Bidirectional scanning force microscopy probes with co-resonant sensitivity enhancement

In dynamic scanning force microscopy (dSFM) an oscillating micromechanical cantilever equipped with an interaction tip is used to provide sub-nanometer spatial resolution data on force gradient related properties of the tip-sample interaction. Our bidirectional dSFM probes provide lateral as well as perpendicular force gradient sensitivity with in-situ-switching capability between the operation directions by employing a special geometry and the first two flexural modes of the cantilever. These probes rely only on basic vertical excitation and detection schemes and are therefore compatible with standard dSFM equipment. A way to increase the sensitivity of the lateral mode is given by a co-resonant detection concept. In the framework of this concept the ease of detection of a micromechanical oscillator, in our case a microstructured cantilever, is combined with the high force gradient sensitivity of a nanomechanical low-stiffness and low-mass carbon nanotube oscillator. The resonance frequency of the nanotube is adjusted via mass deposition to be close to the lateral sensitivity resonance frequency of the cantilever. This ensures that the oscillatory state of the coupled cantilever-nanotube system has an increased sensitivity to force gradients but still retains the easy detection of the oscillation of the cantilever. The amplified sensitivity is experimentally verified by comparing data on magnetic measurements acquired with such a co-resonant sensor with calculations for a standard bidirectional sensor.