The 100 most-cited articles in Journal of Applied Physics, 2020

Following astonishing growth in the last decade, the field of luminescence thermometry has reached the stage of becoming a mature technology. To achieve that goal, further developments should resolve inherent problems and methodological faults to facilitate its widespread use. This perspective presents recent findings in luminescence thermometry, with the aim of providing a guide for the reader to the paths in which this field is currently directed. Besides the well-known temperature read-out techniques, which are outlined and compared in terms of performance, some recently introduced read-out methods have been discussed in more detail. These include intensity ratio measurements that exploit emissions from excited lanthanide levels with large energy differences, dual-excited and time-resolved single-band ratiometric methods, and phase-angle temperature readouts. The necessity for the extension of theoretical models and a careful re-examination of those currently in use are emphasized. Regarding materials, the focus of this perspective is on dual-activated probes for the luminescence…

Luminescent carbon dots (CDs) have received increasing attention from many fields during the past decade. Unfortunately, the luminescent mechanisms of CDs remain unclear due to insufficient experimental and theoretical knowledge, which significantly hinders the development of CDs with desired optical properties. Currently, surface states of CDs, which are based on synergistic hybridization between the carbon backbones and the connected functional groups, have been considered as the dominant luminescence origins. This tutorial paper, thus, aims to offer an overview of the key features on the surface of CDs, such as particle size, surface functional groups, defects and heteroatom doping, and their influences on the photoluminescence of CDs. In addition, optical characteristics of surface state-derived luminescence emissions of CDs are also summarized. Finally, the potential approaches of characterizing surface states of CDs are introduced, followed by an outlook of synthesizing high-quality CDs through modulation of the surface states.

A major challenge for the continued downscaling of integrated circuits is the resistivity increase of Cu interconnect lines with decreasing dimensions. Alternative metals have the potential to mitigate this resistivity bottleneck by either (a) facilitating specular electron interface scattering and negligible grain boundary reflection or (b) a low bulk mean free path that renders resistivity scaling negligible. Recent research suggests that specular electron scattering at the interface between the interconnect metal and the liner layer requires a low density of states at the interface and in the liner (i.e., an insulating liner) and either a smooth epitaxial metal-liner interface or only weak van der Waals bonding as typical for 2D liner materials. The grain boundary contribution to the room-temperature resistivity becomes negligible if the grain size is large (>200 nm or ten times the linewidth for wide or narrow conductors, respectively) or if the electron reflection coefficient is small due to low-energy boundaries and electronic state matching of neighboring grains.…

This paper provides a tutorial overview over recent vigorous efforts to develop computing systems based on spin waves instead of charges and voltages. Spin-wave computing can be considered a subfield of spintronics, which uses magnetic excitations for computation and memory applications. The Tutorial combines backgrounds in spin-wave and device physics as well as circuit engineering to create synergies between the physics and electrical engineering communities to advance the field toward practical spin-wave circuits. After an introduction to magnetic interactions and spin-wave physics, the basic aspects of spin-wave computing and individual spin-wave devices are reviewed. The focus is on spin-wave majority gates as they are the most prominently pursued device concept. Subsequently, we discuss the current status and the challenges to combine spin-wave gates and obtain circuits and ultimately computing systems, considering essential aspects such as gate interconnection, logic level restoration, input–output consistency, and fan-out achievement. We argue that spin-wave circuits need to…

Hybrid dynamic systems have recently gained interest with respect to both fundamental physics and device applications, particularly with their potential for coherent information processing. In this perspective, we will focus on the recent rapid developments of magnon-based hybrid systems, which seek to combine magnonic excitations with diverse excitations for transformative applications in devices, circuits, and information processing. Key to their promising potentials is that magnons are highly tunable excitations and can be easily engineered to couple with various dynamic media and platforms. The capability of reaching strong coupling with many different excitations has positioned magnons well for studying solid-state coherent dynamics and exploiting unique functionality. In addition, with their gigahertz frequency bandwidth and the ease of fabrication and miniaturization, magnonic devices and systems can be conveniently integrated into microwave circuits for mimicking a broad range of device concepts that have been applied in microwave electronics, photonics, and quantum…

In the field of high-power electronics, gallium oxide (Ga2O3) is attracting attention due to its wide bandgap and ability to be doped n-type. Point defects, including vacancies, impurities, and dopants, play important roles in optimizing device performance. This tutorial discusses the fundamental properties of point defects in monoclinic β-Ga2O3 and the methods employed to study them. Oxygen vacancies are deep donors that do not cause n-type conductivity but may compensate acceptors. Gallium vacancies are deep acceptors that can be partially passivated by hydrogen. Substitutional magnesium is a promising acceptor that produces a semi-insulating material and also forms a complex with hydrogen. Calcium and iron also have deep acceptor levels. Iridium deep donors are introduced into crystals grown from a melt in an Ir crucible. Other defects are introduced by irradiation with energetic particles such as neutrons or protons. In addition to altering the electronic properties, defects give rise to UV/visible emission bands in photoluminescence and cathodoluminescence spectra.

The ability of piezoelectric devices to convert mechanical energy to electrical energy and vice versa has inspired remarkable growth in research on piezoelectric materials. However, based on the Restriction of Hazardous Substances legislation, it is necessary to eliminate the lead from currently used piezoelectric ceramics. Together with the increasing market share and improved performance of lead-free piezoelectrics, this growing recognition that the use of lead should be limited in piezoelectric materials has promoted the development of lead-free piezoelectric ceramics. Some devices with excellent performance based on lead-free piezoelectric ceramics have been reported, and their applications are expected to increase in the near future. This perspective provides an overview of key advances related to the structures and properties of lead-free piezoelectrics, including (K,Na)NbO3, BaTiO3, Bi0.5Na0.5TiO3, and BiFeO3. Future prospects are also discussed based on the performances of lead-free piezoelectric materials investigated to date.

Polymer Genome is a web-based machine-learning capability to perform near-instantaneous predictions of a variety of polymer properties. The prediction models are trained on (and interpolate between) an underlying database of polymers and their properties obtained from first principles computations and experimental measurements. In this contribution, we first provide an overview of some of the critical technical aspects of Polymer Genome, including polymer data curation, representation, learning algorithms, and prediction model usage. Then, we provide a series of pedagogical examples to demonstrate how Polymer Genome can be used to predict dozens of polymer properties, appropriate for a range of applications. This contribution is closed with a discussion on the remaining challenges and possible future directions.

The ferroelectricity of (Al1−xScx)N (x = 0–0.34) thin films with various thicknesses was investigated. (Al1−xScx)N films were prepared at 400 °C on (111)Pt/TiOx/SiO2/(001)Si substrates by the radio frequency dual-source reactive magnetron sputtering method using Al and Sc targets under pure N2 gas or a mixture of N2 and Ar gases. The film deposited under N2 gas showed larger remanent polarization than those under N2 + Ar mixture. Ferroelectricity was observed for films with x = 0.10–0.34 for about 140-nm-thick films deposited under N2 gas. The x = 0.22 films showed ferroelectricity down to 48 nm in thickness from the polarization–electric field curves and the positive-up-negative-down measurement. The ferroelectricity of the 9 nm-thick film also was ascertained from scanning nonlinear dielectric microscopy measurement. These results reveal that ferroelectric polarization can switch for films with much broader compositions and thicknesses than those in the previous study.

Manipulating the properties of the isofrequency contours (IFCs) of materials provides a powerful means of controlling the interaction between light and matter. Hyperbolic metamaterials (HMMs), an important class of artificial anisotropic materials with hyperbolic IFCs, have been intensively investigated. Because of their open dispersion curves, HMMs support propagating high-k modes and possess an enhanced photonic density of states. As a result, HMMs can be utilized to realize hyperlenses breaking the diffraction limit, metacavity lasers with subwavelength scale, high-sensitivity sensors, long-range energy transfer, and so on. Aimed at those who are about to enter this burgeoning and rapidly developing research field, this tutorial article not only introduces the basic physical properties of HMMs but also discusses dispersion manipulation in HMMs and HMM-based structures such as hypercrystals. Both theoretical methods and experimental platforms are detailed. Finally, some potential applications associated with hyperbolic dispersion are introduced.

Triboelectric nanogenerators (TENGs), using Maxwell's displacement current as the driving force, can effectively convert mechanical energy into electricity. In this work, an extensive review of theoretical models of TENGs is presented. Based on Maxwell's equations, a formal physical model is established referred to as the quasi-electrostatic model of a TENG. Since a TENG is electrically neutral at any time owing to the low operation frequency, it is conveniently regarded as a lumped circuit element. Then, using the lumped parameter equivalent circuit theory, the conventional capacitive model and Norton's equivalent circuit model are derived. Optimal conditions for power, voltage, and total energy conversion efficiency can be calculated. The presented TENG models provide an effective theoretical foundation for understanding and predicting the performance of TENGs for practical applications.

