Estimation of interaction energy and contact stiffness in atomic-scale sliding on a model sodium chloride surface in ethanol

Abstract

Friction force microscopy (FFM) in aqueous environments has recently proven to be a very effective method for lattice-resolution imaging of crystal surfaces. Here we demonstrate the use of ethanol for similar measurements on water-soluble materials. Lattice resolved frictional stick-slip traces of a cleaved NaCl(100) surface submerged in ethanol are compared with previous obtained FFM results in ultrahigh vacuum (UHV). We use the Prandtl-Tomlinson framework to estimate the amplitude of the corrugation potential and the contact stiffness. The surface potential amplitude scales with the applied normal loads are in good agreement with data obtained for NaCl measured under UHV conditions, but demonstrates deviations from the ideal periodic potential given by the Prandtl-Tomlinson model. An additional finding is that the use of ethanol allows us to explore higher load ranges without detectable evidence of surface wear. The contact stiffness does not vary significantly with the normal load up to 38 nN, while above it a sudden increase by almost one order of magnitude was observed. Comparing this to previous results suggests that considerable atom rearrangements may occur in the contact region, although the (100) surface structure is preserved by ethanol-assisted diffusion of Na and Cl ions.

Comparative Study of Dimensionality and Symmetry Breaking on Nanoscale Friction in the Prandtl–Tomlinson Model with Varying Effective Stiffness

“…Here, with the intention to better understand these differences, as expressed through the PT parameter and the effective contact stiffness, we observe an unexpected non-monotonous behavior of η with the applied normal load, as it increases to a maximal value and then slowly decreases. The resulting effective stiffness shows an increase with the applied load, as previously reported [57].…”

“…Going to higher values of normal loads (13-91 nN) under the similar conditions, η linearly grew to 14.5 [56]. Measured in liquid (ethanol) surrounding, the lateral interaction between NaCl surface and silicon tip showed a different behavior, as the effective stiffness increased with the applied normal loads (up to ~ 60 nN), and the maximal value calculated for η was ~ 14 [57]. Comparison between FFM measurements and simulations under UHV and liquid surrounding concluded a close proximity in the friction signals [58], and corrugation energy amplitudes [57].…”

“…Comparison between FFM measurements and simulations under UHV and liquid surrounding concluded a close proximity in the friction signals [58], and corrugation energy amplitudes [57]. While these two properties are related to the applied load, the increase of the effective stiffness observed at high loads in liquid surroundings [57] may indicate that the applied load may be manifested also in the elastic term within the PT model.…”

“…Previous work on the interaction between NaCl surface and a silicon tip in ethanol using FFM [57] showed an increase of both corrugation energy and effective stiffness with the applied load. PT parameter, calculated from the experimental data (mean slip forces and stick force-position slopes), deviated from the PT parameter which was calculated from the reconstructed parameters (corrugation amplitude and effective stiffness).…”

“…Measured in liquid (ethanol) surrounding, the lateral interaction between NaCl surface and silicon tip showed a different behavior, as the effective stiffness increased with the applied normal loads (up to ~ 60 nN), and the maximal value calculated for η was ~ 14 [57]. Comparison between FFM measurements and simulations under UHV and liquid surrounding concluded a close proximity in the friction signals [58], and corrugation energy amplitudes [57]. While these two properties are related to the applied load, the increase of the effective stiffness observed at high loads in liquid surroundings [57] may indicate that the applied load may be manifested also in the elastic term within the PT model.…”