Micropatterned Liquid Crystalline Networks for Multipurpose Color Pixels

Abstract

Materials that can visually report changes in the surrounding environments are essential for future portable sensors that monitor temperature and detect hazardous chemicals. Ideal responsive materials for optical sensors are defined by a rapid response and readout, high selectivity, the ability to operate at room temperature, and simple microfabrication. However, because of the lack of viable materials and approaches, compact, passive, and multipurpose practical devices are still beyond reach. To address this challenge, we develop a methodology to fabricate colored and responsive micropixels printed by digital light projection lithography on gold substrates. These structures are made by polymeric Liquid Crystalline Networks (LCNs) whose birefringence and external stimuli responsiveness allow for micrometric devices with visual and fast response that we here apply to a few applications. First, we show how varying the projected geometrical shape can become an effective tool to engineer symmetric disclination lines in the liquid crystal order. Depending on the thickness of the micropixels, LCNs give rise to a birefringence color under polarized light or a structural color under white light due to thin-film interference. By exposing the micropatterns to temperature variation and solvents, we demonstrate a real-time optical temperature detection and differentiation between selected organic chemicals. The proposed materials and fabrication method could be scaled up and extended to roll-to-roll printing, enabling future real-life applications of liquid crystalline polymers in affordable microdevices and optical sensors with a net advantage with respect to traditional lithographic techniques in terms of fabrication speeds and costs.

Keywords: color pixels; digital light projection lithography; liquid crystalline networks; microprinting; multiresponsive materials.

Figure 1

Materials and fabrication technique of LCN microscale pixels. (a) Mesogens used in the formulation. (b) Main steps for fabrication of microscale LCN pixels. (c) CAD layout of a square micropattern. (d) Optical and POM images of microscale pixels with LC director, n, oriented at 0 and 45° with respect to the polarizer and analyzer.

Figure 2

Formation of multidomain and topological defects via micropixel shape design. Microprinting of 2D pixels with different shapes (left panel) impresses complex LC alignment (central and right panel). One-fold rotational symmetry and donut geometry preserves initially homeotropic alignment while multifaceted geometry introduces nontrivial disclination lines and point defects.

Figure 3

Thermoresponsive behavior of LCN micropixels. POM images during a heating cycle at 25, 50, 75, and 100 °C. The color is reversible by cooling (blue arrow). Images are taken with the nematic director oriented at 45° with respect to the polarizers. Scale bar: 200 μm.

Figure 4

Chemo-responsive behavior of LCN micropixels. (a) POM images of LCN micropixels in air (top line) and after immersion in different solvents (bottom lines). Depending on the solvents, the LCNs acquire a different color and shape. (b) Insight into chemo-responsive behavior of a LCN pattern. After immersion in toluene, the micropixels change color from green to orange-brown in 5 s. After solvent evaporation at room temperature (RT), the pattern acquired different colors to recover the initial (green) state by drying at 50 °C.

Figure 5

Structural colors by ultrathin LCN micropixels. (a) Optical image of LCN pixels showing fundamental subtractive colors: cyan, yellow, and magenta that correspond to different heights (b). Optical profilometer measurements and their standard deviation (shaded area) obtained for four consecutive measurements of the LCN height for each color reported in Figure 5a. The dashed lines refer to the average height that for the different colors is 165, 195, and 215 nm, respectively. (c) 3D reconstruction of the topography of the LCN micropixels that shows how different colors correspond to different thicknesses of the lossy dielectric cavities. (d) Optical image of LCN micropixels with different heights, the arrow indicated the increase in height. By tuning the structure height, the whole palette of subtractive colors can be obtained.