STM-theory: Image potential, chemistry and surface relaxation

A critical review of the different methods used nowadays for calculating tunneling currents and STM-images is presented with a special emphasis on the role played by the interface image potential and the interaction between the tip and the sample at short distances. After presenting the most commonly used approaches to this problem, we discuss in full detail how the image potential modifies critically the interface tip-sample barrier and how neglecting this effect underestimates the tunneling currents by several orders of magnitude. Although interface non-local image potential effects are difficult to introduce in a plane-wave Density Functional approach, we show how a Green's function Density Functional formalism based on a local-orbital basis set allows us to introduce those image effects with a good accuracy. The effect of the interaction between the tip and the sample is illustrated for an Al-tip approaching an Al surface; and the role of the electronegative atoms adsorbed on the tip is discussed considering the O/Pd(1 1 1) interface and the effect of having an O-atom adsorbed on the tip apex. Finally, by analyzing the Si(1 1 2)-Ga interface we also show how the Green's function Density Functional approach based on a local orbital basis can also be reliably used to analyze surface steochiometries.