Biologists and nanotechnology researchers at Rice University have been working for years on a U.S. Navy-funded project to create a material that can visually adapt to its surroundings in real-time. The goal is to allow ships, vehicles and eventually soldiers to become invisible—or nearly invisible—just like some species of squid and other cephalopods.
With squid skin as their model, the scientists developed a flexible, high-resolution, low-power display that could realistically mimic its environment. The new display technology actually makes individual pixels (the tiny colored dots that make up the image on your television and smartphone) invisible to the human eye. Using aluminum nanorods of precise lengths and spacing, the researchers found they could create vivid dots of various colors that are 40 times smaller than the pixels found in today’s TVs.
How it Works
In a study recently published in the early edition of the Proceedings of the National Academy of Sciences (PNAS), the authors illustrate how they used a technique called electron-beam deposition to create arrays of nanorods and five-micron-square pixels—roughly the size of a plant or mold spore—that produce bright colors without the use of dyes, which can fade over time. The color of each of these tiny pixels can be finely tuned by varying either the distances between the rods in the arrays or the lengths of individual rods.
The color of the pixel is produced when light hits the nanorods and scatters at specific wavelengths. By varying the arrangement and length of the surrounding nanorods, the team is able to precisely control how the light bounces around, narrowing the spectrum of light and, in effect, adjusting the visible light each pixel gives off. The pixels the team created are also plasmonic, meaning they get brighter and dimmer depending on the surrounding light, much like the colors in stained glass. This could be useful in creating lower-power displays in consumer devices, which should also be less stressful on the eyes.
Because the technology relies mostly on aluminum, which is inexpensive and easy to work with, these types of displays shouldn’t be prohibitively expensive or exceedingly difficult to manufacture.
Room for Improvement
Stephan Link, an associate professor of chemistry at Rice University and the lead researcher on the PNAS study, says the team didn’t set out to solve any fundamental problems with existing display technology, but to work toward smaller pixels for use in a wearable, low-power material that is thin and responsive to ambient light.
“Now that we have these nice colors,” he says in an email, “we’re thinking of all the ways we can improve them, and how we can work toward the nano squid skin that is the ultimate goal of this collaboration.”
According to Link, one way to improve the technology would be to partner with experts in the commercial display industry. While the technology for making the pixels is very different, the team expects many of the other display components, like the liquid crystals that determine a display’s refresh rate and pixel response time, will remain the same or similar to those used today.
To make a flexible display, the researchers may try to build the pixels like scales, so that the underlying material can bend, but the liquid crystals and aluminum nano-array can remain flat. But to get to that point, the team may need help.
“It seems kind of funny to say it, but one major hurdle is to scale down the size of the liquid crystal part of our displays,” writes Link. “You see very tiny LCD screens all the time in technology, but we don’t have the fancy industrial machines capable of making those with such high precision and reproducibility, so that’s a major hurdle on our part.”
Another potential hurdle is to replicate the vast array of colors possible in today’s high-end displays. While the researchers aren’t quite there yet, Link seems confident that their technology is up to the task.
“The great thing about color is that there are two ways to make it,” says Link. “For example, the color yellow: The wavelength of light that looks yellow is 570 nanometers, and we could make a pixel that has a nice sharp peak at 570 nm and give you yellow that way. Or, we can make yellow by placing a red pixel and a green pixel next to each other, like what is done in current RGB displays. For an active display, RGB mixing is the way to do it efficiently, but for permanent displays, we have both options.”
RGB mixing has visible drawbacks in existing displays, because the pixels are often visible to the naked eye. But with this technology, you’d need a microscope to see them and to discern which color-creating method is being used.
Applying the Finding to Consumer Technology
The ability to precisely create and manipulate the tiny nano-scale rods plays a large role in the team’s breakthrough. Getting the length or spacing of these tiny rods even slightly off would affect the color output of the completed display. So, scaling manufacturing up to mass-produce these types of displays could also pose a problem—at least at first. Link is hopeful though, pointing to two existing manufacturing technologies that could be used to build these kinds of displays—UV lithography, which uses high-energy light to produce tiny structures, and nanoimprint lithography, which uses stamps and pressure (much like the way the digits on a license plate are embossed, but on a microscopic scale).
“Other than finding the right method so we can pattern larger areas,” says Link, “the rest of the manufacturing process is actually pretty straightforward.”
Link didn’t want to guess as to when we might see these nano-scale pixels used in commercial displays and devices. At this point, he and his fellow researchers are still focused on refining the technology toward their goal of squid-like camouflage. A collaboration with commercial display makers could help the team get closer to that goal though while also leading to new kinds of displays for consumer devices.
Perhaps Link's group at Rice should team up with researchers at MIT, who are also working on replicating the properties of cephalopod skin. The scientists and engineers there recently demonstrated a material that can mimic not only color, but also texture. This will be an important feature for the military's goal of making vehicles invisible. A flexible display could, for example, make a tank look like rocks or rubble from afar. But if its sides are still smooth and flat, it will still stand out on closer inspection.