A method to cost-effectively upgrade the performance of an established small-bandgap solar technology is to deposit a large-bandgap polycrystalline semiconductor on top to make a tandem solar cell. Metal-halide perovskites have recently been demonstrated as large-bandgap semiconductors that perform well even as a defective and polycrystalline material. We review the initial experimental and modeling work performed on these tandems. We also discuss in-depth the challenges of perovskite-based tandems and the innovations needed from the solar research community to propel perovskite-based tandems into the high-efficiency (>25%) regime and reach commercial competitiveness.
Materials play a critical enabling role in many energy technologies, but their development and commercialization often follow an unpredictable and circuitous path. In this article, we illustrate this concept with the history of lithium-ion (Li-ion) batteries, which have enabled unprecedented personalization of our lifestyles through portable information and communication technology. These remarkable batteries enable the widespread use of laptop and tablet computers, access to entertainment on portable devices such as hand-held music players and video game consoles, and enhanced communication and networking on personal devices such as cellular telephones and watches. A similar transformation of transportation to electric cars and of the electricity grid to widespread deployment of variable renewable solar and wind generation, effortless time-shifting of energy generation and demand, and a transition from central to distributed energy services requires next-generation energy storage that delivers much higher performance at lower cost. The path to these next-generation batteries is likely to be as circuitous and unpredictable as the path to today's Li-ion batteries. We analyze the performance and cost improvements needed to transform transportation and the electricity grid, and we evaluate the outlook for meeting these needs with next-generation beyond Li-ion batteries.
The ability to pattern materials in three dimensions is crucial for structural, optical, electronic, and energy applications. Three-dimensional printing allows one to design and rapidly fabricate materials in complex shapes without the need for expensive tooling, dies, or lithographic masks. A growing palette of printable materials, coupled with the ability to programmably control mesoscale architecture, open new avenues for creating designer materials with unprecedented performance.
In situ transmission electron microscopy (TEM) has become an increasingly important tool for materials characterization. It provides key information on the structural dynamics of a material during transformations and the ability to correlate a material's structure and properties. With the recent advances in instrumentation, including aberration-corrected optics, sample environment control, the sample stage, and fast and sensitive data acquisition, in situ TEM characterization has become more powerful. In this article, a brief review of the current status and future opportunities of in situ TEM is provided. The article also introduces the six articles in this issue of MRS Bulletin exploring the frontiers of in situ electron microscopy, including liquid and gas environmental TEM, dynamic four-dimensional TEM, studies on nanomechanics and ferroelectric domain switching, and state-of-the-art atomic imaging of light elements (i.e., carbon atoms) and individual defects.
Dielectric capacitors have been the major enabler for many applications in advanced electronic and electrical power systems because of their capability for ultrafast charging/discharging and ultrahigh power density. The low energy densities of polymer dielectrics used in these capacitors have not been able to meet the ever-increasing demands for compact, reliable, and efficient electrical power systems. Polymer nanocomposites, in which high-dielectric-constant (k) nanofillers are incorporated in the polymer matrix, have been actively pursued. In this article, we begin with two theoretical considerations for concomitantly increasing the dielectric permittivity and breakdown strength of nanocomposites: critical interfacial polarization and local electric-field distribution. In the framework of these considerations, we review recent progress toward polymer nanocomposites with high energy densities based on two approaches: core-shell-structured polymer nanocomposites and dielectric anisotropy. In addition, the potential for the enhanced elastic properties of nanocomposites to improve the dielectric strengths of capacitor films is also discussed.
Carbon, with its variety of allotropes and forms, is the most versatile material, and virtually any combination of mechanical, optical, electrical, and chemical properties can be achieved with carbon by controlling its structure and surface chemistry. The goal of this article is to help readers appreciate the variety of carbon nanomaterials and to describe some engineering applications of the most important of these. Many different materials are needed to meet a variety of performance requirements, but they can all be built of carbon. Considering the example of supercapacitor electrodes, zero- and one-dimensional nanoparticles, such as carbon onions and nanotubes, respectively, deliver very high power because of fast ion sorption/desorption on their outer surfaces. Two-dimensional (2D) graphene offers higher charge/discharge rates than porous carbons and a high volumetric energy density. Three-dimensional porous activated, carbide-derived, and templated carbon networks, with high surface areas and porosities in the angstrom or nanometer range, can provide high energy densities if the pore size is matched with the electrolyte ion size. Finally, carbon-based nanostructures further expand the range of available nanomaterials: Recently discovered 2D transition-metal carbides (MXenes) have already grown into a family with close to 20 members in about four years and challenge graphene in some applications.
Multiferroic magnetoelectric nanostructures with coupled magnetization and electric polarization across their interfaces have stimulated intense research activities over the past decade. Such interface-based magnetoelectric coupling can be exploited to significantly improve the performance of many devices such as memories, tunable radio-frequency/microwave devices, and magnetic sensors. In this article, we introduce a number of current or developing technologies and discuss their limitations. We describe how the use of magnetoelectric nanostructures can overcome these limitations to optimize device performance. We also present challenges that need to be addressed in pursuing practical applications of magnetoelectric devices.
