The first transistors to be fashioned from a single "buckyball"
- a molecule of carbon-60 - have been reported by scientists with
the Lawrence Berkeley National Laboratory (Berkeley Lab).
by Lynn Yarris.
Taking advantage of a phenomenon that is largely viewed as a problem
by the electronics industry, the team of Berkeley Lab and UC Berkeley
researchers created a separation between two gold electrodes that
was about one nanometer (one billionth of a meter) across. This
tiny gap could accommodate the insertion of a single buckyball in
order to create a molecular-sized electronic device.
"Nature long ago solved the problem of making electronic devices
on a molecular-scale and we're now beginning to learn how to do
things the way Nature does," says Paul McEuen, a physicist who holds
joint appointments with Berkeley Lab's Materials Sciences Division,
and with UC Berkeley's Physics Department.
McEuen was one of the co-authors of a paper in the journal Nature
(September 7, 2000) that described this research. The other authors
were Hongkun Park, Jiwoong Park, Andrew Lim, Erik Anderson, and
The ability to use individual molecules as functional electronic
devices is a much coveted prize in the computer industry because
of the potential for dramatically shrinking the silicon-based microelectronic
systems of today. As electronic devices are reduced in size to a
nanometer scale, the atoms with which silicon must be doped will
eventually begin to move about, resulting in poor or uneven performances.
Nanoscale sizes should not pose a problem for devices based on single
large molecules of carbon as the atoms in these molecules are covalently
bonded and therefore firmly locked in place.
Within the past few years, a number of research groups, including
McEuen's, have made transistors from carbon nanotubes - tiny sheets
of graphite that have been curled and connected along the seam.
Although considered a single molecule of carbon, these elongated
tubes were several times larger than the soccer-ball shaped carbon-60
molecules used by McEuen and his colleagues to make their newest
transistors. Buckyballs are so tiny that, as transistors, they only
permit one electron at a time to move through them. This opens the
door to the study of single-electron transport effects.
carbon-60 buckyball is shaped like a soccer ball.
"Transport measurements of these single carbon-60 transistors provide
evidence for coupling between the center-of-mass motion of the carbon-60
and single-electron hopping, a novel conduction mechanism that has
not been observed in previous quantum-dot studies," the authors
stated in their Nature paper. "The transport measurements demonstrate
that single-electron tunneling events can be used both to excite
and probe the motion of a molecule."
McEuen likens the carbon-60 molecule to a ball tethered to a spring
that rests on the surface of a gold electrode. When an electron
hops onto the carbon-60, the "spring" is compressed as the charge
of the additional electron draws the molecule closer to the gold
surface. When the electron hops off the carbon-60, the spring is
released. In this manner, electron-hopping causes the molecule to
oscillate, like a ball on a spring bouncing up and down. McEuen
says this quantized nano-mechanical movement of the carbon-60 might
serve as a logic gate, a means of storing information in the position
of the molecule that would be more stable and much faster than the
To make their transistors, McEuen and his colleagues capitalized
on a phenomenon known as "electromigration." If two electrodes are
physically connected to one another and a large current is sent
through them, the movement of the electrons can create nanometer-sized
fissures between the electrodes. Opening up cracks between the electrodes
is not usually desirable when making electronic devices, but this
was a case, McEuen says, of using lemons to make lemonade, as the
cracks in the gold electrodes were a good fit for buckyballs. Transport
measurements showed that the conductance across the cracks was substantially
enhanced when a solution of carbon-60 was deposited onto the connected
electrodes, indicating that individual buckyballs had filled those
cracks. Measurements were also found to be in excellent agreement
with theoretical predictions.
The gold electrodes used in this study were fabricated on Berkeley
Lab's "Nanowriter," an ultra-high resolution lithography machine
that can generate an electron beam at energies up to 100,000 volts
with a diameter of only five nanometers.
C60, the molecule that started it all.
Says Erik Anderson of the Center for X-ray Optics, a collaborator
on this study who helped design the Nanowriter's pattern generator
and control system, "The Nanowriter's high-resolution, excellent
placement accuracy, and modest throughput capabilities enabled us
to make a large number of high quality gold electrode structures
which we could then break apart with good reliability."
The devices created with the buckyballs are analogous MOSFETs (metal-oxide
semiconductor field effect transistors). Though McEuen says they
probably hold no commercial use at this time (the carbon-60 molecules
can be readily blown out of the junction between the electrodes
with too much voltage), they do represent one of the first actual
experiments with a device for the upcoming age of nanoelectromechanical
systems or NEMS.