Table of contents

We present surface analysis of Cu₂ZnSn(S,Se)₄ (CZTSSe) thin films deposited on Mo/glass substrates. X-ray photoelectron and energy dispersive x-ray spectroscopy has been performed on CZTSSe thin-film solar cell absorbers for surface and bulk compositional analysis, respectively. It is observed that the surface of the CZTSSe absorber is Cu deficient. For further verification of Cu deficiency, spectroscopic ellipsometry has been used to determine the extinction coefficient of CZTSSe thin films with Cu variation. The surface layer has a higher bandgap of 1.79 eV in reference to the bulk film bandgap of 1.5 eV. This surface bandgap increase is beneficial for solar cell performance. The thin film with a Cu deficient surface has a noticeably higher open circuit voltage of 562 mV using a very thin absorber layer of 300 nm in thickness. The open circuit voltage for the Cu deficient surface is 25% higher than that of the Cu rich surface. This analysis gives an understanding into the significance of surface layer engineering for photovoltaic device.

Pure spin currents, i.e. the transport of angular momentum without an accompanying charge current, represent a promising new avenue in modern spintronics from both a fundamental and an application point of view. Such pure spin currents are not only able to flow in electrical conductors via mobile charge carriers, but also in magnetically ordered electrical insulators as a flow of spin excitation quanta. Over the course of the last few years, remarkable results have been obtained in heterostructures consisting of magnetically ordered insulators interfaced with a normal metal, where a pure spin current flows across the interface.

This topical review article deals with the fundamental principles, experimental findings and recent developments in the field of pure spin currents in magnetically ordered insulators. Here, we focus on four different manifestations of pure spin currents in such heterostructures: the spin pumping effect, the longitudinal spin Seebeck effect, the spin Hall magnetoresistance and the all-electrical injection and detection of magnon transport in non-local device concepts. In this article, we utilize a common theoretical framework to explain all four effects and explain the important material systems (especially rare-earth iron garnets) used in the experiments. For each effect, we introduce basic measurement techniques and detection schemes and discuss their application in the experiment. We account for the remarkable progress achieved in each field by reporting recent developments and by discussing the research highlights obtained by our group. Finally, we conclude the review article with an outlook on future challenges and obstacles in the field of pure spin currents in magnetically ordered insulator/normal metal heterostructures.

This topical review aims to be a useful introductory resource for readers from the outside or just starting in this field, and also provides perspective for those who already have an established understanding of the underlying physics.

The presence of organic contaminants in water raises considerable concern due to the negative impact on the environment, as well as to potential human exposure. In addition, scarcity of freshwater resources leads to serious consideration for water reuse by wastewater treatment. Non-thermal plasma treatment could address these problems by reducing water pollution. Highly reactive species generated in plasma in liquid or gas–liquid environment attack the pollutant molecules and degrade them. This paper reviews the literature results on the removal of pesticides in water by plasma treatment. The work is mainly focused on the attempts to improve the efficiency of the degradation process and on the mechanisms responsible for the degradation of pesticides belonging to various classes.

We present the results of an experimental determination of the quenching rate of the metastable ²P state of atomic nitrogen by nitrogen molecules. 2D density profiles of N(²P) and N(⁴S) atoms were measured by absolute optical emission spectroscopy inside a tube placed downstream of an atmospheric pressure pulsed discharge in nitrogen. The results are analyzed by means of a detailed kinetic model taking into account the major processes of production and depletion of N(²P). The quenching rate constant of N(²P) by N₂ molecules determined with this method at atmospheric pressure and T  =  300 K is k  =  (3  ±  1)  ×  10⁻¹⁷ cm³ s⁻¹.

