Showing papers in "Journal of Applied Physics in 2020"

Trends in luminescence thermometry

Abstract: 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 intensity ratio (LIR) and transition-metal-ion-activated phosphors for both lifetime and LIR thermometry. Several particularly important applications of luminescence thermometry are presented. These include temperature measurement in catalysis, in situ temperature mapping for microfluidics, thermal history measurement, thermometry at extremely high temperatures, fast temperature transient measurement, low-pressure measurement via upconversion nanoparticle emission intensity ratios, evaluation of the photothermal chirality of noble metal clusters, and luminescence thermometry using mobile devices. Routes for the development of primary luminescence thermometry are discussed in view of the recent redefinition of the kelvin.

Introduction to spin wave computing

Abstract: 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 be embedded in conventional complementary metal–oxide–semiconductor (CMOS) circuits to obtain complete functional hybrid computing systems. The state of the art of benchmarking such hybrid spin-wave–CMOS systems is reviewed, and the current challenges to realize such systems are discussed. The benchmark indicates that hybrid spin-wave–CMOS systems promise ultralow-power operation and may ultimately outperform conventional CMOS circuits in terms of the power-delay-area product. Current challenges to achieve this goal include low-power signal restoration in spin-wave circuits as well as efficient spin-wave transducers.

Surface states of carbon dots and their influences on luminescence

Abstract: 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.

Hyperbolic metamaterials: From dispersion manipulation to applications

Abstract: 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.

Hybrid magnonics: Physics, circuits, and applications for coherent information processing

Abstract: 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 information. We will discuss a few potential directions for advancing magnon hybrid systems, including on-chip geometry, novel coherent magnonic functionality, and coherent transduction between different platforms. As a future outlook, we will discuss the opportunities and challenges of magnonic hybrid systems for their applications in quantum information and magnonic logic.

The search for the most conductive metal for narrow interconnect lines

Abstract: 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. First-principles calculations provide a list of metals (Rh, Pt, Ir, Nb, Ru, Ni, etc.) with a small product of the bulk resistivity times the bulk electron mean free path ρo × λ, which is an indicator for suppressed resistivity scaling. However, resistivity measurements on epitaxial layers indicate considerably larger experimental ρo × λ values for many metals, indicating the breakdown of the classical transport models at small (<10 nm) dimensions and suggesting that Ir is the most promising elemental metal for narrow high-conductivity interconnects, followed by Ru and Rh.

Point defects in Ga2O3

Abstract: 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.

Perovskite lead-free piezoelectric ceramics

Abstract: 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.

Effects of deposition conditions on the ferroelectric properties of (Al1−xScx)N thin films

Abstract: 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.

Machine-learning predictions of polymer properties with Polymer Genome

Abstract: 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.

Persistent phosphors for the future: Fit for the right application

Abstract: 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 temperature monitoring in, for example, cryopreservation. Deeper traps are useful for x-ray imaging and dosimetry. Next to these applications, a critical evaluation is made of the possibilities of persistent phosphors for applications such as solar energy storage and photocatalysis.

Theoretical modeling of triboelectric nanogenerators (TENGs)

Abstract: 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.

Physical chemistry of the TiN/Hf0.5Zr0.5O2 interface

Abstract: 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 / Hf 0.5 Zr 0.5 O 2 / 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 ( V O) 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 V O concentration at the top interface but causes the generation of additional V O in the film, leading to a decrease of the Schottky Barrier Height for electrons. The interface chemistry and n-type film doping are believed to be at the origin of several phenomena, including wake-up, imprint, and fatigue. Our results give insights into the physical chemistry of the top interface with the accumulation of defective charges acting as electronic traps, causing a local imprint effect. This may explain the wake-up behavior as well and also can be a possible reason of the weaker endurance observed in these systems when increasing the annealing temperature.

Defects chemistry in high-efficiency and stable perovskite solar cells

Abstract: 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 perovskite optoelectronics.

Advanced polymer dielectrics for high temperature capacitive energy storage

Abstract: 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

Self-trapped hole and impurity-related broad luminescence in β-Ga2O3

Abstract: 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-Ga 2O 3 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 Mg Ga, Be Ga, Ca Ga, Cd Ga, Zn Ga, Li Ga, and N O acceptor impurities, as well as their complexes with hydrogen donors. We also explore V Ga acceptors complexed with hydrogen and Si Ga 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-Ga 2O 3.

Thermoelectric properties, phonon, and mechanical stability of new half-metallic quaternary Heusler alloys: FeRhCrZ (Z = Si and Ge)

Abstract: 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 of FeRhCrSi and FeRhCrGe compounds are 2.3 μW/cm K2 and 0.83 μW/m K2, respectively. Present investigations certainly allow the narrow bandgap, spin polarization, and high PF values to be looked upon for suitable applications in thermoelectrics and spintronics.

Thermoelectric properties of Janus MXY (M = Pd, Pt; X, Y = S, Se, Te) transition-metal dichalcogenide monolayers from first principles

Abstract: 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 = S, Se, Te) are potentially TE materials.

Uni-traveling-carrier photodiodes

Abstract: 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.

Multi-component and high-entropy nitride coatings—A promising field in need of a novel approach

Abstract: 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 forward and suggests that there is a need to combine the traditional knowledge-driven materials science methodology with new data-driven methodologies. The latter would include advanced data-handling with artificial intelligence and machine learning to assist in the evaluation of large, shared datasets from both experimental and theoretical work. Such a change in the methodology will be a challenge but will be needed in order to fully realize the full potential of multi-component (nitride) materials.

Dissipative couplings in cavity magnonics

Abstract: 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 present an outlook for developing open cavity systems by engineering and harnessing dissipative couplings.

Hot electron and thermal effects in plasmonic photocatalysis

Abstract: 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 constraints and thus different experimental strategies to reveal the effects of localized heating and hot electrons.

Harmonic Generation at the Nanoscale

Abstract: 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.

Direct laser writing of graphene electrodes

Abstract: 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.

A perspective on MXenes: Their synthesis, properties, and recent applications

Abstract: 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.

Engineering lattice metamaterials for extreme property, programmability, and multifunctionality

Abstract: 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, optical, piezoelectric, and negative-index material properties. These emergent directions portend the transitioning of lattice metamaterials from the stage of conventional materials to smart, adaptive, and versatile materials, which provide solutions to realistic problems in transport systems, wearable devices, and robotics, and continue to push the boundary of possibilities of architected metamaterials.

Electron dynamics in low pressure capacitively coupled radio frequency discharges

Abstract: 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 analysis (aided by animations) with respect to the spatial and temporal dynamics. Finally, the rendered picture provides an overview of how electrons gain and lose their energy in CCRF discharges.

Engineering of defects in resistive random access memory devices

Abstract: 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.

Particle tracking of nanoparticles in soft matter

Abstract: 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.

A practical guide for crystal growth of van der Waals layered materials

Abstract: 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 crystal growers, however, because any researcher who collaborates with a crystal grower can benefit from having a greater understanding and appreciation of the processes of crystal growth and materials development.