Tuning the electronic structure of solids by means of nanometer-sized microstructures

Most materials studied and/or used technologically today are electrically neutral, i.e the positive and negative electric charges are balanced. The properties of these materials are varied predominantly by modifying the atomic structure (e.g. by alloying, introduction of lattice defects, etc.) Limited attention has been paid so far to the option of influencing the properties of materials by deviating from charge neutrality. In fact, solids with nanometer-sized microstructures may open the way to generate materials with an excess of a deficit of electrons or holes of up to 0.3 electrons/holes per atom. Such deviations from charge neutrality may be achieved either by means of an extremely applied voltage or by space charges at interfaces: between materials with different chemical compositions (or combinations of both). As many properties of solid materials depend on their electronic structure, significant deviations from charge neutrality result in materials with new, yet mostly unexplored properties such as modified electric, ferromagnetic, optical etc. properties. In fact, two types of property variations may be distinguished: Variations that last as long as the external voltage is applied and subsequently vanish reversibly once the external voltage is removed. These properties: may be tuned by tuning the applied voltage. The second type of property changes are the persistent ones. They remain even if the material returns to the electrically neutral state. New types of alloys or materials with new types of atomic structures seem to be examples of persistent variations. Existing and conceivable new technological applications of solids deviation from charge neutrality are discussed. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd All rights reserved.