Organic photovoltaic cells typically employ an interlayer sandwiched between the active donor or acceptor materials and the respective electrodes. Conventionally, the employed anode interlayer (AIL) adjacent to the anode has a wide optical band gap and a significantly higher lowest unoccupied molecular orbital (LUMO) energy level, compared to the adjunct electron donor material, such as boron subphthalocyanine chloride (SubPc) studied here. In this report, we synthesized three novel AIL materials, NP-β-PCN, NPAPMLI and NPAPMLI, having narrow optical band gaps but matching the LUMO energy levels of SubPc in a planar heterojunction solar cell of ITO/SubPc/C₆₀/BCP/Al. Upon insertion of an ultrathin (2 nm) AIL, the device power conversion efficiency is increased from 3.98% to as high as 4.92%, mainly due to the significant increase of short-circuit current density (J_SC) from 5.97 to 7.11–7.65 mA cm⁻². From the detailed morphological and photophysical studies, we have demonstrated that the employed unconventional materials of AIL are effective in exciton (of SubPc) blocking, thereby enhancing exciton diffusion towards the charge-separating interface of SubPc/C₆₀ and hence J_SC. Since these AIL materials all have a LUMO energy level very close to that of SubPc, the study reported here clarifies that the electron blocking is not a necessary property of an AIL material.

Dielectric barrier discharges (DBDs) are well-established and useful tools for scientific as well as industrial applications. They have been of high interest for analytical applications due to the fact that DBDs can produce small, low temperature/power, and atmospheric plasmas. These kind of plasmas can be applied for the detection and quantification of analytes in several ways: either, DBDs can be used for example as fragmentation and excitation sources to detect elements via optical emission spectrometry (OES), or as ionization sources of molecules for the detection via mass spectrometry (MS). ISAS has developed several of these discharges and studied the impact of the DBD itself on the subsequent application. This work summarizes the development from low pressure DBD implemented in diode laser atomic absorption spectrometry to atmospheric DBDs that can be used for different ambient applications such as the trace detection of arsenic species via OES or the soft ionization of molecular compounds via MS.

The development of synaptic devices with biologically-inspired information processing functions and low power consumption is critically important for the hardware implementation of highly anticipated brain-like computing systems. Organic materials are regarded as the most promising candidates for synaptic devices and bio-electronics due to several advantages such as low cost, easy processability, mechanical flexibility and ductility. In this review, a description of the current advances in organic synaptic devices, including two-terminal memristors and three-terminal transistors, is provided. Organic two-terminal memristors with the characteristics of non-volatility and reasonable on/off switching ratio are reported to be popular synaptic devices. On the other hand, organic memristive and electrochemical electric-double-layer transistors can accurately select working devices by applying a gate spike to the corresponding gate electrode. Therefore, these three-terminal organic devices provide an alternative approach to the development of neuromorphic systems. Lastly, the novel applications of organic synaptic devices are discussed, and some current challenges are presented.

In this paper, we present the design, fabrication and characterization of a dual band tunable metamaterial absorber (MMA), which can be tuned by the applied magnetic field and modulated by the permittivity of its components. The MMA consists of a periodic array of cuboid ferrites and a metal ground plane. Unlike other MMAs, the proposed MMA avoids utilizing resonant metallic parts, and only employ ferrites as resonators, making fabrication easy and tuning more convenient. When the incident electromagnetic wave and applied magnetic field act on the ferrites simultaneously, cuboid ferrites can produce ferromagnetic resonance. Modulated by its permittivity, the proposed MMA deduces a dual band perfect absorption. The experiment test agrees well with the simulation results and shows two distinct absorption peaks of 99.9% at 9.14 GHz and 97% at 9.48 GHz, respectively. Further investigation shows that the interval of the two distinct absorption peaks can be enlarged by increasing the permittivity of the ferrite, which clearly shows how absorptivity is influenced by its dielectric properties. This work provides an alternative route to realize tunable perfect absorbers with all-ferrite structures. With the artificial design of ferrite materials, the tunable perfect absorbers can be designed freely and are well served in a high power environment.

The reverse recovery characteristics of vertical bulk GaN-based Schottky rectifiers, which were fabricated on free-standing GaN substrates, were systematically investigated. The corrected reverse recovery time was obtained to be about 13.4 ns, 41.0 ns and 81.2 ns for 1 mm, 2 mm, and 3 mm diameter Schottky diodes, respectively. The dominant factor that affects the reverse recovery time of a bulk GaN-based Schottky rectifier was found to be the RC time constant, which was the product of the circuit resistance and capacitance. Besides that, the reverse recovery characteristics of the bulk GaN-based Schottky rectifiers at different temperatures were also measured and analyzed. It was found that the reverse recovery time of the bulk GaN-based Schottky diodes was less influenced by temperature, since the intrinsic exitation in the wide bandgap semiconductor of GaN was almost not influenced by temperature.

