In recent years, tremendous interest has been generated in the fabrication and characterization of nanoscale structures such as quantum dots and wires. For example, there is interest in the electronic, magnetic, mechanical, and chemical properties of materials with reduced dimensions. In the case of nanoscale semiconductors, quantum effects are expected to play an increasingly prominent role in the physics of nanostructures, and a new class of electronic and optoelectronic devices may be possible. In addition to new and interesting physics, the formation and characterization of nanoscale magnetic structures could result in higher-density storage capacity in hard disks and optical-recording media. Likewise, phonon confinement leads to a drastic reduction of thermal conductivity and can be used to improve the performance of thermoelectric devices.

In 1980, H. Sakaki predicted theoretically that quantum wires may have applications in high-performance transport devices, due to their sawtoothlike density of states (E1/2), where E is the electron energy. Since then, most quantum wires have been made by fabricating a gratinglike gate on top of a two-dimensional (2D) electron gas contained in a semiconductor heterojunction or in metal-oxide-semiconductor structures. By applying a negative gate voltage to the system, its structure can be changed from a 2D to a one-dimensional (1D) regime, where electron confinement is achieved by an electrostatic confining potential. It was not until recently that “physical” semiconductor quantum wires with the demonstrated 1D confinement by physical boundaries began to be fabricated.

In the past, many unexpected failures of components were due to poor quality control or a failure to calculate—or to miscalculate—the stresses or fatigue stresses a component would experience in service. Today, improved manufacturing, fracture mechanics, and computational finite element methods combine to provide a solid framework for reducing safety factors, enabling leaner design. In this context, residual stress—that is, stress that equilibrates within the structure and is always present at some level due to manufacturing—presents a real problem. It is difficult to predict and as hard to measure. If unaccounted for in design, these stresses can superimpose upon in-service stresses to result in unexpected failures.

Neutron diffraction is one of the few methods able to provide maps of residual stress distributions deep within crystalline materials and engineering components. Neutron strain scanning, as the technique is called, is becoming an increasingly important tool for the materials scientist and engineer alike. Point, line-scan, area-scan, and full three-dimensional (3D) maps are being used to design new materials, optimize engineering processes, validate finite element modeis, predict component life, and diagnose engineering failures.

Vapor-phase processes such as chemical vapor deposition (CVD) and reactive ion etching are the primary methods for the production-scale synthesis and processing of many high-quality thin-film materials. For example, these processes are widely used in the microelectronics industry for synthesis and lithography of the various semiconducting, insulating, and conducting layers in devices. Understanding the means of controlling the microstructure and composition of these materials is of great technological interest. However a difficulty often encountered in developing vapor-phase processes is an undesirable dependence on trial-and-error methods for optimizing the many process parameters. These parameters include gas composition, flow rate, pressure, and substrate temperature, all of which are typically changing with time. This reliance on empirical methods can be attributed to the tremendous chemical and physical complexity of vapor-phase processes and the lack of appropriate in situ measurement techniques for the vapor-phase environment.

We have initiated a program to apply synchrotron x-ray analysis techniques as real-time probes of film and surface structure during vapor-phase processing. X-rays have a combination of properties which makes them particularly well-suited for these studies. Unlike electrons, x-rays have a sufficiently low absorption to penetrate vapor-phase processing environments and chamber walls. Unlike visible light, x-rays have wavelengths and energies suitable for study of atomic-scale structure and chemistry. A growing number of in situ synchrotron x-ray investigations of film growth and processing demonstrate the power of these techniques.

The corrosion properties of alloys are of enormous practical importance: modern life would be very different without stainless steels. Alloy corrosion is also an intriguing field of scientific study that combines electrochemical kinetics with fashionable aspects of the morphological evolution of surfaces and even a dash of ancient history, via the studies of Forty and Lechtman on “depletion gilding” practiced by Early Andeans during pre-Columbian times in South America.

