Graphene Speed Record

Andre Geim and his colleagues at the University of Manchester have observed the highest electron mobility for an electron in any electronic material. In this case the electrons were moving through graphene, single-atom-thick sheets of carbon, with an electron mobility of 200,000 cm^2/volt-second. Graphene was only discovered a few years ago (by Geim: see http://www.aip.org/pnu/2006/split/769-2.html ). A true two-dimensional material is striking enough, but even more unusual was the observed ease with which electrons moved in graphene.

Electrons moving through any crystal lattice are constantly interacting with the atoms in that lattice, especially if there are irregularities present. This causes the electrons to slow. Their effective mass will be different for each type of crystal. In graphene, the effective mass of electrons is zero. Still another way of quantitatively describing an electron’s journey through the alleyways of a crystal is in terms of its “mobility,” in units of square centimeters per volt/sec.

The charge-carrier mobility is perhaps the most important figure of merit for an electronic material, so researchers have sought a larger mobility. To take some examples: the mobility in silicon is 1500, while in GaAs it is 8500. That’s why the circuitry in cell phones is based on GaAs. For InSb, the mobility is even higher: 80,000. Geim’s new mobility record of 200,000 won’t cause the electronics industry to ditch Si or GaAs any time soon.

The problems with early graphene circuits right now, says Geim, are, first, that graphene can’t yet be made into uniform high-quality wafers; and second that prototype graphene transistor switching (going from Off to On) is too slow. However, Geim predicts that over the short run (3-5 years) graphene might emerge as a basis for chemical sensors and for generators of terahertz-range light-a frequency span (and not yet achieved in any practical way) where human bodies are transparent-making possible security or medical scanning machines. (Morozov et al., Physical Review Letters)