Dual-Peak Friction Upon Traversing a Monolithic Subsurface Step in Layered Materials

Area of Science:

• Surface science
• Tribology
• Materials science

Background:

• Conventional understanding of friction at topographic steps predicts a single peak.
• Few-layer two-dimensional (2D) materials present unique surface properties influencing nanoscale friction.
• Understanding friction at step edges is crucial for 2D material-based devices.

Purpose of the Study:

• To investigate the anomalous friction behavior of atomic force microscopy (AFM) tips scanning across steps on 2D materials.
• To elucidate the underlying mechanism responsible for the observed dual-peak friction.
• To provide insights into friction challenges in 2D material lubrication and superlubricity.

Main Methods:

• High-resolution atomic force microscopy (AFM) measurements.
• Systematic high-resolution stick-slip measurements.
• Nanoindentation experiments.
• Molecular dynamics (MD) simulations.

Main Results:

• A distinctive anomalous dual-peak friction force feature was observed when scanning across a monolithic step covered by few-layer 2D materials.
• This bifurcated frictional response deviates from the conventional single-peak friction expected for topographic obstacles.
• The dual-peak friction is attributed to the tip being transiently pinned at both the front and rear of the step due to asynchronous force evolution.

Conclusions:

• The dual-peak friction mechanism at step edges in 2D materials is identified.
• This finding offers new perspectives for overcoming step-related challenges in 2D material lubrication.
• Understanding this mechanism is essential for advancing structural superlubricity in 2D material devices.