Graphene’s merits in electronic devices and as a light bulb coating are still being debated. But new results suggest the atom-thick carbon sheet has one clear advantage: precise but practical calibrations of electrical resistance.
This might seem like a minor use for the world’s most celebrated wonder material, but it’s one that sits at the very base of the electrical engineering pyramid. A more practical calibration could help national standards laboratories and industries that depend on those standards. It may also help disseminate the International System of Units (SI), which could be overhauled as early as 2018.
The most exacting metrology laboratories calibrate their electrical units based on quantum mechanical phenomena. The ohm, the SI unit of electrical resistance, is calibrated by taking advantage of the quantum Hall effect. The Hall effect occurs when a magnetic field is applied perpendicular to the flow of current. The resulting force on electrons causes them to migrate to the side, which in turn raises a voltage perpendicular to the flow of current.
In the quantum version of the Hall effect, which occurs in a thin layer of material, the voltage and resulting resistance are “quantized” and so take on discrete integer values. Conveniently, the quantum Hall resistance is expected to be completely independent of the kind of device that’s built. Instead it depends only on two unvarying constants of nature: the fundamental charge of the electron and a quantum mechanical measure dubbed the Planck constant.
Unfortunately, the physical conditions required to take advantage of the quantum resistance standard are exacting. State-of-the-art measurements, which are taken using a device made of thin layers of gallium arsenide and aluminum gallium arsenide, can require a 10-Tesla magnetic field (and so a massive superconducting magnet) and temperatures within a few degrees of absolute zero.
Researchers have long suspected that the unique behavior of electrons in graphene, namely the big spacing between electron energy levels when the material is exposed to a magnetic field, could be exploited to produce precise measurements of resistance under less extreme physical conditions.
Several recent results support that idea. In August, Jan-Theodoor Janssen at the UK’s National Physical Laboratory and colleagues reported a way to build a small system for a graphene resistance standard that can operate at a higher temperature and lower magnetic field. This week, a team based at France’s National Metrology and Testing Laboratory and various departments at the National Center for Scientific Research showed that graphene can indeed be used to calibrate resistance with great accuracy, rivalling that of gallium devices, and that it can do so over an even wider range of operational conditions.
The French team constructed its resistance device from a high-quality sheet of graphene grown on a silicon carbide wafer. The resulting 100-by-420-micrometer “Hall bar” contained a source and drain to raise a voltage across the device, as well as a set of metallic pads along the sides that were used to measure resistance.
The team found they could measure resistance with a level of accuracy rivaling those yielded by gallium arsenide devices, but with a magnetic field one-third as strong, with a temperature as high as 10 Kelvin, or with measurement currents 10 times as high (though not all three at the same time).
The temperature the graphene device operates at is high enough that a lab could accurately measure resistance without needing liquid helium as a refrigerant. “These results support graphene as the material of choice for the next generation of easy-to-use, helium-free, and affordable quantum electrical standards, approaching an ideal standard that would be invariant, available to anyone, at any place and any time,” the French team wrote this week in Nature Nanotechnology.
Graphene could also help bring about the realization of a simplified ampere, one of the seven SI base units. In the new SI, this unit of current will be redefined in terms of the fundamental charge of the electron, and quantum electrical standards will play a closer, more integrated role.
The kilogram will also be redefined. Instead of being measured against a physical artifact locked in a vault, it will be realized directly through the watt balance, which measures the weight of an object against an opposing electromagnetic force. That force is calibrated against quantum electrical standards. (Read the strange story of the kilogram change here.)
In the long term, it could be possible for any national metrology laboratory to measure the kilogram from scratch, says Jon Pratt of the U.S. National Institute for Standards and Technology, which is also investigating graphene’s potential as a resistance standard. “If we are to make watt balances available to everyone, we will need quantum standards that are low cost and easy to use,” he says. “Graphene appears to move us towards this, at least for resistance.”
This article was updated on 14 September 2015 to clarify the NPL and LNE/CNRS findings.