Journal of Physics D: Applied Physics

Over the past decade, the global cumulative installed photovoltaic (PV) capacity has grown exponentially, reaching 591 GW in 2019. Rapid progress was driven in large part by improvements in solar cell and module efficiencies, reduction in manufacturing costs and the realization of levelized costs of electricity that are now generally less than other energy sources and approaching similar costs with storage included. Given this success, it is a particularly fitting time to assess the state of the photovoltaics field and the technology milestones that must be achieved to maximize future impact and forward momentum. This roadmap outlines the critical areas of development in all of the major PV conversion technologies, advances needed to enable terawatt-scale PV installation, and cross-cutting topics on reliability, characterization, and applications. Each perspective provides a status update, summarizes the limiting immediate and long-term technical challenges and highlights breakthroughs that are needed to address them. In total, this roadmap is intended to guide researchers, funding agencies and industry in identifying the areas of development that will have the most impact on PV technology in the upcoming years.

Outbreaks of airborne infectious diseases such as measles or severe acute respiratory syndrome can cause significant public alarm. Where ventilation systems facilitate disease transmission to humans or animals, there exists a need for control measures that provide effective protection while imposing minimal pressure differential. In the present study, viral aerosols in an airstream were subjected to non-thermal plasma (NTP) exposure within a packed-bed dielectric barrier discharge reactor. Comparisons of plaque assays before and after NTP treatment found exponentially increasing inactivation of aerosolized MS2 phage with increasing applied voltage. At 30 kV and an air flow rate of 170 standard liters per minute, a greater than 2.3 log reduction of infective virus was achieved across the reactor. This reduction represented ~2 log of the MS2 inactivated and ~0.35 log physically removed in the packed bed. Increasing the air flow rate from 170 to 330 liters per minute did not significantly impact virus inactivation effectiveness. Activated carbon-based ozone filters greatly reduced residual ozone, in some cases down to background levels, while adding less than 20 Pa pressure differential to the 45 Pa differential pressure across the packed bed at the flow rate of 170 standard liters per minute.

The special mechanical properties of nanoparticles allow for novel applications in many fields, e.g., surface engineering, tribology and nanomanufacturing/nanofabrication. In this review, the basic physics of the relevant interfacial forces to nanoparticles and the main measuring techniques are briefly introduced first. Then, the theories and important results of the mechanical properties between nanoparticles or the nanoparticles acting on a surface, e.g., hardness, elastic modulus, adhesion and friction, as well as movement laws are surveyed. Afterwards, several of the main applications of nanoparticles as a result of their special mechanical properties, including lubricant additives, nanoparticles in nanomanufacturing and nanoparticle reinforced composite coating, are introduced. A brief summary and the future outlook are also given in the final part.

Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

Journal of Physics D: Applied Physics published the first Plasma Roadmap in 2012 consisting of the individual perspectives of 16 leading experts in the various sub-fields of low temperature plasma science and technology. The 2017 Plasma Roadmap is the first update of a planned series of periodic updates of the Plasma Roadmap. The continuously growing interdisciplinary nature of the low temperature plasma field and its equally broad range of applications are making it increasingly difficult to identify major challenges that encompass all of the many sub-fields and applications. This intellectual diversity is ultimately a strength of the field. The current state of the art for the 19 sub-fields addressed in this roadmap demonstrates the enviable track record of the low temperature plasma field in the development of plasmas as an enabling technology for a vast range of technologies that underpin our modern society. At the same time, the many important scientific and technological challenges shared in this roadmap show that the path forward is not only scientifically rich but has the potential to make wide and far reaching contributions to many societal challenges.

Phase-change memory (PCM) is an emerging non-volatile memory technology that has recently been commercialized as storage-class memory in a computer system. PCM is also being explored for non-von Neumann computing such as in-memory computing and neuromorphic computing. Although the device physics related to the operation of PCM have been widely studied since its discovery in the 1960s, there are still several open questions relating to their electrical, thermal, and structural dynamics. In this article, we provide an overview of the current understanding of the main PCM device physics that underlie the read and write operations. We present both experimental characterization of the various properties investigated in nanoscale PCM devices as well as physics-based modeling efforts. Finally, we provide an outlook on some remaining open questions and possible future research directions.