Ferroelectric hafnia-based thin films are promising candidates for emerging high-density embedded nonvolatile memory technologies, thanks to their compatibility with silicon technology and the possibility of 3D integration. The electrode–ferroelectric interface and the crystallization annealing temperature may play an important role in such memory cells. The top interface in a TiN/Hf0.5Zr0.5O2/TiN metal–ferroelectric–metal stack annealed at different temperatures was investigated with X-ray photoelectron spectroscopy. The uniformity and continuity of the 2 nm TiN top electrode was verified by photoemission electron microscopy and conductive atomic force microscopy. Partial oxidation of the electrode at the interface is identified. Hf is reduced near the top interface due to oxygen scavenging by the top electrode. The oxygen vacancy (VO) profile showed a maximum at the top interface (0.71%) and a sharp decrease into the film, giving rise to an internal field. Annealing at higher temperatures did not affect the VO concentration at the top interface but causes the generation of…

When the bright green-emitting SrAl2O4:Eu,Dy persistent phosphor was described in the literature in 1996, this presented a real breakthrough in performance, both in terms of initial brightness and afterglow duration. Since then, many new persistent phosphors, with emission spanning from the ultraviolet to the near infrared, have been developed. Very few materials, however, reach a similar afterglow time and intensity as SrAl2O4:Eu,Dy, which is still considered the benchmark phosphor. The present paper discusses the reasons for this—seemingly—fundamental limitation and gives directions for further improvements. An overview is given of the preparation methods of persistent phosphors and their properties. Much attention is paid to the correct evaluation of a persistent phosphor in absolute units rather than vague terms or definitions. State of the art persistent phosphors are currently used extensively in emergency signage, indicators, and toys. Many more applications could be possible by tuning the range of trap depths used for energy storage. Very shallow traps could be used for…

The uni-traveling-carrier photodiode (UTC-PD) is a kind of pin junction photodiode that selectively uses electrons as active carriers. The diode structure has a relatively thin p-type absorber where electrons are generated as minority carriers, and then they diffuse and/or field-accelerate toward the collector. Since the electrons travel in the depleted collector at a ballistically high velocity, the photoresponse performance of a UTC-PD is superior to that of a conventional pin-PD. In this tutorial, the basics of the current response in a UTC-PD, the electron transport in the p-type absorber, and the performance of a terahertz-wave UTC photomixer, as a representative, are described.

Making materials lightweight while attaining a desirable combination of mechanical, thermal, and other physical properties is the “holy grail” of material science. Lattice materials, because of their porous structures and well-defined unit cell geometries, are suitable candidates to achieve lightweight with precisely tailored material properties. Aided by additive manufacturing techniques, a variety of lattice metamaterials with exceptional and unusual properties have been fabricated recently, yet, the rational designs of lattice metamaterials with programmability and multifunctionality are still challenging topics. In this perspective, we identify three emerging directions for lattice metamaterials: (1) developing architected lattice metamaterials with extreme and unusual properties that are non-typical in bulk materials, (2) designing lattice metamaterials with programmable mechanical properties that respond differently at different environments, loading paths, or controls, and (3) exploiting lattice metamaterials with multifunction, including tailorable thermal, mechanical,…

Multi-component and high-entropy nitrides are a growing field with a promise of new functional materials. The interest in the field was sparked by the adjacent field of high-entropy and multi-component alloys, and the promise consists of both demonstrated properties and a possibly very large freedom for materials design. These promises, however, also come with new challenges connected to the vast available experimental space, which is inherent in multi-component materials. Traditional materials science methodologies will be slow to make appreciable progress in such an environment. A novel approach is needed to meet the challenges of the hyperdimensional compositional space. Recent developments within the fields of information technology can give materials science the tools needed. This Perspective article summarizes the state of the art in the field of multi-component nitride materials, focusing on coatings where solid solution phases with simple crystal structures are formed. Furthermore, it outlines the present research challenges that need to be addressed to move the field…

This work explores the luminescence properties of self-trapped holes and impurity-related acceptors using one-dimensional configuration coordinate diagrams derived from hybrid functional calculations. The photoluminescence spectrum of as-grown β-Ga2O3 typically consists of a broad band in the wavelength region from ultraviolet to green and is often dominated by an impurity independent ultraviolet band that is commonly attributed to self-trapped holes. Here, we use the self-trapped hole as a benchmark to evaluate the accuracy of the theoretical defect luminescence spectra and estimate the optical properties of MgGa, BeGa, CaGa, CdGa, ZnGa, LiGa, and NO acceptor impurities, as well as their complexes with hydrogen donors. We also explore VGa acceptors complexed with hydrogen and SiGa donor impurities. The results show that these defects can give rise to broad luminescence bands peaking in the infrared to visible part of the spectrum, making them potential candidates for the defect origin of broad luminescence bands in β-Ga2O3.

Cavity magnonics is an emerging field that studies the strong coupling between cavity photons and collective spin excitations such as magnons. This rapidly developing field connects some of the most exciting branches of modern physics, such as quantum information and quantum optics, with one of the oldest sciences on Earth, the magnetism. The past few years have seen a steady stream of exciting experiments that demonstrate novel magnon-based transducers and memories. Most of such cavity magnonic devices rely on coherent coupling that stems from the direct dipole–dipole interaction. Recently, a distinct dissipative magnon–photon coupling was discovered. In contrast to coherent coupling that leads to level repulsion between hybridized modes, dissipative coupling results in level attraction. It opens an avenue for engineering and harnessing losses in hybrid systems. This article gives a brief review of this new frontier. Experimental observations of level attraction are reviewed. Different microscopic mechanisms are compared. Based on such experimental and theoretical reviews, we…

Surface plasmons have shown increasingly widespread applications in the last decade, especially in the field of solar energy conversion, recently leading to the use of metal nanoparticles as plasmonic photocatalysts. The latter offers great potential in overcoming traditional catalysts by providing localized heating and unconventional reaction pathways leading to improved product selectivity. A complete understanding of the underlying mechanisms remains, however, elusive due to the close resemblance between thermal and non-thermal effects, both leading to enhanced reaction rates. In this tutorial, we will introduce the basic physics of surface plasmons and the interaction mechanisms with surrounding molecules. We will then discuss the main strategies to evaluate photothermal effects and the main signatures of hot electron-driven processes. These aspects will be covered in specific examples of plasmonic photocatalysis for energy-relevant chemical reactions in the case of colloidal suspensions and at the solid/gas interphase in solid pellets, which involve different thermal…

Perspectives about growth of bulk gallium nitride crystals, fabricating high structural quality gallium nitride wafers and the market demand for them are presented. Three basic crystal growth technologies, halide vapor phase epitaxy, sodium flux, and ammonothermal, are described. Their advantages and disadvantages, recent development, and possibilities are discussed. The main difficulty with crystallization of thick GaN is determined. Some new solutions for bulk growth are proposed. It is shown that only crystallization on high structural quality native seeds will ensure proper progress. New ideas for fabricating gallium nitride crystals and wafers with a better control of their structural properties and point defect concentration are proposed.

It is the defects that determine the physicochemical properties and photoelectrical properties of the corresponding semiconductors. Controlling defects is essential to realize high-efficiency and stable solar cells, particularly in those based on hybrid halide perovskite materials. Here, we review the defect chemistry in perovskite absorbers, most of which take effects at grain boundaries and surfaces. These defects impact kinetics and/or thermodynamics during the courses of charge recombination, ion migration, and degradation in the corresponding devices, which inevitably influences their efficiency and stability. The effective suppression of harmful defects in perovskite photovoltaics not only reduces non-radiative recombination centers to improve the efficiency, but also retards their degradation under aging stresses to dramatically improve their long-term operational stability. Finally, the future challenges with regard to the in-depth understanding of defects formation, migration, and their passivation are presented, which shed light on realizing high-efficiency and stable…

Computer simulations within the framework of density functional theory are performed to study the electronic, dynamic, elastic, magnetic, and thermoelectric properties of a newly synthesized FeRhCrGe alloy and a theoretically predicted FeRhCrSi alloy. From the electronic structure simulations, both FeRhCrZ (Z = Si and Ge) alloys at their equilibrium lattice constants exhibit half-metallic ferromagnetism, which is established from the total magnetic moment of 3.00 μB, and that the spin moment of FeRhCrGe is close to the experimental value (2.90 μB). Their strength and stability with respect to external pressures are determined by simulated elastic constants. The Debye temperatures of FeRhCrSi and FeRhCrGe alloys are predicted to be 438 K and 640 K, respectively, based on elastic and thermal studies. The large power factors (PFs) of the two investigated alloys are in contour with those of the previously reported Heusler compounds. Besides, the conservative estimate of relaxation time speculated from the experimental conductivity value is 0.5 × 10−15 s. The room temperature PF values…

Dielectric polymers are critical to meet the increasing demands for high-energy-density capacitors operating in harsh environments, such as aerospace power conditioning, underground oil and gas exploration, electrified transportation, and pulse power systems. In this perspective article, we present an overview of the recent progress in the field of polymer dielectrics for high temperature capacitive energy storage applications. Particular attention is placed on the underlying physical mechanisms of the rational design and the material structure–dielectric property–capacitive performance relationship. The scientific and technological challenges that remain to be addressed and the opportunities for future research are also presented.

In this paper, the thermoelectric (TE) properties of Janus MXY monolayers (M = Pd, Pt; X, Y = S, Se, Te) are systematically studied using first principles and the Boltzmann transport theory. The thermal conductivity (k), Seebeck coefficient (S), power factor (PF), and TE figure of merit (ZT) are calculated accurately for various carrier concentrations. The lattice thermal conductivities of these six materials sequentially decrease in the order PtSSe, PtSTe, PtSeTe, PdSSe, PdSTe, and PdSeTe. PdSeTe and PtSeTe monolayers have a high ZT close to one at 300 K. In addition, we predicted the TE properties at high temperatures and found that the maximum ZT (2.54) is achieved for a monolayer of PtSeTe at 900 K. The structural and electronic properties of these six Janus transition-metal dichalcogenide (TMD) monolayers were systematically studied from first principles. Our results show that all six materials are semiconductors with bandgaps between 0.77 eV and 2.26 eV at the Heyd-Scuseria-Ernzerhof (HSE06) level. The present work indicates that the Janus MXY TMD monolayers (M = Pd, Pt; X, Y…

Since 2011, after the discovery of new ceramic two-dimensional materials called MXenes, the attention has been focused on their unique properties and various applications, from energy storage to nanomedicine. We present a brief perspective article of the properties of MXenes, alongside the most recent studies regarding their applications on energy, environment, wireless communications, and biotechnology. Future needs regarding the current knowledge about MXenes are also discussed in order to fully understand their nature and overcome the challenges that have restricted their use.

