In the last two decades low-dimensional (low-d) physics has matured into a major branch of science. Quite generally we may define a system with restricted dimensionality d as an object that is infinite only in one or two spatial directions (d = 1 and 2). Such a definition comprises isolated single chains or layers, but also fibres and thin layers (films) of varying but finite thickness. Clearly, a multitude of physical phenomena, notably in solid state physics, fall into these categories. As examples, we may mention: * Magnetic chains or layers (thin-film technology). * Metallic films (homogeneous or heterogeneous, crystalline, amorphous or microcristalline, etc.). * I-d or 2-d conductors and superconductors. * Intercalated systems. * 2-d electron gases (electrons on helium, semiconductor interfaces). * Surface layer problems (2-d melting of monolayers of noble gases on a substrate, surface problems in general). * Superfluid films of ~He or 'He. * Polymer physics. * Organic and inorganic chain conductors, superionic conductors. * I-d or 2-d molecular crystals and liquid crystals. * I-d or 2-d ferro- and antiferro electrics.

The study of clusters is one of the most exciting topics in the rapidly developing field of nanostructured materials. As discussed in this book, nanometer-sized metal particles can be obtained not only by evaporation methods, producing atomic or molecular beams, but also by the chemical synthesis of metal cluster compounds. These have a well-defined stoichiometry and are composed of metal clusters of a given, homogeneous size, each cluster being surrounded by a shell of ligand molecules. Accordingly, the compounds provide excellent model systems for macroscopic assemblies of small metal particles embedded in a dielectric solid. The underlying physical properties are described in terms of a three-dimensional matrix of mutually separated quantum wells. In going from one compound to another, the size and separation of the quantum wells may be varied. In this way, one may study such fundamental properties as the size-induced transition from metallic to molecular behaviour. At the same time, the electronic level structure may be changed in a controlled way, which should confer tunable optical, electrical or magnetic properties.

In the last two decades low-dimensional (low-d) physics has matured into a major branch of science. Quite generally we may define a system with restricted dimensionality d as an object that is infinite only in one or two spatial directions (d = 1 and 2). Such a definition comprises isolated single chains or layers, but also fibres and thin layers (films) of varying but finite thickness. Clearly, a multitude of physical phenomena, notably in solid state physics, fall into these categories. As examples, we may mention: * Magnetic chains or layers (thin-film technology). * Metallic films (homogeneous or heterogeneous, crystalline, amorphous or microcristalline, etc.). * I-d or 2-d conductors and superconductors. * Intercalated systems. * 2-d electron gases (electrons on helium, semiconductor interfaces). * Surface layer problems (2-d melting of monolayers of noble gases on a substrate, surface problems in general). * Superfluid films of ~He or 'He. * Polymer physics. * Organic and inorganic chain conductors, superionic conductors. * I-d or 2-d molecular crystals and liquid crystals. * I-d or 2-d ferro- and antiferro electrics.