Plasma catalysis is gaining increasing interest for various gas conversion applications, such as CO ₂ conversion into value-added chemicals and fuels, CH ₄ activation into hydrogen, higher hydrocarbons or oxygenates, and NH ₃ synthesis. Other applications are already more established, such as for air pollution control, e.g. volatile organic compound remediation, particulate matter and NO _x removal. In addition, plasma is also very promising for catalyst synthesis and treatment. Plasma catalysis clearly has benefits over ‘conventional’ catalysis, as outlined in the Introduction. However, a better insight into the underlying physical and chemical processes is crucial. This can be obtained by experiments applying diagnostics, studying both the chemical processes at the catalyst surface and the physicochemical mechanisms of plasma-catalyst interactions, as well as by computer modeling. The key challenge is to design cost-effective, highly active and stable catalysts tailored to the plasma environment. Therefore, insight from thermal catalysis as well as electro- and photocatalysis is crucial. All these aspects are covered in this Roadmap paper, written by specialists in their field, presenting the state-of-the-art, the current and future challenges, as well as the advances in science and technology needed to meet these challenges.

Andreas Berger CICnanoGUNE BRTA

Following the success and relevance of the 2014 and 2017 Magnetism Roadmap articles, this 2020 Magnetism Roadmap edition takes yet another timely look at newly relevant and highly active areas in magnetism research. The overall layout of this article is unchanged, given that it has proved the most appropriate way to convey the most relevant aspects of today’s magnetism research in a wide variety of sub-fields to a broad readership. A different group of experts has again been selected for this article, representing both the breadth of new research areas, and the desire to incorporate different voices and viewpoints. The latter is especially relevant for thistype of article, in which one’s field of expertise has to be accommodated on two printed pages only, so that personal selection preferences are naturally rather more visible than in other types of articles. Most importantly, the very relevant advances in the field of magnetism research in recent years make the publication of yet another Magnetism Roadmap a very sensible and timely endeavour, allowing its authors and readers to take another broad-based, but concise look at the most significant developments in magnetism, their precise status, their challenges, and their anticipated future developments.

While many of the contributions in this 2020 Magnetism Roadmap edition have significant associations with different aspects of magnetism, the general layout can nonetheless be classified in terms of three main themes: (i) phenomena, (ii) materials and characterization, and (iii) applications and devices. While these categories are unsurprisingly rather similar to the 2017 Roadmap, the order is different, in that the 2020 Roadmap considers phenomena first, even if their occurrences are naturally very difficult to separate from the materials exhibiting such phenomena. Nonetheless, the specifically selected topics seemed to be best displayed in the order presented here, in particular, because many of the phenomena or geometries discussed in (i) can be found or designed into a large variety of materials, so that the progression of the article embarks from more general concepts to more specific classes of materials in the selected order. Given that applications and devices are based on both phenomena and materials, it seemed most appropriate to close the article with the application and devices section (iii) once again. The 2020 Magnetism Roadmap article contains 14 sections, all of which were written by individual authors and experts, specifically addressing a subject in terms of its status, advances, challenges and perspectives in just two pages. Evidently, this two-page format limits the depth to which each subject can be described. Nonetheless, the most relevant and key aspects of each field are touched upon, which enables the Roadmap as whole to give its readership an initial overview of and outlook into a wide variety of topics and fields in a fairly condensed format. Correspondingly, the Roadmap pursues the goal of giving each reader a brief reference frame of relevant and current topics in modern applied magnetism research, even if not all sub-fields can be represented here.

The first block of this 2020 Magnetism Roadmap, which is focussed on (i) phenomena, contains five contributions, which address the areas of interfacial Dzyaloshinskii–Moriya interactions, and two-dimensional and curvilinear magnetism, as well as spin-orbit torque phenomena and all optical magnetization reversal. All of these contributions describe cutting edge aspects of rather fundamental physical processes and properties, associated with new and improved magnetic materials’ properties, together with potential developments in terms of future devices and technology. As such, they form part of a widening magnetism ‘phenomena reservoir’ for utilization in applied magnetism and related device technology. The final block (iii) of this article focuses on such applications and device-related fields in four contributions relating to currently active areas of research, which are of course utilizing magnetic phenomena to enable specific functions. These contributions highlight the role of magnetism or spintronics in the field of neuromorphic and reservoir computing, terahertz technology, and domain wall-based logic. One aspect common to all of these application-related contributions is that they are not yet being utilized in commercially available technology; it is currently still an open question, whether or not such technological applications will be magnetism-based at all in the future, or if other types of materials and phenomena will yet outperform magnetism. This last point is actually a very good indication of the vibrancy of applied magnetism research today, given that it demonstrates that magnetism research is able to venture into novel application fields, based upon its portfolio of phenomena, effects and materials. This materials portfolio in particular defines the central block (ii) of this article, with its five contributions interconnecting phenomena with devices, for which materials and the characterization of their properties is the decisive discriminator between purely academically interesting aspects and the true viability of real-life devices, because only available materials and their associated fabrication and characterization methods permit reliable technological implementation. These five contributions specifically address magnetic films and multiferroic heterostructures for the purpose of spin electronic utilization, multi-scale materials modelling, and magnetic materials design based upon machine-learning, as well as materials characterization via polarized neutron measurements. As such, these contributions illustrate the balanced relevance of research into experimental and modelling magnetic materials, as well the importance of sophisticated characterization methods that allow for an ever-more refined understanding of materials. As a combined and integrated article, this 2020 Magnetism Roadmap is intended to be a reference point for current, novel and emerging research directions in modern magnetism, just as its 2014 and 2017 predecessors have been in previous years.

