Research How a New Electronic "Skin" Makes Night Vision Goggles Lighter

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An ultra-thin electronic film could make electronic devices lighter without limiting their functions. This opens up possibilities for smart contact lenses and wearable sensor fabrics, as well as stretchable solar cells and flexible displays.

Engineers at the Massachusetts Institute of Technology (MIT) have developed a technique to create ultra-thin "skins" from electronic material. The method could drive the development of new electronic devices, such as ultra-thin, wearable sensors, flexible transistors and computing elements, as well as highly sensitive and compact imaging devices.

To demonstrate, the team created a thin membrane made of pyroelectric material. It is heat-sensitive and reacts to temperature changes by generating an electric current. The thinner the pyroelectric material, the better it can perceive subtle thermal fluctuations.

Thinnest Pyroelectric Membrane to Date

With their new method, the researchers have created the thinnest pyroelectric membrane to date with a thickness of ten nanometers and demonstrated that it reacts very sensitively to heat and radiation in the far-infrared range.

The newly developed film could, for example, lead to lighter, more portable, and highly precise far-infrared sensors, which could be used for night vision goggles and autonomous driving in fog. The current modern far-infrared sensors require bulky cooling elements. In contrast, the new pyroelectric thin film requires no cooling and responds to significantly lower temperature fluctuations. "The film significantly reduces weight and costs. It is lightweight, mobile, and easy to integrate," explains Xinyuan Zhang, a doctoral student at MIT's Department of Materials Science and Engineering (DMSE). "For example, it could be worn directly on glasses."

The heat-sensitive film could also be used in environmental and biosensing, as well as in the imaging of astrophysical phenomena that emit far-infrared radiation. Furthermore, the new lift-off technique can be generalized beyond pyroelectric materials. The researchers plan to apply the method to produce other ultra-thin, high-performance semiconductor films.

Researchers Experiment with Semiconductor Elements

Their results were published in an article in the journal Nature. The MIT co-authors of the study include lead author Xinyuan Zhang, Sangho Lee, Min-Kyu Song, Haihui Lan, Jun Min Suh, Jung-El Ryu, Yanjie Shao, Xudong Zheng, Ne Myo Han, and Jeehwan Kim, associate professor of mechanical engineering and materials science and engineering, along with researchers from the University of Wisconsin in Madison led by Professor Chang-Beom Eom and authors from several other institutions.

Kim is seeking new ways to create smaller, thinner, and more flexible electronics. Such ultra-thin computer "skins" are envisioned by researchers to be built into everything in the future—from smart contact lenses and wearable sensor fabrics to stretchable solar cells and flexible displays. Kim and his colleagues are experimenting with semiconductor elements to produce ultra-thin, multifunctional electronic thin-film membranes.

A method in which Kim has pioneered is "remote epitaxy"—a technique where semiconductor materials are grown on a single-crystal substrate with an ultra-thin layer of graphene in between. The crystal structure of the substrate serves as a framework on which the new material can grow. Graphene acts like a non-stick layer, similar to Teflon, allowing researchers to easily peel off the new film and transfer it onto electronic devices. After removing the new layer, the underlying substrate can be reused for producing additional layers.

Using remote epitaxy, Kim has produced thin films with different properties. While experimenting with various combinations of semiconductor elements, the researchers discovered that a certain pyroelectric material, called PMN-PT, does not require an intermediate layer to detach from its substrate. By growing PMN-PT directly on a single-crystal substrate, the researchers were able to remove the grown film without the delicate lattice cracking. "It worked amazingly well," says Zhang. "We found that the peeled film is atomically smooth."

Pyroelectric Approach Versus Photodetectors

In their new study, researchers from MIT and UW Madison took a closer look at the process and found that the property of easy peeling is mainly due to lead atoms. Together with colleagues at Rensselaer Polytechnic Institute, the team discovered that the pyroelectric film, as part of its chemical structure, contains an arrangement of lead atoms that have a high "electron affinity." This means that lead attracts electrons and prevents the charge carriers from migrating and bonding with other materials, such as an underlying substrate. The lead acts like a tiny non-stick coating, allowing the material to peel off completely intact.

The team executed the idea and produced several ultra-thin films of PMN-PT, each about ten nanometers thick. They peeled off the pyroelectric films and transferred them onto a small chip to form an array of 100 ultra-thin heat-sensitive pixels, each approximately 60 square micrometers (about 0.006 square centimeters) in size. They subjected the films to ever smaller temperature changes and found that the pixels were very sensitive to small changes in the far-infrared spectrum.

The sensitivity of the pyroelectric array is comparable to that of modern night vision devices. These devices currently rely on photodetector materials, where a temperature change causes the material's electrons to make an energy leap, allowing them to cross an energy "bandgap" temporarily before returning to their ground state. This electron jump serves as an electric signal for the temperature change. However, this signal can be affected by noise in the environment. To avoid such effects, photodetectors must also include cooling devices that bring the instruments down to the temperature of liquid nitrogen.

Current night vision goggles and scopes are heavy and bulky. With the new pyroelectric approach, night vision devices could have the same sensitivity without being too heavy.

We envision our ultra-thin films being processed into high-performance night vision goggles, as they exhibit a wide spectrum of infrared sensitivity at room temperature, allowing for a lightweight design without a cooling system.

Xinyuan Zhang, MIT

Other Possible Applications—e.g., in Vehicles

The scientists also found that the pyroelectric film can respond to wavelengths across the entire infrared spectrum. It is suggested that the films could thus be incorporated into small, lightweight, and portable devices for various applications requiring different infrared ranges. For instance, the films could be integrated into autonomous vehicle platforms, enabling the vehicles to "see" pedestrians and other vehicles in complete darkness or during fog and rain.

The film could also be used in gas sensors for real-time and on-site environmental monitoring, helping to detect pollutants. In electronics, it could monitor thermal changes in semiconductor chips to identify signs of malfunction early.

The team states that the new lift-off method can also be applied to materials that do not contain lead themselves. In these cases, the researchers speculate that they could introduce Teflon-like lead atoms into the underlying substrate to achieve a similar peel-off effect. Currently, the team is actively working on integrating the pyroelectric films into a functioning night vision system.

"We envision that our ultra-thin films can be processed into high-performance night vision goggles, as they exhibit a broad spectrum of infrared sensitivity at room temperature, allowing for a lightweight design without a cooling system," says Zhang. "To transform this into a night vision system, a functioning device array with a readout circuit should be integrated. Additionally, testing under various environmental conditions is essential for practical applications."

The work was supported by the U.S. Air Force Office of Scientific Research.

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