Semiconductor wires with nanometer widths have attracted much attention in recent years for their potential applications in mesoscopic research and nanodevices. Since the 1960s, Si whiskers grown from the vapor-liquid-solid (VLS) reaction have been extensively studied. In the VLS reaction, Au particles are generally used as the mediating solvent on a Si substrate since Au and Si form a molten alloy at a relatively low temperature. Si in the vapor phase diffuses into the liquid-alloy droplet and bonds to the solid Si at the liquid-solid interface, which results in the growth of Si whiskers. The diameter of the whisker is determined by the diameter of the liquid-alloy droplet at its tip. Si whiskers generally grow along ⟨111⟩ directions epitaxially on Si(111) substrates in the form of single crystals by the VLS reaction.

In different materials systems, however, a variety of whisker forms can be obtained. For example, GaP whiskers display rotational twins around their ⟨111⟩ growth axes, while GaAs whiskers grow in the form of the wurtzite structure.

Thus far, the synthesis of one-dimensional nanostructured materials on a large scale remains a challenge. In recent years, many efforts have been made to synthesize Si nanowires by employing different methods such as photolithography and etching techniques and scanning tunneling microscopy. One method of particular interest is a recently developed laser ablation of metal-containing semiconductor targets, by which bulk quantities of semiconductor nanowires can be readily obtained. Our recent studies show that oxides play a dominant role in the nucleation and growth of high-quality semiconductor nanowires in bulk quantities by laser ablation, thermal evaporation, or chemical vapor deposition. A new growth mechanism called oxide-assisted nanowire growth has therefore been established. The ability to synthesize large quantities of high-purity (no contamination), ultra-long (in millimeters), and uniform-sized semiconductor nanowires (a few nanometers to tens of nanometers in diameter) from this new technique offers exciting possibilities in fundamental and applied research.

The liquid-phase lipid bilayer is a unique engineering material. Biologically, it holds a central position in cellular life, providing the structural basis for the membrane that surrounds every cell on the planet. From a materials perspective, it is essentially a 4-nm-thick, water-insoluble sheet of 2-poise oil. Artificial membranes were “discovered” 35 years ago. It was soon recognized that liposomes could have a range of potential uses, and investigators sought to exploit the obvious capsular and biocompatibility properties of the membrane in applications such as liposome drug delivery. Since 1966, some 18,000 papers on liposomes have appeared in the literature (listed on Medline, and see References 4–7 for reviews), and 260 patents have been issued describing the use of liposomes in the pharmaceutical industry. These patented applications have included the delivery of cancer drugs, intracellular drug delivery, inhalation, topical drugs, gene therapy, proteins, peptides, amino acids, vaccines, targeted liposomes, lipophilic drugs, and liposome production, separation, and analysis. A huge database therefore exists with which to establish boundary conditions for predicting under which circumstances the encapsulation of drugs in liposomes or other carriers may be expected to result in improved therapy. Despite this enormous effort, only a few formulations, principally for amphotericin (an antifungal drug) and anthra-cyclins (anticancer drugs), have been approved and marketed, heralding the promise and potential that these versatile lipid-bilayer materials present. The reasons for this limited success are many, not the least of which is that the cost of developing a new pharmaceutical product can be several hundred million dollars.

Despite the fact that thermodynamic calculations, strictly speaking, apply only to equilibrium, they may of ten be used in nonequilibrium situations. If one or several of the stable phases are suppressed in the calculations, we have a metastable equilibrium, which is often of practical interest. For example, one may calculate the driving force available to form the more stable phases and model nucleation.

Thermodynamic calculations may be performed stepwise to predict microseg-regation during solidification by a Scheil-type calculation (no diffusion in the solid State, infinite diffusion in the liquid, and equilibrium at the interface). In such a calculation, no information other than the thermodynamic properties of the System is used.

A more ambitious approach is to com-bine the thermodynamic calculations with kinetic modeis (e.g., diffusion calculations) and thereby predict the rate of reactions. This approach is extremely powerful and may be used to simulate a wide range of different phenomena, including precipitation, homogenization, and diffusional interactionsbetween Substrate and coating.

It is usually assumed that thermodynamic equilibrium holds locally at the migrating phase interface between two phases, and the rate of transformation is calculated at each instant by solving a set of flux-balance equations. The fluxes are obtained from a numerical Solution of the multicomponent diffusion equations (see Reference 3).

Metal/polymer composites are used in numerous technical applications. For example, polymer coatings on metal surfaces are used for corrosion protection, metal films on polymers inhibit static buildup, and polymers between two metals can serve as a “glue” for connecting materials that cannot be welded. Polymer/metal composites also play an important role in modern electronics. In condensers, polymers serve as insulating layers between metallic leads and are used to encapsulate entire electronic circuits. In all circumstances, interfaces are formed between the two different materials, and since the chemistry and structure change abruptly, interfacial failure is frequently observed.

