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Showing papers in "Applied physics reviews in 2018"


Journal ArticleDOI
TL;DR: The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed in this article.
Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

1,535 citations


Journal ArticleDOI
TL;DR: A comprehensive review on the state-of-the-art of piezoelectric energy harvesting is presented, including basic fundamentals and configurations, materials and fabrication, performance enhancement mechanisms, applications, and future outlooks.
Abstract: The last decade has witnessed significant advances in energy harvesting technologies as a possible alternative to provide a continuous power supply for small, low-power devices in applications, such as wireless sensing, data transmission, actuation, and medical implants. Piezoelectric energy harvesting (PEH) has been a salient topic in the literature and has attracted widespread attention from researchers due to its advantages of simple architecture, high power density, and good scalability. This paper presents a comprehensive review on the state-of-the-art of piezoelectric energy harvesting. Various key aspects to improve the overall performance of a PEH device are discussed, including basic fundamentals and configurations, materials and fabrication, performance enhancement mechanisms, applications, and future outlooks.

513 citations


Journal ArticleDOI
TL;DR: In this article, the design, fabrication, and performance of chip-scale atomic clocks, magnetometers, and gyroscopes are discussed and many applications in which these novel instruments are being used.
Abstract: Chip-scale atomic devices combine elements of precision atomic spectroscopy, silicon micromachining, and advanced diode laser technology to create compact, low-power, and manufacturable instruments with high precision and stability. Microfabricated alkali vapor cells are at the heart of most of these technologies, and the fabrication of these cells is discussed in detail. We review the design, fabrication, and performance of chip-scale atomic clocks, magnetometers, and gyroscopes and discuss many applications in which these novel instruments are being used. Finally, we present prospects for future generations of miniaturized devices, such as photonically integrated systems and manufacturable devices, which may enable embedded absolute measurement of a broad range of physical quantities.

288 citations


Journal ArticleDOI
TL;DR: In this article, a multidisciplinary approach addressing underlying physics responsible for their magnetic performance, limitations, and future trends is presented, with a focus on soft magnetic composites and their applications in sensing, energy generation, and conversion.
Abstract: Power saving has been a central driving force for the development of new materials. In this framework, soft magnetic composites appear as a feasible concept to be applied in strategic topics for modern society including sensing, energy generation, and conversion. With a unique freedom regarding material selection based on powder metallurgy techniques, this engineering magnetic material class allows novel designs able to drive operation conditions to new limits, unlocking the potential of novel applications. This document reviews soft magnetic composites by using a multidisciplinary approach addressing underlying physics responsible for their magnetic performance, limitations, and future trends.

236 citations


Journal ArticleDOI
TL;DR: In this paper, the authors highlight the most promising developments reported at the 2017 International Workshop on Micropropulsion and Cubesats (MPCS-2017) by leading world-reputed experts in miniaturized space propulsion systems.
Abstract: Rapid evolution of miniaturized, automatic, robotized, function-centered devices has redefined space technology, bringing closer the realization of most ambitious interplanetary missions and intense near-Earth space exploration. Small unmanned satellites and probes are now being launched in hundreds at a time, resurrecting a dream of satellite constellations, i.e., wide, all-covering networks of small satellites capable of forming universal multifunctional, intelligent platforms for global communication, navigation, ubiquitous data mining, Earth observation, and many other functions, which was once doomed by the extraordinary cost of such systems. The ingression of novel nanostructured materials provided a solid base that enabled the advancement of these affordable systems in aspects of power, instrumentation, and communication. However, absence of efficient and reliable thrust systems with the capacity to support precise maneuvering of small satellites and CubeSats over long periods of deployment remains a real stumbling block both for the deployment of large satellite systems and for further exploration of deep space using a new generation of spacecraft. The last few years have seen tremendous global efforts to develop various miniaturized space thrusters, with great success stories. Yet, there are critical challenges that still face the space technology. These have been outlined at an inaugural International Workshop on Micropropulsion and Cubesats, MPCS-2017, a joint effort between Plasma Sources and Application Centre/Space Propulsion Centre (Singapore) and the Micropropulsion and Nanotechnology Lab, the G. Washington University (USA) devoted to miniaturized space propulsion systems, and hosted by CNR-Nanotec—P.Las.M.I. lab in Bari, Italy. This focused review aims to highlight the most promising developments reported at MPCS-2017 by leading world-reputed experts in miniaturized space propulsion systems. Recent advances in several major types of small thrusters including Hall thrusters, ion engines, helicon, and vacuum arc devices are presented, and trends and perspectives are outlined.

