An estimated 11 million people in the United States have at least one medical-device implant. Orthopedic implants account for 51.3% of all implants. They include fixation devices (usually fracture fixation) and artificial joints, used in 77% and 23% of the cases, respectively. Among the joint-replacement procedures, hip and knee surgeries represent 90% of the total and in 1988 were performed 310,000 times in this country. Currently more than half the joint-reconstruction devices are used with bone cement, which is a polymer grout that keeps the prosthesis components in place in the bone. The fixation in the other cases depends on the bone's ability to grow in contact with the device. This can be achieved by making the prosthesis surface porous such that bone grows into interstices or by making the surface chemically reactive with bone tissue such that a continuous, uninterrupted transition is formed from tissue to device. Bioactive glasses (BGs) and ceramics are the materials of choice to achieve this effect on bone-tissue bonding. Bone-growth stimulation is also sought in the treatment of difficult fractures. In the United States alone, there are 1.23 million fractures that require a bone plate. Of that total, approximately 1 million require between 10 cm3 and 100 cm3 of graft material to stimulate bone repair. At this time, autogenous bone graft represents the gold standard: This graft is typically bone tissue taken from the patient's own pelvic bone. Given the morbidity associated with this procedure and the frequently insufficient quantities available, extensive efforts for suitable alternatives are currently under way. Calcium phosphate ceramics and glasses, either by themselves or as carriers for bone (or “osteogenic”) cells or various bone-growth factors, are also prime candidates for these applications.

It has been generally believed that hydrogen plays a central role in the various processes that have been developed over the years for the chemical vapor deposition (CVD) of diamond films. In particular it has been thought that atomic hydrogen is an absolutely essential ingredient of the vapor from which the films are grown. Typically in diamond CVD, gas mixtures consisting of l-vol% CH3 in 99-vol% H2 have been used in which atomic hydrogen is generated either by thermal decomposition or by collisional processes in a plasma. With a hydrocarbon precursor such as CH3, gas-phase hydrogen-abstraction reactions lead to the generation of the methyl radical CH3, which adsorbs on a carbon radical site also created by hydrogen abstraction from the hydrogen-terminated growing diamond surface. Additional hydrogen-abstraction reactions allow the carbon in the adsorbed methyl radical to form carbon-carbon bonds and thus be incorporated into the diamond lattice. Because graphite is thermodynamically more stable than diamond, the growth of metastable diamond has been thought to require the presence of atomic hydrogen, which has been said to stabilize the diamond lattice and to remove graphitic nuclei when they do form because of the preferential etching or regasification of graphite over diamond. This description of diamond-film growth from hydrocarbon–hydrogen mixtures is of course a very highly condensed version of the detailed experimental and theoretical work that has been carried out in the field over the years. However the predominant conclusion of most of that work is that, particularly in the absence of oxygen and perhaps halogens, atomic hydrogen plays a crucial and decisive role in diamond CVD.

Two of the most widely employed approaches for enhancing the lifetime and tribological performance of mechanical devices involve the introduction of lubricants and solid/hard coatings to the interface between two contacting surfaces. These approaches have been employed for centuries, and their design has been widely accomplished through empirical methods. In the last 20 years however, the general trend of device miniaturization and the development of advanced technology applications have placed more stringent demands on the performance of many tribological interfaces. For example, many of the hard-disk storage products now shipped in computers rely upon a single layer of lubricant molecules adsorbed on a submicron-thick layer of amorphous carbon to control the static friction and wear between the read/write head and the disk surface. In other advanced technologies, hard-coating materials are introduced into tribological interfaces that will operate in remote or harsh environments to improve the durability and lifetime of the devices. In the previous examples, the introduction of a thin coating or a change in surface composition dramatically influences the tribological properties and indicates that many tribological interfaces can be understood and described on a molecular scale. This article will review recent progress made in understanding the molecular events associated with tribological phenomena from both experimental and theoretical approaches.

In general, advances in experimental studies have been realized through developing a greater sensitivity to the relatively few number of species found within a tribological contact.