The basic alloy corrosion process, as used by the metalsmiths to gold-coat artifacts, is de-alloying. This is defined as the selective electrolytic dissolution of one or more components from a metallic solid solution. For this to happen, there must be a significant difference in the equilibrium metal/metal-ion electrode potentials for the two metals, taking into account any complex ions that might be formed in the electrolyte. For example, we can expect de-alloying in Au-Cu alloys, but not in Au-Pt alloys.

De-alloying shows sharp parting limits, expressed as critical atom percentages of the more reactive component above which that component can be removed from the alloy by electrochemical dissolution in an oxidizing environment such as nitric acid. Parting limits range from about 20 at.% to 60 at.%. This concept is still used in noble metal technology to separate noble metals from base metals. For example, an alloy of 55 at.% gold and 45 at.% silver does not de-alloy, but if it is re-melted with additional silver so that the atom fraction of Ag is greater than 60%, the gold can be separated almost completely by nitric acid immersion.

The capability of membranes to affect differently, both qualitatively and quantitatively, the transport rates of chemical species of dissimilar chemical structure through their interior space renders them attractive for use in many separation problems. Extensive research efforts have thus been undertaken on the preparation and characterization of membrane materials and the study of the transport processes involved in their use in separation applications. The study of the transport of gaseous species through the pore space of porous membranes and the analysis and understanding of the mechanisms that are involved in this process are a very important, if not the most important, element in the development of membranebased separation processes.

The resistance that a gaseous species encounters as it is transported through the pore space of a porous membrane is a function of its molecular properties, of its interaction with the material that makes up the walls of the pores, and of the membrane pore structure. Gaseous transport in pores can take place through various mechanisms, whose contribution to the overall transport rate of a particular species is, in general, determined by the strength of the interactions of the molecules of that species with the pore walls and by the relative magnitudes of three length scales that characterize the molecular size, the distance between pore walls, and the density of the fluid in the pore space.

It is widely recognized that the presence of defects in crystals and other solid materials can have a profound effect upon their chemical and physical properties and, consequently, that defects have a major impact on the practical utility of many technological materials. The presence of defects in a crystalline material implies the presence of disorder, and the extent of such disorder can range from very minor, such as the occurrence of Schottky defects in a crystal of sodium chloride, to maximum disorder, as in an amorphous material. The focus of this overview is on systems that are partially disordered, spanning the range between—but not including—sodium chloride and an amorphous material. Even within this range, the aim is not to be comprehensive, since for space reasons we have restricted our coverage to inorganic materials and hybrid inorganic-organic systems. The choice of this topic stems from both its fundamental and practical importance; it is also a very timely topic. For example, there is a great deal of current interest in complex, partially ordered materials such as the surfactant-mediated mesoporous silicas, biominerals, and hybrid organic-inorganic composites. Research on such materials has presented challenges that cannot easily be addressed by characterization tools that have been developed for well-ordered materials. The same situation is found in other areas such as carbons (including nanotubes) and glassy metal oxides.

Recent reports of very high, reversible adsorption of molecular hydrogen in pure nanotubes, alkali-doped graphite, and pure and alkali-doped graphite nanofibers (GNFs) have aroused tremendous interest in the research community, stimulating much experimental work and many theoretical calculations worldwide. The U.S. Department of Energy (DOE) Hydrogen Plan has seta standard for this discussion by providing a commercially significant benchmark for the amount of reversible hydrogen adsorption. This benchmark requires a system-weight efficiency (the ratio of stored H2 weight to system weight) of 6.5-wt% hydrogen and a volumetric density of 63 kg H2/m. If the encouraging experimental reports (summarized in Table I) are reproducible, it may be possible to reach the goals of the DOE Hydrogen Plan. On the other hand, the community still awaits confirmation of these experimental results by workers in other laboratories. Of additional concern is the fact that theoretical calculations have been unable to identify adsorption mechanisms compatible with the requirements of the DOE Hydrogen Plan.

An economical, safe, hydrogen-storage medium is a critically needed component of a hydrogen-fueled transportation system. Hydrogen storage in a carbon-based material offers further advantages associated with its low mass density. Furthermore, fuel cell technology involving the conversion of hydrogen into protons, or hydrogen and oxygen into electric current, is being vigorously researched for both transportation and small power-plant applications.

