With the electronics packaging industry shifting increasingly to three-dimensional packaging, microbumps have been adopted as the vertical interconnects between chips. Consequently, solder volumes have decreased dramatically, and the solder thickness has reduced to a range between a few and 10 microns. The solder volume of a microbump is approximately two orders of magnitude smaller than a traditional flip-chip joint. In contrast, the thickness of the under-bump metallization (UBM) remains almost the same as that in flip-chip solder joints. Therefore, many issues concerning materials and reliability of microbumps arise. This article reviews the challenges related to microbump materials for vertical interconnects, including transformation of solder joints into intermetallic (IMC) joints, necking or voiding induced by side wetting/diffusion on the circumference of the UBM, formation of porous Cu3Sn IMCs, early electromigration failures caused by specific orientations of Sn grains, and precipitation of plate-like Ag3Sn IMCs. An alternative way of fabricating vertical interconnects using direct Cu-to-Cu bonding is also discussed.

Silicon carbide (SiC) is the next-generation power semiconductor material, enabling an energy-efficient future society by drastically reducing energy loss in existing power electronics systems. SiC metal oxide semiconductor field-effect transistors (MOSFETs) are promising low-loss power-electronics devices. We have developed precisely controlled doping techniques for ultrathin and highly doped SiC epitaxial films and applied this to form the channels of SiC-MOSFETs. This has resulted in the successful formation of diode-integrated MOSFETs (DioMOSs), a structure enabled using SiC semiconductor material. The MOSFET current together with the diode current of the DioMOS is designed to maintain low conduction loss and good temperature stability. The high threshold voltage of MOSFETs and the perfect unipolar conduction with fast switching contribute to safety and low-loss operation. DioMOS offers a single-chip solution for next-generation power electronics compared to the parallel connection using a diode and a transistor in a conventional semiconductor device. DioMOS achieves a small and light direct current converter with high voltage, current, efficiency, and power density applicable to electric and hybrid vehicles.

Selection of materials systems for aerospace applications, such as airframes or propulsion systems, involves multiple and challenging requirements that go beyond essential performance attributes (strength, durability, damage tolerance, and low weight). Materials must exhibit a set of demanding properties, be producible in multiple product forms, and demonstrate consistent high quality. Furthermore, they must be both commercially available and affordable. The list of materials meeting these requirements is not long. Integration and transformation of such highly engineered materials into airframe structures is likewise complex. The Boeing 747, for instance, requires more than 6,000,000 components from numerous materials systems and suppliers worldwide. This necessitates that materials be stable and that material design and structure engineering close on effective solutions simultaneously. High-temperature turbine engines demand strong, lightweight, high-temperature materials balanced by high durability and reliability in a severe service environment. Such applications provide remarkable examples of how engineering imperatives influence materials research and development for metallic and composite materials in terms of material chemistry, fabrication, and microstructure.

Precise control of structural parameters through nanoscale engineering to continuously tailor optical and electronic properties of functional nanomaterials remains an outstanding challenge. Previous work focused largely on chemical or physical interactions that occur under ambient pressures. In this article, we introduce a new pressure-directed assembly and fabrication method that uses a mechanical compressive force applied to nanoparticles (NPs) to induce structural phase transitions and consolidate new nanomaterials with precisely controlled structures and tunable properties. By manipulating NP coupling through external pressure instead of through chemistry, a reversible change in assembly structure and properties can be demonstrated. In addition, over a certain threshold, the external pressure forces these NPs into contact, allowing the formation and consolidation of one- to three-dimensional nanostructures. Through stress-induced NP assembly, unusual materials engineering and synthesis, in which morphology and architecture can be readily tuned to produce desired optical and electrical properties, appear feasible.

To meet the challenge of precise nanoscale arrangement of emitter and plasmonic nanoantenna, synthesis and assembly methods continue to evolve in accuracy and reproducibility. This article reviews some of the many strategies being developed for “soft” chemical approaches to precision integration and assembly. We also discuss investigations of the Purcell effect, emission directionality control, and near-unity collection efficiency of photons, emitter–emitter coupling, and higher-order emission processes that have been most deeply explored using individual-emitter– (or several-emitter–) nanoantenna pairs fabricated using traditional lithographic methods or dynamically and controllably manipulated using scanning probe methods. Importantly, these results along with theoretical analyses inspire and motivate continued advancements in large-scale synthesis and assembly. We emphasize assembly approaches that have been used to create nanosemiconductor–nanometal hybrids and, in particular, those that have afforded specific plasmonic effects on excitonic properties. We also review direct-synthesis and chemical-linker strategies to creating discrete, though less spatially extended, semiconductor–metal interactions.

