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The atomic force microscope (AFM) may be used as a tool to probe hardness, elastic and viscous properties of the surface at the nanoscale. In-plane properties such as friction or shear stiffness of the sample can be measured with a lateral force modulation microscopy or by analyzing torsional vibrations. The torsional resonance mode (TR mode) allows shear force microscopy with standard cantilevers [1]. The torsion along the cantilever long axis promotes a small pendulum-like oscillation of the tip apex parallel to the sample surface with typical amplitudes of 0.2nm to 2.0nm. In this study, the dynamics of the torsional vibrated cantilever are analysed by the transfer function method and finite element analysis (FEA).These results explain the fundamental mechanism for the topography feedback in TR-mode. In amplitude modulation TR-mode, the z-feedback regulates the piezo actuator depending on the amplitude variation as induced by the damping in the tip-sample contact. In the experimental part, we have demonstrated the measurement of the nano scale energy dissipation on different types of surface with torsionally vibrated cantilever in frequency modulation (FM) technique in ambient air. To regulate the FM detection scheme, we used a Nanosurf(Liestal, Switzerland) Phase-Loop-Lock controller/detector (PLL). By monitoring the changes in the resonance frequency and excitation amplitude required to keep the oscillation amplitude at a constant value, we were able to measure frequency shift and dissipation caused by the tip-sample interaction. The measurement of the energy dissipation on PS-b-PB surface shows a clear difference for the domains which has different mechanical properties. The high sensitivity of this technique to lateral stiffness (i.e., mechanical properties) suggest that it can be used to measure the frictional (high amplitude), and visco-elastic properties of the sample surface. Experimental results for the dependence of the dissipation on extension and the oscillation amplitude are presented. The success of the energy dissipation mapping of the FM-TR-AFM imaging in air opens a wide variety of new applications that can not be obtained by the FM-NC-AFM in vertical direction to the sample surface.
[1] L. Huang , C. Su ,Ultramicroscopy 100 (2004) 277-285
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