Controlled lateral and perpendicular motion of atoms on metal surfaces

Abstract

Nanoscale modification of matter has been the subject of interest. Recently, several experimental studies have demonstrated that by using a scanning tunneling microscope one can translate atoms on metal surfaces to a desired position. Furthermore, it has been shown that an atom between surface and tip can be transferred reversibly which results in bistable conductance. The controlled dynamics of adsorbed species has opened a new field of research. This thesis work provides a theoretical investigation of the controlled lateral and perpendicular motion of an inert gas atom (Xe) on metal surfaces. The lateral motion of Xe on the Ni(llO) and P t(lll) surfaces is manipulated by a W tip. The interaction energy of the physisorbed atom with the tip and metal surface is described by an empirical potential. Using molecular statics the energy surfaces are calculated and the adsorbtion sites are determined. By using the molecular dynamics calculations, the variation in the coordinates of the adsorbate Xe with the tip moving at a given height are obtained. Three different modes of Xe translation are distinguished depending on the height of the tip. These are i) carriage on the tip, ii) pushing and, iii) pulling modes. The range of the tip height where one of these modes occur is strongly depended on the relaxation of electrodes and the geometry of the tip. Controlled and reversible transfer of atoms between the metal surface and the tip is studied by the transfer of Xe between two flat P t(lll) surfaces. Physisorption of Xe on the P t(lll) surface is studied by an empirical potential including short and long-range interactions and yielding correct account of several experimental data. Effective charge on Xe and the dipole moment constructed therefrom are calculated as a function of the Xe-surface separation. The potential energy curve of Xe between two P t(lll) surfaces and quantum states of Xe therein are calculated as a function of the applied voltage and separation between two P t (lll) surfaces. Within this model, various mechanisms, such as tunneling of Xe, dipole excitation and resonant tunneling, electromigration contributing to the transfer of Xe are examined. The transfer rate of Xe is then calculated for different mechanisms. Its dependence on the bias voltage is explored. The overall behavior of the total transfer rate is not a power law. While at low bias voltages thermal assisted atom tunneling is effective, the dipole excitation and resonant tunneling becomes dominant at high bias voltages.