One-dimensional Si chains embedded in Pt(111)and protected by a hexagonal boron-nitride monolayer

Using scanning tunneling microscopy, we show that Si deposition on Pt(111) at 300K leads to a network of one-dimensional Si chains. On the bare Pt(111) surface, the chains, embedded into the Pt surface, are orientated along the -direction. They disappear within a few hours in ultrahigh vacuum due to the presence of residual gas. Exposing the chains to different gases deliberately reveals that CO is largely responsible for the disappearance of the chains. The chains can be stabilized by a monolayer of hexagonal boron nitride, which is deposited prior to the Si deposition. The resulting Si chains are rotated by 30{\deg} with respect to the chains on the bare Pt(111) surface and survive even an exposure to air for 10 minutes.Comment: 8 pages, 4 Figure

Density-functional theory study on the arrangement of adsorbed formate molecules on Cu(110)

The interaction of formate molecules with the Cu(110) surface is investigated using density-functional theory calculations. We find that in the most stable structures for low and high coverage, the formate molecules are sitting perpendicular to the Cu(110) surface, and they are adsorbed in a bridge position, i.e., the O-C-O group forms a bridge between two Cu atoms. Other tested configurations are less stable by at least 0.45 eV per formate molecule. In the case of an oxygen-precovered Cu(110) surface with high formate coverage [two molecules in a (2x2) unit cell] we find a very similar adsorption geometry. We find an attractive interaction between adsorbed formate molecules on the copper surface. Our results are consistent with experimental results by scanning tunneling microscopy and photoelectron diffraction

Cd-vacancy and Cd-interstitial complexes in Si and Ge

The electrical field gradient (EFG), measured e.g. in perturbed angular correlation (PAC) experiments, gives particularly useful information about the interaction of probe atoms like 111In / 111Cd with other defects. The interpretation of the EFG is, however, a difficult task. This paper aims at understanding the interaction of Cd impurities with vacancies and interstitials in Si and Ge, which represents a controversial issue. We apply two complementary ab initio methods in the framework of density functional theory (DFT), (i) the all electron Korringa-Kohn-Rostoker (KKR) Greenfunction method and (ii) the Pseudopotential-Plane-Wave (PPW) method, to search for the correct local geometry. Surprisingly we find that both in Si and Ge the substitutional Cd-vacancy complex is unstable and relaxes to a split-vacancy complex with the Cd on the bond-center site. This complex has a very small EFG, allowing a unique assignment of the small measured EFGs of 54MHz in Ge and 28MHz in Si. Also, for the Cd-selfinterstitial complex we obtain a highly symmetrical split configuration with large EFGs, being in reasonable agreement with experiments

Spin- and energy-dependent tunneling through a single molecule with intramolecular spatial resolution

We investigate the spin- and energy dependent tunneling through a single organic molecule (CoPc) adsorbed on a ferromagnetic Fe thin film, spatially resolved by low-temperature spin-polarized scanning tunneling microscopy. Interestingly, the metal ion as well as the organic ligand show a significant spin-dependence of tunneling current flow. State-of-the-art ab initio calculations including also van-der-Waals interactions reveal a strong hybridization of molecular orbitals and surface 3d states. The molecule is anionic due to a transfer of one electron, resulting in a non-magnetic (S= 0) state. Nevertheless, tunneling through the molecule exhibits a pronounced spin-dependence due to spin-split molecule-surface hybrid states.Comment: Version of Submission, 18-03-201

Graphene on the Ir(111) surface: from van der Waals to strong bonding

We calculated the properties of a graphene monolayer on the Ir(111) surface, using the model in which the periodicities of the two structures are assumed equal, instead of the observed slight mismatch which leads to a large superperiodic unit cell. We used the density functional theory approach supplemented with the recently developed van der Waals-density function (vdW-DF) non-local correlation functional. The latter is essential for treating the vdW interaction, which is crucial for the adsorption distances and energies of the rather weakly bound graphene. When additional iridium atoms are put on top of graphene, the electronic structure of C atoms acquires the sp(3) character and strong bonds with the iridium atoms are formed. We discuss the validity of the approximations used and their relevance to other graphene-metal systems

Controlling the Local Spin-Polarization at the Organic-Ferromagnetic Interface

By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments we show how to manipulate the local spin-polarization of a ferromagnetic surface by creating a complex energy dependent magnetic structure. We demonstrate this novel effect by adsorbing organic molecules containing pi(pz)-electrons onto a ferromagnetic surface, in which the hybridization of the out-of-plane pz atomic type orbitals with the d-states of the metal leads to the inversion of the spin-polarization at the organic site due to a pz - d Zener exchange type mechanism. As a key result, we demonstrate that it is possible to selectively inject spin-up and spin-down electrons from the same ferromagnetic surface, an effect which can be exploited in future spintronic devices

Role of the van der Waals interactions on the bonding mechanism of pyridine on Cu(110) and Ag(110) surfaces: A first-principles study

We performed density-functional calculations aimed to investigate the adsorption mechanism of a single pyridine (C5H5N) molecule on Cu(110) and Ag(110) surfaces. Our ab initio simulations show that, in the ground state, the pyridine molecule adsorbs with its molecular plane perpendicular to these substrates and is oriented along the [001] direction. In this case, the bonding mechanism involves a sigma bond through the lone-pair electrons of the nitrogen atom. When the heterocyclic ring is parallel to the surface, the bonding takes place via pi-like molecular orbitals. However, depending on the position of the N atom on the surface, the planar adsorption configuration can relax to a perpendicular geometry. The role of the long-range van der Waals interactions on the adsorption geometries and energies was analyzed in the framework of the semiempirical method proposed by Grimme [J. Comput. Chem. 27, 1787 (2006)]. We demonstrate that these dispersion effects are very important for geometry and electronic structure of flat adsorption configurations

Real-space electronic-structure calculations with full-potential all-electron precision for transition-metals

We have developed an efficient computational scheme utilizing the real-space finite-difference formalism and the projector augmented-wave (PAW) method to perform precise first-principles electronic-structure simulations based on the density functional theory for systems containing transition metals with a modest computational effort. By combining the advantages of the time-saving double-grid technique and the Fourier filtering procedure for the projectors of pseudopotentials, we can overcome the egg box effect in the computations even for first-row elements and transition metals, which is a problem of the real-space finite-difference formalism. In order to demonstrate the potential power in terms of precision and applicability of the present scheme, we have carried out simulations to examine several bulk properties and structural energy differences between different bulk phases of transition metals, and have obtained excellent agreement with the results of other precise first-principles methods such as a plane wave based PAW method and an all-electron full-potential linearized augmented plane wave (FLAPW) method.Comment: 29 Page

Tuning the van der Waals Interaction of Graphene with Molecules via Doping

We use scanning tunneling microscopy to visualize and thermal desorption spectroscopy to quantitatively measure that the binding of naphthalene molecules to graphene (Gr), a case of pure van der Waals (vdW) interaction, strengthens with $n$- and weakens with $p$-doping of Gr. Density functional theory calculations that include the vdW interaction in a seamless, ab initio way accurately reproduce the observed trend in binding energies. Based on a model calculation, we propose that the vdW interaction is modified by changing the spatial extent of Gr's $\pi$ orbitals via doping