Optical Properties of Slot‐Die Coated Hybrid Colloid/Cellulose‐Nanofibril Thin Films

• Published in: Advanced Optical Materials Vol. 11; no. 13
• Main Authors: Harder, Constantin, Alexakis, Alexandros E., Bulut, Yusuf, Xiong, Shuxian, Sochor, Benedikt, Pan, Guangjiu, Zhong, Huaying, Goordeyeva, Korneliya, Reus, Manuel A., Körstgens, Volker, Jeromin, Arno, Keller, Thomas F., Söderberg, L. Daniel, Malmström, Eva, Müller‐Buschbaum, Peter, Roth, Stephan V.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01-07-2023
• Subjects: Annealing; Biomedical materials; Cellulose; cellulose nanofibrils; colloidal films; Colloids; Glass transition temperature; grazing incidence X‐ray and neutron scattering; Materials science; Nanostructure; Optical properties; Optics; refractive index; Refractivity; Spectroellipsometry; Thin films
• ISSN: 2195-1071; 2162-7568; 2195-1071
• DOI: 10.1002/adom.202203058

Correlating nanostructure and optical properties of thin hybrid films is the crucial ingredient for designing sustainable applications ranging from structural colors in anticounterfeiting to sensors. Here, the tailoring of the refractive index of hybrid cellulose nanofibril/water‐dispersed colloidal ink thin films is presented. The authors apply scalable, layer‐by‐layer slot‐die coating for preparing the cellulose nanofibril and hybrid thin films. Making use of the mobility of the polymer chains in the colloids upon annealing, the influence of the different colloid sizes and their glass transition temperature on the refractive index of the hybrid material is shown. The complex refractive indices of the thin films are characterized by spectroscopic ellipsometry and correlated to the different nanostructures of the thin films. The authors find that post‐deposition annealing changes the colloidal nanostructure from particulate to agglomerates. Depending on the size of the colloids, imbibition of the colloids into the cellulose nanofibril template is observed. This scalable approach offers new avenues in structural color functional biomaterial hybrid layers.
The refractive index of colloid/cellulose nanofibril thin films is tailored by the size of colloids and nanoscale porosity of the cellulose nanofibril thin film. The imbibition properties are governed by the colloids’ size with respect to the nanopores’ size. Scalability and imbibition are achieved by the sequential slot‐die coating of cellulose nanofibrils and colloids.