[Paper Review] Friction at Atomic-Scale Surface Steps: Experiment and Theory

Hendrik Hölscher, Daniel Ebeling+1 more|ArXiv.org|Oct 1, 2008

Force Microscopy Techniques and Applications3 citations

TL;DR

This study reveals a universal, load-dependent frictional asymmetry at atomic-scale surface steps: friction increases linearly with load when scanning upward (overcoming a Schwoebel-Ehrlich barrier), but remains constant when scanning downward (due to a potential minimum at the step base). A modified Prandtl-Tomlinson model incorporating step-induced potential barriers explains this directional asymmetry, providing a fundamental mechanism for atomic-scale friction in rough surfaces.

ABSTRACT

Experiments performed by friction force microscopy at atomic-scale surface steps on graphite, MoS$_2$, and NaCl in ambient conditions are presented. Both step-down and step-up scans exhibit higher frictional forces at the edge, but distinguish in their load dependence: While the additional frictional force due to the step edge increases linearly with load if the tip has to jump a step up, it remains constant for downward jumps. This phenomena represents a universal effect that can be explained in terms of a modified Prandtl-Tomlinson model featuring a Schoebel-Ehrlich barrier at steps.

Motivation & Objective

• To investigate the load dependence of atomic-scale friction at surface steps, a key feature of real, non-ideal surfaces.
• To resolve the discrepancy between experimental observations and classical friction models on rough surfaces.
• To establish a theoretical framework explaining the directional asymmetry in friction at atomic steps.
• To generalize the findings to other surface defects like vacancies and grain boundaries.

Proposed method

• Conducted friction force microscopy (FFM) on freshly cleaved graphite, MoS2, and NaCl(001) under ambient conditions.
• Measured lateral forces during left-right and right-left scans across single, double, and five-layer steps.
• Calibrated normal and lateral spring constants using established procedures (Schwarz et al., 1996).
• Developed a modified Prandtl-Tomlinson model incorporating a Schwoebel-Ehrlich barrier at step edges.
• Simulated tip motion across a potential landscape with asymmetric barriers and minima on either side of the step.
• Used Morse potential-based tip-sample interactions to validate robustness across different interaction models.

Experimental results

Research questions

• RQ1 Why does friction at atomic-scale surface steps depend differently on load depending on scan direction?
• RQ2 What physical mechanism explains the linear load dependence for upward scans versus load independence for downward scans?
• RQ3 How do changes in atomic coordination at step edges influence the local potential energy landscape and frictional forces?
• RQ4 Can a modified Prandtl-Tomlinson model account for the observed directional asymmetry in friction at steps?
• RQ5 To what extent is this effect generalizable to other surface defects such as vacancies or grain boundaries?

Key findings

• The frictional increase at step edges grows linearly with load for upward scans, indicating a load-dependent barrier height.
• The frictional increase remains constant for downward scans, indicating a load-independent barrier height due to a potential minimum at the step base.
• The directional asymmetry arises from the asymmetric potential landscape: a high barrier for upward motion and a shallow minimum for downward motion.
• The model explains that the force required to overcome the Schwoebel-Ehrlich barrier increases with load when approaching from the lower terrace.
• Experimental data from graphite, MoS2, and NaCl show consistent directional friction behavior across different materials and step heights.
• The effect is attributed to changes in atomic coordination at step edges, creating a potential barrier (Schwoebel-Ehrlich) that dominates frictional response in rough surfaces.