In this report, we study factors that dominate the mode transformation of resistive switching (RS) in yttria based memristive devices. It is found that amorphous yttria films are more suitable for RS whereas highly crystalline films are counterproductive for RS. The transformation from unipolar to bipolar resistive switching mode is demonstrated in our devices via moving from a system of single Schottky barrier diode (SBD) to double SBD. The conduction mechanism behind these transformation mechanisms is found to be predominantly interfacial. We also report a forming-free Al/Y₂O₃/Al based memristor fabricated by the dual ion beam sputtering without any post-processing steps for the first time. It shows stable switching behavior for  >29 000 cycles with good retention (10⁵ s) characteristics.

In this paper, a comprehensive scheme of a Salisbury screen is proposed which makes full use of dispersion engineering of spoof surface plasmon polariton (SSPP) to tailor its multi-order absorptions. As a proof, the symmetrical corrugated groove structures made of metal strips are adhered on dielectric grid periodically as plasmonic metamaterial (PM), and then used as a spacer in a Salisbury screen. Owing to the enhanced k-vector as well as nonlinear k-dispersion relationship inspired by PM spacer, the multi-order absorptions of the proposed SSPP Salisbury screen can be adjusted to lower frequency in a nonlinear scale. Simulation and experimental measurement demonstrate that the fundamental and first-order absorptions can be adjusted from 7.5 and 22.5 GHz to 4.7 and 9.0 GHz, and their according frequency ratio is also changed from 1.0:3.0 to 1.0:1.9. The proposed attempt opens a door for further optimization of conventional absorbing structure based on dispersion engineering of SSPP, enabling a wide range of applications.

A microwave discharge (2.45 GHz) is used to study the conversion of methane in a nitrogen afterglow. Investigations are performed both by means of emission spectroscopy and mass spectrometry.

We show that methane is injected in the 'early afterglow' of the nitrogen discharge where the energy transfer between and is the dominant process producing . Comparing experimental to theoretical results obtained for different vibrational levels v' of the state at different pressures, we determined the reaction rate constant values corresponding to the energy transfer between and , assuming a Treanor distribution (Tr  =  400 K, Tv_0–1  =  2500 K) for the vibrational levels of . The reaction rate constant values range from 1.63  ×  10⁻¹⁸ m³ s⁻¹ for v'  =  0 to 3.62  ×  10⁻¹¹ cm³ s⁻¹ for v'  =  8. The mean value is equal to 1.93  ×  10⁻¹⁷ m³ s⁻¹ when v' ranges from 0 to 10.

The emission intensity decay of the first positive system is studied for bands corresponding to Δv  =  3 versus methane concentration. The reaction rate constant value measured for the quenching of by CH₄ is close to values proposed in literature in the case of collisions between and CH₄. We studied the formation of CN, HCN and C₂N₂ species in the afterglow, comparing experimental to theoretical results and we measured the reaction rate constant value corresponding to:

(1) The production of HCN, by reaction of CH₃ with N, k₉  =  8.7  ×  10⁻¹⁸ m³ s⁻¹.

(2) The production of CN, by reaction of CH_x<4 with N, γk₁₀  =  1.2  ×  10⁻¹⁷ m³ s⁻¹, with γ  =  .

(3) The production of C₂N₂, we show that it is probably due to the reaction between gaseous and adsorbed cyanogen radicals on the reactor wall. The product of the reaction rate constant by the surface density of CN adsorbed is equal to k₁₄(CN_s)  =  55 ms⁻¹.

(4) The global destruction of CN and C₂N₂ when CH₄ in injected in the afterglow, k₁₁  <  1  ×  10⁻¹⁹ m³ s⁻¹ and k₁₅  =  9  ×  10⁻²⁰ m³ s⁻¹, respectively.

