| In this thesis, we examine the effect of an external electric field and surface charging on surface properties. We use density functional techniques. Although conceivable empirical techniques such as tight-binding methods can be formulated to consider electric field effects, they are generally not accurate enough since an electric field would always induce charge rearrangement. The charge rearrangement can be handled self-consistently and accurately by first-principles methods such as density functional techniques, but not by empirical methods. One of the key parameters of a surface that determines its electric field response is the work function. We shall establish the relationship between surface dipole density and work function within the framework of local density formalism to study the change of work functions induced by a layer of adsorbates. While many systems obey the commonly accepted rule that electronegative adsorbates increase the work function of the surface, we find that subtle details of the charge transfer can determine the sign and magnitude of surface dipole change, leading to a strong dependence on the orientation of the substrate, with the consequence that work function changes are not always governed by the sign and quantity of adsorbate induced charge transfer. Electric field is implemented into first principles density functional calculations of metal surfaces to study the surface energetics under an externally imposed field. We use Au surface as the prototype, since there are well-known surface morphology surfaces in an electrochemical environment where strong electric field is present. For Au(100) and hexagonal reconstructed Au(100)-hex, their surface energy changes under electric field are mainly contributed by interaction between surface dipole and the electric field. Work function and surface dipole density of Au(100) are lower so that its surface energy is favorable at a high positive electric field. The calculated critical electric field of reconstruction transition agrees with the corresponding electrode potential observed in the electrochemical experiment. We also consider the adsorption of polar molecules under the influence of an external electric field. Adsorption energies of CO at different sites on Pt(111) surface are calculated, and we find that at some particular field strength, the adsorption energies are nearly equal to each other. The site dependence is traced to interaction between the electric field and surface dipole of the CO/Pt(111) surface and the screening properties of the substrate. The field dependence of adsorption energies implies that the diffusion constants and pathways can be modified by external fields, which can be used as an extra degree of freedom to mediate surface processes. |
