Metallic adhesion and tunnelling at the atomic scale

We simultaneously measured the distance dependence of the force and the tunnelling current between a W(111) tip and a Au(111) sample in an ultrahigh vacuum at T = 150 K. The tip was characterized by field ion microscopy. Even at atomically close contact no evidence of structural instabilities was found. The scaling of the force curves show an unexpectedly long distance scaling parameter of λ = 0.2 nm. We conclude that not only the apex atoms contribute to the adhesion forces, but the first three layers play an almost equal role. Using a model that correlates the force and the tunnelling current, we are able to extract the tip density of states. Possible reasons for the long scaling length are discussed. The study of metallic adhesion on the atomic scale lays the foundation for the understanding of the physics of nanometre-sized structures. In particular, issues surrounding nanoscale tribology find widespread interest in newly developing fields such as nanotechnology. Adhesion has also been found to play a crucial role in understanding imaging mechanisms in scanning tunnelling microscopy (STM) [1,2] and atomic force microscopy (AFM) [3]– [5] with special attention given to the influence of the geometric and chemical nature of the probe itself [6]– [8]. Earlier adhesion studies [9]– [11] were handicapped by the inability to analyse the geometric structure and chemical nature of the tip. This limitation was finally addressed using in situ field ion microscopy (FIM) [12]. It was found that the typical range over which metallic exchange correlation forces act is much larger than expected from theoretical models [14, 15] and simulations [16]. In this paper we take a further step and present for the first time simultaneously acquired force and tunnelling current of a defined tip–sample junction as a function of the tip–sample separation. † Present address: IBM Research Division, Zurich Research Laboratory, CH-8803 Rüschlikon, Switzerland. ‡ Present address: iT security AG, 8048 Zurich, CH, Switzerland. New Journal of Physics 2 (2000) 29.1–29.10 PII: S1367-2630(00)15669-1 1367-2630/00/000029+10$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft