Molecular-Resolution FM-AFM Imaging of Biological Systems

Frontier Science Organization, Kanazawa University, Kakuma-machi, 

Nanoscale Liquid Interfaces Wetting, Patterning, and Force Microscopy at the Molecular Scale Edited by Thierry Ondar uhu and Jean-Pierre Aime Copyright 2013 Pan Stanford Publishing Pte. Ltd. 

In 1991, Albrecht et al. invented frequency modulation atomic force microscopy (FM-AFM) for operating d3mamic-mode AFM in vacuum environments. Before this invention, it was common to operate d3mamic-mode AFM with the amplitude detection method, which is referred to as amplitude modulation AFM (AM-AFM). In AM-AFM, the tip-sample distance is regulated such that the oscillation amplitude of the cantilever (A) is kept constant. 

AM-AFM was mainly used in air and liquid, and its applications in vacuum were very limited. This is because of the high Q factor in vacuum. For a t3q ical cantilever, Q = 1,000-100,000 in vacuum, Q = 100-1,000 in air and Q = 1-10 in liquid. In dynamic-mode AFM, a high Q factor provides higher force sensitivity. In addition, the vacuum environment provides a clean surface and a well-defined environment, which are essential for the surface science studies. Thus, there were strong demands for operating d3mamic-mode AFM in vacuum. However, the time response of A is inversely proportional to the Q factor, so the use of AM-AFM in vacuum was difficult in most of the applications because of the slow time response. 

To resolve this issue, Albrecht et al. proposed to detect the frequency shift A/ of the cantilever resonance instead of A. In FM-AFM, the cantilever is always oscillated at its resonance frequency using a self-oscillation circuit. Thus, the A/ induced by the tip-sample interaction results in the frequency shift of the cantilever vibration. The A/ is detected from the cantilever deflection signal and used for the tip-sample distance regulation feedback. As the time response of A/ is free from the influence of Q, the time response of FM-AFM in vacuum is much faster than that of AM-AFM. Owing to the high force sensitivity obtained by the high Q factor and the atomically well-defined environment provided by vacuum, true atomic-resolution imaging by dynamic-mode AFM has become possible in 1995.  

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