Reflection of circularly polarized light and the effect of particle distribution on circular dichroism in evaporation induced self-assembled cellulose nanocrystal thin films

This is the final version of the article. Available from the publisher via the DOI in this record.Evaporation induced self-assembled (EISA) thin films of cellulose nanocrystals\ud(CNCs) have shown great potential for displaying structural colour across the visible\udspectrum. They are believed primarily to reflect left handed circularly polarised\ud(LCP) light due to their natural tendency to form structures comprising left handed\udchirality. Accordingly the fabrication of homogenously coloured CNC thin films is\udchallenging. Deposition of solid material towards the edge of a dried droplet, via\udthe coffee-stain effect, is one such difficulty in achieving homogenous colour across\udCNC films. These effects are most easily observed in films prepared from droplets\udwhere observable reflection of visible light is localised around the edge of the dry\udfilm. We report here, the observation of both left and right hand circularly polarised\ud(LCP/RCP) light in reflection from distinct separate regions of CNC EISA thin\udfilms and we elucidate how these reflections are dependent on the distribution of\udCNC material within the EISA thin film. Optical models of reflection are presented\udwhich are based on structures revealed using high resolution transmission electron\udmicroscopy (TEM) images of film cross sections. We have also employed spectroscopic\udcharacterisation techniques to evaluate the distribution of solid CNC material\udwithin a selection of CNC EISA thin films and we have correlated this distribution\udwith polarised light spectra collected from each film. We conclude that film regions\udfrom which RCP light was reflected were associated with lower CNC concentrations\udand thicker film regions.The authors would like to thank the University of Exeter for providing funding for a doctoral studentship