Nanocrystalline materials have been attracting rapidly increasing interest in the last decade and have the potential of revolutionizing traditional materials design in many applications via atomic-level structural control to tailor the engineering properties. In addition to interesting physical properties in the areas of magnetics, catalysis, and optics, this class of materials exhibits a broad range of fascinating mechanical behavior. Superplastic deformation behavior has been observed at significantly lower temperatures in ceramic nanoscale powders. Ultrahigh hardnesses have been measured in nanoscale superlattices made of metallic and ceramic materials. Tensile and compressive strengths in nearly all material systems studied have shown anomalously high values at the nanometer-length scale.

The development of nanostructured materials is now raising the question of how the different properties change as the microstructural scale is reduced to nanometer dimensions. Among potential applications of nanostructured materials, the design to achieve optimum mechanical properties is a common concern. Traditionally the mechanical strength σ of crystalline materials is believed to be largely controlled by the grain size d, often in the manner described by the Hall-Petch relationship σ= kd−1/2 +σ0. As the structural scale reduces to the nanometer range, the limits to the conventional descriptions of yielding need to be established, and new mechanisms that may come into play at these very small dimensions need to be explored and studied. In addition the intrinsically high interface-to-volume ratio of the nanostructured materials may enhance interface-driven processes to extend the strain to failure and plasticity. These potential gains will have profound technological impact in a wide range of engineering applications, and need to be validated and exploited.

The discovery and development of new materials is the foundation of the science and technology “food chains.” Examples of new materials with novel properties that have stimulated new scientific questions and/or led to new technologies include liquid crystals, advanced batteries, structural ceramics, dielectrics, ferroelectrics, catalysts, high-temperature superconductors, har dmagnets, and magnetoresistive devices. Establishing the crystal structure of a newly discovered Compound is a mandatory first step, but the most important contribution of diffraction techniques is to provide an understanding of the relationships among chemical composition, crystal structure, and physical behavior. In this way, diffraction experiments provide critical Information for testing theories that explain novel behavior and guide the optimization of new materials to meet the demands of emerging technologies.

The first samples of newly discovered materials are often polycrystalline. With state-of-the-art neutron powder diffraction data and Rietveld refinement techniques, for structures of modest complexity, the precision for atom positions rivals that obtained by single-crystal diffraction. Rietveld refinement is a method of obtaining accurate values for atom positions and other structural parameters from powder diffraction data by least-squares fitting of a calculated model to the full diffraction pattern. As evidence of thi s success, the Inorganic Crystal Structure Database contains 6044 entries from neutron powder diffraction, 7096 from laboratory x-ray powder diffraction, an d 228 from Synchrotron x-ray powder diffraction. Other reasons for the rapidly growing impact of neutron diffraction include the favorable neutron-scattering cross sections for light elements, the sensitivity to magnetic moments, and the ability to penetrate special sample environments for in situ studies. These strengths are widely accepted and have been exploited for many years. Previous reviews have focused on these topics.

Most metals are thermodynamically unstable in aqueous environments. They are kinetically stabilized by the formation of very thin “passive” oxide films that provide protection from corrosion. The technological importance of passive films has led to widespread investigation of their structure and chemistry. However, despite decades of work since Michael Faraday first described the existence of the passive film on iron, the atomic structure of these films is still poorly understood. This is because the films are so thin (typically only a few nanometers thick), making measurements difficult, and because the structure can only be investigated in the (wet) electrochemical environment in which these films form.

The electrochemical nature of the formation of these oxide films is shown in Figure 1, which presents the current response of an iron electrode in an aqueous solution to an applied voltage (potential). As the potential is slowly increased from negative (cathodic) to positive (anodic), the iron first begins to dissolve. This area of dissolution is known as the active region, indicating that the iron is corroding freely as ferrous ions (Fe2+). Then, at a sufficiently anodic (oxidizing) voltage, a thin (30 Å) oxide film forms at the iron surface, preventing further dissolution. This results in a marked decrease in the measured current, a phenomenon known as passivity. Over the passive region, the current is virtually independent of applied voltage and remains extremely low, indicating that the iron is protected from additional oxidation or corrosion.

