Is there a Hidden World of New Materials and Effects “between” the Elements of the Periodic Table?

Abstract

A constrained state of solid matter is 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 propertywise 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 is 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 becomes comparable to the total volume of the residual lattice regions between the defect cores. The resulting solids are 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 neighbouring defects is reduced to a few interatomic spacings. As the properties of solids depend on 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. In this paper attention will be focused on the tuning 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 are 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 on their electronic structure, significant deviations from charge neutrality may lead the way into a world of materials with new, yet mostly unexplored properties such as modified electric, ferromagnetic, optical etc. properties. Some existing and conceivable new technological applications of solids deviation from charge neutrality are briefly discussed.