With a tensile strength of 200 times steel, graphene is one of the strongest materials of its kind. However, there are a two key issues with this unique material comprised of a carbon layer as thick as an atom: the price tag and the time it takes to develop graphene-based solar cells. That could change according to a new Caltech study.
"With this new technique, we can grow large sheets of electronic-grade graphene in much less time and at much lower temperatures," says David Boyd, the Caltech staff scientist who created the new method.
Boyd’s study, which was published on March 18 in the Nature Communications journal, claims a new method can drastically reduce the time it takes to produce strain-free graphene.
The disadvantage of high temperatures
"Previously, people were only able to grow a few square millimeters of high-mobility graphene at a time, and it required very high temperatures, long periods of time, and many steps," says Nai-Chang Yeh, the corresponding author of the study. "Our new method can consistently produce high-mobility and nearly strain-free graphene in a single step in just a few minutes without high temperature. We have created sample sizes of a few square centimeters, and since we think that our method is scalable, we believe that we can grow sheets that are up to several square inches or larger, paving the way to realistic large-scale applications."
Today, it typically takes 1,800 degrees Fahrenheit (or 1,000 degrees Celsius) to produce greaphene that can be used for engineering purposes. This process tends to deform the graphene, which in turn compromises its intrinsic properties.
An accidental discovery
Boyd admitshe discovered this new manufacturing method practically by chance. Back in 2012, when the researcher worked as a professor of mechanical engineering and applied physics, he attempted to reproduce a graphene manufacturing process he had heard about, which utilizes heated copper.
"I was playing around with it on my lunch hour," shares Boyd. "But the recipe wasn't working. It seemed like a very simple process. I even had better equipment than what was used in the original experiment, so it should have been easier for me."
Boyd picked up his phone during the experiment and as a result, he let a copper foil heat for longer than intended. During the examination of the copper plate, he realized a graphene layer had formed. "It was an 'A-ha!' moment," he says. "I realized then that the trick to growth
is to have a very clean surface, one without the copper oxide."
This method isn’t necessarily new. In fact, the system Boyd utilized was developed more than 50 years ago to generate hydrogen plasma (electrified hydrogen gas) with the purpose of removing copper oxide at a lower temperature. In addition to removing the copper oxide, this method also produces graphene.
According to Boyd, the reason this process works so well is due to leaky vales. The valves were letting in just the right amount of methane for graphene to grow," he explains.
Impact on the manufacturing industry
The researchers say aside from reducing manufacturing costs, this new method for producing graphene could also eliminate deffects caused by thermal expansion. As a result, manufacturers won’t have to spend as much time reversing these defects. Typically, it takes about ten hours and nine to ten different steps to make a batch of high-mobility graphene using high-temperature growth methods," Yeh says. "Our process involves one step, and it takes five minutes."
According to Yeh and Boyd, their production method yields higher quality graphene thanks to fewer defects and high electrical mobility. The answer to why their technique is efficient could be the result of a chemical reaction involving the hydrogen plasma. Another difference between this new method and convention ones is that the graphene tends to grow in a more orderly format.
This could essentially allow graphene to be used in electronics (for example, to act as a shield of sorts against exposure and environment). "In the future, you could have graphene-based cell-phone displays that generate their own power," adds Yeh.
Manufacturers might also want to manipulate specific imperfections into the graphene to develop unique attributes. “If you can strain graphene by design at the nanoscale, you can artificially engineer its properties,” says Yeh. “But for this to work, you need to start with a perfectly smooth, strain-free sheet of graphene. "You can't do this if you have a sheet of graphene that has uncontrollable defects in different places."