This issue of MRS Bulletin is devoted to the subject of supramolecular materials. The term supramolecular is widely used to describe the intentional use of noncovalent interactions to bring about a desired arrangement of molecules. As the articles in this issue illustrate, this type of molecular engineering can provide structural control on the nanoscale and beyond, broadly impacting the properties of the resulting materials. The goal is to create a collection of molecules in which the whole possesses characteristics that are different and unattainable from the individual components.

The enhancement of engineering materials by surface modification has extended the operational envelope for many structures in terms of resistance to corrosion, wear, fatigue, and other forms of surface degradation and has made feasible the introduction of novel materials into high-performance applications. Surface engineering, using thermal-spray coatings, represents a pragmatic and highly costeffective means of satisfying stringent design criteria (e.g., in aerospace applications), operating under extreme environments (e.g., high temperatures, wear, and corrosion), and introducing a multiplicity of functions (e.g., thermal barriers, biomedical implants, and electronic multilayers). Thermal-spray technology as a family of processes is distinguished by its ability to deposit overlays of metals, ceramics, polymers, and composites of these materials in layers of substantial thickness (e.g., >25 µm) for engineering applications, often with equipment that can operate in the atmosphere and can be portable for use in the field. While traditional applications of thermal-spray coatings have addressed issues of surface protection, there is a growing activity in the use of the basic technology concepts for producing engineered functional surfaces and devices that offer the materials engineer a new scale of construction between thin films and macroscopic structures.

The dominant device used in the semiconductor industry today is the silicon-based metal oxide semiconductor (MOS) transistor. The MOS transistor consists of a source, drain, channel, and gate region fabricated in single-crystal silicon (Figure 1). The source region provides a supply of mobile charge when the device is turned “on.” The source is electrically isolated from the drain by the channel region, which is oppositely charged. An insulating oxide layer between the gate and the channel region forms a capacitor. During operation, a voltage is applied to the gate. By applying the appropriate voltage, a conductive layer of charge can be attracted in the channel region at the oxide/silicon interface. This layer of charge acts as a wire that effectively connects the source and drain regions. By changing the voltage on the gate, the conducting layer of charge can be removed. Thus the transistor acts like a switch, with the gate electrode controlling the connection from the source to the drain. These individual switches can be connected to form the basic building blocks for circuit design. These building blocks are used to create the high-performance microprocessors and memory chips in today's computers.

In the interim between the conception of this issue of MRS Bulletin on transparent conducting oxides (TCOs) and its publication, the remarkable applications dependent on these materials have continued to make sweeping strides. These include the advent of larger flat-screen high-definition televisions (HDTVs), larger and higher-resolution screens on portable computers, the increasing importance of low emissivity (“low-e”) and electrochromic windows, a significant increase in the manufacturing of thin-film photovoltaics (PV), and a plethora of new hand-held and smart devices, all with smart displays.1-7 Coupled with the increased importance of TCO materials to these application technologies has been a renaissance over the last two years in the science of these materials. This has included new n-type materials, the synthesis of true p-type materials, and the theoretical prediction and subsequent confirmation of the applicability of codoping to produce p-type ZnO. Considering that over the last 20 years much of the work on TCOs was empirical and focused on ZnO and variants of InxSn1-xO2, it is quite remarkable how this field has exploded. This may be a function of not only the need to achieve higher performance levels for these devices, but also of the increasing importance of transition-metal-based oxides in electro-optical devices. This issue of MRS Bulletin is thus well timed to provide an overview of this rapidly expanding area. Included are articles that cover the industrial perspective, new n-type materials, new p-type materials, novel deposition methods, and approaches to developing both an improved basic understanding of the materials themselves as well as models capable of predicting performance limits.

The interdisciplinary area of science and engineering dealing with solid electrolytes and mixed conductors is frequently known as solid-state ionics. It concerns materials that show rapid ionic motion with or without electronic conductivity, from basic science through application. Interest in solid-state ionic materials has continued for the past few decades due to several important, promising applications, such as fuel cells, batteries, sensors, and electrochemical pumps. The principle behind these applications is simply either the Nernst law (as exemplified by Equation 1 in the article by Singhal in this issue) or Faraday's laws of electrochemistry (which connect current flow to mass flow), as applied to a cell consisting of an electrolyte and two electrodes. However, the technological issues are complex, and demands on materials can be very diverse, as illustrated by the five articles in this issue. These articles are based, in part, on the invited talks presented at a symposium in April 1999 on the same subject at Lehigh University to commemorate G.C. Farrington's inauguration as its president.

Molecule-based magnets are a broad, emerging class of magnetic materials that expand the materials properties typically associated with magnets to include low density, transparency, electrical insulation, and low-temperature fabrication, as well as combine magnetic ordering with other properties such as photoresponsiveness. Essentially all of the common magnetic phenomena associated with conventional transition-metal and rare-earth-based magnets can be found in molecule-based magnets. Although discovered less than two decades ago, magnets with ordering temperatures exceeding room temperature, very high (∼27.0 kOe or 2.16 MA/m) and very low (several Oe or less) coercivities, and substantial remanent and saturation magnetizations have been achieved. In addition, exotic phenomena including photoresponsiveness have been reported. The advent of molecule-based magnets offers new processing opportunities. For example, thin-film magnets can be prepared by means of low-temperature chemical vapor deposition and electrodeposition methods.