225 citations


Journal ArticleDOI
TL;DR: The Wigner function has been widely used in quantum information processing and quantum physics as discussed by the authors, where it has been used to model the electron transport, to calculate the static and dynamical properties of many-body quantum systems.
Abstract: The Wigner function was formulated in 1932 by Eugene Paul Wigner, at a time when quantum mechanics was in its infancy. In doing so, he brought phase space representations into quantum mechanics. However, its unique nature also made it very interesting for classical approaches and for identifying the deviations from classical behavior and the entanglement that can occur in quantum systems. What stands out, though, is the feature to experimentally reconstruct the Wigner function, which provides far more information on the system than can be obtained by any other quantum approach. This feature is particularly important for the field of quantum information processing and quantum physics. However, the Wigner function finds wide-ranging use cases in other dominant and highly active fields as well, such as in quantum electronics—to model the electron transport, in quantum chemistry—to calculate the static and dynamical properties of many-body quantum systems, and in signal processing—to investigate waves passing through certain media. What is peculiar in recent years is a strong increase in applying it: Although originally formulated 86 years ago, only today the full potential of the Wigner function—both in ability and diversity—begins to surface. This review, as well as a growing, dedicated Wigner community, is a testament to this development and gives a broad and concise overview of recent advancements in different fields.

211 citations


Journal ArticleDOI
TL;DR: In this article, the authors present a review of the state-of-the-art in the field of thermoelectric materials with a focus on bulk inorganic materials, including powder and single crystal synthesis, error analysis, and modeling of transport data using an effective mass model.
Abstract: The study of thermoelectric materials spans condensed matter physics, materials science and engineering, and solid-state chemistry. The diversity of the participants and the inherent complexity of the topic mean that it is difficult, if not impossible, for a researcher to be fluent in all aspects of the field. This review, which grew out of a one-week summer school for graduate students, aims to provide an introduction and practical guidance for selected conceptual, synthetic, and characterization approaches and to craft a common umbrella of language, theory, and experimental practice for those engaged in the field of thermoelectric materials. This review does not attempt to cover all major aspects of thermoelectric materials research or review state-of-the-art thermoelectric materials. Rather, the topics discussed herein reflect the expertise and experience of the authors. We begin by discussing a universal approach to modeling electronic transport using Landauer theory. The core sections of the review are focused on bulk inorganic materials and include a discussion of effective strategies for powder and single crystal synthesis, the use of national synchrotron sources to characterize crystalline materials, error analysis, and modeling of transport data using an effective mass model, and characterization of phonon behavior using inelastic neutron scattering and ultrasonic speed of sound measurements. The final core section discusses the challenges faced when synthesizing carbon-based samples and the measuring or interpretation of their transport properties. We conclude this review with a brief discussion of some of the grand challenges and opportunities that remain to be addressed in the study of thermoelectrics.

208 citations


Journal ArticleDOI
TL;DR: The ability of spintronic devices to utilize an electric current for manipulating the magnetization has resulted in large-scale developments, such as magnetic random access memories and boosted the spintronics research area as mentioned in this paper.
Abstract: The ability of spintronic devices to utilize an electric current for manipulating the magnetization has resulted in large-scale developments, such as magnetic random access memories and boosted the spintronic research area. In this regard, over the last decade, magnetization manipulation using spin-orbit torque has been devoted a lot of research attention as it shows a great promise for future ultrafast and power efficient magnetic memories. In this review, we summarize the latest advancements in spin-orbit torque research and highlight some of the technical challenges for practical spin-orbit torque devices. We will first introduce the basic concepts and highlight the latest material choices for spin-orbit torque devices. Then, we will summarize the important advancements in the study of magnetization switching dynamics using spin-orbit torque, which are important from scientific as well as technological aspects. The final major section focuses on the concept of external assist field-free spin-orbit torque switching which is a requirement for practical spin-orbit torque devices.

166 citations


Journal ArticleDOI
TL;DR: In this article, the authors outline similarities and differences between polycrystalline thin-film photovoltaic materials from both the materials and the industrial point of view and address the materials characteristics and device concepts for each technology, including a description of recent developments that have led to very high efficiency achievements.
Abstract: Already, several technologies of polycrystalline thin‐film photovoltaic materials have achieved certified record small‐cell power conversion efficiencies exceeding 22%. They are CdTe, Cu(In,Ga)(S,Se)2 (CIGS), and metal halide perovskite (PSC), each named after the light-absorbing semiconductor material. Thin-film solar cells and modules require very little active material due to their very high absorption coefficient. Efficient production methods with low materials waste, moderate temperatures, attractive cost structures, and favorable energy payback times will play a strong role in market development as thin-film technologies reach full maturity, including mass production and the standardization of production machineries. In fact, the first two technologies have already been developed up to the industrial scale with a market share of several GW. In this review article, we outline similarities and differences between these high‐efficiency thin‐film technologies from both the materials and the industrial point of view. We address the materials characteristics and device concepts for each technology, including a description of recent developments that have led to very high efficiency achievements. We provide an overview of the CIGS industry players and their current status. The newcomer PSC has demonstrated its potential in the laboratory, and initial efforts in industrial production are underway. A large number of laboratories are experimenting through a wide range of options in order to optimize not only the efficiency but also stability, environmental aspects, and manufacturability of PSC. Its high efficiency and its high bandgap make PSC particularly attractive for tandem applications. An overview of all these topics is included here along with a list of materials configurations.

