Rui Huang, Aerospace Engineering & Engineering Mechanics, UT-Austin

"Mechanical Interactions between 2D Materials: Adhesion, Friction and Moiré"

Abstract: Atomically thin 2D materials such as graphene are promising for a wide range of applications. Mechanical interactions at the interfaces of 2D materials, including adhesion and friction, are critical for manufacturing (e.g., synthesis and transfer), integration, functional performance and reliability of 2D materials. While van der Waals interactions have been commonly assumed to be the primary mechanism, challenges remain for experimental characterization of the adhesion and friction properties of 2D materials, partly due to the intimate coupling of the normal and shear interactions as well as elastic deformation of the 2D materials. Three related phenomena will be discussed to highlight the coupled mechanical interactions between 2D materials. First, formation of micro/nanoscale bubbles has provided a robust and versatile platform for measuring mechanical properties of the ultrathin 2D materials, including adhesion energy, shear strength, and bending modulus, based on a simple mechanics model. Controlled formation of such bubbles could also be useful for strain engineering applications. Second, peeling and sliding of 2D nanoribbons exhibit remarkably rich dynamics with coupled adhesion and stick-slip behaviors. Based on a continuum model and numerical simulations with a periodic potential energy function for the van der Waals interactions, we show that stick-slip sliding of a graphene nanoribbon (GNR) is facilitated by formation and gliding of strain solitons, and different types of strain solitons may form in the zigzag and armchair GNRs, including tensile, shear and mixed types. Consequently, the peak pulling force as a measure of the sliding friction becomes nearly independent of the ribbon length for relatively long GNRs. For the third phenomenon, formation of 2D moiré supperlattices will be discussed from a mechanics perspective. Based on the same continuum model for peeling and sliding, we have simulated twisted graphene bilayers, forming a set of metastable moiré patterns at particular twisting angles. Similar moiré patterns are obtained by stretching one graphene monolayer relative to the other, with tunable symmetry by two independently controlled strain components for biaxial stretch. A combination of twisting and stretching could be a promising route for strain engineering of the 2D moiré crystals.