Studies of Nanowire Friction using AFM-based Manipulation

Abstract: In this thesis we present friction studies on InAs nanowires pushed laterally across surfaces by an AFM tip. The contact length in the direction of movement is of the order of a few tens of nanometers, and is thus comparable to that found in existing point-contact studies, but perpendicular to the motion the wire can be several microns in length. We are therefore able to investigate friction in the mesoscopic regime, with a geometry that closely models proposed MEMS structures and devices. The shape of the wire after manipulation is determined by an equilibrium between internal elastic restoring forces and friction with the substrate. We have developed a method to calculate the friction force per unit length by measuring the diameter and curvature of the bent nanowire and applying standard elasticity theory to calculate the stresses within the wire. We have studied InAs nanowires on three different substrates, silicon dioxide, fluoro-silanised silicon dioxide, and silicon nitride. We have measured the static and sliding friction force per unit length for a range of nanowire diameters on all three substrates. The system displays behaviors typical of friction studies at other scales, such as differentiated sliding and static friction and a transition to stick-slip motion, the details being an interesting mix of those from both atomic-scale and macroscopic regimes. We have also studied how the friction between InAs nanowires and an insulating silicon nitride layer on a Si substrate varies when a DC voltage is applied to the tip during a manipulation sequence. A monotonic increase of the sliding friction with the voltage applied to the tip was observed, caused by the electrostatic attraction of the wire to the substrate. The increase in friction with normal force implies that the mesoscopic nanowire-surface system behaves like a macroscopic interface, despite the nanometer size of the contact in the direction of motion. In the third part, we have investigated the friction between InAs nanowires and atomically-flat mica substrates. The dependence of the static friction force per unit length on the nanowire diameter is very different than the previously observed behavior on silicon nitride substrates and is consistent with models for atomically-smooth nanoscale friction. The results suggest that for the technologically relevant silicon-based surfaces the details of the surface roughness down to the atomic scale are important, and will affect the observed friction behavior.