Showing papers in "Applied physics reviews in 2019"

A quantum engineer's guide to superconducting qubits

Abstract: The aim of this review is to provide quantum engineers with an introductory guide to the central concepts and challenges in the rapidly accelerating field of superconducting quantum circuits. Over the past twenty years, the field has matured from a predominantly basic research endeavor to a one that increasingly explores the engineering of larger-scale superconducting quantum systems. Here, we review several foundational elements—qubit design, noise properties, qubit control, and readout techniques—developed during this period, bridging fundamental concepts in circuit quantum electrodynamics and contemporary, state-of-the-art applications in gate-model quantum computation.

Trapped-ion quantum computing: Progress and challenges

Abstract: Trapped ions are among the most promising systems for practical quantum computing (QC). The basic requirements for universal QC have all been demonstrated with ions, and quantum algorithms using few-ion-qubit systems have been implemented. We review the state of the field, covering the basics of how trapped ions are used for QC and their strengths and limitations as qubits. In addition, we discuss what is being done, and what may be required, to increase the scale of trapped ion quantum computers while mitigating decoherence and control errors. Finally, we explore the outlook for trapped-ion QC. In particular, we discuss near-term applications, considerations impacting the design of future systems of trapped ions, and experiments and demonstrations that may further inform these considerations.

Radiative sky cooling: Fundamental principles, materials, and applications

Abstract: Radiative sky cooling cools an object on the earth by emitting thermal infrared radiation to the cold universe through the atmospheric window (8–13 μm). It consumes no electricity and has great potential to be explored for cooling of buildings, vehicles, solar cells, and even thermal power plants. Radiative sky cooling has been explored in the past few decades but limited to nighttime use only. Very recently, owing to the progress in nanophotonics and metamaterials, daytime radiative sky cooling to achieve subambient temperatures under direct sunlight has been experimentally demonstrated. More excitingly, the manufacturing of the daytime radiative sky cooling material by the roll-to-roll process makes large-scale deployment of the technology possible. This work reviews the fundamental principles of radiative sky cooling as well as the recent advances, from both materials and systems point of view. Potential applications in different scenarios are reviewed with special attention to technology viability and benefits. As the energy situation and environmental issues become more and more severe in the 21st century, radiative sky cooling can be explored for energy saving in buildings and vehicles, mitigating the urban heat island effect, resolving water and environmental issues, achieving more efficient power generation, and even fighting against the global warming problem.

Distributed optical fiber sensing: Review and perspective

Abstract: Over the past few decades, optical fibers have been widely deployed to implement various applications in high-speed long-distance telecommunication, optical imaging, ultrafast lasers, and optical sensors. Distributed optical fiber sensors characterized by spatially resolved measurements along a single continuous strand of optical fiber have undergone significant improvements in underlying technologies and application scenarios, representing the highest state of the art in optical sensing. This work is focused on a review of three types of distributed optical fiber sensors which are based on Rayleigh, Brillouin, and Raman scattering, and use various demodulation schemes, including optical time-domain reflectometry, optical frequency-domain reflectometry, and related schemes. Recent developments of various distributed optical fiber sensors to provide simultaneous measurements of multiple parameters are analyzed based on their sensing performance, revealing an inherent trade-off between performance parameters such as sensing range, spatial resolution, and sensing resolution. This review highlights the latest progress in distributed optical fiber sensors with an emphasis on energy applications such as energy infrastructure monitoring, power generation system monitoring, oil and gas pipeline monitoring, and geothermal process monitoring. This review aims to clarify challenges and limitations of distributed optical fiber sensors with the goal of providing a pathway to push the limits in distributed optical fiber sensing for practical applications.

Photonic quantum information processing: A concise review

Abstract: Photons have been a flagship system for studying quantum mechanics, advancing quantum information science, and developing quantum technologies. Quantum entanglement, teleportation, quantum key distribution, and early quantum computing demonstrations were pioneered in this technology because photons represent a naturally mobile and low-noise system with quantum-limited detection readily available. The quantum states of individual photons can be manipulated with very high precision using interferometry, an experimental staple that has been under continuous development since the 19th century. The complexity of photonic quantum computing devices and protocol realizations has raced ahead as both underlying technologies and theoretical schemes have continued to develop. Today, photonic quantum computing represents an exciting path to medium- and large-scale processing. It promises to put aside its reputation for requiring excessive resource overheads due to inefficient two-qubit gates. Instead, the ability to generate large numbers of photons—and the development of integrated platforms, improved sources and detectors, novel noise-tolerant theoretical approaches, and more—have solidified it as a leading contender for both quantum information processing and quantum networking. Our concise review provides a flyover of some key aspects of the field, with a focus on experiment. Apart from being a short and accessible introduction, its many references to in-depth articles and longer specialist reviews serve as a launching point for deeper study of the field.

