Nanotechnology offers the promise of enabling revolutionary advances in diverse areas ranging from electronics, optoelectronics, and energy to healthcare. Underpinning the realization of such advances are the nanoscale ma te rials and corresponding nanodevices central to these application areas. Semiconductor nanowires and nanobelts are emerging as one of the most powerful and diverse classes of functional nanoma terials that are having an impact on science and technology. In this issue of MRS Bulletin, several leaders in this vibrant field of research present brief reviews that highlight key aspects of the underlying materials science of nanowires, basic device functions achievable with these materials, and developing applications in electronics and at the interface with biology. This article introduces the controlled synthesis, patterned and designed self-assembly, and unique applications of nanowires in nanoelectronics, nano-optoelectronics, nanosensors, nanobiotechnology, and energy harvesting.

Polymer nanocomposites (PNCs)–that is, nanopar ticles (spheres, rods, plates) dispersed in a polymer matrix–have garnered substantial academic and industrial in terest since their inception, circa 1990. This is due in large part to the incredible promise demonstrated by these early efforts: PNCs will not only expand the per form ance space of traditional filled polymers, but introduce completely new combinations of properties and thus enable new applications for plastics. Low volume additions (1–5%) of nanopar -ticles, such as layered silicates or carbon nanotubes, provide property enhancements with respect to the neat resin that are comparable to those achieved by conventional loadings (15–40%) of traditional fillers. The lower loadings facilitate proc essing and re duce component weight. Most important, though, is the unique value - added properties not normally possible with traditional fillers, such as reduced permeability, optical clarity, self - passivation, and increased re sis tance to oxidation and ablation. These characteristics have been transformed into numerous commercial suc cesses, including automotive parts, coatings, and flame retardants. This issue of the MRS Bulletin provides a snapshot of these exemplary successes, future opportunities, and the critical scientific challenges still to be addressed for these nanoscale multiphase systems. In addition, these ar ticles provide a perspective on the current status and future directions of polymer nanocomposite science and technology and their potential to move beyond additive concepts to designed ma te rials and devices with prescribed nanoscale composition and morphology.

Many insects and some larger animals, such as geckos, skinks, and tree frogs, can easily climb vertical walls and even walk on the ceiling. These abilities require a method to attach the feet strongly but reversibly to a variety of surfaces—smooth or rough, hydrophilic or hydrophobic, clean or containing contaminants. This issue of MRS Bulletin examines how fibrils, absorbed water layers, geometry, and other factors make reversible adhesion possible, and how this understanding might be applied to robots and other artificially created structures that can climb walls, walk on ceilings, and get to other hard-to-reach places.

By 2050, world annual energy consumption is predicted to grow from the present 13 terawatt - years (TWyr) to nearly 30 TWyr. Although all energy sources must be considered in meeting this challenge, solar energy may arguably be the only carbon-free source capable of supplying a significant fraction of energy at these levels. This issue of MRS Bulletin reviews the status and future development of solar photovoltaic technologies based on inorganic materials. The discussion begins with materials and cell designs for second-generation photovoltaics based on thin films [a-Si:H, Si, Cu(In, Ga)(Se, S)2, CdTe]. Recent advances in tandem cells and concentrators are alsoreported, along with photovoltaic approaches involving nanoscale materials such as quantum dot arrays. Finally, work on transparent conducting oxides that are critical to nearly all cell designs are discussed.

Direct-write techniques enable computer-controlled two- and three-dimensional pattern formation in a serial fashion. Among these techniques, the versatility offered by laser-based direct-write methods is unique, given their ability to add, remove, and modify different types of materials without physical contact between a tool or nozzle and the material of interest. Laser pulses used to generate the patterns can be manipulated to control the composition, structure, and even properties of individual three-dimensional volumes of materials across length scales spanning six orders of magnitude, from nanometers to millimeters. Such resolution, combined with the ability to process complex or delicate material systems, enables laser direct-write tools to fabricate structures that are not possible to generate using other serial or parallel fabrication techniques. The goal of the articles in this issue of MRS Bulletin is to illustrate the range of materials processing capabilities, fundamental research opportunities, and commercially viable applications that can be achieved using recently developed laser direct-write techniques. We hope that the articles provide the reader with a fresh perspective on the challenges and opportunities that these powerful techniques offer for the fabrication of novel devices and structures.

Glassy alloys (metallic glasses) are currently the focus of intense research in the international metals community. Setting aside elevated-temperature applications, these amorphous metals have exciting potential for structural applications. When metallic glasses were first widely studied in the 1960s, the alloy compositions then known to be quenchable into the glassy state from the liquid required high cooling rates on the order of 106 K s−1 and were consequently restricted to thin sections. The current interest in metallic glasses has its origin mainly in the increasing range of compositions that can now be cast into glasses at much lower cooling rates, permitting minimum sections of 1 mm to 1 cm or even larger. These bulk metallic glasses (BMGs) are the focus of the articles in this issue of MRS Bulletin. Our goal is to illustrate the major materials issues for BMGs, from processing to structures to properties and from the fundamental science of glasses to viable industrial applications. We hope that the articles, in providing a snapshot of a rapidly moving field, show why BMGs are attracting such intense interest and serve to highlight some challenging issues awaiting resolution.

