Dynamic force microscopy by means of the phase-controlled oscillator method

Dynamic force microscopy, a technique also known as non-contact force microscopy, has proved to be a powerful tool for atomic resolution imaging. A number of schemes have been developed, but recently the oscillator method has become the preferred operating mode. Here, the force sensor acts as resonator in an active feedback circuit. A practical implementation of the method is described and the underlying key concepts are discussed. It is shown that
a tracking oscillator excitation scheme is superior to the more standard direct feedback method for cases in which
the force sensor exhibits only a weak resonance enhancement. Furthermore, the simultaneous measurement of dissipative interaction channels is an important extension of dynamic force microscopy. It allows one to differentiate between sample materials via their plasto-mechanical response. As an example, a Cr test grating has been imaged
in the constant force gradient mode. The dissipation measured on Cr-covered areas is significantly lower than that
on the bare quartz glass substrate, which enables one to distinguish between the two materials with a lateral resolution comparable to that of the topographic image.