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 pi between the first and the second resonance. Although, the optimum phase is almost constant -pi/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. (C) 2003 Elsevier Science B.V. All rights reserved.