Recent progress of study on optical solitons in fiber lasers

Abstract: Solitons are stable localized wave packets that can propagate long distance in dispersive media without changing their shapes. As particle-like nonlinear localized waves, solitons have been investigated in different physical systems. Owing to potential applications in optical communication and optical signal processing systems, optical solitons have attracted intense interest in the past three decades. To experimentally study the formation and dynamics of temporal optical solitons, fiber lasers are considered as a wonderful nonlinear system. During the last decade, several kinds of theoretically predicted solitons were observed experimentally in fiber lasers. In this review, we present a detailed overview of the experimentally verified optical solitons in fiber lasers, including bright solitons, dark solitons, vector solitons, dissipative solitons, dispersion-managed solitons, polarization domain wall solitons, and so on. An outlook for the development on the solitons in fiber lasers is also provided and discussed.

Conformality in atomic layer deposition : current status overview of analysis and modelling

Abstract: Atomic layer deposition (ALD) relies on alternated, self-limiting reactions between gaseous reactants and an exposed solid surface to deposit highly conformal coatings with a thickness controlled at the submonolayer level. These advantages have rendered ALD a mainstream technique in microelectronics and have triggered growing interest in ALD for a variety of nanotechnology applications, including energy technologies. Often, the choice for ALD is related to the need for a conformal coating on a 3D nanostructured surface, making the conformality of ALD processes a key factor in actual applications. In this work, we aim to review the current status of knowledge about the conformality of ALD processes. We describe the basic concepts related to the conformality of ALD, including an overview of relevant gas transport regimes, definitions of exposure and sticking probability, and a distinction between different ALD growth types observed in high aspect ratio structures. In addition, aiming for a more standardized and direct comparison of reported results concerning the conformality of ALD processes, we propose a new concept, Equivalent Aspect Ratio (EAR), to describe 3D substrates and introduce standard ways to express thin film conformality. Other than the conventional aspect ratio, the EAR provides a measure for the ease of coatability by referring to a cylindrical hole as the reference structure. The different types of high aspect ratio structures and characterization approaches that have been used for quantifying the conformality of ALD processes are reviewed. The published experimental data on the conformality of thermal, plasma-enhanced, and ozone-based ALD processes are tabulated and discussed. Besides discussing the experimental results of conformality of ALD, we will also give an overview of the reported models for simulating the conformality of ALD. The different classes of models are discussed with special attention for the key assumptions typically used in the different modelling approaches. The influence of certain assumptions on simulated deposition thickness profiles is illustrated and discussed with the aim of shedding light on how deposition thickness profiles can provide insights into factors governing the surface chemistry of ALD processes. We hope that this review can serve as a starting point and reference work for new and expert researchers interested in the conformality of ALD and, at the same time, will trigger new research to further improve our understanding of this famous characteristic of ALD processes.

A PUF taxonomy

Abstract: Authentication is an essential cryptographic primitive that confirms the identity of parties during communications. For security, it is important that these identities are complex, in order to make them difficult to clone or guess. In recent years, physically unclonable functions (PUFs) have emerged, in which identities are embodied in structures, rather than stored in memory elements. PUFs provide “digital fingerprints,” where information is usually read from the static entropy of a system, rather than having an identity artificially programmed in, preventing a malicious party from making a copy for nefarious use later on. Many concepts for the physical source of the uniqueness of these PUFs have been developed for multiple different applications. While certain types of PUF have received a great deal of attention, other promising suggestions may be overlooked. To remedy this, we present a review that seeks to exhaustively catalogue and provide a complete organisational scheme towards the suggested concepts for PUFs. Furthermore, by carefully considering the physical mechanisms underpinning the operation of different PUFs, we are able to form relationships between PUF technologies that previously had not been linked and look toward novel forms of PUF using physical principles that have yet to be exploited.

