Breakthrough technique at NC State creates flexible, nanoscale, transparent circuits using printed metal oxides.

Imagine printing transparent circuits at room temperature – no heat, no fancy equipment – just a trail of liquid metal creating a conductive masterpiece. In a breakthrough that feels too simple to be true, scientists have cracked the code on how to print metal oxide films that are both transparent and incredibly durable, opening the door to flexible, high-performance electronics.

In the world of electronics, while many think of semiconductors, circuit boards, or transistors, there's another material that quietly works behind the scenes – metal oxides. Most metal oxides are electrically insulating (like glass) and are crucial for transparent electronics, enabling technologies like touch screens and displays.

"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," said 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."

Traditionally, creating metal oxides requires high heat and specialized equipment, but a recent discovery may have changed the game.

"In principle, metal oxide films should be easy to make," said Dickey. "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."

Flickr.com; Alex Brown

A glass of water nearly overflowing, showcasing the meniscus held in place by surface tension.

To tap into this natural phenomenon, a team at North Carolina State University devised a method to harness meniscus tension to separate the metal oxide layer from the liquid.

If you fill a glass with liquid, a meniscus is the curved surface of the liquid that extends beyond the rim of the glass. It curves because surface tension prevents the liquid from spilling out completely. In the case of liquid metals, the surface of the meniscus is covered with a thin metal oxide skin that forms where the liquid metal meets air.

Once separated, it was simply a matter of depositing the metal oxide film onto a new surface, much like a printer would deposit ink onto a piece of paper.

"We fill the space between two glass slides with liquid metal so that a small meniscus extends beyond the ends of the slides," said Dickey. "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."

The result is that the printer lays down a two-layer thin film of metal oxide that is approximately four nanometers thick.

"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," the research lead said. "The film adheres to the substrate – it's not something you could smudge or smear. That's important for printing circuits."

The researchers demonstrated that different liquid metals and alloys can alter the composition of the metal oxide film, and by making multiple passes with the printer, they could stack these films into layered, thin structures.

More incredible finds

Even after their remarkable discovery, the research team encountered further surprises: these newly printed films, while transparent, exhibited metallic properties, making them 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," said 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."

This incorporation of gold not only enhances the film's conductive properties but also ensures that these properties remain stable over time. The team believes that this remarkable conductivity stems from the unique composition of the thin films.

"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," said Jeong. "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."

Further, the researchers found that the thin films retained their conductivity even at high temperatures. For example, if the thin film is four nanometers thick, it would retain its conductivity up to almost 600 degrees Celsius (1,112 degrees Fahrenheit). If the thin film is 12 nanometers thick, it would retain its conductive properties up to at least 800 degrees Celsius (1,472 degrees Fahrenheit).

The researchers also demonstrated the utility of their technique by printing metal oxides onto a polymer, creating highly flexible circuits that were robust enough to retain their integrity even after being folded 40,000 times.

"The films can also be transferred to other surfaces, such as leaves, to create electronics in unconventional places," said Dickey. "We're preserving the intellectual property on this technique and are open to working with industry partners to explore potential applications."

This groundbreaking technique stands to revolutionize the electronics industry, enabling the creation of more adaptable, resilient, and integrated devices. With the potential to enhance everything from wearable tech to transparent displays, this innovation could redefine electronic design, pushing the boundaries of what's possible and promising a new era of creativity and functionality in electronics.