Nanostructures can be in the form of nanoparticles or nanograins, nanowires or nanotubes, and nanoplates or multilayers. These nanostructures may be used individually or embedded in a bulk material. In both cases, they share two common features. First, the small dimensions minimize or even eliminate the presence of defects. Second, nanostructures entail large surface or interface areas. The absence of defects makes nanostructure materials stronger than their bulk counterparts, leading to the eventual realization of ideal strength. The presence of surfaces and interfaces may either reduce or increase the strength. Atomistic simulations can provide insight into the deformation mechanism at the atomic and electronic level, something that is very difficult to obtain from experiments. This article describes generic features of nanostructures and summarizes the five areas presented in the articles in this issue.

Many properties of functional materials are quite different at the nanoscale, because at this length scale, the surface/interface energy becomes comparable to the bulk energy. Thus, the nature of various phase transitions at the nanoscale (e.g., structural, electronic, magnetic, metal-insulator) is altered. In addition, in functional materials with many coupled order parameters, quantum effects can dominate the response. We use the term nanoscale with three different context-specific connotations: it could refer to a cluster of atoms or molecules, a confined geometry as in a nanoscale grain or a superlattice, and a nanoscale region in the bulk. This field is still in its infancy, and much needs to be learned in terms of nucleation and thermodynamics at this scale. Materials of interest that we consider in this issue include, but are not limited to, ferroics (ferroelectrics, ferromagnets, ferroelastics), multiferroics (magnetoelectrics, ferrotoroidics), and complex functional materials such as those that exhibit colossal magnetoresistance and high-temperature superconductivity, including the recently discovered iron pnictide superconductors. Superconductors provide a fertile ground for quantum phase transitions.

Atom-probe tomography (APT) is in the midst of a dynamic renaissance as a result of the development of well-engineered commercial instruments that are both robust and ergonomic and capable of collecting large data sets, hundreds of millions of atoms, in short time periods compared to their predecessor instruments. An APT setup involves a field-ion microscope coupled directly to a special time-of-flight (TOF) mass spectrometer that permits one to determine the mass-to-charge states of individual field-evaporated ions plus their x-, y-, and z-coordinates in a specimen in direct space with subnanoscale resolution. The three-dimensional (3D) data sets acquired are analyzed using increasingly sophisticated software programs that utilize high-end workstations, which permit one to handle continuously increasing large data sets. APT has the unique ability to dissect a lattice, with subnanometer-scale spatial resolution, using either voltage or laser pulses, on an atom-by-atom and atomic plane-by-plane basis and to reconstruct it in 3D with the chemical identity of each detected atom identified by TOF mass spectrometry. Employing pico- or femtosecond laser pulses using visible (green or blue light) to ultraviolet light makes the analysis of metallic, semiconducting, ceramic, and organic materials practical to different degrees of success. The utilization of dual-beam focused ion-beam microscopy for the preparation of microtip specimens from multilayer and surface films, semiconductor devices, and for producing site-specific specimens greatly extends the capabilities of APT to a wider range of scientific and engineering problems than could previously be studied for a wide range of materials: metals, semiconductors, ceramics, biominerals, and organic materials.

Despite recent progress in the treatment of cancer, far too many cases are still diagnosed only after tumors have metastasized. As a result, patients with cancer face a grim prognosis and often need to endure toxic and uncomfortable whole-body chemotherapy and/or other radiation treatments with the hope that their cancers will be eliminated. If the disease can be detected early enough, statistics have shown that the burden of cancer is drastically reduced. Nanotechnology applied to cancer, by way of nanofunctional materials, is in a unique position to significantly transform the way the disease is diagnosed, imaged, and treated and is the focus of this issue of MRS Bulletin. Materials research in nanotechnology is already successfully implemented in several applications. For instance, photocatalysis using TiO2 nanoparticles is becoming the dominant method for the “self-cleaning” of material surfaces such as glass, ceramics, and fabrics. The nanomaterial carbon nanotubes is a promising candidate in sensor technology and field-emission technology. Our goal is to illustrate the promising new methods being developed in the research community and the challenges that need to be overcome in order to reach clinical utility. More importantly, we hope this issue helps educate and invoke the materials science community to tackle some of the hard issues in diagnosing and treating this disease.