Today, surface acoustic waves (SAWs) and bulk acoustic waves are already two of the very few phononic technologies of industrial relevance and can been found in a myriad of devices employing these nanoscale earthquakes on a chip. Acoustic radio frequency filters, for instance, are integral parts of wireless devices. SAWs in particular find applications in life sciences and microfluidics for sensing and mixing of tiny amounts of liquids. In addition to this continuously growing number of applications, SAWs are ideally suited to probe and control elementary excitations in condensed matter at the limit of single quantum excitations. Even collective excitations, classical or quantum are nowadays coherently interfaced by SAWs.

This wide, highly diverse, interdisciplinary and continuously expanding spectrum literally unites advanced sensing and manipulation applications. Remarkably, SAW technology is inherently multiscale and spans from single atomic or nanoscopic units up even to the millimeter scale.

The aim of this Roadmap is to present a snapshot of the present state of surface acoustic wave science and technology in 2019 and provide an opinion on the challenges and opportunities that the future holds from a group of renown experts, covering the interdisciplinary key areas, ranging from fundamental quantum effects to practical applications of acoustic devices in life science.

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to ‘real world’ applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

Today, surface acoustic waves (SAWs) and bulk acoustic waves are already two of the very few phononic technologies of industrial relevance and can been found in a myriad of devices employing these nanoscale earthquakes on a chip. Acoustic radio frequency filters, for instance, are integral parts of wireless devices. SAWs in particular find applications in life sciences and microfluidics for sensing and mixing of tiny amounts of liquids. In addition to this continuously growing number of applications, SAWs are ideally suited to probe and control elementary excitations in condensed matter at the limit of single quantum excitations. Even collective excitations, classical or quantum are nowadays coherently interfaced by SAWs.

This wide, highly diverse, interdisciplinary and continuously expanding spectrum literally unites advanced sensing and manipulation applications. Remarkably, SAW technology is inherently multiscale and spans from single atomic or nanoscopic units up even to the millimeter scale.

The aim of this Roadmap is to present a snapshot of the present state of surface acoustic wave science and technology in 2019 and provide an opinion on the challenges and opportunities that the future holds from a group of renown experts, covering the interdisciplinary key areas, ranging from fundamental quantum effects to practical applications of acoustic devices in life science.

The mechanical properties of materials are not only indispensable key factors in their application fields, but are also fundamentally important in terms of materials science. Since the successful isolation of graphene with an atomic thickness, two-dimensional (2D) materials have attracted enormous attention over the past decade due to their unique properties. In particular, 2D materials are of interest owing to their outstanding mechanical properties, such as high Young’s modulus and strength, despite their ultrathinness and low weight in comparison to their bulk counterparts. However, studies on the mechanical properties of various 2D materials have been limited, with the exception of graphene, leaving many open questions and challenges. In this article, recent empirical and theoretical advances in studies of the mechanical properties of 2D materials and their applications are reviewed. First, mechanical characterization methods, which are widely used for ultrathin membranes, are summarized. The effects of defects on the mechanical properties of 2D materials are reviewed, including naturally (or intentionally) generated defects and chemically functionalized 2D materials. Finally, we discuss recent advances and the possibility of using 2D materials in diverse mechanical applications. The summary of the unique mechanical properties of 2D materials and their derivatives in this article would be beneficial for the study of 2D materials and their applications in lightweight, flexible, and transparent systems.

