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Bad Münster am Stein-Ebernburg, Germany

Monig H.,University of Munster | Monig H.,Center for Nanotechnology ech | Monig H.,Yale University | Todorovic M.,Autonomous University of Madrid | And 8 more authors.
ACS Nano | Year: 2013

A comprehensive analysis of contrast formation mechanisms in scanning tunneling microscopy (STM) experiments on a metal oxide surface is presented with the oxygen-induced (2√2×√2)R45 missing row reconstruction of the Cu(100) surface as a model system. Density functional theory and electronic transport calculations were combined to simulate the STM imaging behavior of pure and oxygen-contaminated metal tips with structurally and chemically different apexes while systematically varying bias voltage and tip-sample distance. The resulting multiparameter database of computed images was used to conduct an extensive comparison with experimental data. Excellent agreement was attained for a large number of cases, suggesting that the assumed model tips reproduce most of the commonly encountered contrast-determining effects. Specifically, we find that depending on the bias voltage polarity, copper-terminated tips allow selective imaging of two structurally distinct surface Cu sites, while oxygen-terminated tips show complex contrasts with pronounced asymmetry and tip-sample distance dependence. Considering the structural and chemical stability of the tips reveals that the copper-terminated apexes tend to react with surface oxygen at small tip-sample distances. In contrast, oxygen-terminated tips are considerably more stable, allowing exclusive surface oxygen imaging at small tip-sample distances. Our results provide a conclusive understanding of fundamental STM imaging mechanisms, thereby providing guidelines for experimentalists to achieve chemically selective imaging by properly selecting imaging parameters. © 2013 American Chemical Society. Source

Falter J.,Center for Nanotechnology ech | Langewisch G.,Center for Nanotechnology ech | Holscher H.,Karlsruhe Institute of Technology | Fuchs H.,Center for Nanotechnology ech | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

Direct comparison of tip-sample forces obtained by dynamic force spectroscopy experiments with theoretical simulations is extremely difficult, since the precise tip shape and chemical identity of the apex atoms of the force sensing tip remain unknown in most experiments. Here, we present force curves measured with a tungsten tip on a Ag(111) surface obtained in a low-temperature atomic force microscope using tips that were analyzed by field ion microscopy down to atomic levels. The resulting van der Waals and electrostatic forces were found to be in quantitative agreement with analytical models, if the tip shape parameters from the field ion microscopy analysis were used. Furthermore, our analysis shows an additional long-range force interaction at tip-sample distances above 1.3 nm. We suggest that this unexpected force is related to patch charges arising from the inhomogeneous work function distribution on the surface of highly faceted sharp tips. © 2013 American Physical Society. Source

Feldmann M.,Justus Liebig University | Dietzel D.,Justus Liebig University | Fuchs H.,Center for Nanotechnology ech | Fuchs H.,University of Munster | And 2 more authors.
Physical Review Letters | Year: 2014

One of the oldest concepts in tribology is stick-slip dynamics, where a disruptive sequence of stick and slip phases determine the overall resistance in sliding friction. While the mechanical energy dissipates in the sudden slip phase, the stick phase has been shown to be characterized by contact strengthening mechanisms, also termed contact aging. We present experiments of sliding nanoparticles, where friction is measured as a function of sliding velocity and interface temperature. The resulting complex interdependence is in good agreement with Monte Carlo simulations, in which the energy barrier for contact breaking increases logarithmically with time, at a rate governed by thermal activation. © 2014 American Physical Society. Source

Lee Y.,University of Munster | Jung G.-E.,Park Systems Corporation KANC | Cho S.J.,Park Systems Corporation KANC | Cho S.J.,University of Suwon | And 4 more authors.
Nanoscale | Year: 2013

Halloysite nanotube (HNT)-based supramolecular complexes are synthesized and evaluated with respect to their cytotoxicity and effects on cellular structures. As HNTs are water-insoluble, DNA is applied for wrapping the surface of HNTs to enhance their water-dispersibility. To investigate the potential of DNA-wrapped HNTs (HD) as a promising drug delivery carrier, doxorubicin (DOX) is introduced as a model anticancer agent and loaded onto HD. The DOX-loaded, DNA-wrapped HNTs (HDD) show sustained DOX release over two weeks without initial burst of DOX indicating delayed DOX release inside cells. In addition, effects of DNA-wrapped HNTs (HD) or HDD on the cytoskeleton organization of A549 cells are studied by visualizing the distribution of F-actin filaments using confocal laser scanning microscopy, and cellular morphological changes are observed by scanning electron microscopy and scanning ion conductance microscopy. © 2013 The Royal Society of Chemistry. Source

Schmutz J.-E.,Center for Nanotechnology ech | Schmutz J.-E.,University of Munster | Fuchs H.,Center for Nanotechnology ech | Fuchs H.,University of Munster | Holscher H.,Karlsruhe Institute of Technology
Wear | Year: 2010

We introduce a technique for measuring wear on the nano-scale by combining friction force and dynamic force microscopy. By measuring the resonance frequency of the cantilever after scratching over a sample surface we are able to detect the increase or decrease of the tip's worn mass down to some picograms. Applying a recently developed technique to attach a small sphere to the upper end of the cantilever's tip we are able to measure the nano-wear of several material combinations with this approach. © 2009 Elsevier B.V. All rights reserved. Source

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