Electrically and electrohydrodynamically driven phase transition and structural color switching of oligomer tethered 2D colloid

Two-dimensional (2D) nanoparticles in an oligomer-tethered alpha zirconium phosphate (alpha ZrP) colloid self-assemble to form a cofacial lamellar structure with regular spacing parallel to the surface and exhibit high reflectance and vivid structural colors within the visible frequency spectrum. Here, we demonstrate electrical switching of the structural color reflection by electrical control of the liquid crystalline phase of the alpha ZrP colloid. At low frequency (less than 15 Hz, optimally at 1 Hz), electrohydrodynamic flow in the colloid destroys the photonic crystalline lamellar phase and creates an apparently disordered dynamic state with local nematic orientation. The method using electrohydrodynamic flow is a better approach to erase the photonic crystalline ordering of nanoparticles, than application of a high-frequency field, which has been proposed previously, in terms of the required voltage and color uniformity. The field-induced disordered particle orientation can be spontaneously recovered to the initial photonic crystal state by removing the applied voltage, but this method requires quite a long time and does not work in materials with a high nanoplatelet concentration. On the other hand, by applying a horizontal high-frequency field (approximately 10 kHz), the initial lamellar ordering can be forcibly recovered. In this way, the structural color in the 2D nanoparticle colloid can be repeatedly erased or rewritten by switching the frequency of the applied voltage from 10 kHz to 1 Hz and vice versa, respectively. Our method of switching a 2D colloid using both electrohydrodynamic flow and frequency modulation is expected to be a promising approach to control the photonic crystallinity of colloidal photonic crystals.