Super-resolution microscopy (SRM) has become essential for the study of nanoscale biological processes. This type of imaging often requires the use of specialised image analysis tools to process a large volume of recorded data and extract quantitative information. In recent years, our team has built an open-source image analysis framework for SRM designed to combine high performance and ease of use. We named it NanoJ—a reference to the popular ImageJ software it was developed for. In this paper, we highlight the current capabilities of NanoJ for several essential processing steps: spatio-temporal alignment of raw data (NanoJ-Core), super-resolution image reconstruction (NanoJ-SRRF), image quality assessment (NanoJ-SQUIRREL), structural modelling (NanoJ-VirusMapper) and control of the sample environment (NanoJ-Fluidics). We expect to expand NanoJ in the future through the development of new tools designed to improve quantitative data analysis and measure the reliability of fluorescent microscopy studies.

Two-dimensional van der Waals (vdW) heterostructures were constructed using MoSSe and XN (X  =  Ga, Al) via density functional calculations to serve as water splitting photocatalysts. Both the MoSSe/GaN and MoSSe/AlN heterostructures are energetically, dynamically and thermally stable. Interestingly, they exhibited type-II band structures, indicating the ability to continuously separate photogenerated electrons and holes. They also have appropriate band edge positions for redox reaction potentials of water splitting at pH 0. Remarkably, the MoSSe/GaN vdW heterostructure possessed excellent carrier mobility for holes along both the transport directions. Besides, the charge transfer between the MoSSe and XN layers induced a strong built-in electric field, which further separated the photogenerated carriers. In addition, the MoSSe/GaN and MoSSe/AlN vdW heterostructures exhibit good optical absorption ability toward solar irradiation. All these excellent properties render the MoSSe/GaN and MoSSe/AlN heterostructures are high-efficiency photocatalysts for water splitting.

In this paper, a broadband and high-efficient tri-layered chiral structure metasurface (CSM) for linear polarization conversion and multi-functional wavefront manipulation was proposed and investigated numerically in terahertz region. The unit-cell of the proposed CSM consist of two orthogonal metal wires sandwiched with square split-ring resonator (SRR) structure, which can manipulate the polarization and wavefront of the transmitted wave simultaneously. Based on the Fabry–Perot-like cavity-enhanced effect of proposed CSM, broadband and high-efficiency cross-polarization conversion can be achieved. Numerical simulation results indicate that the cross-polarization transmission coefficient is higher than 0.9 from 0.4 THz to 1.0 THz, with a fractional bandwidth of 85.7%. The proposed CSM can achieve complete 2 π phase coverage by adjusting the geometric parameters of the unit-cell structure. Anomalous refraction with a wide angle, two kinds of focusing metalenses and vortex beam generation are investigated to verify the multi-functional wavefront manipulation performance of the proposed CSM. Our work provides an effective method of enhancing the performance of transmission-type metasurface and the proposed devices show endless potential in wavefront control and communication applications in terahertz region.

The paper presents, for the first time, an analysis of the optimum arrangement of a coplanar waveguide-fed monopole antenna and a metasurface absorber (MSA), aiming to reduce the antenna's in-band radar cross section (RCS) without perturbing its radiation properties. The proposed arrangement will show that the efficiency of the antenna, of great interest in many applications (such as the internet of things), can be preserved, contrary to results of previous papers focused on combining these structures. Moreover, the final structure will be easily embeddable (with an optimum number of unit-cells) as well as conformable. A proper analysis of the losses and currents on the structure will be provided for a better understanding of the interaction phenomena that arise. Good agreement between simulation and measurement results can be observed, corroborating the proper performance of the structure. Furthermore, bistatic reductionis obtained as well as monostatic RCS reduction, due to the angular stability of the employed MSA. Finally, it will be shown that the introduction of loaded resistors will be preferable to the use of a lossy dielectric to improve the RCS reduction whilst maintaining the antenna performance.

Gold–tantalum alloy films are attractive for hohlraums used in indirect drive magnetized inertial confinement fusion. A high electrical resistivity of over ~100 µΩ cm at cryogenic temperatures is an essential requirement for allowing an externally imposed pulsed magnetic field to soak through a hohlraum and magnetize the fusion fuel. Here, we systematically study properties of Au–Ta alloy films in the entire compositional range from pure Au to pure Ta with thicknesses up to 30 µm. These films are made by direct current magnetron co-sputtering on planar substrates. Films are characterized by a combination of high-energy ion scattering, x-ray diffraction, electron microscopy, nanoindentation, and electrical transport measurements. Results show that an alloy with ~80 at.% of Ta forms a metallic glass exhibiting a maximum electrical resistivity of ~300 µΩ cm with a weak temperature dependence in the range of 5–400 K. The deposition of a film with ~80 at.% of Ta onto a sphero-cylindrical substrate for hohlraum fabrication is also demonstrated.