Colloids have long been used in applications such as paints, coatings, foods, and many manufacturing processes. Recently, synthetic crystalline arrays of colloidal particles have been used as novel optical materials such as diffractive filters, mimicking the optical properties of opals—natural colloidal crystals made from silica spheres. Colloidal assembly has been proposed to manufacture photonic bandgap materials that can be tailored and that could have many uses in optical devices. The advantages of using colloids to do the self-assembly of novel materials are the relative ease with which monodisperse spheres comparable in size to the wavelength of light can be manufactured and also the demonstrated ease by which some suspensions of monodisperse colloidal spheres crystallize when placed under favorable conditions. Before we can use colloidal crystallization as a controlled self-assembly technique for making novel optical materials, we need (1) to create a means of manufacturing large quantities of monodisperse particles of the desired dielectric behavior, (2) to understand the phase diagram and nucleation phenomena of colloidal suspensions, and (3) to find an easy means to fix the particles in place once they selforganize. In this article, I focus on the second point just mentioned, I give an overview of the phases and some of the complex phenomena encountered in three-dimensional (3D) suspensions and in thin layers of monodisperse colloidal spheres between smooth walls, and I then briefly mention the greater complexity encountered in bidisperse systems. The first and third points will be dealt with elsewhere in this issue.

The stability and precision of modern scanning-tunneling-microscope (STM) systems allow positioning of the tip on a subnanometer scale. This advancement has stimulated diverse efforts on surface modifications in the nanometer and even atomic range, as recently reviewed by Avouris. The lateral movement of individual adatoms and molecules in a controlled manner on solid surfaces and the construction of structures on a nanoscale were first demonstrated by Eigler and collaborators at 4 K. The reason for operating the STM at low temperatures (apart from increased stability and sensitivity of the STM setup itself) is the necessity to freeze the motion of single adsorbates, which are very often mobile at ambient temperatures. By selecting strongly bonded adsorbate/substrate combinations and large molecules, it was possible to extend the lateral manipulation technique even to room temperature. In the case of large molecules, not only their translational motion but also internal flexure of the molecule during the positioning process must be considered. In general, different physical and chemical interaction mechanisms between tip and sample can be exploited for atomic-scale manipulation. We will concentrate in the following on lateral manipulation where solely the forces that act on the adsorbate because of the proximity of the tip are used. This means that long-range van der Waals and short-range chemical forces can be used to move atoms or molecules along the surface. No bias voltage or tunneling current is necessary. Apart from this technique, additional advances using the effects caused by the electric field generated by the bias voltage between tip and sample and by the current flowing through the gap region can be used for atomic or molecular modification.

Quantization effects in semiconductor structures were first demonstrated in the early 1970s in III-V quantum wells; these structures consisted of a thin epitaxial film of a smaller bandgap (Eg) semiconductor (e.g., GaAs, Eg = 1.42 eV) sandwiched between two epitaxial films of a larger bandgap semiconductor (e.g., Al0.3Ga0.7As, Eg = 2.0 eV). The conduction- and valence-band offsets of the two semiconductor materials produce potential barriers for electrons and holes, respectively. The smaller bandgap semiconductor constitutes the quantum-well region and the larger bandgap material the potential barrier region. If the film of the smaller bandgap material is sufficiently thin (thickness less than the de-Broglie wavelength of the charge carriers, which typically requires thicknesses less than about 300 Å for III-V semiconductors), then the charge carriers are confined in one dimension by the potential barriers, and quantization of the energy levels for both electrons and holes can occur (Figure 1).

For the past seven years I have taught the Engineering Materials course (EM 380) at the United States Military Academy (USMA), Department of Civil and Mechanical Engineering (CME). The course has been modified to introduce modern materials technology to the cadets and to provide the opportunity to fabricate, design, test, and evaluate advanced materials in this course, in capstone design projects, and in independent-study projects. Theoretical and experimental studies of sporting equipment have been a sound and compelling basis to develop student ability in applied mechanics and to gain insight into the importance of materials choice. This article describes the methods and results of several of these projects.

The science and technology of ferroelectric thin films is currently attracting worldwide attention because of its application to a new generation of novel devices. Prime, among these applications are nonvolatile ferroelectric random-access memories (NVFRAMs), which have high speed and extended endurance. The core of an NVFRAM is a capacitor with a ferroelectric film sandwiched between two electrode layers. The polarization of the ferroelectric layer in two possible opposite directions, upon application of an electric field between the two electrodes, provides the logic “1” and “0” states needed for binary-code memory. In spite of the advances in the science and technology of ferroelectric thin films and their integration into ferroelectric capacitors, some materials-related integration strategies as well as manufacturability issues have delayed commercialization of NVFRAMs. High-density memories require storage elements that approach submicron lateral dimensions. Thus, improved understanding of the materials properties and polarization phenomena is needed in conjunction with the development of new characterization tools that can enable such an understanding. For example, a fundamental issue in ferroelectric thin-film capacitors is the exact nature of the complex domain structure in the polarizable ferroelectric layer and its dynamics under high-speed switching conditions. The miniaturization of NVFRAMs requires understanding of granularity in polarization reversal dynamics, fatigue, and retention characteristics. In this respect, theoretical models and electrical measurements (e.g., polarization hysteresis loops and transient currents) have provided substantial insights into the nature of the switching processes. However, the models (phenomenological in nature) and the electrical measurements provide only a global or macroscopic view of the switching process.

