Researchers from MIT, University of Wisconsin-Madison, Rensselaer Polytechnic Institute, University of Louisville, University of Illinois Urbana−Champaign, Yonsei University and Seoul National University have developed a technique for peeling ultra-thin crystalline electronic membranes away from their substrates to facilitate the high-throughput production of scalable, ultrathin, freestanding perovskite systems. The team used thin film membranes developed in their experiments to create a record-breaking infrared-detecting sensor that could be used in night vision eyewear or autonomous vehicles.
Study author Chang-Beom Eom, a professor of materials science and engineering at the University of Wisconsin-Madison, is an expert in crystalline perovskite oxides, containing oxygen and, typically, transition metals in a distinct atomic arrangement. These materials are particularly stable and strong when produced as thin films and can be precisely engineered at the atomic level. They also offer a wide range of tunable functions, including superconductivity, oxygen catalysis, magnetoresistance, and insulating behaviors. If incorporated into next-generation devices, these films could lead to a whole range of new gadgets, including improved fuel cells, field-effect transistors, spintronic-based memory devices and a wide range of detectors.
Until now, however, it has not been possible to produce membranes of oxide films at an industrial scale. That’s because they are typically grown on a substrate, or base, that helps determine their epitaxial atomic arrangement and properties. Efficiently detaching these delicate thin films from their substrates has proved challenging.
Previously, researchers placed a sacrificial layer between the substrate and the crystalline film. Then they chemically etched away that layer, releasing the crystalline membrane. The process, however, is a balancing act: It’s very slow and can cause the films to crack. As a result, it’s not practical for large crystals or on an industrial scale.
Another technique, placing a layer of graphene between the crystal and substrate to act as an atomic-scale non-stick coating, also faces hurdles and scalability issues.
That’s why Eom and colleagues decided to try a completely different method. Using theoretical techniques and lessons learned from previous experiments, they determined that lead atoms in some complex perovskite oxides can bind up electrons, weakening the atomic bonds between the films and the substrate. They concluded that by incorporating lead into the oxide materials, they could lift or peel the films directly off their substrates without damaging them.
To test the idea, the team grew a lead-containing oxide crystal, PMN-PT, on a common substrate. Indeed, as predicted, the thin film detached perfectly. The analysis showed the crystal was atomically smooth and an exceptionally thin 10 nanometers thick. Because there were no intermediary sacrificial layers to contaminate it, the film was also purer than films produced using other techniques.
Even more, PMN-PT has a practical application; the researchers found their ultra-thin lead-based film has a record-high pyroelectric coefficient, meaning it’s ideal for infrared sensing.
The team then grew multiple batches of the film, affixing 100 squares of the material to a small chip, using each as a heat-sensitive pixel. They found that the sensitivity of the array was on par with current state-of-the art liquid-nitrogen-cooled night-vision devices and could detect an even wider swath of the infrared spectrum. Even more, it worked at room temperature, with no need to cool the sensor to -300 Fahrenheit like the other leading pyroelectric materials.
That means the film could be integrated into portable, lightweight devices for a range of applications, including real-time environmental monitoring or to keep tabs on the heat of semiconductor chips.
Eom says that, besides identifying an exciting new type of ultrathin membrane, the research demonstrates a path for incorporating other complex oxide crystals into real-world devices. The team hopes that this work will help them identify or engineer other oxides with “peel off” properties or to develop a way to incorporate lead atoms into the substrate to keep heavy metals out of finished films.