Coupling between mechanical and electrical phenomena is ubiquitous at the nano-and molecular scales, with examples ranging from piezoelectricity and flexoelectricity in perovskites to complex molecular transformations in redox active molecules and ion channels. This article delineates the field of nanoelectromechanics enabled by recent advances in scanning probe, indentation, and interferometric techniques and provides a unified outlook at a number of related topics, including membrane and surface flexoelectricity, local piezoelectricity in ferroelectrics and associated devices, and electromechanical molecular machines. It also summarizes experimental and theoretical challenges on the pathway to visualize, control, and manipulate electromechanical activity on the nanoscale and molecular levels.

Over the years, many new materials have been introduced in advanced complementary metal oxide semiconductor (CMOS) processes in order to continue the trend of reducing the gate length and increasing the performance of CMOS devices. This is clearly evidenced in the International Technology Roadmap for Semiconductors (ITRS), which indicates the requirements and technological challenges in the microelectronics industry in various technology nodes. Every new technology node, characterized by the minimal device dimensions that are used, has required innovations in new materials and transistor design. The introduction of deposited high-κ gate dielectrics and metal gates as replacements for the thermally grown SiO2 and poly-Si electrode was a major challenge that has been met in the transition toward the 32 nm technology node since it replaced the heart of the metal oxide semiconductor structure. For the next generation of technology nodes, even bigger hurdles will need to be overcome, since new device structures and high-mobility channel materials such as Ge and III–V compounds might be needed, according to the ITRS roadmap, to meet the power and performance specifications of the 16 nm CMOS node and beyond. The basic properties of these high-mobility channel materials and their impact on the device performance have to be fully understood to allow process integration and full-scale manufacturing. In addition to thermal stability, compatibility with other materials, electronic transport properties, and especially the passivation of electronically active defects at the interface with a high-κ dielectric, are enormous challenges. Many encouraging results have been obtained, but the stringent demands in terms of electrical performance and oxide thickness scaling needed for highly scaled CMOS devices are not yet fully met. Other areas where breakthroughs will be needed are the formation of low-resistivity contacts, especially on III–V materials, and III–V materials suited for pMOS channels. An overview of the major successes and remaining critical issues in the materials research on high-mobility channel materials for advanced CMOS devices is given in this issue of MRS Bulletin.

Recalling some of the progress that has been made in understanding the mechanical properties of materials over the past 50 years or so reveals the importance of remembering and applying old lessons when addressing new opportunities in materials research. Often, the classical lessons of the past are especially useful as a guide for thinking about new problems. Such an approach to new problems is intimately connected to the creation of simple models that capture the essential features of the phenomena involved. Experience shows that, although such efforts might not pay off immediately, they come to be useful many years later when new problems are confronted. The merit of applying old lessons to new problems is described herein by using examples from the author's career in characterizing and understanding the mechanical properties of materials. It is hoped that these lessons are sufficiently general to be applied to other areas of materials research. Problems ranging from the high-temperature creep resistance of titanium aluminides, to the residual stresses in deposited thin films, to diffusive relaxation processes in thin films, to the size dependence of the strength of crystalline materials at the nanometer scale, all provide examples of how applying lessons of the past can help to understand new problems. An effort is also made to identify new, emerging problems in materials research where the application of the lessons of the past, together with new capabilities of the future, can come together to produce a fresh understanding of material behavior.

