A team of researchers from Korea's Pusan National University, led by Assistant Professor Songkil Kim, have examined the relationship between surface structures on chemical vapor deposition (CVD) grown graphene and its properties, specifically friction.
They correlated surface structures with nanoscale friction of a multi-layered graphene island. By cleaning the graphene surface using mechanical scratching of polymeric surface contamination, the team unveiled the surface structures such as small-scale and large-scale (folded) wrinkles on graphene using atomic force microscopy (AFM) and investigated their effect on nanoscale friction.
"Correlating surface characteristics with a material's properties is really important," explains Dr. Kim, "Imagine you are stacking papers, and there is a huge compressive strain over these papers. This could cause massive structural deformations within the stacked layers and the surface. Similarly, the structural changes that occur in multi-layered graphene can affect its surface properties, such as its friction, which is what we focused on".
The team first used the atom-sized tip of an atomic force microscope (AFM) to scratch the surface of CVD multi-layer graphene, cleaning off any polymeric residues. Then, they used AFM imaging, friction force microscopy, and Raman spectroscopy to identify and study various surface structures and how they affect nanoscale friction. Interestingly, they found that only the top-most layer of graphene was twisted with respect to the rest, which influenced the layer-dependent nanoscale friction in a way that varied according to the applied load.
Overall, this study's findings could open the door to interesting mechanical applications for CVD graphene. "Graphene and similar materials can be used as solid lubricants," comments Dr. Kim, "Whereas liquid lubricants like motor oils are not suitable for harsh environment such as outer space or the polar regions, the excellent robustness and frictional properties of graphene make it an attractive non-toxic alternative."
Developing high-performance lubricants can have environmental benefits since reducing friction is essential to preventing energy losses in mechanical systems. Another potential application for multi-layered CVD graphene is in micro/nano-devices, where a precise control of friction is necessary.