Researchers have developed a method for printing thin metal oxide films at room temperature. This approach has been successfully employed to produce transparent and flexible circuits that are both robust and capable of operating at high temperatures.
“Creating metal oxides that are useful for electronics has traditionally required making use of specialized equipment that is slow, expensive, and operates at high temperatures,” says Michael Dickey, co-corresponding 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 create and deposit metal oxide thin films at room temperature, essentially printing metal oxide circuits.”
The Role of Metal Oxides in Electronics
Metal oxides represent a significant material category, ubiquitous in the majority of electronic devices. The majority of metal oxides exhibit insulating properties, akin to those observed in glass. However, some metal oxides exhibit both conductive and transparent properties, which are essential for the functionality of touchscreens in smartphones and computer monitors.
“In principle, metal oxide films should be easy to make,” Dickey says. “After all, they form naturally on the surface of nearly every metal object in our homes – soda cans, stainless steel pots, and forks. Although these oxides are everywhere, they are of limited use since they can’t be removed from the metals they form on.”
In order to achieve this objective, the research team devised an innovative methodology for the separation of metal oxide from a meniscus of liquid metal. In the case of a tube filled with liquid, the meniscus is defined as the curved surface of the liquid that extends beyond the end of the tube. The curvature is a result of surface tension, which prevents the liquid from spilling out completely. In the case of liquid metals, the surface of the meniscus is coated with a thin layer of metal oxide, which forms where the liquid metal meets the air.
“We fill the space between two glass slides with liquid metal so that a small meniscus extends beyond the ends of the slides,” Dickey says. “Think of the slides as the printer, and the liquid metal is the ink. The meniscus of liquid metal can then be brought into contact with a surface. The meniscus is covered with oxide on all sides, analogous to the thin rubber that encases a water balloon. When 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 behind by a snail. As this happens, the exposed liquid on the meniscus constantly forms fresh oxide to enable continuous printing.”
Creating Thin and Solid Metal Oxide Films
The outcome is the deposition of a two-layer thin film of metal oxide, with an approximate thickness of 4 nm.
“It’s important to note that even though we use a liquid, the metal oxide film deposited on the substrate is solid and incredibly thin,” Dickey says. “The film adheres to the substrate – it’s not something you could smudge or smear. That’s important for printing circuits.”
The researchers demonstrated the efficacy of this technique with a number of liquid metals and metal alloys, noting that each metal had a distinct impact on the composition of the metal oxide film. Furthermore, the researchers demonstrated the ability to deposit a stack of layered thin films by making multiple passes with the printer.
“One of the things we found surprising was that the printed films are transparent but have metallic properties,” Dickey says. “They are highly conductive.”
“Because the films have a metallic character, gold bonds to the printed oxide, which is unusual – gold normally doesn’t stick to oxides,” says Unyong Jeong, co-corresponding author of a paper on the work and a professor of materials science and engineering at Pohang University of Science and Technology (POSTECH). “When you introduce a small amount of gold to these thin films, the gold is essentially incorporated into the film. This helps prevent the conductive properties of the oxide from degrading over time.”
“We think these films are so conductive because the center of the two-layer thin film contains very little oxygen, it’s more metallic and less of an oxide,” Jeong says. “Without the presence of gold, more oxygen makes its way to the center of the layered thin film over time, which causes the film to become electrically insulating. Adding gold to the thin film helps prevent the central part of the film from oxidizing. The fact that this works so well is surprising because we’re using so little gold – the oxide thin film is still highly transparent.”
High Temperature and Flexibility Performance
Furthermore, the researchers discovered that the thin films retained their conductive properties at elevated temperatures. A thin film with a thickness of 4 nanometers has been observed to retain its conductive properties up to temperatures of approximately 600 degrees Celsius. A film of 12 nanometers in thickness is capable of retaining its conductive properties up to a temperature of at least 800 degrees Celsius.
Furthermore, the researchers illustrated the efficacy of their methodology by printing metal oxides onto a polymer, resulting in highly flexible circuits that demonstrated resilience to folding, retaining their structural integrity even after 40,000 cycles.
“The films can also be transferred to other surfaces, such as leaves, to create electronics in unconventional places,” Dickey says. “We’re preserving the intellectual property on this technique and are open to working with industry partners to explore potential applications.”
- See also: Toward a code-breaking quantum computer
Reference: “Ambient printing of native oxides for ultrathin transparent flexible circuit boards” by Minsik Kong, Man Hou Vong, Mingyu Kwak, Ighyun Lim, Younghyun Lee, Seong-hun Lee, Insang You, Omar Awartani, Jimin Kwon, Tae Joo Shin, Unyong Jeong and Michael D. Dickey, 15 August 2024, Science.
DOI: 10.1126/science.adp3299
This work was done with support from the National Research Foundation of Korea, funded by the Ministry of Science, under grants 2022M3C1A3081359 and RS-2024-00338686.