Nanomagnets form the building blocks for a gamut of miniaturized energy-efficient devices including data storage, memory, wave-based computing, sensors, and biomedical devices. They also offer a span of exotic phenomena and stern challenges. The rapid advancements of nanofabrication, characterization, and numerical simulations during the last two decades have made it possible to explore a plethora of science and technology applications related to nanomagnet dynamics. The progress in the magnetization dynamics of single nanomagnets and one- and two-dimensional arrays of nanostructures in the form of nanowires, nanodots, antidots, nanoparticles, binary and bi-component structures, and patterned multilayers have been presented in detail. Progress in unconventional and new structures like artificial spin ice and three-dimensional nanomagnets and spin textures like domain walls, vortex, and skyrmions has been presented. Furthermore, a huge variety of new topics in the magnetization dynamics of magnetic nanostructures are rapidly emerging. A future perspective on the steadily evolving…

In low temperature plasmas, the interaction of the electrons with the electric field is an important current research topic that is relevant for many applications. Particularly, in the low pressure regime (≤10 Pa), electrons can traverse a distance that may be comparable to the reactor dimensions without any collisions. This causes “nonlocal,” dynamics which results in a complicated space- and time-dependence and a strong anisotropy of the distribution function. Capacitively coupled radio frequency (CCRF) discharges, which operate in this regime, exhibit extremely complex electron dynamics. This is because the electrons interact with the space- and time-dependent electric field, which arises in the plasma boundary sheaths and oscillates at the applied radio frequency. In this tutorial paper, the fundamental physics of electron dynamics in a low pressure electropositive argon discharge is investigated by means of particle-in-cell/Monte Carlo collisions simulations. The interplay between the fundamental plasma parameters (densities, fields, currents, and temperatures) is explained by…

This Perspective surveys the state-of-the-art and future prospects of science and technology employing nanoconfined light (nanophotonics and nanoplasmonics) in combination with magnetism. We denote this field broadly as nanoscale magnetophotonics. We include a general introduction to the field and describe the emerging magneto-optical effects in magnetoplasmonic and magnetophotonic nanostructures supporting localized and propagating plasmons. Special attention is given to magnetoplasmonic crystals with transverse magnetization and the associated nanophotonic non-reciprocal effects and to magneto-optical effects in periodic arrays of nanostructures. We also give an overview of the applications of these systems in biological and chemical sensing, as well as in light polarization and phase control. We further review the area of nonlinear magnetophotonics, the semiconductor spin-plasmonics, and the general principles and applications of opto-magnetism and nano-optical ultrafast control of magnetism and spintronics.

Direct laser writing of graphene electrodes is an emerging research field for the rapid fabrication of two-dimensional carbon electronic materials with wide applications, ranging from supercapacitors and batteries to sensors, electrocatalysts, actuators, etc. Many types of carbon-containing raw materials can be converted to graphene by one-step laser scribing, without complicated chemical synthesis routines, using a variety of lasers. This perspective categorizes the principles of direct laser writing of graphene, according to the different types of raw materials, different types of lasers, and different applications. The future directions of laser synthesized graphene are also discussed.

The widespread use of nanostructures and nanomaterials has opened up a whole new realm of challenges in thermal management, but also leads to possibilities for energy conversion, storage, and generation, in addition to numerous other technological applications. At the microscale and below, standard thermal measurement techniques reach their limits, and several novel methods have been developed to overcome these limitations. Among the most recent, contactless photothermal methods have been widely used and have proved their advantages in terms of versatility, temporal and spatial resolution, and even sensitivity in some situations. Among them, thermoreflectance and Raman thermometry have been used to measure the thermal properties from bulk materials to thin films, multilayers, suspended structures, and nanomaterials. This Tutorial presents the principles of these two techniques and some of their most common implementations. It expands to more advanced systems for spatial mapping and for probing of non-Fourier thermal transport. Finally, this paper concludes with discussing the…

This article offers a perspective on photoacoustic tomography (PAT) under realistic scenarios. While PAT has gained much attention in preclinical and clinical research, most early works used image reconstruction techniques based on ideal assumptions, and thus these techniques may not be fully effective in real environments. In this work, we consider such non-ideal conditions as a limited view, limited bandwidth, lossy medium, or heterogeneous medium. More importantly, we use k-Wave simulation to numerically evaluate the effects of these limiting factors on various image reconstruction algorithms. Then, to enable more reliable PAT image reconstruction, we introduce recent techniques for mitigating each of the limiting conditions. We seek to emphasize the importance of working within these realistic limitations, and we encourage researchers to develop compensating solutions that advance PAT’s translation to real clinical environments.

The fabrication processes of p-type regions for vertical GaN power devices are investigated. A p-type body layer in a trench gate metal-oxide-semiconductor field-effect transistor requires precise control of the effective acceptor concentration, which is equal to the difference between the Mg acceptor concentration (Na) and the compensating donor concentration (Nd). The carbon atoms incorporated during growth via metalorganic vapor phase epitaxy substitute nitrogen sites (CN) and function as donor sources in a p-type GaN layer. Since interstitial H atoms (Hi) also compensate holes, their removal from an Mg-doped layer is crucial. Extended anneals to release H atoms cause the formation of extra hole traps. The p+ capping layer allows effective and rapid removal of H atoms from a p-type body layer owing to the electric field across the p+/p– junction. On the other hand, selective area p-type doping via Mg ion implantation is needed to control the electrical field distribution at the device edge. Ultrahigh-pressure annealing (UHPA) under a nitrogen pressure of 1 GPa enables…

Evaporation plays a critical role in a range of technologies that power and sustain our society. Wicks are widely used as passive, capillary-fed evaporators, attracting much interest since these devices are highly efficient, compact, and thermally stable. While wick-based evaporators can be further improved with advanced materials and fabrication techniques, modeling of heat and mass transport at the device level is vital for guiding these innovations. In this perspective, we present the design and optimization of capillary-fed, thin film evaporation devices through a heat and mass transfer lens. This modeling framework can guide future research into materials innovations, fabrication of novel architectures, and systems design/optimization for next generation, high-performance wick-based evaporators. Furthermore, we describe specific challenges and opportunities for the fundamental understanding of evaporation physics. Finally, we apply our modeling framework to the analysis of two important applications—solar vapor generation and electronics cooling devices.

An ultra-broadband absorber is introduced based on a multilayer graphene structure. Wave absorption of the optimized device is >90% in the bandwidth between 1.12 THz and 3.78 THz. Electric field distributions are studied to analyze the working mechanism. Because of the symmetrical structure, the designed absorber is not sensitive to the polarization state of the incident wave under small angles. The absorber has a reasonable tolerance for incident angles. The working intensity and bandwidth of the absorber are modulated with a wide range of tunability by different Fermi energy levels of graphene. This kind of ultra-broadband terahertz absorbers could have potential applications in a variety of areas, such as terahertz imaging, anti-radar stealth, and electrically adjustable modulation.

Defects are essential to switch the resistance states in resistive random-access memory (RRAM) devices. Controlled defects in such devices can lead to the stabilization of the switching performance, which is useful for high-density memory and neuromorphic computing applications. In contrast, uncontrolled defects in RRAM can generate randomness and increase intrinsic entropy, which are useful for security applications. In this tutorial, we explain how to engineer defects in RRAM devices. More specifically, we focus on defect engineering of the oxide layer and how the defects can affect the switching mechanism. Defect engineering processes include the doping effect, nanocrystal-based switching layer design, embedded metals in switching oxide, defective electrode design, etc. We explain how defects can improve the electrical performance of RRAM devices and the recent development of applications using defect-based RRAM devices.

In this article, we provide a perspective overview of the iconic properties, recent application-oriented research, and future commercialization opportunities of thin film metallic glasses (TFMGs). A brief review on the preparation and fundamental properties of TFMGs will be given first. TFMGs possess attractive properties such as corrosion resistance, extremely low roughness, and antibacterial characteristics, which give rise to various applications in biomedical devices, sensors, and tribology. Therefore, a number of our representative works will be reviewed to showcase the benefits of TFMGs over traditional materials and processing in these applications. In addition, new perspectives in the research and development of TFMGs and opportunities for commercialization will also be highlighted.

Native defects in semiconductors play an important role in their optoelectronic properties. Nickel oxide (NiO) is one of the few wide-gap p-type oxide semiconductors and its conductivity is believed to be controlled primarily by Ni-vacancy acceptors. Herein, we present a systematic study comparing the optoelectronic properties of stoichiometric NiO, oxygen-rich NiO with Ni vacancies (NiO:VNi), and Ni-rich NiO with O vacancies (NiO:VO). The optical properties were obtained by spectroscopic ellipsometry, while valence band spectra were probed by high-resolution x-ray photoelectron spectroscopy. The experimental results are directly compared to first-principles density functional theory + U calculations. Computational results confirm that gap states are present in both NiO systems with vacancies. Gap states in NiO:Vo are predominantly Ni 3d states, while those in NiO:VNi are composed of both Ni 3d and O 2p states. The absorption spectra of the NiO:VNi sample show significant defect-induced features below 3.0 eV compared to NiO and NiO:VO samples. The increase in sub-gap absorptions in…

Minimally invasive surgeries often require complicated maneuvers and delicate hand-eye coordination and ideally would incorporate "x-ray vision" to see beyond tool tips and underneath tissues prior to making incisions. Photoacoustic imaging has the potential to offer this feature but not with ionizing x-rays. Instead, optical fibers and acoustic receivers enable photoacoustic sensing of major structures-such as blood vessels and nerves-that are otherwise hidden from view. This imaging process is initiated by transmitting laser pulses that illuminate regions of interest, causing thermal expansion and the generation of sound waves that are detectable with conventional ultrasound transducers. The recorded signals are then converted to images through the beamforming process. Photoacoustic imaging may be implemented to both target and avoid blood-rich surgical contents (and in some cases simultaneously or independently visualize optical fiber tips or metallic surgical tool tips) in order to prevent accidental injury and assist device operators during minimally invasive surgeries and…