157 citations


Journal ArticleDOI
TL;DR: In this article, the authors discuss the latest advancements in quartzenhanced photoacoustic spectroscopy (QEPAS) based trace-gas sensing, and present a comparison of the QEPAS performance of different spectrophone configurations based upon signal-to-noise ratio.
Abstract: This review aims to discuss the latest advancements in quartz-enhanced photoacoustic spectroscopy (QEPAS) based trace-gas sensing. Starting from the QEPAS basic physical principles, the most used QEPAS configurations will be described. This is followed by a detailed theoretical analysis and experimental study regarding the influence of quartz tuning forks (QTFs) geometry on their optoacoustic transducer performance. Furthermore, an overview of the latest developments in QEPAS trace-gas sensor technology employing custom QTFs will be reported. Results obtained by exploiting novel micro-resonator configurations, capable of increasing the QEPAS signal-to-noise ratio by more than two orders of magnitude and the utilization of QTF overtone flexural modes for QEPAS based sensing will be presented. A comparison of the QEPAS performance of different spectrophone configurations is reported based upon signal-to-noise ratio. Finally, a novel QEPAS approach allowing simultaneous dual-gas detection will be described.

152 citations


Journal ArticleDOI
TL;DR: A review of the plasma physics of liquids can be found in this paper, where the main difficulties in understanding revolve around the basic mechanisms of plasma initiation in the liquid phase and the electrical interactions at a plasma-liquid interface, which require an interdisciplinary approach.
Abstract: The interaction of plasma with liquids has led to various established industrial implementations as well as promising applications, including high-voltage switching, chemical analysis, nanomaterial synthesis, and plasma medicine. Along with these numerous accomplishments, the physics of plasma in liquid or in contact with a liquid surface has emerged as a bipartite research field, for which we introduce here the term “plasma physics of liquids.” Despite the intensive research investments during the recent decennia, this field is plagued by some controversies and gaps in knowledge, which might restrict further progress. The main difficulties in understanding revolve around the basic mechanisms of plasma initiation in the liquid phase and the electrical interactions at a plasma-liquid interface, which require an interdisciplinary approach. This review aims to provide the wide applied physics community with a general overview of the field, as well as the opportunities for interdisciplinary research on topics, such as nanobubbles and the floating water bridge, and involving the research domains of amorphous semiconductors, solid state physics, thermodynamics, material science, analytical chemistry, electrochemistry, and molecular dynamics simulations. In addition, we provoke awareness of experts in the field on yet underappreciated question marks. Accordingly, a strategy for future experimental and simulation work is proposed.

Journal ArticleDOI
TL;DR: In this article, a review of the available results and generalization of the physical mechanisms of the guided ionization waves is presented, which are of particular interest to practical applications of atmospheric-pressure plasma discharges, in general.
Abstract: Free to read on publisher website Guided ionization waves, or plasma streamers, are increasingly important for many applications in spanning materials processing and biomedicine. The highly reproducible, repeatable behavior of the most puzzling kind of the streamers–plasma bullets is highly attractive as it promises a high degree of control in many applications. However, despite a dozen years since the discovery of this phenomenon, the exact reasons for such behavior still remain essentially unclear. To understand the dynamics of the guided ionization wave (plasma bullet), a large number of works have been carried out and many interesting results have been reported. Here, we critically examine the available results and generalize the physical mechanisms of the guided ionization waves, which are of particular interest to practical applications of atmospheric-pressure plasma discharges, in general. The critical examination of the fundamental principles will show that, in order to propagate in a repeatable-mode, the plasma bullet must propagate in a channel with a high seed electron density (HSED), which is on the order of 109 cm−3. This review concludes that to distinguish guided ionization waves from traditional positive streamer discharges, it is most appropriate to describe an atmospheric-pressure discharge featuring a plasma bullet behavior as an HSED discharge. When the HSED condition is met, the dynamics of a plasma plume appears to be repeatable. On the contrary, it propagates in an unrepeatable mode and emerges more like a positive streamer discharge when the HSED condition is not satisfied. According to this theory, the transition of the propagation mode of the plasma bullet between the repeatable mode and the stochastic mode can be well explained. Besides by controlling the seed electron density around the transition region between the HSED discharge and the traditional positive streamer, this knowledge will help in better understanding of the positive streamer discharges in air, in cases relevant to practical applications of such plasma discharges in materials processing technologies, industrial chemistry, nanotechnology, and health care.

