Atomic force microscopy (AFM) has been widely used to investigate structures and mechanical properties of materials on surfaces. Static force mode and the so-called tapping mode are two most used operation modes. Our lab has made a lot of efforts to develop high-resolution AFM imaging techniques. In our work, we would like to improve the force sensitivity and spatial resolution as compared with the conventional AFM modes, especially for imaging of biomaterials in aqueous environment. In these techniques, we use the frequency-modulation (FM) detection scheme, which can track the frequency shift of the vibrating cantilever during scanning. Moreover, we made the cantilever-tip to execute a torsion resonance oscillation by introducing a alternative excitation mechanism. In this vibration mode, the cantilever vibrates laterally as compared to conventional vertically vibrating in flexural-vibration mode. We have successfully developed frequency-modulation flexure resonance mode and frequency-modulation torsion resonance mode. High-resolution imaging with FM-Torsion resonance mode can successfully resolve molecular surface features of biomaterials and atomic lattice structure of mica surface. In summary, these new imaging modes are promising and potential high-resolution imaging techniques for more detailed surface characterization.
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