Figure 1.

Schematic setup of our AFM operated in AM-OL or AM-FM mode. AM-OL mode can be accomplished by adding the drive signals of two function generators for exciting the cantilever beam and using two lock-in amplifiers to track the amplitudes and phase shifts at the corresponding eigenmodes. For AM-FM mode external PLL electronics were used, which provided a drive signal with a constant phase shift of 90° between excitation and response while tracking changes in the instantaneous resonant frequency.