Three-Dimensionally Printed, Vertical Full-Color Display Pixels for Multiplexed Anticounterfeiting

Since counterfeiting has emerged as a global issue, novel and advanced strategies are in great demand for data storage and encryption to prevent information leakage. To this end, various materials, structuring strategies, and data processing algorithms have been devised. Creating high-resolution, sophisticated anticounterfeiting patterns has mostly relied on photolithography, nanoimprinting, and laser engraving, which are energy-intensive and/or chemically deleterious, difficult to perform on valuable products^1–4. Alternatively, inkjet printing has also been utilized by taking advantage of its cost-effectiveness and high compatibility with ink and substrate materials. However, the resulting patterns are easily duplicated due to their simple geometry constrained in-plane. Advancing the structural complexity for improving security level remains a longstanding challenge. The utilization of luminescent materials is a strategy to advance anticounterfeiting labels. The intrinsic or stimulus-responsive optical properties of semiconductor quantum dots, perovskites, or organic dyes can be exploited to configure multilevel anticounterfeiting^5,6. A luminescent-based anticounterfeiting label consists of an array of pixels which are the fundamental element to encode/display information. Each pixel is typically composed of red, green, and blue subpixels with lateral patchwork to render multi-channel information encryption. However, this lateral configuration is unavoidable for the existence of a subpixel pitch, not only being easily copied but also limiting the improvement of pixel density.<br/><br/>Herein, we propose vertically stacked multicolor micropixels for high-resolution, multiplexed anticounterfeiting. The micropixel is produced by sequentially stacking luminescent dye-doped red-green-blue (RGB) subpixels, realized with high-resolution 3D printing. Our 3D printing approach based on the use of a femtoliter ink meniscus enables vertical stacking of micron-sized subpixels without the existence of a subpixel pitch, drastically increasing the pixel density up to 13,400 pixels per inch (PPI), which is not easily attainable by the traditional lateral subpixel layout. Furthermore, a full-scale color synthesis for each micropixel is achieved by modulating the height ratio of R, G, and B subpixels, which is reflected in the CIE 1931 diagram. These features create high-resolution, multiplexed anticounterfeiting labels, inaccessible by conventional optical imaging, and 3D printing offers the simplest, most versatile route to manufacture. This work demonstrates the exceptional inclusivity of 3D printing to construct vertically stacked structures and highlights the possibility to devise 3D printing for photonic device manufacturing.<br/>1. Choi, M. K. <i>et al.</i> Wearable red–green–blue quantum dot light-emitting diode array using high-resolution intaglio transfer printing. <i>Nature Communications 6:1</i> <b>6</b>, 1–8 (2015).<br/>2. Zhu, C. <i>et al.</i> An Integrated Luminescent Information Encryption–Decryption and Anticounterfeiting Chip Based on Laser Induced Graphene. <i>Adv Funct Mater</i> <b>31</b>, 2103255 (2021).<br/>3. Arppe, R. & Sørensen, T. J. Physical unclonable functions generated through chemical methods for anti-counterfeiting. <i>Nature Reviews Chemistry 1:4</i> <b>1</b>, 1–13 (2017).<br/>4. Hu, Z. <i>et al.</i> Physically unclonable cryptographic primitives using self-assembled carbon nanotubes. <i>Nature Nanotechnology 11:6</i> <b>11</b>, 559–565 (2016).<br/>5. Ho, S. J., Hsu, H. C., Yeh, C. W. & Chen, H. S. Inkjet-Printed Salt-Encapsulated Quantum Dot Film for UV-Based RGB Color-Converted Micro-Light Emitting Diode Displays. <i>ACS Appl Mater Interfaces</i> <b>12</b>, 33346–33351 (2020).<br/>6. Chen, M. <i>et al.</i> Three-Dimensional Perovskite Nanopixels for Ultrahigh-Resolution Color Displays and Multilevel Anticounterfeiting. <i>Nano Lett</i> <b>21</b>, 5186–5194 (2021).