The design and synthesis of solid-state materials constructed from molecules has emerged as an important frontier of materials research. Molecular materials promise an unparalleled opportunity for systematic manipulation of solid-state properties and functions by using molecular design principles and capitalizing on the versatility of organic synthesis. Furthermore, the use of molecular components may produce considerable economic benefits, whether by reducing fabrication cost or through increases in performance. The articles in this issue of MRS Bulletin cover recent discoveries and developments based on materials with properties and functions that hinge on the characteristics of their molecular constituents. These materials promise significant advances in several technologies of substantial commercial interest, including organic light-emitting diodes, nonlinear optics, gas separations, chiral separations, and molecular magnets.

The fairly recent availability of commercial focused ion beam (FIB) microscopes has led to rapid development of their applications for materials science. FIB instruments have both imaging and micromachining capabilities at the nanometer–micrometer scale; thus, a broad range of fundamental studies and technological applications have been enhanced or made possible with FIB technology. This introductory article covers the basic FIB instrument and the fundamentals of ion–solid interactions that lead to the many unique FIB capabilities as well as some of the unwanted artifacts associated with FIB instruments. The four topical articles following this introduction give overviews of specific applications of the FIB in materials science, focusing on its particular strengths as a tool for characterization and transmission electron microscopy sample preparation, as well as its potential for ion beam fabrication and prototyping.

The cosmetics industry is increasingly using sophisticated materials and technologies in their products and to measure their effectiveness. Understanding how these products interact with skin, adhere to skin, and deliver a benefit or create a visible effect leads to better and safer products. An understanding of the optics and mechanics of skin and the cosmetics–skin interface is important for achieving various visual and functional effects. This issue reviews the underlying science and the technical challenges behind the development of materials for the personal care and cosmetics industry.

This article is an edited transcript of the MRS Medal presentation given by Mark Thompson (University of Southern California) on November 28, 2006, at the Materials Research Society Fall Meeting in Boston. Thompson was awarded the Medal for the “development of highly efficient heavy-metal phosphor complexes.” The MRS Medal recognizes a specific outstanding recent discovery or advancement which is expected to have a major impact on the progress of any materials-related field.

Successful research efforts have led to improvements in the internal efficiencies of organic light-emitting devices (OLEDs) from 25% to 100%. The electroluminescence process in OLEDs involves the formation of both singlet and triplet excitons, formed in a ratio of 1:3. There is a drive to improve spin statistics by developing compounds in which triplet excitons, in addition to singlet excitons, can be used efficiently. Success with the incorporation of heavy-metal–based phosphors into OLEDs, in which the strong spin-orbit coupling of the metal atom allows for efficient molecular phosphorescence from triplet excitons, resulted in the identification and synthesis of an iridium complex, fac-tris-phenylpyridine iridium, with internal efficiencies of 100%. This, in turn, has led to the synthesis of more than 100 iridium- and platinum-based compounds, which have become the most efficient light-emitting compounds yet discovered. Intellectual property from Thompson's research in this field has led to more than 50 U.S. patents and substantial entrepreneurial investment toward commercial applications and devices.

During the past few years, substantial effort has been devoted to developing new experimental techniques capable of delivering atomic-scale information on surfaces and nanoparticles under catalytic reaction conditions. Since the advent of surface science, pioneering experiments under highly idealized conditions have been performed (at very low gas pressures, <10−6 mbar), and idealized model material systems (e.g., single crystals) have been investigated. However, understanding chemical reactions on singlecrystal surfaces close to ultrahigh vacuum does not always enable prediction of the performance of nanoparticles operating at gas pressures near or above atmospheric pressure. Therefore, this MRS Bulletin issue focuses on the capabilities of atomic-scaleresolution, high-gas-pressure- and high-temperature-compatible in situ probes sensitive to the structure, chemical composition, and dynamical properties of nanomaterials. It will be demonstrated how novel in situ techniques enable one to bridge the combined pressure and materials gaps from ultrahigh vacuum to atmospheric pressures and from metal single-crystal surfaces to nanoparticles or oxides.

Thin-film solar cell technologies based on Si with a thickness of less than a few micrometers combine the low-cost potential of thin-film technologies with the advantages of Si as an abundantly available element in the earth's crust and a readily manufacturable material for photovoltaics (PVs). In recent years, several technologies have been developed that promise to take the performance of thin-film silicon PVs well beyond that of the currently established amorphous Si PV technology. Thin-film silicon, like no other thin-film material, is very effective in tandem and triple-junction solar cells. The research and development on thin crystalline silicon on foreign substrates can be divided into two different routes: a low-temperature route compatible with standard float glass or even plastic substrates, and a high-temperature route (>600°C). This article reviews the material properties and technological challenges of the different thin-film silicon PV materials.

