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

Abstract

Two-dimensional (2D) nanoparticles in an oligomer-tethered alpha zirconium phosphate (αZrP) 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 αZrP 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.

Fig. 1. Characterization of synthesized αZrP nanocrystals and exfoliated nanoplatelets. (a) The XRD pattern of synthesized αZrP crystals. (b) The FESEM image shows hexagonal shaped crystals of αZrP having average diameter of 1300 ± 250 nm. (c) Exfoliated αZrP particles that are tethered with oligomer. (d) AFM image of (∼1 mm) diameter oligomer coated αZrP nanoplatelet.

Fig. 2. Schematic of cells. (a) Type-A cell with fully covered ITO glass on top and bottom substrates. (b) Type-B cell with interdigitated electrodes.

Fig. 3. Bottles of αZrP-DMF dispersions with the concentration from 0.4 to 1.09 wt%, and their spectral reflectance. Inset image shows the linear dependence of inter layer spacing on inverse volume fraction of colloidal concentration.

Fig. 4. (a) Application of 10 kHz vertical electric field removes the structural color in αZrP colloid (left three images), and enhances the birefringence (right two images). (b) The application of 1 Hz vertical electric field also removes the structural colors of αZrP colloid, but the birefringence is reduced by the 1 Hz field, differently from 10 kHz field applications. Concentrations: 0.93, 0.6, and 0.4 for blue, green, and red colloids. (c) Increasing reflectance as a function of applied voltage. (d) Spontaneous recovery of structural color with time. (e) Increasing reflectance with time during the spontaneous recovery of structural color.

Fig. 5. (a) (i) 10 kHz (ii) 1 Hz vertically applied electric field, electrode configuration (Black = GND and Red = signal) and particles orientation within two adjacent electrodes. Red and blue dotted lines in cell represent the location of ground and signal electrodes respectively. (b) (i) Initial state of cell with electrodes configuration for horizontal field signal, horizontal field induced color variations at (ii) V = 10 V, f = 1 Hz (iii) V = 10 V, f = 10 kHz and corresponding birefringence pattern. (c) Dynamic state oscillatory motion of αZrP colloid when subjected to 1 Hz electric field. Group of particles in marked area is oscillating right and left along the field direction.

Fig. 6. (a) Normalized reflectance percentage of structural color removal as a function of time. (b) Normalized reflectance percentage of structural color recovery as a function of time. (c) Normalized reflectance spectra of initial state, at 1 Hz and 10 kHz. (d) Normalized reflectance percentage of ten consecutive field induced color switching ON and OFF cycles.