With Applications Looming, Is
Graphene the New Silicon?
by Simon Mitton
Physics
Top Ten Papers
Rank
Papers
Cites
Mar-Apr 08
Rank
Jan-Feb 08
1
D.N. Spergel, et al.,
"Three-year Wilkinson Microwave
Anisotropy Probe (WMAP)
observations: Implications for
cosmology," Astrophys. J. Suppl.
Ser., 170(2): 377-408, June 2007.
[13 U.S. and Canadian institutions]
*178TD
201
1
2
C. Berger, et al.,
"Electronic confinement and coherence
in patterned epitaxial graphene,"
Science, 312(5777): 1191-6, 26
May 2006. [Georgia Tech., Atlanta;
CNRS, Grenoble, France] *048OW
44
5
3
F.H.L. Koppens, et al.,
"Driven coherent oscillations of a
single electron spin in a quantum dot,"
Nature, 442(7104): 766-71, 17
August 2006. [Delft U. Technol.,
Netherlands] *074DK
39
7
4
L. Page, et al.,
"Three-year Wilkinson Microwave
Anisotropy Probe (WMAP)
observations: Polarization analysis,"
Astrophys. J. Suppl. Ser.,
170(2): 335-76, June 2007. [13 U.S. and
Canadian institutions] *178TD
35
†
5
K. Mitsuda, et al., "The
X-ray observatory Suzaku," Pub.
Astron. Soc. Japan, 59(SP1): S1-7,
25 January 2007. [42 institutions
worldwide] *139ON
35
†
6
G. Sansone, et al.,
"Isolated single-cycle attosecond
pulses," Science, 314(5798):
443-6, 20 October 2006. [Politec.
Milan, Italy; CNR-IMIP, Rome, Italy; U.
Padua, Italy; U. Naples, Italy]
*096MW
33
†
7
J.B. Pendry, D. Schurig, D.R.
Smith, "Controlling electromagnetic
fields," Science, 312(5781):
1780-2, 23 June 2006. (Imperial College
London, U.K.; Duke U., Durham, NC]
*055LS
30
4
8
M. Tegmark, et al.,
"Cosmological constraints from the SDSS
luminous red galaxies," Phys. Rev.
D, 74(12): no. 123507, December
2006. [36 institutions worldwide]
*121QJ
29
6
9
K. Koyama, et al.,
"X-ray imaging spectrometer (XIS) on
board Suzaku," Pub. Astron. Soc.
Japan, 59(SP1): S23-33, 25 January
2007. [12 Japanese and U.S.
institutions] *139ON
29
†
10
J.K. Adelman-McCarthy, et
al., "The fifth data release of
the Sloan Digital Sky Survey,"
Astrophys. J. Suppl. Ser.,
172(2): 634-44, Octobre 2007. [73
institutions worldwide] *212HY
Physicists and materials scientists are excited about potential
applications in the semiconductor industry for a new form of carbon, known
as graphene. Whenever you use an ordinary lead pencil, flakes of graphene
slide off the graphite and leave their mark. Graphene is a form of carbon
that is only one atom thick. The atomic structure of this two-dimensional
crystal resembles a chicken wire of benzene rings, with one carbon atom
residing at each 120° corner. Carbon nanotubes are graphene cylinders,
and buckyballs are graphene spheres. Although nanotubes exhibit enormous
strength, they are difficult to make on an industrial scale, and it is hard
to incorporate them into electronic circuits. It's entirely possible that
graphene technology can overcome both of these problems.
Graphene has been around in the lab only since 2004, when scientists from
the University of Manchester first isolated and named it. (Note: see the
recent interview with Manchester's Andre Geim, Science Watch,
19[4]: 3-4, July/August 2008.) The pioneers produced graphene by repeated
exfoliation, or peeling, of graphite crystals. Once graphene films were in
the lab, their remarkable properties became apparent: almost no electrical
resistivity, and very high electron mobility at room temperatures. These
features alone mark out graphene as an attractive alternative to silicon,
because the semiconductor industry has just about reached the limit of what
can be achieved with silicon. Graphene's electrical qualities exceed all
known materials, thus offering the potential of cooler, smaller, faster
circuits.
Graphene produced by exfoliation consists of tiny flakes that are not fixed
to anything. Researchers with an eye to applications and ease of handling
have therefore been turning to epitaxial graphene (EG)—that is to
say, graphene deposited as a film on a substrate. There's no surprise that
the substrate is silicon, since the technology to handle this element is
well established. Graphitic films can be grown on hexagonal SiC crystals
heated to 1300° C in ultra-high vacuum. EG is multi-layered, which
makes it a more complex material than exfoliated graphene. The two
materials are rather different, and the electronics community is now
focused on EG as a platform for nanoelectronics.
The way ahead is marked out in Hot Paper #2 from Walt de Heer's
nanotechnology group at the Georgia Institute of Technology. They have
investigated the electronic properties of a single layer of EG sandwiched
between a SiC substrate, and an overburden of graphite. Paper #2 gives
clear instructions for making EG on diced (3 mm by 4 mm) commercial SiC
wafers. The later stages of the production process include electron-beam
patterning, oxygen plasma etching, and wire bonding. These techniques
enabled the production of a variety of devices.
Ribbons of EG have remarkable properties. For example, electron transport
is via electromagnetic waves, as in a waveguide. The width of a ribbon and
its crystallographic structure can be tuned to make the material behave as
either a metal or a semiconductor. The team at Georgia Tech have already
demonstrated that EG can survive the processing necessary for the creation
of patterned ribbons.
The results in #2 show that EG electronics can be delivered on a nanoscale
and at high temperature. The material and its transport properties are
suitable for electronic devices and their interconnections. Mass production
of real devices requires the precision use of standard lithographical and
chemical methods, and the signs are that EG will handle these requirements.
Just outside the Top Ten, the papers ranked at #11 and #12 are steadily
advancing, and both tell us more about graphene in practice. In #11,
Taisuke Ohta and four colleagues give the properties of the electronic band
structure in bilayer graphene deposited on SiC (Science,
313[5789]: 951-4, 2006; with 23 citations during March-April 2008). They
show how the energy gap between the valence and conduction bands can be
controlled.
The band gap theme is taken up in #12, with Melinda Han as lead author,
which reports on electronic transport in lithographically patterned
graphene ribbons (Phys. Rev. Lett., 98[20]: no. 206805, 2007; also
cited 23 times this period). They find that the energy gap scales inversely
with the ribbon width, thus demonstrating the ability to engineer the band
gap of graphene nanostructures by lithographic processes.
Control of the band gap will be of critical importance in switching devices
made from graphene, because switching functionality is indispensable in
applications to computers. All three papers make a strong case for graphene
electronics being just around the corner. Certainly the citations show that
the race is on. The blue skies phase is already over for this field, with
the research groups now chasing potential applications.
Dr. Simon Mitton is a Fellow of St. Edmund’s College,
Cambridge, U.K.
Keywords: graphene, epitaxial graphene, Walt de Heer, Melinda Han,
graphitic films, alternative to silicon.