Electrochromic photonic crystals (ECPs) have advantages such as a wide range of color regulation, convenient control approach, low power consumption and fast response. Therefore, these crystals have found applications in reflective display devices.
Study: In situ dynamic study of color change in liquid colloidal crystals for electrophoretic screens. Image credit: All for you friend/Shutterstock.com
In a paper recently published in the journal ACS Applied Nano Materials, colloidal liquid crystals based on silica (SiO2) nanospheres were prepared using a solvent evaporation-induced self-assembly method. These colloidal crystals were used as a reflective unit to fabricate ECP crystal devices. Under low voltages, this device exhibited exceptional controllable structural color across the entire visible spectrum.
Regulatory rules have been established for ECP crystal devices with respect to their response speed, tunable color range, cycling performance, and reflectance intensity, based on the results of ultra-small-angle X-ray scattering ( USAXS). In addition, a relationship was established between electrode spacing, slurry concentration and viewing angle.
To explain the response behavior of the electrical modulation-induced reflection spectra, a dynamic mechanism was elaborated that may help to develop various ECP crystal devices in the future. In addition, an electrochromic prototype device was built whose operation was regulated by pressure. The constructed device exhibited dynamic structural color, fast response, and good reversibility.
ECP crystals in reflective display devices
ECP crystals are promising optically active materials with continuous color changing capability, fast response, and convenient tuning approach and can be easily integrated into electronic devices. Therefore, these crystals are used in the construction of smart devices.
In addition, the modulation principle of ECP crystals is based on the Bragg-Snell law, in which an external electric field manipulates the microstructure and optical properties of the photonic crystal material. Four types of ECP crystals were reported based on the electrochemical process, liquid crystal components, electrophoresis process, and other stimuli.
In comparison, electrophoretic ECP crystals take advantage of colloidal nanospheres in a three-dimensional (3D) array of photonic crystals, which realizes the wide tuning range of the photonic bandgap at low voltage. To this end, colloidal crystal arrays (CCA) constructed from monodisperse nanospheres serve as the active material of the electrophoretic ECP crystal.
ECP electrophoretic crystals are operated at low voltage to avoid the effects of degradation of optical properties caused by the reduction of indium tin oxide (ITO) electrodes. In addition, the solvent can adjust the microstructure of the photonic crystal under an electric field and prevent the electrochemical reaction. For example, water electrolysis can have a negative impact on the electrical double layer of the colloidal nanosphere, which is essential to maintain the structural color tuning range and cycling stability of the ECP crystal device.
Liquid colloidal crystals for electrophoretic screens
In the present work, ECP crystal devices were constructed from colloidal liquid crystals to investigate their mechanism and dynamic properties. As a first step, SiO2-based colloidal nanospheres were fabricated using a modified Stöber method. The slurry of SiO2-based liquid colloidal crystals was then prepared by evaporation-induced self-assembly in propylene carbonate (PCb).
Transmission electron microscope (TEM) images showed that the 110, 200, and 300 microliter seed solutions had SiO2 nanospheres with an average diameter of 234, 197, and 165 nanometers, respectively, and a coefficient of variation (CV) of 0.010, 0. , and 0.013, respectively. Furthermore, each seed sample of the SiO2 nanospheres showed a polydispersity index (PDI) of less than 0.05. These values indicated the narrow size distribution of the SiO2 nanospheres. In addition, the zeta potential of each SiO2 nanosphere seed sample was greater than – 30 millivolts, revealing its colloidal stability and good dispersibility.
The prepared liquid colloidal crystal slurry was sealed between the ITO electrodes with different thicknesses, followed by the application of varied voltage for structural color control. In addition, the application of in situ measurement technology during electrical modulation helped study the kinetic process and the impact of slurry concentration, viewing angle, cyclic voltage, and electrode spacing in response behaviors. The ECP crystal devices showed excellent performance demonstrating the potential of these devices in the display field.
conclusion
In summary, ECP crystal devices were fabricated from colloidal liquid crystals to study their corresponding electrical response. SiO2 colloidal nanospheres with abundant surface charge and controllable monodisperse size were used as the basic units to construct liquid colloidal crystals via solvent evaporation-induced self-assembly in PCb.
In addition, USAXS in situ characterization revealed that suspended amorphous nanospheres and microcrystal arrays coexisted within the liquid colloidal crystal slurry. Furthermore, ECP crystal devices were fabricated using a colloidal crystal slurry with a volume fraction of 25% SiO2 nanospheres, which was sealed between the ITO electrodes.
Furthermore, a microscopic mechanism was detailed to understand the kinetic process during electrical modulation. The results revealed that the mechanism was based on the damping effect of the electric field caused by the deposition of the insulating layer with a low dielectric constant (ε).
reference
Fang, Y., Li, H., Wang, X., Zhu, M., Guo, J., Wang, C. (2022). In situ dynamic study of color change in colloidal liquid crystals for electrophoretic displays. Nanomaterials applied ACS. https://doi.org/10.1021/acsanm.2c02391
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