The growth of high-quality single crystals remains a challenging endeavor of materials science. Crystals of suitable size (from fiber crystals with diameters of tens of micrometers up to crystalline ingots or blocks with volumes up to 1 m3) and perfection (free from precipitates, inclusions, and twins with good uniformity and low concentration of dislocations) are required for fundamental research and practical implementation in microelectronic circuits, electro-optic switches and modulators, solid-state lasers, light-emitting diodes, sensors, and many other devices. In this introductory article of this issue of MRS Bulletin, we describe the two main challenges of today's crystal growth, namely (1) the production of well-established crystalline materials with improved structural perfection and larger size at a lower cost and (2) the bulk growth of new categories of materials with extreme thermodynamic characteristics, such as a very high melting point, high melting dissociation pressure, incongruent phase diagram, and anisotropic segregation. The subsequent six articles provide examples of how the crystal growers took up these challenges, which led to new experimental approaches and technological advances.

GaN and its alloys with InN and AlN are materials systems that have enabled the revolution in solid-state lighting and high-power/high-frequency electronics. GaN-based materials naturally form in a hexagonal wurtzite structure and are naturally grown in a (0001) c-axis orientation. Because the wurtzite structure is polar, GaN-based heterostructures have large internal electric fields due to discontinuities in spontaneous and piezoelectric polarization. For optoelectronic devices, such as light-emitting diodes and laser diodes, the internal electric field is generally deleterious as it causes a spatial separation of electron and hole wave functions in the quantum wells, which, in turn, likely decreases efficiency. Growth of GaN-based heterostructures in alternative orientations, which have reduced (semipolar orientations) or no polarization (nonpolar) in the growth direction, has been a major area of research in recent years. This issue highlights many of the key developments in nonpolar and semipolar nitride materials and devices.

The phenomenon of ion migration in solids forms the basis for a wide variety of electrochemical applications, ranging from power generators and chemical sensors to ionic switches. Solid-state ionics (SSI) is the field of research concerning ionic motions in solids and the materials properties associated with them. Owing to the ever-growing technological importance of electrochemical devices, together with the discoveries of various solids displaying superior ionic conductivity at relatively low temperatures, research activities in this field have grown rapidly since the 1960s, culminating in “nanoionics”: the area of SSI concerned with nanometer-scale systems. This theme issue introduces key research issues that we believe are, and will remain, the main research topics in nanoionics and SSI during the 21st century. These include the application of cutting-edge experimental techniques, such as secondary ion mass spectroscopy and nuclear magnetic resonance, to investigate ionic diffusion in both bulk solids and at interfaces, as well as the use of atomic-scale modeling as a virtual probe of ionic conduction mechanisms and defect interactions. We highlight the effects of protonic conduction at the nanometer scale and how better control of interfaces can be employed to make secondary lithium batteries based on nanoionics principles. Finally, in addition to power generation and storage, the emergence of atomic switches based on cation diffusion shows great promise in developing next-generation transistors using SSI.

Porous metals and ceramic materials are of critical importance in catalysis, sensing, and adsorption technologies and exhibit unusual mechanical, magnetic, electrical, and optical properties compared to nonporous bulk materials. Materials with nanoscale porosity often are formed through molecular self-assembly processes that lock in a particular length scale; consider, for instance, the assembly of crystalline mesoporous zeolites with a pore size of 2–50 nm or the evolution of structural domains in block copolymers. Of recent interest has been the identification of general kinetic pattern-forming principles that underlie the formation of mesoporous materials without a locked- in length scale. When materials are kinetically locked out of thermodynamic equilibrium, temperature or chemistry can be used as a “knob” to tune their microstructure and properties. In this issue of the MRS Bulletin, we explore new porous metal and ceramic materials, which we collectively refer to as “hard” materials, formed by pattern-forming instabilities, either in the bulk or at interfaces, and discuss how such nonequilibrium processing can be used to tune porosity and properties. The focus on hard materials here involves thermal, chemical, and electrochemical processing usually not compatible with soft (for example, polymeric) porous materials and generally adds to the rich variety of routes to fabricate porous materials.