The cause of failure may just be mechanical (e.g., shrinkage of the polymer during curing), or the interface stability may be degraded by attack of aggressive species, resulting in delamination. More specifically, loss of adhesion is directly caused by interfacial electrochemical reactions that nucleate at a defect and progress into intact regions of the interface. This occurs for encapsulated electronic parts in humid atmospheres as well as for lacquers on automotive parts.

Thus the investigation of corrosion reactions at a buried interface is an important area of research, but it is made very difficult by the fact that most electrochemical methods do not give information on localized reaction kinetics at a buried (metal/polymer) interface. This situation has changed with the invention and development of the scanning Kelvin probe (SKP). This method allows, for the first time, local analysis of reactions occurring at a buried metal/polymer interface. Based on the results obtained with the SKP, a detailed reaction model for the delamination process has been developed. This understanding has led to the development of new approaches that protect the interface from delamination. The idea is to chemically modify the interface using Afunctional molecules that promote adhesion between metal and polymer surfaces.

Primarily to meet the dramatic increase in Internet traffic, a substantial expansion in new fiber-optic networks has been seen in the last few years, increasing the total amount of transmission fiber deployed in the field. However, increased capacity has also been achieved by utilizing more of the available bandwidth present in the currently installed fiber. A key component in facilitating this increase in bandwidth is the erbium-doped fiber amplifier (EDFA), which provides efficient broad-band gain in the 1530–1560-nm telecommunications window. Erbium-doped glasses can be drawn into low-loss fiber, and the width of the gain band can be controlled with glass composition. With appropriate composition and design, EDFAs can simultaneously amplify 32 or more wavelengths, providing a 32-fold increase in data capacity over single-channel systems. These devices can boost signal strength by a factor of 1000, with high reliability and low noise at data rates exceeding 1 Tbit/s. In this article, we review some of the properties that are key to the success of EDFAs and discuss the potential for other rare-earth-doped glass-fiber combinations that may find possible applications in future telecommunications networks.

Fiber optics have revolutionized the telecommunications industry, providing more information capacity and greater distances between signal boosters than copper wire and coaxial cable. The attenuation in coaxial systems increases exponentially with signal frequency, making high-speed transmission over long distances impractical. The best copper systems have a bandwidth of about 10 Mbit/s and are limited to lengths of less than 200 m at high data rates. In contrast, the attenuation of SiO2 optical fibers is low and independent of signal frequency, thus optical fiber can easily support 100 Gbit/s (10,000 times the capacity of copper) over 80 km and is currently only limited by the speed of the transmission and receiving electronics, with capacities in excess of 50 Tbit/s theoretically possible.1 For links in excess of 80 km, signal amplification is necessary to prevent total loss of the signal. In the 1980s, amplification was done with electronic devices called repeaters that detected the light, converted it to an electronic signal, amplified, retimed, and then retransmitted it as an optical pulse.

The field of optical telecommunications has itself undergone a revolution. In the late 1980s, the invention of the all-optical amplifier allowed for simultaneous amplification of multiple channels in a single optical fiber each at a different wavelength or color of light. SiO2 fibers have a minimum in attenuation in the infrared (IR) portion of the optical spectrum near 1550 nm, as shown in Figure 1. The EDFA fortuitously provides high gain and low noise in the 1530-1560-nm spectral window. This technology now enables simultaneous amplification of 32 channels in a single fiber without the need for optical-to-electronic conversion. Thus single-fiber capacities of 320 Gbit/s are currently being deployed today. To perform this electronically, each channel would have to be separated (demultiplexed), amplified by its own costly repeater, and then recombined (multiplexed) in the fiber. Researchers are now perfecting 100-channel EDFAs in the lab.

The substantial power of both elastic and inelastic neutron-scattering techniques for the investigation of the structure and dynamics of biological systems and related biomolecular-based materials—as with soft matter in the previous article by Lindner and Wignall—arises primarily from the essentially isomorphous nature of the substitution of deuterium for selected hydrogen atoms in these systems, coupled with the exquisite sensitivity of neutron scattering to this isotopic substitution. Since these systems are comprised of large macromolecules and supramolecular assemblies thereof, their essential structures and dynamics extend from the atomic scale up to very large length scales of the Order of 101–104 Å. Hence neutron sources and neutron-scattering spectrometers optimized for longer wavelength (or “cold”) thermal neutrons are necessary in order to most effectively address the structure and dynamics at the longer length scales inherent to these Systems.

