Nanoscopic Friction under Electrochemical Control

Abstract

We propose a theoretical model of friction under electrochemical conditions focusing on the interaction of a force microscope tip with adsorbed polar molecules of which the orientation depends on the applied electric field. We demonstrate that the dependence of friction force on the electric field is determined by the interplay of two channels of energy dissipation: (i) the rotation of dipoles and (ii) slips of the tip over potential barriers. We suggest a promising strategy to achieve a strong dependence of nanoscopic friction on the external field based on the competition between long range electrostatic interactions and short range chemical interactions between tip and adsorbed polar molecules. PACS numbers:Control of friction during sliding is extremely important for a large variety of applications [1,2]. A unique path to control and ultimately manipulate the forces between material surfaces is through an applied electric field. By varying the applied potential, the electrode surface can quickly and reversibly be modified either with adsorbed (sub)monolayer or multilayers, or via the oxidation and reduction of surfaces, or deposition of ultrathin films [3,4]. Thus, friction force microscopy (FFM) measurements under electrochemical conditions [5-10] may offer significant advantages in comparison to those between dry surfaces.Several experimental and theoretical studies of electrochemical interfaces demonstrated that the orientation of polar molecules adsorbed at electrode surfaces is potential dependent. Water molecules at electrode/electrolyte interfaces reorient from "oxygen-up" to "oxygen-down" as the potential on the electrode changes from negative to positive [11][12][13][14]. Another extensively studied system is pyridine adsorbed on gold electrodes [15,16]. Recent FFM measurements combined with cyclic voltammetry [17] have shown that friction depends strongly on the orientation of the molecules and is five times higher when the molecules are parallel to the substrate compared to their vertical orientation. The molecule orientation can be changed either by changing their concentration or by an external field. In the latter case, the friction shows an intense peak around values of the field where the change of orientation takes place. Recent FFM measurements in UHV have also shown strong sensitivity of nanoscopic friction to the orientation of surface molecules [18]. Investigating the impact of potentialdependent orientation of adsorbed molecules on friction offers a new perspective on active control of friction forces through reversible molecular reorientation.In spite of the first successful experimental studies of nanoscopic friction under potential control [5][6][7][8][9][10], so far there have been no theoretical or numerical studies of the effect of electric fields on friction. We do not know what the mechanism is behind the observed variation of friction with electrostatic potential, nor in which systems significant reversible variation of the friction can be achieved.In this Letter we propose a m...

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“…The cation-tip interaction pushes the cation to turn into the flat orientation, and as a result the orientation of the cations located below the tip may differ from the orientation on the rest of the substrate surface (seeFigures 4c and d). The tipinduced reorientation of cations provides an additional channel for energy dissipation in FFM experiments, which may influence the effect of substrate charge on the friction 43.…”

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“…The charge-induced reorientation of the cations has a profound effect on friction, since the increase of the fraction of tilted cations leads to a reduction of the electrostatic attraction between the cation and the anion layers, hence facilitating the shift of the slippage plane from the solid-cation interface to the interface between the contacting cation and anion layers. In FFM experiments, where the electric field acting on the cations adsorbed on the electrode surface, and correspondingly their orientation, change when the tip approaches them, the reorientation of cations may lead to an additional route for energy dissipation, thus strongly influencing the potential dependency of friction [25].…”

Nanoscale Lubrication of Ionic Surfaces Controlled via a Strong Electric Field

“…In the former case, the system will try to minimize its free energy by adsorbing ions on the tip surface (electrode) and forming an electric double layer (EDL). The ordered structure of the EDL can be responsible for a decrease in the friction coefficient between the tip and surface 27 . Solutions of molecular solids will form a very weak double layer consisting of polarized water molecules, thus leaving friction unchanged.…”