Atomic Friction Modulation on the Reconstructed Au(111) Surface

Abstract

Friction between a nanoscale tip and a reconstructed Au(111) surface is investigated both by atomic force microscopy (AFM) and molecular statics calculations. Lateral force AFM images exhibit atomic lattice stick-slip behavior with a superstructure corresponding to the herringbone reconstruction pattern. However, the superstructure contrast is not primarily due to variations in the local frictional dissipation (which corresponds to the local width of the friction loop). Rather, the contrast occurs primarily because the local centerline position of the friction loop is periodically shifted from its usual value of zero. Qualitatively, similar behavior is reproduced in atomistic simulations of an AFM tip sliding on the reconstructed Au(111) substrate. In both simulations and experiments, this centerline modulation effect is not observed on unreconstructed surfaces. Similarly, using a topographically flat surface as a hypothetical control system, the simulations show that the centerline modulation is not caused by variations in the reconstructed surface's topography. Rather, we attribute it to the long-range variation of the local average value of the tip-sample interaction potential that arises from the surface reconstruction. In other words, surface atoms located at unfavorable sites, i.e., in the transition between face-centered-cubic (FCC) and hexagonal-close-packed (HCP) regions, have a higher surface free energy. This leads to a varying conservative force which locally shifts the centerline position of the friction force. This demonstrates that stick-slip behavior in AFM can serve as a rather sensitive probe of the local energetics of surface atoms, with an attainable lateral spatial resolution of a few nanometers.

Chemical effects on the sliding friction of Ag and Au(111)

“…Also, in the case of the PtSi-tip a further modulation of the friction signal was observed that has been attributed in Ref. [23] to the Herringbone reconstruction of the Au(111) surface. In our statistical analysis, the presence of the Herringbone reconstruction resulted in a splitting of the PSD-peak, into two symmetrical peaks around the value corresponding to the interatomic distance of Au atoms in the [110] direction.…”

“…4(d), 6(a) and 6(b) the maximum in the PSDfunctions are also found to split into two equidistant peaks with corresponding wavelength-values  3 = 0.277 nm and  1 = 0.312 nm, respectively. We attribute these peaks to the herringbone reconstruction of the Au(111) surface and the resulting different tilt angles of the fcc and hcp domains with respect to the unreconstructed surface [23]. For the Pt-coated tip three main characteristic wavelengths can be determined [14].…”

Analytical Models for Atomic Friction

“…However, this does not directly answer the question of where the modulated potential comes from. Initial work has been done by Li et al [58] to explain the friction offset modulation on a reconstructed Au (111) surface using MD simulation. The fully atomistic model revealed that the cause of the friction modulation is not the tiny rumpling in the normal direction, but rather can be attributed to the transitions from face-centered-cubic (FCC) to hexagonal-closepacked (HCP) structure on the surface.…”

Experimental Studies of Nanometer-Scaled Single-Asperity Contacts with Metal Surfaces

“…The peak in the PSD functions was also found to split into two equidistant peaks at F n = 10 nN, with corresponding wavelength values λ 3 = 0.277 nm and λ 1 = 0.312 nm, respectively. These peaks may correspond to the herringbone reconstruction of the Au(111) surface and the resulting different tilt angles of the fcc and hcp domains with respect to the unreconstructed surface [30]. AFM scratch testing was used to demonstrate the transitions between different wear regimes on Au(111) during single-asperity sliding contact.…”