difficult. This paper proposed a wide band design method on the scaled absorbing material of the thin absorption coating with added carbonyl iron particles. According to the theoretical radar cross section (RCS) of the plate, the reflection loss determined by the permittivity and permeability was chosen as the main design factor. Then, the parameters of the scaled absorbing materials were designed using the effective medium theory, and the scaled absorbing material was constructed. Finally, the full-size coating plate and scaled coating plates (under three different scale factors) were simulated; the RCSs of the coating plates were numerically calculated and measured at 4 GHz and a scale factor of 2. The results showed that the compensated RCS of the scaled coating plate was close to that of the full-size coating plate, that is, the mean deviation was less than 0.5 dB, and the design method for the scaled material was very effective.

The tetragonal crystalline structure and magnetic properties of MgO/Rh/(Fe_1−xCo_x)_0.9V_0.1 and MgO/Rh/(Fe_1−xCo_x)_0.9V_0.05C_0.05 films (0.4 ${\leqq}$x$\leqq$ 0.7, thickness t  =  2–50 nm) were studied. For both the systems, the films with t  =  5 nm showed a tetragonal distortion (c/a) of ~1.15 at x  =  0.6. Furthermore, in the case of (Fe_1−xCo_x)_0.9V_0.05C_0.05, the films showed a c/a value of ~1.06 even at t  =  50 nm. The magnetic anisotropy induced by the tetragonal distortion of films became 1.4  ×  10⁷ erg cm⁻³ for (Fe_1−xCo_x)_0.9V_0.1 (t  =  5 nm) and (Fe_1−xCo_x)_0.9V_0.05C_0.05 (t  =  5 nm) films. We also investigated the tetragonal distortion stability of the films using their enthalpy of formation (ΔH) values obtained from density functional theory calculations.

Herein, we report the appearance of a large exchange bias effect in a moderate cooling field (cooling field, $H_{\rm FC} = 1$ kOe) for the 314—Sr₃YCo₄O_10.5 material. The exchange bias has started to appear near room temperature and reaches a maximum value of 5.5 kOe at 4 K. The existence of ferrimagnetic clusters in the compensated host in this layered structure originates the large exchange anisotropy. Remarkably, the observed value of moderate magnetic field induced exchange bias field is extremely large in comparison with material systems which are recognized to exhibit giant exchange bias effect. In combination with the feasibility of room temperature application, the appearance of large exchange bias in a moderate cooling field exemplifying the present material system as a promising class of compounds for designing coherent magnetic materials with huge exchange bias in low/moderate magnetic field.

In this paper, we study the domain wall dynamics in high magnetic flux density ($B_S \sim 1.82$–1.85 T) of soft magnetic ribbons Fe_81.2Co₄Si_0.5B_9.5P₄Cu_0.8 prepared with various annealing conditions. It is observed that annealing these ribbons beyond a temperature ($T_a \sim 400~^{\circ}$C) crystalizes the alloy with the appearance of α-Fe(-Co) phase. The post annealing leads to a change from amorphous to nanocrystalline phase accompanied by a reduction in internal stress in the ribbons. Domain imaging has been performed under both dc and ac fields to study the domain wall behavior in these ribbons. A wide variety of domain structures are observed in ribbons prepared with different annealing temperature as a result of the change in the intrinsic anisotropy. Under an ac magnetic field, the 'stress patterns' in the amorphous ribbons are robust. However, ribbons with reduced stress (i.e. nanocrystalline) exhibit domain wall propagation which hints at a significant reduction in magnetocrystalline and magnetoelastic anisotropies due to uniform nanocrystallization. The ribbon annealed at $T_a \sim 460~^{\circ}$C exhibits static domain patterns resulting from strong pinning centers created by Fe-B precipitates.

Dielectrics based on amorphous sub-nanometric laminates of TiO₂ and Al₂O₃ are subject to elevated dielectric losses and leakage currents, in large parts due to the extremely thin individual layer thickness chosen for the creation of the Maxwell–Wagner relaxation and therefore the high apparent dielectric constants. The optimization of performances of the laminate itself being strongly limited by this contradiction concerning its internal structure, we will show in this study that modifications of the dielectric stack of capacitors based on these sub-nanometric laminates can positively influence the dielectric losses and the leakage, as for example the nature of the electrodes, the introduction of thick insulating layers at the laminate/electrode interfaces and the modification of the total laminate thickness. The optimization of the dielectric stack leads to the demonstration of a capacitor with an apparent dielectric constant of 90, combined with low dielectric loss (tan δ) of 7 · 10⁻² and with leakage currents smaller than 1  ×  10⁻⁶ A cm⁻² at 10 MV m⁻¹.

