Water-Enabled Lithography Creates Long Graphene Nanoribbons

James Tour and his lab at Rice University have been tinkering with graphene nanoribbons (GNR) since they started unzipping carbon nanotubes to create them back in 2009. In the ensuing four years, they have been hard at work developing applications for the material, such as increasing the storage capacity of Li-ion batteries.

Tour, along with two of his graduate students,  Vera Abramova and Alexander Slesarev, have developed a method for producing GNRs that ensures the creation of long nanowires of the material simply by using water.

The research, which was published in the journal ACS Nano, (“Meniscus-Mask Lithography for Narrow Graphene Nanoribbons”),  essentially uses water as the mask in a lithography process that—when followed by ion etching—cuts up graphene into nanoribbons. The process does not require any high-resolution lithography tools—just atmospheric water collected at the edge of a lithography pattern.

Under the influence of surface tension water is forced to curve, forming a meniscus. Because the meniscus serves as the mask for the lithography, the researchers have dubbed the process: meniscus-mask lithography (MML).

Tour believes that because this process can generate long graphene nanoribbons it should be of interest to anyone working in microelectronics.

“They can never take advantage of the smallest nanoscale devices if they can’t address them with a nanoscale wire,” Tour said in the press release covering the research. “Right now, manufacturers can make small features, or make big features and put them where they want them. But to have both has been difficult. To be able to pattern a line this thin right where you want it is a big deal because it permits you to take advantage of the smallness in size of nanoscale devices.”

In ironic twist, the water that most lithography processes try avoid and eliminate at great cost is the same water that makes this new lithography process work.

“There are big machines that are used in electronics research that are often heated to hundreds of degrees under ultrahigh vacuum to drive off all the water that adheres to the inside surfaces,” Tour added in the release. “Otherwise there’s always going to be a layer of water. In our experiments, water accumulates at the edge of the structure and protects the graphene from the reactive ion etching (RIE). So in our case, that residual water is the key to success.

This counter-intuitive use of water in a lithography process, as one might suspect, was developed when another method was not working out as hoped.

Tour’s graduate students, Abramova and Slesarev, had actually intended to duplicate another process for creating GNRs that had been developed at Rice. This method, which is also new, exploits the ability of certain metals to form a native oxide layer. This layer expands and protects the material at the edge of the metal mask.

After observing their results with this method, they discovered that some metals didn’t expand as much as others and others showed no expansion at all. Desperate to find something that would change their results, the researchers worked on the project for two years before they tested and developed their meniscus theory. They confirmed in that time that MML method produces sub-10-nanometer wires from different materials, including platinum.

In further research the aim is to gain better control over the width of the nanoribbons and to better refine the edges of the nanoribbons, because it's those edges that dictate the nanoribbon's electronic properties.

Credit: Tour Group/Rice University

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