High entropy oxides (HEOs) are a recently introduced class of oxide materials, which are characterized by a large number of elements (i.e., five or more) sharing one lattice site, which crystallize in a single phase structure. One complex example of the rather young HEO family is the rare-earth transition metal perovskite high entropy oxides. In this comprehensive study, we provide an overview of the magnetic properties of three perovskite type high entropy oxides. The compounds have a rare-earth site that is occupied by five different rare-earth elements, while the transition metal site is occupied by a single transition metal. In this way, a comparison to the parent binary oxides, namely, the orthocobaltites, -chromites, and -ferrites, is possible. X-ray absorption near edge spectroscopy, magnetometry, and Mössbauer spectroscopy are employed to characterize these complex materials. In general, we find surprising similarities to the magnetic properties of the binary oxides despite the chemical disorder on the rare-earth site. However, distinct differences and interesting magnetic…

Self-heating in AlGaN/GaN high electron mobility transistors (HEMTs) negatively impacts device performance and reliability. Under nominal operating conditions, a hot-spot in the device channel develops under the drain side corner of the gate due to a concentration of volumetric heat generation leading to nonequilibrium carrier interactions and non-Fourier heat conduction. These subcontinuum effects obscure identification of the most salient processes impacting heating. In response, we examine self-heating in GaN-on-Si HEMTs via measurements of channel temperature using above-bandgap UV thermoreflectance imaging in combination with fully coupled electrothermal modeling. The methods together highlight the interplay of heat concentration and subcontinuum thermal transport showing that channel temperature cannot be determined solely by continuum scale heat transfer principles. Under conditions of equal power dissipation (PDISS = VDS × IDS = 250 mW), for example, a higher VDS bias (∼23 V) resulted in an ∼44% larger rise in peak junction temperature compared to that for a lower VDS (∼7.5…

This report discusses the design and demonstration of β-(Al0.17Ga0.83)2O3/Ga2O3 modulation doped heterostructures to achieve high sheet charge density. The use of a thin spacer layer between the Si delta-doping and the heterojunction interface was investigated in a β-(AlGa)2O3/Ga2O3 modulation doped structure. It is shown that this strategy enables a higher two-dimensional electron gas (2DEG) sheet charge density up to 4.7 × 1012 cm−2 with an effective mobility of 150 cm2/V s. The presence of a degenerate 2DEG channel was confirmed by the measurement of a low temperature effective mobility of 375 cm2/V s and the lack of carrier freeze out from low temperature capacitance voltage measurements. The electron density of 4.7 × 1012 cm−2 is the highest reported 2DEG density obtained without parallel conducting channels in a β-(AlxGa(1−x))2O3/Ga2O3 heterostructure system.

Liquid crystal elastomers (LCEs) are a class of stimuli-responsive polymers that undergo reversible shape-change in response to environmental changes. The shape change of LCEs can be programmed during processing by orienting the liquid crystal phase prior to crosslinking. The suite of processing techniques that has been developed has resulted in a myriad of LCEs with different shape-changing behavior and mechanical properties. Aligning LCEs via mechanical straining yields large uniaxial actuators capable of a moderate force output. Magnetic fields are utilized to control the alignment within LCE microstructures. The generation of out-of-plane deformations such as bending, twisting, and coning is enabled by surface alignment techniques within thin films. 4D printing processes have emerged that enable the fabrication of centimeter-scale, 3D LCE structures with a complex alignment. The processing technique also determines, to a large extent, the potential applications of the LCE. For example, 4D printing enables the fabrication of LCE actuators capable of replicating the forces…

Motivated by the recently observed topological Hall effect in ultra-thin films of SrRuO3 (SRO) grown on the SrTiO3 [001] substrate, we investigate the magnetic ground state and anomalous Hall response of the SRO ultra-thin films by virtue of spin density functional theory (DFT). Our findings reveal that in the monolayer limit of an SRO film, a large energy splitting of Ru-t2g states stabilizes an anti-ferromagnetic (AFM) insulating magnetic ground state. For the AFM ground state, our Berry curvature calculations predict a large anomalous Hall response upon doping. From the systematic symmetry analysis, we uncover that the large anomalous Hall effect arises due to a combination of broken time-reversal and crystal symmetries caused by the arrangement of non-magnetic atoms (Sr and O) in the SRO monolayer. We identify the emergent Hall effect as a clear manifestation of the so-called crystal Hall effect in terminology of Šmejkal et al., Crystal Hall effect in collinear antiferromagnets (2019), and demonstrate that it persists at finite frequencies, which is the manifestation of the…

Strain inevitably exists in practical GaN-based devices due to the mismatch of lattice structure and thermal expansion brought by heteroepitaxial growth and band engineering, and it significantly influences the thermal properties of GaN. In this work, thermal transport properties of GaN considering the effects from biaxial strain and electron-phonon coupling (EPC) are investigated using the first principles calculation and phonon Boltzmann transport equation. The thermal conductivity of free GaN is 263 and 257 W/mK for in-plane and cross-plane directions, respectively, which are consistent better with the experimental values in the literature than previous theoretical reports and show a nearly negligible anisotropy. Under the strain state, thermal conductivity changes remarkably. In detail, under +5% tensile strain state, average thermal conductivity at room temperature decreases by 63%, while it increases by 53% under the −5% compressive strain, which is mostly attributed to the changes in phonon relaxation time. Besides, the anisotropy of thermal conductivity changes under…

Reconfigurable plasmonics constitutes an exciting and challenging new horizon in nanophotonics. This blooming field aims at providing plasmonic nanostructures that present a dynamic and active plasmonic response that can be switched or manipulated by external stimuli to induce a controllable change in the optical properties. Most common plasmonic materials, such as the noble metals gold and silver, cannot deliver this type of behavior. Therefore, significant effort is being invested in developing alternative materials whose optical properties can be controllably modified to provide a reconfigurable plasmonic response. In this perspective, several materials including non-noble metals, transition metal oxides and nitrides, and chalcogenide compounds will be analyzed. The selected materials share interesting characteristics like low cost, good chemical and thermal stabilities, and CMOS compatibility while presenting a reconfigurable plasmonic response governed by different phase-change mechanisms.

The formation of periodic patterns on the surfaces of many solid materials undergoing ion-beam irradiation has long been known. The advent of high resolution characterization techniques elucidated the nanoscopic traits of this self-organization process, enabling a wide range of applications for the nanostructures thus produced, from optoelectronic to biomedical. Meanwhile, full theoretical understanding of the technique has been challenged by its multiscale nature, whereby the external perturbation implemented by the ion beam acts at a much slower rate (typically, one ion arrives per square-nm every second) than the microscopic processes, like collision cascades and material transport, which try to relax such external perturbations (collision cascades or surface diffusion attempts usually relax after a few picoseconds). Here, we present a Perspective on the main developments that have led to the current understanding of nanoscale pattern formation at surfaces by ion-beam irradiation, from the points of view of experiments, applications, and theory, and offer an outlook on future…

Nonlinear photon conversion is a fundamental physical process that lies on the basis of many modern disciplines, from bioimaging and theranostics in nanomedicine to material characterization in materials science and nanotechnology. It also holds great promise in laser physics with applications in information technology for optical signal processing and in the development of novel coherent light sources. The capability to efficiently generate harmonics at the nanoscale will have an enormous impact on all these fields, since it would allow one to realize much more compact devices and to interrogate matter in extremely confined volumes. Here, we present a perspective on the most recent advances in the generation of nonlinear optical processes at the nanoscale and their applications, proposing a palette of future perspectives that range from material characterization and the development of novel compact platforms for efficient photon conversion to bioimaging and sensing.

In dielectric materials, due to the flexoelectric effect, a large inhomogeneous deformation can break or significantly alter the inversion symmetry of the material and induce net polarization. Such a phenomenon is distinctive comparing to other forms of electromechanical coupling, such as piezoelectricity and electrostriction, in that it is an intrinsic and universal effect whose strength scales up with the decrease in the sample size. Research works in the past decade indicate that the emergence of flexoelectricity not only provides a new way of realizing electromechanical coupling, but also has impact on multiple fields including smart materials, devices, and even physics. Our objective of this perspective is to briefly introduce the field of flexoelectricity and discuss on its future development. Also, current challenges and ambiguities that may impede the further development of the field are pointed out. Furthermore, our opinions on why flexoelectricity matters and how can we use it in novel ways are presented.

While artificial intelligence, capable of readily addressing cognitive tasks, has transformed technologies and daily lives, there remains a huge gap with biological systems in terms of performance per energy unit. Neuromorphic computing, in which hardware with alternative architectures, circuits, devices, and/or materials is explored, is expected to reduce the gap. Antiferromagnetic spintronics could offer a promising platform for this scheme. Active functionalities of antiferromagnetic systems have been demonstrated recently and several works indicated their potential for biologically inspired computing. In this perspective, we look through the prism of these works and discuss prospects and challenges of antiferromagnetic spintronics for neuromorphic computing. Overview and discussion are given on non-spiking artificial neural networks, spiking neural networks, and reservoir computing.

Superhard materials are of great interest in various practical applications, and an increasing number of research efforts are focused on their development. In this article, we demonstrate that machine learning can be successfully applied to searching for such materials. We construct a machine learning model using neural networks on graphs together with a recently developed physical model of hardness and fracture toughness. The model is trained using available elastic data from the Materials Project database and has good accuracy for predictions. We use this model to screen all crystal structures in the database and systematize all the promising hard or superhard materials, and find that diamond (and its polytypes) are the hardest materials in the database. Our results can be further used for the investigation of interesting materials using more accurate ab initio calculations and/or experiments.