Zinc oxide (ZnO) has recently attracted attention for its potential application to high speed electronics. In this work, a high speed Schottky diode rectifier was fabricated based on a ZnO thin film deposited by plasma-enhanced atomic layer deposition and a PtO_x Schottky contact deposited by reactive radio-frequency sputtering. The rectifier shows an ideality factor of 1.31, an effective barrier height of 0.79 eV, a rectification ratio of 1.17  ×  10⁷, and cut-off frequency as high as 550 MHz. Low frequency noise measurements reveal that the rectifier has a low interface-state density of 5.13  ×  10¹² cm⁻² eV⁻¹, and the noise is dominated by the mechanism of a random walk of electrons at the PtO_x/ZnO interface. The work shows that the rectifier can be used for both noise sensitive and high frequency electronics applications.

In this study, high conductivity and transparent multi-layer (AZO/Al/AZO-/Al/AZO) source/drain (S/D) electrodes for thin film transistors were fabricated via conventional physical vapor deposition approaches, without toxic elements or further thermal annealing process. The 68 nm-thick multi-layer films with excellent optical properties (transparency: 82.64%), good electrical properties (resistivity: 6.64  ×  10⁻⁵ Ω m, work function: 3.95 eV), and superior surface roughness (R_q  =  0.757 nm with scanning area of 5  ×  5 µm²) were fabricated as the S/D electrodes. Significantly, comprehensive performances of AZO films are enhanced by the insertion of ultra-thin Al layers. The optimal transparent TFT with this multi-layer S/D electrodes exhibited a decent electrical performance with a saturation mobility (µ_sat) of 3.2 cm² V⁻¹ s⁻¹, an I_on/I_off ratio of 1.59  ×  10⁶, a subthreshold swing of 1.05 V/decade. The contact resistance of AZO/Al/AZO/Al/AZO multi-layer electrodes is as low as 0.29 MΩ. Moreover, the average visible light transmittance of the unpatterned multi-layers constituting a whole transparent TFT could reach 72.5%. The high conductivity and transparent multi-layer S/D electrodes for transparent TFTs possessed great potential for the applications of the green and transparent displays industry.

Double perovskite materials have been studied in detail by many researchers, as their magnetic and electronic properties can be controlled by the substitution of alkaline earth metals or lanthanides in the A site and transition metals in the B site. Here we report the temperature-driven, p–n-type conduction switching assisted, large change in thermopower in La³⁺-doped Sr₂TiFeO₆-based double perovskites. Stoichiometric compositions of La_xSr_2−xTiFeO₆ (LSTF) with 0  ⩽  x  ⩽  0.25 were synthesized by the solid-state reaction method. Rietveld refinement of room-temperature XRD data confirmed a single-phase solid solution with cubic crystal structure and $Pm\bar{3}m$ space group. From temperature-dependent electrical conductivity and Seebeck coefficient (S) studies it is evident that all the compositions underwent an intermediate semiconductor-to-metal transition before the semiconductor phase reappeared at higher temperature. In the process of semiconductor–metal–semiconductor transition, LSTF compositions demonstrated temperature-driven p–n-type conduction switching behavior. The electronic restructuring which occurs due to the intermediate metallic phase between semiconductor phases leads to the colossal change in S for LSTF oxides. The maximum drop in thermopower (ΔS ~ 2516 µV K⁻¹) was observed for LSTF with x  =  0.1 composition. Owing to their enormous change in thermopower of the order of millivolts per kelvin, integrated with p–n-type resistance switching, these double perovskites can be used for various high-temperature multifunctional device applications such as diodes, sensors, switches, thermistors, thyristors, thermal runaway monitors etc. Furthermore, the conduction mechanisms of these oxides were explained by the small polaron hopping model.

The recombination processes of excitons in Fe-doped GaN have been characterized by time-resolved photoluminescence measurement. The photoluminescence shape at different excitation powers revealed that the hole capture process by Fe²⁺ centers has always existed in doped GaN. Decreasing of the fast component in the biexponential decay curves with increasing iron concentration indicates the enhancive contribution of the hole capture process. Furthermore, the structures of an iron-related acceptor and complex bound exciton were confirmed by the dependence of lifetime constants on the localization energy. Also, the extended wave function of the hole from the complex bound exciton will enable spin coupling between isolated iron ions.