The interface of quantum mechanics methods with classical atomistic simulation techniques, such as molecular dynamics and Monte Carlo, continues to be an area of considerable promise and interest. Such coupled quantum–atomistic approaches have been developed and employed, for example, to gain a comprehensive understanding of the energetics, kinetics, and dynamics of chemical processes involving surfaces and interfaces of hard materials. More recently, it has become possible to directly couple first-principles electronic structure techniques to continuum solid mechanics, either on the fly with feedback between length scales or by information passing between length scales. We discuss, with tutorial examples, the merging of quantum mechanics with molecular dynamics and Monte Carlo simulations, as well as quantum–continuum coupled techniques. We illustrate the opportunities offered by incorporation of information from quantum mechanics (reducing assumptions in higher length-scale models) and outline the challenges associated with achieving full predictive capability for the behavior of materials.

Zinc oxide is a unique material that exhibits semiconducting, piezoelectric, and pyroelectric multifunctionalities. By controlling the size and orientation of the polar surfaces of ZnO nanobelts, single - crystal nanocombs, nanorings, nanohelices, nanosprings, and nanobows of ZnO have been synthesized. This article centers on the fundamental growth mechanism and fabrication of electromechanical devices based on piezoelectric ZnO nanostructures, including a nanogenerator using aligned ZnO nanowires for converting nanoscale mechanical energy into electric energy. The mechanism of the electric nanogenerator relies on the unique coupling of the piezoelectric and semiconducting properties of ZnO, which is the fundamental principle of nano - piezotronics, a new field using the piezoelectric effect for fabricating electronic devices and components. The approach has the potential of converting biological mechanical energy, acoustic/ultrasonic vibration energy, and biofluid hydraulic energy into electricity, demonstrating a new pathway for self - powering of wireless nanodevices and nanosystems.

This article summarizes research on laser-based processing and structuring of materials at the nanoscale using optical near-field schemes. Both apertureless and tapered fiber near-field scanning optical microscope probes can deliver highly confined irradiation at sufficiently high intensities to cause morphological and structural changes in materials at the nanometer level. The energy emitted by the probes and the absorption within the target material are predicted by carrying out calculations of the near-field electromagnetic distribution. The effects of shrinking laser beam dimensions compete with the energy diffusion into the target material. Experimental results have shown well-controlled subtractive material modification with minimum feature size in the neighborhood of 10 nm. Precise patterning can be achieved via laser-assisted chemical etching. Control of the nucleation of nanostructures via rapid melting and crystallization is demonstrated. The article concludes with an outlook to applications.

The following article is based on the Symposium X presentation given by David A. Weitz (Harvard University) on April 11, 2007, at the Materials Research Society Spring Meeting in San Francisco. The article describes how simple microfluidic devices can be used to control fluid flow and produce a variety of new materials. Based on the concepts of coaxial flow and hydrodynamically focused flow, used alone or in various combinations, the devices can produce precisely controlled double emulsions (droplets within droplets) and even triple emulsions (double emulsions suspended in a third droplet). These structures, which can be created in a single microfluidic device, have various applications such as encapsulants for drugs, cosmetics, or food additives.

In recent years, a new class of porous materials based on a combination of organic components and metal centers has emerged, namely, microporous coordination polymers (MCPs), in which the chemical properties as well as the pore dimensions affect the incorporation of “guest” molecules within the pores. In this article, we describe the ability of MCPs to store gas molecules, which is ascribed to framework regularity and high porosity, and the unique capacity of certain MCPs to capture molecules selectively by well-defined interactions with organic functional groups.

This article gives the foundation for understanding the structure and function of human skin and its layers. Skin is our first line of defense against the penetration of external chemicals and bacteria into the body and of water loss from the body. Skin's visual and tactile properties are very much linked to appearance and beauty. Skin treatments as simple as washing with soap affect the mechanics of skin properties, making it dry and brittle versus smooth and elastic. Likewise, natural aging, hormonal changes, and sun exposure affect the properties of skin. Mild cleansing, moisturizer treatments, and regular use of sunscreens can, however, help skin maintain and even improve its quality, appearance, and functional properties.

Geckos attach and detach their adhesive toes in milliseconds while running with reckless abandon on nearly any surface. The adhesive on gecko toes differs dramatically from that of conventional adhesives. Conventional pressure-sensitive adhesives (PSAs) are soft viscoelastic polymers that degrade, foul, self-adhere, and attach accidentally to inappropriate surfaces. In contrast, gecko toes bear angled arrays of branched, hair-like fibers (setae) formed from stiff, hydrophobic keratin that act as a bed of angled springs with an effective stiffness similar to that of PSAs. Setae are selfcleaning and maintain function for months during repeated use in dirty conditions. Setae are an anisotropic “frictional adhesive” in that adhesion requires maintenance of a proximally directed shear load. Thus, gecko setae resist inappropriate bonding and are capable of easy and rapid attachment and detachment. Engineered adhesive nanostructures inspired by geckos may become the glue of the future—and perhaps the screw of the future as well.