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

Most materials studied and/or used technologically today are electrically n eutral, i.e the positive and negative electric charges are balanced. The pr operties of these materials are varied predominantly by modifying the atomi c structure (e.g. by alloying, introduction of lattice defects, etc.) Limit ed attention has been paid so far to the option of influencing the properti es of materials by deviating from charge neutrality. In fact, solids with n anometer-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 p er atom. Such deviations from charge neutrality may be achieved either by m eans of an extremely applied voltage or by space charges at interfaces: bet ween materials with different chemical compositions (or combinations of bot h). As many properties of solid materials depend on their electronic struct ure, significant deviations from charge neutrality result in materials with new, yet mostly unexplored properties such as modified electric, ferromagn etic, optical etc. properties. In fact, two types of property variations ma y be distinguished: Variations that last as long as the external voltage is applied and subsequently vanish reversibly once the external voltage is re moved. These properties: may be tuned by tuning the applied voltage. The se cond 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 pe rsistent variations. Existing and conceivable new technological application s of solids deviation from charge neutrality are discussed. (C) 2001 Acta M aterialia Inc. Published by Elsevier Science Ltd All rights reserved.