Researchers have revealed a new method to print thin metal oxide films at room temperature, enabling the creation of transparent, flexible circuits that are durable and can operate at elevated temperatures.
Researchers have revealed a new method to print thin metal oxide films at room temperature, enabling the creation of transparent, flexible circuits that are durable and can operate at elevated temperatures.
“Traditionally, producing useful metal oxides for electronics has relied on specialized machinery that is slow, costly, and requires high temperatures,” explains Michael Dickey, co-lead author of a study on this innovation and the Camille and Henry Dreyfus Professor of Chemical and Biomolecular Engineering at North Carolina State University. “Our aim was to create a method for fabricating and applying metal oxide thin films at room temperature, effectively printing metal oxide circuits.”
Metal oxides are crucial components in almost all electronic devices. While many metal oxides are electricity-insulating (like glass), some are both transparent and conductive, making them essential for the touch screens of smartphones and computer monitors.
“Theoretically, metal oxide films should be straightforward to produce,” Dickey states. “After all, they naturally form on the surfaces of nearly all metal items in our homes—like soda cans, stainless steel pots, and forks. Although these oxides are ubiquitous, their utility is limited since they can’t be extracted from the metals they form on.”
In this study, the team devised an innovative method to separate metal oxide from a meniscus of liquid metal. When a tube is filled with liquid, the meniscus is the curved surface that bulges beyond the tube’s end. This curve occurs due to surface tension, which prevents the liquid from spilling out entirely. For liquid metals, this meniscus is covered with a thin layer of metal oxide, which forms at the boundary of the liquid metal and air.
“We create a space between two glass slides filled with liquid metal, allowing a small meniscus to extend beyond the ends of the slides,” Dickey describes. “You can think of the slides as the printer and the liquid metal as the ink. The meniscus of liquid metal can then touch a surface. With oxide coating on all sides of the meniscus—similar to how rubber encases a water balloon—moving the meniscus across the surface allows the oxide to adhere and peel off, resembling a trail left by a snail. As this process occurs, the exposed liquid constantly produces new oxide, enabling uninterrupted printing.”
This method allows the printer to deposit a two-layer thin film of metal oxide that measures around 4 nm in thickness.
“It’s crucial to emphasize that even though we’re using a liquid, the metal oxide film that is applied to the substrate is solid and extremely thin,” Dickey points out. “The film firmly adheres to the substrate—it’s not something that can be easily smudged or smeared. This is vital for circuit printing.”
The team successfully utilized this technique with various liquid metals and alloys, where each metal changed the composition of the resulting metal oxide film. They were even able to create layered thin films by making multiple passes with the printer.
“One surprising discovery was that the printed films are transparent yet exhibit metallic characteristics,” Dickey notes. “They are highly conductive.”
“Due to the metallic nature of the films, gold adheres to the printed oxide, which is unusual—gold typically does not bond with oxides,” explains Unyong Jeong, co-lead author of the study and a professor of materials science and engineering at Pohang University of Science and Technology (POSTECH). “When gold is introduced in small amounts to these thin films, it effectively integrates into the film, helping to maintain the conductive properties of the oxide over time.”
“We believe the high conductivity of these films is due to the center of the two-layer thin film containing very little oxygen, resulting in a more metallic profile rather than an oxide,” Jeong adds. “Without gold, higher oxygen levels infiltrate the center of the layered thin film over time, causing it to become electrically insulating. Incorporating gold helps prevent oxidation in the center of the film. Remarkably, this works well even though we use very little gold—the oxide thin film remains highly transparent.”
Furthermore, the researchers discovered that the thin films maintained their conductive properties at high temperatures. For a 4 nanometer thick film, conductivity persists up to nearly 600 degrees Celsius. For a 12 nanometer thick film, the properties remain effective at least up to 800 degrees Celsius.
The researchers also showcased the practical applications of their technique by printing metal oxides onto a polymer, resulting in highly flexible circuits that maintained their integrity even after being flexed 40,000 times.
“These films can also be transferred onto other surfaces, like leaves, allowing electronics to be created in unique settings,” Dickey states. “We are currently maintaining the intellectual property on this technique and are open to collaborating with industry partners to investigate potential applications.”
