Self‐Assembly of Cellulose Nanocrystals into Semi‐Spherical Photonic Cholesteric Films

Spherical‐shape cholesteric (Ch) photonic films offer promising applications such as omnidirectional lasing and broadband reflection. Here, a strategy is reported for the fabrication of photonic Ch films by the self‐assembly of cellulose nanocrystals (CNCs) in semi‐spherical cavities. It is shown that film structure and photonic properties are controlled by the film curvature, the composition of the precursor CNC suspension, and its liquid crystalline or isotropic state. The effect of curvature of the drying Ch‐CNC films coupled with the restricted mobility of CNCs during film preparation results in the variation in pitch and helical axis inclination, thus leading to broadband reflectance properties of the films. The role of curvature of the Ch‐CNC films is underlined by comparing their distinct properties with those of planar films with the same composition. These findings provide a novel insight into the design and development of biomimetic photonic Ch films. Semi‐spherical cholesteric cellulose nanocrystal (CNC) films with tunable optical properties are fabricated by exploring the interplay between the curvature, the composition and state of the precursor CNC suspension, and the film structure and optical properties. The combination of photonic properties and spherical shape results in a broadband reflection covering the entire spectral range.