Mechanics of Interaction and Atomic-Scale Wear of Amplitude Modulation Atomic Force Microscopy Probes

Abstract

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 have developed a protocol for conducting consistent and quantitative AM-AFM wear experiments. The protocol involves controlling the tip-sample interaction regime during AM-AFM scanning, 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 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 transmission electron microscopy and blind tip reconstruction. The contact stress is estimated by applying Derjaguin-Müller-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. Contact stresses are significantly affected by the tip geometry while the peak repulsive force is mainly determined by experimentally controlled parameters, specifically, the free oscillation amplitude and amplitude ratio. 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; wear could be characterized as a gradual atom-by-atom process. In contrast, silicon nitride wears through removal of the cluster of atoms and plastic deformation.

Controlled tip wear on high roughness surfaces yields gradual broadening and rounding of cantilever tips

“…S1. A reported disadvantage of blind tip reconstruction is that typically only a small region of the tip around the apex can be resolved (~10 nm)1011, however by using the high roughness surfaces reported in this study we are able to reconstruct tip shapes up to 100 nm below the apex. Clearly, we can observe an increase in the radius of the tip, before and after wear.…”

“…Therefore, AFM tip wear is a process that is currently subject of extensive research, using various tip materials and AFM operating modes67. Tip wear can occur by several mechanisms, such as adhesive wear8, abrasive wear8, fracture910 and plastic deformation11. Recently, it was found that nanoscale wear with low forces can be described accurately based on an atom-by-atom attrition process, which implies a very gradual wear mechanism12131415.…”

AFM Imaging of Molecular Spin‐State Changes through Quantitative Thermomechanical Measurements

“…[ 17 ] A few scanning tunneling microscopy (STM) observations on spin-state switching phenomena have also been published. [ 28 ] In this study, the authors show that the radius of the silicon tip changes from the fi rst scan, and after nine scans the radius is doubled. Here, we describe a completely different approach, based on the strong electron-lattice coupling in these compounds, wherein we determine the spin-state dependence of the Young's modulus ( E ) and use this property for high-spatial-resolution imaging of the spin transition.…”

“…[ 28 ] In this study, the authors show that the radius of the silicon tip changes from the fi rst scan, and after nine scans the radius is doubled. For a quantitative study of the mechanical properties during the course of a phase transition (i.e., through a large number of scans) in relatively stiff materials, the wearing of the tip becomes a critical issue.…”

Peak forces and lateral resolution in amplitude modulation force microscopy in liquid

“…We deduce a rule to image soft materials with a lateral resolution below 3 nm that involves the application of forces in the sub-100 pN regime, the use of cantilevers with force constants below 0.1 N/m, free amplitudes below 2 nm and relative sharp tips ( R t ≤ 5nm). AM-AFM operation at relatively high amplitudes can also lead to tip blunting [35–36]. The estimation of the peak force prior to performing the experiment could prevent tip damage.…”