Nanostructured materials: State of the art and perspectives

Nanostructured materials (NsM) are solids composed of structural elements - mostly crystallites - with a characteristic size (in at least one direction) of a few nanometers. NsM may be classified into twelve groups according to the shape and chemical composition of their constituent structural elements. The atomic structure and properties of NsM deviate from the ones of a single crystal and/or glass with the same chemical composition. This deviation results from the reduced size of the crystallites, dimensionality effects due to the shape of the crystallites (thin plates, needles or equiaxed shape), and the reduced density and/or modified coordination numbers in the interfaces between the structural elements. NsM are non-equilibrium solids and, hence, their structure and properties depend on the mode of preparation and on the time-temperature history. Some of the observations supporting this view are discussed. Technological applications of metallic, macromolecular and semiconducting NsM are described.