INVESTIGATION ON TRANSITION BETWEEN TIP-SAMPLE INTERACTION REGIMES IN BIMODAL AMPLITUDE MODULATION ATOMIC FORCE MICROSCOPY

Bimodal amplitude modulation atomic force microscopy (AM-AFM) has two interaction regimes in measurements or imaging, i.e., the attractive regime and the repulsive regime. The investigation on the transition between the interaction regimes is critical for the setting of parameters for imaging in a specific interaction regime, the control of the ranges of the interaction regimes as well as the correct understanding and interpretation of the imaging results in bimodal AM-AFM. Combining finite difference method and the in-phase and quadrature method, the influences of the magnitudes setting of modal free resonance amplitudes, the variation of mechanical properties of the sample, and the setting of excitation frequencies on the transition between the interaction regimes in bimodal AM-AFM are studied by a numerical simulation. Results show that the higher the sum of the modal free resonance amplitudes, the larger the critical setpoint when the attractive regime transitions to the repulsive regime, i.e., the setpoint range of the attractive regime becomes smaller. The higher the elastic modulus and the smaller the viscosity coefficient of the sample, the earlier the attractive regime transitions to the repulsive regime during the approach of the probe to the sample surface. When the probe is excited at the frequencies away from the modal free resonance frequencies, the range of the attractive regime is smaller than that excited at the free resonance frequencies. The saltation of the state of motion of the probe does not necessarily correspond to the transition between the interaction regimes. Furthermore, the phase values are invalid to be employed to determine the attractive or repulsive regime by judging whether the phase value is higher or lower than 90°.