The large majority of previous neutron-scattering experiments on biological systems have been performed with reactor neutron sources. Some of the more significant of these are briefly summarized in the following sections. They may be categorized in terms of the nature of the intermolecular order, both orientational and positional, within the System of interest and either the elastic neutron-scattering technique employed to investigate their time-averaged structures or the inelastic neutron-scattering technique employed to investigate their dynamics.

Almost all thin films deposited on a substrate are in a state of stress. Fifty years ago pioneering work concerning the measurement of thin-film stresses was conducted by Brenner and Senderoff. They electroplated a metal film onto a thin metal substrate strip fixed at one end and measured the deflection of the free end of the substrate with a micrometer. Using a beam-bending analysis, they were able to calculate a residual stress from the measured deflection of the bimetallic film-substrate system. A variety of other, more sensitive methods of measuring the curvature of the surface of a film-substrate system have since been developed using, for example, capacitance measurements and interferometry techniques.

When a monochromatic x-ray beam is incident onto a curved single crystal, the diffraction condition is satisfied only for regions of the crystal where the inclination angle with respect to the incident beam exactly matches the Bragg angle. When a parallel beam plane-wave source is used, the diffracted beam from a particular set of (hkl) planes gives rise to a single narrow-contour band. If the crystal is rocked by an angle ω, the contour band will move by a certain distance D. The radius of curvature R of the crystal lattice planes is given by

where θ is the Bragg angle. Equal rocking angles produce equivalent D values for uniform curvature, or varied D values for nonuniform curvature. Using this procedure, detailed contour maps of the angular displacement field of the crystal can be mapped in two dimensions.

Circularly polarized x-ray radiation is attracting increasing interest as a tool for the characterization of the electronic, magnetic, and chiral properties of low-dimensional structures. Using circular light (with electric field vector parallel to the orbital plane), a dependence of the measured quantity by changing either the orientation of the light polarization or the magnetization is indicative of the existence of magnetic circular dichroism. It can be observed in x-ray absorption spectroscopy (XAS), in which the photon energy is scanned through an absorption threshold exciting a core electron into an unoccupied valence state using circularly polarized light. Synchrotron radiation sources have made this technique possible. It can also be observed in photo-emission spectroscopy from core and valence levels. Here we focus on magnetic circular x-ray dichroism (MCXD) in XAS as an element-specific tool to investigate magnetic properties of ultrathin films in situ. The application of magneto-optical sum rules enables the determination of the orbital and spin magnetic moments per atom from XAS spectra, as well as the easy magnetization direction.

MCXD-based magnetometry in XAS is extensively used by measuring the L absorption edges of 3d-transition metals, where large intensity changes (up to 60%) of the L-edge white lines are observed upon reversal of either the sample magnetization or the light helicity. The high magnetic contrast obtained, combined with the elemental specificity of the technique, allows for the study of very dilute samples such as ultrathin films. We first concentrate on the selection rules governing MCXD in XAS.

Despite the obvious importance of understanding the chemistry of oxide glass materials, predictive thermochemical modeis of complex glasses have not yet been developed. Such modeis are important for technologies such as the disposal of high-level nuclear and transuranic waste (HLW), which are currently fore-seen as being incorporated in a host glass for permanen t Sequestration. A large number of glasses have been explored, with a borosilicate glass being the typical base composition. An example of the complexity of such a HLW glass is given in Table I. This article discusses our at-tempts to develop an accurate, easy to understand and use glass Solution model for describing the thermodynamic stability of such HLW glasses. Critical for such a model is the availability of reliable thermodynamic data that can be used in generating accurate values for thermodynamic activities of glass components as a function of temperature and glass composition. Therefore, a major part of this article focuses on developing reliable sets of thermodynamic data for complex HLW glass Systems and Subsystems. With such Information and a model, we can make predictions of the stability of these waste forms, including their volatility, leaching behavior, and corrosion reactions, and understand crystallization behavior during both the initial glass processing and long-term storage.

Using an equilibrium thermodynamic model is offen questioned, since HLW is to be stored as part of a glass phase, and glass is a nonequilibrium material. Our model uses a pseudoequilibriu map-proach in which we thermochemically treat the glass as a supercooled liquid. This is a more accurate approach than assuming a global System equilibrium, as it describes the behavior of the metas-table glass phase using thermodynamic data for the liquid phase and excludes the formation of crystalline species. As a result, developing an accurate model and data for representing the thermodynamic properties of oxide liquid phases is critical to understanding the limiting chemi-cal behavior of the nuclear waste glass.

The methodology requires that a critically assessed thermodynamic database be created for binary and ternary combinations of the major constituents in a typical waste glass. These data can then be combined to represent the thermodynamic behavior of the more complex multicomponent HLW glass Systems. If a crystalline phase is experimentally observed to precipitate from the glass under certain conditions, a thermodynamic description can be used to calculate the composition-temperature conditions under which this specific crystalline phase can exist in equilibrium with the metas-table glass phase.

