Non-fading and non-toxic structural color can now be applied to flat or 3D surfaces using an inkjet printer. The Kobe University development also opens possibilities for novel display and anti-counterfeiting technologies.
While traditional printer pigments fade and most structural color can't be printed, the Kobe University material engineer SUGIMOTO Hiroshi has been working on nothing short of a revolution in the way color is produced. Over the past several years, his team has worked on spherical silicon crystals that reflect color specifically based on their precise size in the range between 100 and 200 nanometers. Paint using this technology is extremely light-weight, non-toxic, sustainably sourced and, importantly, non-fading. However, a key element in coloring the world with this new technology has been missing: printing.
"We wanted to develop a structural color material that can be processed in a similar way to conventional inks or paints," says Sugimoto. A key issue they had to overcome was that, as the solvent dries, the nanospheres tend to clump together. This changes how the material interacts with light and degrades its coloration. The team now tried to overcome this challenge by coating each crystal with a silica shell whose material properties won't cause light to bend at the interface to the surrounding resin. Essentially, they created a transparent bumper between the crystals.
In the journal Advanced Materials, the Kobe University engineer and his team now report on their achievement. They show impressive images with vibrant colors printed at resolutions between 250 and 125 dots per inch onto a flat PET film as well as on a 3D metallic surface. Sugimoto says, "I am really excited that the creation of an inkjet-compatible suspension for silicon nanospheres finally enables full-color printing without using pigments or dyes."
When printed onto a PET film, they showed that the images exhibit different colors when the light passes through the image compared to when light from above is reflected. This is due to the way the silicon nanospheres interact with light in a process called "Mie refraction," but it also enables a feat that is impossible with conventional inks: Other than the colors being different, the images can be made both reflective with vivid colors and at the same time highly transparent to transmitted light. "These two properties are typically considered mutually exclusive in conventional colorants," remarks Sugimoto.
When applied to a monitor, such images would be virtually invisible while the display is on, but be clearly visible when it is powered off, enabling zero-energy information display possibilities. Another possible application is anti-counterfeiting technology. Looking at the future, Sugimoto thus says, "This work represents an important step towards scalable structural color technologies that are compatible with existing printing and coating processes."
This research was funded by the Japan Society for the Promotion of Science (grants 24K01287, 25K01608, JPNP20004), the New Energy and Industrial Technology Development Organization and the Japan Science and Technology Agency (grant JPMJSF2405).
Kobe University is a national university with roots dating back to the Kobe Higher Commercial School founded in 1902. It is now one of Japan's leading comprehensive research universities with over 16,000 students and over 1,700 faculty in 11 faculties and schools and 14 graduate schools. Combining the social and natural sciences to cultivate leaders with an interdisciplinary perspective, Kobe University creates knowledge and fosters innovation to address society's challenges.
