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

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.

Figure 1

(a) Crystal structure of KAP [gray (large dark balls): C atoms; red (small dark balls): O atoms and ions; blue (large bright balls): K ions; white (small bright balls): H atoms] and unit cell axes. (b) Representation with a depth-dependent color scale of the cleavage surface of KAP; the surface unit cell is indicated by a black rectangle; significative crystallographic directions are indicated by green (bright) arrows; $\varphi$ is the angle between KAP[100] and the scan direction (black arrow). (c) Structural models of the cleavage surface of KAP as viewed along the [001] and [101] directions, pointing out the main surface corrugations.

Figure 2

Solid line: experimental TSM signal collected on the KAP(010) surface as a function of the angle $\varphi$ (10° steps). Dotted line: (a) TSM signal calculated from the in-plane elastic constants of KAP [10] using Eq. (1), (b) TSM signal calculated from a nonlinear model of anisotropic friction with six principal directions and constant friction coefficients.

Figure 3

Molecular-scale TSM images ($7\times 3.5{\mathrm{nm}}^2$) collected on the KAP(010) surface at $\varphi =345°$. The cross-sectional profiles taken along the gray (red and blue) lines are reported in the graph (offset has been subtracted), showing the periodicity corresponding to the (100) corrugation and the inverse relation between the trace and retrace signal, respectively.