Beyond solid-state lighting: Miniaturization, hybrid integration, and applications of GaN nano- and micro-LEDs

Abstract: Gallium nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting, and ubiquitous white light sources has developed around the inception of the Nobel-prize-winning blue GaN LEDs. Today, GaN optoelectronics is developing beyond solid-state lighting, leading to new and innovative devices, e.g., for microdisplays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (micro-LEDs and nano-LEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN micro-LEDs with silicon microelectronics in a hybrid approach. GaN LED technology is now even spreading into the realm of display technology, which has been occupied by organic LEDs and liquid crystal displays for decades. In this review, the technological transition toward GaN micro- and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art.

Extrusion bioprinting of soft materials: An emerging technique for biological model fabrication

Abstract: Bioprinting has attracted increasing attention in the tissue engineering field and has been touted to potentially become the leading technology to fabricate, and regenerate, tissues and organs. Bioprinting is derived from well-known additive manufacturing (AM) technology, which features layered deposition of materials into complex three-dimensional geometries that are difficult to fabricate using conventional manufacturing methods. Unlike the conventional thermoplastics used in desktop, AM bioprinting uses cell-laden hydrogel materials, also known as bioinks, to construct complex living biological model systems. Inkjet, stereolithography, laser-induced forward transfer, and extrusion are the four main methods in bioprinting, with extrusion being the most commonly used. In extrusion-based bioprinting, soft materials are loaded into the cartridges and extruded from the nozzle via pneumatic or mechanical actuation. Multiple materials can be printed into the same structure resulting in heterogeneous models. In this focused review, we first review the different methods to describe the physical mechanisms of the extrusion process, followed by the commonly employed bioprintable soft materials with their mechanical and biochemical properties and finally reviewing the up-to-date heterogeneous in vitro models afforded via bioprinting.

Two-dimensional pnictogens: A review of recent progresses and future research directions

Abstract: Soon after the synthesis of two-dimensional (2D) ultrathin black phosphorus and fabrication of field effect transistors thereof, theoretical studies have predicted that other group-VA elements (or pnictogens), N, As, Sb, and Bi can also form stable, single-layer (SL) structures. These were nitrogene in a buckled honeycomb structure, arsenene, antimonene, and bismuthene in a buckled honeycomb, as well as washboard and square-octagon structures with unusual mechanical, electronic, and optical properties. Subsequently, theoretical studies are followed by experimental efforts that aim at synthesizing these novel 2D materials. Currently, research on 2D pnictogens has been a rapidly growing field revealing exciting properties, which offers diverse applications in flexible electronics, spintronics, thermoelectrics, and sensors. This review presents an evaluation of the previous experimental and theoretical studies until 2019, in order to provide input for further research attempts in this field. To this end, we first reviewed 2D, SL structures of group-VA elements predicted by theoretical studies with an emphasis placed on their dynamical and thermal stabilities, which are crucial for their use in a device. The mechanical, electronic, magnetic, and optical properties of the stable structures and their nanoribbons are analyzed by examining the effect of external factors, such as strain, electric field, and substrates. The effect of vacancy defects and functionalization by chemical doping through adatom adsorption on the fundamental properties of pnictogens has been a critical subject. Interlayer interactions in bilayer and multilayer structures, their stability, and tuning their physical properties by vertical stacking geometries are also discussed. Finally, our review is concluded by highlighting new research directions and future perspectives on the challenges in this emerging field.

Review of mid-infrared mode-locked laser sources in the 2.0 μm–3.5 μm spectral region

Abstract: Ultrafast laser sources operating in the mid-infrared (mid-IR) region, which contains the characteristic fingerprint spectra of many important molecules and transparent windows of atmosphere, are of significant importance in a variety of applications. Over the past decade, a significant progress has been made in the development of inexpensive, compact, high-efficiency mid-IR ultrafast mode-locked lasers in the picosecond and femtosecond domains that cover the 2.0 μm–3.5 μm spectral region. These achievements open new opportunities for applications in areas such as molecular spectroscopy, frequency metrology, material processing, and medical diagnostics and treatment. In this review, starting with the introduction of mid-IR mode-locking techniques, we mainly summarize and review the recent progress of mid-IR mode-locked laser sources, including Tm3+-, Ho3+-, and Tm3+/Ho3+-doped all-solid-state and fiber lasers for the 2.0 μm spectral region, Cr2+:ZnSe and Cr2+:ZnS lasers for the 2.4 μm region, and Er3+-, Ho3+/Pr3+-, and Dy3+-doped fluoride fiber lasers for the 2.8 μm–3.5 μm region. Then, some emerging and representative applications of mid-IR ultrafast mode-locked laser sources are presented and illustrated. Finally, outlooks and challenges for future development of ultrafast mid-IR laser sources are discussed and analyzed. The development of ultrafast mid-IR laser sources, together with the ongoing progress in related application technologies, will create new avenues of research and expand unexplored applications in scientific research, industry, and other fields.