Journal ArticleDOI
TL;DR: A review of continuum and mesoscale models for high explosives can be found in this paper, where it is argued that porosity and thermodynamics can both be explained by the combined effect of thermodynamics and hydrodynamics, rather than the traditional hotspot-based explanations linked to...
Abstract: The shock and detonation response of high explosives has been an active research topic for more than a century. In recent years, high quality data from experiments using embedded gauges and other diagnostic techniques have inspired the development of a range of new high-fidelity computer models for explosives. The experiments and models have led to new insights, both at the continuum scale applicable to most shock and detonation experiments, and at the mesoscale relevant to hotspots and burning within explosive microstructures. This article reviews the continuum and mesoscale models, and their application to explosive phenomena, gaining insights to aid future model development and improved understanding of the physics of shock initiation and detonation propagation. In particular, it is argued that “desensitization” and the effect of porosity on high explosives can both be explained by the combined effect of thermodynamics and hydrodynamics, rather than the traditional hotspot-based explanations linked to ...

Journal ArticleDOI
TL;DR: The relevance of the material in high power fiber laser technologies is reviewed, and where appropriate, materials-based paths to the enhancement of laser performance will be underscored.
Abstract: Over the past two decades, fiber laser technologies have matured to such an extent that they have captured a large portion of the commercial laser marketplace. Yet, there still is a seemingly unquenchable thirst for ever greater optical power to levels where certain deleterious light-matter interactions that limit continued power scaling become significant. In the past decade or so, the industry has focused mainly on waveguide engineering to overcome many of these hurdles. However, there is an emerging body of work emphasizing the enabling role of the material. In an effort to underpin these developments, this paper reviews the relevance of the material in high power fiber laser technologies. As the durable material-of-choice for the application, the discussion will mainly be limited to silicate host glasses. The discussion presented herein follows an outward path, starting with the trivalent rare earth ions and their spectroscopic properties. The ion then is placed into a host, whose impact on the spectroscopy is reviewed. Finally, adverse interactions between the laser lightwave and the host are discussed, and novel composition glass fiber design and fabrication methodologies are presented. With deference to the symbiosis required between material and waveguide engineering in active fiber development, this review will emphasize the former. Specifically, where appropriate, materials-based paths to the enhancement of laser performance will be underscored.

Journal ArticleDOI
TL;DR: In this article, a review of the existing knowledge/technological gaps identified from the current literature and suggestions for the future work is presented, followed by the physics behind solid sampling of laser ablation plumes, optical methods for isotope measurements, the suitable physical conditions of laser-produced plasma plumes for isotopic analysis, and the current status.
Abstract: Rapid, in-field, and non-contact isotopic analysis of solid materials is extremely important to a large number of applications, such as nuclear nonproliferation monitoring and forensics, geochemistry, archaeology, and biochemistry. Presently, isotopic measurements for these and many other fields are performed in laboratory settings. Rapid, in-field, and non-contact isotopic analysis of solid material is possible with optical spectroscopy tools when combined with laser ablation. Laser ablation generates a transient vapor of any solid material when a powerful laser interacts with a sample of interest. Analysis of atoms, ions, and molecules in a laser-produced plasma using optical spectroscopy tools can provide isotopic information with the advantages of real-time analysis, standoff capability, and no sample preparation requirement. Both emission and absorption spectroscopy methods can be used for isotopic analysis of solid materials. However, applying optical spectroscopy to the measurement of isotope ratios from solid materials presents numerous challenges. Isotope shifts arise primarily due to variation in nuclear charge distribution caused by different numbers of neutrons, but the small proportional nuclear mass differences between nuclei of various isotopes lead to correspondingly small differences in optical transition wavelengths. Along with this, various line broadening mechanisms in laser-produced plasmas and instrumental broadening generated by the detection system are technical challenges frequently encountered with emission-based optical diagnostics. These challenges can be overcome by measuring the isotope shifts associated with the vibronic emission bands from molecules or by using the techniques of laser-based absorption/fluorescence spectroscopy to marginalize the effect of instrumental broadening. Absorption and fluorescence spectroscopy probe the ground state atoms existing in the plasma when it is cooler, which inherently provides narrower lineshapes, as opposed to emission spectroscopy which requires higher plasma temperatures to be able to detect thermally excited emission. Improvements in laser and detection systems and spectroscopic techniques have allowed for isotopic measurements to be carried out at standoff distances under ambient atmospheric conditions, which have expanded the applicability of optical spectroscopy-based isotopic measurements to a variety of scientific fields. These technological advances offer an in-situ measurement capability that was previously not available. This review will focus on isotope detection through emission, absorption, and fluorescence spectroscopy of atoms and molecules in a laser-produced plasma formed from a solid sample. A description of the physics behind isotope shifts in atoms and molecules is presented, followed by the physics behind solid sampling of laser ablation plumes, optical methods for isotope measurements, the suitable physical conditions of laser-produced plasma plumes for isotopic analysis, and the current status. Finally, concluding remarks will be made on the existing knowledge/technological gaps identified from the current literature and suggestions for the future work.