Nuclear energy holds the promise to provide vast amounts of reliable baseline electricity at commercially competitive costs with modest environmental impact. However, the future of nuclear energy lies beyond the current generation of light water reactors. Future reactors will be expected to provide additional improvements in safety, maintain high reliability, use uranium resources more efficiently, and produce lower volumes of less toxic solid wastes. Several advanced reactor concepts are under development to meet these demands. In most cases, these designs translate into higher operating temperatures and longer lifetimes, more corrosive environments, and higher radiation fields in which materials must reliably perform. This issue focuses on the materials challenges that will determine the feasibility of these advanced concepts and define the long-term future of nuclear power.

Nanoporous metals (NPMs) made by dealloying represent a class of functional materials with the unique structural properties of mechanical rigidity, electrical conductivity, and high corrosion resistance. They also possess a porous network structure with feature dimensions tunable within a wide range from a few nanometers to several microns. Coupled with a rich surface chemistry for further functionalization, NPMs have great potential for applications in heterogeneous catalysis, electrocatalysis, fuel cell technologies, biomolecular sensing, surface-enhanced Raman scattering (SERS), and plasmonics. This article summarizes recent advances in some of these areas and, in particular, we focus on the discussion of microstructure, catalytic, and optical properties of nanoporous gold (NPG). With advanced electron microscopy, three-dimensional tomographic reconstructions of NPG have been realized that yield quantitative characterizations of key morphological parameters involved in the intricate structure. Catalytic and electrocatalytic investigations demonstrate that bare NPG is already catalytically active for many important reactions such as CO and glucose oxidation. Surface functionalization with other metals, such as Pt, produces very efficient electrocatalysts, which have been used as promising fuel cell electrode materials with very low precious metal loading. Additionally, NPG and related materials possess outstanding optical properties in plasmonics and SERS. They hold promise to act as highly active, stable, and economically affordable substrates in high-performance instrumentation applications for chemical inspection and biomolecular diagnostics. Finally, we conclude with some perspectives that appear to warrant future investigation.

Secondary ion mass spectrometry (SIMS) is a powerful analytical technique for determining elemental and isotopic distributions in solids. One of its main attractions to researchers in the field of solid-state ionics is its ability to distinguish between isotopes of the same chemical element as a function of position in a solid. With enriched stable isotopes as diffusion sources, this allows self-diffusion kinetics in solids to be studied. In this article, taking oxygen isotope diffusion in oxides as our main example, we present the standard experimental method, and, subsequently, we discuss several promising developments, in particular the opportunities offered by thin-film geometries, and the investigation of inhomogeneous systems, including possible fast diffusion along grain boundaries and making space-charge layers at interfaces “visible.” These examples demonstrate that SIMS is capable of probing mass transport processes over various length scales, ranging from some nanometers to hundreds of micrometers.

The remarkable progress in nonpolar and semipolar devices based on gallium nitride (GaN) in recent years has been driven by not only advancements in the epitaxial growth technique but also improvements in the quality of bulk nonpolar and semipolar GaN substrates. At present, high-quality nonpolar/semipolar substrates are only made by slicing thick bulk GaN crystals grown by hydride vapor-phase epitaxy (HVPE). Although HVPE is currently the most successful method for obtaining high-quality bulk GaN crystals, it is still difficult to obtain uniform crystals with large diameters and thicknesses. The size of the nonpolar/semipolar substrates has been limited by the growth thickness along the c-axis of bulk GaN crystals. Here we review the growth of bulk GaN crystals by HVPE to achieve high-quality and large-sized nonpolar and semipolar substrates.

