Wrinkle formation in synthesized graphene and 2D materials

Chemical vapor deposition (CVD) is widely used to produce high-quality, large-area graphene and two-dimensional (2D) materials at low cost, making them suitable for a wide range of applications in mechanical, electronic and photonic devices due to their unique physical properties. However, wrinkles always form when the as-grown materials on the substrate cool from the growth temperature to room temperature because of the difference in the coefficient of thermal expansion (CTE) between the materials and the substrates, significantly degrading device performance. In this review, we first discuss several typical surface corrugations of graphene and 2D materials that are commonly observed on a substrate, including minor, standing, contacted and lying wrinkles, as well as wrinkle networks. We then introduce a criterion to ascertain the wrinkle-free state by equating the total compressive strain obtained from CTE mismatch to the sum of the strain released by the wrinkles and the residual strain, which can be deduced from the morphology of wrinkles and Raman spectroscopy, respectively. Based on this criterion, we review several notable endeavors in the synthesis of wrinkle-free graphene and finally answer the question of where the lost compressive strain goes. We aim that this review provides readers with a thorough understanding of the mechanism underlying wrinkle formation and stimulates further exploration into the synthesis of wrinkle-free 2D materials.