Nanofriction mechanisms derived from the dependence of friction on load and sliding velocity from air to UHV on hydrophilic silicon

This paper examines friction as a function of the sliding velocity and applied normal load from air to UHV in a scanning force microscope (SFM) experiment in which a sharp silicon tip slides against a flat Si(100) sample. Under ambient conditions, both surfaces are covered by a native oxide, which is hydrophilic. During pump-down in the vacuum chamber housing the SFM, the behavior of friction as a function of the applied normal load and the sliding velocity undergoes a change. By analyzing these changes it is possible to identify three distinct friction regimes with corresponding contact properties: (a) friction dominated by the additional normal forces induced by capillarity due to the presence of thick water films, (b) higher drag force from ordering effects present in thin water layers and (c) low friction due to direct solid-solid contact for the sample with the counterbody. Depending on environmental conditions and the applied normal load, all three mechanisms may be present at one time. Their individual contributions can be identified by investigating the dependence of friction on the applied normal load as well as on the sliding velocity in different pressure regimes, thus providing information about nanoscale friction mechanisms

Photothermal Micro- and Nanopatterning of Organic/Silicon Interfaces

Photothermal laser processing of organic monolayers on oxide-free silicon substrates under ambient conditions is investigated. Organic monolayers on Si(100) and Si(111) substrates are prepared via hydrosilylation of H-terminated silicon samples in neat 1-hexadecene and 1-hexadecyne, respectively. Laser processing at ¿ = 514 nm and a 1/e2 spot diameter of 2.6 µm results in local decomposition of the monolayers and oxidation of the exposed substrate. In agreement with the high thermal and chemical stability of these monolayers, a thermokinetic analysis of the data from experiments at distinct laser powers and pulse lengths points to a highly activated process. As a result, processing is strongly nonlinear and allows for subwavelength patterning, with line widths between 0.4 and 1.4 µm. Most remarkably, upon fabrication of dense line patterns, narrow organic monolayer stripes with sharp edges and lateral dimensions of 80 nm are formed. This opens up new perspectives in photothermal engineering of organic/silicon interfaces, e.g., for hybrid microelectronic and sensor application

Fabrication of chemical templates via selective laser-induced desorption of hexadecanethiol self-assembled monolayers

A nonlinear photothermal laser patterning technique for rapid fabrication of chemical templates is demonstrated. Hexadecanethiol monolayers on Au-coated Si substrates are processed at lambda = 532 nm, a 1/e(2) spot diameter of d(1/e)(2) = 2.8 mu m and ambient conditions. Local laser irradiation at high laser powers and short irradiation times in the micro-/millisecond range induces desorption of thiol molecules. The laser-depleted areas are backfilled with mercaptohexadecanoic acid in order to build up chemical templates. Atomic force microscopy, scanning electron microscopy and scanning Auger electron spectroscopy are used for characterization of these templates. In agreement with a selective laser process, the results indicate the formation of flat chemical patterns with well-defined boundaries. Complementary condensation experiments demonstrate the functionality of the patterns as hydrophilic/hydrophobic templates. In particular, upon decreasing the temperature below the dew point, selective formation of water droplets on the backfilled areas is observed. (C) 2013 Elsevier B. V. All rights reserved

Electrochemical Oxidation as Vertical Structuring Tool for Ultrathin (d < 10 nm) Valve Metal Films

Stepwise potentiostatic oxidation is used to reduce the thickness of thin aluminum and tantalum films from an initial thickness of 10 nm down to 2 nm. The thicknesses of the oxide and the residual metal are adjusted by the finite potential of an electrochemical oxidation procedure which consumes the initially 10 nm thick metal films. The metal–metal oxide interfaces are smooth and sharply defined. The metal consumption and oxide formation are proportional to each other by the ratio of their specific densities. This enables the derivation of a metal consumption factor for the residual metal film. Residual aluminum films show a significant increase of the specific resistivity with decreasing film thicknesses. This can be explained by modified electronic transport in the residual aluminum for example by changed electronic scattering processes at the metal–metal oxide interface or in the metal. Residual tantalum films show a weaker dependence of the specific resistivity down to 3 nm pointing to only slightly changed transport properties for electrons in the thin tantalum layers