Applying innovative measurement techniques, researchers from the Georgia Institute
of Technology and the National
Institute of Standards and Technology (NIST) have directly measured the
unusual energy spectrum of graphene, a technologically promising, two-dimensional
form of carbon that has tantalized and puzzled scientists since its discovery
Published in this week's issue of Science,* their work adds new detail to help
explain the unusual physical phenomena and properties associated with graphene,
a single layer of carbon atoms arrayed in a repeating, honeycomb-like arrangement.
Graphene's exotic behaviors present intriguing prospects for future technologies,
including high-speed, graphene-based electronics that might replace today's
silicon-based integrated circuits and other devices. Even at room temperature,
electrons in graphene are more than 100 times more mobile than in silicon.
Graphene apparently owes this enhanced mobility to the curious fact that its
electrons and other carriers of electric charges behave as though they do not
have mass. In conventional materials, the speed of electrons is related to their
energy, but not in graphene. Although they do not approach the speed of light,
the unbound electrons in graphene behave much like photons, massless particles
of light that also move at a speed independent of their energy.
This weird massless behavior is associated with other strangeness. When ordinary
conductors are put in a strong magnetic field, charge carriers such as electrons
begin moving in circular orbits that are constrained to discrete, equally spaced
energy levels. In graphene these levels are known to be unevenly spaced because
of the "massless" electrons.
The Georgia Tech/NIST team tracked these massless electrons in action, using
a specialized NIST instrument to zoom in on the graphene layer at a billion
times magnification, tracking the electronic states while at the same time applying
high magnetic fields. The custom-built, ultra-low-temperature and ultra-high-vacuum
scanning tunneling microscope allowed them to sweep an adjustable magnetic field
across graphene samples prepared at Georgia Tech, observing and mapping the
peculiar non-uniform spacing among discrete energy levels that form when the
material is exposed to magnetic fields.
The team developed a high-resolution map of the distribution of energy levels
in graphene. In contrast to metals and other conducting materials, where the
distance from one energy peak to the next is uniformly equal, this spacing is
uneven in graphene.
The researchers also probed and spatially mapped graphene's hallmark "zero
energy state," a curious phenomenon where the material has no electrical
carriers until a magnetic field is applied.
The measurements also indicated that layers of graphene grown and then heated
on a substrate of silicon-carbide behave as individual, isolated, two-dimensional
sheets. On the basis of the results, the researchers suggest that graphene layers
are uncoupled from adjacent layers because they stack in different rotational
orientations. This finding may point the way to manufacturing methods for making
large, uniform batches of graphene for a new carbon-based electronics.