We have investigated the temperature-dependent photoluminescence (TDPL) profiles of Eu³⁺ ions implanted in an HVPE-grown bulk GaN sample doped with Mg and of donor–acceptor pairs (DAP) involving the shallow Mg acceptor in GaN(Mg) (unimplanted) and GaN(Mg):Eu samples. Below 125 K, the TDPL of Eu³⁺ in GaN(Mg):Eu correlates with that of the DAP. Below 75 K, the intensity of Eu³⁺ emission saturates, indicating a limitation to the numbers of Eu–Mg defects available to receive excitation transferred from the host, while the DAP continues to increase, albeit more slowly in the implanted than the unimplanted sample. Prolonged exposure to UV light at low temperature results in the photodissociation of Eu–Mg defects in their Eu1(Mg) configuration, with a corresponding increase in shallow DAP emission and the emergence of emission from unassociated Eu_Ga (Eu2) defects.

A comprehensive study on the whispering gallery mode (WGM) lasing characteristics of a rhodamine 6G (R6G)-based flexible bottle-like microcavity has been performed. The closed-loop WGMs excited inside the microcavity were found strongly confined along the radial direction. Thus, the spacing of ultralow loss lasing modes can be realized by controlling the diameter of the bottle-like microcavity. By studying the WGM lasing action, the dependence of lasing threshold and Q-factor on the radius of the microcavity was analyzed. The lasing threshold for the random gain medium was reduced under the influence of radial optical confinement by increasing diameter. In addition, the WGM lasing characteristics of the bottle-like microcavity were also explored by the three-dimensional finite-difference time-domain method. Our approach provides a solution scheme towards the research for lasing characteristic of novel nanocrystal fluorescence or quantum dot materials based on the flexible ultrahigh Q factor bottle-like microcavity.

Incorporation of fluorine (F) into the AlGaN layer is crucial to the fabrication of enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistors (HEMTs). However, the understanding of properties of F doping in AlGaN alloys is rather limited. Using first-principles calculations and the special quasirandom structure (SQS) approach, we investigate the alloying effects on the doping properties of F-incorporated Al_xGa_1−xN alloys. We find that substitutional F on N sites (F_N) and interstitial F (F_i) are dominant defects for F in Al_xGa_1−xN alloys. For these two types of defects, both the global composition x and the local motif surrounding the dopant play important roles. On contrary, the incorporation of substitutional F on Ga sites (F_Ga) or Al sites (F_Al) are affected only by the composition x. We also find that there exists a large asymmetric bowing for the effective formation energies of F_N and F_i. These results are explained in terms of local structural distortion and electronic effects. The mechanism discussed in this paper can also be used in understanding doping in other semiconductor alloys.

As one of the prominent applications in intelligent systems, gas sensing technology has attracted great interest in both industry and academia. In the current study, the pristine graphyne (GY) without and with a single Mn atom is investigated to detect the gas molecules (CO, CH₄, CO₂, NH₃, NO and O₂). The pristine GY is promising to detect O₂ molecules because of its chemical adsorption on GY with large electron transfer. The great stability of the Mn/GY is found, and the Mn atom prefers to anchor at the alkyne ring as a single atom. Upon single Mn atom anchoring, the sensitivity and selectivity of GY based gas sensors is significantly improved for various molecules, except CH₄. The recovery time of the Mn/GY after detecting the gas molecules may help to appraise the detection efficiency for the Mn/GY. The current study will help to understand the mechanism of detecting the gas molecules, and extend the potentially fascinating applications of GY-based materials.

Although MoS₂ field-effect transistors (FETs) with high-k dielectrics are promising for electron device applications, the underlying physical origin of interface degradation remains largely unexplored. Here, we present a systematic analysis of the energy distribution of the interface state density (D_it) and the quantum capacitance (C_Q) in a dual-gate monolayer exfoliated MoS₂ FET. The C_Q analysis enabled us to construct a D_it extraction method as a function of E_F. A band tail distribution of D_it with the lowest value of 8  ×  10¹¹ cm⁻² eV⁻¹ suggests that D_it is not directly related to the sharp peak energy distribution of the S vacancy. Therefore, the Mo–S bond bending related to the strain at the interface or the surface roughness of the SiO₂/Si substrate might be the origin. It is also shown that ultra-thin 2D materials are more sensitive to interface disorder due to the reduced density of states. Since all the constituents for the measured capacitance are well understood, I–V characteristics can be reproduced by utilizing the drift current model. As a result, one of the physical origins of the metal/insulator transition is suggested to be the external outcome of interface traps and quantum capacitance.

