Mechanical instabilities in the blistering and buckling of polymer-supported two-dimensional materials: Graphene and beyond

Abstract

Graphene is the zero band gap semimetal though it has remarkable electronic, optical, and mechanical properties. Researchers started to look for ways to open its band gap. In 2008, researchers showed the opening of the band gap of graphene by ap plying tensile strain. Thereafter, such strain engineering techniques have constantly been searched for, which have significant implications in tunable electronics. The strain engineering in 2D materials offers a versatile toolbox for tailoring their physical properties to suit specific applications. It enables researchers and engineers to explore new avenues for device design, fundamental research, and technological innovation. Blistering and wrinkling of 2D materials are the most efficient techniques to tailor their physical properties. Moreover, the formation of such nano/microstructures of 2D elastic sheets has huge implications for fundamental research. Blistering occurs when a 2D material detaches from its substrate due to the accumulation of strain resulting from the confinement of molecules or lattice-mismatch. Blistering can lead to localized areas of high strain, which can significantly alter the electronic and op tical properties of the material within the blister region. This provides a means to create strain gradients and non-uniform strain patterns, which are desirable for engi neering specific electronic and optoelectronic properties. The accumulated strain gets relaxed by the formation of blisters, potentially leading to a local change in its lattice structure and band gap. This can be exploited to create locally engineered properties within the material. On the other hand, wrinkling occurs when a 2D material forms periodic or irregular folds or wrinkles due to the relaxation of applied strain. These wrinkles can have various sizes and orientations, and they can be controlled to achieve specific strain distributions. The wrinkling can introduce controlled strain patterns in a 2D material, enabling precise manipulation of its electronic properties. The re sulting strain-induced modifications can be used to tailor band gaps, carrier mobility, and other characteristics for specific applications. It increases the surface area of the 2D material, which can be advantageous for applications in catalysis, sensing, and energy storage. The increased surface area provides more active sites for chemical reactions and interactions. Moreover, the mechanical properties of wrinkled 2D ma terials can be different from their flat counterparts, offering enhanced flexibility and stretchability. This is important for applications in flexible electronics and wearable devices. In summary, both blistering and wrinkling are manifestations of how 2D materials respond to applied strain, and they provide unique opportunities for strain engineering. Researchers can exploit these effects to create tailored strain distribu tions and modify the electronic, optical, and mechanical properties of 2D materials. By carefully designing and controlling blistering and wrinkling, scientists can develop novel devices and materials with customized functionalities for various technological applications, ranging from electronics and photonics to sensors and energy conversion. In the presented thesis work, we mainly focus on the blistering and wrinkling of 2D materials: graphene and beyond. The 2D material blisters are formed by the confinement of molecules. The blisters form either spontaneously or intentionally. We utilized a polymer curing-assisted technique to form optically visible sub-micron blisters of 2D materials spontaneously. A suitable blister-test model is used by taking into account the shape profile, thickness, and aspect ratio of the blister to deduce the mechanical properties of 2D material. We also strategically investigated the instabil ities that arose while blister formation. The instabilities could be either based on the 2D material or the substrate. The choice of a viscoelastic substrate in this blistering process led to the emergence of an unconventional phenomenon i.e., viscous finger ing. Besides this, we offer intriguing insights into the formation of wrinkles, buckles, and folds in 2D elastic nanosheets over a soft substrate under a prestretch-release process. The acquired information would advance our understanding of the mechan ics of 2D materials adhering to various solid or soft substrates, which is potentially advantageous in nanopatterning and strain engineering of 2D materials.