Josef Redinger

Atomic resolution by STM on ultra-thin films of alkali halides: experiment and local density calculations

Abstract Atomically resolved scanning tunneling microscopy (STM) of ultra-thin NaCl films on Al(111) and Al(100) demonstrates that only one atomic species of NaCl is imaged as a protrusion. By comparison of the constant current STM images with ab initio calculations of the local density of states by means of the full-potential linearized augmented plane wave method, the protrusions could be attributed to the anion Cl – . The calculations shows that a higher density of occupied states at the Cl-sites than for the Na-sites around the Fermi level causes the STM contrast between Cl and Na. With increasing number of NaCl layers the density of states in the band gap is reduced and the apparent height of additional NaCl layers decreases. The maximum film thickness allowing successful imaging by STM was found to be three layers.

Nickel carbide as a source of grain rotation in epitaxial graphene.

Graphene has a close lattice match to the Ni(111) surface, resulting in a preference for 1 × 1 configurations. We have investigated graphene grown by chemical vapor deposition (CVD) on the nickel carbide (Ni(2)C) reconstruction of Ni(111) with scanning tunneling microscopy (STM). The presence of excess carbon, in the form of Ni(2)C, prevents graphene from adopting the preferred 1 × 1 configuration and leads to grain rotation. STM measurements show that residual Ni(2)C domains are present under rotated graphene. Nickel vacancy islands are observed at the periphery of rotated grains and indicate Ni(2)C dissolution after graphene growth. Density functional theory (DFT) calculations predict a very weak (van der Waals type) interaction of graphene with the underlying Ni(2)C, which should facilitate a phase separation of the carbide into metal-supported graphene. These results demonstrate that surface phases such as Ni(2)C can play a major role in the quality of epitaxial graphene.

Vacancy induced changes in the electronic structure of titanium carbide—I. Band structure and density of states

Abstract Results of a self-consistent “Augmented Plane Wave” (APW) band-structure calculation are presented for substoichiometric titanium carbide with 25% vacancies on the carbon sublattice sites (TiC 0 75 ) assuming a model structure with ordered vacancies Comparison with an earlier APW study on stoichiometric TiC reveals that the carbon vacancies induce two pronounced peaks in the density of states (DOS), 0.4 eV below and 0.8 eV above the Fermi energy E γ , thus forming electronic states in a region where the DOS for stoichiometric TiC exhibits a minimum So-called “vacancy states” with an important amount of charge on the vacancy site are found to be derived from Ti 3 d states extending into the vacancy muffin tin sphere An angular momentum decomposition with respect to the center of the vacancy muffin tin sphere shows that the s character predominates for the occupied and the p character for the unoccupied “vacancy states” The theoretical findings explain features near E γ , observed in recently published X-ray emission spectra Furthermore, we find a slight increase of electronic charge in the carbon muffin tin spheres as compared with stoichiometnc TiC.

Scanning tunneling microscopy of binary alloys: first principles calculation of the current for PtX (100) surfaces.

To probe the influence of realistic tip models on the tunnel current and the corrugation of binary alloy surfaces we have calculated the electronic structure of PtX (100) sample surfaces and realistic STM tips with different tip atoms. We then used the Bardeen integral to calculate the tunnel current from the electronic structure of sample and tip numerically. Apart from the usual approximations of the perturbation approach the method developed is therefore fully ab initio. It can be shown that the currents obtained in the limit of low bias voltage are within the range of measurements, and equally, that including a realistic tip improves the agreement between measurements and calculations.

FLAPW: applications and implementations.

Modern material design involves a close collaboration between experimental and computational materials scientists. To be useful, the theory must be able to accurately predict the stability and properties of new materials, describe the physics of the experiments, and be applicable to new and complex structures-the all-electron full-potential linearized augmented plane wave (FLAPW) is one such method that provides the requisite level of numerical accuracy, albeit at the cost of complexity. Technical aspects and modifications related to the choice of basis functions (energy parameters, core-valence orthogonality, extended local orbitals) that affect the applicability and accuracy of the method are described, as well as an approach for obtaining k-independent matrix elements. The inclusion of external electric fields is illustrated by results for the induced densities at the surfaces of both magnetic and non-magnetic metals, and the relationship to image planes and to nonlinear effects such as second harmonic generation. The magnetic coupling of core hole excitations in Fe, the calculation of intrinsic defect formation energies, the concentration-dependent chemical potentials, entropic contributions, and the relative phase stability of Zr-rich Zr-Al alloys are also discussed.

