Ageing of a Microscopic Sliding Gold Contact at Low Temperatures

Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300 K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170 K. Below 170 K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact.

Figure 1

Friction as a function of tip velocity recorded on a Au(111) surface at different sample temperatures. Full symbols indicate data from a series with increasing velocities, open symbols from a subsequently recorded series with decreasing velocities. The lines are fits to the data assuming a logarithmic increase of friction with velocity.

Figure 2

(a) Variation of friction as a function of scanning time at lower temperatures. The sharp drop in friction is observed when the tip is pulled away from the surface and then the contact is restored. (b) Pull-off curves obtained in high friction regimes ($A$, $C$) and low-friction regimes ($B$, $D$), revealing significant reduction in adhesion after renewing the contact.

Figure 3

Lateral force loop recorded before (a) and after (b) the pull-off experiment in Fig. 2 are compared to lateral force loops from the simulation of a Au(111) (c) and a Ni(111) (d) tip in commensurate contact with a Au(111) surface. Both simulated lateral curves are the third forward and backward loop of the respective simulation. Snapshots from the atomistic simulation of the Au and Ni cone tips are displayed in (e) and (f), respectively. The shading of the balls indicates fixed atoms of the support, atoms in the thermostat region, and atoms which can move freely.