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Controlling microscopic friction on gold surfaces by electrochemical potential Florian Hausen, Johannes A. Zimmet, and Roland Bennewitz INM – Leibniz-Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany ABSTRACT The nano-scale friction on crystalline gold surfaces can be systematically varied by changing the oxidation state of the surfaces through an applied electrochemical potential. We present experimental results from high-resolution friction force microscopy, where the atomic structure of the surface is reflected in lateral force maps. While the oxidation of gold surfaces always brings upon a significant increase in friction, the situation is more complex in the potential regime where only sulfate anions are adsorbed. The influence of adsorbed anions on friction depends on electrochemical potential and on normal load, demonstrating that electrochemical processes and sliding dynamics are altered in the confinement of the tip-sample contact. INTRODUCTION The relation between friction, wear, and electrochemistry has been a longstanding problem in Materials Science due to the technological importance of tribocorrosion. However, one aspect of the relation has attracted interest more recently: the control of friction through reversible electrochemical modification of surfaces. Fundamental studies in this field rely on the methods of nanotribology, in particular on single-asperity experiments by means of friction force microscopy [1]. The power of atomic force microscopy to resolve electrochemically induced changes of surface structures has been demonstrated early in its development [2, 3]. Later studies found a sensitivity of friction force microscopy to the specific adsorption of anions [4], to electrochemical deposition of metals [5], and to ionic adsorbates [6]. Recently, dedicated instrumentation was developed to study atomic-scale friction processes under electrochemical control [7], including a switching of nano-scale friction between high and low values for the oxidized and the reduced state of Au(111) surfaces [8]. On the same surface, we have elucidated details of the relation between anion adsorption and atomic stick-slip behaviour [9]. We found indications that the ordering of the adsorbed layer and its influence on friction depend on potential and normal load, i.e. that electrochemical processes and sliding dynamics are altered in the confinement of the tip-sample contact. In this contribution we present experimental results for atomic friction processes on the Au(100) surface under electrochemical control. The influence of electrochemical oxidation on friction is demonstrated and results from switching experiments are presented. Furthermore, we present new data confirming the load-dependence of sulfate adsorption in the contact and of its influence on friction on Au(111). EXPERIMENT All experiments have been performed using an atomic force microscope (Agilent 5500) with a home-built electrochemical cell. Sample surface and force sensor where fully immersed in the electrolyte. The Au(100) a