Scientific publications

An electrostatically actuated double-clamped cantilever beam test structure has been designed and fabricated to determine the adhesion forces between co-planar, impacting polycrystalline silicon (polysilicon) surfaces. To examine the effect of apparent contact area, dimples of varying sizes have been included in the test structure. By measuring the cantilever beam profile, through optical interferometric methods, as a function of applied bias, the force of adhesion has been determined for various device geometries. The results reveal a weak dependence of adhesion on apparent contact area, rather than a linear dependence. Fabrication process artifacts, observed and discussed here, contribute to but do not suffice to explain this observed weak scaling. The results strongly suggest that contact on the micrometer scale between rough, rigid materials such as polysilicon involves only a few asperities.

The epitaxial growth of 3C-SiC films on Si(100) substrates is demonstrated using a two-step chemical vapor deposition (CVD) process. A thin (50 nm) SiC buffer layer grown at 925 °C using 1,3-disilabutane is shown to enable the growth of a high crystalline quality epitaxial 3C-SiC film using methyltrichlorosilane at 1200 °C. The ability to deposit high-quality epitaxial film is traced to the suppression of void defects and to the improvement in film adhesion obtained by the deposition of the buffer layer at low temperature.

The effect of prior deformation on mechanical behavior as a function of size is investigated for body-centered cubic (bcc) molybdenum (Mo) pillars. Experiments were performed using focused ion beam (FIB) manufactured [0 0 1] and [2 3 5] Mo micro/nanopillars, which were compressed, re-FIB machined, and compressed again. Unlike in bulk materials, pre-straining has a negligible effect on stress-strain behavior of the pillars, suggesting that dislocation storage does not occur in small-scale bcc specimens. The prevailing mechanism behind the size effect is attributed to dislocation nucleation mechanisms.

This manuscript explores the possibility of exploiting polymer morphology (thermal or flow-induced) as materials inherent template, and domain-selective plasma etching as pattern developer, to obtain nanopatterned surfaces with different and controlled geometries, with a particular focus on nanofibrillar patterns. Oxidative plasma treatment of PET films has rendered patterned surfaces with different geometries depending on the crystallinity and orientation of the PET sample and plasma treatment time (or etching ratio). Homogeneous patterns with either randomly distributed or aligned nanofibrils with diameters between 20 and 40 nm and lengths up to 1 µm (after extensive etching) were observed depending on the sample pretreatment. Our results demonstrate the potential of oxidative plasmas as templateless nanopatterning technique and reveal a complex interplay between plasma etching parameters and polymer microstructure driving the pattern formation mechanism. These results open the possibility of fabricating gecko-inspired surfaces in a cost-effective manner.

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 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.