# Free-Standing Iridescent Films of Cellulose Nanocrystal Doped with Eu3+ and Tb3+ Ions for Photonic Applications

## TL;DR

This paper describes the creation of iridescent films using cellulose nanocrystals doped with rare earth ions for potential use in optical devices.

## Contribution

The novel contribution is the integration of iridescence and light emission in cellulose nanocrystal films doped with Eu3+ and Tb3+ ions.

## Key findings

- SEM and reflectance analysis confirmed the chiral nematic structure of the CNC films.
- The cholesteric pitch can be controlled, influencing the emission color through the Bragg band shift.
- The films show potential for applications in polarized luminescence and tunable optical devices.

## Abstract

The simultaneous integration of iridescence and light
emission
into a photonic material is an attractive proposal for designing novel
optical devices. These properties could be controlled by the action
of chiral nematic liquid crystals, and the self-assembly of cellulose
nanocrystals is a smart methodology for the development of this new
material. Herein, bacterial cellulose (BC) has been used as a cellulose
source, mainly due to its biocompatibility, nontoxicity, and high
purity. In this context, cellulose nanocrystals (CNCs), obtained by
acid hydrolysis methodology, have received significant interest in
the production of new optical materials due to their controllable
chiral nematic self-organization. The work aims to obtain new iridescent
films based on CNCs with adjustable cholesteric pitch, in the presence
of lanthanide ions (Ln3+), specifically Eu3+ and Tb3+ ions, as a new platform for optical systems.
The results from scanning electron microscopy (SEM) confirmed the
chiral nematic structure of the CNC film, and it was corroborated
by the Bragg diffraction band observed at specular reflectance analysis.
In addition, the following methodology allows control of the cholesteric
pitch. The luminescence results from the spectroscopic probe, Eu3+ ion, show the lanthanide in a low symmetry environment.
The possibility of the shift of the Bragg band may influence Tb3+ ion emission, and as a result, the emission color changes.
In conclusion, the ability to control the cholesteric pitch of the
doped iridescent CNC films is associated with the angle-dependent
excitation wavelength and consequently the emission of the material;
these new films show an innovative path with potential applications
in polarized luminescence and CNC-based optical devices, sensors,
tunable filters, and lasers.