[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: The dependence of the contrast and symmetry of scanning tunneling microscope images of O∕Pd(111)−2×2 on the structure of the tunneling tip and on tunneling parameters is explained using first-principles density functional theory. Experimentally, the contrast changes in different ways when a metal-terminated tip over hcp and top sites changes its bias and tip-sample distance. These changes are also reflected in the symmetry of the image. A detailed analysis of the tunneling contributions indicates that for the metallic tips, the Pd d orbitals are determining the image symmetry at close range and low bias, while at larger separations and high bias the Pd pz orbitals are the ones that control the image contrast. For oxygen-terminated tips, we predict a positive image contrast, associated with the tip oxygen bonds, as opposed to the negative contrast images obtained with metallic tips.
[show abstract][hide abstract] ABSTRACT: The operation of the STM on metallic surfaces from the tunneling to the contact regime has been explored with a combination of first-principles total energy methods and a calculation of the electronic currents based on nonequilibrium Keldish-Green’s function techniques. Our calculations for the behavior of the total energy, forces, atomic relaxations, and currents for an Al tip on an Al(111) surface as a function of the tip-sample distance indicate that atomic relaxations and saturation effects become relevant in a similar distance range where the onset of a short-range chemical interaction between the tip apex and the surface atoms is taking place. These two factors, that have an opposite influence in the current, lead to corrugations of the order of 0.2 Å, similar to the ones found experimentally in other (111) metal surfaces, for the closer distances (around 4.25 Å) where stable operation can be achieved.