Researchers demonstrate “no-ink” colour printing with nanomaterials

Researchers at Missouri University of Science and Technology are giving new meaning to the term “read the fine print” with their demonstration of a colour printing process using nanomaterials.

Metamaterials interact with light

In this case, the print features are very fine – visible only with the aid of a high-powered electron microscope. The researchers illustrate their “no-ink” printing method by reproducing the Missouri S&T athletic logo on a nanometer-scale surface. The technique described in the Scientific Reports article Structural colour printing based on plasmonic metasurfaces of perfect light absorption involves the use of thin sandwiches of nanometer-scale metal-dielectric materials known as metamaterials that interact with light in ways not seen in nature. Experimenting with the interplay of white light on sandwich-like structures, or plasmonic interfaces, the researchers developed what they call “a simple but efficient structural colour printing platform” at the nanometer-scale level. They believe the process holds promise for future applications, including nanoscale visual arts, security marking and information storage.

Thin films of silver as printing surface

The researchers’ printing surface consists of a sandwich-like structure made up of two thin films of silver separated by a “spacer” film of silica. The top layer of silver film is 25 nanometers thick and is punctured with tiny holes created by a microfabrication process known as focused ion beam milling. The bottom layer of silver is four times thicker than the top layer but still minuscule at 100 nanometers. Between the top and bottom films lies a 45-nanometer silica dielectric spacer. The researchers created a scaled-down template of the athletic logo and drilled out tiny perforations on the top layer of the metamaterial structure. Under a scanning electron microscope, the template looks like a needlepoint pattern of the logo. The researchers then beamed light through the holes to create the logo using no ink – only the interaction of the materials and light.

Light was beamed into the material

By adjusting the hole size of the top layer, light at the desired frequency was beamed into the material with a perfect absorption. This allowed researchers to create different colours in the reflected light and thereby accurately reproduce the S&T athletic logo with nanoscale colour palettes. The researchers further adjusted the holes to alter the logo’s official green and gold colour scheme to introduce four new colours (an orange ampersand, magenta “S” and “T,” cyan pickaxe symbol and navy blue “Missouri”). “To reproduce a colourful artwork with our nanoscale colour palettes, we replaced different areas in the original image with different nanostructures with specified hole sizes to represent various visible colours,” says Dr. Xiaodong Yang, an assistant professor at Missouri S&T. “We chose the athletic logo to fill that need.”

Pure colours with high brightness

“Unlike the printing process of an inkjet or laserjet printer, where mixed colour pigments are used, there is no colour ink used in our structural printing process – only different hole sizes on a thin metallic layer,” says Dr. Jie Gao, an assistant professor of mechanical and aerospace engineering at Missouri S&T. In their paper, the authors note that the process resulted in “pure colours with high brightness” with little need for protective coatings. The researchers believe the process could lead to “high-performance, pigment-free colour printing and relevant applications such as security marking and information storage.”