PUB - Publikationen an der Universität Bielefeld

In frequency modulated non-contact atomic force microscopy, the change of the cantilever frequency (Delta f) is used as the input signal for the topography feedback loop. Around the Delta f(z) minimum, however, stable feedback operation is challenging using a standard proportional-integral-derivative (PID) feedback design due to the change of sign in the slope. When operated under liquid conditions, it is furthermore difficult to address the attractive interaction regime due to its often moderate peakedness. Additionally, the Delta f signal level changes severely with time in this environment due to drift of the cantilever frequency f(0) and, thus, requires constant adjustment. Here, we present an approach overcoming these obstacles by using the derivative of Delta f with respect to z as the input signal for the topography feedback loop. Rather than regulating the absolute value to a preset setpoint, the slope of the Delta f with respect to z is regulated to zero. This new measurement mode not only makes the minimum of the Delta f(z) curve directly accessible, but it also benefits from greatly increased operation stability due to its immunity against f(0) drift. We present isosurfaces of the Delta f minimum acquired on the calcite CaCO3(1014) surface in liquid environment, demonstrating the capability of our method to image in the attractive tip-sample interaction regime. (C) 2014 AIP Publishing LLC.