Super potential for super power
Just one atom thick, graphene could revolutionize mobile phone chips IBM expert Shu-jen Han explains how
Graphene. According to one professor, it’s so strong that a sheet one atom thick could support an elephant balanced on the eraser end of a pencil. It’s also flexible, conducts electricity better than any other material and is the world’s first two-dimensional substance. Almost since its discovery in 2004, graphene has been touted as the next big thing in displays, computer chips and other electronics. There’s no doubt that the material is both fascinating and promising, but its miracles still appear to be some years away.
Shu-jen Han is part of a team working in IBM Research to make graphene productive rather than semi-mythical. Han says the material has a future, although perhaps not as broad and as bright as portrayed in the press.
Lacks band gaps
“Graphene,” says Han, “is a single layer of carbon atoms. So it is only .3 to .4 nanometers (0.00000001181 to 0.00000001575 inches) thick. It has very high electronic mobility, but it doesn’t have band gaps.”
That’s good and bad. Electrical mobility is the ability of charged particles (such as electrons) to move through a medium, in this case graphene, in response to an electric field that is pulling them. High mobility means the particles, really an electrical signal, move quickly through the graphene. That’s good. ‘Band gap’ is a term that describes a sort of zone where no electron states can exist. A large band gap means the substance is probably an efficient insulator. Semiconductors, the heart of most electronic chips, need smaller band gaps. And conductors, anything that lets electricity flow freely, have little or no band gap. Graphene’s lack of a band gap is problematic, at least to some degree.
“It’s very difficult to use for logic applications,” says Han. Such applications generally need semiconductors, such as silicon. Silicon transistors can turn off and on, generating the 1’s and 0’s at the heart of computer logic. Graphene can’t turn off the flow.
Never too thin
60,000 times thinner than Saran Wrap.
1 million times as thin as a human hair
Graphene’s super properties
Conductivity world’s most conductive material
Strength: roughly 200 times stronger than steel
Flexiblity: extremely bendable and stretchable
Transparency: absorbs 2.3 percent of the visible light, virtually eliminating glare
2-D: the first two-dimensional material, only one atom thick
Potential with radio frequency
However, other uses, especially radio frequency (RF), don’t need that on/off capability. For example, take your mobile phone. Despite its name, it really is a radio frequency transmitter and receiver, and someday it may run faster and use less power (prolonging battery life) with a graphene receiver inside.
“The receiver is a very important component in any mobile device,” says Han. “A cell phone can’t process the signal that’s sent to it without a receiver. It amplifies the signal and filters out unwanted noise and interference. The most important part is called the mixer. That converts the high frequency signal sent over the air to a much lower frequency that’s easy to process.”
And all this is not idle speculation. In January of 2014, the IBM team made and tested the world’s first multi-stage graphene RF receiver and the most sophisticated graphene integrated circuit to date. The device’s performance is 10,000 times better than previously reported efforts for graphene integrated circuits.
Full graphene circuit
“We made a full circuit—transistor, inductor, capacitor and resistor--all on a single wafer. It worked pretty well,” reports Han. “Everything was processed in our 200mm fab (fabrication facility).
“We even used the circuit to receive some simple text: the letters I-B-M, transmitted at 4.3 GHz.”
The researchers see this as a major step in graphene technology that could lead to better performing and lower cost wireless communication systems.
And graphene has potential in other areas, particularly photonics—technology that uses or interacts with light. “Graphene can absorb all sorts of light wavelengths,” says Han. “And its high mobility can lead to high speed photo detectors.” Those characteristics could open up new, less expensive, devices that can detect infrared light which is invisible to the human eye.
How graphene is made
Although it’s a sophisticated substance, graphene is surprisingly easy to make in small quantities. You can actually produce graphene at home with a few simple materials.
“There are several videos on YouTube of how to make graphene using Scotch tape,” says Han. “You apply the tape to graphite and exfoliate a layer at a time. Eventually you get a single layer.” That was the method that led to the first production of graphene.
Obviously, the tape approach is unsuited for large scale production. IBM researchers now make graphene in two ways. In the first, Han says, “we can grow it on a silicon carbide substrate. We heat that to 1000 or 1200 degrees Centigrade, and eventually the silicon atoms will leave the surface.” That process, however, is very expensive and produces a maximum yield of 4 to 6 inches of the material. The second approach builds on research from the University of Texas, Austin. It uses copper as a catalyst. That method produces graphene “almost as large as we want, up to 12 inches,” says Han. “We’re really limited by the size of the furnace chamber.”