On the Origin of Nonclassical Ripples in Draped Graphene Nanosheets: Implications for Straintronics

Ever since the discovery of graphene and subsequent explosion of interest in single-atom-thick materials, studying their mechanical properties has been an active area of research. Atomistic length scales often necessitate a rethinking of physical laws, making such studies crucial for understanding and ultimately utilizing material properties. Here, we report on the investigation of nanoscale periodic ripples in suspended, single-layer graphene sheets by scanning tunneling microscopy and atomistic scale simulations. Unlike the sinusoidal ripples found in classical fabrics, we find that graphene forms triangular ripples, where bending is limited to a narrow region on the order of a few unit cell dimensions at the apex of each ripple. This nonclassical bending profile results in graphene behaving like a bizarre fabric, which regardless of how it is draped, always buckles at the same angle. Investigating the origin of such nonclassical mechanical properties, we find that unlike a thin classical fabric, both in-plane and out-of-plane deformations occur in a graphene sheet. These two modes of deformation compete with each other, resulting in a strain-locked optimal buckling configuration when draped. Electronically, we see that this in-plane deformation generates pseudo electric fields creating a ∼3 nm wide pnp heterojunction purely by strain modulation.