Graphene wrinkles weakest point
Graphene is hailed as a top material for technology due to its electrical, optical, and mechanical properties. Its high carrier mobility, optical transparency and tensile strength yield an unprecedented combination of properties favorable for applications ranging from high-speed electronics to construction. However, even with its high durability and flexibility, graphene can break when pulled at large forces, or due to friction when rubbed against. It is important to know the limits of wear resistivity when considering applications of this material. Now researchers from Graphenea and the Institute of Physics in Belgrade have shown that wear and breaking of graphene always start from wrinkles formed naturally during fabrication. The results were published in the journal Carbon.
Using atomic force microscopy (AFM), researchers scanned a very sharp needle probe across the surface of graphene. By gradually increasing the contact force that the probe exerts on graphene, effectively increasing friction until the surface ripped, the scientists observed that tears always start from the wrinkles. Wrinkles are out-of-plane deformations of graphene that probably occur during the transfer of graphene from its growth substrate to a practical substrate such as SiO2. The experiments were performed with CVD graphene.
Aside from wrinkles, CVD graphene contains naturally occurring terraces, which form because graphene conforms to the terraces of the copper substrate during CVD growth. It was shown that terraces have no effect on wear resistance.
Image: Graphene topography and local current flow. Current is reduced at the position of wrinkles (copyright Elsevier).
Electric measurements with nanometer-scale resolution, using the same AFM method, further showed that no current is conducted through wrinkles, and that islands enclosed by wrinkles have different electrical potential compared to the surrounding area, indicating that wrinkles impede electrical conductance in graphene. This is an important finding, because technological applications in electronics require efficient transport of carriers across graphene devices.
This careful study of electrical and mechanical effects of wrinkles in graphene is expected to lead to new experiments on improving graphene uniformity and flatness, helping to speed up the adoption of graphene technology.