The spatial control of the nano-emitters in novel light harvesting platforms offers great potential for the manipulation of the excitonic interaction amongst the donor–acceptor pairs of energy transferring agents. In this work, we report colloidal quantum dot loaded electrospun nanofibers as a light harvesting platform to study the excitonic interaction among them. The donor emission lifetime modified from 12.46 ns to 7.45 ns with the change in the ratio of green and red quantum dots in the nanofiber, as a result of confining acceptor quantum dots in close proximity. The spectrally narrow emitter luminescent nanofiber platforms have further been investigated for their potential of white light generation. The hybrid platform of blue LED integrated electrospun nanofibers has been shown to demonstrate a correlated color temperature of 3632.5 K, luminous efficacy of optical radiation value of 307.7 lm/W_opt along with color rendering index value of 60.

Broadband THz radiation emission from photoexcited GaSb_1−xBi_x (0  <  x  <  1.65%) was investigated. We observed an enhancement in the THz emission with Bi incorporation in GaSb. GaSb_1−xBi_x alloy with $x\sim 1.65\%$ showed an emission amplitude comparable to that from LT-GaAs, which is over an order of magnitude higher compared to GaSb. Transient photocurrents due to p-type surface band bending was found to be the dominant emission mechanism in these alloys. Strengthening of the surface field due to Bi induced p-doping is responsible for the observed enhancement. The NIR pump incident angle and THz detection angle study suggests that the photocarrier momentum is preferentially aligned along the refracted excitation beam direction inside the semiconductor alloy. GaSb_1−xBi_x emitter showed better spectral performance compared to a commercially available photoconductive antenna in a THz time domain spectroscopy system. This demonstrates the application potential of this material in THz spectroscopy.

Conductive solid material sampling by micro-plasma under ambient atmosphere was studied experimentally. A high-voltage pulse generator was utilized to drive discharge between a tungsten needle and metal samples. The effects of pulse width on discharge, micro-plasma and sampling were investigated. The electrical results show that two discharge current pulses can be formed in one voltage pulse. The duration of the first current pulse is of the order of 100 ns. The duration of the second current pulse depends on the width of the voltage pulse. The electrical results also show that arc micro-plasma was generated during both current pulses. The results of the emission spectra of different sampled materials indicate that the relative emission intensity of elemental metal ions will increase with pulse width. The excitation temperature and electron density of the arc micro-plasmas increase with the voltage pulse width, which contributes to the increase of relative emission intensity of metal ions. The optical images and energy dispersive spectroscopy results of the sampling spots on metal surfaces indicate that discharge with a short voltage pulse can generate a small sputtering crater.

The article describes the complex study of the interaction of a helium plasma jet with distilled water and saline. The discharge development, spatial distribution of the excited species, electric field measurement results and the results of the Schlieren imaging are presented. The results of the experiments showed that the plasma–liquid interaction could be prolonged with the proper choice of the gas composition between the jet nozzle and the target. This depends on the gas flow and the target distance. Increased conductivity of the liquid does not affect the discharge properties significantly. An increase of the gas flow enables an extension of the plasma duration on the liquid surface up to 10 µs, but with a moderate electric field strength in the ionization wave. In contrast, there is a significant enhancement of the electric field on the liquid surface, up to 30 kV cm⁻¹ for low flows, but with a shorter time of the overall plasma liquid interaction. Ignition of the plasma jet induces a gas flow modification and may cause turbulences in the gas flow. A significant influence of the plasma jet causing a mixing in the liquid is also recorded and it is found that the plasma jet ignition changes the direction of the liquid circulation.

With a plasma assisted gas condensation system it is possible to achieve high-purity nanoporous Au (np-Au) structures with minimal contaminations and impurities. The structures consist of single Au-nanoparticles, which partially sintered together due to their high surface to volume ratio. Through electron microscopy investigations a porosity  >50% with ligament sizes between 20–30 nm was revealed. The elastic modulus of the np-Au was determined via peak force quantitative nanomechanical mapping and resulted in values of 7.5  ±  1.5 GPa. The presented structures partially sintered at room temperature, but proved to be stable to electron irradiation with energies of 7 MeV up to doses of 100 MGy. The electron irradiation stability opens the venue for electron assisted functionalization with biomolecules.

