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Using combined low temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we demonstrate hydrogen passivation of individual, selected dangling bonds (DBs) on a hydrogen-passivated Si(100)-2$\times$1 surface (H-Si) by atom manipulation. This method allows erasing of DBs and thus provides an error-correction scheme for hydrogen lithography. Si-terminated tips (Si tips) for hydrogen desorption and H-terminated tips (H tips) for hydrogen passivation are both created by deliberate contact to the H-Si surface and are assigned by their characteristic contrast in AFM. DB passivation is achieved by transferring the H atom that is at the apex of an H tip to the DB, reestablishing a locally defect-free H-Si surface.

Si dangling bonds without H termination at the interface of quasi-free standing monolayer graphene (QFMLG) are known scattering centers that can severely affect carrier mobility. In this report, we study the atomic and electronic structure of Si dangling bonds in QFMLG using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and density functional theory (DFT) calculations. Two types of defects with different contrast were observed on a flat terrace by STM and AFM. Their STM contrast varies with bias voltage. In STS, they showed characteristic peaks at different energies, 1.1 and 1.4 eV. Comparison with DFT calculations indicates that they correspond to clusters of 3 and 4 Si dangling bonds, respectively. The relevance of these results for the optimization of graphene synthesis is discussed.

C60-functionalized tips are used to probe C60 molecules on Cu(111) with scanning tunneling and atomic force microscopy. Distinct and complex intramolecular contrasts are found. Maximal attractive forces are observed when for both molecules a [6,6] bond faces a hexagon of the other molecule. Density functional theory calculations including parameterized van der Waals interactions corroborate the observations., Comment: 6 pages, 3 figures

The formation of extended electron states in one-dimensional nanostructures is of key importance for the function of molecular electronics devices. Here we study the effects of strong electron-phonon interaction on the formation of extended electronic states in intentionally created Cl vacancy pairs and chains in a NaCl bilayer on Cu(111). The interaction between the vacancies was tailored by fabricating vacancy pairs and chains of different orientation and separation with atomic precision using vertical manipulation. Small separation of divacancies led to the formation of symmetric and antisymmetric vacancy states and localized interface-states. By scanning tunneling spectroscopy (STS) we measured their energy splitting and broadening as a function of the inter-vacancy separation. Unexpectedly, the energy splitting between the vacancy states is enlarged by level repulsion resulting from phonon dressing of the electronic states, as evidenced by theory. Already for a few coupled vacancies we observe an emerging band structure of the defect band., Comment: 9 pages, 4 figures

Noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) have become important tools for nanotechnology; however, their contrast mechanisms on the atomic scale are not entirely understood. Here we used chlorine vacancies in NaCl bilayers on Cu(111) as a model system to investigate atomic contrast as a function of applied voltage, tip height, and tip functionalization. We demonstrate that the AFM contrast on the atomic scale decisively depends on both the tip termination and the sample voltage. On the contrary, the local contact potential difference (LCPD) acquired with KPFM showed the same qualitative contrast for all tip terminations investigated, which resembled the contrast of the electric field of the sample. We find that the AFM contrast stems mainly from electrostatic interactions but its tip dependence cannot be explained by the tip dipole alone. With the aid of a simple electrostatic model and by density functional theory we investigate the underlying contrast mechanisms., Comment: 14 pages, 12 figures

Force and conductance were simultaneously measured during the formation of Cu-C60 and C60-C60 contacts using a combined cryogenic scanning tunneling and atomic force microscope. The contact geometry was controlled with submolecular resolution. The maximal attractive forces measured for the two types of junctions were found to differ significantly. We show that the previously reported values of the contact conductance correspond to the junction being under maximal tensile stress.

Chemisorption of CO on the stepped Cu(211) surface is studied within ab-initio density functional theory (DFT) and scanning tunneling microscopy (STM) imaging as well as manipulation experiments. Theoretically we focus on the experimentally observed ordered (2x1) and (3x1) CO-phases at coverages 1/3, 1/2 and 2/3 monolayer (ML). To obtain also information for isolated CO molecules found randomly distributed at low coverages, we also performed calculations for a hypothetical (3x1) phase with 1/3 ML. The adsorption geometry, the stretching frequencies, the work functions and adsorption energies of the CO molecules in the different phases are presented and compared to experimental data. Initially and up to a coverage of 1/2 ML CO adsorbs upright on the on-top sites at step edge atoms. Determining the most favorable adsorption geometry for the 2/3 ML ordered phase turned out to be nontrivial both from the experimental and the theoretical point of view. Experimentally, both top-bridge and top-top configurations were reported, whereby only the top-top arrangement was firmly established. The calculated adsorption energies and the stretching frequencies favor the top-bridge configuration. The possible existence of both configurations at 2/3 ML is critically discussed on the basis of the presently accessible experimental and theoretical data. In addition, we present observations of STM manipulation experiments and corresponding theoretical results, which show that CO adsorbed on-top of a single Cu-adatom, which is manipulated to a location close to the lower step edge, is stronger bound than CO on-top of a step edge atom., Comment: resubmitted to PRB, ver. 2, 9 pages, 4 figures

On NaCl(100)/Cu(111) an interface state band is observed that descends from the surface-state band of the clean copper surface. This band exhibits a Moire-pattern-induced one-dimensional band gap, which is accompanied by strong standing-wave patterns, as revealed in low-temperature scanning tunneling microscopy images. At NaCl island step edges, one can directly see the refraction of these standing waves, which obey Snell's refraction law., Comment: 4 pages, 4 figures

A combined low-temperature scanning tunneling microscopy and density functional theory study on the binding and diffusion of copper monomers, dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers are found in fcc sites only, monomers as well as atoms in dimers can occupy the stable fcc as well as the metastable hcp site. In fact the dimer fcc-hcp configuration was found to be only 1.3 meV less favorable with respect to the fcc-fcc configuration. This enables a confined intra-cell dimer motion, which at temperatures below 5 K is dominated by thermally assisted tunneling., Comment: 4 pages, 4 figures