Carbon quantum dots (CQDs) have been extensively explored in diverse fields because of their exceptional features. The nanometric particles with photoluminescence (PL) benefit various optical and photonic applications. However, the majority of previous reports have mainly focused on either unpolarized or circular-polarized (CP) PL. Linearly polarized (LP) emission of CQDs is limited mainly because of their isometric shape and difficulties in macroscopic orientation control. Herein, we report syntheses of anisometric CQDs and facile control of the uniaxial orientation on a macroscopic scale, which results in linearly polarized photoluminescence (LP-PL). The anisometric CQDs are synthesized from rigid-rod-shaped precursors and evenly dispersed in the rod-like liquid crystal (LC) host. As-synthesized CQDs exhibit a PL quantum yield as high as 35% in chloroform. In addition to uniform alignment, facile directional switching of the elongated CQD is established by employing the electrical responsiveness of the CQD and host LC. Therefore, the dichroic photophysical properties of anisometric CQDs have been beneficially adopted for fabrications of polarization-sensitive and electrically switchable PL devices. Also, anisometric CQDs are embedded in polymer films with molecular orientational patterns and clearly recognized by LP-PL.

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Orientation control of two-dimensional (2D) colloidal liquid crystals in both microscopic and macroscopic scales holds massive promise for improved multifield applications. In this respect, graphene oxide (GO) is used as a platform substance for 2D carbon-based lyotropic liquid crystal (LC) materials. Here, we design the directed alignment of aqueous GO-LC dispersion confined in a rectangular capillary tube. The isotropic GO dispersion is loaded into a horizontal capillary and subsequently concentrated to the LC phase by slow water evaporation at open ends of the tube. An obtained surprising result, GO layers organize normal to the inner surfaces of the capillary with a highly ordered orientation of the optic axis over tens of millimeters. Such transverse orientation of GO layers originates from the meniscus region and then moves toward the center, caused by the interfacial effect and the sequential isotropic-to-nematic transition. This aligned mesomorphic system is characterized via optical retardation, nematic order parameter, absorption, and modified Cauchy’s transmission equation about the dependency of both birefringence and wavelength with concentration. Finally, the aligned GO-LC layers are preserved in a polymer composite matrix by photopolymerizing the evaporation-aligned GO/acrylamide-LC dispersion confined in a capillary. This composite shows higher thermal stability than polyacrylamide until 500 °C of about 6.1%. Our experimental findings provide efficient advantages for controlling the orientation of GO-LC and 2D materials, beneficial for diverse optical applications due to their directional ordering.

Photonic crystals (PC) are of great importance in technology, especially in optics and photonics. In general, the structural color of PCs responds to external stimuli primarily by changing their periodicity. Herein, the authors report on refractive index (RI) adaptive PCs. Cross‐linked cholesteric films with interconnected nanopores exhibit a very low RI without light scattering. Transparent PC films with maximum reflectance in the ultravoilet (UV) region respond to various chemicals by changing the reflective color of the PC. The authors demonstrate its unique colorimetric chemical detections of hazardous organic liquids. Loading various chemicals into nanopores significantly shifts the structural color into the visible range depending on the chemical's RI. These results are unique in that the structural color of photonic films is mediated by RI changes rather than periodicity changes. In principle, nanoporous photonic crystal films can detect the RI of a chemical substance by its unique color. In contrast to volumetric changes, this sensing mechanism offers several advantages, including durability, excellent sensitivity, fast response time, and wide detection range. These results provide useful insight into stimulus‐responsive PCs. The structural color of PC films can be effectively tuned by adjusting average RIs instead of changing periodicity.