Great things come in small packages: data storage for digital files, anti-counterfeiting microtext, even intricate microimages. Now the packages have gotten even smaller. An accidental discovery led materials scientists to achieve 100,000 dots per inch, 10 times the previous record for high-resolution color printing. (Magazines typically print images at 300 dots per inch.)
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“If you have a metal, say silver, and you carve it into tiny pieces, you get all kinds of colors based on the size of the nanostructure,” said Joel Yang, a scientist at the Institute of Materials Research and Engineering in Singapore whose group developed the record-breaking method. Their paper (PDF) was published in the latest issue of Nature Nanotechnology.
Yang’s group at the Institute focuses on nanoplasmonics, an area of research that involves shining light onto extremely small metal structures and causing their electrons to start oscillating. This holds promise for sensing, optics and communication technologies. They were working on metal nanostructures for another experiment when one of Yang’s colleagues looked at them under the microscope.
“He observed a whole range of colors,” Yang said. “So we thought, ‘Wow we have colors, why don’t we try to make a picture?’” The scientists replicated a well-known picture of a woman’s face, called Lena. Originally published in Playboy, Lena has become a traditional test for imaging processing quality.
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When they presented their Lena image at a conference, participants asked how small could they make the color pixel elements, and how closely could they put two colors together without it appearing as a single color. “When we thought about this question a little bit more and did some calculations, we found that indeed we were printing at very high resolutions already without realizing it,” Yang said.
Other researchers have successfully put different colors extremely close together, Yang said, but those approaches involved dyes and pigments. The processes to place color molecules side-by-side were also tedious. Instead, the scientists in Singapore simply used nanoprinting techniques.
“What we did here was to basically encode color information into nanostructures,” Yang said. Then they applied a uniform metal, silver, on top of those structures to produce the colors. An analogy would be filling up glasses with water at different heights to play notes around the rims. When filled correctly, each glass has a true resonance. The scientists arranged a version of glassware that resonates optically, not sonically. As a result, they can tune the colors of light reflected back to the viewer.
"We were making these metal nanostructures for another experiment and when one of my group mates looked at these structures under the optical microscope, he observed a whole range of colors. So we thought, ‘Wow we have colors, why don’t we try to make a picture?"
-- Joel Yang, Institute of Materials Research and Engineering scientist on his A-Ha! Moment
High-resolution color printing could be used as an improved security measure on expensive items like a bottle of wine, Yang said. The microscopic marking might also be used as an anti-counterfeiting measure to label authentic items such as currency and medicines. Teri Odom is a professor of chemistry, materials science, and engineering at Northwestern University. Her group specializes in manipulating structures on the nanoscale, in particular trying to get metals to exhibit unexpected and superior properties, something she calls making precious metals more precious.
Odom, who wrote a commentary for Nature Nanotechnology about the resolution record, compared this new tunable color palate to stained glass windows in medieval art. Those panes have small metal particles embedded in glass, but now Yang’s group can do just one level of printing by organizing the metal particles and holes in such a way that they produce distinct color types, she said.
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This nanoprinting method also has digital storage potential. “Say you want data to be stored in a certain way, the structure should be robust enough to hold fairly high density optical data storage,” she said.
“This will probably get you to at least a factor of four to 10 increase in storage compared to state-of-the-art.” The downside is that the data can’t be rewritten or erased, she added. “But that’s OK because a lot of times once you’ve done something, you want to preserve it.”
Yang said he’s not entirely sure how his printing method will be received, or whether it will be commercialized. That depends in part on how the images look once they’ve been scaled up. “We are still actively researching all these things, trying to scale it up to something that you can see with the human eye, and to see if we can make more beautiful colors.”