Recent progress in ultrafast lasers based on 2D materials as a saturable absorber

Abstract: Two-dimensional (2D) materials are crystals with one to a few layers of atoms and are being used in many fields such as optical modulator, photodetector, optical switch, and ultrafast lasers. Their exceptional optoelectronic and nonlinear optical properties make them as a suitable saturable absorber for laser cavities. This review focuses on the recent progress in ultrafast laser use 2D materials as a saturable absorber. 2D materials traditionally include graphene, topological insulators, transition metal dichalcogenides, as well as new materials such as black phosphorus, bismuthene, antimonene, and MXene. Material characteristics, fabrication techniques, and nonlinear properties are also introduced. Finally, future perspectives of ultrafast lasers based on 2D materials are also addressed.

Colors with plasmonic nanostructures: A full-spectrum review

Abstract: Since ancient times, plasmonic structural coloring has inspired humanity; glassmakers achieved vibrant colors by doping glass with metal nanoparticles to craft beautiful objects such as the Roman Lycurgus cup and stained glass. These lovely color filtering effects are a consequence of the resonant coupling of light and free electrons in metal nanoparticles, known as surface plasmons. Thanks to the continuing improvement of nanofabrication technology, the dimensions of nanoparticles and structures can now be precisely engineered to form “optical nanoantennas,” allowing for control of optical response at an unprecedented level. Recently, the field of plasmonic structural coloring has seen extensive growth. In this review, we provide an up-to-date overview of various plasmonic color filtering approaches and highlight their uses in a broad palette of applications. Various surface plasmon resonance modes employed in the plasmonic color filtering effect are discussed. We first review the development of the pioneering static plasmonic colors achieved with invariant optical nanoantennas and ambient environment, then we address a variety of emerging approaches that enable dynamic color tuning, erasing, and restoring. These dynamic color filters are capable of actively changing the filtered colors and carrying more color information states than the static systems. Thus, they open an avenue to high-density data storage, information encryption, and plasmonic information processing. Finally, we discuss the challenges and future perspectives in this exciting research area.

Two-dimensional infrared and terahertz detectors: Outlook and status

Abstract: Since the discovery of graphene, its applications to electronic and optoelectronic devices have been intensively and thoroughly researched. Extraordinary and unusual electronic and optical properties make graphene and other two-dimensional (2D) materials promising candidates for infrared and terahertz (THz) photodetectors. Until now, however, 2D material-based performance is lower in comparison with those of infrared and terahertz detectors existing in the global market. This paper gives an overview of emerging 2D material detectors' performance and comparison with the traditionally and commercially available ones in different applications in high operating temperature conditions. The most effective single graphene detectors are THz detectors utilizing the plasma rectification effect in the field-effect transistors. Most of the 2D layered semiconducting material photodetectors operate in the visible and near-infrared regions, and generally, their high sensitivity does not coincide with the fast response time, which limits real detector functions.

Microsphere enhanced optical imaging and patterning: From physics to applications

Abstract: The diffraction limit is a fundamental barrier in optical science and engineering. It limits the minimum feature size in surface patterning technologies, such as lithography and laser direct writing. It also restricts the resolution for optical imaging, which includes different kinds of microscopes. Microspheres have been demonstrated as a powerful platform to challenge the diffraction limit. Microspheres can manipulate the light in a novel way that conventional optical components cannot achieve. In this review, we summarize the fundamental physical mechanisms and the related applications of microspheres in two primary research directions: first, to focus light energy on the sample surface, which leads to nano-patterning and achieves a sub-100 nm feature size and second, to manipulate light reflected back from the sample surface, which forms the foundation of super-resolution optical imaging to observe nano-structures. We also analyze key features, development, limitation, and opportunities of the nano-patterning and nano-imaging systems based on the microsphere.

