Seeing images captured by an ultraviolet sensor is like looking at the world with new eyes. UV images reveal spots that presage rot on mushrooms, dark lines along flower petals that guide insects to nectar, and clouds of acetone in water. And with their relatively short wavelengths, UV sensors could be well suited to more precise navigation for flying swarms of tiny drones.
“There’s important information hidden in the UV,” says Rihito Kuroda, a researcher at Tohoku University in Japan who develops UV imagers in his lab. But that information has been difficult to capture; silicon doesn’t absorb ultraviolet wavelengths very well, and other semiconductors that play well with ultraviolet light make slow imagers with low frame rates. But that’s about to change. This week at the International Electron Devices Meeting in San Francisco, two research groups presented ultrathin, flexible UV sensor designs they hope will help make these devices more widespread.
A group from King Abdullah University of Science and Technology in Saudi Arabia presented their research aimed at making UV sensors from specially formulated paper. Electrical engineering student Chun-Ho Lin explained that it’s difficult to make flexible UV sensors because they heat up under the high-energy rays. Typical flexible substrates, like plastic and paper, can’t wick away that heat quickly enough. He and other electrical engineers in Jr-Hau He’s lab made thermally conductive, UV-sensitive paper by combining boron nitride nanosheets with cellulose fibers. Flexible sensors made from this formulation can take the heat, withstanding temperatures up to 200 degrees Celsius. What’s more, they are blind to wavelengths above the deep UV band.
Other researchers are sticking with silicon, but using graphene to help it along. Yang Xu, an electrical engineer at Zhejiang University in China, says there are good reasons to work with silicon, even though in its native state it is a strong reflector of UV rays. Silicon photodetectors can work quickly, enabling higher frame rates, and they can draw on a vast manufacturing infrastructure. Xu says his philosophy is, “Why not help silicon do better?” With that in mind, his team is pairing the semiconductor with graphene, which absorbs UV light like a champ.
To make flexible silicon-graphene UV photodetectors, the Zhejiang University group uses etching and rubber stamps to transfer ultrathin silicon microstructures to a flexible plastic substrate, then coats the silicon with graphene and adds electrodes. This photodetector is blind to visible light, because the silicon layer is just 20 nanometers thick and cannot absorb it.
This ultrathin device is flexible and performs as well as state-of-the-art UV photodetectors, says Xu. His lab is currently working on shrinking the size of the photodetectors to improve their resolution.