Three-dimensional (3D) printing represents the direct fabrication of parts layer-by-layer, guided by digital information from a computer-aided design file without any part-specific tooling. Over the past three decades, a variety of 3D printing technologies have evolved that have transformed the idea of direct printing of parts for numerous applications. Three-dimensional printing technology offers significant advantages for biomedical devices and tissue engineering due to its ability to manufacture low-volume or one-of-a-kind parts on-demand based on patient-specific needs, at no additional cost for different designs that can vary from patient to patient, while also offering flexibility in the starting materials. However, many concerns remain for widespread applications of 3D-printed biomaterials, including regulatory issues, a sterile environment for part fabrication, and the achievement of target material properties with the desired architecture. This article offers a broad overview of the field of 3D-printed biomaterials along with a few specific applications to assist the reader in obtaining an understanding of the current state of the art and to encourage future scientific and technical contributions toward expanding the frontiers of 3D-printed biomaterials.
Complex oxides provide an ideal playground for exploring the interplay among the fundamental degrees of freedom: structural (lattice), electronic (orbital and charge), and magnetic (spin). In thin films and heterostructures, new states of matter can emerge as a consequence of such interactions. Over the past decade, the ability to synthesize self-assembled nanocomposite thin films of metal oxides has provided another pathway for creating new interfaces and, thus, new physical phenomena. In this article, we describe examples of such materials systems explored to date and highlight the fascinating multifunctional properties achieved. These include enhanced flux pinning in superconductors, strain-enhanced ferroelectricity, strain- and charge-coupled magnetoelectrics, tunable magnetotransport, novel electrical/ionic transport, memristors, and tunable dielectrics.
This article reviews the application of nanomaterials for radiation shielding to protect humans from the hazards of radiation in space. The focus is on protection from space radiation, including galactic cosmic radiation (GCR), solar particle events (SPEs), and neutrons generated from the interactions of the GCR and SPEs with the intervening matter. Although the emphasis is on protecting humans, protection of electronics is also considered. There is a significant amount of work in the literature on materials for radiation shielding in terrestrial applications, such as for neutrons from nuclear reactors; however, the space environment poses additional and greater challenges because the incident particles can have high charges and extremely high energies. For materials to be considered for radiation shielding in space, they should perform more than just the radiation-shielding function; hence the emphasis is on multifunctional materials. In space, there is also the need for materials to be very lightweight and capable of surviving temperature extremes and withstanding mechanical loading. Nanomaterials could play a significant role as multifunctional radiation-shielding materials in space.
Additive manufacturing (also known as 3D printing) is considered a disruptive technology for producing components with topologically optimized complex geometries as well as functionalities that are not achievable by traditional methods. The realization of the full potential of 3D printing is stifled by a lack of computational design tools, generic material feedstocks, techniques for monitoring thermomechanical processes under in situ conditions, and especially methods for minimizing anisotropic static and dynamic properties brought about by microstructural heterogeneity. This article discusses the role of interdisciplinary research involving robotics and automation, process control, multiscale characterization of microstructure and properties, and high-performance computational tools to address each of these challenges. Emerging pathways to scale up additive manufacturing of structural materials to large sizes (>1 m) and higher productivities (5-20 kg/h) while maintaining mechanical performance and geometrical flexibility are also discussed.
Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) such as MoS2, WS2, MoSe2, and WSe2 present an unprecedented excitonic materials family. These materials promise to open up a new age of atomic-scale photonics where devices can be scaled down to the truly atomic level and provide novel functionalities that cannot be obtained with conventional materials systems. Knowledge of the exciton dynamics in these materials is key to the development of the photonic devices. This article reviews recent studies on the excitonic properties of 2D TMDCs and the strategies used to manipulate the exciton dynamics. It also highlights many important scientific questions that remain to be answered for the realization of atomic-scale photonics.
Phase-change materials (PCMs) are promising candidates for novel data-storage and memory applications. They encode digital information by exploiting the optical and electronic contrast between amorphous and crystalline states. Rapid and reversible switching between the two states can be induced by voltage or laser pulses. Here, we review how density-functional theory (DFT) is advancing our understanding of PCMs. We describe key DFT insights into structural, electronic, and bonding properties of PCMs and into technologically relevant processes such as fast crystallization and relaxation of the amorphous state. We also comment on the leading role played by predictive DFT simulations in new potential applications of PCMs, including topological properties, switching between different topological states, and magnetic properties of doped PCMs. Such DFT-based approaches are also projected to be powerful in guiding advances in other materials-science fields.
