Ionically Modified Cellulose Nanocrystals Chiral Structures for Electronics

Many approaches exist in the quest for efficient, abundant, cheap and eco-friendly photonics that can be easily integrated with other platforms for practical applications.<br/>Approaches using 3D metamaterials, mesoporous semiconductors, or plasmonics are promising. Nevertheless, these methods often involve complicated fabrication processes that need high energy consumption (such as vacuum and physical vapour deposition) to deposit these materials, which are often not so green, sustainable, typically expensive, and colour-limited.<br/>Researchers have sought eco-friendly materials with similar performance and lower energy costs during the past few years, and Nature offers many examples of such materials for our innovation. Natural micro and nanostructures, such as the ones found in beetles or butterflies, have evolved over millions of years to produce periodic patterns that can reflect light. These structures served as inspiration to create artificial analogues using cellulose nanocrystals (CNCs), which can form a chiral nematic liquid crystalline phase when dispersed in water. The chiral order is preserved as the water evaporates, and solid films with 1D photonic crystal properties and iridescent colours are obtained. The intrinsic left-handedness of such structures can interact selectively with left and right-circular polarized light (LCPL and RCPL).<br/>The pitch control and, therefore, the reflected colour is particularly interesting. This manipulation can be done using external stimuli such as magnetic and electric fields, drying processes with different interfaces or playing with the chemistry behind the acid hydrolysis. With this work, we demonstrate that CNC suspensions with low concentrations (~4-5wt.%) with proper evaporation-induced self-assembly conditions lead to the formation of films with interesting photonic properties with a left-handed twisted arrangement. The use of strong acid cation resins to mediate the counterion exchange on commercial CNCs. It was observed that adding different alkali salts to the CNCs anisotropic suspension results in a blue shift of the photonic bandgap with higher salt concentrations and proper distinction between LCPL and RCPL (ΔCPL ≈ 42%).<br/>Combining the photonic character of the tailored CNC films, whose photonic bandgap can be matched with the bandgap of light sensitivity of semiconductors, it will be possible to create devices capable of discrimination between LCPL and RCPL signals in the blue region.