Exactly 50 years ago, E.W. Müller became the first person to observe single atoms, with the aid of the field-ion microscope (FIM). In 1967, with John Panitz and S. Brooks McLane, Müller's invention of the atom probe meant that, in his words, “We can now really deal much more intimately with the individual atoms which we encounter, since we know their names.” By combining position-sensitive detection with the time-of-flight mass spectrometry of single atoms in the atom probe, Cerezo and co-workers in the late 1980s built an instrument capable of reconstructing the three-dimensional (3D) atomic distribution of elements present in a material. The instruments that are capable of microanalysis at this level, called generically 3D atom probes (3DAPs), were the subject of two articles published in MRS Bulletin in 1994. In this article, we review some of the progress in the field since that time, in particular, the expansion of the range of materials problems that can be addressed by this powerful technique.

Plutonium symbolizes everything we associate with the nuclear age. It evokes the entire gamut of emotions from good to evil, from hope to despair, and from the salvation of humanity to its utter destruction. No other element bears such a burden. Its discovery in 1941, following the discovery of fission in 1938, unlocked the potential and fear of the nuclear age. During the Cold War, the primary interest in plutonium was to provide triggers for thermonuclear weapons that formed the basis of nuclear deterrence. Beginning in the 1950s, plutonium also became an integral part of the quest for nearly limitless electrical power. The end of the Cold War has dramatically altered the military postures of the United States and Russia, allowing each to reverse the engines fueling the nuclearweapons buildup. Now, both countries face the challenge of keeping the remaining stockpile of nuclear weapons safe and reliable without nuclear testing, as well as cleaning up nuclear contamination and preventing the spread of nuclear weapons and terrorism. Moreover, current concerns about energy availability and global warming have rekindled interest in nuclear power.

The planispheric astrolabe occupies a special place among early scientific instruments because of its mathematical sophistication, its elegant appearance, and its usefulness to astronomers in pre-telescopic times. The operation of the astrolabe is based on the mathematical properties of stereographic projection, concepts that were understood by the time of Ptolemy in the second century a.d., although surviving instruments are from much later periods.

Hybrid materials lie at the interface of the organic and inorganic realms. These materials offer exceptional opportunities not only to combine important properties from both worlds, but also to create entirely new compositions with truly unique properties.

Since the confirmation that nonmetallic single crystals with a diamond-like structure, such as SiC, BP, and AlN, have high intrinsic thermal conductivities of over 300 W m−1 K−1,1,2 a great deal of effort has been focused on the development of nonoxide polycrystalline ceramics with high thermal conductivity.

There has been increasing interest in photonic crystals in which the refractive index changes periodically. A photonic bandgap can be formed in the crystals, and the propagation of electromagnetic waves is prohibited for all wave vectors in this bandgap. Various important scientific and engineering applications, such as control of spontaneous emission, sharp bending of light, trapping of photons, and so on, may be realized by creating photonicbandgap crystals and engineering the defects and light-emitters. In the field of two-dimensional (2D) photonic crystals, some important contributions aiming at device applications have included Scherer et al.'s demonstration that a single defect can be utilized as a very tiny cavity for light emission, and Joannopoulos et al.'s work on 2D photonic-crystal circuits. Here, the present status of our work in III–V semiconductor-based 2D and 3D photonic crystals is briefly reviewed.

Plutonium metallurgy lies at the heart of science-based stockpile stewardship. One aspect is concerned with developing predictive capabilities to describe the properties of stockpile materials, including an assessment of microstructural changes with age. Yet, the complex behavior of plutonium, which results from the competition of its 5f electrons between a localized (atomic-like or bound) state and an itinerant (delocalized bonding) state, has been challenging materials scientists and physicists for the better part of five decades. Although far from quantitatively absolute, electronic-structure theory provides a description of plutonium that helps explain the unusual properties of plutonium, as recently reviewed by Hecker. (See also the article by Hecker in this issue.) The electronic structure of plutonium includes five 5f electrons with a very narrow energy width of the 5f conduction band, which results in a delicate balance between itinerant electrons (in the conduction band) or localized electrons and multiple lowenergy electronic configurations with nearly equivalent energies. These complex electronic characteristics give rise to unique macroscopic properties of plutonium that include six allotropes (at ambient pressure) with very close free energies but large (∼25%) density differences, a lowsymmetry monoclinic ground state rather than a high-symmetry close-packed cubic phase, compression upon melting (like water), low melting temperature, anomalous temperature-dependence of electrical resistance, and radioactive decay. Additionally, plutonium readily oxidizes and is toxic; therefore, the handling and fundamental research of this element is very challenging due to environmental, safety, and health concerns.

The demand for precise micro-optical elements and subsystems is increasing drastically, influenced mainly by the exploding information and communications markets. For many applications, technologies capable of high-volume production of these elements are needed. Favored methods are replication technologies such as injection molding, hot embossing, or UV molding. The challenge for materials scientists is to synthesize materials that can be formed into micro-optical structures and that can resist extreme environmental conditions during operation and stocking (e.g., temperature changes from −45°C to +125°C).

