Measurements of In-Plane Material Properties with Scanning Probe Microscopy
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Measurements of
In-Plane Material Properties with Scanning Probe Microscopy
Robert W. Carpick and Mark A. Eriksson Abstract Scanning probe microscopy (SPM) was originally conceived as a method for measuring atomic-scale surface topography. Over the last two decades, it has blossomed into an array of techniques that can be used to obtain a rich variety of information about nanoscale material properties. With the exception of friction measurements, these techniques have traditionally depended on tip–sample interactions directed normal to the sample’s surface. Recently, researchers have explored several effects arising from interactions parallel to surfaces, usually by deliberately applying a modulated lateral displacement. In fact, some parallel motion is ubiquitous to cantilever-based SPM, due to the tilt of the cantilever. Recent studies, performed in contact, noncontact, and intermittent-contact modes, provide new insights into properties such as structural anisotropy, lateral interactions with surface features, nanoscale shear stress and contact mechanics, and in-plane energy dissipation. The understanding gained from interpreting this behavior has consequences for all cantilever-based scanning probe microscopies. Keywords: atomic force spectroscopy, mechanical properties, nanomechanics, scanning probe microscopy.
Introduction Atomic force microscopy (AFM) refers to techniques that use a force-sensing probe to measure surface properties. Typically, AFM cantilevers are designed to be extremely sensitive to forces normal to the sample surface, and they enable a user to determine properties such as topography with high resolution. Lateral, or friction, force microscopy (LFM/FFM)1,2 is a modification of AFM that involves measuring the interaction force parallel to the sample surface; recent advances in FFM research are reviewed by Perry in this issue of MRS Bulletin. One of the most important consequences of the development of FFM is the emergence of the field of nanotribology (see the May 1993 and June 1998 issues of MRS Bulletin), from which new, fundamental insights into the origins of friction continue to be uncovered.3
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In addition to FFM, a range of other distinct methods has been developed in which lateral motion of the tip is used. These inplane methods span the entire range of AFM techniques, from contact to intermittent- and noncontact regimes, and are applicable to a wide range of materials including thin organic films, ceramics, and metals. This article is motivated by a recent burst of results in the area of in-plane material properties. Examples include lateral stiffness measurements, which are related to the contact area and shear modulus of the materials; intermittent-contact phase measurements, where in-plane structure and dissipation are measured; and noncontact lateral modulation to determine interaction forces in the plane of the sample between the AFM tip and adsorbates or surface features. The
emergence of in-plane techniques is highlighted not only by this research, but also by the fac
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