Journal ArticleDOI
TL;DR: Optomechanically induced transparency (OIT) as discussed by the authors is an analog to atomic electromagnetic induced transparency that a transmission window for the propagation of the probe field is induced by a strong control field when the resonance condition is met.
Abstract: Cavity optomechanical systems have been shown to exhibit an analogon to atomic electromagnetically induced transparency that a transmission window for the propagation of the probe field is induced by a strong control field when the resonance condition is met. Sharp transmission features controlled by the control laser beam enable many applications ranging from force sensors to quantum communication. In recent years, there has been significant progress in both theoretical and experimental studies of this phenomenon, driven by the development of nanophotonics as well as the improvement of nano-fabrication techniques. Optomechanically induced transparency has been found to manifest in numerous different physical mechanisms, e.g., nonlinear optomechanically induced transparency, double optomechanically induced transparency, parity-time symmetric optomechanically induced transparency, and optomechanically induced transparency in various hybrid optomechanical systems, etc. These results offer a pathway towards an integrated quantum optomechanical memory, show the utility of these chip-scale optomechanical systems for optical buffering, amplification, and filtering of microwave-over-optical signals, and may be applicable to modern optical networks and future quantum networks. Here, we systematically review the latest research progress on the fundamentals and applications of optomechanically induced transparency. Perspectives and opportunities on future developments are also provided by focusing on several promising topics.

Journal ArticleDOI
TL;DR: Superlubricity as discussed by the authors is the state of ultra-low friction between surfaces in relative motion, and various approaches to achieving this state are considered in a broad sense, including structural superlubrication, superl lubricity via normal force control, and contact actuation.
Abstract: We present a review of superlubricity: the state of ultra-low friction between surfaces in relative motion. Various approaches to achieving this state are considered in a broad sense, including structural superlubricity, superlubricity via normal force control, and contact actuation, as well as thermolubricity, liquid superlubricity, and quantum lubricity. An overview of the physical fundamentals associated with each approach is presented, with particular emphasis on recent theoretical and experimental developments that constitute milestones in our scientific understanding. The review also includes a discussion of perspectives on future research in the context of existing challenges. It is projected that interest in superlubricity from the basic science and engineering communities will continue to accelerate in the near future, accompanied by a transition from fundamental studies to technologically relevant applications.

Journal ArticleDOI
TL;DR: A comprehensive overview of the studies of bioink printability during representative 3D bioprinting processes, including inkjet printing, laser printing, and micro-extrusion is presented, with a focus on the understanding of the underlying physics during the formation of bioinks.
Abstract: Three-dimensional (3D) bioprinting, as a freeform biomedical manufacturing approach, has been increasingly adopted for the fabrication of constructs analogous to living tissues. Generally, materials printed during 3D bioprinting are referred as bioinks, which may include living cells, extracellular matrix materials, cell media, and/or other additives. For 3D bioprinting to be an enabling tissue engineering approach, the bioink printability is a critical requirement as tissue constructs must be able to be printed and reproduce the complex micro-architecture of native tissues in vitro in sufficient resolution. The bioink printability is generally characterized in terms of the controllable formation of well-defined droplets/jets/filaments and/or the morphology and shape fidelity of deposited building blocks. This review presents a comprehensive overview of the studies of bioink printability during representative 3D bioprinting processes, including inkjet printing, laser printing, and micro-extrusion, with a focus on the understanding of the underlying physics during the formation of bioink-based features. A detailed discussion is conducted based on the typical time scales and dimensionless quantities for printability evaluation during bioprinting. For inkjet printing, the Z (the inverse of the Ohnesorge number), Weber, and capillary numbers have been employed for the construction of phase diagrams during the printing of Newtonian fluids, while the Weissenberg and Deborah numbers have been utilized during the printing of non-Newtonian bioinks. During laser printing of Newtonian solutions, the jettability can be characterized using the inverse of the Ohnesorge number, while Ohnesorge, elasto-capillary, and Weber numbers have been utilized to construct phase diagrams for typical non-Newtonian bioinks. For micro-extrusion, seven filament types have been identified including three types of well-defined filaments and four types of irregular filaments. During micro-extrusion, the Oldroyd number has been used to characterize the dimensions of the yielded areas of Herschel-Bulkley fluids. Non-ideal jetting behaviors are common during the droplet-based inkjet and laser printing processes due to the local nonuniformity and nonhomogeneity of cell-laden bioinks.

