Intrinsic Wrinkling of Free-Standing Polycrystalline Atomically Thin Films

Atomically thin films, like transition metal dichalcogenides, can now be synthesized at wafer scale while maintaining monolayer thickness. At such extreme aspect ratios, atomistic simulations and existing experimental techniques are unable to directly predict or measure the mechanical effects of intrinsic features like polycrystallinity without the interference of pinned boundaries or solid substrates. To address this challenge, here we introduce a versatile approach: we realize large scale free-floating membranes on water and measure their mechanical properties using atomic force microscopy (AFM) and Raman spectroscopy adapted to water's surface. We reveal that free-standing polycrystalline membranes spontaneously form large athermal wrinkles. These wrinkles differ from those of extrinsic origin in that their size and shape depend on an intrinsic mesoscopic feature of the 2D material: the polycrystalline grain size. We rationalize the relationship between grain size and wrinkle shape using continuum theory and minimal mathematical models. Finally, we demonstrate experimentally that the wrinkles alter the mechanical properties of the sheet, introducing dramatic softening and spatial heterogeneity in response to a point probe. The present work illuminates the mechanics of polycrystalline nanomaterials at extreme aspect ratios and suggests principles for engineering strain-controlled nanomechanical responses.