Atomic-scale friction modulated by a buried interface: Combined atomic and friction force microscopy experiments

Combined atomic and friction force microscopy reveals a significant modulation of atomic-scale friction related to the small periodic rumpling induced at the interface between heteroepitaxial films of KBr on NaCl(100). Transitions from dissipative atomic-scale stick slip to smooth sliding with ultralow friction are observed within the $6\times 6$ surface unit cell of the underlying superstructure. Scanning across atomic-scale defects confirms the high-resolution capabilities of friction force microscopy close to the ultralow friction state. Strong variations of the tip-surface interaction energy across the superstructure demonstrate that subsurface chemical and size inhomogeneities dramatically change the frictional properties of the surface probed by the microscope tip.

Figure 1

(Color online) Simultaneously recorded images of (a) the surface topography and of (b) the lateral force at a normal force of $-0.01\text{nN}$ on a KBr double layer deposited on NaCl(100). They reveal the underlying superstructure, as well as atomic-scale modifications (indicated by the circle and arrows) induced by an unidentified defect at the same locations in both images. (c) Topography and lateral force traces across the unidentified defect along the yellow line in (b).

Figure 2

(Color online) Constant height lateral force image of (a) a KBr double layer on NaCl(100) and [(c) and (d)] corresponding forward and backward traces along the lines indicated in (a). (b) Difference between lateral forces recorded in successive forward and backward scans. The average normal force was reduced from $-0.01$ to $-0.32\text{nN}$ in the lower half of the image.

Figure 3

(a) Forward lateral force trace along a line of maximal modulation from Fig. 2a (top). The gradual change from a nearly symmetric shape to a sawtooth shape indicates a transition in atomic-scale friction from smooth sliding to stick slip. (b) Simulated lateral force trace assuming a modulated energy corrugation with period $6a$ but constant stiffness in a one-dimensional Tomlinson model (Ref. 21). (c) Simulated lateral force trace assuming a modulated stiffness with period $6a$ but constant energy corrugation. (d) Corrugation amplitude $E_0$, (e) effective stiffness $k$, and (f) dimensionless parameter $\eta$ determined for every oscillation within the supercell on the KBr-terminated superstructure. The transition from stick slip to smooth sliding is consistent with the change in $\eta$ from values below one to values larger than one. The effective stiffness $k$ was found to be nearly constant. The error bar for the last value of $k$ is large because the corresponding value of $\eta$ is low.