Prof. Dr. Roland Bennewitz

Molecular processes in the frictional response of an alkanethiol monolayer, self-assembled on a Au(111) surface, are studied by means of high-resolution friction force microscopy in ultrahigh vacuum. With increasing load, three regimes are observed on defect-free domains of the monolayer: smooth sliding with negligible friction, regular molecular stick-slip motion with increasing friction, and the onset of wear in the monolayer. Molecular contrast in the lateral force is found for inequivalent molecules within the unit cell of the c(4 × 2) superstructure. Significant differences in the frictional response are found between defect-free domains and areas including a domain boundary. Friction increases by an order of magnitude on domain boundaries in connection with irregular stick-slip motion. This increased friction at domain boundaries is observed at loads below the onset of wear.

Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300 K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170 K. Below 170 K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact.

The influence of anion adsorption on friction forces in an electrochemical environment has been studied by means of lateral force microscopy on Au(1 1 1) surfaces. Sensitivity to atomic stick-slip motion allows to reveal sulphate adsorption in ordered layers under the sliding tip at potentials lower than expected from cyclic voltammetry for the open surface. No ordered adsorption is found in lateral force measurements for the weakly adsorbed perchlorate anions. Correspondingly, some increase in friction in the anion adsorption regime is observed for sulphate but none for perchlorate adsorption. Friction increases significantly at the onset of oxidation in both sulphuric and perchloric acid solutions.

Friction between the sliding tip of an atomic force microcope and a gold surface changes dramatically upon electrochemical oxidation of the gold surface. Atomicscale variations of the lateral force reveal details of the friction mechanisms. Stick-slip motion with atomic periodicity on perfect Au(111) terraces exhibits extremely low friction and almost no dependence on load. Significant friction is observed only abouve a load threshold at which wear Of the surface is initiated. In contrast, irregular stick slip motion and a linear increase of friction with load are. observed on electrochemically oxidized surfaces. The observations are discussed with reference to the amorphous structure of the oxo-hydroxide surface and atomic place exchange Mechanisms upon oxidation. Reversible, fast switching between the two states of friction has been achieved in both perchloric and sulfuric acid solutions.

Nanotribology explores the mechanical properties of materials at small length scales, where deviations from the scaling laws of macroscopic descriptions are observed. Atomic force microscopy is introduced as an important instrument in nanotribology for imaging friction contrasts on heterogeneous surfaces, for quantitative friction studies, and for the observation of single dislocation processes in plastic deformation. Recent experimental results for the frictional properties of carbon-based materials are discussed. Friction studies using microstructured surfaces are presented as an attempt to bridge the gap between nanotribological and macroscopic friction studies.

Structural and frictional properties of single-layer and bilayer graphene films on a SiC(0001) substrate are studied by means of atomic force microscopy with atomic resolution. Friction on single-layer graphene is found to be a factor of two larger than on bilayer films for a variety of experimental situations. The friction contrast is found not to originate in differences in structural properties, in lateral contact stiffness, or in contact potential. The transition from atomic stick-slip friction to a regime of ultralow friction is found to occur at normal loads of 40 nN when the tip-sample interaction potential approaches 0.1-0.2 eV.

Atomically flat and clean metal surfaces exhibit a regime of ultra-low friction at low normal loads. Atomic force microscopy, performed in ultra-high vacuum on Cu(100) and Au(111) surfaces, reveals a clear stick-slip modulation in the lateral force but almost zero dissipation. Significant friction is observed only for higher loads (∼4–6 nN above the pull-off force) together with the onset of wear. We discuss the minor role of thermal activation in the low friction regime and suggest that a compliant metallic neck between tip and surface is formed which brings upon the low, load-independent shear stress.

We report the design and development of a friction force microscope for high-resolution studies in electrochemical environments. The design choices are motivated by the experimental requirements of atomic-scale friction measurements in liquids. The noise of the system is analyzed based on a methodology for the quantification of all the noise sources. The quantitative contribution of each noise source is analyzed in a series of lateral force measurements. Normal force detection is demonstrated in a study of the solvation potential in a confined liquid, octamethylcyclotetrasiloxane. The limitations of the timing resolution of the instrument are discussed in the context of an atomic stick-slip measurement. The instrument is capable of studying the atomic friction contrast between a bare Au(111) surface and a copper monolayer deposited at underpotential conditions in perchloric acid.

We review recent friction measurements on ordered superstructures performed by atomic force microscopy. In particular, we consider ultrathin KBr films on NaCl(001) and Cu(001) surfaces, single and bilayer graphene on SiC(0001), and the herringbone reconstruction of Au(111). Atomically resolved friction images of these systems show periodic features spanning across several unit cells. Although the physical mechanisms responsible for the formation of these superstructures are quite different, the experimental results can be interpreted within the same phenomenological framework. A comparison between experiments and modeling shows that, in the cases of KBr films on NaCl(001) and of graphene films, the tip-surface interaction is well described by a potential with the periodicity of the substrate which is modulated or, respectively, superimposed with a potential with the symmetry of the superstructure.

The incipient stages of plasticity in KBr single crystals have been examined in ultra-high vacuum by means of atomic force microscopy and Kelvin probe force microscopy (KPFM). Conducting diamond-coated tips have been used to both indent the crystals and image the resulting plastic deformation. KPFM reveals that edge dislocations intersecting the surface carry a negative charge similarly to kinks in surface steps, while screw dislocations show no contrast. The charges are attributed to trapped cation vacancies which compensate the charge of divalent impurities. Furthermore, the site of indentation has been found to carry a large positive charge. Weak topographic features extending in the < 110 > direction from the indentation are identified by atomic-resolution imaging to be pairs of edge dislocations of opposite sign, separated by a distance similar to the indenter radius. They indicate the glide of two parallel {110} planes perpendicular to the surface, a process which allows for a slice of KBr to be pushed away from the indentation site.