During a lecture on the optical properties of gold in 1857, Michael Faraday remarked that “a mere variation in the size of its particles gave rise to a variety of resultant colors.” Since Faraday was referring to his studies of nanometer-scale gold particles (now commonly referred to as nanocrystals), it is clear that the topic of this issue of MRS Bulletin is an old field with an important history. Indeed, the study of Faraday's gold sols was critical for the development of several aspects of modern materials science, including how light interacts with matter and how colloids can be formed, manipulated, and controlled.

Magnetic colloids, or ferrofluids, have been studied to probe the fundamental size-dependent properties of magnetic particles and have been harnessed in a variety of applications. The magnetorheological properties of magnetic colloids have been exploited in high-performance bearings and seals. The deposition of magnetic dispersions on platters and tapes marked the earliest embodiments of magnet information storage. Magnetic particles enhance contrast in magnetic resonance imaging and promise future diagnostic and drug delivery applications. The need to explore the scaling limits of magnetic storage technology has motivated the preparation of size-tunable monodisperse magnetic nanoparticles with controlled internal structures. The study of these nanoparticles is critical to efforts to separate the role of defects from intrinsic, finite size effects.

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.

Semiconductor materials are widely used in both optically and electrically pumped lasers. The use of semiconductor quantum wells (QWs) as optical-gain media has resulted in important advances in laser technology. QWs have a two-dimensional, step-like density of electronic states that is nonzero at the band edge, enabling a higher concentration of carriers to contribute to the band-edge emission and leading to a reduced lasing threshold, improved temperature stability, and a narrower emission line. A further enhancement in the density of the band-edge states and an associated reduction in the lasing threshold are in principle possible using quantum wires and quantum dots (QDs), in which the confinement is in two and three dimensions, respectively. In very small dots, the spacing of the electronic states is much greater than the available thermal energy (strong confinement), inhibiting thermal depopulation of the lowest electronic states. This effect should result in a lasing threshold that is temperatureinsensitive at an excitation level of only 1 electron-hole (e-h) pair per dot on average. Additionally, QDs in the strongconfinement regime have an emission wavelength that is a pronounced function of size, adding the advantage of continuous spectral tunability over a wide energy range simply by changing the size of the dots.

Modern semiconductor technology has been enabled by the ability to control the number of carriers (electrons and holes) that are available in the semiconductor crystal. This control has been achieved primarily with two methods: doping, which entails the introduction of impurity atoms that contribute additional carriers into the crystal lattice; and charging, which involves the use of applied electric fields to manipulate carrier densities near an interface or junction. By controlling the carriers with these methods, the electrical properties of the semiconductor can be precisely tailored for a particular application. Accordingly, doping and charging play a major role in most modern semiconductor devices.

Generally, when one looks up the properties of an element such as gold in an encyclopedia, one is immediately confronted with a list of established facts and figures. While many people are aware that most countries do not rely on a “gold reserve” anymore, it is surprising to learn that even basic physical and chemical properties of something as familiar as gold are not as fixed as one may believe. The reason for this is that tabulated values for material properties inevitably refer to macroscopic samples. It turns out that many simple properties such as those listed in the definition given here go spectacularly wrong as the size of the piece of gold is reduced. One of the most dramatic demonstrations of this was provided by Buffat and Borel, who measured the melting point of gold particles as a function of particle size using an electron microscope. They found that by the time gold crystals are just 4 nm across, the melting point drops to 700 K from its “encyclopedia value” of 1337 K. Another example is the noble character of gold metal.

Nanotechnology, a much abused term, refers to the fabrication and assembly of functional objects of nanometer dimensions. In this article, we will refer only to specific aspects of chemical nanotechnology, meaning by this the production of structures of nanometer dimensions by chemical means. In particular, we will center the discussion on the use of metal nanoparticles as chemical building blocks.