Visible Photonic Bandgap Configured by Non-Affine Block Copolymer Gyroid

Abstract

Despite extensive efforts to self-assemble block copolymers (BCPs) into gyroid photonic crystals, achieving a photonic bandgap (PBG) in the visible regime still remains unreachable due to the difficulty in accessing magnificent lattice sizes. Here, giant BCP gyroids with the largest lateral unit-cell size reported to date (335.7 nm) are successfully assembled and their non-affine lattice structures along with the corresponding photonic band structures are theoretically unveiled. The key to realizing this visible PBG is the precise control of non-affine distortion within the largest gyroid lattices, which effectively transforms their morphology toward a high symmetry state. Particularly, high-molecular-weight polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) films are utilized as templates to construct the giant gyroid. The PMMA-removed, PS gyroid films are treated with cosolvent mixtures of tetrahydrofuran and acetic acid to induce a directional contraction along the z-direction, thereby leading to precise fine-tuning of non-affine distortion. Numerical reconstructions of the resulting gyroid lattices revealed that increasing symmetry through non-affine transformation is critical for opening and widening the PBG in the visible regime. By integrating theoretical modeling with experimental validation of a distinct visible PBG, this study fully uncovers the atlas of giant BCP gyroid structures and their PBG characteristics, which had previously remained elusive.