Due to their hydrophobic properties, composite insulators have excellent anti-pollution flashover performance and have been widely used in power systems. For a better understanding of the flashover characteristics of a hydrophobic surface at wet conditions, the hydrophobic surface is covered with water droplets and water film, respectively, in this paper. The initiation and development of partial arc are observed by an ultrahigh-speed camera and the leakage current is also recorded. It is found that there exists a critical water proportion for a given dry band length and water volume conductivity. Beyond this water proportion, the partial arc does not necessarily lead to the flashover. On the contrary, flashover will take place right after the occurrence of partial arc. Furthermore, it is also found that when the water proportion is below the critical water proportion, the flashover voltage is barely related to the volume conductivity and the flashover voltage generally decreases with the increase of water volume and water droplet number. The results obtained are of great significance to understand the flashover process of hydrophobic surfaces.

Compared with traditional optical lenses, metalenses have obvious advantages of ultra-thin and easy-integration. In this paper, resonant phase and geometric phase are combined to design a metalens that can independently control the focus of right-handed circular polarization (RCP) and left-handed circular polarization (LCP) light. This solves the disadvantage that resonant phase or geometric phase alone cannot focus the RCP and LCP light at any two different positions simultaneously and independently. Two types of metalenses have been designed. One is off-axis metalens, which can focus RCP and LCP light in symmetric and asymmetric positions, respectively. The other is on-axis metalens, which can realize the bifocal effect of RCP and LCP light at different positions along the axis. Furthermore, by increasing the numerical aperture, the maximum electric field energy intensity and full width at half maximum of two focal points of the same metalens can gradually approach each other. This provides a new way for the regulation of polarization-dependent imaging, information detection, as well as the possibility of realizing multi-functional metasurface devices.

Halide perovskite materials, which are emerging as some of the most promising candidates for photovoltaics, have been widely studied and have been certified as demonstrating a comparable efficiency to single-crystal silicon solar cells. However, their low stability poses a challenge for commercialization. External impediments, like moisture, heat, and UV light, can be addressed by strict encapsulation; nevertheless, ion migration remains. The migrated ions will bring in a growing number of charged defects and phase segregation to bulk perovskite; they will cause interfacial band doping and degradation of the carrier transport layer, which will greatly hinder carrier transportation. Those effects are the origins of perovskite intrinsic instability. Thus, a thorough understanding of the operational mechanism of ion migration is urgent for the fabrication of perovskite solar cells (PSCs) with improved stability. Here, we systemically summarize the factors governing ion migration in perovskite film and the associated impact on the performance of PSCs. Light illumination, organic cations, grain boundaries, residue lattice strain and moisture have been found to make ion migration easier. Strategies developed to suppress the ion migration are also interspersed in each section.

Excitons in van der Waals (vdW) heterostructures exhibit multiple excellent characteristics which in recent years have attracted researchers to make an in-depth exploration. In this review, we briefly introduce vdW heterostructures based on monolayer transition metal dichalcogenides and the emerging exciton characteristics in them, especially the formation of interlayer excitons. Then, we summarize the main results of a recent study about excitons in vdW heterostructures, including a Bose–Einstein condensate state, moiré excitons, and the complex exciton dynamics. The applications of excitons in vdW heterostructures are also mentioned, particularly the valley devices, which have a promising outlook. Finally, we point out some problems that exist at present, and give some future research directions.

Light detection in the deep-ultraviolet (DUV) solar-blind waveband has attracted interest due to its critical applications, especially in safety and space detection. A DUV photodetector based on wide-bandgap semiconductors provides a subversive scheme to simplify the currently mature DUV detection system. As an ultra-wide-bandgap (4.4–5.3 eV) semiconductor directly corresponding to the DUV solar-blind waveband, Ga ₂O ₃ has an important strategic position in the prospective layout of semiconductor technology owing to its intrinsic characteristics of high breakdown electric field, excellent tolerance of high/low temperature, high resistance to radiation, and rich material systems. As the only native substrate that can be fabricated from melt-grown bulk single crystals, β-Ga ₂O ₃ has attracted a lot of attention both in power-electronic and photo-electronic devices. In addition, other metastable phases (e.g. α, ϵ, γ) of Ga ₂O ₃ have attracted great interest due to their unique properties. In this work, we discuss the advances in achieving bulk and film Ga ₂O ₃ materials with different crystal phases. In addition, the latest achievements with polymorphous Ga ₂O ₃-based solar-blind photodetectors (SBPDs) and the methods to enhance their performance, including doping, annealing, and transparent electrodes, are also discussed. Furthermore, as the most desirable application, DUV imaging technologies based on Ga ₂O ₃ SBPDs are systematically summarized. Finally, conclusions regarding recent advances in Ga ₂O ₃ SBPDs, remaining challenges, and prospects are presented and discussed.