In this paper, we present a novel design of broadband metamaterial microwave absorber (MWA) based on asymmetric sectional resonator structures (SRSs), which has been verified by simulation, calculation based on circuit theory, and experiment. The broadband perfect absorption of the MWA can be obtained by assembling four asymmetric SRSs with different sizes. The numerical results indicate that the bandwidth of the broadband MWA based on asymmetric SRSs is about 16.2 times of the ones based on single SRS or four complete symmetric SRSs. The broadband perfect absorption of the proposed MWA mainly originates from the excitation of the fundamental resonance mode of the SRS. The numerical results also demonstrate that the broadband MWA exhibits an angle-insensitivity microwave absorption capacity, which is manifested as a high absorption level at large incident angles for both transverse electric and transverse magnetic modes. Thus, the proposed broadband MWA turns out to be suitable for a wide variety of potential applications in stealth technology and energy harvesting.

The field of magnonics, which utilizes propagating spin waves for nanoscale transmission and processing of information, has been significantly advanced by the advent of the spin–orbit torque. The latter phenomenon allows one to overcome two main drawbacks of magnonic devices—low energy efficiency of the conversion of electrical signals into spin-wave signals and fast spatial decay of spin waves in thin-film waveguiding structures. At first glance, the excitation and amplification of spin waves by spin–orbit torques seem to be straightforward. Recent research indicates, however, that the lack of the mode selectivity in the interaction of spin currents with dynamic magnetic modes and the onset of dynamic nonlinear phenomena represent significant obstacles. Here, we discuss the possible route to overcoming these limitations, based on the suppression of nonlinear spin-wave interactions in magnetic systems with perpendicular magnetic anisotropy. We show that this approach enables efficient excitation of coherent magnetization dynamics and propagating spin waves in extended spatial…

Janus nanoparticles (JNPs) have two faces with differing physical or chemical characteristics which endow them with unique properties. This anisotropy can lead to phenomena like self-assembly or interfacial activity, and JNPs have been under intense scientific scrutiny since the 1990s. Since then, powerful synthetic techniques have been developed which allow for JNPs with near limitless combinations of materials, properties, and morphologies. As such, JNPs have been featured in a number of applications such as drug delivery, catalysis, interfacial engineering, and optics. This perspective discusses the synthesis of these important anisotropic materials. We will focus exclusively on JNPs that involve polymers. In particular, we outline the various classes of JNP synthetic techniques including some historical context, describe recent developments in this area, and weigh the pros and cons of the different methods with an eye on future directions and applications. We hope this perspective will help acclimate those new to the field who want to make and/or use JNPs.

Phononic crystals (PnCs) have been utilized to amplify the amount of input energy transferred to a piezoelectric energy harvesting (PEH) device by manipulating elastic wave propagation. When introducing a defect that has material properties and/or geometry different from a unit cell, mechanical resonance of the defect leads to localizing elastic waves inside the defect. This is called a defect mode. Several prior studies have explored a defect mode for PEH purpose; however, they have focused only on a single defect. When introducing an additional defect into a PnC, the coupling between two defects leads to splitting the defect band. Incorporating such split defect band phenomena into PEH can potentially widen frequency bandwidth and realize broadband energy harvesting. Thus, this study newly proposes a PnC-based PEH system that uses double defect modes under elastic waves. In particular, this study examines how an electrical circuit connection (i.e., Independent, Series, or Parallel) between two PEH devices attached on each defect affects PEH performances. Key findings from this…

Vanadium oxides (VxOy) are classic “smart functional materials” used in a wide array of thermochromic, electronic, and catalytic applications. Specifically, vanadium dioxide (VO2) class nanomaterials are of enormous interest due to their unique first order reversible metal-insulator phase transition (MIT) behavior accompanied by a structural phase transition, inducing dramatic changes in electrical and optical properties with large lattice deformation. To date, a plethora of reports exemplifying the MIT characteristics of VO2, synthetic methods of VO2, and modulating VO2 phase transition temperatures (Tc) have been published. In this Tutorial Review, we present an overview on the fundamentals of the VO2 band structure and principles of MIT and outline various reported synthetic approaches for VO2 thin films, including dimensionally oriented VO2 nanostructures. Discussion on recent trends in VO2 applications, challenges in VO2 synthesis, and future perspectives are also elaborated in detail.

Two-dimensional (2D) hexagonal boron nitride (h-BN) is one of the most promising materials for many technological applications ranging from optics to electronics. In past years, a property-tunable strategy that involves the construction of electronic structures of h-BN through an atomic-level design of point defects has been in vogue. The point defects imported during material synthesis or functionalization by defect engineering can endow h-BN with new physical characteristics and applications. In this Perspective, we survey the current state of the art in multifunction variations induced by point defects for 2D h-BN. We begin with an introduction of the band structure and electronic property of the pristine h-BN. Subsequently, the formation and characterization of the most obvious point defects and their modulation in electronic structures of h-BN nanomaterials are envisaged in theory. The experimental results obtained by atom-resolved transmission electron microscopy, magnetic measurement, and optical measurements have provided insights into the point defect engineered structures…

Antiferromagnetic materials have recently gained renewed interest due to their possible use in spintronics technologies, where spin transport is the foundation of their functionalities. In that respect, metallic antiferromagnets are of particular interest since they enable complex interplays between electronic charge transport, spin, optical, and magnetization dynamics. Here, we review the phenomena where the metallic conductivity provides a unique perspective for the practical use and fundamental properties of antiferromagnetic materials. The future direction is outlined with respect to the current advances of the field.

As the challenges in continued scaling of the integrated circuit technology escalate every generation, there is an urgent need to find viable solutions for both the front-end-of-line (transistors) and the back-end-of-line (interconnects). For the interconnect technology, it is crucial to replace the conventional barrier and liner with much thinner alternatives so that the current driving capability of the interconnects can be maintained or even improved. Due to the inherent atomically thin body thicknesses, 2D materials have recently been proposed and explored as Cu diffusion barrier alternatives. In this Perspective article, a variety of 2D materials that have been studied, ranging from graphene, h-BN, MoS2, WSe2 to TaS2, will be reviewed. Their potentials will be evaluated based on several criteria, including fundamental material properties as well as the feasibility for technology integration. Using TaS2 as an example, we demonstrate a large set of promising properties and point out that there remain challenges in the integration aspects with a few possible solutions waiting for…

AlScN/GaN heterostructures are worth investigating due to the remarkable high gradients in spontaneous polarization at their interfaces, which brings them into play for a wide field of potential high-power and high-frequency electronic applications. In this work, AlScN/GaN heterostructures for high electron mobility transistor (HEMT) structures were grown by metalorganic chemical vapor deposition. We have investigated the impact of growth parameters on thick AlScN layers and on thin AlScN/GaN heterostructures. Growth parameters, such as temperature, V/III ratio, pressure, and growth mode, were varied with the focus on surface morphology, crystal quality, and incorporation of impurities. High growth temperatures improve the surface quality and reduce impurities incorporation notably. In addition to that, a slight decrease in carbon concentration is obtained by adopting a pulsed supply of metalorganic precursors. V/III ratio and pressure did not influence the layer quality observably. Heterostructures with root mean square surface roughness values as low as 0.38 nm, revealing smooth…

The full InGaN structure is used to achieve red light emitting diodes (LEDs). This LED structure is composed of a partly relaxed InGaN pseudo-substrate fabricated by Soitec, namely, InGaNOS, a n-doped buffer layer formed by a set of InxGa1−xN/GaN superlattices, thin InyGa1−yN/InxGa1−xN multiple quantum wells, and a p doped InxGa1−xN area. p-doped InGaN layers are first studied to determine the optimal Mg concentration. In the case of an In content of 2%, an acceptor concentration of 1 × 1019/cm3 was measured for a Mg concentration of 2 × 1019/cm3. Red electroluminescence was then demonstrated for two generations of LEDs, including chip sizes of 300 × 300 μm2 and 50 × 50 μm2. The differences between these two LED generations are detailed. For both devices, red emission with a peak wavelength at 620 nm was observed for a pumping current density of 12 A/cm2. Red light-emission is maintained over the entire tested current range. From the first to the second LED generation, the maximum external quantum efficiency, obtained in the range of 17–40 A/cm2, was increased by almost one order of…

The Z-pinch configuration offers the promise of a compact fusion device owing to its simple geometry, unity beta, and absence of external magnetic field coils. Increasing the axial current compresses the plasma, resulting in a rapid rise of the fusion reaction rate. Historically, the Z pinch has been plagued by fast growing instabilities that limit plasma lifetimes. Recent progress has resulted from investigating approaches that provide stability. One approach exploits sheared-flow stabilization to produce an equilibrium Z pinch, which sustains the compressed plasma state for durations much longer than other plasma timescales. Recent experimental and simulation results encourage pursuit of the stabilized Z pinch to explore its fusion performance limits.

