During the past decade, scientific and technological interest in the properties of surfaces and interfaces has grown at an astounding rate. On first thought, one might not consider a neutron or even an x-ray photon to be a particularly sensitive surface probe given their relatively weak interactions with matter compared to that of a low-energy electron or atom. Indeed, low-energy electron diffraction and atomic beam scattering techniques have contributed significantly to our understanding of surface phenomena. Nonetheless, the very fact that electrons and atoms are so strongly interacting makes quantitative analysis of their scattering data difficult. The interaction of neutrons or x-rays with matter, on the other hand, is weak enough that the potential can be characterized by a relatively simple scattering amplitude. Presently attainable neutron intensities, though not yet comparable to those of x-ray synchrotron sources, are still of sufficient strength to permit a variety of surface or near surface reflectivity and grazing angle diffraction experiments. Because neutrons can distinguish between different isotopes of the same element, most notably hydrogen and deuterium, as well as couple to atomic magnetic moments via a dipolar interaction, they can be indispensable and complementary probes.

More conventional neutron diffraction techniques can also be applied to the study of interfacial phenomena and the effects of reduced dimensionality and compositional modulation in super-lattice structures grown by a variety of thin film deposition methods. In this article we will differentiate between reflectivity and diffraction measurements as follows: if the scattering occurs at a wavevector transfer low enough that the scattering medium appears as a continuum, so that amorphous and crystalline states are indistinguishable, then it will be considered to be in the reflectivity regime whereas diffraction will be taken to correspond to higher wavevector transfer where the precise arrangement of atoms is discernible.

Although solids are one of the three states of matter, and the solid state is pervasive throughout science and our lives, students would not know it from the standard chemistry curriculum, which still emphasizes small molecules. Despite this education, a significant proportion (more than 30%) of all chemists end up as practitioners of materials chemistry, either in inorganic solids or in polymers, and they must therefore obtain on-the-job education. Not only should this need be reflected in the curriculum, but it should be possible through modern areas of chemistry such as materials to bring some of the excitement of the practicing chemist to the undergraduate student's first chemistry course, perhaps turning around the flight from science, and from chemistry and physics in particular. The American Chemical Society is encouraging this approach through the proposal of a certified BS degree in chemistry with emphasis in materials. To place the present position in perspective, one only needs to look at the recent figures tabulated by the National Science Foundation; there is a tremendous attrition of students planning to major in science and engineering during the freshman year (See Table I).

Potential science majors are indeed there, but they are being lost due to their first experiences, which are usually in general chemistry and calculus, and a lesser number in biology and physics. It is therefore imperative that these courses encourage students rather than kill their enthusiasm.

Before the application of small-angle neutron scattering (SANS) to the study of polymer structure, chain conformation studies were limited to light scattering and small-angle x-ray scattering (SAXS) techniques. These experiments were usually conducted in dilute solution, and the methodology to measure radii of gyration, virial coefficients, molecular weights, etc., was well established in the classical works of Guinier, Zimm, Debye and Kratky, who pioneered these techniques during the 1940s and 1950s. This methodology could not be applied to concentrated solutions or bulk polymers because of the difficulty of separating the intra- and inter-molecular components of the scattering function. One attempt to circumvent this difficulty was the experiment by Krigbaum and Godwin, who end-labeled polystyrene molecules with Ag atoms. When dispersed in unlabeled polystyrene, the excess x-ray scattering could in principle be analyzed to provide the end-to-end distance, though in practice the signal-to-noise ratio of the experiment was insufficient for accurately determining this parameter. To our knowledge the first suggestion to use the difference in coherent scattering lengths of deuterium (bD = 0.66 × 10−12cm) and hydrogen (bH = −0.37 × 10−12cm) to create scattering contrast between deuterated and normal (hydrogenous) molecules and provide a direct determination of molecular dimensions was made independently by at least two groups in the late 1960s. By deuterating the whole molecule, as opposed to end-labeling, this proposal increased the signal-to-noise ratio of the experiment by several orders of magnitude and made possible for the first time the practical analysis of molecular conformations in bulk polymers. Even so, such experiments could not be undertaken until the completion in Europe of the first instruments employing long wavelength neutrons and large distances between the entrance slit, sample and detector, which allowed deuterium labeling methods to be successfully applied to polymers in the early 1970s.

Any finite system is delimited by interfaces. In this trivial sense interfaces are ubiquitous. However, modern epitaxial techniques seek to modify the properties of materials by the creation of interfaces. “Band gap engineering,” the attempt to tailor the electronic properties of semiconductors by interleaving many dissimilar layers, is an example of this approach. Interfaces between lattice-matched, isostructural, crystalline systems, (differing only in composition) are technologically the most advanced and thus most widely used. These “chemical” interfaces are formed by the introduction of dopant impurities, or by the stacking of dissimilar materials. Chemical interfaces formed by the epitaxial growth of dissimilar materials are the subject of this article.

A fundamental tenet of modern epitaxy is the tailoring of properties through control of structure. We will therefore outline how the “structure” of a chemical interface may be defined and experimentally determined. Determining the interfacial structure is a major experimental challenge, inextricably bound with our understanding of how structure affects other properties. The article will thus briefly discuss this vital link. We will conclude with an outline of the way chemical interfaces, in reality systems far from equilibrium, can relax, and how their relaxation mechanisms shed light on the fundamental properties of solids. Because the GaAs/AlGaAs system is the most technologically advanced, and to be concrete, we will illustrate the discussion by reference primarily to this system. Many of the concepts and experimental results, however, are more generally valid.

The development of LDMMs is a significant scientific accomplishment. This article provided an overview of the synthesis, structural characterization, and properties of these materials. Hopefully, the reader gained an appreciation for (1) the diverse morphologies of these materials, (2) their unique structure-property relationships, and (3) the difficulty in simultaneously achieving low densities with small cell sizes. This section will briefly discuss the future development of these materials and their potential commercial applications.

Although substantial progress has been made in the scientific understanding of the formation of LDMMs, many important questions remain. As mentioned earlier, LDMMs rarely display an ideal spinodal morphology even though many of these structures have been established by phase-separation of polymer/solvent systems. Such morphologies appear to coarsen after the initial structure is established. The question arises — on what length scales is phase-separation taking place and how does its temporal evolution affect LDMM structure? In aerogel processing, polymerlike gels are formed, but certain sol-gels do not behave in accordance with theories on the swelling of conventional polymer gels. Why is this, and what is the effect on the final aerogel morphology? These and other questions can only be answered with a better understanding of gel structures and the dynamics of their formation. This is an important area for future research because the information gained can be used to tailor LDMMs for specific applications.