Interactive Surfaces

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 the use of atomic force microscopy (AFM) in liquids to measure oscillatory solvation forces. We find solvation layering can occur for all the liquids studied (linear and branched alkanes) but marked variations in the force and dissipation may arise dependent on: a) the temperature, b) the tip shape/radius of curvature, and c) the degree of molecular branching. Several findings (e.g., the strong temperature dependence in measured solvation forces, solvation oscillations using branched molecules) differ from those observed using the Surface Force Apparatus, because of the nanoscale area probed by AFM. Conduction AFM is used to explore how liquid is squeezed out of the tip-sample gap, and enables the change in contact area of the tip-sample junction to be monitored and compared to mechanical models. We find elastic models provide a good description of the deformation of ordered, solid-like solvation layers but not disordered, liquid-like layers.

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.

We have studied friction and dissipation in single and bilayer graphene films grown epitaxially on SiC. The friction on SiC is greatly reduced by a single layer of graphene and reduced by another factor of 2 on bilayer graphene. The friction contrast between single and bilayer graphene arises from a dramatic difference in electron-phonon coupling, which we discovered by means of angle-resolved photoemission spectroscopy. Bilayer graphene as a lubricant outperforms even graphite due to reduced adhesion.

Using frequency-modulation atomic force microscopy (FM-AFM) at sub-nanometer vibration amplitudes, we find in the system n-dodecanol/graphite that solvation layers may extend for several nanometers into the bulk liquid. These layers maintain crystalline order which can be imaged using FM-AFM. The energy dissipation of the vibrating tip can peak sharply upon penetration of molecular layers. The tip shape appears critical for this effect.

Fundamental processes of wear include the rupture of single chemical bonds and the displacement of atoms or small clusters by mechanical action. Experimental studies of such processes have become feasible with the development of scanning probe microscopy. The small volume affected in these experiments overlaps with the size scale of large atomistic simulations, making a direct comparison possible. The complexity of real-world wear processes is reduced in most nanometer-scale experiments, for example, by probing surfaces of single crystals or by establishing and maintaining carefully controlled environments, including ultraclean conditions. The studies address the onset and topography of wear, the formation of debris structures, the interplay of mechanical and chemical action, the role of ultrathin films, the role of crystal defects in wear processes, and temporal and thermal effects.

The work function difference between single layer and bilayer graphene grown epitaxially on 6H-SiC(0001) has been determined to be 135+-9 meV by means of the Kelvin probe force microscopy. Bilayer films are found to increase the work function as compared to single layer films. This method allows an unambiguous distinction between interface layer, single layer, and bilayer graphene. In combination with high-resolution topographic imaging, the complex step structure of epitaxial graphene on SiC can be resolved with respect to substrate and graphene layer steps.