Atomic Friction at Electrodes: Anisotropy Effects at Regularly Stepped Gold Electrodes

Friction under ambient conditions often involves wet surfaces, and thus electrochemical inter­faces. Yet measurements of friction at an atomic scale under electrochemical condi­tions are scarce though accessible by combination of AFM with an electrochemical cell. For HOPG, a change of friction forces at steps edges was observed with a change of potential.[1] Our inter­est has been in the influence of potential, Cu UPD on Au(111) and organic adsorption on atomic-scale friction.[2]

We will present friction measurements performed on Au(111) single crystal electrodes and the effect of adsorbates on atomic stick-slip resolution. The range of normal force neces­sary to observe stick-slip resolution depends on the type of ionic adsorbates. At the small normal load of 15 nN transition to atomic stick-slip resolution is observed, when sulfate is adsorbed on the surface. The reason for that transition could be a penetration of the tip in to the double layer. At the pzc on clean gold stick-slip behaviour not shows any transition or penetration effects. The adhesion on the force-distance curve disappear with a copper adsorption.

However, with a monolayer of Cu on Au(111) a transition to stick-slip behavior is present, but without atomic resolution. We found 3 different types of stick-slip pattern on copper monolayer in diapason of normal loads between 0 and 100 nN; slip length is equal 4,6; 9,2; and 13,8 Å. The slope of stick combines the stiffness of tip and sample. That stiffness is independent from adsorbat or normal load and always have the same value at about 12 nN/nm, which is much smaller than torsion spring constant of cantilever 190 nN/nm.

Friction is increased at step edges due to the the Schwoebel barrier. This effect is reduced when steps are decorated by Pd on Au. Due to this effect of steps on friction, vicinally stepped surfaces show a large anisotropy for friction, although atomic resolution is obtained even when scanning perpendicular to the steps.

[1]      E. Weilandt, A. Menck, O. Marti, Surface and Interface Analysis 1995, 23, 428; B. Schnyder, D. Alliata, R. Kötz, H. Siegenthaler, Applied Surface Science 2001, 173, 221.

[2]      M. Nielinger, H. Baltruschat, Physical Chemistry Chemical Physics 2007, 9, 3965.; F. Hausen, M. Nielinger, S. Ernst, H. Baltruschat, Electrochimica Acta 2008, 53, 6058.