Table of contents

Realizing manipulation of the photoelectric properties of wide bandgap semiconductors is a main challenge for successful next-generation functional optoelectronics. As an intriguing wide bandgap semiconductor, β-Ga₂O₃ (e.g. ∼4.9 eV) is emerging as a promising candidate for photodetectors operating in the solar-blind region. Here, we show that by selecting substrates with different symmetries and lattice parameters (i.e. (100) MgO, (100) MgAl₂O₄ and (0001) α-Al₂O₃), epitaxial β-Ga₂O₃ films with (100)-or $\left( {\bar 201} \right)$-orientation could be fabricated. We found that the photoresponse characteristics are strongly correlated with the lattice mismatch and film orientation. In particular, (100)-oriented β-Ga₂O₃ film grown on MgO substrate with smaller lattice mismatch exhibited a 254 nm responsivity of 0.1 A · W⁻¹ and detectivity of 4.3 × 10¹² Jones, which are approximately an order of magnitude higher than that of the $\left( {\bar 201} \right)$-oriented β-Ga₂O₃ film. Our work may provide a strategy to develop further high performance solar-blind photodetectors.

The effect of pre-sowing plasma seed treatment of maize (Zea mays L.), narrow-leaved lupine (Lupinus angustifolius L.) and winter wheat (Triticum aestivum L.) on seed germination, plant resistance to common diseases during vegetation and crop yield is studied in laboratory and field experiments. It is shown the efficiency of seed treatment by low-pressure radio frequency plasma in suppression of a number of fungal crop diseases such as boil smut of maize, root rot of lupine and winter wheat at different growth stages. At the stage of V9 (9th leaf visible) the infection level in maize plants from treated seeds was 3 times less than that in control. Root rot disease development of lupine at the first stages (3rd–4th leaves emerged) of growth did not exceed 6.9% in plants from the treated seeds while reached to 47.8% in control. Pre-sowing seed treatment led to suppress the anthracnose spreading on narrow-leaved lupine up to the flowering stage. It was revealed that, due to a decrease in the level of seed infection, stimulation of field germination, early seedling growth and plant resistance to pathogens during the vegetation period, the winter wheat grain yield increased by 2.3%, maize—by 1.7%, narrow–leaved lupine—by 26.8% compared to control plants. Increases in activity of non-enzymatic antioxidants (proline, anthocyanins as well as total phenolic content) in roots of maize seedling were observed which may indicate a significant role of plasma seed treatment in improving the plant resistance to biotic and abiotic stress during the vegetation.

Phase-change memory materials are promising candidates for beyond-silicon, next-generation non-volatile-memory and neuromorphic-computing devices; the canonical such material is the chalcogenide semiconductor alloy Ge₂Sb₂Te₅. Here, we describe the results of an analysis of glassy molecular-dynamics models of this material, as generated using a newly developed, linear-scaling (O(N)), machine-learned, Gaussian approximation potential. We investigate the behaviour of the glassy models as a function of different quench rates (varied by two orders of magnitude, down to 1 K ps⁻¹) and model sizes (varied by two orders of magnitude, up to 24 300 atoms). It is found that the lowest quench rate studied (1 K ps⁻¹) is comparable to the minimum cooling rate needed in order completely to vitrify the models on quenching from the melt.

The family of atomically thin magnets holds great promise for a number of prospective applications in magneto-optoelectronics, with CrI₃ arguably being its most prototypical member. However, the formation of defects in this system remains unexplored to date. Here, we investigate native point defects in monolayer CrI₃ by means of first-principles calculations. We consider a large set of intrinsic impurities and address the atomic structure, thermodynamic stability, diffusion and aggregation tendencies as well as local magnetic moments. Under thermodynamic equilibrium, the most stable defects are found to be either Cr or I atomic vacancies along with their complexes, depending on the chemical potential conditions. These defects are predicted to be quite mobile at room and growth temperatures, and to exhibit a strong tendency to agglomerate. In addition, our calculations indicate that the deviation from the nominal stoichiometry largely impacts the magnetic moments, and the defect-induced lattice distortions can drive local ferromagnetic-to-antiferromagnetic phase transitions. Overall, this work portrays a comprehensive picture of intrinsic point defects in monolayer CrI₃ from a theoretical perspective.

