Scanning polarization force microscopy SPFM

Other noncontact AFM methods have also been used to study the structure of water films and droplets [27,28]. Each has its own merits and will not be discussed in detail here. Often, however, many noncontact methods involve an oscillation of the lever in or out of mechanical resonance, which brings the tip too close to the liquid surface to ensure a truly nonperturbative imaging, at least for low-viscosity liquids. A simple technique developed in 1994 in the authors laboratory not only solves most of these problems but in addition provides new information on surface properties. It has been named scanning polarization force microscopy (SPFM) [29-31]. SPFM not only provides the topographic stracture, but allows also the study of local dielectric properties and even molecular orientation of the liquid. The remainder of this paper is devoted to reviewing the use of SPFM for wetting studies. 

Salmeron and his group, realizing the comphcation introduced by adlayer water on the AFM tip, have developed a technique they call scanning polarization force microscopy (SPFM) [60,61]. In this form of noncontact AFM tip imaging, the metalized AFM tip is electrically biased and responds to the dielectric properties of the water film and any accompanying ions. The method requires the tip to be of the order of 10 nm from the film, so the higher resolution of contact AFM is lost. On the other hand, the electrical properties of the film can be explored. An example of their investigation of thin film water on NaCl (001) [34] is shown in Fig. 6. 

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