Anomalous molecular orbital variation upon adsorption on wide band gap insulator

It is commonly believed that organic molecules are physisorbed on the ideal non-polar surfaces of wide band gap insulators with limited variation of the electronic properties of the adsorbate molecule. On the basis of first principles calculations within density functional theory (DFT) and $GW$ approximation, we show that this is not generally true. We find that the molecular frontier orbitals undergo significant changes when a hydroxy acid (here we chose gluconic acid) is adsorbed on MgSO$_4$$\cdot$H$_2$O(100) surface due to the complex interaction between the molecule and the insulating surface. The predicted trend of the adsorption effect on the energy gap obtained by DFT is reversed when the surface polarization effect is taken into account via the many-body corrections.Comment: 5 pages, 3 figure

Local transport measurements on epitaxial graphene

Growth of large-scale graphene is still accompanied by imperfections. By means of a four-tip STM/SEM the local structure of graphene grown on SiC(0001) was correlated with scanning electron microscope images and spatially resolved transport measurements. The systematic variation of probe spacings and substrate temperature has clearly revealed two-dimensional transport regimes of Anderson localization as well as of diffusive transport. The detailed analysis of the temperature dependent data demonstrates that the local on-top nano-sized contacts do not induce significant strain to the epitaxial graphene films.Comment: 3 figure

Diffraction from disordered vicinal surfaces with alternating terraces

Vicinal surfaces with terraces of alternating stress develop inhomogeneous distributions of terrace sizes which sometimes leads even to the formation of double steps. Both vicinal Si(001) and vicinal Ge(001) are typical examples for this behavior. However, vicinal surfaces of some alloys show this effect, too. It is well established that average terrace sizes can be evaluated from the splitting of peaks in surface sensitive diffraction experiments. More parameters, however, are necessary to obtain an improved characterization of the morphology of the vicinal surface. Therefore, we present a detailed analysis of diffraction patterns from alternating vicinal surfaces to extract more statistical data, e.g., standard deviations of the terrace size distributions, step rms widths, step correlation lengths, and kink densities. This analysis considers both profiles of (split) diffraction peaks and the profile of the diffuse scattering. In addition, the diffraction analysis is applied to vicinal Ge(001) to characterize the morphology in full detail. © 2007 The American Physical Society

Formation of surface color centers at differently coordinated sites: MgO/Ag(1,1,19)

In order to find criteria for discrimination between surface color centers at terraces and steps, and bulk color centers, the characteristic losses of these centers have been investigated by electron energy loss spectroscopy on an epitaxial MgO film grown on a Ag (1,1,19) surface. This film contains a significantly higher step density compared with a film grown on Ag(100). The generation of four distinct losses at 2.0, 2.4, 2.8, and 3.4 eV and a broad loss centered at 5.5 eV have been observed that are induced by electron bombardment of the MgO surface. The latter loss is attributed to bulk color centers (Fb centers). By comparing the measured loss energies with experiments performed on MgO/Ag(100) and with theoretical literature data, the observed losses at 2.0 and 2.8 eV can be consistently assigned to transitions of color centers located on step sites, whereas those at 2.4 and 3.4 eV are attributed to terrace sites. The kinetics of color center formation during electron bombardment, as well as the annihilation of F centers by simple exposure to O2 gas at room temperature, was determined and compared for differently coordinated color centers. While all surface color centers follow the same kinetics, we show that the Fb centers can only decay as long as the existence of surface color centers provides active sites for dissociating oxygen molecules. This corroborates the assignment of Fb centers. © 2003 The American Physical Societ

Coulomb blockade effects in Ag/Si(111): The role of the wetting layer

Single and double barrier structures have been realized with Ag nanostructures grown on Si(111) in combination with scanning tunneling microscopy. The series connection of a Schottky (SB) and tunneling barrier mimics a double barrier structure showing Coulomb blockade oscillations as revealed by scanning tunneling spectroscopy (STS). Although the SB remains in presence of a Ag√3×√3 reconstruction, the dI/dV characteristic turns into a single barrier structure. The Ag reconstruction provides a sufficiently high electron mobility capturing intrinsic defects which shortens the resistance of the SB. These results show that vertical transport properties, as measured with STS, are not only controlled by the structure and the bonding on the atomic scale, but depend strongly on the lateral properties of the interface as well. © 2009 The American Physical Society.DF

Anisotropic conductance oscillations in Pb films on Si(557)

We correlate in this study the growth of Pb films on the stepped Si(557) surface at temperatures of 70 K, studied by low-energy electron diffraction, with the properties of electronic transport, measured by a macroscopic four-point probe technique. Despite a large lattice mismatch, layer-by-layer growth is observed, as most obvious from the characteristic oscillations in conductance with layer periodicity, incipient with the first monolayer both along and across the step direction. These findings demonstrate that lateral misfits (here almost 10%) in heteroepitaxial systems can be effectively compensated by substrate steps and can change the growth mode with respect to flat surfaces. While structurally the layers appear to be isotropic starting already with the third layer, anisotropy is seen in transport up to at least six monolayers with functional dependencies of conductance varying with layer thickness and measurement direction. Maxima of conductance oscillations up to five monolayers do not coincide with completion of individual layers. They are characteristic for the close coupling of structure and quantum effects. © 2010 The American Physical Society.DF

Two-dimensional crossover and strong coupling of plasmon excitations in arrays of one-dimensional atomic wires

The collective electronic excitations of arrays of Au chains on regularly stepped Si(553) and Si(775) surfaces were studied using electron loss spectroscopy with simultaneous high energy and momentum resolution (ELS-LEED) in combination with low energy electron diffraction (SPA-LEED) and tunneling microscopy. Both surfaces contain a double chain of gold atoms per terrace. Although one-dimensional metallicity and plasmon dispersion is observed only along the wires, two-dimensional effects are important, since plasmon dispersion explicitly depends both on the structural motif of the wires and the terrace width. The electron density on each terrace turns out to be modulated, as seen by tunneling spectroscopy (STS). The effective wire width of 7.5\,\AA\ for Si(553)-Au -- 10.2\,\AA\ for Si(775)-Au -- , determined from plasmon dispersion is in good agreement with STS data. Clear evidence for coupling between wires is seen beyond nearest neighbor coupling.Comment: 5 pages, 4 figure

Temperature-driven refacetting phase transition in Pb chains on Si(557)

By using quantitative low energy electron diffraction, we have studied the temperature-driven phase transition of Pb chains grown on Si(557) substrates at a surface concentration of 1.3 ML. This concentration, which is still below one physical monolayer, exhibits a unique switching of electrical conductance from one dimensional to two dimensional above 78 K, which is coupled to this phase transition, and was investigated for this reason. Annealing to 640 K causes a concentration-driven refacetting of the whole surface into large (223) facets at low temperatures, while along the chains a so-called (1,5) linear phase is formed, causing a tenfold periodicity. At Tc=78 K, we analyze a temperature-driven order-order transition along the [¯1¯12] direction in detail, which again turns out to be a refacetting transition. The two-dimensional character of this transition was seen by corresponding structural changes along the [1¯10] direction as well. Refacetting causes a change in periodicity and destroys the conditions of Fermi nesting necessary for one-dimensional conductance. © 2008 The American Physical Society.DF