The temporal character of ion flux of a vacuum arc generation in nanosecond range was investigated by means of a multi-channel energy-mass analyser. The results obtained are the waveforms of arc current, discharge voltage and ion flux for few ion fractions with defined energy-to-charge ratios and several mass-to-charge ratios. The results show that the ion flux is constituted by 10–30 ns elementary bursts. These elementary bursts have durations that coincide in common to cathode spot life time estimations. The ion bursts combinations form both subsequent bursts groups and 100–150 ns super-bursts. The occurrence of the super-bursts usually precedes non-stabilities in the arc current and the attempts of the discharge to extinct. Amplitude Fourier spectrum dependence is close to Brownian random process dependence. The Fourier spectra of ion flux contain several local maxima corresponding to supposed cathode spot cycle period.

Experimental and computational studies of a rail plasma actuator (RailPAc) magnetohydrodynamic flow actuator were performed. The actuator functions by inducing flow around a fast-moving gliding arc, with device current , which is generated between flush mounted copper electrodes. High-speed imaging photometry is used to analyze the composition and internal structure of the arc for flush mounted electrode spacings of , , and , as well as free-floating electrodes with spacing. Results are compared with 2D thermal plasma simulations. The dynamics of the arc movement are found to be dependent on the height of the plasma column above the RailPAc surface and on the presence of prominent anode and cathode jets. Mechanisms are proposed for wall-stabilization of the arc and root-jet formation based on agreement between experimental and computational results.

The presence of occupied intra-band gap states in oxygen-deficient tin dioxide (SnO_x; 1  <  x  <  2) is crucial for efficient manufacturing of multipurpose electronic devices based on transparent conducting oxides. Former experimental determination of these states was conducted for well-defined, usually thick tin oxides obtained under highly controlled vacuum conditions. In this work, we present precise specification of gap defects states for ultra-thin SnO_x layers prepared by sol-gel synthesis followed with spin-coat deposition. Post-deposition drying and annealing processing changed layers' surface morphology and bulk crystalline structure as monitored by scanning electron microscopy, atomic force microscopy and x-ray diffraction. An x-ray photoemission spectroscopy (XPS) analysis of chemical composition revealed the presence of both Sn²⁺ and Sn⁴⁺ species in layers with and without post-drying annealing step. A stronger contribution of SnO was found for dried SnO_x. In the valence band region, XPS studies revealed pronounced O 2p and hybridised Sn 5p/5s–O 2p states as well as deep, overlapping with the O 2p, band gap states resulting from Sn 5s orbitals. These states—attributed to defect states—indicated enhanced presence of Sn²⁺ cations, and were assigned to 'bridging' oxygen vacancies. Complementary photoemission yield spectroscopy (PYS) studies of the SnO_x band gap region revealed an increased effective density of occupied electronic states below the Fermi level E_F for annealed layers. The consequence was a work function reduction by 0.15 eV after the annealing process. PYS results allowed a precise detection of SnO_x shallow band gap states close to E_F. These states were attributed to surface oxygen vacancies, which was confirmed by computer modelling. Finally, the annealed layers exhibited higher calculated charge carrier concentration, hence the increased n-type character.

Halide perovskite materials have scored great successes in a number of optoelectronic devices such as solar cells. The industrial sector requires an inevitable possibility of contacts between the halide perovskite materials and a number of atmospheric gases and volatile organic compounds (VOCs), which could strongly influence the stability and charge transfer properties of perovskite-based optoelectronic devices. Understanding the interactions between various gas molecules and the perovskite layer could thus be of utmost importance to further optimize perovskite solar cells from an industrial perspective. Surprisingly, a systematic study of the interactions between gas molecules highlighting atmospheric and VOCs and the prototypical CH₃NH₃PbI₃ surfaces is unavailable at the moment. Aiming to bridge the gap, we perform first principles and molecular dynamic calculations to investigate the interactions between CH₃NH₃PbI₃ surfaces and gas molecules, employing CO, CO₂, H₂, H₂S, NH₃, NO, NO₂, O₂, and SO₂ gas molecules as the adsorbates. Our study suggests that the NH₃ gas molecules exhibit detrimental effects on the structural and optical properties of the perovskite material; consequently, halide perovskite solar cells should be strongly protected against these gases. However, the influences of gas molecules on the perovskite layer is not always undesirable, since NH₃ molecules can strongly adsorb onto the perovskite surface, increasing the conductivity and initiating optical bleaching in the perovskite underlayer, leading to the viability of the halide perovskite materials as an effective gas sensor. The structure–property relationships of the molecule/perovskite systems investigated in this study could initiate the design protocol of halide perovskite materials toward different applications. This study provides a foundation for the convergence to a fundamental understanding of designing different halide perovskite-based optoelectronic devices.

