For most of human history, the only available colors were those conferred by dyes extracted from nature. Some dyes were so scarce and expensive that they came to be associated with wealth and status, like the purple of royalty. With the mid-1800s came the first synthetic dyes, which made colors more accessible to everyone, but also introduced significant environmental hazards, as well as health risks. For instance, workers in the aniline dye industry were found to be at greater risk of bladder cancer.
Finding a way to produce color without resorting to dyes might seem like a non-starter to many people, but not to a group of researchers led by Joel Yang at A*STAR’s Institute of Materials Research and Engineering (IMRE).
“With the chemical dyes used in everyday consumer products, colors are created by absorption and scattering of the light caused by various chemical compounds and elements. Depending on the wavelength of the light that is absorbed or scattered, various colors emerge,” Yang explained.
In contrast, plasmonic color generation involves the use of metallic nanostructures—something like studs on the top of Lego blocks. When light hits such a surface, it sets in motion the collective oscillation of free electrons, also known as plasmon resonances, on the surface of the metallic nanostructures, causing light to be absorbed and scattered differently, thereby producing different colors. However, the span of colors is limited due to losses in the metals.
Seeking to expand the color palette of plasmonic color printing technology, Yang’s team sandwiched a 30-nm-thick aluminum oxide film between a 100-nm-thick aluminum layer and 40-nm-thick aluminum disks.
They found that by varying the diameter of the disks, the distance between discs and the arrangement (square and hexagonal patterns) of disks, they could increase the color spectrum of their plasmonic color printing system by relying on an effect where each disk absorbs light within an area significantly larger than its size.
“We achieved about 50 percent coverage of the standard red, green, blue (RGB) color space, which is considerably more than previous works,” said Soroosh Daqiqeh Rezaei, a graduate student in Yang’s lab and the first author of the study.
The researchers further showed that the hexagonal arrangement of disks achieved higher color saturation as compared to the square arrangement. They hope that their findings could bring plasmonic color generation closer to commercial use.
“Plasmonic color prints possess ultra-high resolution—you can pack more than 100,000 pixels into one pixel of the iPad Retina display!” said Rezaei. “Therefore, plasmonic color prints can be used to store information and provide anti-counterfeiting features, in addition to providing fade-resistant colors. We are also working on incorporating plasmonic pixels in display technologies and optical sensors.”
Importantly, the researchers noted that scaling up their technology would not result in hazards to the environment. “In this work, we have only used aluminum and aluminum oxide, which are cost-effective, earth-abundant and stable,” said Yang. “Colors from precious metals or chemical dyes instead might be more vibrant but could be less sustainable or environmentally friendly.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).