Abstract
Evaporation induced self-assembled (EISA) thin films of cellulose nanocrystals (CNCs) have shown great potential for displaying structural colour across the visible spectrum. They are believed primarily to reflect left handed circularly polarised (LCP) light due to their natural tendency to form structures comprising left handed chirality. Accordingly the fabrication of homogenously coloured CNC thin films is challenging. Deposition of solid material towards the edge of a dried droplet, via the coffee-stain effect, is one such difficulty in achieving homogenous colour across CNC films. These effects are most easily observed in films prepared from droplets where observable reflection of visible light is localised around the edge of the dry film. We report here, the observation of both left and right hand circularly polarised (LCP/RCP) light in reflection from distinct separate regions of CNC EISA thin films and we elucidate how these reflections are dependent on the distribution of CNC material within the EISA thin film. Optical models of reflection are presented which are based on structures revealed using high resolution transmission electron microscopy (TEM) images of film cross sections. We have also employed spectroscopic characterisation techniques to evaluate the distribution of solid CNC material within a selection of CNC EISA thin films and we have correlated this distribution with polarised light spectra collected from each film. We conclude that film regions from which RCP light was reflected were associated with lower CNC concentrations and thicker film regions.
Highlights
Reflection of circularly polarized light and the effect of particle distribution on circular dichroism in evaporation induced selfassembled cellulose nanocrystal thin films
We report optical variations observed in Cellulose nanocrystals (CNCs) thin films that include the reflection of both left and right hand circularly polarised (LCP/RCP) light from distinct separate regions of CNC EISA thin films
Our TEM analysis of CNC film micro-structures intended to elucidate how this reflection is dependent on the distribution of CNC material within the films, relating the structures observed to inherent optical properties
Summary
Reflection of circularly polarized light and the effect of particle distribution on circular dichroism in evaporation induced selfassembled cellulose nanocrystal thin films. Peiss et al showed that the relationship between the rate of evaporation and the radius of drying drops is non-linear.[21] This property probably contributes to the inhomogeneous distribution of solid material within EISA films and may help to explain variations in optical behaviour.[22] We report optical variations observed in CNC thin films that include the reflection of both left and right hand circularly polarised (LCP/RCP) light from distinct separate regions of CNC EISA thin films.
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