Increased attention has been paid to the thermal management of β-Ga2O3 devices as a result of the large thermal resistance that can present itself in part due to its low intrinsic thermal conductivity. A number of die-level thermal management approaches exist that could be viable for thermal management. However, they have not been assessed for β-Ga2O3 devices exclusively. Here, we explore the limits of various die level thermal management schemes on a β-Ga2O3 metal–semiconductor field-effect transistor using numerical simulations. The effects of the various cooling approaches on the device channel temperature were comprehensively investigated, along with guidance for material selection to enable the most effective thermal solutions. Among various cooling strategies, double side cooling combined with a heat spreader used in the active region of the device can suppress the device thermal resistance to as low as 11 mm °C/W, achieving a maximum dissipated power density as high as 16 W/mm for a junction temperature limit of 200 °C. A multi-finger transistor thermal model was also…

This study demonstrates the application of artificial neural networks (ANNs) and Bayesian support vector regression (BSVR) models for predicting the relative viscosity of nanofluids. The study examined 19 nanofluids comprising 1425 experimental datasets that were randomly split in a ratio of 70:30 as a training dataset and a testing dataset, respectively. To establish the inputs that will yield the best model prediction, we conducted a systematic analysis of the influence of volume fraction of nanoparticles, the density of nanoparticles, fluid temperature, size of nanoparticles, and viscosity of base fluids on the relative viscosity of the nanofluids. Also, we analyzed the results of all possible input combinations by developing 31 support vector regression models based on all possible input combinations. The results revealed that the exclusion of the viscosity of the base fluids (as a model input) leads to a significant improvement in the model result. To further validate our findings, we used the four inputs—volume fraction of nanoparticles, the density of nanoparticles, fluid…

This Tutorial provides an overview of the techniques that are most commonly utilized to grow bulk van der Waals crystals. The materials discussed were selected to highlight various challenges that are often encountered during crystal growth. In relatively equal parts, the text covers melt-based techniques, vapor transport growths, and the characterization of crystal quality with an emphasis on structural and chemical homogeneities. Pertinent details are given regarding the growth and characterization of many specific compounds, with examples mostly drawn from our own research, and an effort is made to highlight cases where the growths offer a particular lesson or the conditions have a significant impact on the crystal’s physical properties. A primary goal is to motivate more researchers to grow crystals by providing general descriptions and considerations for different growth techniques and equipment while sharing some of our own lessons learned and best practices for the growth and characterization of layered van der Waals crystals. The Tutorial is not written solely for aspiring…

Compared with the state-of-the-art Pb(Zr,Ti)O3 ceramics, relaxor-PbTiO3 (PT) based ferroelectric single crystals possess much higher piezoelectricity, thus receiving considerable attention from the ferroelectric community. Recently, it was found that the alternating current electric field poling (AC-poling) could further improve the piezoelectricity of [001]-poled rhombohedral (R) relaxor-PT single crystals. In this work, we investigated the domain structures and electromechanical properties of alternating current electric field poled (AC-poled) relaxor ferroelectric Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT) crystals with different orientations and phase structures, including [001]-oriented R PIN–PMN–PT (23/47/30), [011]-oriented R PIN–PMN–PT (23/47/30), and [001]-oriented monoclinic (MC) PIN–PMN–PT (22/44/34) single crystals. We found that the piezoelectric coefficient (d33), free dielectric permittivity (ɛ33T/ɛ0), and clamped dielectric permittivity (ɛ33S/ɛ0) for AC-poled [001]-oriented R PIN–PMN–PT (23/47/30) crystals were improved by 24.4%, 22.6%, and 8.5%,…

The shock to detonation transition in heterogeneous high energy density solids starts with the spatial localization of mechanical energy into so-called hotspots that form due to the interaction between the leading wave and microstructural features and defects. We used large-scale molecular dynamics to characterize the hotspots resulting from the shock-induced collapse of cylindrical voids and elongated cracks focusing on the effect of shock strength, defect shape, and size. The temperature fields resulting from the collapse of cracks elongated along the shock direction show significantly higher sensitivity to both shock strength and size than cylindrical voids. Cracks 80 nm in length result in temperatures almost three times higher than voids 80 nm in diameter, reaching values corresponding to the ideal case of isentropic recompression of a gas. The molecular dynamics trajectories reveal the atomic origin of this contrasting behavior. While circular voids undergo a transition from viscoelastic pore collapse to a hydrodynamic regime with increasing shock strength, shock focusing in…

Infrared photothermal heterodyne imaging (IR-PHI) is an ultrasensitive technique for achieving super-resolution, infrared imaging throughout the mid-infrared “fingerprint” spectral region (2.5–10μm). An achieved spatial resolution of ∼300nm is up to 30-fold smaller than the infrared diffraction limit. In this study, we establish IR-PHI’s signal contrast mechanism and benchmark the technique’s capabilities using the characteristic infrared vibrational transitions of individual polystyrene (PS) and poly(methyl methacrylate) (PMMA) beads. The analysis of acquired results reveals that IR-PHI contrast originates from a competing, photothermally induced specimen size and refractive index changes to backscattering cross sections. For PS and PMMA, thermo-optic refractive index contributions dominate and are responsible for medium-free IR-PHI signal contrast. Our analysis simultaneously establishes IR-PHI’s current, medium-free peak absorption cross-sectional limit-of-detection to be σabs=3×10−18m2. This improved understanding of IR-PHI signal contrast provides a framework for future…

All semiconductors, whether by design or by accident, contain defects.The fundamental properties of defects such as impurities, native defects, and extended defects, affect a broad range of applications.These technologically important defects may be introduced during growth and processing.Electrical, optical, and magnetic phenomena related to defects have been observed experimentally and studied theoretically in a variety of materials.In addition to traditional semiconductors (elemental, III-V, and II-VI), emerging materials include organic semiconductors, nitrides, oxides, and topological insulators, in bulk crystal and thin-film forms as well as low-dimensional and nanoscale structures.Recent developments have highlighted the importance of understanding defects at a fundamental level.The efficiency of light-emitting diodes and solar cells, in particular those using novel materials, can be enhanced by controlling defect populations.Dopant spins and optically active centers are potential building blocks for quantum computers.III-nitrides continue to get a lot of attention, while…

We study the interaction between mechanical fields and the motion of charge carriers in a composite beam of flexoelectric (nonpiezoelectric) and semiconductor layers. The macroscopic theory of flexoelectricity and the drift-diffusion theory of semiconduction are used. They are coupled by doping and mobile charges in Gauss's equation of electrostatics. A one-dimensional electromechanical model is established for the bending of the beam. Solutions from the model show that bending of the beam induces redistribution of charge carriers through the electric field produced by flexoelectric coupling. This provides a new means for constructing electromechanical semiconductor devices and extends piezotronic devices.

Understanding heat transport in semiconductors and insulators is of fundamental importance because of its technological impact in electronics and renewable energy harvesting and conversion. Anharmonic lattice dynamics provides a powerful framework for the description of heat transport at the nanoscale. One of the advantages of this method is that it naturally includes quantum effects due to atoms vibrations, which are needed to compute the thermal properties of semiconductors widely used in nanotechnology, like silicon and carbon, even at room temperature. While the heat transport picture substantially differs between amorphous and crystalline semiconductors from a microscopic standpoint, a unified approach to simulate both crystals and glasses has been devised. Here, we introduce a unified workflow, which implements both the Boltzmann Transport equation and the quasi-harmonic Green-Kubo methods. We discuss how the theory can be optimized to exploit modern parallel architectures, and how it is implemented in κALDo: a versatile and scalable open-source software to compute phonon…

A subwavelength seismic metamaterial (SM) consisting of a three-component SM plate (SMP) and a half space is proposed to attenuate ultra-low frequency seismic surface waves. The design concept and models are verified first by lab-scale experiments on the SM consisting of a two-component SMP and a half space. Then, we calculate the band structures of one-dimensional and two-dimensional subwavelength SMs and evaluate their ability to attenuate Rayleigh waves. A wide ultra-low frequency bandgap can be found, and the Rayleigh waves are deflected by the subwavelength SM and converted into bulk waves in the frequency range of this bandgap. When the number of unit cells of the subwavelength SM is sufficient, the transmission distance and deflection angle of the Rayleigh waves are constant at the same frequency. This discovery is expected to open up the possibility of pragmatic seismic protection for large nuclear power plants, ancient buildings, and metropolitan areas.

Recent advances in optical microscopy instrumentation and processing techniques have led to imaging that both breaks the diffraction barrier and enables sub-pixel resolution. This enhanced resolution has expanded the capabilities of particle tracking to nanoscale processes in soft matter including biomolecular, colloidal, and polymeric materials. This tutorial provides a basic understanding of particle tracking instrumentation, the fundamentals of tracking analysis, and potential sources of error and bias inherent in analyzing particle tracking. Finally, we provide a brief outlook for the future of particle tracking through the lens of machine learning.

Ferroelectric materials are known to be ideal materials for nonvolatile memory devices, owing to their two electrically switchable spontaneous polarization states. However, difficulties in scaling down devices with ferroelectric materials have hindered their practical applications and research. The discovery of ferroelectricity in fluorite-structured ferroelectrics has revived research on semiconductor devices based on ferroelectrics. With their scalability and established fabrication techniques, the performance of nanoscale electronic devices with fluorite-structured ferroelectrics is being rapidly developed. However, the fundamental physics behind the superior ferroelectricity is yet to be elucidated. From this Perspective, the status of research on fluorite-structured ferroelectrics and state-of-the-art semiconductor devices based on them are comprehensively reviewed. In particular, the fundamental physics of fluorite-structured oxides is critically reviewed based on a newly developed theory as well as on the classical theory on ferroelectrics. A perspective on the establishment…

The study of high-entropy (HE) alloys has seen dramatic growth in recent years as, in some cases, these systems can exhibit exceptional properties, including enhanced oxidation resistance, superior mechanical properties, and desirable magnetic properties. The identification of promising HE alloys is, however, extremely challenging due to the extraordinarily large number of distinct systems that may be fabricated from the available palette of elements. For this reason, machine learning strategies have been employed to reduce the size of the associated chemistry/composition space. In this review, we outline several computational strategies that have led to the identification of useful alloys and discuss the relative merits and shortcomings of these approaches. We also present short tutorials illustrating the use of selected computational approaches to HE characterization and design.

