Pyridine on Au(1 1 1): A frictional transition controlled by electrochemical potential

We present an electrochemical FFM study for pyridine adsorbed on Au(1 1 1). It is shown that at very low potentials, where pyridine is desorbed from the Au surface, friction is low but rises with potential. At potentials where pyridine is expected to be N-bonded vertically to the surface, 0.5 to 1 nm high islands displaying low friction are observed. Shape and number of these islands do not noticeably change with time or applied normal load. Only few cases of islands going across step edges are observed; most extend only on terraces. Disappearance of these islands upon potential reduction documents reversibility of the process. Friction within these islands is negligible. At small scan sizes stick-slip friction is observed for all potentials.Cyclic voltammetry shows that a peak which previously was ascribed to the change of horizontally adsorbed pyridine to vertically adsorbed pyridine splits above 1 mM into a peak which continues to shift downwards with increasing pyridine concentration and a spike which shifts upward with increasing concentration. The formation of the islands correlates with this spike. We conclude that the low-friction islands are domains of well ordered pyridine standing upright on the Au surface. below this spike and between the islands pyridine may also be standing upright, but is less ordered and the tip might penetrate this less ordered adlayer during scanning.

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