Cellulose nanocrystals (CNC) extracted from natural cellulose by sulfuric acid hydrolysis spontaneously form a left-handed chiral nematic structure above a critical concentration that can be retained in solid state. Chiral nematic CNC materials are expected to play important roles in sensing, security pigments and light management. The left-handed chiral nematic CNC films enable selective reflection of left-handed circularly polarized (LCP) light, selective transmission of right-handed circularly polarized (RCP) light and RCP luminescence due to the underlying photonic bandgaps (PBG). The PBG-based circular polarization ability of the left-handed chiral nematic CNC films has been extended to enable optically ambidextrous reflection and luminescence by post processing or synergistic self-assembly and kinetic stabilization. It has been demonstrated that strong magnetic fields (> 5 T) facilitate the formation of chiral nematic domains with helical pitch and orientation uniformity owing to the negative diamagnetic susceptibility anisotropy of CNC and the chiral nematic order-induced corporative effect. To date, approaches to control over the self-assembly of CNC to enable the formation of left-handed chiral nematic films with planar-homeotropic textures that display chiroptical properties on both plane and lateral surfaces, to the best of our knowledge, remain unexplored.

Herein we present the first experimental evidence that magnetic field-directed self-assembly of CNC with Fe₃O₄ nanoparticles produces chiral nematic CNC films displaying tunable LCP light reflection patterns on both plane and lateral surfaces. These unique chiroptical properties arise from a left-handed chiral nematic architecture featuring planar-homeotropic textures with different PBGs. The homeotropic textures with concentric rings of helical axis are intercalated in the planar textures and formed surrounding linear Fe₃O₄ ensembles aligned in the magnetic field direction. Such linear Fe₃O₄ ensembles produce a radial force field with collective magnetism that is responsible for the reorientation of helical axes and the formation of homeotropic concentric fingerprint textures. The homeotropic PBG is tunable from the visible to near-infrared regime. We demonstrate, as a proof-of-principle, that the left-handed chiral nematic CNC films with tunable chiroptical properties on both plane and lateral surfaces can be realized through exquisite design of local magnetic fields.