Color Change Studied in Real Time for Electrophoretic Displays

Electrochromic photonic (ECP) crystals have advantages such as a wide color regulation range, convenient control approach, low power consumption, and fast response. Hence, these crystals have found applications in reflective display devices.

Color Change Studied in Real Time for Electrophoretic Displays

Study: In Situ Dynamic Study of Color-Changing in Liquid Colloidal Crystals for Electrophoretic Displays. Image Credit: All for you friend/Shutterstock.com

In an article recently published in the journal ACS Applied Nano Materials, silica (SiO2) nanospheres-based liquid colloidal crystals were prepared via a self-assembly method induced by solvent evaporation. These colloidal crystals were used as a reflective unit to fabricate ECP crystal devices. Under low voltages, this device exhibited outstanding controllable structural color across the visible spectrum.

Regulatory rules have been set up for ECP crystal devices concerning their response speed, color-tunable range, cyclic performance, and reflectance intensity, based on ultra-small-angle X-ray scattering (USAXS) results. Furthermore, a relationship was established between electrode spacing, slurry concentration, and viewing angle.

To explain the responsive behavior of electrical modulation-induced reflection spectra, a dynamic mechanism was elaborated that can help develop various ECP crystal devices in the future. Additionally, an electrochromic prototype device was constructed whose functioning was regulated by pressure. The constructed device showed dynamic structural color, rapid response, and good reversibility.

ECP Crystals in Reflective Display Devices

ECP crystals are promising optically active materials with continuous color-switching ability, rapid response, and convenient regulation approach and can be integrated into electronic devices easily. Hence, these crystals are used in the construction of smart devices.

Furthermore, the modulation principle of the ECP crystals is based on Bragg−Snell’s law, wherein an external electric field manipulates the microstructure and the photonic crystal material’s optical properties. Four types of ECP crystals were reported based on the electrochemical process, liquid crystal components, electrophoresis process, and other stimuli.

Comparatively, electrophoretic ECP crystals leverage the colloidal nanospheres in a three-dimensional (3D) assembly of photonic crystals, which realizes the photonic band’s wide regulation range at low voltage. To this end, colloidal crystal arrays (CCAs) constructed from monodispersed nanospheres serve as electrophoretic ECP crystal’s active material.

The electrophoretic ECP crystals are driven under low voltage to prevent degradation effects on optical properties caused by indium tin oxide (ITO) electrode reduction. Additionally, the solvent can adjust the photonic crystal’s microstructure under an electric field and prevent the electrochemical reaction. For example, water electrolysis can have a negative impact on the colloidal nanosphere’s electric double layer, which is essential to maintain the structural color tuning range and cyclic stability of the ECP crystal device.

Liquid Colloidal Crystals for Electrophoretic Displays

In the present work, the ECP crystal devices were constructed based on liquid colloidal crystals to investigate their dynamic mechanism and properties. As a first step, the SiO2-based colloidal nanospheres were fabricated via a modified Stöber method. Next, the SiO2-based liquid colloidal crystal slurry was prepared via self-assembly induced by evaporation in propylene carbonate (PCb).

The transmission electron microscope (TEM) images showed that seed solutions of 110, 200, and 300 microliters had SiO2 nanospheres with an average diameter of 234, 197, and 165 nanometers, respectively, and a coefficient of variation (CV) of 0.010, 0.015, and 0.013, respectively. Moreover, 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 SiO2 nanospheres. Furthermore, the zeta potential of each seed sample of SiO2 nanospheres was higher than – 30 millivolts, revealing their colloidal stability and good dispersibility.

The prepared liquid colloidal crystal slurry was sealed between the ITO electrodes with different thicknesses, followed by the varied voltage application for the structural color control. Furthermore, 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 on the responsive behaviors. The ECP crystal devices showed excellent performance demonstrating the potential of these devices in the display field.

Conclusion

To summarize, the ECP crystal devices were fabricated based on liquid colloidal crystals to study their corresponding electrical response. SiO2 colloidal nanospheres with abundant surface charge and controllable monodisperse size were utilized as basic units to construct liquid colloidal crystals via self-assembly induced by solvent evaporation in PCb.

Moreover, in situ USAXS characterization revealed that amorphous suspended nanospheres and microcrystal arrays coexisted inside the slurry of liquid colloidal crystals. Moreover, the ECP crystal devices were fabricated using a colloidal crystal slurry with a 25% volume fraction of SiO2 nanospheres, which was sealed between 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 electric field’s dampening effect caused by the deposition of the isolation layer having 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-Changing in Liquid Colloidal Crystals for Electrophoretic Displays. ACS Applied Nano Materials. https://doi.org/10.1021/acsanm.2c02391

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Bhavna Kaveti

Written by

Bhavna Kaveti

Bhavna Kaveti is a science writer based in Hyderabad, India. She has a Masters in Pharmaceutical Chemistry from Vellore Institute of Technology, India, and a Ph.D. in Organic and Medicinal Chemistry from Universidad de Guanajuato, Mexico. Her research work involved designing and synthesizing heterocycle-based bioactive molecules, where she had exposure to both multistep and multicomponent synthesis. During her doctoral studies, she worked on synthesizing various linked and fused heterocycle-based peptidomimetic molecules that are anticipated to have a bioactive potential for further functionalization. While working on her thesis and research papers, she explored her passion for scientific writing and communications.

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