A research team led by Alex Zettl and Paul McEuen has succeeded
in making novel nanosized electronic devices using carbon nanotubes.
This work represents a significant step towards making functional
electronics on the nanometer scale.
Single-walled carbon nanotubes (SWNTs), tiny wires of pure carbon
only 1.4 nm in diameter, have attracted much attention as possible
building blocks for molecular electronic devices. Previous LBNL
work has shown that depending on the chirality, or twist, in the
graphite lattice as it rolls to form the SWNT, nanotubes can be
either metallic or semiconducting. More recently, the team demonstrated
that individual metallic SWNTs can act as highly efficient conductors
of electrons between metal contacts several microns apart. As
reported in Phys Rev. B and in Science, the Zettl
and McEuen groups have made two important breakthroughs in making
a truly molecular scale electronics technology from SWNTs, namely
the demonstration of transistor function and the connection of
individual nanotubes with each other to make devices.
The LBNL team deposited nanotubes on a silicon wafer coated with
an insulating oxide and labeled with fiduciary marks. Atomic force
microscopy was used to locate the nanotubes and electron-beam
lithography techniques were used to "write" metallic
"source" and "drain" contacts to both ends
of the tubes. The conducting plane beneath the oxide layer served
as the third,"gate" electrode. With "as-grown"
nanotubes, the source-drain conductance as a function of the gate
voltage indicated p-type transistor function, in agreement with
previous experiments. To convert the device to n-type, it was
placed in a specially constructed furnace where a controlled amount
of potassium vapor was deposited onto the nanotube. Electrons
from the potassium were transferred to the nanotube, effectively
doping it n-type. At certain levels of potassium exposure, the
conductance plot became almost a "mirror image" of that
of the p-doped device (see figure), exactly the behavior of an
n-type field effect transistor.
In the next experiment, the team constructed more complicated
devices consisting of two SWNTs forming a cross, with four electrical
contacts, one at each end of each nanotube. This geometry allowed
measurement of the electrical properties of each SWNT individually
and also the electrical properties of the junction. Junctions
between metal-metal, semiconducting-semiconducting and metal-semiconducting
nanotubes were studied. Since the two nanotube "wires"
in a junction are coupled only by weak van der Waals interactions,
it was surprising that the metal-metal and semiconductor-semiconductor
junctions have a very low resistance, approximately 200 kW, and
can reliably pass currents of hundreds of nanoamps. Given the
tiny area of the junction, this corresponds to enormous current
densities on the order of 107 amps/cm2. On the other hand, metal-semiconductor
junctions showed quite different behavior. At low applied voltage
bias, they had a much higher resistance (ca. 30 MW), but when
the voltage was turned up, the resistance dropped, but only for
current flow in one direction. This diode-like behavior, while
characteristic of junctions between macroscopic metals and semiconductors
("Schottky barriers"), had never before been observed
at such a small scale.
Alex Zettl (510.642-4939) and Paul McEuen (510.643-8646), Materials Sciences Division (510.486-4755), E. O. Lawrence Berkeley National Laboratory.
Bockrath, M., J. Hone, A. Zettl, Paul L. McEuen, Andrew G.
Rinzler, and Richard E. Smalley, "Chemical doping of individual
semiconducting carbon nanotube ropes", Phys Rev. B,
61, 16, pp. R10606, 2000.
Fuhrer, M.S., J. Nygård, L. Shih, M. Forero, G.-G. Yoon,
M.S.C. Mazzoni, H.J. Choi, J. Ihm, S.G. Louie, A. Zettl and Paul
L. McEuen, "Crossed Nanotube Junctions," Science,
288, pp. 494, 21 April 2000.
Ernest Orlando Lawrence Berkeley
National Laboratory
One Cyclotron Road, Mail Stop 66, Berkeley, California 94720 USA