Enrico Gnecco, Institute of Physics, University of Basel, Switzerland
Due to their simple structure and the weak interaction with the probing tip, ionic crystals are ideal systems for studying nanoscale phenomena by atomic force microscopy in ultra-high vacuum. In the first part of my talk I will discuss fundamental friction and wear processes observed on KBr and NaCl. At extremely low loads (< 1 nN) a "superlubricated" regime of motion is achieved, in which the probing tip alternately follows or precedes the cantilever driving it without energy dissipation. At higher loads sliding becomes discontinuous. At room temperature the occurrence of discontinuities is modified by thermal vibrations, which leads to a velocity dependence of friction. The elastic regime can be easily overcome on KBr. In such a case the tip starts to pick up material from the surface and to redeposit it in mounds which reproduce the structure of the primeval surface. On larger scales, ripple patterns are formed perpendicular to the scan direction, with wavelengths of the same order of the tip radius of curvature. In the second part of my talk I will show how the surface of KBr can be treated in order to create nanotraps for organic molecules. In a first series of experiments porphyrin and perylene derived molecules were found to decorate the edges of the nanostructures without self-assembling. The situation changed dramatically when polar molecules were introduced. Ordered structures of sub-phtalocyanine molecules were found inside nanoholes less than 15 nm in size. Interesting applications to molecular electronics will be discussed.