A substantial and continuous volume expansion from liquid to rarefied vapor occurs by the change of temperature and pressure surrounding the liquid-vapor critical point without crossing the saturated vapor pressure curve. Many investigations have been carried out with molecular fluids including supercritical fluids, and several variations of the equation of state have been advanced since the famous work by van der Waals. In the expansion process the mean interatomic distance increases by up to 10 times that found under standard conditions. However reconstitution of the molecular units or other major structure changes are rare in these fluids. On the other hand, in the expansion process of metallic or semiconducting liquids, physical properties can drastically change. Liquid Hg, well-known as a prototype of liquid metals, undergoes a metal-insulator transition when the density is reduced to about 9 g/cm3 near the critical point.Elemental Se, which units in its liquid form is a semiconductor with a twofold-coordinated chain structure in which atoms are covalently bonded, also experiences a semiconductor-metal-insulator transition around the critical point (critical temperature and pressure data of Se: Tc = 1615°C, Pc = 385 bar, pc = 1.85 g/cm3).

The first indication of the semiconductormetal (SC-M) transition in fluid Se was found in electrical conductivity data by Hoshino and co-workers. Figure 1 shows the contours of constant dc conductivity plotted on the pressure-temperature (P-T) plane. The high-conductivity region appears in the immediate vicinity of the critical point. Another indication of the SC-M transition was obtained from the measurement of the optical reflectivity by Seyer et al. Drude-like energy dependence of the reflectivity spectra was observed at elevated pressures and temperatures.

Recent advances in the control of thin films and surfaces have brought an intriguing question within reach: Is it possible to tailor the electronic properties of solids by controlling them layer by layer or row by row? Customized molecules are commonplace in biochemistry. Can the same idea be brought to bear on solids and electronic materials? Electronic properties of semiconductor devices have been controlled by hetero-structures, quantum wells, and super-lattices. Magnetism as a cooperative phenomenon lends itself to manipulation in small structures, where neighbor atoms can be replaced systematically by species with stronger or weaker magnetism. In fact, a class of magnetic/nonmagnetic multilayers termed spin valves has recently been introduced into commercial read heads for magnetically stored data. The optimum thickness of their active region lies in the single-digit-nanometer regime.

The smallest nanostructures may be viewed as objects consisting only of interfaces with no bulk behind them. More typically, single-digit-nanometer dimensions are sufficient for realizing the benefits of structuring (e.g., operating a quantum-well device at room temperature). This regime is difficult to reach with lithography methods, particularly when macroscopic amounts are to be fabricated. Self-assembly becomes the method of choice.

Cells are a complex topic of study for materials scientists. They are the fundamental building blocks of living organisms, able to sense their environment and act in response to it. In addition to their many biochemical functions, cells also play a mechanical role: They hold organs in place and move to the locations where they are needed in processes like wound healing, metastasis, or embryogenesis. Their mechanical behavior is mostly determined by a meshwork of three types of connected biopolymers (actin microfilaments, microtubules, and intermediate filaments) that compose a structural framework called the cytoskeleton, surrounded by a lipid membrane (Figure 1). In contrast to this simple picture, cells are very different from polymer gels or liposomes: They are active materials, powered by chemically stored energy. Their mechanical condition is closely linked to their biochemical function; for example, they may “commit suicide,” following a well-defined protocol known as apoptosis, which can be triggered by their mechanical state.

The enormous progress of modern cell biology combined with new micromanipulation techniques is leading researchers toward a more global understanding of the mechanical properties of cells and toward finding a functional link between biochemistry, chemical signaling, and cell mechanics, thus crossing the boundaries between these subjects.

The characterization of cell mechanical behavior has been the object of numerous studies. Red blood cells are a simple model system; if deprived of a nucleus while retaining a constant surface area, they have properties reminiscent of lipid vesicles.