Tetragonal Mn₂Au is a metallic antiferromagnet with high Néel temperature well above 1000 K. The collinear magnetic sublattice with broken inversion symmetry in company with stable in-plane anisotropy in this material favors to control it via the Néel spin–orbital effect and spin-Hall effect. In this paper, we study the spin–orbital torque induced Néel order dynamics in the heavy-metal(HM)/Mn₂Au system based on coupled Landau–Lifshitz-Gilbert-Slonczewski equations. We demonstrate robust picosecond switching of the Néel order of the Mn₂Au in the presence of two charge currents, and close resemblance of the Néel order switching of the HM/Mn₂Au bilayer to short-term plasticity and long-term potentiation observed in the biological synapses. Orders in magnitude reduction of the power consumption in the Pt/Mn₂Au bilayer comparing with the ferromagnetic tunnel junction is demonstrated also. The ultra-high speed terahertz dynamics, ultra-lower power consumption accompanying with feasible electrical manipulation make HM/Mn₂Au system a promising candidate in memory storage and neuromorphic computing applications.

Metasurfaces for beam engineering are usually implemented by using dense-plasmonic or high-contrast dielectric building blocks with deep-subwavelength feature size and strong field confinement. Here, we theoretically and experimentally demonstrate extremely large angle deflection with a very steep phase gradient based on low-contrast ($\delta n = \,0.57$) metagratings composed of very sparse building blocks. Only two ridges are included in a 4π variation supercell, which diffracts the normal excitation to desired angles as large as 80° with a theoretical efficiency of more than 80%. Due to the limited beam radius relative to the metasurfaces, the angular distribution of the intensity is broadened with reduced peak intensity. Such metagratings are 3D-printed for 0.14 THz operation. The measured peak intensities are 57% of the theoretical ones due to material loss and fabrication errors. Repeatability, bandwidth and angular sensitivity are systematically studied, and the results pave the way towards using practical THz elements for large-gradient wavefront shaping.

Nanosecond near resonant excitation in As₅₀S₅₀ thin films leads to strong nonlinear optical response, i.e. nonlinear absorption coefficient up to 4 × 10⁶ cm GW⁻¹ and nonlinear refractive index of 8.5 cm² GW⁻¹, both of which are the strongest ever reported in amorphous semiconductors. We propose a three-level energy band model to explain this effect, which indicates that the nonlinear process is reverse saturable absorption in nature, mediated by excited-state absorption from slow interband transition between the conduction and valence band. On the other hand, observation of negative nonlinear refractive index reveals the occurrence of self-defocusing effect. Finally, benefitting from the strong nonlinear response, we demonstrate a promising application of As₅₀S₅₀ thin films as an optical limiter for optoelectronic sensors.

Reliability issues in metal-oxide-semiconductor field-effect transistor (MOSFET) are closely related to oxide dielectric degradation under electric field stressing, such as stress-induced leakage current (SILC). Some studies show that SILC induced dielectric degradation can be cured by high-temperature annealing while some others show contradictory results. A possible microscopic mechanism could be related to different states of oxygen vacancies in the oxide dielectric because these defects contribute to the leakage current via trap-assisted tunneling (TAT). Here, by first-principles calculations, we demonstrate that the leakage current recovery can be explained in terms of the modulation of trap levels by extra charges. It is found that, with extra holes around the defect, the trap level will be moved far away from the Si valence band and leave the TAT window, which accordingly assists SILC recovery. On the contrary, with extra electrons around the defect, the trap level will be closer to the Si conduction band, having no contribution to the SILC recovery. This study provides a theoretical perspective on the dielectric recovery mechanisms by including the impacts of extra charges.