Journal ArticleDOI
TL;DR: In this article, the effect of parametric pumping and thermal-piezoresistive pumping on the quality factor of a micro-and nano-electromechanical (MEM/NEM) resonator was investigated.
Abstract: Quality factor (Q) is an important property of micro- and nano-electromechanical (MEM/NEM) resonators that underlie timing references, frequency sources, atomic force microscopes, gyroscopes, and mass sensors. Various methods have been utilized to tune the effective quality factor of MEM/NEM resonators, including external proportional feedback control, optical pumping, mechanical pumping, thermal-piezoresistive pumping, and parametric pumping. This work reviews these mechanisms and compares the effective Q tuning using a position-proportional and a velocity-proportional force expression. We further clarify the relationship between the mechanical Q, the effective Q, and the thermomechanical noise of a resonator. We finally show that parametric pumping and thermal-piezoresistive pumping enhance the effective Q of a micromechanical resonator by experimentally studying the thermomechanical noise spectrum of a device subjected to both techniques.

Journal ArticleDOI
TL;DR: In this article, the authors review recent progress in two-dimensional ferromagnetism in detail and predict new possible 2D ferromagnetic materials, and discuss the prospects for applications of atomically thin ferromagnets in novel dissipationless electronics, spintronics, and other conventional magnetic technologies.
Abstract: The inherent susceptibility of low-dimensional materials to thermal fluctuations has long been expected to pose a major challenge to achieve intrinsic long-range ferromagnetic order in two-dimensional materials. The recent explosion of interest in atomically thin materials and their assembly into van der Waals heterostructures has renewed interest in two-dimensional ferromagnetism, which is interesting from a fundamental scientific point of view and also offers a missing ingredient necessary for the realization of spintronic functionality in van der Waals heterostructures. Recently, several atomically thin materials have been shown to be robust ferromagnets. Such ferromagnetism is thought to be enabled by magnetocrystalline anisotropy which suppresses thermal fluctuations. In this article, we review recent progress in two-dimensional ferromagnetism in detail and predict new possible two-dimensional ferromagnetic materials. We also discuss the prospects for applications of atomically thin ferromagnets in novel dissipationless electronics, spintronics, and other conventional magnetic technologies. Particularly, atomically thin ferromagnets are promising to realize time reversal symmetry breaking in two-dimensional topological systems, providing a platform for electronic devices based on the quantum anomalous Hall effect showing dissipationless transport. Our proposed directions will assist the scientific community to explore novel two-dimensional ferromagnetic families which can spawn new technologies and further improve the fundamental understanding of this fascinating area.

Journal ArticleDOI
TL;DR: In this article, the authors comprehensively review both the current knowledge in the context of various applied fields and the global understanding of the bistability and hysteresis physics in the inductively coupled plasmas (ICPs).
Abstract: Many different gas discharges and plasmas exhibit bistable states under a given set of conditions, and the history-dependent hysteresis that is manifested by intensive quantities of the system upon variation of an external parameter has been observed in inductively coupled plasmas (ICPs). When the external parameters (such as discharge powers) increase, the plasma density increases suddenly from a low- to high-density mode, whereas decreasing the power maintains the plasma in a relatively high-density mode, resulting in significant hysteresis. To date, a comprehensive description of plasma hysteresis and a physical understanding of the main mechanism underlying their bistability remain elusive, despite many experimental observations of plasma bistability conducted under radio-frequency ICP excitation. This fundamental understanding of mode transitions and hysteresis is essential and highly important in various applied fields owing to the widespread use of ICPs, such as semiconductor/display/solar-cell processing (etching, deposition, and ashing), wireless light lamp, nanostructure fabrication, nuclear-fusion operation, spacecraft propulsion, gas reformation, and the removal of hazardous gases and materials. If, in such applications, plasma undergoes a mode transition and hysteresis occurs in response to external perturbations, the process result will be strongly affected. Due to these reasons, this paper comprehensively reviews both the current knowledge in the context of the various applied fields and the global understanding of the bistability and hysteresis physics in the ICPs. At first, the basic understanding of the ICP is given. After that, applications of ICPs to various applied fields of nano/environmental/energy-science are introduced. Finally, the mode transition and hysteresis in ICPs are studied in detail. This study will show the fundamental understanding of hysteresis physics in plasmas and give open possibilities for applications to various applied fields to find novel control knob and optimizing processing conditions.

Journal ArticleDOI
TL;DR: In this focused review, some of the more commonly used potential energy functions for molecular simulations are reviewed with an eye toward presenting their general forms, strengths, and weaknesses.
Abstract: Molecular simulation is a powerful computational tool for a broad range of applications including the examination of materials properties and accelerating drug discovery. At the heart of molecular simulation is the analytic potential energy function. These functions span the range of complexity from very simple functions used to model generic phenomena to complex functions designed to model chemical reactions. The complexity of the mathematical function impacts the computational speed and is typically linked to the accuracy of the results obtained from simulations that utilize the function. One approach to improving accuracy is to simply add more parameters and additional complexity to the analytic function. This approach is typically used in non-reactive force fields where the functional form is not derived from quantum mechanical principles. The form of other types of potentials, such as the bond-order potentials, is based on quantum mechanics and has led to varying levels of accuracy and transferability. When selecting a potential energy function for use in molecular simulations, the accuracy, transferability, and computational speed must all be considered. In this focused review, some of the more commonly used potential energy functions for molecular simulations are reviewed with an eye toward presenting their general forms, strengths, and weaknesses.