Bridgman or vertical gradient freeze (VGF) crystal growth processes have several advantages compared to other melt growth methods, especially the possibility to achieve a low level of thermal stress and low dislocation densities in the grown crystals. However, crystals grown in contact with a crucible usually suffer from mechanical stress during cooling, reducing the structural quality. The “detached” or “dewetted” Bridgman growth avoids this problem and has recently been investigated in more detail as a promising tool to improve crystal quality. Detached growth, where the crystal is separated from the crucible wall by a gap of 10–100 μm, was found originally in some microgravity experiments going back to 1975. Considerable improvements of crystal quality were reported for those cases; however, the reasons for the detachment were not fully understood. In the last 10–15 years, theoretical investigations as well as new experiments have shown beyond a doubt that detached growth can, in principle, be achieved in Earth's gravity with the same advantages that were demonstrated in the crystals grown under microgravity. It could be shown that the ability to achieve detachment depends on a complex interplay of the wetting of the melt with the crucible and the crystal as well as the pressure balance in the system, including the hydrostatic pressure, the gas pressure above the melt, and the pressure below the melt. It turns out that for stable detachment, only, specific combinations of meniscus shape, gap size, wetting angle, growth angle, and pressures work. The conditions that lead to detachment are thus highly specific for a given system.

Recent experiments on nanoscale materials, including nanowires, nanopillars, nanoparticles, nanolayers, and nanocrystals, have revealed a host of “ultra-strength” phenomena, defined by stresses in the material generally rising up to a significant fraction of the ideal strength—the highest achievable strength of a defect-free crystal. This article presents an overview of the strength-controlling deformation mechanisms and related mechanics models in ultra-strength nanoscale materials. The critical role of the activation volume is highlighted in understanding the deformation mechanisms, as well as the size, temperature, and strain rate dependence of ultra strength.

In the past two decades, the fact that “small is different” has been established for a wide variety of phenomena, including electrical, optical, magnetic, and mechanical behavior of materials. However, one largely untapped but potentially very important area of nanoscience involves the interplay of electricity and mechanics at the nanoscale. In this article, predicated on both phenomenological approaches and atomistic calculations, we summarize the state-of-the-art in understanding electromechanical coupling at the nanoscale. First, we address flexoelectricity—the coupling of strain gradient to polarization. Flexoelectricity exists in both piezoelectric and nonpiezoelectric dielectrics. As a high-order spatial-dispersion effect, the flexoelectricity becomes more and more important with the reduction of the spatial scale of the problem. Exploitation of this phenomenon and the associated nanoscale size effects can lead to tantalizing applications, such as “piezoelectric nanocomposites without using piezoelectric materials.” The second issue concerns electromechanical effects at the dielectric/metal interface. An interface in solids typically exhibits a lower symmetry compared to that of the associated adhering materials. This symmetry reduction can drastically affect the electromechanical and dielectric behavior of the material at the nanoscale.

The prospect of utilizing alternative transistor channel materials for ultrahigh performance transistors will require suitable gate dielectrics for surface-channel field-effect devices. With the utilization of deposited gate dielectrics in large-scale production for Si-based integrated circuits by atomic layer deposition, extending this technology to channel materials that exhibit high bulk mobility behavior is of interest. A review of the current status for atomic layer deposited high-κ dielectrics on Ge and III–V channel materials is presented.

Nanoparticles with a gold shell and iron core have unique optical and magnetic properties that can be utilized for simultaneous detection and treatment strategies. Several nanoparticles have been synthesized and show an ability to mediate a variety of potential applications in biomedicine, including cancer molecular optical and magnetic resonance imaging, controlled drug delivery, and photothermal ablation therapy. However, to be effective, these nanoparticles must be delivered efficiently into their targets. In this review, we will provide an updated summary of the gold-shelled magnetic nanoparticles that have been synthesized, methods for characterization, and their potential for cancer diagnosis and treatment. We will also discuss the biological barriers that need to be overcome for the effective delivery of these nanoparticles. The desired nanoparticle characteristics needed to evade these biological barriers, such as size, shape, surface charge, and surface coating are also explained.