Breakthrough in Nanostructure Technology for Real-Time Color Display

Abstract
Diblock copolymer (dBCP) particles capable of dynamic shape and color changes have gained significant attention due to their versatility in programmable shapes and intricate nanostructures. However, their application in photonic systems remains limited due to challenges in achieving a sufficient number of defect-free photonic layers over a tens-of-micrometer scale. In this study, we present a pioneering demonstration of photonic dBCP particles featuring over 300 axially stacked photonic layers with responsive color- and shape-transforming capabilities. Our approach leverages the complex interplay between the macrophase separation of multiple incompatible components and the microphase separation of dBCP from solvent-evaporative microemulsions. Specifically, continuous phase separation of silicone oil from polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP), triggered by solvent evaporation, promotes the anisotropic growth of PS-b-P2VP layers. This results in the formation of Janus colloids, where an oil droplet merges with a nanostructured polymer cone and lamellar structures align along the long axis of the cone. We highlight the capability to precisely adjust the particle morphology and the corresponding orientation, dispersion, and structural color window by modulating both the molecular weight of PS-b-P2VP and the volume ratio between PS-b-P2VP and silicone oil. Furthermore, reversible swelling/deswelling of photonic colloids is visualized and correlated with their structural colors. Finally, we demonstrate the potential of this study by presenting a multicolor-patterned array of photonic colloids, highlighting the possibilities for applications in smart photonic ink and devices.

A groundbreaking technology that enables the real-time display of colors and shapes through changes in nanostructures has been developed. This innovative technology, pioneered by Professor Kang Hee Ku and her team in the School of Energy and Chemical Engineering at UNIST, has the potential to revolutionize various fields, such as smart polymer particles.

Utilizing block copolymers, the research team has achieved the self-assembly of photonic crystal structures on a large scale, mimicking natural phenomena observed in butterfly wings and bird feathers. By reflecting the shape and direction of nanostructures, this technology allows for the visualization of vibrant colors and intricate patterns in real time.

Block copolymers, composed of two or more different monomers covalently bonded in a block shape, were strategically employed to induce phase separation using a non-mixing liquid droplet. Professor Ku emphasized the significance of this achievement, stating, “We have successfully generated hundreds of flawless photonic crystal structures through the autonomous organization of block copolymers, eliminating the need for external manipulation.”

Figure 1. Photonic Janus Colloids with Nanostructured Cone l Image Credit: ACS Nano 2024, 18, 6, 5196-5205

Setting itself apart from conventional methods, this cutting-edge technology leverages internal nanostructures to create colors that are vivid, long-lasting, and sustainable. Furthermore, its enhanced applicability in display technology is evident through its capability to pattern large areas efficiently.

The key innovation lies in the use of a polymer that can dynamically adjust the size of microstructures within particles in response to changes in the external environment. By leveraging the unique properties of polystyrene-polyvinylpyridine (PS-b-P2VP) block copolymers, the structure, shape, and color of the particles can be tailored, reverting to their original state despite environmental variations.

Real-time monitoring of structural changes revealed that the size and color of micro-nanostructures adapt to fluctuations in alcohol concentration or pH value. Notably, the particles produced through this technology exhibit an innovative ‘Ice Cream Cone’ shape structure, combining aspects of solids and liquids to visualize fluid vibrations and dynamically alter shape and color in response to external stimuli.

Figure 2. (a) Reflective optical micrographs, (b) photographs of particle suspensions, and (c) corresponding reflectance spectra of Janus colloids prepared with different molecular weights of PS-b-P2VP: PS55k-b-P2VP55k, PS133k-b-P2VP132k, PS250k-b-P2VP200k, and PS240k-b-P2VP296k. (d) SEM and (e) TEM images of PS-b-P2VP cones (fBCP = 0.2 for SEM and 0.6 for TEM) after the removal of silicone oil. (f) A plot of AR values of PS-b-P2VP cones as a function of fBCP depending on the Mn of PS-b-P2VP. (g) Patterned RGB pixel array of a colloidal suspension: PS240k-b-P2VP296k (red), a blend of PS250k-b-P2VP200k and PS132k-b-P2VP133k in a 1:1 weight ratio (green), and PS133k-b-P2VP132k (blue). l  Image Credit: ACS Nano 2024, 18, 6, 5196-5205

Professor Ku showed confidence about the potential applications of this research, stating, “This study opens doors to the creation of self-assembling optical particles, streamlining the complex process conditions typically associated with colloidal crystal structure and pattern formation.” She further noted, “The technology’s practical applications in smart paint and polymer particles across various industries are envisioned.”

Published in the February 2024 issue of ACS Nano, the research received support from the National Research Foundation of Korea (NRF), the Ministry of Science and ICT (MSIT), and the Korea Toray Science Foundation, underscoring collaborative efforts driving this groundbreaking innovation.

Journal Reference
Juyoung Lee, Soohyun Ban, Kyuhyung Jo, et al., “Dynamic Photonic Janus Colloids with Axially Stacked Structural Layers,” ACS Nano, (2024).