Structural color printing via polymer-assisted photochemical deposition

“In nature, light creates the color. In the picture, color creates the light.” This quote by American artist Hans Hofmann precisely describes the fundamental light-matter interaction and color perception. Indeed, light can produce a trichromatic color image in the brain responding to stimuli from red, green and blue wavelengths. On the other hand, the colorful pigments from pictures serve to selectively absorb light within a spectral range, thus modulating the light reflection and color display.

 

In a new paper published in Light Science & Application, a team of scientists, led by Professors Chao Wang and Yu Yao from Arizona State University, and in collaboration with Professor Zhi Zhao from Beijing University of Technology, have demonstrated to use light to “create the color” using an additive manufacturing process. In this invention, ultraviolet light is used to program photochemical reduction of metal (silver) in a solution at room temperature, directly printing color pixels down to micrometer sizes. To vividly display color, the authors designed multi-layered thin-film structures as a Fabry-Perot (FP) cavity, which functions similar to the colorful pigments from paintings to absorb light from specific wavelengths. Here the FP cavity absorption is modulated both by the thicknesses of printed metal films, controllable even when thinner than 10 nanometers, and the chosen dielectric cavity spacer thicknesses. Using this novel strategy, the authors were able to print blue, green, yellow and orange color pictures with a high contrast.

 

This work was built on the team’s recent success in 3D metal printing via UV light triggered photochemical reduction. Uniquely, the team introduced a polymer in the precursor solution that functions as “sticky hands” to connect the reduced metal nanoparticles (MNPs) together into continuous and smooth films. Therefore, the printed metal film is in fact a composite of MNPs and a small amount of the polymer “sticky hands”. Prof. Wang said, “Optically, the refractive indexes of the printed films are very different from that of vacuum-deposited metal. In fact, the composite is more lossy.” Prof. Yao explained, “Usually, loss is a bad thing, but it is used in a positive way in this case. A higher loss from the printed metal means a higher loss of the FP cavity, and therefore a broader cavity absorption. This is seen clearly from a much larger color contrast than using evaporated metal from our simulation.”

 “We performed simulation, modeling, and experiments to determine the optical indexes (of the printed films), because that is crucial in engineering the color display.” Ph.D. student Shinhyuk Choi, the first author of the publication, said. Then Choi went on to optimize the printing process and designed the FP cavity film stack for color printing. To demonstrate the printing versatility, Choi printed pictures of a cartoon character ‘Stitch’ in blue and purple, a symbol of Arizona ‘Cactus’ in green and yellow, and a logo of ‘ASU’ in orange and yellow.

Importantly, because the printing process is a room-temperature, solution-based, nondestructive process, the scientists were able to print not only on rigid glass substrates, but also on flexible substrates, such as polyethylene terephthalate – the material for making water bottles. They also demonstrated the FP cavity could be formed by spin-coating a layer of polymer rather than vacuum deposition, which can further simplify fabrication. To celebrate the 70^th anniversary of the department of electronic engineering of Tsinghua University on this special issue of the Light: Science & Applications, the team also printed the department logo in blue and purple on both glass and plastics.

“It has been a very fun experience to be able to use light to create microstructures that can, in turn, control light. Our work requires understandings in material synthesis, structural characterization, and optics. This is a good example how multidisciplinary (thinking) helps in research.” Professor Wang said. “In terms of applications, our technology can be useful in eliminating complex lithography and vacuum deposition processes in structural color display, and this can be used in colorimetric sensors, surface decoration, wearable optical devices, and flexible display.” the scientists added.

---

---