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.
Highlights
With the advent of friction force microscopy (FFM) as a variant technique of atomic force microscopy (AFM), it became possible to disclose in the sub-nanometer scale information on the underlying tribological mechanisms, on crystal lattices and sometimes even on atomic structures[1,2,3,4,5]
They concluded that the remarkable lattice resolution reported when performing Friction force microscopy (FFM) experiments in liquid is not incidental, and can be regarded as a highly practical methodology. This statement was based on their founding that the liquid molecules do not impair the spatial resolution of the measurement, but merely introduce a stochastic effect that result in a slightly noisier recording. We continue this line of study by performing FFM measurements on NaCl immersed in ethanol, and comparing these measurements with FFM measurements in ultrahigh vacuum (UHV) reported by Socoliuc et al.[9]
We show that our measurements performed in liquid environment agree and scale with FFM measurements of NaCl under UHV conditions
Summary
With the advent of friction force microscopy (FFM) as a variant technique of atomic force microscopy (AFM), it became possible to disclose in the sub-nanometer scale information on the underlying tribological mechanisms, on crystal lattices and sometimes even on atomic structures[1,2,3,4,5]. With the use of AFM apparatus, FFM experiments record atomic-scale forces via the interaction of an ultra-sharp cantilever tip with the surface of interest, while probing it by scanning under an applied normal load This enables a direct measurement of lateral friction forces that exhibit atomic stick-slip pattern as the tip slides across a crystal substrate. In light of this increasing trend, Vilhena et al.[19] addressed the question of the validity of FFM experiments in liquid environment, joining FFM experiments with molecular dynamic (MD) simulations of graphene in water and under UHV conditions They concluded that the remarkable lattice resolution reported when performing FFM experiments in liquid is not incidental, and can be regarded as a highly practical methodology. We show that our measurements performed in liquid environment agree and scale with FFM measurements of NaCl under UHV conditions
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