5.7. Summary and future development

In contrast to contact mode, which senses a force, DFM is a derivative technique that senses an interfacial stiffness. As frequency is increased, DFM also becomes sensitive to increased interfacial damping. The properties of a fluid change significantly at distances on the order of several molecular diameters. In the case of water, this appears to be some ten molecular diameters, though contamination may play a role. In this regard, it is worth noting that the surface energy of mica freshly cleaved in air is an order of magnitude less that that of mica cleaved in ultra high vacuum (Israelachvilli 1991) while gold changes from hydrophilic to hydrophobic on exposure to air (Smith 1980).

While the long decay lengths facilitate non-contact operation, they also make atomic resolution difficult. Experience indicates that resolution is increased as the tip is brought closer to the underlying surface by increased the amplitude damping set-point. On the other hand, this also results in increased disturbance of the sample (Eqn 12). The discussion of section 3 indicated that extremely close approach would be required for atomic resolution. Recent work (Ohnesorge 1999) does suggest that atomic resolution might be possible in fluid. There is, as yet, no comprehensive analysis of noise sources and the optimal choice of cantilever along the lines of that carried out for ultra-high vacuum DFM (Giessibl 1997). Perhaps adequate sensitivity will not be available with low-Q cantilevers.

Nonetheless, progress has been substantial, and much work can be done at the 1nm level of resolution that is routinely obtained for DFM imaging in fluid. Direct (magnetic) drive offers advantages for imaging in fluids and applications for magnetic drive beyond imaging will be discussed below.