The technical sophistication and virtuosity of prehispanic Andean ceramics are so often praised in the literature that it may appear that there is a large body of supportive, empirical findings. To the contrary, in-depth technical and more comprehensive technological studies have been rare. To achieve a comprehensive understanding of the many facets and stages of the technology and organization of craft production requires a correspondingly comprehensive, sustained effort—one built on interdisciplinary collaboration and longterm regional study of environmental, historical, social, and technological factors.

Early in the 1990s in the United States, a series of legislative measures were proposed for banning the use of lead for many applications in industries involving electronic solders. Although electronic solders were ultimately exempted, this argument ignited worldwide concern, and research began on the development of lead-free solders. The U.S. project, organized by the National Center for Manufacturing Sciences (NCMS), was carried out from 1992 to 1996. Major electronics companies and auto manufacturers participated with several national institutes in this project.

While microelectromechanical systems (MEMS) technology has made a substantial impact over the past decade at the device or component level, it has yet to realize the “S” in its acronym, as complex microsystems consisting of sensors and actuators integrated with sense, control, and signal-processing electronics are still beyond the current state of the art. There are several incentives to co-fabricate MEMS devices and electronics on a single silicon chip, which apply to applications such as inertial sensors.

Organic thin films have been studied for their properties as active optoelectronic materials for at least 50 years. Yet, until recently, they have failed to make a significant impact in the commercial world. However, display products based on organic light emission are now available commercially, indicating that a major change in the acceptance of organic optoelectronics is in the offing. I would like to discuss some of the reasons for these developments from an engineering perspective.

We are living in the golden era of materials science. To cite but one example, consider the field of thin-film physics. Crystal growers have been moving inexorably closer to being able to deposit layers and hence to control film properties on an atom-by-atom basis. We are nearing an era in which it will be possible to deposit “designer” materials with a specified set of properties.

The possibility of combining the properties of organic and inorganic components in a unique composite material is an old challenge that dates to the beginning of the industrial era. Some of the earliest and most well-known organic–inorganic representatives are derived from the paint and polymer industries, where inorganic pigments or fillers are dispersed in organic components such as solvents, surfactants, and polymers to yield or improve optical and mechanical properties.

Metallization is the back end of the integrated-circuit (IC) fabrication process where the transistor interconnections are formed. Figure 1 shows the metallized part of a static random-access memory chip. Metal lines for electrical connections (Al and Cu) in Si devices are deposited as blanket films and then etched or polished away to define the conducting lines.

The development of advanced materials from inorganic nanoparticles (NPs) is currently one of the most dynamic areas of science. NPs are attracting significant fundamental and commercial interest, with a wide range of applications including the next generation of optics, electronics, and sensors. In optical, electrical, and magnetic devices, NPs will be mostly used in the form of thin films. Currently, such films are typically made by the spincoating, spraying, or sometimes simple painting of nanoparticle–matrix mixtures. Layer-by-layer (LBL) assembly is one of the new methods of thin-film deposition, often realized as sequentially adsorbed (mono)layers of oppositely charged polyelectrolytes. LBL assembly has also been successfully applied to thin films of NPs and nanocolloids, that is, dispersed species with only one or two spatial dimensions in the nanoscale regime.

The strength of a material is its ability to withstand an applied load without breaking or changing its shape. The strength of an ideal, defect-free crystal can be very high, but except for rather exotic materials such as micrometer-sized whiskers, crystals will fracture and/or deform plastically under stresses that are well below their ideal strength limits.

Catalysts based on cerium oxide are now used as effective oxidation systems in numerous environmental applications. Cerium oxide was introduced into the catalysis field relatively recently, in 1976, and not as a catalyst initially. Rather, it was chosen as the key oxygen-storage component of the three-way catalyst (TWC) used in automotive exhausts. Accordingly, ceria is used to extend the air/fuel ratio window in the exhaust gas, releasing or accepting oxygen, respectively, under fuel-rich or fuellean conditions, so that the noble metal catalyst operates at the desirable stoichiometric air/fuel ratio, at which it effectively converts all three gaseous pollutants—CO, hydrocarbons, and NO—to innocuous products. A solid solution of cerium and zirconium oxides is used in today's catalytic converters because of its higher oxygen-storage capacity (OSC) compared with pure ceria. In the years that followed the introduction of ceria into the catalytic converter, many additional merits of cerium oxide were realized, first as an active catalytic component of the TWC and subsequently as a catalyst and sorbent in various industrial applications. A review article by Trovarelli on ceria-based catalysts is a good recent compilation.

Microelectromechanical systems (MEMS) devices are being manufactured in the hundreds of millions and are widely deployed for pressure sensor, accelerometer, display, and printing applications. This suggests customer confidence in the longterm reliability of MEMS (also known as microsystems or micromachines) under diverse stringent conditions.

Many technologies require load-sustaining components that are as lightweight and compact as possible. This requirement is especially true in the transportation area, where lightness equates with fuel efficiency and range. Concept implementation is dictated by the tradeoff between performance and manufacturing cost, which is manifest in a value factor. The attainment of the minimum weight has a long history. Four technical factors are involved: materials selection, utilization of shape, topology optimization, and multifunctionality. The new developments to be addressed in this article arise in topology optimization. Multifunctionality has been elaborated elsewhere.