p-bits for probabilistic spin logic

Abstract: We introduce the concept of a probabilistic or p-bit, intermediate between the standard bits of digital electronics and the emerging q-bits of quantum computing. We show that low barrier magnets or LBMs provide a natural physical representation for p-bits and can be built either from perpendicular magnets designed to be close to the in-plane transition or from circular in-plane magnets. Magnetic tunnel junctions (MTJs) built using LBMs as free layers can be combined with standard NMOS transistors to provide three-terminal building blocks for large scale probabilistic circuits that can be designed to perform useful functions. Interestingly, this three-terminal unit looks just like the 1T/MTJ device used in embedded magnetic random access memory technology, with only one difference: the use of an LBM for the MTJ free layer. We hope that the concept of p-bits and p-circuits will help open up new application spaces for this emerging technology. However, a p-bit need not involve an MTJ; any fluctuating resistor could be combined with a transistor to implement it, while completely digital implementations using conventional CMOS technology are also possible. The p-bit also provides a conceptual bridge between two active but disjoint fields of research, namely, stochastic machine learning and quantum computing. First, there are the applications that are based on the similarity of a p-bit to the binary stochastic neuron (BSN), a well-known concept in machine learning. Three-terminal p-bits could provide an efficient hardware accelerator for the BSN. Second, there are the applications that are based on the p-bit being like a poor man's q-bit. Initial demonstrations based on full SPICE simulations show that several optimization problems, including quantum annealing are amenable to p-bit implementations which can be scaled up at room temperature using existing technology.

Resolution and shape in bioprinting : strategizing towards complex tissue and organ printing

Abstract: In 3D bioprinting, printing resolution represents the deposited material in the x- and y-axes, while dimensionality defines the structural resolution of printed constructs. Dimensionality in 3D bioprinting can be defined as the resolution in the z-axis. The printing resolution, together with dimensionality, contributes to the overall shape fidelity of the bioprinted constructs. The in-depth understanding of physical processes for different printing technologies is imperative in controlling the print resolution and definition. In this article, bioprinting technologies are classified according to the physical processes that deposit or form the bioprinted construct. Due to the different fabrication processes in forming fundamental printed units (voxels), the definition of printability differs for each bioprinting technique. Another aspect of resolution is the spatial positioning of cells within each fundamental building unit. The proximity of cells in the bioprinted construct affects the physiological outcomes. The second aspect of 3D bioprinting technologies is the ability to control shape fidelity. Different strategies have been used to improve the construction of a 3D engineered tissue or organ. Lastly, moving toward complex tissue printing involves adding functionalities to the bioprinted construct. Data processing, material formulations, and integration of different fabrication technologies are key areas in bioprinting that can recapture the different hierarchical aspects of native tissues. This article presents a comprehensive overview of enhancing the resolution of the bioprinting construct and identifying methods to improve functionalities of bioprinted tissues.

Experimental and theoretical investigation of the fluid behavior during polymeric fiber formation with and without pressure

Abstract: The fabrication of polymeric micro/nanofibers is gaining attention due to their use in an array of applications including tissue engineering scaffolds, nanosensors, and fiber-reinforced composites. Despite their versatile nature, polymeric fibers are widely underutilized due to the lack of reliable, large-scale production techniques. Upon the discovery of centrifugal spinning and, recently, pressurized gyration techniques, new research directions have emerged. Here, we report a comprehensive study detailing the optimal conditions to significantly improve the morphology, homogeneity, and yield of fibers of varying diameters. A series of polymeric fibers was created using a 21 wt. % solution of polyethylene oxide in distilled water and the fluid behavior was monitored inside a transparent reservoir using a high-speed camera. Fabrication of the fibers took less than 1 s. Using centrifugal spinning, we studied the formation of the fibers at three different rotational speeds, and for pressurized gyration, one rotational speed was studied with three different nitrogen gas pressures. Using the pressurized gyration technique at a gas pressure of 0.3 MPa, there was significant improvement in the production yield of the fibers. We found a strong correlation between the variation of pressure and the rate of the solution leaving the reservoir with the improved morphology of the fibers. The use of reduced power techniques, like centrifugal spinning and pressured gyration, to yield high-quality nonwoven nanofibers and microfibers in large quantities is important due to their use in rapidly expanding markets.

