New coating for graphene simplifies manufacturing

13th March 2019

Graphene is an atom-thin material that holds promise for various uses. Researchers are exploring possibilities for using it in circuits for flexible electronics and quantum computers, and in a variety of other devices. It has also been touted in uses as diverse as water filtration and mining. One recent University of Manchester project even used it to turn whisky clear

To protect it from performance-impairing wrinkles and contaminants that mar its surface during device fabrication, MIT researchers have turned to an everyday material: wax.

Removing the fragile material from the substrate it’s grown on and transferring it to a new substrate is particularly challenging. Traditional methods encase the graphene in a polymer that protects against breakage but also introduces defects and particles onto graphene’s surface. These interrupt electrical flow and stifle performance.

In experiments, the MIT researchers’ wax-coated graphene performed four times better than graphene made with a traditional polymer-protecting layer. Performance, in this case, is measured in electron mobility — meaning how fast electrons move across a material’s surface — which is hindered by defects.

“Like waxing a floor, you can do the same type of coating on top of large-area graphene and use it as layer to pick up the graphene from a metal growth substrate and transfer it to any desired substrate,” said Wei Sun Leong, a postdoc in the Department of Electrical Engineering and Computer Science (EECS). “This technology is very useful, because it solves two problems simultaneously: the wrinkles and polymer residues.”

Haozhe Wang, a PhD student in EECS, said using wax may sound like a natural solution, but it involved some thinking outside the box — or, more specifically, outside the laboratory: “As students, we restrict ourselves to sophisticated materials available in lab. Instead, in this work, we chose a material that commonly used in our daily life.”

The protector

To grow graphene over large areas, the 2D material is typically grown on a commercial copper substrate. Then, it’s protected by a sacrificial polymer layer, typically polymethyl methacrylate (PMMA). The PMMA-coated graphene is placed in a vat of acidic solution until the copper is completely gone. The remaining PMMA/graphene is rinsed with water, then dried, and the PMMA layer is ultimately removed.

Wrinkles occur when water gets trapped between the graphene and the destination substrate, which PMMA doesn’t prevent. Moreover, PMMA comprises complex chains of oxygen, carbon, and hydrogen atoms that form strong bonds with graphene atoms. This leaves behind particles on the surface when it’s removed.

Researchers have tried modifying PMMA and other polymers to help reduce wrinkles and residue, but with minimal success. The MIT researchers instead searched for completely new materials — even once trying out commercial shrink wrap. “It was not that successful, but we did try,” Wang said, laughing.

After combing through materials science literature, the researchers landed on paraffin, the common whitish, translucent wax used for candles, polishes, and waterproof coatings, among other applications.

In simulations before testing, the group, which studies the properties of materials, found no known reactions between paraffin and graphene, due to paraffin’s very simple chemical structure. “Wax was so perfect for this sacrificial layer. It’s just simple carbon and hydrogen chains with low reactivity, compared to PMMA’s complex chemical structure that bonds to graphene,” Leong observed.



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