Preparation of Pseudo-Single Grain Double Gyroid Thin Films for Advanced Optical Applications

Materials with three-dimensional network structures (<i>Nets</i>) on sub-micron length scales are of great interest for their optical properties. The vivid colors of insect wing scales and bird feathers arise from naturally occurring <i>Nets</i> with spatial periodicities ranging 400 – 700 nm, which interact with visible light as photonic crystals. On the other hand, <i>Nets</i> with sub-wavelength scale periodicities (&lt; 400 nm) have potential applications as optical metamaterials with exciting properties such as negative refraction and strong circular dichroisms. While <i>Nets</i> may be fabricated by top-down methods such as two-photon lithography, such methods are limited by their achievable lengthscales and the challenges associated with fabrication throughput. Bottom-up self-assembly of block copolymers provides a possible alternative for addressing these limitations. For example, Vignolini <i>et al.</i> prepared a gold single gyroid nanomaterial using a template based on ABC triblock terpolymer self-assembly and demonstrated its strong, orientation-dependent dichroic responses. Although these results suggest the promise of bottom-up self-assembly as means of accessing <i>Nets</i>, a significant challenge facing these approaches is the lack of control over <i>Net</i> grain sizes and their orientations relative to a substrate that are important in their optical applications.<br/>The self-assembly of block copolymers can be directed using chemical patterns or external fields, but few studies have applied these methods to block copolymers that form double gyroid (DG) <i>Nets</i>. Of these few examples, many rely on solvent annealing of block copolymer thin films sometimes in the presence of chemically or topographically patterned surfaces. In this study, we report a solvent-free, thermal processing method for producing pseudo-single grain DG thin films on silicon wafers bearing native oxide layers. Through top-down and cross-section scanning electron microscopy (SEM), we identify the well-defined orientation of the DG mesostructure with respect to the underlying substrate. These findings are corroborated using grazing-incidence small-angle X-ray scattering (GISAXS), and we use azimuthal angle-dependent GISAXS to investigate the degree of microstructural orientation. Thus, this thermal processing approach enables efficient unidirectional alignment of mesoscopic DG structures over large areas for future optical materials applications.