Mar 7 2019
To safeguard graphene from performance-weakening wrinkles and contaminants that deface its surface during device fabrication, MIT scientists have sought the use of an ordinary material: wax.
The image on the right shows a graphene sheet coated with wax during the substrate-transfer step. This method drastically reduced wrinkles on the graphene’s surface compared to a traditional polymer coating (left). (Courtesy of the researchers)
Graphene, an atom-thin material, shows great potential for use in the creation of next-generation electronics. Scientists are investigating possibilities for using the unique material in circuits for quantum computers and flexible electronics, and in a range of other devices.
But taking the delicate material from the substrate it has been grown on and moving it to a new substrate is very challenging. Traditional techniques wrap the graphene in a polymer that defends against breakage but also adds particles and defects onto graphene’s surface. These hinder electrical flow and suffocate performance.
In a paper published in Nature Communications, the scientists describe a fabrication method that applies a coating of wax to a graphene sheet and heats it up. Heat makes the wax to expand, which evens out the graphene to lessen wrinkles. Furthermore, the coating can be rinsed away without leaving behind a lot of residues.
In experiments conducted by the team, the wax-coated graphene did four times better than graphene made with a traditional polymer-protecting layer. Performance, in this case, is measured in “electron mobility”—meaning how quickly electrons travel across a material’s surface—which is obstructed by surface 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. This technology is very useful, because it solves two problems simultaneously: the wrinkles and polymer residues.
Wei Sun Leong, Study First Author and Postdoc, Department of Electrical Engineering and Computer Science (EECS), MIT.
Co-first author Haozhe Wang, a PhD student in EECS, says using wax could sound like a natural solution, but it required thinking differently: “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.”
Besides Leong and Wang, the other authors of the paper are: Jing Kong and Tomas Palacios, both EECS professors; Markus Buehler, professor and head of the Department of Civil and Environmental Engineering (CEE); and six other graduate students, postdocs, and researchers from CEE, EECS, and the Department of Mechanical Engineering.
The “perfect” protector
To grow graphene across large areas, the 2D material is usually grown on a commercial copper substrate. Then, it is secured by a “sacrificial” polymer layer, normally polymethyl methacrylate (PMMA). The PMMA-coated graphene is positioned in a vat of acidic solution until the copper is fully gone. The remaining PMMA-graphene is washed with water, then dried, and the PMMA layer is finally removed.
Wrinkles creep in when water gets caught between the graphene and the destination substrate, which PMMA does not stop. Besides, PMMA consists of complex strings of oxygen, hydrogen, and carbon atoms that form powerful bonds with graphene atoms. This leaves behind particles on the surface when it is removed.
Scientists have tried altering PMMA and other polymers to help minimize wrinkles and residue, but with little success. The MIT scientists instead hunted for totally new materials—even once using commercial shrink wrap. “It was not that successful, but we did try,” Wang says, amused.
After searching through materials science literature, the team chose paraffin, the widely used whitish, translucent wax used for polishes, candles, and waterproof coatings, among other applications.
In simulations prior to testing, Buehler’s group, which examines the properties of materials, discovered no known reactions between graphene and paraffin. That is because of paraffin’s highly basic 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.
Wei Sun Leong, Study First Author and Postdoc, Department of Electrical Engineering and Computer Science (EECS), MIT.
Cleaner transfer
In their method, the scientists initially melted small pieces of the paraffin in an oven. Then, using a spin coater, a microfabrication machine that applies centrifugal force to evenly spread material across a substrate, they put the paraffin solution onto a graphene sheet grown on copper foil. This spread the paraffin into a protective layer, approximately 20 microns thick, across the graphene.
The scientists moved the paraffin-coated graphene into a solution that takes away the copper foil. The coated graphene was then moved to a traditional water vat, which was heated to about 40 °C. They used a silicon destination substrate to take out the graphene from underneath and baked in an oven fixed to the same temperature.
As paraffin has a high thermal expansion coefficient, it expands quite a bit when heated. Exposed to this heat increase, the paraffin expands and stretches the attached graphene beneath, effectively decreasing wrinkles. To finish, the scientists used a different solution to rinse away the paraffin, leaving a single layer of graphene on the destination substrate.
In their paper, the scientists exhibit microscopic images of a small area of the paraffin-coated and PMMA-coated graphene. Paraffin-coated graphene is nearly completely clear of debris, while the PMMA-coated graphene appears severely damaged, similar to a scratched window.
As wax coating is already widely used in a number of manufacturing applications—such as applying a waterproof coating to a material—the team thinks their technique could be easily adapted to practical fabrication processes. Particularly, the increase in temperature to melt the wax should not influence efficiency or fabrication costs, and the heating source could later be substituted with a light, the scientists explain.
Going forward, the scientists want to further lessen the wrinkles and impurities left on the graphene and expanding the system to handle larger sheets of graphene. They are also aiming at applying the transfer method to the fabrication processes of other 2D materials.
We will continue to grow the perfect large-area 2D materials, so they come naturally without wrinkles.
Wei Sun Leong, Study First Author and Postdoc, Department of Electrical Engineering and Computer Science (EECS), MIT.