Improved Manufacturing Techniques for Graphene Becomes a Research Priority
After a decade of characterizing graphene’s properties, attention is turning towards manufacturing it cheaply and effectively
One of the hurdles facing nanomaterials to achieve their promised impact is manufacturing them at a cost that would make them competitive with the materials they are intended to replace.
Now that we’re approaching the 15th anniversary of the National Nanotechnology Initiative (NNI) that helped kickoff the worldwide push to organize efforts in nanotechnology research so as to create the “next industrial revolution,” the status of so-called nanomanufacturing remains unclear.
Even one of the latest reports out of the Government Accountability Office (GAO) on the state of US nanomanufacturing primarily discusses policy issues and hardly touches upon the technologies and engineering that are needed to make nanomanufacturing a reality.
This is not the case for researchers and engineers who are constantly on the hunt to find improved manufacturing solutions for nanomaterials. In the past year, developing manufacturing techniques for graphene and other 2-D materials has been a preoccupation for researchers.
New Techniques for Improving Graphene Production
One of the biggest pushes has been to find ways to produce graphene in bulk quantities and still produce high-quality, polycrystalline single layers of graphene. One company late last year, Graphene Frontiers, announced funding from the National Science Foundation (NSF) to ramp up a new manufacturing technique for producing graphene in a roll-to-roll process. Graphene Frontiers’ technique is a variation on what is known as chemical vapor deposition (CVD).
CVD techniques take a number of forms but are essentially chemical processes in which a solid material in its gaseous form is sprayed onto a substrate (like copper in this case). When the solid materials in their gaseous form come in contact with the heated substrate inside a furnace (or reaction chamber), they turn into their solid form and are deposited on the substrate.
For graphene production, the CVD process results in a graphene film lying on top of copper or nickel. The problem has been pulling that graphene film away from the metal substrate without contaminating it or ruining it.
New CVD Techniques for Polycrystalline Graphene
Near the end of last year, we witnessed a sudden surge of research in which new techniques were announced that were aimed at easing that removal process. In December, researchers from the National University of Singapore (NUS) reported in the journal Nature a way in which the graphene did not need to be ripped off the surface of the copper. Instead the graphene was grown on copper, which itself sat on top of a silicon substrate. This meant that the copper could just be etched away through a wet-etching process, leaving the graphene layered on top of the silicon.
“The direct growth of graphene film on silicon wafer is useful for enabling multiple optoelectronic applications, but current research efforts remain grounded at the proof-of-concept stage,” says Professor Loh Kian Ping, who heads the Department of Chemistry at the NUS Faculty of Science, in a press release.
Researchers at the Massachusetts Institute of Technology (MIT) and the University of Michigan at the end of May developed a technique similar to the work out of NUS, but instead of relying on a wet etching process to remove the metal, the graphene is mechanically peeled off of the metal layer. The US researchers were able to achieve this by growing the graphene on both sides of the metal substrate.
The researchers believe that it could be superior to the NUS approach, which some believe could result in the graphene becoming contaminated and unable to produce large, uniform films.
One of the MIT researchers, A. John Hart, noted in the press release: “The ability to produce graphene directly on nonmetal substrates could be used for large-format displays and touch screens, and for ‘smart’ windows that have integrated devices like heaters and sensors.”
Samsung Raises the Bar With Single-Crystal Monolayer Graphene Production
While being able to produce single-layer polycrystalline graphene in bulk is an achievement, being able to produce single-crystal, monolayer graphene on the scale of a wafer actually opens graphene’s prospects in electronics applications.
In April, researchers at Samsung’s Advanced Institute of Technology reported in the journal Science a process for producing wafer-scale growth of wrinkle-free single-crystal monolayer graphene on a silicon wafer.
While there had been previous reports of producing single-crystal monolayer graphene, it was believed that method was too costly and could not lend itself to bulk-scale production.
Like the other techniques for producing graphene in bulk, the Samsung researchers used a CVD technique to produce the graphene wafer. The graphene was grown on the surface of a germanium-coated silicon wafer. The Korean researchers did not use a wet-etching process to transfer the graphene from the germanium. Instead, they deposited a thin film layer of gold on top of the graphene using a thermal evaporator. They then attached the gold/graphene/germanium sandwich to a thermal release tape and applied some pressure. This allowed the graphene and germanium substrate to easily separate.
Exfoliated Graphene Made in Your Kitchen
In April, a story grabbed a lot of headlines when researchers at Trinity College Dublin announced that it was possible to produce graphene using your kitchen blender.
The technique is essentially a liquid-phase exfoliation technique in which graphite is placed in a solvent and then mixed in a blender to chip off bits of graphene. The issue with graphene produced in this and other exfoliation methods like it is that they produce multi-layered graphene sheets that really don’t have a lot of the attractive electronic properties that are so sought after in graphene.
However, the researchers claim that is the fastest way to produce defect-free graphene. And, if you wanted, you could use your kitchen blender with dishwashing liquid as your solvent.