New technique offers more choice for 2D nanomaterials

A group of materials scientists from the University of Wisconsin (UW)-Madison believe that the tiny sheets of the semiconductor zinc oxide they're growing could have huge implications for the future of a host of electronic and biomedical devices.

The group – led by Xudong Wang, a UW-Madison professor of materials science and engineering, and postdoctoral researcher Fei Wang – has developed a technique for creating nearly two-dimensional (2D) sheets from compounds that do not naturally form such thin materials. It is the first time such a technique has been successful and the researchers describe it in a paper in Nature Communications.

Essentially the microscopic equivalent of a single sheet of paper, a 2D nanosheet is just a few atoms thick. The electronic and chemical properties of these nanomaterials differ from the properties of the same materials at larger, conventional scales. "What's nice with a 2D nanomaterial is that because it's a sheet, it's much easier for us to manipulate compared to other types of nanomaterials," says Xudong Wang.

Until now, materials scientists were limited to working with naturally occurring 2D nanosheets, which include graphene, a single layer of graphite, and a limited number of other compounds. Developing a reliable method to synthesize and manufacture 2D nanosheets from other materials has been a goal of materials researchers and the nanotechnology industry for years.

In their new technique, the UW-Madison team applied a specially-formulated surfactant – a detergent-like substance – onto the surface of a liquid containing zinc ions. Due to its chemical properties, the surfactant assembles itself into a single layer at the surface of the liquid, with negatively charged sulfate ions pointing down into the liquid. These sulfate ions attract the positively-charged zinc ions from within the liquid to the surface, and within a couple of hours enough zinc ions are drawn up to form continuous zinc oxide nanosheets only a few atomic layers thick.

Xudong Wang first had the idea for using a surfactant to grow nanosheets during a lecture he was giving in a course on nanotechnology in 2009. "The course includes a lecture about self-assembly of monolayers," he explains. "Under the correct conditions, a surfactant will self-assemble to form a monolayer. This is a well-known process that I teach in class. So while teaching this, I wondered why we wouldn't be able to reverse this method and use the surfactant monolayer first to grow the crystalline face."

After five years of trial and error with different surfactant solutions, the idea paid off. "We are very excited about this," says Xudong Wang. "This is definitely a new way to fabricate 2D nanosheets, and it has great potential for different materials and for many different applications."

Already, the researchers have found that their 2D zinc oxide nanosheets are able to function as a type of semiconductor transistor known as a p-type, which displays the opposite electronic behavior of naturally-occurring zinc oxide. Researchers have for some time attempted to produce zinc oxide with reliable p-type semiconductor properties.

Zinc oxide is a very useful component for electronic materials, and the new nanosheets have potential for use in sensors, transducers and optical devices. But the zinc oxide nanosheets are only the first of what could be a revolution in 2D nanomaterials. Already, the UW-Madison team is applying its surfactant method to growing 2D nanosheets of gold and palladium, and the technique holds promise for growing nanosheets from all sorts of metals that wouldn't form them naturally.

"It brings a lot of new functional material to this 2D material category," Xudong Wang says.

This story is adapted from material from the University of Wisconsin-Madison, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.