The electrostatic field of CO functionalized metal tips

Abstract

This chapter conclusively shows that the electric field created by CO functionalized metal tips cannot be described by a single dipole. It is necessary to take into account both the positive dipole that describes the electric field created by the metal tip and the negative charge cloud strongly localized in front of the oxygen atom. We have incorporated this insight into a theoretical model that allows the efficient simulation of AFM measurements retaining a first-principles accuracy. Using this model, we have identified the contrast formation mechanisms for localized ionic defects (Cl vacancies on a metal–supported NaCl bilayer). The opposite sign and different spatial extension of the associated electric fields explain the rich contrast observed. While both terms compete to determine the contrast of uncompensated, extended defects like the Cl vacancy, atomic–scale resolution of the ionic lattice arises mainly from the CO electric field as the more extended field created by the metal apex averages out the contribution coming from those periodic and rapidly varying charge distributions. The insight gained from our analysis is used to address the apparent contradiction in the interpretation of previous experiments involving CO molecules either as a tip on a metallic apex probing ionic surfaces or as an adsorbate probed with a pure metallic tip.