Physics in High Electric Fields

Abstract

This thesis is a theoretical study on the effects of high electrostatic fields, fields of the order of Volts per Angstrøm, on the structure and stability of water, MgO, ZnO, and Si nanoclusters. A high electrostatic field applied across a material will raise the energy levels of a molecular orbital relative to another molecular orbital in the field direction. This rearrangement of the bonding and antibonding orbitals or field-induced chemistry can cause: (i) the electronic transformation of one material into another creating a continuous periodic table, (ii) the formation of new structures which are stable in the field, and (iii) field-induced dissociation or field evaporation. We perform detailed quantum mechanical calculations using density functional theory to explain experimental observations and explore the above effects. To understand Atom Probe results on the field evaporation of semiconductors we calculate the effects of high electrostatic fields on MgO, ZnO, and Si clusters to follow the structural changes during field evaporation and to obtain potential energy curves, partial charges and desorption pathways. We obtain field evaporation products seen in experiment, address the question of what happens to the oxygen during field evaporation of the oxide, and explain why the O goes missing. It can migrate down the (metallic) surface of the tip and eventually desorb either as atoms or molecules (neutral). Evidence of field-induced metallization of the semiconductors is seen through closure of the HOMO-LUMO gaps, increases in dielectric constant and polarizability, and field expulsion inside the clusters. Strong band gap shrinkage of a dielectric nanostructure in a field was experimentally confirmed. Finally, it has been observed in field ion microscopy experiments that long whiskers of up to 12 water molecules can form. We present whisker structures and energetics, lower and upper threshold fields, and fragmentation patterns. Our results are in general agreement with experiments including the fact that predominantly small protonated clusters break off. We also look at the quantum mechanics of charge transport along proton wires both with free ends and donor/acceptor terminated. The charge transfer times and the conductivity of the proton wire were calculated in agreement with experimental results.