Food is such a familiar “material” to all of us that it is easy to overlook the science input required at all stages in its manufacture, from production to consumption. Nevertheless, as the articles in this special issue of MRS Bulletin seek to prove, the materials-science approach is every bit as valid—and as useful—for food as it is for any more conventional system studied in our field. The familiar linkage of structure–property–processing serves equally well in the case of a chocolate bar as a steel bar.

Research into polymers began slowly. While polymers have long been considered to be very useful materials, today they are receiving greater recognition as subjects for conducting good science.

The silicon at the heart of highperformance integrated circuits (ICs) is the most precisely engineered material in mass production today. Crystalline silicon, purified to the level required for manufacturing modern, high-performance microelectronics (better than parts-per-billion purity), is arguably the most perfect material known. It is produced in large quantities and very economically. Continuing progress in basic silicon materials science has been central to this achievement. At the same time, access to such a perfect material and detailed knowledge of silicon's properties have provided the experimentalist with excellent opportunities to devise rather elegant experiments in whichthe parameters can be controlled very precisely. Silicon has similarly provided condensed-matter theorists with a nearly ideal test bed for new theoretical approaches. This issue of MRS Bulletin is dedicated to the materials physics that has both enabled the development of such a highly perfect material and benefited from its availability.

The mechanical properties of materials are ultimately determined by events occurring on the atomic scale. In the case of brittle fracture, this connection is obvious, since the crack in a perfectly brittle material must be atomically sharp at its tip. The crack moves by breaking individual bonds between atoms and can therefore be regarded as a macroscopic probe for the atomic bonding. Nevertheless, traditional analysis of brittle-fracture processes resorts to the treatment of Griffith,1 which implies thermodynamic equilibrium. The Griffith criterion for the mechanical stability of a crack can be formulated as a balance of the crack driving force, the energyrelease rate G, and the surface energy ɣs of the two freshly exposed fracture surfaces: G = 2ɣs. The crack driving force can be obtained from elasticity theory. Within linear elasticity, the crack is characterized by a singularity in the stress field that decays as the inverse square root of the distance R from the crack. The strength of the singularity is characterized by the stressintensity factor K, the square of which directly gives access to the energy-release rate (G = K2/E′, where E′ is an appropriate elastic modulus). While this linear elastic description of the material is not disputed for brittle materials, except for a few atomic bonds around the crack, the assumption that the resistance of the material to crack propagation will only be characterized by the surface energy of the fracture surfaces is certainly worth some further consideration. Such considerations should range from examining atomic details at the tip of a single brittle crack to the relevance of more complex fracture events involving additional irreversible processes and complex crack geometries.

The term “materials cycle” is generally used to describe the synthesis of substances from raw materials (sometimes including the synthesis of manufactured raw materials), the fabrication of shaped materials, their use, and their eventual disposal. It is well known that all materials are extracted from the earth, then are converted to functional products through various fabrication processes, usually involving a high expenditure of energy, which in turn contributes to environmental problems such as global warming.

The notion of micromachines made up of miniature gears and motors may seem like a fairy tale. In 1959, Richard Feynman delivered a lecture entitled “There's Plenty of Room at the Bottom,” a bold prediction of what was to come in the future, where one could fit the entire Encyclopaedia Britannica onto the head of a pin or use ions focused through a microscope lens in reverse to etch away silica to create patterns on a submicron scale. In retrospect, Feynman's hypotheses were amazingly accurate. He was describing the techniques of microlithography and the manipulation of atoms by scanning tunneling electron microscopy, methods that are commonplace today. Advances in biochemistry have revealed the enormous complexity of “simple” organisms, leading one to conclude that “micromachines” (of a complexity that a visionary giant such as Feynman had foreseen) have long been operating quite happily without any assistance from chemists!

Molecule-based magnets display the full range of phenomena observed in conventional transition-metal and rareearth-based magnets, as well as unusual phenomena unique to themselves. These materials form in a wide range of one-, two-, and three-dimensional network structures, with disorder often playing a substantial role.

The first report of a transparent conducting oxide (TCO) was published in 1907, when Badeker reported that thin films of Cd metal deposited in a glow discharge chamber could be oxidized to become transparent while remaining electrically conducting. Since then, the commercial value of these thin films has been recognized, and the list of potential TCO materials has expanded to include, for example, Al-doped ZnO, GdInOx, SnO2, F-doped In2O3, and many others. Since the 1960s, the most widely used TCO for optoelectronic device applications has been tin-doped indium oxide (ITO). At present, and likely well into the future, this material offers the best available performance in terms of conductivity and transmissivity, combined with excellent environmental stability, reproducibility, and good surface morphology. The use of other TCOs in large quantities is application-specific. For example, tin oxide is now widely used in architectural glass applications.

Starch is a versatile food ingredient and is widely used in numerous food and industrial applications. Whether in its native form, or modified1 through chemical substitution and/or physical modification (e.g., annealing), starch is used as a texturizer, gelling agent, thickener, adhesive, and moisture-retainer. Starches also provide an essential carbohydrate energy source and are obtained from roots, rootlike or tuberous plants, and cereals, all of which possess unique starch chemistries that impact their properties and function as food ingredients.