The thermoelectric property of ZnO is severely limited due to its high thermal conductivity, in spite of its enormously extraordinary characteristics. Two-dimensional (2D) materials usually act as a good strategy to suppress heat transfer by introduction of interfacial scattering in low dimensional systems. The dependence of thermal conductivity on thickness in 2D ZnO is investigated using a first-principle study, combined with the Boltzman phonon transport equation. The thermal conductivity of ZnO for the first three layers sharply decreases with the thickness increasing, which is attributed to the broken phonon selection rule and stronger anharmonicity. The suppressed processes can be reactivated in contrast to the monolayer, such as ZA  +  ZA/ZO ZA/ZO and ZA  +  LA/TA/LO/TO LA/TA/LO/TO. The scattering channels for flexural phonons increase, which suppresses the phonon transport and decreases the thermal conductivity. In addition, the decrease of in-plane interactions outweighs the enhancement of out-of-plane interactions, which leads to the reduced acoustic-optical gap. More A  +  A O phonon scattering may take place and hinder thermal transfer due to a smaller phonon gap. Furthermore, the variation of bond strength in-plane and out-of-plane in multilayer ZnO leads to the reduction of phonon group velocity and the enhancement of anharmonicity.

The ability to concentrate heat fluxes has recently attracted a great deal of interests due to its innumerable benefits in thermal management such as thermoelectricity, solar cells and other fields. In this work, we propose a practical design method of thermal concentration utilizing fiber reinforced composite microstructures. The effective thermal conductivity (ETC) of the microstructure is determined using effective medium theory, and the ETC is designed to meet the perfect conductivity profile calculated by the transformation thermodynamics approach. In the design, we choose microstructures with appropriate fiber volume fractions to match with the required conductivity distribution. Numerical examination is performed to verify the thermal concentrating effects. In the numerical model, we use stainless steel and air as the fiber material and matrix material, respectively. The proposed thermal concentrator can be easily fabricated by naturally available materials, which paves a new avenue for thermal harvesting in solar cells, thermal energy storage and other related fields.

Magnesium alloys appear as one of the most promising materials for many applications such as degradable implants. However, their mechanical, corrosion and integration behaviors need to be optimized to comply with this application. The present paper focuses on laser-assisted maskless microdeposition (LAMM) of silver nanoparticles, a surface treatment aiming to modify the surface characteristics for better integration of the magnesium implant. The LAMM process parameters for obtaining desirable depositions are reported. The impacts of the LAMM process on the deposit and the substrate microstructure have been investigated using various characterization techniques. The results show that laser processing, which can lead to particle sintering in the deposit, can be fine-tuned to achieve necking between nano particles, while the nano-scale characteristics of the deposited layer is retained. Microstructural characterization reveals significant grain refinement in the immediate vicinity of the surface, providing evidence for the thermal impact of the laser process on the substrate. The thermal profiles of the deposit and the substrate during processing are further investigated by developing a 3D finite element modelling method. The implementation of the model allows us to better understand the origin of the fine-grained sublayer as well as the overall thermal impact of the current laser processing method on the substrate.

The electronic and phonon transport properties of quaternary tetradymite BiSbSeTe₂ are investigated by using a first-principles approach and Boltzmann transport theory. Unlike the binary counterpart Bi₂Te₃, we obtain a pair of Rashba splitting bands induced by the absence of an inversion center. Such unique characteristics could lead to a large Seebeck coefficient even at a relatively higher carrier concentration. Besides, we find an ultralow lattice thermal conductivity of BiSbSeTe₂, especially along the interlayer direction, which can be traced to the extremely small phonon relaxation time mainly induced by the mixed covalent bonds. As a consequence, a considerably large ZT value of ~2.0 can be obtained at 500 K, indicating that the unique lattice structure of BiSbSeTe₂ caused by isoelectronic substitution could be an advantage to achieve high thermoelectric performance.