Spin-orbit torques offer a promising mechanism for electrically controlling magnetization dynamics in nanoscale heterostructures. While spin-orbit torques occur predominately at interfaces, the physical mechanisms underlying these torques can originate in both the bulk layers and at interfaces. Classifying spin-orbit torques based on the region that they originate in provides clues as to how to optimize the effect. While most bulk spin-orbit torque contributions are well studied, many of the interfacial contributions allowed by symmetry have yet to be fully explored theoretically and experimentally. To facilitate progress, we review interfacial spin-orbit torques from a semiclassical viewpoint and relate these contributions to recent experimental results. Within the same model, we show the relationship between different interface transport parameters. For charges and spins flowing perpendicular to the interface, interfacial spin-orbit coupling both modifies the mixing conductance of magnetoelectronic circuit theory and gives rise to spin memory loss. For in-plane electric fields,…

Multiple particle tracking microrheology (MPT) is a passive microrheological technique that measures the Brownian motion of probe particles embedded in a sample to characterize material rheological properties. MPT is a powerful tool that quantifies material rheology in the low moduli range while requiring only small sample volumes and relatively simple data acquisition using video microscopy. MPT quantitatively characterizes spatiotemporal rheological properties and is particularly well suited for the investigation of evolving materials with complex microenvironments. MPT has expanded the study of a variety of materials including biofilms, colloidal gels, hydrogels, stimuli-responsive materials, and cell-laden biomaterials. The aim of this Tutorial is to summarize the fundamentals, illustrate the versatility, and highlight recent advances in MPT. In each application, we will highlight how MPT is uniquely positioned to gather rheological properties, which would be difficult, if not impossible, to attain with other rheological characterization techniques and highlight how MPT can be…

Acoustic metamaterials (AMs) have attracted a substantial amount of attention in recent decades where the parameter design plays an important role. However, conventional design methods generally rely on analytical physical models and require a very large number of evaluations of acoustic performance. Here, we propose and experimentally demonstrate an inverse-design method for AMs based on machine learning using a Gauss–Bayesian model. As a result of the cycle of training and prediction and the use of adaptive acquisition functions, this method allows the parameters of AMs to be efficiently designed for specific functionalities without the need for physical models. Considering the significance of low-frequency ventilated sound absorption, we present a design for a typical acoustic metamaterial absorber with multiple structural parameters that facilitate high sound absorption at low frequencies. In the design process, the parameters were adaptively adjusted to improve the sound absorption performance at low frequencies using only 37 evaluations, and this high absorption performance…

This tutorial paper focuses on the physical origin of thermal droop, i.e., the decrease in the luminescence of light-emitting diodes (LEDs) induced by increasing temperature. III-nitride-based LEDs are becoming a pervasive technology, covering several fields from lighting to displays, from automotive to portable electronics, and from horticulture to sensing. In all these environments, high efficiency is a fundamental requirement, for reducing power consumption and system cost. Over the last decade, a great deal of effort has been put in the analysis of the efficiency droop, the decrease in LED internal quantum efficiency (IQE) induced by high current density. On the other hand, an IQE decrease is observed also for increasing temperature, a phenomenon usually referred to as thermal droop. For commercial LEDs, the IQE decrease related to thermal droop can be comparable to that of efficiency droop: for this reason, understanding thermal droop is a fundamental step for making LEDs capable of operating at high temperature levels. In several fields (including street lighting, automotive,…

Surface passivation of organic–inorganic halide perovskites (OIHPs) is a crucial step to annihilate the surface defects and to control the deteriorated ion migration phenomenon. Here, we study the role of lead sulfate (PbSO4) as an effective passivator in OIHP single crystals (SCs). Using impedance spectroscopy, we evaluate the ion migration and electrical properties of lead sulfate-passivated methylammonium lead tri-bromide (MAPbBr3) SCs. We found that the low-frequency impedance response that is assigned to the ionic motion in the MAPbBr3 SC is strongly affected by the inorganic PbSO4 surface treatment. The activation energy corresponding to the ion migration of MAPbBr3 SC is increased from 0.28 to 0.36 eV after PbSO4 surface treatment. The temperature-dependent I–V hysteresis of the MAPbBr3 SCs upon PbSO4 passivation was also measured. We found that such PbSO4 surface treatment stabilizes the crystal surface and improves the hysteresis properties of the crystals at elevated temperatures.

This tutorial introduces systematically the foundational concepts undergirding the recently formulated AI (artificial intelligence)-based materials knowledge system (AI-MKS) framework. More specifically, these concepts deal with features engineering the heterogeneous material internal structure to obtain low-dimensional representations that can then be combined with machine learning models to establish low-computational cost surrogate models for capturing the process–structure–property linkages over a hierarchy of material structure/lengths scales. Generally referred to as materials knowledge systems (MKS), this framework synergistically leverages the emergent AI/ML (machine learning) toolsets in conjunction with the modern experimental and physics-based simulation toolsets employed currently by the domain experts in the materials field. The primary goal of this tutorial is to present to the domain expert the foundations needed to understand and take advantage of the impending opportunities arising from a synergistic integration of AI/ML tools into the current materials innovation…

This Perspective outlines theoretical, simulation, and experimental studies linking electron emission and gas breakdown. Many studies have investigated field emission-driven microscale gas breakdown, including recent reviews [Garner et al., IEEE Trans. Plasma Sci. 48, 808–824 (2020); Fu et al., Plasma Res. Express 2, 013001 (2020)]. This Perspective focuses on generalizing field emission-driven microscale gas breakdown to consider the contribution of other forms of electron emission, specifically thermionic and space-charge limited. Recent theoretical studies have unified thermionic, field, and space-charge limited emission with and without collisions to derive “nexuses” where the individual solutions match, indicating transitions in the mechanisms. Reducing device size to nanoscale at atmospheric pressure leads to a transition from field emission to space-charge limited emission for nitrogen at ∼250 nm. This Perspective summarizes the derivation of these nexuses and future extensions. We next describe simulation and theoretical studies for field emission-driven microscale gas…

Transparent conducting oxides (TCOs), combining the mutually exclusive functionalities of high electrical conductivity and high optical transparency, lie at the center of a wide range of technological applications. The current design strategy for n-type TCOs, making wide bandgap oxides conducting through degenerately doping, obtains successful achievements. However, the performances of p-type TCOs lag far behind the n-type counterparts, primarily owing to the localized nature of the O 2p-derived valence band (VB). Modulation of the VB to reduce the localization is a key issue to explore p-type TCOs. This Perspective provides a brief overview of recent progress in the field of design strategy for p-type TCOs. First, the introduction to principle physics of TCOs is presented. Second, the design strategy for n-type TCOs is introduced. Then, the design strategy based on the concept of chemical modulation of the valence band for p-type TCOs is described. Finally, through the introduction of electron correlation in strongly correlated oxides for exploring p-type TCOs, the performance of…

The field of atomistic simulations of multicomponent materials and high entropy alloys is progressing rapidly, with challenging problems stimulating new creative solutions. In this Perspective, we present three topics that emerged very recently and that we anticipate will determine the future direction of research of high entropy alloys: the usage of machine-learning potentials for very accurate thermodynamics, the exploration of short-range order and its impact on macroscopic properties, and the more extensive exploitation of interstitial alloying and high entropy alloy surfaces for new technological applications. For each of these topics, we briefly summarize the key achievements, point out the aspects that still need to be addressed, and discuss possible future improvements and promising directions.

Up until recently, thin film growth by magnetron sputtering relied on enhancing adatom mobility in the surface region by gas-ion irradiation to obtain dense layers at low deposition temperatures. However, an inherently low degree of ionization in the sputtered material flux during direct-current magnetron sputtering (DCMS), owing to relatively low plasma densities involved, prevented systematic exploration of the effects of metal-ion irradiation on the film nanostructure, phase content, and physical properties. Employing only gas-ion bombardment results in an inefficient energy and momentum transfer to the growing film surface. Also, for enhanced substrate biasing, the higher concentration of implanted noble gas atoms at interstitial lattice positions causes elevated compressive stress levels. High-power impulse magnetron sputtering (HiPIMS), however, provides controllable metal-ion ionization and, more importantly, enables the minimization of adverse gas-ion irradiation effects. The latter can be realized by the use of pulsed substrate bias applied synchronously with the…

One-dimensional piezoelectric semiconductor (PSC) nanowires have been widely used in smart structures and devices; however, few theoretical studies on their nano-size effects have been carried out. In this paper, a theoretical analysis of one-dimensional n-type PSC nanowires under an axial force was carried out, with consideration of the flexoelectric and strain gradient effects. Exact solutions were obtained based on the differential operator theory. The flexoelectric and strain gradient effects on the internal electromechanical field and carrier concentration have been discussed. It can be shown that the flexoelectric effect weakens the piezoelectricity of a PSC nanowire while the strain gradient has the opposite effect, with an amplitude enhancement that is dependent on the flexoelectric and inner scale coefficients.

A comprehensive understanding of the oxidation of Cu thin films in the low-temperature regime is of fundamental interest and particularly relevant for applications in the fields of micro- and nanoelectronics, sensors, catalysis, and solar cells. The current study reports on the oxidation kinetics of PVD grown Cu thin films (20–150 nm thick) and the oxide phase evolution from Cu2O to CuO upon thermal oxidation in the temperature range of 100–450 °C. XRD investigations in the laboratory and at the synchrotron show that the oxide phase formation critically depends on the oxidation conditions such as temperature and oxygen partial pressure. The real-time synchrotron XRD measurements reveal that the formation of the CuO phase only starts after complete oxidation of the Cu films to Cu2O films. In situ resistance measurements were performed to follow the oxide growth rate of Cu2O on Cu films in the temperature range of 100–300 °C in air and in 10 mbar pO2. It is found that the oxidation kinetics of Cu films to Cu2O films follows the linear rate law, which is attributed to surface reaction…

In this paper, the strain gradient is introduced to tune the semiconducting performance of piezoelectric semiconductor (PSC) nanowires by changing their cross sections. A one-dimensional model of the PSC nanowire with a non-homogeneous cross section under axial extension is established. The combined influences of piezoelectricity and flexoelectricity resulting from the strain gradient are taken into account. Approximate closed-form solutions for the distribution of carriers and electromechanical fields of the PSC nanowire are given. The effects of strain gradient, flexoelectricity, and initial carrier concentration on the distribution of carriers, electric field, electric potential, and displacement are discussed. It is observed that non-homogeneous PSC nanowires show strong size-dependent behaviors in connection with their cross-sectional diameters. The strain gradient and flexoelectric effect enhance the electromechanical coupling effect. This research provides a new way to tune inner carrier distributions and electromechanical characteristics for piezoelectric semiconductor…