We present an experimental investigation on the origin of coherent random laser (RL) emission from 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminos-tyryl)-4H-pyran (DCM) doped polyvinyl alcohol (PVA) thin film (DCM-PVA) planar waveguide using various characterization techniques. Absorption and fluorescence spectra of the DCM-PVA thin film confirm the presence of aggregates at dye concentrations greater than 0.08 wt. %. Time correlated single photon counting measurements were perfomed to confirm the presence of dye aggregates in PVA matrix. x-ray diffraction studies reveal the semi-crystalline nature of the DCM-PVA thin film. The optical gain coefficient was determined by the variable stripe length method under 532 nm pulsed laser excitation and was found to be 2.1 cm⁻¹ using the one-dimensional amplifier model for the 0.08 wt. % thin film. The RL emission of the planar waveguide depends on the geometry of the excitation spot. The emission spectrum consists of randomly positioned narrow spectral lines under stripe excitation geometry whereas, a smooth spectrum lacking the narrow peaks is observed under circular spot excitation, both above the lasing thresholds. The origin of weak scattering in DCM-PVA waveguide is attributed to the formation of dye aggregates and inhomogeneities created by the semi-crystalline nature of the film. The RL threshold decreases with an increase of the stripe length due to weak waveguiding in the planar film under stripe excitation. The high optical gain and low lasing thresholds attainable in DCM-PVA waveguides make them a promising candidate for the fabrication of polymer waveguide based photonic devices.

In this paper a GST-based perfect absorber is investigated for sensing applications. The simulation results show the high sensitivity and figure of merit of 900 nm per refractive index unit (RIU) and 15 RIU⁻¹, respectively. The sensing parameters are also studied for different biomedical applications such as detection of glucose in water, malaria infection, bacillus bacteria, and cancer cells. The phase change material GST makes the structure tunable and switchable by changing the phase through different annealing temperatures. By changing temperature, the optical characteristics of GST are altered and so the perfect absorption can be tuned at different near-infrared wavelengths. Moreover, the sweep between the amorphous and crystalline states of GST results in switching capability with the high extinction ratio of 13.97 dB at λ = 1724 nm. Therefore, the perfect tunable absorber supports our new idea of multi-application plasmonic devices with two capabilities of sensing and switching, simultaneously. Also, the E-field distributions and analytical method of equivalent circuit model are utilized to verify the simulation results and demonstrate a better insight of the proposed structure performance. The polarization independency of the proposed structure, tunability, good sensing and switching performances are some other benefits of our proposed structure, which can pave the way for development of new investigations about the phase change material applications in plasmonic studies.

The effect of 1D photonic crystals on the optical transmission of VO₂ is studied by depositing thin films of VO₂ nanoparticles on SiO₂/TiO₂ distributed Bragg reflectors (DBRs) in the near-infrared (IR) spectrum as per the earlier theoretical predictions of Rashidi et al (2018 J. Phys. D: Appl. Phys.51 375102). Monoclinic VO₂ nanoparticles with tuned crystallinity are synthesized using a facile solution processing method. Moderately crystalline (MC) and highly crystalline (HC) VO₂ nanostructures are obtained by varying the synthesis temperature and post-growth annealing conditions. Both the MC VO₂ and HC VO₂ films exhibit the expected reduction in optical transmission in the IR region due to the structural phase transition from monoclinic (insulator) to rutile (metallic) around the critical temperature of 68 °C. By combining VO₂ films on a 40% transmitting DBR structure, the average optical transmission decreases further to ~20%. The number of stacks of DBRs plays a key role in such an effective reduction of optical transmission in the IR. When the number of stacks of DBRs is further increased from 4 to 7, the optical transmission of metallic VO₂ films on DBRs nearly vanishes in the near-IR spectrum in such vanadium dioxide 1D photonic crystal based composite photonic structures. Such a temperature-controlled, enhanced, broad band optical response could provide a promising design for VO₂ nanoparticle based hybrid photonic absorbers for various smart window applications.

