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Scanning probe imaging in a shear force mode enables the characterization of in-plane surface properties. In a standard AFM, such a shear force imaging can be realized by taking advantage of the torsional eigenmodes of atomic force microscope (AFM) cantilevers that are highly sensitive toward in-plane material properties. Sample viscosity and lateral contact stiffness lead to variations in resonance frequency, oscillation amplitude, or phase response. To gain more insight into image contrast formation in torsional resonance shear force AFM, we examined various types of specimen such as mineral surfaces, polymers, and DNA. In both modes, amplitude (AM) and frequency modulation (FM) feedback, a compositional contrast can be achieved on hard surfaces. A separation of conservative and dissipative forces, however, can only be achieved by constant amplitude mode of torsional FM-AFM (1). On soft surfaces, such as polymers, the tip indents into the specimen. The imaging contrast is then more complex because the indentation depth varies in the case of a constant frequency shift measurement (2). Approach - retract curves reveal further details on the contact mechanics (1-3). The tip oscillations can be minimized by passive torsional measurements, i.e. by analyzing the thermomechanical noise of the force sensor. At minimum vibration amplitudes it is possible to obtain insights into the details of static friction (sticktion) (4). In summary, torsional shear force AFM provides a powerfull toolbox to investigate nanoscale friction phenoma at surfaces and interfaces. (1) A. Yurtsever, A.M. Gigler, R.W. Stark, Ultramicroscopy, 109 (3), 275-279, 2009; (2) A. Yurtsever, A. M. Gigler, C. Dietz, R.W. Stark, Appl. Phys. Lett., 92, art. 143103, 2008 (3) A. Yurtsever, A. M. Gigler, E. Macias, R. W. Stark, Appl. Phys. Lett. 91, art. 253120, 2007 (4) O. von Sicard, A.M. Gigler, T. Drobek, R. W. Stark, Langmuir, 25 (5), 2924-2927, 2009. |
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