Researchers have demonstrated a technique for printing thin metal oxide films at room temperature and used this technique to create transparent, flexible circuits that are both robust and able to function at high temperatures.
“Previously, making metal oxides useful for electronics required the use of specialized equipment that is slow, expensive, and operates at high temperatures,” says Michael Dickey, co-author of a paper on the work and the Camille and Henry Dreyfus Professor of Chemical and Biomolecular Engineering at North Carolina State University. “We wanted to develop a technique to make and deposit thin metal oxide films at room temperature, essentially printing metal oxide circuits.”
Metal oxides are an important material found in almost every electronic device. Most metal oxides are electrically insulating (like glass). However, some metal oxides are both conductive and transparent, and these oxides are crucial for the touchscreen of your smartphone or the monitor of your computer.
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“In principle, it should be easy to create metal oxide films,” says Dickey. “After all, they form naturally on the surface of almost all metal objects in our homes – soda cans, stainless steel pots and forks. Although these oxides are everywhere, they are of limited use because they cannot be removed from the metals on which they form.”
For this work, the researchers developed a novel method to separate metal oxide from a meniscus of liquid metal. When you fill a tube with liquid, a meniscus is the curved surface of the liquid that extends beyond the end of the tube. The curvature is due to surface tension, which prevents the liquid from completely draining out. In liquid metals, the surface of the meniscus is covered with a thin metal oxide skin that forms where the liquid metal meets the air.
“We fill the space between two slides with liquid metal, leaving a small meniscus extending beyond the ends of the slides,” explains Dickey. “Think of the slides as printers and the liquid metal as ink. The liquid metal meniscus can then be brought into contact with a surface. The meniscus is covered with oxide on all sides, analogous to the thin rubber that covers a water balloon. As we move the meniscus across the surface, the metal oxide on the front and back of the meniscus sticks to the surface and peels off, like the trail left by a snail. As we do so, the exposed liquid on the meniscus continually forms fresh oxide to enable continuous printing.”
The result is that the printer deposits a two-layer thin film of metal oxide with a thickness of about 4 nm.
“It’s important to note that the metal oxide film deposited on the substrate is solid and incredibly thin, even though we’re using a liquid,” says Dickey. “The film sticks to the substrate – you can’t smudge or smear it. This is important for printing circuits.”
The researchers demonstrated this technique using multiple liquid metals and metal alloys, with each metal changing the composition of the metal oxide film. The researchers were also able to deposit a stack of multilayered thin films by making multiple passes with the printer. A video of the technique can be found at https://youtu.be/yT7Jzw5Ak2U?si=mqOBoWn4dKnApWgw.
“One of the things that surprised us was that the printed films, although transparent, have metallic properties,” says Dickey. “They are highly conductive.”
“Because the films have a metallic character, gold bonds to the printed oxide, which is unusual – usually gold does not stick to oxides,” says Unyong Jeong, co-author of a paper on the work and a professor of materials science and engineering at Pohang University of Science and Technology (POSTECH). “By adding a small amount of gold to these thin films, the gold is essentially incorporated into the film. This prevents the conductivity of the oxide from deteriorating over time.”
“We believe these films are so conductive because the center of the two-layer thin film contains very little oxygen, it is more metallic and less oxide,” says Jeong. “Without the presence of gold, more oxygen gets into the center of the multilayer thin film over time, making the film electrically insulating. Adding gold to the thin film helps prevent oxidation of the central part of the film. That this works so well is surprising, since we use so little gold – the oxide thin film is still highly transparent.”
In addition, the researchers found that the thin films retain their conductivity even at high temperatures. If the thin film is four nanometers thick, it retains its conductivity up to almost 600 degrees Celsius. If the thin film is 12 nanometers thick, it retains its conductivity up to at least 800 degrees Celsius.
The researchers also demonstrated the usefulness of their technique by printing metal oxides onto a polymer, creating highly flexible circuits that were robust enough to maintain their integrity even after 40,000 folds.
“The films can also be transferred to other surfaces, such as leaves, to create electronics in unconventional places,” says Dickey. “We retain the intellectual property on this technique and are open to collaborating with industry partners to explore potential applications.”
Reference: Kong M, Vong MH, Kwak M et al. Ambient printing of native oxides for ultra-thin transparent flexible circuit boards. Science. 2024;385(6710):731-737. doi: 10.1126/science.adp3299
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