As an ultrawide bandgap semiconductor, gallium oxide (Ga ₂O ₃) has superior physical properties and has been an emerging candidate in the applications of power electronics and deep-ultraviolet optoelectronics. Despite numerous efforts made in the aspect of material epitaxy and power devices based on β-Ga ₂O ₃ with rapid progresses, the fundamental understanding of defect chemistry in Ga ₂O ₃, in particular, acceptor dopants and carrier compensation effects, remains a key challenge. In this focused review, we revisited the principles of popular approaches for characterizing defects in semiconductors and summarized recent advances in the fundamental investigation of defect properties, carrier dynamics and optical transitions in Ga ₂O ₃. Theoretical and experimental investigations revealed the microstructures and possible origins of defects in β-Ga ₂O ₃ bulk single crystals, epitaxial films and metastable-phased α-Ga ₂O ₃ epilayers by the combined means of first-principle calculation, deep level transient spectroscopy and cathodoluminescence. In particular, defects induced by high-energy irradiation have been reviewed, which is essential for the identification of defect sources and the evaluation of device reliability operated in space and other harsh environments. This topic review may provide insight into the fundamental properties of defects in Ga ₂O ₃ to fully realize its promising potential in practical applications.

Live-cell imaging has become indispensable in modern cell biology. However, many cellular processes can only be resolved by the means of super-resolution microscopy. Among the microscopy techniques that enable resolution beyond the diffraction limit of light, stimulated emission depletion (STED) microscopy is well suited for multicolour imaging of nanoscale dynamics. Here, we describe the development of live-cell STED imaging from proof-of-principle experiments into a versatile tool for multi-colour microscopic studies in living cells and even in living multicellular organisms. We summarise the advances in probes and hardware and discuss the current challenges and limitations that need to be addressed to realise the full potential of this technique.

Confidence and integrity are critical in the physical and chemical analyses of tissues and living cells. However, many of the probes targeting biological markers for confocal spectroscopy affect cells’ molecular identity. Hence, we combined photonics with electrical analysis in an assisted laser impedance spectroscopy facility and applied it to characterize two breast cancer cell lines (BT-474 and MCF-7) and lymphocytes (as a normal control). The setup comprised a sample holder with a ∼15 000 cell capacity fitted with two isolated conducting electrodes arranged concentrically and connected to an impedance analyser with a 20 Hz–1 MHz sweeping frequency. Capacitive transconductance measurements showed bands at 3491, 3494 and 3470 Hz corresponding to the BT-474, MCF-7, and lymphocytes, respectively. Under photonic stimulation by a 532 nm laser, these dark reference bands shifted to 3518, 3566 and 3674 Hz, respectively, reflecting optical transitions favouring ionic transport in the cells. Based on the experimental Nyquist diagrams and taking into account the roughness nature of the cell membrane, a constant phase element (CPE) was introduced in the circuit. The CPE was explained through a fractional parameter, α, based on fractional calculus. Results showed that, under photonic stimulation, α is less than ½, and the minimum change of series and membrane resistances are about 28.95% and 58.88%, respectively.

This study analyzes different strategies of plasma actuation of premixed swirl flames at pressures up to 3 bar. A wide range of applied voltages and pulse repetition frequencies (PRF) is considered, resulting in different combinations of nanosecond repetitively pulsed (NRP) discharge regimes, NRP glow and NRP spark discharges. Electrical characterization of the discharges is performed, measuring voltage and current, and deposited energy and power are evaluated. The effectiveness of the plasma actuation is assessed through images of OH* chemiluminescence from the flame. From these images, the distance of the center of gravity of the flame to the burner plate is evaluated, with and without plasma actuation. The results show that strategies which involve a high percentage of NRP sparks are effective at improving flame anchoring at atmospheric pressure, while they are detrimental at higher pressures. Therefore, high applied voltage and low PRF are preferable at atmospheric pressure, while the opposite is observed at elevated pressures. Moreover, it is found that a ratio of plasma power to thermal power of the flame around 1% is the best compromise between a strong actuation of the flame and a reasonable deposited electrical power. Explanations for these results are proposed.

