Simultaneously Measuring Force and Displacement: Calibration of Magnetic Torque Actuated Microcantilevers for Nanomechanical Mapping

Force-distance curve-based atomic force microscopy (FD-based AFM) has been extensively used to study the interactive phenomena and mechanical properties of various materials at the nanoscale. Unlike the commonly used piezodrive FD-based AFM that uses two sensors (cantilever and piezoactuator), the magnetic-drive FD-based AFM uses only one sensor of an off-resonant oscillating cantilever to measure the FD curve. However, due to inconsistent probe sensitivities induced by the force and magnetic torque, locations of the laser spot and magnetic bead need to be specified for broad modulus range nanomechanical mapping. Here, a calibration method is presented to obtain a uniform probe sensitivity induced by the force and magnetic torque. In this method, the torque-induced deflection sensitivity is accurately determined by the deflection tracking approach. Impacts of the laser spot and magnetic bead location on deflection sensitivity are investigated, and a straight protocol is developed to locate the laser spot for unifying two deflection sensitivities. The proposed calibration methods are applied in mapping nanomechanical properties of polystyrene, highly oriented pyrolytic graphite surfaces at driving frequencies of 5 kHz in air, and polydimethylsiloxane surface at 0.5 kHz in liquid environment.