Non-contact AFM imaging in water using electrically driven cantilever vibration.

An atomic force microscopy (AFM) imaging mode is presented that can simultaneously record surface topography and local electrical properties in aqueous solutions without mechanical contact between the AFM tip and the sample. The interaction between the electrically biased tip and the grounded sample in aqueous medium causes the AFM cantilever to vibrate. This operation mode is based on the previously developed SPFM technique, though using water as the medium instead of air introduces some important practical and theoretical differences, and also greatly extends the applicability of this technique. There are two vibration modes, one at the frequency of the applied voltage (ω) and one at twice this frequency (2ω). The surface topography can be imaged using feedback control of the 2ω vibration amplitude, which is very sensitive to the tip-sample separation distance in the range of 1-10 nm. The amplitude and phase of the 1ω vibration can be recorded simultaneously during imaging to obtain information on local surface charge or potential differences. Similar techniques exist for imaging in air or vacuum, but the addition of a polarizable medium such as water adds significant theoretical and practical complexities. This paper addresses those complexities and demonstrates the effectiveness of the technique for surface imaging and analysis in aqueous environments.