True atomic-resolution imaging by frequency modulation atomic force microscopy (FM-AFM) has become possible by using ultra low noise cantilever deflection detection system and thereby oscillating a cantilever with extremely small oscillation amplitude [1]. This novel technique has also enabled direct imaging of individual hydration layers [2] and mobile ions [3] on biological membranes under physiological conditions. Whilst the striking results have highlighted the advantages of using FM-AFM over conventional AFM techniques, they have also given rise to a number of questions from the technical point of view.
First, this novel technique utilizes FM detection as a force detection scheme rather than commonly used amplitude modulation (AM) detection or static-mode detection. This has raised a question if the demonstrated high-resolution and high-force-sensitivity are due to the small oscillation amplitude or the use of FM detection. Secondly, the novel technique utilized very stiff cantilever (40 N/m) whereas a cantilever with a spring constant of less than 0.1 N/m have commonly been used for imaging biological sample. Thus it has been questioned if the use of stiff cantilever is essential or what is the optimal spring constant to be used for imaging biological systems. Thirdly, the spatial resolution demonstrated by FM-AFM in liquid is as high as the one demonstrated in vacuum in spite of the extremely small Q-factor in liquid. It has not been quantitatively explained why FM-AFM can provide such a high-resolution and high-sensitivity in low-Q environment.
In order to answer these questions, I have carried out detailed and quantitative characterization of the performance of FM-AFM in low-Q environment such as in liquid and air. Signal-to-noise ratio and time response of the force detection, and contributions from the long-range and short-range forces are discussed in detailed for FM-AFM operations with different implementations.
[1] T. Fukuma, K. Kobayashi, K. Matsushige, H. Yamada, Appl. Phys. Lett. 87 (2005) 034101.
[2] T. Fukuma, M. J. Higgins, S. P. Jarvis, Biophys. J. in press.
[3] T. Fukuma, M. J. Higgins, S. P. Jarvis, Phys. Rev. Lett. 98 (2007) 106101.
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