Efficient structural design and thermal management for aerospace structures demand next-generation lightweight thermally conductive and mechanically robust materials to withstand high-velocity impacts and distribute localized heat fluxes from spacecraft components. Notwithstanding the excellent mechanical, electrical, and thermal properties of individual carbon nanotubes (CNTs), bulk CNT-based composites suffer from CNT anisotropy and high interjunction resistance. We provide a brief overview of scalable methods that can tune electrical and thermal connectivity in bulk CNT composites by tuning CNT shape, intertubular bonding, and packing density. These scalable production methods are posited to open new avenues for incorporating CNTs into thermal interface materials, structural reinforcement, and auxiliary power units in the form of energy-storage devices, especially for use in aerospace applications.

This article describes a new microscope, based on angle-resolved cathodoluminescence (CL) imaging spectroscopy, which enables optical imaging and spectroscopy at deep-subwavelength spatial resolution. We used a free electron beam in a scanning electron microscope as a direct excitation source for polarizable materials, and we collected the emitted coherent visible/infrared CL radiation using a specially designed optical collection system that is integrated in the electron microscope. We have demonstrated the use of this new technique for the excitation of plasmons in single metal nanoparticles, surface plasmon polaritons at metal surfaces, resonant Mie modes in dielectric nanostructures, and cavity modes and Bloch modes in photonic crystals. Using angle-resolved detection, we are able to derive the nature of localized modes and the dispersion of propagation modes in dielectric and plasmonic geometries. An outlook about new directions and applications of CL imaging spectroscopy is also provided.

The field of plasmonics has transformed the ability to control nanoscale light-matter interactions with applications ranging from high-efficiency photovoltaic modules to ultrasensitive biodetectors, electromagnetic cloaks, and subwavelength integrated photonic circuits. This article summarizes my group’s efforts to contribute to this burgeoning field, with emphasis on our research in quantum plasmonics and optical-frequency magnetism. First, we explore the plasmon resonances of individual nanoparticles as they transition from a classical to a quantum-influenced regime. We then utilize these results to directly monitor hydrogen absorption and desorption in individual palladium nanocrystals. Subsequently, using real-time manipulation of plasmonic particles, we investigate plasmonic coupling between pairs of particles separated by nanometer- and angstrom-scale gaps. For sufficiently small separations, we observe the effects of quantum tunneling between particles on their plasmonic resonances. Finally, using the properties of coupled metallic nanoparticles, we demonstrate the colloidal synthesis of an isotropic metafluid or “metamaterial paint” that exhibits a strong optical-frequency magnetic response and the potential for negative permeabilities and negative refractive indices.

High-temperature electronic applications are presently limited to a maximum operational temperature of 225°C for commercial integrated circuits (ICs) using silicon. One promise of silicon carbide (SiC) is high-temperature operation, although most commercial efforts have targeted high-voltage discrete devices. Depending on the technology choice, several processing challenges are involved in making ICs using SiC. Bipolar, metal oxide semiconductor field-effect transistors, and junction field-effect transistor technologies have been demonstrated in operating temperatures of up to 600°C. Current technology performance and processing challenges relating to making ICs in SiC are reviewed in this article.

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.

In situ transmission electron microscopy (TEM) nanomechanical testing has benefited from a number of recent technical developments related to both how deformation is imaged and how deformation is induced and measured inside a TEM instrument. These developments have led to new insights into the deformation mechanisms of a wide range of metals and alloys, as well as measurements of the unusual mechanical properties of small-scale objects such as whiskers and nanocrystals. Herein, we describe this recent progress through selected highlights of recent findings on the dynamic behavior of defects such as dislocations, twins, and grain boundaries.

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.

In his famous 1959 lecture “There’s plenty of room at the bottom,” Richard Feynman put out this challenge: “Is there no way to make the electron microscope more powerful?” He called for “improving the electron microscope by a hundred times,” which, given that the resolution then was about 10 Å, meant he was calling for a resolution in the range of 0.1 Å. Today’s aberration-corrected microscopes have come a long way, achieving a resolution of around 0.5 Å. This has enormously improved our ability to see atomic arrangements in crystals, measure ferroelectric displacements, and even determine valence and spin states with electron energy-loss spectroscopy. However, there remain many structures crucial to materials properties that we cannot yet see. Continuing the road toward Feynman’s goal would bring these structures to light, with yet more dramatic impacts on the entire field of materials science.