Metallic adhesion and tunneling at the atomic scale

The metallic adhesion and tunneling properties of an atomically defined junction were measured and analyzed. The junction consisted of a tip opposing a flat surface in the scanning probe microscopy (SPM) configuration. Measurements were performed in ultrahigh vacuum (UHV) at 150 K. Sub-nN force resolution was achieved on a stiff cantilever beam employing an in-situ differential interferometer. Tips were prepared from W and Ir wire and imaged with atomic resolution in-situ using field ion microscopy (FIM). Ultrasharp tips with an apex radius of 20--30 A were fabricated from single crystal W(111) wire and engineered with FIM to terminate in only three atoms. Calculations indicate that for those tips metallic adhesion forces dominate over van der Waals and capacitive electrostatic forces. The sample was a thin (111) oriented Au film. Metallic adhesion forces and the tunneling current were measured simultaneously for the W-Au system as a function of tip sample separation. In contrast to theoretical simulations the system featured exceptional mechanical stability with adhesive forces of up to 5 nN. In particular no indications of a sudden jump-to-contact, which is commonly believed to be an inherent property of metallic contacts, were found. Furthermore, the range over which the metallic adhesion forces act is four times larger than expected. Experiments with sharp but not atomically defined W tips corroborate those results. The observed long interaction range is discussed in the framework of various models. Some of the consequences of this new property for force microscopy applications are pointed out