Nam’s graphene origami improves chemical detection for soldiers
Soldiers often need to see through smoke, fog, dust, or any other airborne obscurant and detect the presence of toxins or other chemicals in the field or on the front lines. To identify those chemicals, they use infrared (IR) sensors and spectroscopy, which allow a specific color of light to shine at a particular frequency corresponding to each chemical. Identifying each chemical requires a soldier to coat the goggle with a unique filter, enabling the chemical signature to come through at a specific frequency (i.e., a specific color).
MechSE Assistant Professor SungWoo Nam, however, has successfully developed a tunable infrared filter made from graphene, which would allow a solider to change the frequency of a filter simply by controlled mechanical deformation of the filter (i.e., graphene origami), instead of by replacing the substance on the goggles used to filter a spectrum of colors.
The research is funded by the Air Force Office of Scientific Research, which is interested in sensors that are not only sensitive to different IR wavelengths, but that are also mechanically controllable and tunable. The results are published in a paper titled “Mechanically Reconfigurable Architectured Graphene for Tunable Plasmonic Resonances” in Light: Science & Applications.
The application from Nam is another in a series of his discoveries related to the “wonder material,” graphene.
“Typically when you place graphene on a substrate, it is extremely transparent and absorbs only about three percent of light,” Nam said. “At certain angles, you can see it. We use this versatility to make other structures like flexible and transparent sensors out of graphene.”
Because it’s one-atom thin, graphene is normally used while flat. Nam’s research team asked several questions: What would happen if, through origami, we wrinkled the graphene? Could we change the properties of graphene by altering its topography?
Nam said scientists haven't tried this idea before with other conventional materials because they are brittle and break when bent. Graphene is not only thin, but also resilient.
“Let's say we create graphene wrinkles by mechanical deformation,” Nam said. “If you get a certain dimension, will there be any changes in the way the light will be absorbed by the graphene? We wanted to link the dimensions of the wrinkled graphene to its optical absorption.”
Nam’s team discovered that wrinkled graphene does absorb light differently depending on the structure and dimensions through plasmonic resonances, thus producing different colors. In addition, unlike paper, which can't easily be flattened after folding or crumpling, graphene can be re-stretched to become flat and wrinkle-free again. The amount of light absorption also can be altered by a factor of approximately 10.
“By changing the shape, you can absorb the light of a different frequency by controlling plasmonic resonance conditions,” said Pilgyu Kang, first author of the paper and now an assistant professor of mechanical engineering at George Mason University. “And by mechanically controlling the height and wavelength of the graphene wrinkles, I can excite different surface plasmons and thus absorb different frequency. At the end of the day, you get a tunable filter.”
By choosing graphene as a filter for infrared goggles, the user can turn a knob to mechanically stretch and compress the graphene. That allows for a change of the light wavelength being absorbed. As an example of its application, a solider can easily tune the graphene filter to a desired wavelength to match the desired chemical.
“In a conventional filter, once you make the filter, you are done,” Nam said. “No matter the size, there is one unique light wavelength. With graphene, depending on how much you stretch and release, you can communicate in different light wavelengths.”
This work is based on an international collaboration with Dr. Kyoung-Ho Kim and Professor Hong-Gyu Park at Korea University, and is supported by the AFOSR and National Science Foundation.