A modified Ångström method was used to determine the thermal diffusivity and thermal conductivity of aqueous dispersions of multiwalled carbon nanotubes as a function of their weight fraction concentration and in the presence of an externally applied electric field. Measurements were performed in planar samples, with a fixed thickness of 3.18 mm applying an AC voltage in the range from 0 to $70~V_{\rm RMS}$ and for concentrations of carbon nanotubes from 0 to 2 wf%. It is shown that this field induces the formation of clusters followed by their alignment along the electric field, which can favor heat transfer in that direction. Heat transfer measurements show two regimes, in the first one under 0.5 wf%, voltages lower than $30~V_{\rm RMS}$ are not strong enough to induce the adequate order of the carbon nanostructures, and as a consequence, thermal diffusivity of the dispersion remains close to the thermal diffusivity of water. In contrast for higher concentrations (above 1.5 wf%), $10~V_{\rm RMS}$ are enough to get a good alignment. Above such thresholds of concentrations and voltages, thermal diffusivity and conductivity increase, when the electric field is increased, in such a way that for an applied voltage of $20~V_{\rm RMS}$ and for a concentration of 1.5 wf%, an increase of 49% of the thermal conductivity was obtained. It is also shown that this approach exhibits limits, due to the fact that the electric-field induced structure, can act as a heating element at high electric field intensities and carbon nanotubes concentrations, which can induce convection and evaporation of the liquid matrix.

Several members of a large family of perovskite-like halides with a common chemical formula, ABX₃ (A  =  monovalent, B  =  divalent, and X  =  halogen ion), are being investigated for their interesting properties and potential technological applications. CsCaI₃ and KCaI₃ are two such ionic compounds who are of interest in the quest for superior and cost-effective alternatives to NaI or CsI based scintillators. They are the subject of this first-principles based computational study. Both are wide-gap materials having primarily I 5p and Ca 3d characters near the valence and conduction band edges, respectively. Although built from [CaI₆] octahedral motifs, structural differences between the two compounds is reflected in anisotropic electron effective mass and distinctive formation and migration of self-trapped holes. We discuss these properties as they relate to scintillation decay and proportional light yield.

Laser-shock experiments were performed on a ternary ${\rm {Zr}_{50}{Cu}_{40}{Al}_{10}}$ bulk metallic glass. A spalling process was studied through post-mortem analyses conducted on a recovered sample and spall. Scanning electron microscopy magnification of fracture surfaces revealed the presence of a peculiar feature known as cup-cone. Cups are found on sample fracture surface while cones are observed on spall. Two distinct regions can be observed on cups and cones: a smooth viscous-like region in the center and a flat one with large vein-pattern in the periphery. Energy dispersive spectroscopy measurements conducted on these features emphasized atomic distribution discrepancies both on the sample and spall. We propose a mechanism for the initiation and the growth of these features but also a process for atomic segregation during spallation. Cup and cones would originate from cracks arising from shear bands formation (softened paths). These shear bands result from a quadrupolar-shaped atomic disorder engendered around an initiation site by shock wave propagation. This disorder turns into a shear band when tensile front reaches spallation plane. During the separation process, temperature gain induced by shock waves and shear bands generation decreases material viscosity leading to higher atomic mobility. Once in a liquid-like form, atomic clusters migrate and segregate due to inertial effects originating from particle velocity variation (interaction of release waves). As a result, a high rate of copper is found in sample cups and high zirconium concentration is found on spall cones.

The inverse problem of determining three parameters: the film thickness, the film and substrate Young's moduli of a film/substrate bilayer by indentation, is formulated and solved. The physical mechanism for the solvability of the inverse problem is that these three parameters have different impacts at different indentation depth. Their impacts are systematically studied, which also provides a different approach of finding the three parameters or refining their range. Compared with various atomic force microscopy based techniques of detecting subsurface structures, which have to deal with an extremely difficult or even an insurmountable inverse problem with the integral equation of dynamics, the inverse problem here formulated by statics is much more straightforward and simpler. Formulating and solving such an inverse problem can be of some help to the applications such as characterizing subsurface structures, the out-of-plane properties of two-dimensional (2D) materials and various bilayer structures.