Photoacoustic tomography (PAT) has become a fast-evolving biomedical imaging modality in recent years. In PAT, image reconstruction is a critical step to produce high-quality photoacoustic images from raw photoacoustic signals. To date, algorithms based on back projection are the most widely used image reconstruction techniques due to their simplicity and computational efficiency. However, images reconstructed by back projection contain negative values, especially at the edge of photoacoustic sources, which have no physical meaning and are essentially undesired artifacts. In this work, we study the formation mechanism, fundamental causes and removal strategies of the negativity artifacts in back-projection-based PAT. Our results show that limited detector bandwidth and limited view angle are two fundamental causes of negativity artifacts. When the bandwidth of a detector is limited, back-projection signals will be distorted as a result of the loss of frequency contents and negativity artifacts thus appear. When the view angle of the detector is limited, photoacoustic signals propagating in three-dimensional space will be partially lost, resulting in negativity artifacts. Post-processing strategies, such as envelope detection and forced zeroing can be used to remove the negativity artifacts but may cause problems. Since negativity artifacts are a common image quality degradation factor in PAT, understanding their characteristics may expedite the development of novel artifact-removal techniques and artifact-free image reconstruction algorithms, which are of importance to correct image interpretation and quantitative imaging.

A conductive wire can explode by rapidly heating it to vaporization temperature by flowing a current through it. This process is utilized to generate high-temperature high-density plasmas. The temperature and pressure distributions at the time of the explosion are not easily measured. Moreover, the amount of metal vapor from the wire that remains within the arcing area is unknown. This work presents the whole-process model of a single-wire electrical explosion from solid-state to plasma formation. For this purpose, the voltage drop and resistance of the exploding copper wire in solid-state are simulated through a zero-dimensional thermo-electrical model. Then, compressible Euler equations are implemented with nodal discontinuous Lagrange shape functions in a one-dimensional model to compute the flow of the generated copper vapor (due to the wire explosion) in surrounding nitrogen gas. The aim is to calculate the distributions of pressure, density, velocity, temperature, and mass flow along the cylindrical shock waves to estimate the arc’s copper/nitrogen mixture ratio in free burning and nozzle constricted arcs. This mixture ratio is used to calculate the precise percentage of the metal vapor in the arcing area and to calculate Townsend growth coefficients utilizing to estimate the streamer breakdown of the mixture. The simulation results show good agreement with the experimental results in terms of the temporal evolution of the plasma channel boundary, the shock front speed estimation as well as the arc voltage magnitude numerically calculated deploying the extracted mixture percentage from this study, manifesting the validity of the model. It shows that despite the low-pressure studies, the exploding wire method is not suitable for circuit breakers employing supercritical fluids as the insulation.

Optimization of the laser shock peening (LSP) and LASer Adhesion Test (LASAT) processes requires control of the laser-induced target’s loading. Improvements to optical and laser technologies allow plasma characterization to be performed with greater precision than 20 years ago. Consequently, the processes involved during laser–matter interactions can be better understood. For the purposes of this paper, a self-consistent model of plasma pressure versus time is required. The current approach is called the inverse method, since it is adjusted until the simulated free surface velocity (FSV) corresponds to the experimental velocity. Thus, it is not possible to predict the behavior of the target under shock without having done the experiments. For the first time, experimental data collected in different labs with the most up-to-date laser parameters are used to validate a self-consistent model for temporal pressure-profile calculation. In addition, the parameters characterizing the plasma (temperature, thickness and duration) are obtained from the ESTHER numerical code, together with the amount of ablated matter. Finally, analytic fits are presented that can reproduce any pressure–temporal profiles in the following domains of validity: intensities, I, ranging from 10 to 500 GW cm ⁻² and pulse durations, T _pul, between 5 and 40 ns for the direct-illumination regime at 1053 nm, I ranging from 1 to 6 GW cm ⁻² and T _pul between 10 to 40 ns in the water-confined regime at 1053 nm, and I from 1 to 10 GW cm ⁻² and T _pul between 7 and 20 ns in the water-confined regime at 532 nm. These temporal pressure profiles can then be used to predict the aluminum target’s behavior under laser shock using mechanical simulation software.

