Nanoscale chemical effect on friction force

Area of Science:

• Surface Science
• Nanotechnology
• Tribology

Background:

• Self-assembled alkylsilane monolayers on silicon surfaces are known to reduce friction.
• Bias-assisted nanolithography enables chemical patterning via local oxidation of these monolayers.
• This oxidation process converts terminal methyl groups to carboxyl groups through a redox reaction.

Purpose of the Study:

• To develop and utilize a specialized sample for precise nanoscale chemical friction measurements on silicon.
• To eliminate topography effects and minimize inconsistencies in friction force measurements.
• To directly compare friction between oxidized (carboxyl-terminated) and unmodified (methyl-terminated) regions within a single scan.

Main Methods:

• Preparation of a unique silicon substrate with adjacent regions of methyl-terminated and carboxyl-terminated alkylsilane monolayers.
• Utilizing an atomic force microscope for nanoscale friction force measurements under varying load and tip velocity.
• Employing bias-assisted nanolithography to create chemically distinct surface patterns.

Main Results:

• Friction force increases with applied load for both methyl- and carboxyl-terminated surfaces.
• The coefficient of friction for carboxyl-terminated regions is approximately five times higher than for methyl-terminated regions.
• Friction force decreases with increasing tip velocity for both surfaces, with a more pronounced effect on carboxyl-terminated surfaces.

Conclusions:

• Nanoscale chemical friction is significantly influenced by surface terminal groups (methyl vs. carboxyl).
• Tip/sample bonding and localized condensation are identified as key factors governing chemically induced friction.
• The findings provide critical insights into controlling friction at the nanoscale through surface chemistry modification.