Recent advances in fabrication strategies, phase transition modulation, and advanced applications of vanadium dioxide

Abstract: Vanadium dioxide (VO2), with the first-order metal-insulator phase transition at near room temperature, has attracted increasing attention in the past few decades. With rapid electrical switching, the phase transition in VO2 also triggers the colossal property changes in various aspects, such as optical properties, magnetic properties, and strain, and, thus, enables a wide range of modern applications. In this review, we present a complete picture of the latest advances of VO2, including the fabrication strategies, property modulation, and advanced applications. This review summarizes several typical fabrication methods of VO2 crystals as well as some common problems and their possible solutions. The strategies for the fabrication of single-crystalline VO2 arrays are also discussed to meet the requirements of the high-performance devices at the macro-scale. This review concerns the typical approaches for the modulation of (metal-insulator transition) MIT and emphasizes on the domain study of VO2 single crystals at the nanoscale. We aim at a clear explanation of the effect of various inhomogeneities on the MIT behavior of VO2 and the importance of the accurate control of MIT at the domain level. After summarizing the preparation and modification of VO2, we focus on the applications of this amazing smart material in various aspects, including strain engineering, optical modulation, electrical switching, and multi-responsive sensing.

Stretchable sensors for environmental monitoring

Abstract: The development of flexible and stretchable sensors has been receiving increasing attention in recent years. In particular, stretchable, skin-like, wearable sensors are desirable for a variety of potential applications such as personalized health monitoring, human-machine interfaces, and environmental sensing. In this paper, we review recent advancements in the development of mechanically flexible and stretchable sensors and systems that can be used to quantitatively assess environmental parameters including light, temperature, humidity, gas, and pH. We discuss innovations in the device structure, material selection, and fabrication methods which explain the stretchability characteristics of these environmental sensors and provide a detailed and comparative study of their sensing mechanisms, sensor characteristics, mechanical performance, and limitations. Finally, we provide a summary of current challenges and an outlook on opportunities for possible future research directions for this emerging field.

3D bioprinting for high-throughput screening: Drug screening, disease modeling, and precision medicine applications.

Abstract: High-throughput technologies have become essential in many fields of pharmaceutical and biological development and production. Such technologies were initially developed with compatibility with liquid handling-based cell culture techniques to produce large-scale 2D cell culture experiments for the compound analysis of candidate drug compounds. Over the past two decades, tools for creating 3D cell cultures, organoids, and other 3D in vitro models, such as cell supportive biomaterials and 3D bioprinting, have rapidly advanced. Concurrently, a significant body of evidence has accumulated which speaks to the many benefits that 3D model systems have over traditional 2D cell cultures. Specifically, 3D cellular models better mimic aspects such as diffusion kinetics, cell-cell interactions, cell-matrix interactions, inclusion of stroma, and other features native to in vivo tissue and as such have become an integral part of academic research. However, most high throughput assays were not developed to specifically support 3D systems. Here, we describe the need for improved compatibility and relevant advances toward deployment and adoption of high throughput 3D models to improve disease modeling, drug efficacy testing, and precision medicine applications.

Synthesis and emerging properties of 2D layered III–VI metal chalcogenides

Abstract: Atomically thin layered III–VI metal chalcogenides are an emerging class of 2D materials that have attracted increasing attention in recent years due to their remarkable physical properties and technological applications. Thanks to the recently developed theoretical and experimental methods, a number of exciting discoveries for these materials have revealed their new phases, a unique “Mexican hat”-shaped electronic band structure, and superior optical and electronic properties that distinguish them from other 2D materials such as transition metal dichalcogenides. This review summarizes the novel properties, structures, and synthesis strategies for these materials and emphasizes the most cutting-edge and seminal achievements in this rapidly growing field in order to provide input for future research works. We first present the rich crystal structure and phases that have been found in these materials, with an emphasis on the possibility of phase engineering. Then, we discuss the synthesis strategies for 2D layered III–VI metal chalcogenides from the top-down, bottom-up, and template-based chemical conversion approaches. We focus on the highly controlled synthesis methods that provide fine-tuning of the thickness, phase, edge structure, and other morphological characteristics. Third, we discuss the properties and applications of these materials, focusing on their unique electronic structure including the Mexican hat-shaped valence band, their superior nonlinear optical properties, high-performance electronic devices, promising photoelectrochemical properties, and emerging quantum properties such as quantum emission, exciton condensation, ferromagnetism, and topological quantum phase transition. Finally, we provide our perspective on the current challenges and future directions in this field.

