Origin of contrast in STM images of oxygen on Pd(111) and its dependence on tip structure and tunneling parameters

Abstract

The dependence of the contrast and symmetry of scanning tunneling microscope images of O / Pd͑111͒ −2 ϫ 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 p z 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.

Surface Species Formed by the Adsorption and Dissociation of Water Molecules on a Ru(0001) Surface Containing a Small Coverage of Carbon Atoms Studied by Scanning Tunneling Microscopy

“…A decomposition of the current in terms of adsorbate and substrate contributions reveals that the triangular shape of the C atoms in the images is the result of direct tunneling into the surrounding Ru atoms, whose electronic structure is strongly perturbed by presence of the C. We also note that the orientation of the triangular shape of C in the images is inverted with respect to the triangle of 3 Ru atoms around the hollow site (Figure 3 (a)). Similar shapes have been reported for O on several metal surfaces, 18,19 although in many cases the triangle orientation was the opposite.…”

Interactions of oxygen and hydrogen on Pd(111) surface

“…Examples of these structures are shown in the STM images of Figure 2. In the images O appears as a depression, about 0.4 Å when isolated, a well know electronic effect due to the loss of density of states at the Fermi edge near the oxygen atoms [30]. The broad brighter spots in the figure between the O structures correspond to ~ 0.1 Å protrusions due to subsurface impurities.…”

Advances and applications in the FIREBALLab initio tight‐binding molecular‐dynamics formalism

“…In particular, the FIREBALL approach allows to perform complex studies of dependence of atomic scale contrast on atomic and chemical structure of probe [133,137].…”

“…In this approach, the tip sample interaction is obtained calculating the hopping integrals between the orbitals of both sides. The approach allows us to treat on the same foot both the tunneling and the contact regimes, where commonly used perturbation methods give incorrect results [133]. In this approach the Hamiltonian is written as the sum of three terms:Ĥ ¼Ĥ s þĤ t þĤ tÀs , with one term associated with the sample (s), one with the tip (t), and finally a term that takes into account the interaction between tip and sample (t-s).…”