One of the greatest advantages of the AFM, compared to other imaging techniques, is the possibility of modifying the morphology of a surface while scanning it. This possibility is well exemplified by the number of nanolithography and nanomanip-ulation experiments reported in the literature. However, to use these techniques for practical applications, the motion of the probing tip must be controlled in order to pattern the surfaces in a desired way or to rearrange the manipulated objects in a desired configuration. In the case of nanomanipulation, assembling nanoparticles in a well-defined arrangement is usually a difficult and time-consuming task. Fric- tion and adhesion forces between particles and substrates indeed play a major role in the motion on the nanoscale and, even if one works in a controlled environment, the size of the nanoparticles is usually comparable to that of the tip apex, which makes any attempt of controlling the manipulation process quite challenging.

Contact mode manipulation

One of the first examples of AFM manipulation was reported by Lüthi et al. [199], who succeded in moving compact C60 islands on a NaCl(001) surface by pushing them with the probing tip (Fig. 19.1). From the area of the islands determined by the AFM topographies and the values of the kinetic friction force recorded while sliding, a shear stress between C60 and NaCl of the order of 0.1 MPa was estimated. A larger shear stress, caused by the static friction and more difficult to quantify, accompanied the onset of motion. In another experiment, Sheehan and Lieber recognized the importance of the misfit angle in the manipulation of MoO3 islands on an MoS2 surface [307]. In this case, the islands could only move along low index directions of the substrate. Another experiment on Sb islands manipulated on the same substrate is presented in Section 19.3. Note that, instead of pushing a nanoisland, the tip can also be positioned on top of it.