It has been recognized that the application of magnetic shielding technology deteriorates the plume divergence efficiency of a Hall thruster. To try to alleviate this effect, three magnetic shielding configurations with different inclination directions of the lines of force in the strong magnetic field region are designed via the introduction of a peripheral coil surrounding the thruster body, and their influence on the performance and plume divergence are experimentally evaluated in this study. The measured results show that the performance and plume distribution vary significantly with the change in the inclination direction of the lines of force. Particularly, different to the unshielded Hall thruster that exhibits the best performance and minimal divergence when the strong magnetic field lines are non-inclined (perpendicular to the thruster axis), the magnetically shielded Hall thruster has the largest thrust and the smallest divergence angle when the strong magnetic field lines are inclined inward. Further theoretical research indicates that this is mainly due to the change in the ion acceleration process under the magnetic shielding discharge. This study is significant and instructive for the optimized design of magnetically shielded Hall thrusters.

We present a multi-physics model of combustion ignition phenomena in an atmospheric pressure hydrogen-air mixture ignited by a microwave surface plasma discharge. The surface plasma is generated over a resonant metasurface structure that provides sufficient field intensification to break down and sustain a discharge. Specifically, a surface electromagnetic (EM) wave mode known as the spoof surface plasmon polariton (SSPP) is excited to yield a hybrid resonance that results from coupling of cavity and surface EM wave modes. Motivated by the need for a large, volumetric ignition kernel for applications in combustion ignition, we numerically demonstrate the volumetric surface plasma discharge enabled by the use of this particular EM wave mode in a high pressure operating regime. We discuss the transient evolution of a centimeter scale plasma kernel and subsequent ignition kernel formation. High density combustion enhancing radical species (O, H, OH) are produced throughout the bulk plasma, which leads to successful ignition. A parametric study shows that the large size of a plasma kernel is attributed to the shortening of ignition delay.

The inactivation of Bacillus subtilis (ATCC 6633) spores deposited on a filter membrane was studied by using low-temperature plasma produced via surface dielectric barrier discharge. Spore samples were carefully prepared to avoid the formation of cell aggregates, and their inactivation was induced by multiple surface streamer discharge driven in a coplanar dielectric barrier discharge electrode geometry by an amplitude-modulated AC high voltage waveforms in humid air at atmospheric pressure. At a discharge duty cycle of 0.4, the surface dielectric barrier discharge is characterised by an average total power of 1.7 W (power density 1.5 W cm⁻² and energy density ∼0.3 Wh l⁻¹) and a low gas temperature of the plasma filaments of about 320 K. The spores were exposed by placing a sample holder at a fixed distance of 3 mm from the electrode surface covered by plasma filaments. Particular attention was paid to identifying sporicidal agents employed in the process of inactivation. Since treated samples did not come into direct contact with the streamer filaments and excessive heating was excluded thanks to the low energy density, our results indicate that the spores were inactivated mainly by reactive oxygen and nitrogen species such as O₃, H₂O₂ and NO₂^–. Discharge-induced damage of the spore structure was evidenced via the detection of dipicolinic acid and leaking of intracellular components. We therefore conclude that B. subtilis spores were inactivated chemically, probably due to failure of the coat structure or membrane of the spore.

