Synthesis of Chiral Metasurface by Chemical Vapor Polymerization into the Self-Assembled Square Array of Smectic Liquid-Crystalline Defects

Metasurfaces with emergent chiral properties are drawing increasing interest because of their inherent ability to interact and change the state of the incident light. Surfaces decorated with periodic nanostructures having twists or handedness demonstrate the ability to act as a metasurface. Although the twist in the individual nanostructure usually originates from the intrinsic chirality of the molecules, recent advances have shown the fabrication of structurally twisted nanostructures consisting of achiral molecules using techniques including photolithography, electron or ion beam lithography, glancing angle deposition, self-assembly, etc. Self-assembly of nanostructures is one of the fast and potentially low-cost approaches to achieve externally controlled structurally chiral metasurfaces.
Smectic liquid crystal forms a square array of symmetric Toric focal conic defects on bi-directionally rubbed polymer substrates following a self-assembly pathway. We have removed the conventional mirror symmetry of these defect structures and introduced extrinsic chirality by fabricating a square array of fragmented focal conic defects. Higher rubbing strength and the thickness limitation of the LC film can be utilized to confine each quadrant of the full Toric focal conic defect within the rubbing-induced nano grooves. We have achieved large area (1.5x1.5 cm²) ultra-thin (60nm-1.2µm) stable liquid crystalline coatings using spin coating. These defects could be used for targeted synthesis of nanostructures since the defect or singularity points are energetically more favorable to accommodate foreign molecules. Chemical vapor polymerization (CVP) using paracyclophane precursors into these defect structures and subsequent removal of the LC templating phase using critical point drying can fabricate nanostructures made of individual nanofibers that follow the LC director profile as confirmed by scanning electron microscope (SEM) cross-section imaging. This fast, cost-effective approach can demonstrate extrinsic chirality because of the bending in individual nanofibers and the anisotropic shape of the nanostructures. Altering the orientation of fragmented FCDs through varied rubbing directions could change the handedness of the nanostructures. The optical characteristics and the light-matter interactions have been analyzed by Mueller matrix polarimetry (MMP) and circular dichroism spectroscopy (CD) to map spatially the optical parameters of the defect array and explore the fundamental origin of the chirality. Finally, the physical properties of the nanostructures including shape factor, lattice spacings, symmetry, and orientation have been correlated to the optical properties. In conclusion, this work will provide deeper insight into self-assembly-associated design principles for fabricating inducible, structure-based chiral metasurfaces.