Thickness dependent friction on few-layer MoS₂, WS₂, and WSe₂

Area of Science:

• Materials Science
• Nanotechnology
• Tribology

Background:

• Nanoscale friction on two-dimensional (2D) materials is critical for their mechanical and electronic properties.
• Friction is known to be influenced by material thickness and tip geometry.

Purpose of the Study:

• To investigate the thickness-dependent nanoscale friction on few-layer molybdenum disulfide (MoS2), tungsten disulfide (WS2), and tungsten selenide (WSe2).
• To understand the underlying mechanisms governing friction variations with sample thickness and tip shape.

Main Methods:

• Atomic force microscopy (AFM) was employed to measure friction on 2D materials under ambient conditions.
• Density functional theory (DFT) calculations were performed to model and interpret the observed friction behaviors.
• Experiments utilized both sharp and pre-worn, flat-ended AFM tips to probe different contact regimes.

Main Results:

• A regular trend of decreasing friction with increasing thickness was observed with a sharp tip.
• An abnormal trend was found with a flat-ended tip: friction increased with thickness for WS2 and WSe2, and showed a non-monotonic behavior for MoS2.
• DFT calculations suggest friction is a result of competing puckering effects and intrinsic energy corrugation.

Conclusions:

• The study reveals complex thickness-dependent friction behaviors in 2D materials, strongly influenced by tip shape.
• A competition between the puckering effect and intrinsic energy corrugation dictates nanoscale friction.
• The findings offer a method to tune friction by altering tip geometry and suggest ways to measure intrinsic frictional properties with reduced puckering impact.