While plasmas are now routinely employed to synthesize or remove nano- to micron-sized particles, the charge state (polarity and magnitude) of the particles remains relatively unknown. In this study, charging of nanoparticles was systematically characterized in low-temperature, atmospheric-pressure, flow-through plasmas previously applied for synthesis. Premade, charge-neutral nanoparticles of MgSO₄, NaCl, and sea salt were introduced into the plasma to decouple other effects such as the reactive vapor precursor, and MgSO₄ was selected as the focus because of its stability (i.e., no evaporation) in the plasma environment. The charge fraction and distribution of the particles was examined at the reactor outlet for different particle diameters (10–250 nm) as a function of plasma power and two types of power source, alternating current (AC) and radio frequency (RF). We found that the overall charge fraction increased with increasing plasma power and diameter for the RF plasma. A similar increasing trend was observed for the AC plasma with increasing particle diameter in the range of 50–250 nm, but the charge fraction increased with decreasing particle diameter in the range of 10–50 nm. The charge distribution was revealed to be bipolar, with particles supporting multiple charges for both the RF and AC plasmas, but the RF plasma produced a higher fraction of multiple charges. Differences in the characteristic timescales for particle charging in the AC and RF plasmas are a possible explanation of the trends observed in the experiments.

Through first-principle calculations, we investigate the stability, electronic structures, elasticity, piezoelectricity, and mobility of 2D BiTeI monolayer. Our calculations show that 2D BiTeI monolayer is energetically, mechanically, thermodynamically, and dynamically stable. In addition to its well-known large Rashba spin-splitting, the monolayer exhibits a high-tensile ductility and mechanical flexibility. Formation of BiTeI/BN van der Waals heterostructure makes it freestanding without perturbing the electronic structure. The broken mirror symmetry structure induces out-of-plane internal electric field of 0.289 eV Å⁻¹, yielding out-of-plane piezoelectric coefficients of 0.556 pm V⁻¹, which is larger than that of Janus group III and transition metal chalcogenide monolayers. Furthermore, the effective masses and mobility of electrons are 0.176 m_e and 392 cm² V^–1 s^–1, respectively, which are half and six times of those of the MoS₂ monolayer, respectively. These charateristics make 2D BiTeI a potential candidate for a wide variety of applications in nanoscale spin and electromechanical devices.

Small-angle x-ray scattering (SAXS) can identify a material based on its scattering features. However, SAXS techniques have been limited to the study of thin samples in the mm scale. We investigated the use of spectral SAXS (sSAXS) in the 30–45 keV energy range to identify embedded materials in up to 5 cm thick objects. A prototype sSAXS system was built by integrating a polychromatic x-ray source and a 2D spectroscopic detector in a two-pinhole collimation setup. The elastically scattered x-rays at deflection angles smaller than 10^° were measured without collimating the scattered rays or filtering the energy spectrum. Using caffeine as a target material in 1 to 5 cm polymethyl methacrylate (PMMA) slabs, we demonstrate the capability of sSAXS technique to identify caffeine targets in all cases with 600 s acquisition times. The distinct Bragg peaks of caffeine at 8.44 and 18.64 nm⁻¹ were recovered with a q-resolution of 0.6 nm⁻¹ after attenuation and background corrections. Furthermore, we show the effect of PMMA thickness and target location to estimate the caffeine amount from the recovered signal using area-under-the-peak (AUP) analysis. We found that AUPs were slightly overestimated due to signal contamination by PMMA when the scattered photons from the caffeine target traverse through 1 to 3 cm PMMA slabs. This effect was more significant in the 5 cm thick PMMA with an average transmission factor of 24%, where the AUP was overestimated approximately by 34%.

A novel magnetic drug-loaded osteoinductive Fe₃O₄/CaCO₃ hybrid microspheres system (MDHMs) with excellent drug delivery property has been fabricated by a modified coprecipitation method. The MDHMs are uniform vaterite microspheres with well-defined mesoporous nanostructures. The formation of vaterite is based on the templating effect of casein micelles, which is promising in bone tissue engineering. Cyclodextrins (CDs) increase the porosity of the microspheres with a pore size of about 7.3 nm and a surface area of about 50.73 m² g⁻¹. The mesoporous structure gives the MDHMs great drug loading efficiency of about 80.75–88.74% and the minocycline-loaded MDHMs display a sustained drug release property (about 95% in 5 d). Moreover, the MDHMs also possess stable magnetic property with a saturation magnetization of about 4.41 emu g⁻¹ and a remarkable magnetism-responsive behavior. These results demonstrate that with superior drug delivery property, osteoinductive potential and magnetic property, the MDHMs have a great potential in the treatment of peri-implantitis.

