Sharpening contact resonance spectra in UAFM using Q-control

Ultrasonic atomic force microscopy (UAFM) is a new scientific tool for reliable measurement of nano-scale elasticity based on the resonance frequency measurement of AFM cantilever in the contact mode. This paper proposes Q-control for improving resolution of contact mode resonance spectra. First, we perform a theoretical analysis using a distributed mass model for the UAFM cantilever. Next, we confirm experimentally that the Q factor is actually improved by applying Q-control. In the theoretical analysis, we analyze the effect of Q-control for the higher resonance, which cannot be analyzed by the point mass model, and we demonstrate that the optimum phase is different by almost π between the first and the second resonance. Although, the optimum phase is almost constant −π/2 in the first resonance, it turns out that it changes with damping in the second resonance. This behavior is discussed in terms of the vibration mode shape.