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Yurtsever, Ayhan (2008): Nanotribological surface characterization by frequency modulated torsional resonance mode AFM. Dissertation, LMU München: Faculty of Geosciences
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Abstract

The aim of this work is to develop an experimental method to measure in-plane surface properties on the nanometer scale by torsional resonance mode atomic force microscopy and to understand the underlying system dynamics. The invention of the atomic force microscope (AFM) and the advances in development of new AFM based techniques have significantly enhanced the capability to probe surface properties with nanometer resolution. However, most of these techniques are based on a flexural oscillation of the force sensing cantilever which are sensitive to forces perpendicular to the surface. Therefore, there is a need for highly sensitive measurement methods for the characterization of in-plane properties. To this end, scanning shear force measurements with an AFM provide access to surface properties such as friction, shear stiffness, and other tribological surface properties with nanometer resolution. Dynamic atomic force microscopy utilizes the frequency response of the cantilever-probe assembly to reveal nanomechanical properties of the surface. The frequency response function of a cantilever in torsional motion was investigated by using a numerical model based on the finite element method (FEM). We demonstrated that the vibration of the cantilever in a torsional oscillation mode is highly sensitive to lateral elastic (conservative) and visco-elastic (non-conservative) in-plane material properties, thus, mapping of these properties is possible in the so-called torsional resonance mode AFM (TR-mode). The theoretical results were then validated by implementing a frequency modulation (FM) detection technique to torsion mode AFM. This method allows for measuring both conservative and non-conservative interactions. By monitoring changes of the resonant frequency and the oscillation amplitude, we were able to map elastic properties and dissipation caused by the tip-sample interaction. During approach and retract cycles, we observed a slight negative detuning of the torsional resonance frequency, depending on the tilt angle between the oscillation plane and the surface before contact to the HOPG surface. This angle leads to a mixing of in-plane (horizontal) and out-of-plane (vertical) sample properties. These findings have a significant implication for the imaging process and the adjustment of the microscope and may not be ignored when interpreting frequency shift or energy dissipation measurements. To elucidate the sensitivity of the frequency modulated torsional resonance mode AFM (FM-TR-AFM) for the energy dissipation measurement, different types of samples such as a compliant material (block copolymer), a mineral (chlorite) and a macromolecule (DNA) were investigated. The measurement of energy dissipation on these specimens indicated that the TR-AFM images reveal a clear difference for the domains which have different mechanical properties. Simultaneously a topographic and a chemical contrast are obtained by recording the detuning and the dissipation signal caused by the tip-surface interaction. Using FM-TR-AFM spectroscopically, we investigated frequency shift versus distance curves on the homopolymer polystyrene (PS). Depending on the molecular weight, the frequency detuning curve displayed two distinct regions. Firstly, a rather compliant surface layer was probed; secondly, the less mobile bulk of the polymer was sensed by the oscillatory motion of the tip. The high sensitivity of this technique to mechanical in-plane properties suggests that it can be used to discriminate different chemical properties (e.g. wetting) of the material by simultaneously measuring energy dissipation and surface topography.