Friction Anisotropy of the Surface of Organic Crystals and Its Impact on Scanning Force Microscopy

Abstract

The transverse component of the friction forces acting on the tip of an atomic force microscope scanning on the surface of an organic crystal was monitored as a function of the scan direction. The relation between friction and the crystallographic system is disclosed, revealing that the symmetry of the friction phenomenon is dictated by the direction of the prominent corrugations of the crystal surface. It is also illustrated that molecular-resolution images can be collected through the monitoring of the motion of the tip in a transverse direction with respect to the scan direction.

Anisotropic surface coupling while sliding on dolomite and calcite crystals

“…5 Systematic measurements of such "friction anisotropy" were reported on highly oriented pyrolitic graphite, 6 metallic quasicrystals, 7 and organic crystals. 8 However, the results of the first study could only be explained by assuming the presence of a graphite flake attached to the tip apex, the second investigation did not report any friction maps on the atomic scale, and the third one did not reveal sublattice features in the complex surface structure. This was indeed possible on another organic crystal, where Fessler et al were recently able to distinguish between two differently oriented molecules forming the unit cell of a surface lattice, although the direction of motion of the tip was kept fixed.…”

Nanoscale Mapping of Frictional Anisotropy

“…The first is the higher longitudinal friction force measured for sliding perpendicular to the corrugation lines compared with that measured for sliding parallel to them [21]. The second is the finite transverse friction force measured for sliding along intermediate directions [8]. It must be clear that the relation between these two observations is inherent to the constitutive relation between friction force and sliding direction [1] (Eq.1): whenever there is a difference in the friction forces measured along the two principal directions, a transverse component of friction raises for sliding along any other direction.…”

“…In addition to structural aspects, surface electrical polarity [19] and electronic and phononic contributions [4] can enhance friction anisotropy. Finally, the presence of marked surface corrugations inherent to the surface crystal structure [8,20] or due to extrinsic factors [21] was considered a source of friction anisotropy. In this latter case, friction anisotropy explicates through two related phenomena.…”

“…It was also recognized that, while scanning along a direction parallel to the cantilever axis (conventional AFM constant-force contact-mode), the tangential component of friction causes a buckling of the cantilever which is translated as a pseudovariation of the surface height [4,6]. On the other hand, the normal component of the friction force causes a torsion of the cantilever, which is detected in the transverse shear mode (TSM) [7,8]. In this study, we show that, under specific conditions, the vertical displacements of the sample recorded during AFM scanning in constant force mode can be directly related to the magnitude of the component of the friction force vector parallel to the cantilever axis.…”

Formation of intra-island grain boundaries in pentacene monolayers

“…It tracks the twisting of the cantilever due to the lateral forces acting perpendicular to the scan vector and can be used to visualise the relative orientation of grains in thin films with high contrast. 19,21,[35][36][37] Fig. 3 shows transverse shear for the different substrate preparations and pentacene deposition conditions, the molecular island count was averaged from five AFM images collected in different regions on each sample.…”