Scientists and engineers still debate the relative contributions of roughness and interlocking of asperities, adhesion and interatomic forces, and elastic and plastic deformation in friction phenomena. However a central point of agreement does exist: Friction involves energy dissipation, or the conversion of mechanical work into heat. Therefore any mechanism that allows for this energy dissipation during sliding interactions of materials deserves consideration, whether it occurs strictly at the nominal sliding interface or within material at some distance from that interface.

If the interactions are limited to atomic distances, it is entirely appropriate to focus attention on atoms at the two surfaces (see the articles by M.O. Robbins and J. Krim and J.A. Harrison and S.S. Perry in this issue). Friction is then a surface phenomenon. However if the sliding conditions lead to extensive plastic deformation of either material—over distances readily detectable by eye or by optical microscopy—then the associated energy dissipation is likely to account for most of the frictional losses. In such cases, friction involves more than processes occurring at the surface.

Watanabe and Tooi first reported on the formation of silicon oxide by oxygenion implantation into silicon in 1966. Further investigations of such oxygen-implanted oxide layers have been carried out by several workers, with the result that the oxide has equivalent isolation characteristics to those for thermally grown silicon oxide. Practical applications of the oxygen-implanted oxide to semiconductor devices have been reported by only a few workers, with a suggestion of their usefulness. Unfortunately Watanabe and Tooi's work has not been followed by additional silicon-on-insulator (SOI) studies.

In 1978 Izumi, Doken, and Ariyoshi succeeded in fabricating a complementary-metal-oxide-semiconductor (CMOS) ring oscillator using a buried SiO2 layer formed by oxygenion (16O+) implantation into silicon. They named the new SOI technology “SIMOX,” which is short for separation by implanted oxygen. Since then Izumi and his research group have continued their study of SIMOX technology.

Because of their size and ability to selforganize, colloidal particles are ideal building blocks for the creation of three-dimensional (3D) structures that can have feature sizes of the order of the wavelength of electrons, photons, or both. This article is too short to provide an extensive literature survey but instead will give some illustrative examples, based on work of the author and co-workers, of how specially developed core-shell particles might be organized on a 3D lattice. These examples are only intended to give an impression of how colloidal-particle systems can be used in the design of new materials with interesting photonic properties.

Generally particles are considered colloidal if their size is between several nm and several μm. This range is more or less defined by the importance of Brownian motion—that is, the irregular, overdamped, random displacements the particles make as a result of the not completely averaged-out bombardment of solvent (or gas) molecules. Consequently the lower size range is determined by the size of the solvent molecules. Compared to the particle size, the solvent molecules need to be so small that the time scales of the solvent molecules and particles are so far apart that the solvent molecules can be “integrated out” in a description of the particles. If such a description holds, the solvent can be approximated well by a continuum. The upper size limit is determined by the size at which external fields, like gravity, start to overshadow the effects of Brownian motion.

In the absence of an applied magnetic field, magnetorheological (MR) fluids typically behave as nearly ideal Newtonian liquids. The application of a magnetic field induces magnetic dipole and multipole moments on each particle. The anisotropic magnetic forces between pairs of particles promote the head-to-tail alignment of the moments and draws the particles into proximity. These attractive interparticle forces lead to the formation of chains, columns, or more complicated networks of particles aligned with the direction of the magnetic field. When these structures are deformed mechanically, magnetic restoring forces tend to oppose the deformation. Substantial field-dependent enhancements of the rheological properties of these materials result, as demonstrated in Figure 1.

The myriad potential applications of MR and electrorheological (ER) fluids provide considerable motivation for research on these materials. The availability of fluids with yield stresses or apparent viscosities that are controllable over many orders of magnitude by applied fields enables the construction of electromechanical devices that are engaged and controlled by electrical signals and that require few or no moving parts. Potential automotive applications include electrically engaged clutches for vehicle powertrains and engine accessories as well as semiactive shock absorbers that can adapt in real time to changing road conditions. Semiactive dampers for rotorcraft control surfaces are among the potential aerospace applications. The critical need to mitigate the structural vibrations of large structures has led to the construction of large, high-force MR-fluid-based dampers. A promising application in manufacturing processes is the computer-aided polishing of precision optics in which abrasive particles are suspended in an MR fluid so that the polishing rate is determined in part by the strength of an applied magnetic field.