The past 30 years have seen spectacular progress in the development of techniques for measuring the complete temporal field, and even the complete spatiotemporal field, of ultrashort laser pulses. The challenge has been to measure a pulse without the use of a shorter event or an independent known reference pulse, neither of which is typically available. We begin with autocorrelation, the first such “self-referenced” pulse-measurement method ever proposed, which measures only a rough pulse length, and we describe its limitations. One such limitation is the presence of a somewhat unintuitive “coherent artifact,” which occurs for complicated pulses and also when averaging over a train of pulses whose shapes vary from pulse to pulse. We then describe the most important modern techniques capable of measuring the complete temporal intensity and phase of even complicated ultrashort pulses, as well as their ability (or inability) to measure such unstable pulse trains. A pulse reliably measured with such a device can then be used as a reference pulse in conjunction with another technique,…

Magnetotactic bacteria (MTB) are aquatic microorganisms that are able to biomineralize membrane-enclosed magnetic nanoparticles called magnetosomes. Inside the MTB, magnetosomes are arranged in a chain that allows MTB to align and navigate along the Earth's magnetic field. When isolated from the MTB, magnetosomes display a number of potential applications for targeted cancer therapies, such as magnetic hyperthermia, localized drug delivery, or tumor monitoring. The characteristics and properties of magnetosomes for these applications exceed in several aspects those of synthetic magnetic nanoparticles. Likewise, the whole MTB can also be considered as promising agents for cancer treatment, taking advantage of their self-propulsion capability provided by their flagella and the guidance capabilities ensured by their magnetosome chain. Indeed, MTB are envisaged as nanobiots that can be guided and manipulated by external magnetic fields and are naturally attracted toward hypoxic areas, such as the tumor regions, while retaining the therapeutic and imaging capacities of the isolated…

Propylammonium lead iodide (C3H7NH3PbI3), a promising hybrid perovskite, is successfully synthesized by a solgel technique. Structural, optical, and dielectric properties have been studied in detail. The dielectric constant, loss factor, electric modulus, and AC and DC conductivity of this hybrid perovskite exhibit strong temperature dependence over the frequency range of 10 Hz ≤ f ≤ 8 MHz. The Nyquist plot reveals the distinct contributions of grain and grain boundary to the total impedance. The dielectric constant is found to increase with temperature in the high frequency region. The modified Cole–Cole plot shows that the space charge and free charge conductivity increase with the elevation of temperature, whereas the relaxation time decreases with the rise in temperature. From the modified Kohlrausch–Williams–Watts equation, we perceived asymmetrical nature in electric modulus spectra at various temperatures, which corresponds to the non-Debye type nature of perovskite. It has also been found that, with the elevation of temperature, the imaginary part of electric modulus spectra…

Impedance and electric modulus spectroscopy is exploited over a broad frequency and temperature range to find the relaxation phenomenon in LaFeO3 (LFO), which otherwise was concealed by the dc conductivity in dielectric ɛ*(ω) representation. The impedance measurements and the ac resistivity determined from Z′(ω) indicate that LFO is an insulator at room temperature and divulges the negative temperature coefficient of resistance. At higher temperatures, capacitive behavior flips to inductive behavior. The ac resistivity is exploited to determine the activation energy using the Arrhenius model. The relaxation peaks appear in the imaginary parts of electric modulus [M*(ω)] and impedance [Z(ω)], which have been exploited to determine the activation energy. The single distorted semicircle in the Nyquist and complex plots of electric modulus is evidence of the contribution of grains in the conduction process. At higher temperatures, data corresponding to the grain interior transform from an arc to a line with an intercept on the Z′(ω) axis and is parallel to the imaginary axis Z″(ω).…

Multicatalytic activities (photocatalysis, piezocatalysis, and pyrocatalysis) of ferroelectric ceramics Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZTO) were studied. Maximum degradations (89% and 81%) were achieved in piezo-photocatalytic experiments for degradation of Rhodamine B (RB) and ciprofloxacin. Similarly, 95% degradation of RB was achieved during pyrocatalysis in 250 heating/cooling cycles. Antibacterial performance of ceramics was analyzed with the help of Gram-positive and Gram-negative bacteria-killing processes. The bacterial colony formation drops to zero in 90 min with poled samples of BCZTO ceramics. The poled samples performed much better than that of the unpoled samples in all the catalytic reactions as well as in the bacterial killing process.

Thermal rectification is a nonreciprocal thermal transport phenomenon, which typically takes place in asymmetric structures or hetero-junctions. In this work, we propose a new route to achieve remarkable thermal rectification even in pristine single-layer graphene without asymmetry by engineering the thermal contacts at the two ends. When setting a fixed long thermal contact at one end and varying the length of thermal contact at the other end, our molecular dynamics simulations demonstrate that notable thermal rectification efficiency can be achieved with very short thermal contact, which vanishes in the limit of long thermal contact. Such a strategy of asymmetric thermal contact can provide a significant enhancement of thermal rectification efficiency, achieving around 920% thermal rectification in the short sample with a length of 200 nm and around 110% thermal rectification in the micrometer scale sample. Phonon participation ratio analysis reveals that the strong localization of low-frequency acoustic phonons is induced by the short thermal contact in the backward direction,…

In this work, a polarization conversion metasurface is proposed, which can convert linearly polarized electromagnetic (EM) waves into cross-polarized waves in ultra-wideband at high polarization conversion efficiency. The proposed sandwiched metasurface, composed of a top anisotropic resonator structure, an intermediate dielectric substrate, and a bottom metallic ground, has an extremely thin thickness (only 6.6% of the longest wavelength of the working frequency range). Both numerical simulation and experimental results illustrate that the metasurface can work in the ultra-wideband (ranging from 9.04 to 20.83 GHz) with a high polarization conversion ratio (>0.9). Polarization conversion in a wide bandwidth is attributed to multiple resonances in the operating frequency range. Additionally, the proposed metasurface also shows good performance for oblique incidence. It is expected that the designed metasurface will be more convenient for integration within ultra-thin EM devices.

The absence of both the inversion symmetry and out-of-plane mirror symmetry together with spin–orbit coupling (SOC) can induce novel electronic and piezoelectric properties. In this work, the piezoelectric properties along with carrier mobilities of Janus monolayer XTeI (X=Sb and Bi) are studied by density functional theory. By using generalized gradient approximation (GGA) plus SOC, they are found to be indirect gap semiconductors with the Rashba spin splitting. The piezoelectric tensors of Janus monolayer XTeI (X=Sb and Bi) are reported by using the density functional perturbation theory. Due to lacking both the inversion symmetry and out-of-plane mirror symmetry for Janus monolayer XTeI (X=Sb and Bi), both in-plane and out-of-plane piezoelectric effects can be observed, and the large piezoelectric coefficients are predicted (e.g., d11=12.95pm/V for SbTeI and d11=8.20pm/V for BiTeI), which are comparable and even higher than the ones of many other two-dimensional materials and other well-known bulk piezoelectric materials, especially for out-of-plane piezoelectric coefficients.…

The group-IV monochalcogenides are a set of binary materials MX composed by a group-IV element (M=Sn or Ge) and a chalcogen (X=S or Se). Strong anisotropy is one of the most interesting facets of these materials, especially in their low-dimensional form. In this Tutorial, we present some of the recent studies on the electronic and optical properties of 2D group-IV monochalcogenides, including predictions from first-principles density functional theory calculations and available experimental observations. We discuss the variation of the bandgap from bulk down to monolayer and the respective band structures, which are characterized by multiple valence and conduction band valleys, making these materials suitable for a variety of applications, including valleytronics. We also discuss the emergence of spin–orbit splitting, piezoelectricity, and ferroelectricity as a result of the polar character of the monolayers. Current predictions of carrier mobilities in monolayers and their potential application as thermoelectric materials are also presented.

The refractive index (RI) is an important material property and is necessary for making informed materials selection decisions when optical properties are important. Acquiring accurate empirical measurements of RI is time consuming, and while semi-empirical and computational determination of RI is generally faster than empirical determination, predictions are less accurate. In this work, we utilized experimentally measured RI data of polymers to build a machine learning model capable of making accurate near-instantaneous predictions of RI. The Gaussian process regression model is trained using data of 527 unique polymers. Feature engineering techniques were also used to optimize model performance. This new model is one of the most chemically diverse and accurate RI prediction models to date and improves upon our previous work. We also concluded that the model is capable of providing insights about structure–property relationships important for estimating the RI when designing new polymer backbones.

Wrinkled surfaces have attracted enormous interest during the past years due to their various wrinkling patterns and impressive multifunctional properties. With the growing demand of numerous potential applications, it is desirable to uncover the formation mechanism and develop fabrication methods for tunable wrinkles, in particular, for hierarchical wrinkle that has spatially varying wavelength and amplitude. In this tutorial, we comprehensively discuss the possible mechanism of the formation of hierarchical wrinkles, including the role of elasticity gradient in film–substrate systems, the effect of boundary confinement, the sequential multistep strain-releasing method in a multilayer system, etc. The formation conditions and morphological features of various hierarchical wrinkling patterns are outlined. Lastly, representative applications of hierarchical wrinkling are briefly summarized as well.

Phononic crystals are artificially structured materials that can possess special vibrational properties that enable advanced manipulations of sound and heat transport. These special properties originate from the formation of a bandgap that prevents the excitation of entire frequency ranges in the phononic band diagram. Unfortunately, identifying phononic crystals with useful bandgaps is a problematic process because not all phononic crystals have bandgaps. Predicting if a phononic crystal structure has a bandgap, and if so, the gap's center frequency and width is a computationally expensive process. Herein, we explore machine learning as a rapid screening tool for expedited discovery of phononic bandgap presence, center frequency, and width. We test three different machine learning algorithms (logistic/linear regression, artificial neural network, and random forests) and show that random forests performs the best. For example, we show that a random phononic crystal selection has only a 17% probability of having a bandgap, whereas after incorporating rapid screening with the random…