Energy dissipation offers the very important physical properties relevant to the viscoelasticity, elasticity, adhesion, hydrophobicity/hydrophilicity, and surface charges of biological molecules, etc. So far, in liquids, tapping-mode AFM has been widely used to observe the soft sample surface, and phase shift image offers the information of compositional properties concerning to the energy dissipation.
Recently, we have proposed the phase modulation AFM (PM-AFM) to detect the very weak tip-sample interaction force [1]. PM-AFM detects the phase shift of the oscillating cantilever relative to the excitation signal and uses it as a feedback signal. From the theory as well as experiments, we have clarified that the minimum detectable force in PM-AFM had about 30 times improved as compared with that of AM-AFM. This feature of PM-AFM is very useful to observe the fragile soft sample with high resolution. However, energy dissipation method has not been established.
In this study, we develop high-speed simultaneous measurement method of topography and energy dissipation in PM-AFM. In this method, phase shift (cosθ and sinθ) and vibration amplitude are simultaneously detected with sampling technique, and energy dissipation is estimated with arithmetic circuit. This method can carry out high-speed measurement of the energy dissipation for each cycle of the cantilever vibration. We demonstrate the high-speed simultaneous imaging of topography and energy dissipation for various biological samples in liquid.
Reference:
[1] N. Kobayashi, Y. J. Li, Y. Naitoh, M. Kageshima and Y. Sugawara. Jpn. J. Appl. Phys. Vol. 45, L793 (2006).
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