The room temperature resistive switching behavior in 50 keV O⁺-ion irradiated TiO_2−x layers at an ion fluence of 5  ×  10¹⁶ ions cm⁻² is reported. A clear transformation from columnar to layered polycrystalline films is revealed by transmission electron microscopy with increasing ion fluence, while the complementary electron energy loss spectroscopy suggests an evolution of oxygen vacancy (OV) in TiO_2−x matrix. This is further verified by determining electron density with the help of x-ray reflectivity. Both local and device current–voltage measurements illustrate that the ion-beam induced OVs play a key role in bistable resistive switching mechanism.

The effect of water absorption on the dielectric response of polyethylene/hexagonal boron nitride nanocomposites has been studied by dielectric spectroscopy. The nanocomposites have been prepared with hBN concentrations ranging from 2 wt% to 30 wt%. Fourier transform infrared spectroscopy and thermogravimetric analysis revealed a very small amount of hydroxyl groups on the surface of hBN. Mass loss measurements showed that the nanocomposites did not absorb any water under ambient and dry conditions while there was some water absorption under wet conditions. The dielectric spectroscopy results showed a broad relaxation peak, indicative of different states of water with water shells of different thickness, which moved to higher frequencies with increasing water content. However, the dielectric losses were significantly lower than the losses reported in the literature of nanocomposites under wet conditions. In addition, all the absorbed water was successfully removed under vacuum conditions which demonstrated that the interactions between the water and the nanocomposites were very weak, due to the hydrophobic nature of the hBN surface. This is a highly useful property, when considering these materials for applications in electrical insulation.

We demonstrate the immobilisation of polystyrene nanoparticles on vertical nano-electrodes by means of dielectrophoresis. The electrodes have diameters of 500 nm or 50 nm, respectively, and are arranged in arrays of several thousand electrodes, allowing many thousands of experiments in parallel. At a frequency of 15 kHz, which is found favourable for polystyrene, several occupation patterns are observed, and both temporary and permanent immobilisation is achieved. In addition, a histogram method is applied, which allows to determine the number of particles occupying the electrodes. These results are validated with scanning electron microscopy images. Immobilising exactly one particle at each electrode tip is achieved for electrode tip diameters with half the particle size. Extension of this system down to the level of single molecules is envisaged, which will avoid ensemble averaging at still statistically large sample sizes.

Novel duct graphitic carbon nitride (DCN) was successfully prepared using the temperature control method in a quartz tube furnace from commercially available melamine and evaluated against the photo-degradation of latent organic pollutants, acarbose (ACB). These prepared materials were characterized by UV–Vis spectroscopy, Fourier transform infrared spectra, x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy and scanning electron microscopy. The characterization results indicated that the synthesized material was in the form of a duct-like structure and has greater adsorption capacity and photocatalytic ability as compared to traditionally synthesized graphitic carbon nitride materials. The DCN split theACB completely into many intermediates, which were depicted in the HPLC-MS spectrum for knowing the acarbose photo-degrdation pathway. The duct-like morphology of graphitic carbon nitride has improved properties, such as increasing the surface area and decelerating the e⁻/h⁺ recombination, which increase the light absorbance ability with enhanced photoactivity.

Electrolyzer stacks in a bipolar architecture (cells connected in series) are desirable since power provided to a stack can be transferred at high voltages and low currents and thus the losses in the power bus can be reduced. The anode electrodes (active electrodes) considered as part of this study are single sided but there are manufacturing cost advantages to implementing double side anodes in the future. One of the main concerns with a bipolar stack implementation is the existence of leakage currents (bypass currents). The leakage current is associated with current paths that are not between adjacent anode and cathode pairs. This leads to non uniform current density distributions which compromise the electrochemical conversion efficiency of the stack and can also lead to unwanted side reactions. The objective of this paper is to develop modelling tools for a bipolar architecture consisting of two single sided cells that use single sided anodes. It is assumed that chemical reactions are single electron transfer rate limited and that diffusion and convection effects can be ignored. The design process consists of the flowing two steps: development of a large signal model for the stack, and then the extraction of a small signal model from the large signal model. The small signal model facilitates the design of a controller that satisfies current or voltage regulation requirements. A model has been developed for a single cell and two cells in series but can be generalized to more than two cells in series and to incorporate double sided anode configurations in the future. The developed model is able to determine the leakage current and thus provide a quantitative assessment on the performance of the cell.