Journal ArticleDOI
TL;DR: Time-domain Brillouin scattering is an all-optical experimental technique based on ultrafast lasers applied for generation and detection of coherent acoustic pulses on time durations of picoseconds and length scales of nanometers as mentioned in this paper.
Abstract: Time-domain Brillouin scattering is an all-optical experimental technique based on ultrafast lasers applied for generation and detection of coherent acoustic pulses on time durations of picoseconds and length scales of nanometers. In transparent materials, scattering of the probe laser beam by the coherent phonons permits imaging of sample inhomogeneity. The transient optical reflectivity of the sample recorded by the probe beam as the acoustic nanopulse propagates in space contains information on the acoustical, optical, and acousto-optical parameters of the material under study. The experimental method is based on a heterodyning where weak light pulses scattered by the coherent acoustic phonons interfere at the photodetector with probe light pulses of significantly higher amplitude reflected from various interfaces of the sample. The time-domain Brillouin scattering imaging is based on Brillouin scattering and has the potential to provide all the information that researchers in materials science, physics, chemistry, biology, etc., get with classic frequency-domain Brillouin scattering of light. It can be viewed as a replacement for Brillouin scattering and Brillouin microscopy in all investigations where nanoscale spatial resolution is either required or advantageous. Here, we review applications of time-domain Brillouin scattering for imaging of nanoporous films, ion-implanted semiconductors and dielectrics, texture in polycrystalline materials and inside vegetable and animal cells, and for monitoring the transformation of nanosound caused by nonlinearity and focusing. We also discuss the perspectives and the challenges for the future.

Journal ArticleDOI
TL;DR: Semiconductor nanowires (NWs) represent a new class of materials and a shift from conventional two-dimensional bulk thin films to three-dimensional devices as discussed by the authors, where lattice mismatch strain in NWs can be relaxed elastically at the NW free surface without dislocations.
Abstract: Semiconductor nanowires (NWs) represent a new class of materials and a shift from conventional two-dimensional bulk thin films to three-dimensional devices. Unlike thin film technology, lattice mismatch strain in NWs can be relaxed elastically at the NW free surface without dislocations. This capability can be used to grow unique heterostructures and to grow III-V NWs directly on inexpensive substrates, such as Si, rather than lattice-matched but more expensive III-V substrates. This capability, along with other unique properties (quantum confinement and light trapping), makes NWs of great interest for next generation optoelectronic devices with improved performance, new functionalities, and reduced cost. One of the many applications of NWs includes energy conversion. This review will outline applications of NWs in photovoltaics, thermoelectrics, and betavoltaics (direct conversion of solar, thermal, and nuclear energy, respectively, into electrical energy) with an emphasis on III-V materials. By transitioning away from bulk semiconductor thin films or wafers, high efficiency photovoltaic cells comprised of III-V NWs grown on Si would improve performance and take advantage of cheaper materials, larger wafer sizes, and improved economies of scale associated with the mature Si industry. The thermoelectric effect enables a conversion of heat into electrical power via the Seebeck effect. NWs present an opportunity to increase the figure of merit (ZT) of thermoelectric devices by decreasing the thermal conductivity (κ) due to surface phonon backscattering from the NW surface boundaries. Quantum confinement in sufficiently thin NWs can also increase the Seebeck coefficient by modification of the electronic density of states. Prospects for III-V NWs in thermoelectric devices, including solar thermoelectric generators, are discussed. Finally, betavoltaics refers to the direct generation of electrical power in a semiconductor from a radioactive source. This betavoltaic process is similar to photovoltaics in which photon energy is converted to electrical energy. In betavoltaics, however, energetic electrons (beta particles) are used instead of photons to create electron-hole pairs in the semiconductor by impact ionization. NWs offer the opportunity for improved beta capture efficiency by almost completely surrounding the radioisotope with semiconductor material. Improving the efficiency is important in betavoltaic design because of the high cost of materials and manufacturing, regulatory restrictions on the amount of radioactive material used, and the enabling of new applications with higher power requirements.

