Tuning frictional properties of molecularly thin erucamide films through controlled self-assembling

Self-assembled films (SAFs) have been proposed to be a promising candidate for molecularly thin lubricants. However, the frictional performance of SAFs is sensitively dependent on their molecular structures that are susceptible to external environments. Taking erucamide, a fatty amide widely used as a macroscale slip additive, as an example, we demonstrate that SAFs can be readily formed on various substrates, including silicon oxide, sapphire, copper, and graphite. Through high-resolution topography and friction measurements, two types of erucamide SAFs are identified. The first type is atomically flat and exhibits clear atomic stick-slip friction behavior and ultra-low frictional dissipation; while the second type has a stripe-like nanoscale pattern and shows much higher (8 times higher) frictional dissipation. The sharp contrast between these two types of SAFs is attributed to their distinct molecular structures originating from different interactions between erucamide molecules and substrates. We further demonstrate that, by proper surface functionalization, we can actively and reliably control the molecular structures of SAFs through guided self-assembling, achieving rational friction tuning with patterning capability.