Printed Twisted Thin Films with Near-Infrared Bandgaps and Tailored Chiroptical Properties

Abstract

In this work, twisted helical cellulose nanocrystals films with preprogrammed circular polarization and near-infrared reflectance are fabricated via a blade-based 3D printing method. The films are composed of stacked nanoscale slabs with high birefringence from unidirectionally organized cellulose nanocrystals. By changing the printing director, we achieved two types of films: twisted helical stacks and anisotropic Bragg stacks. These films are highly transparent and clear, and the achiral anisotropic Bragg stack shows near-infrared spectral region reflectance (1.3–1.4 μm). In contrast, the twisted helical films show concurrent left- and right-handed circularly polarized properties, as opposed to left-handed natural cellulose nanocrystals films. We observe dual chiroptical properties with circular dichroism peaks due to circular Bragg reflectance in the visible region and suggest that the circularly polarized properties are extended to the near-infrared region. These observations prompted us to explore the transition between anisotropic Bragg stacks and continuous helical films via simulations. We show that the printed twisted films can act as optical metamaterials with dual helicity and fill the gap between known photonic structures─the conventional continuous chiral nematic material with a chiroptical appearance and the achiral Bragg stack with a controlled photonic bandgap. These printed twisted stacked films hold the potential of larger-scale printed ordering of unique anisotropic nanostructures for circularly polarized-sensitive photonic, thermal, and energy management applications.