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TL;DR: The potential to use single-crystal sapphire optical fiber as an alternative to silica optical fibers for sensing in high-temperature, high-pressure, and chemically aggressive harsh environments has been recognized for several decades as mentioned in this paper.
Abstract: The potential to use single-crystal sapphire optical fiber as an alternative to silica optical fibers for sensing in high-temperature, high-pressure, and chemically aggressive harsh environments has been recognized for several decades A key technological barrier to the widespread deployment of harsh environment sensors constructed with sapphire optical fibers has been the lack of an optical cladding that is durable under these conditions However, researchers have not yet succeeded in incorporating a high-temperature cladding process into the typical fabrication process for single-crystal sapphire fibers, which generally involves seed-initiated fiber growth from the molten oxide state While a number of advances in fabrication of a cladding after fiber-growth have been made over the last four decades, none have successfully transitioned to a commercial manufacturing process This paper reviews the various strategies and techniques for fabricating an optically clad sapphire fiber which have been proposed

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TL;DR: In this paper, the main difficulty is efficient doping of films with aluminum-rich compositions; the problem is particularly severe for p-type doping, which is essential for Ohmic contacts to bipolar device structures.
Abstract: The group III-nitride (InN, GaN, and AlN) class of semiconductors has become one of two that are critical to a number of technologies in modern life—the other being silicon. Light-emitting diodes made from (In,Ga)N, for example, dominate recent innovations in general illumination and signaling. Even though the (In,Ga)N materials system is fairly well established and widely used in advanced devices, challenges continue to impede development of devices that include aluminum-containing nitride films such as (Al,Ga)N. The main difficulty is efficient doping of films with aluminum-rich compositions; the problem is particularly severe for p-type doping, which is essential for Ohmic contacts to bipolar device structures. This review briefly summarizes the fundamental issues related to p-type doping, and then discusses a number of approaches that are being pursued to resolve the doping problem or for circumventing the need for p-type doping. Finally, we discuss an approach to doping under liquid-metal-enabled gro...

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TL;DR: The need for very black coatings is persistent for a variety of applications ranging from baffles and traps to blackbodies and thermal detectors as discussed by the authors, which is attributable to the intrinsic properties of graphitic material as well as the morphology (density, thickness, disorder, and tube size).
Abstract: Coatings comprising carbon nanotubes are very black, that is, characterized by uniformly low reflectance over a broad range of wavelengths from the visible to far infrared. Arguably, there is no other material that is comparable. This is attributable to the intrinsic properties of graphitic material as well as the morphology (density, thickness, disorder, and tube size). We briefly describe a history of other coatings such as nickel phosphorous, gold black, and carbon-based paints and the comparable structural morphology that we associate with very black coatings. The need for black coatings is persistent for a variety of applications ranging from baffles and traps to blackbodies and thermal detectors. Applications for space-based instruments are of interest and we present a review of space qualification and the results of outgassing measurements. Questions of nanoparticle safety depend on the nanotube size and aspect ratio as well as the nature and route of exposure. We describe the growth of carbon nano...

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TL;DR: In this article, a simple classification of spin-gapless semiconductors (SGSs) based on their unique band structures is proposed, which can be divided into two types: parabolic and Dirac-like linear.
Abstract: Spin-gapless semiconductors (SGSs), the new generation of spintronic materials, have received increasing attention recently owing to their various attractive properties such as fully spin-polarization and high carrier mobility. Based on their unique band structures, SGSs can be divided into two types: parabolic and Dirac-like linear. The linear-type SGSs, also called Dirac SGSs (DSGSs), have real massless fermions and dissipation-less transport properties, and thus are regarded as promising material candidates for applications in ultra-fast and ultra-low-power spintronic devices. DSGSs can be further classified into p-state type or d-state type depending on the degree of contribution of either the p-orbitals or d-orbitals to the Dirac states. Considering the importance of the research field and to cover its fast development, we reviewed the advances in DSGSs and proposed our own viewpoints. First, we introduced the computational algorithms of SGSs. Second, we found that the boundaries between DSGSs and Dirac half-metals were frequently blurred. Therefore, a simple classification is proposed in this work. Third, we collected almost all the studies on DSGSs published in the past six years. Finally, we proposed new guidance to search for DSGSs among 3D bulk materials on the basis of our latest results.

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TL;DR: The achievements and the challenges for the CPV module technology and its components are reviewed and the module designs that have shown the highest efficiencies are presented.
Abstract: Concentrator photovoltaics (CPV) is a special high efficiency system technology in the world of PV-technologies. The idea of CPV is to use optical light concentrators to increase the incident power on solar cells. The solar cell area is comparatively tiny, thus saving expensive semiconductor materials and allowing the use of more sophisticated and more costly multi-junction solar cells. The highest CPV module efficiency achieved is 38.9%. This CPV module uses four-junction III-V-based solar cells. Moreover, mini-modules have already achieved an efficiency of 43.4%. The interaction between optics, cells, and layout of the module and tracker determines the overall field performance. Today, some utility scale CPV plants are installed. The CPV technology allows for many technical solutions for system designs and for optimizing performance while maintaining the economics. This paper will review the achievements and discuss the challenges for the CPV module technology and its components. We discuss the different components and the most important effects regarding the module design. Furthermore, we present the module designs that have shown the highest efficiencies.

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TL;DR: In this paper, a review of 2D GaN and AlN structures is presented, starting from three-dimensional (3D) GaN/AlN crystals and multilayer (ML) structures.
Abstract: Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were...