A constrained state of solid matter was known to exist in the cores of crystal defects - for example - in the cores of intercrystalline interfaces. This constrained state of solid matter differs structurally and property-wise from other (unconstrained) solid states such as perfect crystals, glasses etc. in terms of its atomic and electronic structure as well as in its chemical composition. It was the basic idea of nanocrystalline solids to generate a novel type of materials by incorporating such a high density of defect cores into a - formerly - perfect crystal that the total volume of these defect cores became comparable to the total volume of the residual lattice regions between the defect cores. The resulting solids were called nanocrystalline solids. Due to the large volume fraction of defect cores, nanocrystalline solids differ from other forms of solids (e.g. single crystals, coarse-grained polycrystals, glasses) in terms of their atomic and electronic structure, their chemical composition and by the fact that the size of the crystalline regions between neighboring defects was reduced to a few interatomic spacings. As the properties of solids depended upon exactly those four parameters (atomic structure, electronic structure, chemical composition and crystal size), the properties of nanocrystalline solids deviate from the ones of crystalline or glassy materials.
Attention here was focused on theTuning of the electronic structure of solids by means of their nanostructure. In fact, solids with nanometer-sized microstructures may open the way to generate materials with an excess or a deficit of electrons or holes of up to 0.3 electrons/holes per atom i.e. elements that were electronically “between” the (electrically neutral) elements of the periodic table. Large deviations from charge neutrality may be achieved either by means of an externally applied voltage or by the space charges at interfaces between materials with mobile charge carriers (such as metals or semiconductors) and with different chemical compositions (or combinations of both). As many properties of solid materials depend upon their electronic structure, significant deviations from charge neutrality may led the way into a world of materials with new, yet mostly unexplored properties such as modified electric, ferromagnetic, optical properties, etc.. Some existing and conceivable new technological applications of solids deviation from charge neutrality were briefly discussed.