Capillary confinement produces organized photonic films

Photonic thin films—in which ordered nanostructures interact with light to produce color—can be used to make lightweight and flexible color filters, sensors, display components, and more. But ensuring that the nanocrystals within them align uniformly to produce well-defined color can be difficult and time consuming. Casting such a film in a dish, for example, can take weeks and still result in poorly controlled nanostructures and colors. Now, Georgia Institute of Technology’s Vladimir V. Tsukruk and colleagues at the Air Force Research Lab and Kent State University have developed a capillary-based method that makes highly ordered photonic thin films in a matter of hours (Nano Lett. 2018, DOI: 10.1021/acs.nanolett.8b02522). The team puts an isotropic suspension of cellulose nanocrystals in a 50-µm-thick rectangular capillary that is open on one end to allow water to evaporate. Within the confined space of the capillary, the solution dries asymmetrically and the crystals organize in a uniform pattern as the film solidifies. The film can be peeled away after breaking the capillary. The highly ordered nanostructures, which take on left-handed chirality, refract light in a predictable, uniform way. Tsukruk says his team has scaled up the technique to make films of up to 25 cm². Mark MacLachlan at the University of British Columbia, who works with cellulose nanocrystals, says he thinks the work is an elegant contribution to the field, and it could be important for developing new microdevices, including ones that use light for biological or chemical detection.