Tissue-engineering of vascular grafts containing endothelium and smooth-muscle using triple-coaxial cell printing

Abstract: Tissue engineering has emerged as a promising approach to viable small-diameter vascular grafts that may be used to treat cardiovascular diseases. One challenge in constructing such blood vessels is proper localization of endothelial cells and smooth muscle cells, as well as promotion of their cellular functions to generate functional tissues. Thus far, construction of small-diameter vascular substitutes with both endothelial and muscular tissues, which is essential for the grafts to acquire antithrombosis function and sufficient strength to avoid thrombus formation as well as to withstand blood pressure, has not yet been demonstrated. In this study, we engineer small-diameter blood vessel grafts containing both functional endothelial and muscular cell layers, which has been demonstrated in vivo in a living rat model. Our construction of the blood vessel grafts uses vascular-tissue-derived extracellular matrix bioinks and a reservoir-assisted triple-coaxial cell printing technique. The prematured vessel was implanted for three weeks as a graft of rat abdominal aorta in a proof-of-concept study where all implants showed great patency, intact endothelium, remodeled smooth muscle, and integration with host tissues at the end of the study. These outcomes suggest that our approach to tissue-engineered biomimetic blood vessels provides a promising route for the construction of durable small-diameter vascular grafts that may be used in future treatments of cardiovascular diseases.

Performance boosting strategy for perovskite light-emitting diodes

Abstract: Low-dimensional (low-D) luminescent materials have attracted significant attention due to the high photoluminescent quantum yields. However, it is unclear whether low-D materials are superior to 3D materials for electroluminescent (EL) devices given that low-D materials have poor charge transport nature due to their highly localized electronic structures. We noticed a significant phenomenon that EL performances for 3D materials, such as CsPbX3, are governed by adjacent charge transport layers, which is possibly due to nonradiative recombination resulting from the small exciton binding energy. This finding encouraged us to develop new electron transport layers (ETLs) that satisfy not only the energy alignment to confine excitons but also an efficient electron injection into 3D CsPbX3 layers. This strategy enables one to exploit the good charge transport nature of 3D CsPbX3. The proposed amorphous Zn-Si-O ETL has sufficiently shallow electron affinity (∼3.2 eV) to confine excitons and sufficiently high electron mobility (∼0.8 cm2/V s) to transport electrons. Furthermore, the controllable conductivity and electron affinity of amorphous Zn-Si-O enable fine-tuning of charge balance. Consequently, the very low operating voltage of 2.9 V at 10 000 cd/m2 and high power efficiency of 33 lm/W were achieved for a green perovskite (CsPbBr3) EL (PeLED). The obtained ultrahigh brightness of ∼500 000 cd/m2 demonstrates the effectiveness of the proposed strategy. We also extend this strategy into 3D CsPbBrI2 (red) and 3D CsPbBrCl2 (blue) PeLEDs, and demonstrate a record high brightness of 20 000 cd/m2 for the red PeLED. We believe this study provides new insight into the realization of practical PeLEDs.

Novel mechanocaloric materials for solid-state cooling applications

Abstract: Current refrigeration technologies based on compression cycles of greenhouse gases are environmentally threatening and cannot be scaled down to on-chip dimensions. Solid-state cooling is an environmentally friendly and highly scalable technology that may solve most of the problems associated with current refrigerant methods. Solid-state cooling consists of applying external fields (magnetic, electric, and mechanical) on caloric materials, which react thermally as a result of induced phase transformations. From an energy efficiency point of view, mechanocaloric compounds, in which the phase transitions of interest are driven by mechanical stresses, probably represent the most encouraging type of caloric materials. Conventional mechanocaloric materials like shape-memory alloys already display good cooling performances; however, in most cases they also present critical mechanical fatigue and hysteresis problems that limit their applicability. Finding new mechanocaloric materials and mechanisms that are able to overcome those problems, while simultaneously rendering large temperature shifts, is necessary to further advance the field of solid-state cooling. In this article, we review novel families of mechanocaloric materials that in recent years have been shown to be especially promising in the aspects that conventional mechanocaloric materials are not, and that exhibit unconventional but significant caloric effects. We emphasize elastocaloric materials, in which the targeted cooling spans are obtained through uniaxial stresses, since from an applied perspective they appear to be the most accomplished ones. Two different types of mechanocaloric materials emerge as particularly hopeful from our analysis: (1) compounds that exhibit field-induced order-disorder phase transitions involving either ions or molecules (polymers, fast-ion conductors, and plastic crystals), and (2) multiferroics in which the structural parameters are strongly coupled with the polar and/or magnetic degrees of freedom (magnetic alloys and oxide perovskites).