From a systematic study of the threshold current density as a function of temperature and hydrostatic pressure, in conjunction with theoretical analysis of the gain and threshold carrier density, we have determined the wavelength dependence of the Auger recombination coefficients in InGaAsSb/GaSb quantum well lasers emitting in the 1.7–3.2 µm wavelength range. From hydrostatic pressure measurements, the non-radiative component of threshold currents for individual lasers was determined continuously as a function of wavelength. The results are analysed to determine the Auger coefficients quantitatively. This procedure involves calculating the threshold carrier density based on device properties, optical losses, and estimated Auger contribution to the total threshold current density. We observe a minimum in the Auger rate around 2.1 µm. A strong increase with decreasing mid-infrared wavelength (<2 µm) indicates the prominent role of intervalence Auger transitions to the split-off hole band (CHSH process). Above 2 µm, the increase with wavelength is approximately exponential due to CHCC or CHLH Auger recombination, limiting long wavelength operation. The observed dependence is consistent with that derived by analysing literature values of lasing thresholds for type-I InGaAsSb quantum well diodes. Over the wavelength range considered, the Auger coefficient varies from a minimum of 1 × 10 ⁻¹⁶cm ⁴ s ⁻¹ at 2.1 µm to ∼8 × 10 ⁻¹⁶cm ⁴ s ⁻¹ at 3.2 µm.

Environment-friendly solid-state cooling technology necessitates the search for energy-efficient electrocaloric (EC) materials. In this regard, the EC effect and energy storage performance have been investigated on a site-engineered lead-free Ba _{1- x} (Bi _0.5Li _0.5) _x TiO ₃ ( x = 0.0, 0.10, 0.125, 0.15 and 0.175) system from the perspective of its enhanced characteristic parameters. The ferroelectric and dielectric studies reveal the tunable polarization and Curie temperature as a function of composition. The EC measurements on these samples display superior EC parameters compared to the values reported for other polycrystalline ferroelectric systems. The observed EC parameters for the x = 0.10 sample, such as the change in entropy (Δ S), adiabatic temperature change (Δ T) and EC coefficient are 2.63 J kg ⁻¹ K, 2.03 K and 0.68 K mm ⁻¹ kV, respectively. Notably, the x = 0.15 sample displays near room-temperature (307 K) EC response with Δ T ≥ 0.30 K over a broad 24 K temperature range. In addition, the energy storage performance studies elucidate that the Ba _{1- x} (Bi _0.5Li _0.5) _x TiO ₃ compound with x = 0.175 displays large energy storage efficiency (96.7%) with 144 mJ cm ⁻³ as the storage density. The tunable EC characteristics and high energy storage efficiency demonstrated in this work illustrate the application potential of site-engineered BaTiO ₃ samples in efficient cooling and storage devices.

We report the application of phasor analysis and nonlinear iterative fitting to complex spatial and spectroscopic luminescence decay data obtained from multidimensional microscopy of a CVD diamond grown on a HPHT substrate. This spectral and lifetime-resolved analysis enabled spatial mapping of variations in concentrations of nitrogen vacancy (NV) defects in both charge states and the quenching of NV ⁻ defects, as well as the identification of SiV ⁻ luminescence. These imaging and spectroscopic modalities may be important for reliable fabrication of quantum devices based on such defects in diamond, which will require well-defined and characterised quantum electronic properties.

Hafnium oxide (HfO ₂), zirconium oxide (ZrO ₂), and the solid-solution (Hf _1-xZr _xO ₂) system continue to be some of the most relevant ferroelectric materials, in particular, for their promising application in CMOS integrated ferroelectric memories. Recent understanding of the influence of oxygen supplied during film deposition on the structural phase formation process in Hf _1-xZr _xO ₂ films has drawn attention to a commonly overlooked parameter for tuning ferroelectric and electrical properties of these films. In this paper, a comparison of O ₃ and O ₂ plasma used as the oxygen source in an atomic layer deposition process for Hf _1-xZr _xO ₂ films within the full compositional range is discussed. A combination of structural and electrical characterization methods grant insight on the influence of each of the oxygen sources on the crystalline phase formation during deposition of Hf _1-xZr _xO ₂ films. These observations are then correlated to the material’s behavior regarding its ferroelectric and electrical properties; mainly, dielectric constant, ferroelectric remanent polarization, and number of electric field cycles to breakdown.

A high-performance III–V quantum-dot (QD) laser monolithically grown on Si is one of the most promising candidates for commercially viable Si-based lasers. Great efforts have been made to overcome the challenges due to the heteroepitaxial growth, including threading dislocations and anti-phase boundaries, by growing a more than 2 µm thick III–V buffer layer. However, this relatively thick III–V buffer layer causes the formation of thermal cracks in III–V epi-layers, and hence a low yield of Si-based optoelectronic devices. In this paper, we demonstrate a usage of thin Ge buffer layer to replace the initial part of GaAs buffer layer on Si to reduce the overall thickness of the structure, while maintaining a low density of defects in III–V layers and hence the performance of the InAs/GaAs QD laser. A very high operating temperature of 130 °C has been demonstrated for an InAs/GaAs QD laser by this approach.