Date of Award
Doctor of Philosophy (PhD)
Mechanical Engineering & Applied Mechanics
Robert W. Carpick
Wear is one of the main factors that hinders the performance of probes for atomic force microscopy (AFM), including for the widely-used amplitude modulation (AM-AFM) mode. Unfortunately, a comprehensive scientific understanding of nanoscale wear is lacking. We initially investigate and discuss the mechanics of the tip-sample interaction in AM-AFM. Starting from existing analytical formulations, we introduce a method for conveniently choosing an appropriate probe and free oscillation amplitude that avoids exceeding a critical contact stress to minimize tip/sample damage. We then introduce a protocol for conducting consistent and quantitative AM-AFM wear experiments. The protocol involves determining the tip-sample contact geometry, calculating the peak repulsive force and normal stress over the course of the wear test, and quantifying the wear volume using high-resolution transmission electron microscopy (TEM) imaging. The peak repulsive tip-sample interaction force is estimated from a closed-form equation accompanied by an effective tip radius measurement procedure, which combines TEM and blind tip reconstruction. The contact stress is estimated by applying Derjaguin-MÃ¼ller-Toporov contact mechanics model and also numerically solving a general contact mechanics model recently developed for the adhesive contact of arbitrary axisymmetric punch shapes. We discuss the important role that the assumed tip shape geometry plays in calculating both the interaction forces and the contact stresses. We find that contact stresses are significantly affected by the tip geometry, while the peak repulsive force is mainly determined by experimentally-controlled parameters, most critically, the free oscillation amplitude. The applicability of this protocol is demonstrated experimentally by assessing the performance of diamond-like carbon-coated and silicon nitride-coated silicon probes scanned over ultrananocrystalline diamond substrates in repulsive-mode AM-AFM. There is no sign of fracture or plastic deformation in the case of diamond-like carbon (DLC); wear could be characterized as a gradual atom-by-atom process. In contrast, silicon nitride wears through apparent removal of the cluster of atoms and plastic deformation. DLC's gradual wear mechanism can be described using reaction rate theory, which predicts an exponential dependence of the rate of atom removal on the contact average normal stress, allowing us to estimate kinetic parameters governing the wear process.
Vahdat, Vahid, "Mechanics of Interactions and Atomic-Scale Wear of Tips ian Amplitude Modulation Atomic Force Microscopy" (2013). Publicly Accessible Penn Dissertations. 812.