Active meta-optics and nanophotonics with halide perovskites

Abstract: Meta-optics based on optically resonant all-dielectric structures is a rapidly developing research area driven by its potential applications for low-loss efficient metadevices. Active, light-emitting subwavelengh nanostructures and metasurfaces are of particular interest for meta-optics, as they offer unique opportunities for novel types of compact light sources and nanolasers. Recently, the study of “halide perovskites” has attracted enormous attention due to their exceptional optical and electrical properties. As a result, this family of materials can provide a prospective platform for modern nanophotonics and meta-optics, allowing us to overcome many obstacles associated with the use of conventional semiconductor materials. Here, we review the recent progress in the field of halide-perovskite meta-optics with the central focus on light-emitting nanoantennas and metasurfaces for the emerging field of “active metadevices.”

A comprehensive study on the mechanism of ferroelectric phase formation in hafnia-zirconia nanolaminates and superlattices

Abstract: Many applications, most notably memory and optical devices use ferroelectric materials. For many years the evolution of the field has revolved around understanding the materials science behind complex structures like artificial superlattices based mainly on perovskite-structure oxides. The recent discovery of ferroelectricity in fluorite-structure oxides has opened a new research direction. However, the formation of unstable or metastable phases in atomic layer deposited fluorite oxides has inhibited a full understanding of the origin of ferroelectricity in these materials. This work reports a comprehensive study of the structural and electrical properties of HfO2 and ZrO2 nanolaminates and superlattices of various layering combinations and thicknesses. The structural investigations provide insight into how to optimize conditions during atomic layer deposition to avoid the formation of unstable phases. Investigations showed that the starting layer of the material, the thickness ratio between HfO2 and ZrO2 layers, and the single-layer thickness strongly effected the ferroelectric properties. The influence of single-layer thickness related most strongly to the presence of interfacial nonferroelectric layers between the HfO2 and ZrO2 deposits. These features make the structures highly promising candidates for next-generation memory applications. Potentially other fluorite-structure oxides might also function as building blocks for nanolaminates and superlattices.

Atomic layer deposition of metals: Precursors and film growth

Abstract: The coating of complex three-dimensional structures with ultrathin metal films is of great interest for current technical applications, particularly in microelectronics, as well as for basic research on, for example, photonics or spintronics. While atomic layer deposition (ALD) has become a well-established fabrication method for thin oxide films on such geometries, attempts to develop ALD processes for elemental metal films have met with only mixed success. This can be understood by the lack of suitable precursors for many metals, the difficulty in reducing the metal cations to the metallic state, and the nature of metals as such, in particular their tendency to agglomerate to isolated islands. In this review, we will discuss these three challenges in detail for the example of Cu, for which ALD has been studied extensively due to its importance for microelectronic fabrication processes. Moreover, we give a comprehensive overview over metal ALD, ranging from a short summary of the early research on the ALD of the platinoid metals, which has meanwhile become an established technology, to very recent developments that target the ALD of electropositive metals. Finally, we discuss the most important applications of metal ALD.

Sensing and imaging using laser feedback interferometry with quantum cascade lasers

Abstract: Quantum cascade lasers (QCLs) are high-power sources of coherent radiation in the midinfrared and terahertz (THz) bands. Laser feedback interferometry (LFI) is one of the simplest coherent techniques, for which the emission source can also play the role of a highly-sensitive detector. The combination of QCLs and LFI is particularly attractive for sensing applications, notably in the THz band where it provides a high-speed high-sensitivity detection mechanism which inherently suppresses unwanted background radiation. LFI with QCLs has been demonstrated for a wide range of applications, including the measurement of internal laser characteristics, trace gas detection, materials analysis, biomedical imaging, and near-field imaging. This article provides an overview of QCLs and the LFI technique, and reviews the state of the art in LFI with sensing using QCLs.