Amorphous chalcogenides, of which selenium is the simplest representative, exhibit a number of unique properties such as the ability to undergo various transformations under the action of the bandgap light. On illumination, the absorption edge shifts to lower energies, and subsequent annealing near the glass-transition temperature leads to a recovery of the initial parameters as demonstrated in Figure 1. Such a photo-induced change could not be observed either in amorphous group IV semiconductors or a-As or in crystalline chalcogenides. A good review of the initial stage of these studies was made by de Neufville.

Reversible photodarkening can also be observed in pure chalcogens, but this process can be achieved only at lower temperatures, which is understandable if one takes into account that the glass-transition temperature of selenium is just above room temperature. This result indicates that there is a correlation between the temperature at which photodarkening is annealed out and the flexibility of the glassy network.

Reversible changes in the optical absorption are accompanied by (reversible) changes in electrical and photoelectric properties, volume, microhardness, glass-transition temperature, and dissolution rate in various solvents, to name a few. The totality of these changes has led investigators to the conclusion that the photoinduced changes in optical absorption are caused by changes in structure.

Flywheels are mechanical devices that store kinetic energy in a rotating mass. A simple example is the potter's wheel. For energy storage and conversion, an efficient method to exchange energy with a flywheel device is by converting the energy between mechanical and electrical forms. Typically a flywheel designed to perform this type of energy exchange is a combination of a motor and a generator. Energy is transferred into the device for storage by using it as a motor to consume electrical energy and spin the mass. This energy can be recovered with an efficiency exceeding 80% by using the flywheel as an electrical generator. Although the concept of storing energy in a rotating mass is an ancient idea, the relatively recent advent of advanced fiber-composite materials offers the potential for improved energy storage and conversion using rotating electromechanical devices.

The achievable energy density (energy/weight) of a simple flywheel design, such as that shown schematically in Figure 1, is proportional to the specific strength (strength/density) of the material. The particular type of composite flywheel shown in this figure is composed entirely of circumferentially wrapped fiber. Although other designs have been suggested and constructed, this version is the most common for energy-storage applications. As discussed in the earliest papers on the subject, such as the article by Post and Post, the proportionality between energy density and specific strength favors the use of fiber composites. The remarkable combination of mechanical properties and low density achieved with fiber composites has made them attractive and nearly essential for aerospace applications.

A scaling-down of feature sizes into the nanometer range is a common trend in silicon and compound semiconductor advanced devices. That this trend will continue is clearly evidenced by the fact that the “roadmap” for the Si ultralarge-scale-integration circuit (USLI) industry targets production-level realization of a 70-nm minimum feature size for the year 2010. GaAs- and InP-based heterostructure devices such as high-electron-mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs) have made remarkable progress by miniaturization, realizing ultrahigh speeds approaching the THz range with ultralow power consumption. Due to progress in nanofabrication technology, feature sizes of scaled-down transistors are rapidly approaching the Fermi wavelength of electrons in semiconductors, even at the production level. This fact may raise some concerns about the operation of present-day devices based on semiclassical principles.

However, the progress of nanofabrication technology has opened up the exciting possibility of constructing novel quantum devices, based directly on quantum mechanics, by utilizing artificial structures such as quantum wells, wires, and dots. In these structures, new physical effects appear, such as the formation of new quantum states in single and coupled quantum structures, artificial miniband formation in superlattices, tunneling and resonant tunneling in single and multiple barriers, propagation of phase-coherent guided electron waves in quantum wires, conductance oscillations in small tunnel junctions due to single-electron tunneling, and so on. We expect that these effects will offer rich functionality in next-generation semiconductor quantum ULSIs based on artificial quantum structures, with feature sizes in the range of one to a few tens of nanometers. Beyond this, molecular-level ULSIs using exotic materials and various chemical and electrochemical processes other than the standard semiconductor ones may appear, butat present, they still seem to be too far in the future for realistic consideration for industrial applications.