The range of applications for ionically conducting materials in technology is very broad and includes batteries; fuel cells; sensors; electrochromic displays; catalysts, oxygen, and hydrogen pumps; and bionics. In most of these applications, the mobility of the ions is a determining factor for optimum performance. Examples are liquid-electrolyte and plastic Li+ ion batteries for portable electronics, solid oxide fuel cells such as the oxygen ion-conducting stabilized zirconia electrolyte for energy conversion, and exchange of different ions in oxide glasses for the fabrication of micro-optic lenses. Therefore research on the dynamics of ionic transport that clarifies the mechanisms limiting the mobility of the ions is useful. These dynamic processes are also fascinating from a basic research point of view and qualitatively follow general patterns independent of the chemical and physical structures of the materials, suggesting that some fundamental physical mechanism is at work. On a quantitative level, the ionic-transport properties depend on the structures of the materials seemingly in well-defined patterns, which can be used to advantage in choosing materials for specific applications.

Ternary-chalcopyrite crystals contain a variety of point defects—the most common of which are vacancies, antisite ions, and impurities. Usually these defects are isolated, but they can also appear as complexes involving two or more of the simple defects. Depending on the material, the concentrations of these defects may vary from a few hundred parts per billion to a few hundred parts per million. Many of the point defects in the ternary chalcopyrites have associated optical-absorption bands with significant oscillator strengths. It is these absorption features that become important when the crystals are exposed to intense laser beams during device operation. Even a small amount of absorption will seriously degrade the performance of the device if any of the wavelengths of the various propagating beams happen to overlap an absorption band. This phenomenon can be a problem for both second-harmonic-generator and optical-parametric-oscillator applications. In general the absorption leads to heating of the crystal and results in-thermal lensing (due to temperature dependence of the index of refraction) and dephasing of the beams, and it can ultimately lead to thermal fracturing of the crystal. Thus it is important to develop a fundamental understanding of the defect structure of the ternary-chalcopyrite crystals if they are to serve as the critical component in midinfrared frequency-conversion devices. Once the nature and behavior of the point defects are established, processes can be developed to remove the defects from the crystals either during the growth itself or during post-growth treatments.

The achievement of efficient room-temperature light emission from crystalline Si is a crucial step toward the achievement of fully Si-based optoelec-tronics. However Si, the leading semiconductor in microelectronic applications, is unable to perform as well in the optical arena. In fact due to its indirect bandgap, Si does not exhibit efficient light emission and has been considered unsuitable for optoelectronic applications. Several efforts have been dedicated to overcoming this limitation. Among them, luminescence through the incorporation of rare-earth impurities has been considered In particular, erbium doping has been demonstrated as a valid approach toward achievement of efficient light emission from Si.1−43 Erbium is a rare-earth ion that, in its 3+ state, can emit photons at 1.54 μm because of an intra-4f shell transition between the first excited state (4I13/2) and the ground state (4I15/2). This emission is particularly attractive because its wavelength falls inside a window of maximum transmission for the silica optical fibers. When Er ions are inserted within a Si matrix, the excitation (4I15/2 → 4I13/2) can be achieved through the carriers provided by the host, whereas the subsequent deexcitation (4I13/2 → 4I15/2) can result in a sharp, atomlike light emission.

The development and application of semiconductor light-emitting and laser diodes has been a huge success during the last 30 years in key areas of modern technology like communications, recording, and printing. Still there is ample room for improvement through combination of the atomlike properties for zero-dimensionally localized carriers in quantum dots (QDs) with state-of-the-art semiconductor-laser technology. Low, temperature-insensitive threshold current; high gain; and differential gain have been predicted since the early 1980s.

In the past two decades, the fabrication of QDs has been attempted using colloidal techniques (see the article by Nozik and Mićić in this issue), patterning, etching, and layer fluctuations (see the article by Gammon in this issue). However a break-through occurred recently through the employment of self-ordering mechanisms during epitaxy of lattice-mismatched materials (see the next section) for the creation of high-density arrays of QDs that exhibit excellent optical properties, particularly high quantum efficiency, up to room temperature. The zero-dimensional carrier confinement and subsequent atomlike electronic properties have a drastic impact on optical properties (see the section on Spectroscopy). Also intimately connected is the applicability of QDs as a novel gain medium in state-of-the-art laser diodes with superior properties (see the section on Lasers).