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)
Source: http://www.aip.org/pnu/2008/split/854-2.html