Scanning tunneling microscopy of binary-alloy surfaces: is chemical contrast a consequence of alloying?

Abstract Recent STM studies achieved chemical resolution on PtRh and PtNi alloy surfaces. By a first-principles method employing the Tersoff–Hamann model, we have simulated STM scans on PtRh and PtNi(100) surfaces by calculating the apparent heights of individual surface atoms. The difference in apparent heights between Pt and Rh atoms is caused by changes in the density of states due to alloying. The simulations for the PtNi(100) surface, however, yield apparent heights of Pt and Ni atoms below atomic resolution, indicating that in the experiment, tip–sample interactions are responsible for chemical and atomic resolution.

Vacancy-induced changes in the electronic structure of transition metal carbides and nitrides: calculation of X-ray photoemission intensities

Starting from KKR-CPA and KKR-GF densities of states and cross-sections within the single-scatterer final-state approximation, X-ray photoemission intensities were calculated for a series of stoichiometric and substoichiometric transition metal carbides and nitrides. For all compounds nonmetal vacancies give rise to an additional peak in the XPS spectra. The theoretical results are compared to several experimental XPS measurements. In most cases very good agreement is found. The discrepancies between theory and experiment are discussed in detail.

Vacancy induced changes in the electronic structure of titanium nitride

Abstract A self-consistent APW band structure calculation has been performed for TiN0.75, assuming long-range ordered vacancies at the nonmetal lattice sites. Two different kinds of titanium atoms occur in this model: Ti[6] atoms that are octahedrally surrounded by six nitrogen atoms and Ti[4] atoms that have only four nitrogen neighbors and are adjacent to two vacancies. The model structure can be described as Ti[4]3Ti[6]N3□N, where □N denotes a nitrogen vacancy. In the densities of states, two sharp vacancy peaks have been found which are not present in stoichiometric TiN. The bonding situation is discussed by means of electron density plots. It is found that the chemical bonding is characteristically influenced by the introduction of vacancies. The calculated XPS and K XES are shown to be in good agreement with the experimental spectra.

Vacancy induced changes in the electronic structure of titanium carbide—II. Electron densities and chemical bonding

Abstract In a recent paper, we presented the band structure and the state densities for an ordered model structure for TiCn 0.75 and discussed the changes which occur in comparison with stoichiometric TiC. Starting from these results, we investigate in the present paper, on the basis of calculated electron densities, how the bonding situation is influenced by the vacancies on the carbon sublattice in TiC 0.75 . The results can be summarized as follows: The presence of empty lattice sites leads to the formation of new types of bonds not present in TiC; i.e., weak bonds between second nearest Ti neighbors across the vacancy sites and stronger Ti-Ti bonds in the Ti octahedra around the vacancies caused by the reduction of the number of C-Ti bonds. An analysis of the electron densities also shows a strengthening of the covalent Ti-Ti bonds involving Ti atoms not immediately adjacent to a vacancy as well as an increase of the ionic character of the C-Ti bonds.

Artificially lattice-mismatched graphene/metal interface: Graphene/Ni/Ir(111)

We report the structural and electronic properties of an artificial graphene/Ni(111) system obtained by the intercalation of a monoatomic layer of Ni in graphene/Ir(111). Upon intercalation, Ni grows epitaxially on Ir(111), resulting in a lattice mismatched graphene/Ni system. By performing Scanning Tunneling Microscopy (STM) measurements and Density Functional Theory (DFT) calculations, we show that the intercalated Ni layer leads to a pronounced buckling of the graphene film. At the same time an enhanced interaction is measured by Angle-Resolved Photo-Emission Spectroscopy (ARPES), showing a clear transition from a nearly-undisturbed to a strongly-hybridized graphene -band. A comparison of the intercalation-like graphene system with flat graphene on bulk Ni(111), and mildly corrugated graphene on Ir(111), allows to disentangle the two key properties which lead to the observed increased interaction, namely lattice matching and electronic interaction. Although the latter determines the strength of the hybridization, we find an important influence of the local carbon configuration resulting from the lattice mismatch.