It is generally recognized that the capillary forces associated with internal and external interfaces affect both the shapes of liquid-vapor surfaces and wetting of a solid by a liquid. It is less commonly understood that the same phenomenology often applies equally well to solid-solid or solid-vapor interfaces.

The fundamental quantity governing capillary phenomena is the excess free energy associated with a unit area of interface. The microscopic origin of this excess free energy is often intuitively simple to understand: the atoms at a free surface have “missing bonds”; a grain boundary contains “holes” and hence does not have the optimal electronic density; an incoherent interface contains dislocations that cost strain energy; and the ordering of a liquid near a solid-liquid interface causes a lowering of the entropy and hence an increase in the free energy. In what follows we shall show how this fundamental quantity determines the shape of increasingly complex bodies: spheres, wires, thin films, and multilayers composed of liquids or solids. Crystal anisotropy is not considered here; all interfaces and surfaces are assumed isotropic.

Consideration of the equilibrium of a spherical drop of radius R with surface free energy γ shows that pressure inside the droplet is higher than outside. The difference is given by the well-known Laplace equation:

This result can be obtained by equating work done against internal and external pressure during an infinitesimal change of radius with the work of creating a new surface.

Conventional optical data-storage techniques, such as magneto-optic disks and CD-ROMs, record a single bit of information at each particular substrate location. In order to produce the gigabyte-class storage substrates demanded by today's computers using such conventional technologies, access to tens of billions of individual material locations is required. This brute-force approach to optical data storage has produced impressive results. However, there is increasing interest in methods for more efficiently accessing storage materials. One approach is to record multiple bits at a single storage-material location. This can be accomplished by multiplexing the bits spectrally, using differing optical frequencies to record data bits. It has been realized for over 20 years that when certain materials are cooled to appropriate temperatures, typically below 20 K, the possibility of spectrally multiplexing large numbers of bits in a single material location arises. Although this approach, known as spectral hole-burning, has been proposed as a data-storage mechanism, to date it has primarily been used as a tool to study material properties. Rare-earth-doped crystals have been demonstrated to have properties that lend themselves to a variety of different spectral hole-burning-based data-storage applications. In this article, we will review the principles of spectral hole-burning, discuss some specific material systems in which spectral hole-burning is of particular interest, and describe methods for producing high-capacity, high-data-rate spectral memories.

Spectral hole-burning, and spectral memories based on spectral hole-burning, depend on a material property referred to as inhomogeneous absorption line broadening. Materials exhibiting this property contain active atoms or molecules that individually respond to (absorb) very specific frequencies of light, but the collective response of all of the material's active atoms or molecules covers a spectral region that is broad compared with the response of a particular active atom or molecule. Inhomogeneous absorption line broadening is caused by local variations in the structure of the host, which in turn lead to variations in the electronic levels of the active atoms or molecules. The absorption linewidth of an individual absorber is referred to as the homogeneous linewidth Γh, and the absorption width of a collection of inhomogeneously broadened absorption centers is referred to as the inhomogeneous linewidth Γi. Application of monochromatic light to such a material has the effect of exciting only a very small subset of active absorbing atoms—those residing in the illuminated spatial volume within a homogeneous width of the exciting light's specific frequency. If the frequency of the imposed light is shifted, a different subset of active absorbing atoms in the illuminated volume responds.

The following article is based on the MRS Medal talk presented by William L. Johnson at the 1998 MRS Fall Meeting on December 2, 1998. The MRS Medal is awarded for a specific outstanding recent discovery or advancement that has a major impact on the progress of a materials-related field. Johnson received the honor for his development of bulk metallic glass-forming alloys, the fundamental understanding of the thermodynamics and kinetics that control glass formation and crystallization of glass-forming liquids, and the application of these materials in engineering.

The development of bulk glass-forming metallic alloys has led to interesting advances in the science of liquid metals. This article begins with brief remarks about the history and background of the field, then follows with a discussion of multicomponent glass-forming alloys and deep eutectics, the chemical constitution of these new alloys, and how they differ from metallic glasses of a decade ago or earlier. Recent studies of deeply undercooled liquid alloys and the insights made possible by their exceptional stability with respect to crystallization will then be discussed. Advances in this area will be illustrated by several examples. The article then describes some of the physical and specific mechanical properties of bulk metallic glasses (BMGs), and concludes with some interesting potential applications.

The first liquid-metal alloy vitrified by cooling from the molten state to the glass transition was Au-Si, as reported by Duwez at Caltech in 1960. Duwez made this discovery as a result of developing rapid quenching techniques for chilling metallic liquids at very high rates of 105–106 K/s.