“We found that adsorbed gases modify graphene’s low-frequency noise spectra by inducing ‘Lorentzian’ components with distinctive features,” explained team leader Alexander Balandin of the University of California Riverside. “The characteristic frequency, fc of the Lorentzian noise bulges in graphene devices and is different for different chemicals, varying from fc = 10–20 Hz to fc = 1300–1600 Hz for tetrahydrofuran and chloroform vapours respectively.

Researchers have known for some time that graphene can be used to detect individual gas molecules adsorbed on its surface. However, although the material is highly sensitive, it cannot easily distinguish between different molecules. “In other words, although we can see that some molecules attached to the graphene surface change the resistivity of a field-effect transistor made from graphene, we cannot say exactly what kind of molecules are attached,” says Balandin.

The fact that the low-frequency noise signal of graphene changes as different gases are absorbed on the material opens the way for practicable and reliable simple gas sensors made from the material, he adds.

Clear peak signatures

“It is interesting that the motivation for this work came not from the desire to make a better sensor but rather from us wanting to know how the low-frequency noise response of graphene changes when it is exposed to various vapours,” Balandin told nanotechweb.org. “It turns out that some gases induce very clear peaks in the noise spectra that could be used as their signatures in sensor devices.”

While the gas signatures are reproducible, the physics of the processes leading to the characteristic peaks in the noise spectra are not completely understood. The team, which includes researchers from the Rensselaer Polytechnic Institute, the Russian Academy of Sciences and the GE Global Research Center, now plans to find out exactly how the properties of the molecules attached to the graphene surface are related to the energies of the peaks.

The current work is detailed in Nano Letters.