Directional Alignment of Quasi-Single-Crystalline 2D Dot Array in Diblock Copolymer Thin Films via Epitaxial Cylinder-to-Sphere Transition

Despite the great applicability of block copolymer (BCP) thin films capable of producing dense periodic nanostructures, the difficulty to control the pattern orientation acts as an obstacle to the successful bottom-up lithography of BCPs. The production of guiding pattern on the substrate such as grapho- or chemo- epitaxy has been suggested to create highly ordered nanostructures. However, these methods can be experimentally complicated and expensive, particularly in a large-area.
In this work, we suggest a facile method to fabricate highly aligned quasi-single-crystalline 2D dot array of BCP thin films over large-area (cm²). We utilized the epitaxial morphology transition from BCP cylinders to spheres. First, sphere forming polystyrene-b-poly(2-vinyl pyridine) thin films were transformed to cylinders by shear; shear results in the nicely aligned parallel cylinders along the shear direction. Then, the subsequent solvent vapor annealing (SVA) induces a cylinder-to-sphere transition where the dot pattern of BCP spheres is epitaxially grown on the aligned stripe pattern of the cylinders. We found that the increase of d-spacing with SVA is a key factor for the successful generation of highly ordered 2D array, which could compensate the lattice mismatch between dot and stripe pattern.