Graphene is a unique material in many aspects and has recently been applied in various ways. In tribology, it has been shown to lead to a superlubricity – an ultra low friction state. In laboratory air, graphene was reported to show twofold symmetry in friction and anisotropy exceeding 200%. This behavior was associated with structural features and it was believed, that the periodic ripples, induced by the stress from the substrate, enhance the generation of puckering around the sliding tip and are responsible for the anisotropy.
Recently, an international team of researchers from China, USA and Japan have studied the friction of graphene using high-resolution Atomic Force Microscopy. Thanks to high resolution, they could directly image a supperlattice of nanoscale stripes on the surface of graphene. This nanotexture with a period of 4nm and amplitude of 10-100pm was reported to be responsible for the friction anisotropy, since the the friction domains on graphene clearly followed the symmetry of the superlattice.
The periodic stripes were also observed on multilayer graphene and hexogonal Boron Nitride(h-BN). The behavior of stripes was reported to be extremely similar for graphene and h-BN – materials with different material properties. Based on this observation, it was speculated that the stripes are self-assembled environmental adsorbates rather than features of the crystals. Further, the use of Scanning Tunneling Microscopy did not show the presence of the stripes, indicating a possibility of the adsorbates to be highly affected by the pressure from the tip. As it was discussed in the original article, various adsorbates can self-assemble into nanoscale stripes.
The key result of of the stripes formation is the possibility to manipulate these features by a physical contact, as they are sensitive to pressure. By scanning the substrate at high normal force in a certain region, the frictional response can be manipulated in repeatable and predictable way. This may be helpful in nanolitography and nanografting applications.
Further details can be found in the original article: “Switchable friction enabled by nanoscale self-assembly on graphene”, 10.1038/ncomms10745.
Credit for image (edited from): “Switchable friction enabled by nanoscale self-assembly on graphene”, 10.1038/ncomms10745.