Two-dimensional (2D) transition-metal dichalcogenides (TMDs) consist of over 40 compounds. Complex metal TMDs assume the 1T phase where the transition-metal atom coordination is octahedral. The 2H phase is stable in semiconducting TMDs where the coordination of metal atoms is trigonal prismatic. Stability issues have hampered the study of interesting phenomena in two-dimensional 1T phase TMDs. Phase conversion in TMDs involves transformation by chemistry at room temperature and pressure. It is possible to convert 2H phase 2D TMDs to the 1T phase or locally pattern the 1T phase on the 2H phase. The chemically converted 1T phase 2D TMDs exhibit interesting properties that are being exploited for catalysis, source and drain electrodes in field-effect transistors, and energy storage. We summarize the key properties of 2D 1T phase TMDs and their applications as electrodes for energy and electronics.
Carbon nanotubes (CNTs) have captured the imagination of the research community because of their many superior properties. In the nearly 25 years since their novelty was recognized, however, progress toward their utility as superlightweight structural materials, especially for aerospace applications, has been disappointing. Recent advancements have revived some of the anticipation for the touted systems payoffs. The purpose of this article is to examine how close CNTs have come to fulfilling expectations for lightweight aerospace structures in the two decades since the initial report stimulated intense interest in this material. This article also proposes areas of study to bridge knowledge gaps that can realize the potential for these CNT composites to be part of the lightweight structures technology suite for aerospace use.
The global automotive industry is facing challenges in several key areas, including energy, emissions, safety, and affordability. Lightweighting is one of the key strategies used to address these challenges. Maximizing the weight reduction (i.e., minimizing vehicle weight) requires a systems-engineering design optimization and iteration process that combines material properties and manufacturing processes to meet product requirements at the lowest mass and/or cost. Advanced high-strength steels, aluminum and magnesium alloys, and carbon-fiber-reinforced polymers have emerged as important materials for automotive lightweighting. This article presents examples of how coupling materials science with innovative manufacturing processes can provide lightweight solutions in automotive engineering.
Efficient, reproducible, and precise methodologies for fabricating tissue engineering (TE) scaffolds using three-dimensional (3D) printing techniques are evaluated. Fusion deposition modeling, laser sintering, and photo printing each have limitations, including the materials that can be used with each printing system. However, new and promising resorbable materials are surfacing as alternatives to previously studied resorbable TE materials for 3D printing. One such resorbable polymer is poly(propylene fumarate) (PPF), which can be printed using photocross-linking 3D printing. The ability to print new materials opens up TE to a wide range of possibilities not previously available. The ability to control precise geometries, porosity, degradation, and functionalities present on 3D printable polymers such as PPF shows a new layer of complexity available for the design and fabrication of TE scaffolds.
Three-dimensional (3D) integration has emerged as a potential solution to the wiring limits imposed on chip performance, power dissipation, and packaging form factor beyond the 14 nm technology node. In 3D integrated circuits (ICs), the through-silicon via (TSV) is a critical element connecting die-to-die in the integrated stack structure. The thermal expansion mismatch between copper (Cu) vias and silicon (Si) can induce complex stresses in TSV structures to drive interfacial failure and Cu extrusion, degrading the performance and reliability of 3D interconnects. This article reviews current studies on thermal stresses and their effects on reliability of TSV structures. Recent results from measurements of stress and plasticity characteristics of Cu TSV structures are reviewed, including wafer curvature, micro-Raman spectroscopy, and synchrotron x-ray microdiffraction techniques. The effects of the Cu microstructure on stress and reliability, particularly on via extrusion and the device keep-out zone in TSV structures, are discussed. Based on the analysis of the reliability impact, we explore the potential of material and processing optimization to build reliable TSV structures for 3D ICs.
Three-dimensional powder printing (3DP) is attractive for the direct fabrication of bioceramic implants and scaffolds from a computer aided design file for bone tissue engineering by localized deposition of a reactive binder liquid onto thin powder layers. This article reviews recent findings on novel material developments for the three-dimensional (3D) printing process using either sintering regimes or cement setting reactions. Customized ceramic implants can be fabricated by 3DP using computer tomography data obtained from a patient, whereas further drug modification of such implants can be achieved either in situ or post-printing. The excellent biological in vitro and in vivo behavior of 3D-printed bioceramics together with processing at ambient conditions may give the opportunity to directly produce cell-seeded patient-specific implants for accelerated and enhanced bone regeneration in the future.
This article reviews the development of SiC and GaN devices for power-switching applications in the context of four specifically identified application requirements: (1) high-blocking voltage, (2) high-power efficiency, (3) high-switching speed, and (4) normally OFF operation. Specific device and material characteristics, such as ON resistance, parasitic capacitances, and energy-gap values, are compared and discussed in relation to the identified application requirements. Following a review of the fundamental limitations of silicon as a material, this article describes the material advantages that motivated the development of commercially available Schottky diodes and transistors using SiC. The last section analyzes the potential of GaN to enable further technical progress beyond the theoretical limit of Si and to significantly reduce the cost of power-electronic switches.