Silver nanocubes produce full-color images

Using plasmonic silver nanocubes, researchers have developed a novel technique for printing and imaging across a range of colors

Original source: Materials Today

Researchers at Duke University believe they have overcome a longstanding hurdle to producing cheaper, more robust ways for printing and imaging across a range of colors extending into the infrared.

As any mantis shrimp will tell you, there are a wide range of ‘colors’ along the electromagnetic spectrum that humans cannot see but which can provide a wealth of information. Sensors that extend into the infrared can, for example, identify thousands of plants and minerals, diagnose cancerous melanomas and predict weather patterns, simply by analyzing the spectrum of reflected light.

Current imaging technologies that detect infrared wavelengths are expensive and bulky, requiring numerous filters or complex assemblies in front of an infrared photodetector. The need for mechanical movement in such devices reduces their expected lifetime and can be a liability in harsh conditions, such as those experienced by satellites.

In a new paper, a team of Duke engineers reveals a manufacturing technique that promises to bring a simplified form of multispectral imaging into daily use. Because the process uses existing materials with fabrication techniques that are inexpensive and easily scalable, it could revolutionize any industry where multispectral imaging or printing is used. The engineers report the novel technique in a paper in Advanced Materials.

“It’s challenging to create sensors that can detect both the visible spectrum and the infrared,” said Maiken Mikkelsen, assistant professor of electrical and computer engineering and physics at Duke. “Traditionally you need different materials that absorb different wavelengths, and that gets very expensive. But with our technology, the detectors’ responses are based on structural properties that we design rather than a material’s natural properties. What’s really exciting is that we can pair this with a photodetector scheme to combine imaging in both the visible spectrum and the infrared on a single chip.”

The new technology relies on plasmonics – the use of nanoscale physical phenomena to trap certain wavelengths of light. The Duke engineers fashioned silver cubes just 100nm wide and placed them a few nanometers above a thin gold foil. When incoming light strikes the surface of a nanocube, it excites the silver atoms’ electrons, trapping the light’s energy – but only at a certain wavelength.

The size of the silver nanocubes and their distance from the base layer of gold determines that wavelength, while the spacing between the nanoparticles determines the strength of the absorption. By precisely tailoring these spacings, researchers can make the system respond to any specific color they want, all the way from visible wavelengths out to the infrared.

The engineers were then faced with the challenge of how to build a useful device that could be scalable and inexpensive enough to use in the real world. For that, Mikkelsen turned to her research team, including graduate student Jon Stewart.

“Similar types of materials have been demonstrated before, but they’ve all used expensive techniques that have kept the technology from transitioning to the market,” said Stewart. “We’ve come up with a fabrication scheme that is scalable, doesn’t need a clean room and avoids using million-dollar machines, all while achieving higher frequency sensitivities. It has allowed us to do things in the field that haven’t been done before.”

To build a detector, Mikkelsen and Stewart used light etching and adhesives to fabricate pixels made from different sized silver nanocubes that are sensitive to specific wavelengths of light. When incoming light strikes this array, each area responds differently depending on the wavelength of light it is sensitive to. By teasing out how each part of the array responds, a computer can reconstruct the wavelengths of the original light.

The technique can be used for printing as well, the team showed. Instead of creating pixels tuned to respond to specific colors of light, they created pixels with three bars that reflect just three colors: red, green and blue (RGB). By controlling the relative lengths of each bar, they can dictate what combination of colors the pixel reflects. It’s a novel take on the classic RGB scheme first used in photography in 1861.

But unlike most other RGB technologies, this plasmonic color scheme promises never to fade over time and can be reliably reproduced with high accuracy time and again. It also allows its adopters to create color schemes in the infrared.

“Again, the exciting part is being able to print in both visible and infrared on the same substrate,” said Mikkelsen. “You could imagine printing an image with a hidden portion in the infrared, or even covering an entire object to tailor its spectral response.”