This paper offers an in-depth look into beam shaping and polarization control as two of the most promising techniques for improving industrial laser cutting of metal sheets. An assessment model is developed for the study of such effects. It is built upon several modifications to models as available in literature in order to evaluate the potential of a wide range of considered concepts. This includes different kinds of beam shaping (achieved by extra-cavity optical elements or asymmetric diode staking) and polarization control techniques (linear, cross, radial, azimuthal). A fully mathematical description and solution procedure are provided. Three case studies for direct diode lasers follow, containing both experimental data and parametric studies. In the first case study, linear polarization is analyzed for any given angle between the cutting direction and the electrical field. In the second case several polarization strategies are compared for similar cut conditions, evaluating, for example, the minimum number of spatial divisions of a segmented polarized laser beam to achieve a target performance. A novel strategy, based on a 12-division linear-to-radial polarization converter with an axis misalignment and capable of improving cutting efficiency with more than 60%, is proposed. The last case study reveals different insights in beam shaping techniques, with an example of a beam shape optimization path for a 30% improvement in cutting efficiency. The proposed techniques are not limited to this type of laser source, neither is the model dedicated to these specific case studies. Limitations of the model and opportunities are further discussed.

In this study, multi-walled carbon nanotube (MWCNT) filled polyevinelidenefluoride-trifluoroethylene-chlorofluoroethylene composites are used to realize fractional-order capacitors (FOCs). A solution-mixing and drop-casting approach is used to fabricate the composite. Due to the high aspect ratio of MWCNTs, percolation regime starts at a small weight percentage (wt%), 1.00%.The distributed MWCNTs inside the polymer act as an electrical network of micro-capacitors and micro-resistors, which, in effect, behaves like a FOC. The resulting FOCs' constant phase angle (CPA) can be tuned from $-65{\hspace{0pt}}^\circ$ to $-7{\hspace{0pt}}^\circ$ by changing the wt% of the MWCNTs. This is the largest dynamic range reported so far at the frequency range from 150 kHz to 2 MHz for an FOC. Furthermore, the CPA and pseudo-capacitance are shown to be practically stable (with less than 1% variation) when the applied voltage is, changed between 500 µV and 5 V. For a fixed value of CPA, the pseudo-capacitance can be tuned by changing the thickness of the composite, which can be done in a straightforward manner via the solution-mixing and drop-casting fabrication approach. Finally, it is shown that the frequency of a Hartley oscillator built using an FOC is almost 15 times higher than that of a Hartley oscillator built using a conventional capacitor.

Lossless coding metasurfaces and metamaterial absorbers have been widely used for radar cross section (RCS) reduction and stealth applications, which merely depend on redirecting electromagnetic wave energy into various oblique angles or absorbing electromagnetic energy, respectively. Here, an absorptive coding metasurface capable of both the flexible manipulation of backward scattering and further wideband bistatic RCS reduction is proposed. The original idea is carried out by utilizing absorptive elements, such as metamaterial absorbers, to establish a coding metasurface. We establish an analytical connection between an arbitrary absorptive coding metasurface arrangement of both the amplitude and phase and its far-field pattern. Then, as an example, an absorptive coding metasurface is demonstrated as a nonperiodic metamaterial absorber, which indicates an expected better performance of RCS reduction than the traditional lossless coding metasurface and periodic metamaterial-absorber. Both theoretical analysis and full-wave simulation results show good accordance with the experiment.

One of the established means of achieving complementary properties in soft matter is polymer blending, and hence studies on polymer blends have gained immense significance. Despite a growing body of research, the mechanisms responsible for imparting properties to a polymer blend consisting of semi-crystalline and amorphous polymer remain poorly understood. Herein, we report the dielectric and mechanical properties of a semi-crystalline polymer poly(vinylidene fluoride) (PVDF), an amorphous polymer poly(vinyl formal) (PVF) and their blends. These blends were prepared by solution mixing and solid state mixing techniques and were found to be immiscible in both cases. Their immiscibility was further confirmed by time–temperature superposition analysis. A reduction of 75% in the energy dissipation factor of PVDF was observed in the blend. On the other hand, 45% enhancement was observed in the modulus value of the PVDF–PVF blend compared with pure PVDF, which could be due to inclusion of PVF which has a high glass transition temperature. Thus, this blend can be used for high-energy-density storage, energy harvesting and celestial applications, where low losses and robust devices are desirable.