Lead telluride (PbTe) is an excellent thermoelectric material in the intermediate temperature zone and has been applied to deep space exploration, waste heat recovery and other fields. However, the low thermoelectric conversion efficiency of the n-type PbTe alloys limits its applications. Here, the thermoelectric performances have been enhanced in n-type PbTe alloys through trace bismuth (Bi) and iodine (I) co-doping. The Pb_1−xBi_xTe_1−xI_x (x = 0.00%, 0.05%, 0.10%, 0.20% and 0.50%) alloys are synthesized in the single phase compounds by a stepwise synthesis method. The carrier concentration has reached an optimal concentration range within the order of 10¹⁹ cm⁻³. The highest absolute Seebeck coefficient of 244 μV/K is obtained for 0.05% doped alloy at 730 K. The highest absolute Seebeck coefficient leads to high power factor for 0.05% doped, especially in low- and middle-temperature range. The highest power factor ∼25 μW K⁻² cm⁻¹ has been obtained at 329 K. Complex micro-scale grain boundaries and point defects strongly increase the phonon scattering and then lead to the lowest lattice thermal conductivity of 0.64 W mK⁻¹ at 674 K for x = 0.50%, which is 26% lower than that of pristine PbTe. As a result, the highest figure of merit, zT ∼ 0.9 has been determined in 0.20% doped samples at 725 K. Moreover, the highest average figure of merit, zT_ave ∼ 0.7 has been achieved in 0.05% doped samples in the 323–723 K temperature range, which is about two or three times higher than reported for single Bi or I doped PbTe samples.

The modelling and analysis of induction heating process is a strongly coupled nonlinear electromagnetic-thermodynamic problem. It is of great theoretical and practical significance to develop an effective numerical technique for solving the coupled nonlinear governing equations. This paper presents a mathematical model of the coupled electromagnetic-thermodynamic fields of induction heating problems, taking into account the nonlinearity due to temperature dependent physical properties and the heat sources due to eddy currents. Due to the temperature dependent nonlinearity, the conventional approach to dynamic problems is to reconstruct the global stiffness matrix for each time step, which increases dramatically the computational burden. A new method is proposed in this paper to reduce the computational burden by modifying the mathematical model to marginalize the nonlinearity throughout the calculation domain to the external surfaces. This method can effectively reduce the degree of nonlinearity, and simplify efficaciously the calculation process. The proposed model and method are combined with the algebraically formed cell method to analyze the induction heating process of an aluminum cylindrical billet. The calculation results are validated by the 3D finite element analysis.

An efficient and clean solid-state cooling technique based on elastocaloric effect (eCE) has received great interests as a promising refrigeration technology for the last decade. The eCE materials release and absorb heat under the application/removal of uniaxial stress due to the stress-induced martensite transformation (MT). Ferromagnetic shape memory alloys have great potential to produce large eCE because they require low stress to drive the stress-induced MT. In this report, we employed replication casting technique to fabricate Ni-Fe-Ga alloy foams with NaAlO₂ as space holder. The martensite transformation, superelasticity and compressive elastocaloric properties of the annealed foam were investigated. A completely reversible superelastic behavior with a total recovery strain of 3.3% was found at temperatures slightly higher than austenite finishing temperature. As a result, the foam exhibited an adiabatic temperature change (ΔT_ad) 3.4 K and large specific eCE strength |ΔT/Δσ| = 56.7 K GPa⁻¹ under an external stress 60 MPa. The enhanced eCE properties of the foam over bulk alloys, and the large specific surface area for better heat exchange, imply that the present Ni-Fe-Ga foam has great application potential for room-temperature cooling.