Soft-contact imaging in liquid with frequency-modulation torsion resonance mode atomic force microscopy.

In this work, we demonstrate that high-resolution imaging in water with a soft contact between the tip and the sample can be achieved with frequency-modulation torsional resonance (FM-TR) mode atomic force microscopy (AFM). This mode is very sensitive to the contact of the tip with the sample surface. A sharp jump in the resonance frequency shift occurs when the tip is getting in touch with the sample. Individual atomic features on mica surfaces can be resolved with a relatively large tip. The tip applies very small normal and lateral forces on the surface. In addition, even a long and compliant AFM cantilever can achieve a high quality factor and a high resonant frequency for the torsional oscillation in water. Along with several other advantages, this mode is very suitable for future development of high-sensitivity, high-resolution, high-speed AFM for the study of dynamic biological processes in liquid.