Yorktown Heights, N.Y.,
August 26 , 2001 ... IBM researchers today announced they have created
and demonstrated the world's first logic-performing computer circuit
within a single molecule, which may someday lead to a new class
of smaller and faster computers that consume less power than today's
machines. The IBM team made a " voltage inverter " -- one of the
three fundamental logic circuits that are the basis for all of today's
computers -- from a carbon nanotube, a tube-shaped molecule of carbon
atoms that is 100,000 times thinner than a human hair. IBM scientists
will present the achievement today at the 222nd National Meeting
of the American Chemical Society being held in Chicago and it appears
in the web edition of the ACS' journal Nano Letters.
This is the second major research breakthrough this year by IBM
scientists using carbon nanotubes to make tiny electronic devices.
In April, the same IBM team became the first to develop a ground
breaking technique (Science, Vol. 292, Issue 5517, April 27, 2001)
to produce arrays of carbon nanotube transistors, bypassing the
need to meticulously separate metallic and semiconducting nanotubes.
The team used these nanotube transistors to make the circuit revealed
today.
"Carbon nanotubes are now the top candidate to replace silicon when
current chip features just can't be made any smaller, a physical
barrier expected to occur in about 10 to 15 years," said Dr. Phaedon
Avouris, lead scientist on the project and manager of nanoscale
science, IBM Research. "Such 'beyond silicon' nanotube electronics
may then lead to unimagined progress in computing miniaturization
and power."
Building a Computer Circuit "Inverter" Out of Carbon Nanotubes
The IBM scientists used nanotubes to make a "voltage inverter" circuit,
also known as a "NOT" gate . They encoded the entire inverter logic
function along the length of a single carbon nanotube, forming the
world's first intra-molecular -- or single-molecule -- logic circuit.
In the binary digital world of zeros and ones, a voltage inverter
changes a '1' into a '0', and a ' 0' into a '1' inside computer
chips. The processors at the heart of today' s computers are basically
vast and intricate combinations of the NOT gate, with two other
basic functions, "AND" and "OR" gates, which perform other computations.
Voltage inverters typically comprise two types of transistors with
different electronic properties – "n-type" (in which electrons carry
the electrical current) and "p-type" (in which electron-deficient
regions called "holes" carry the electrical current). All previous
carbon nanotube transistors have been p-type only. These transistors,
while fine for scientific studies, are not sufficient to build logic-performing
computer circuits. Scientists at IBM and elsewhere have been able
to alter the properties of nanotube transistors by adding atoms
of another element, such as potassium, to the carbon nanotube. However,
Avouris' team recently discovered a new, simpler way to convert
p-type nanotube transistors into n-type transistors. They found
that they could simply heat p-type transistors in a vacuum, which
turns them into n-type transistors and that they could reverse this
process by exposing the transistors to air.
The team also discovered that in addition to converting an entire
nanotube from p-type to n-type, they could also selectively convert
part of a single nanotube to n-type, leaving the remaining part
of the single nanotube p-type. The researchers used this process
to build the world's first single-molecule logic circuit.
More importantly, the output signal from IBM's new nanotube circuit
is stronger than the input. This phenomenon, called "gain," is essential
for assembling gates and other circuit elements into useful microprocessors.
Circuits with a gain less than one are ultimately useless -- the
electrical signal becomes so faint that it cannot be detected. Since
IBM's nanotube circuit has a gain of 1.6, Avouris is hopeful that
even more complex circuits could be made along single nanotubes.
The IBM team is now working to create these more complex circuits,
which is the next step toward molecular computers. In addition,
the team is working to further improve the performance of individual
nanotube transistors, and further integrate them into more complex
circuits.
The report on this work "Carbon nanotube inter- and intra-molecular
logic gates" by Vincent Derycke, Richard Martel, Joerg Appenzeller
and Phaedon Avouris of IBM's T.J. Watson Research Center in Yorktown
Heights, N.Y. was published in the August 26 Web edition of Nano
Letters, a peer reviewed journal of the American Chemical Society,
the world's largest scientific society. The online version is available
at http://pubs.acs.org/nano. The work was also presented in Chicago
at the 222nd national meeting of the American Chemical Society ,
August 26 during a symposium on "Molecular Electronics."
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Atomic Force Microscope image showing the design of
an intra-molecular logic gate. A single carbon nanotube (shaded
in blue) is positioned over gold electrodes to produce two p-type
carbon nanotube field-effect transistors in series. The device is
covered by an insulated layer (called PMMA) and a window is opened
by e-beam lithography to expose part of the nanotube. Potassium
is then evaporated through this window to convert the exposed p-type
nanotube transistor into an n-type nanotube transistor, while the
other nanotube transistor remains p-type.
Characteristics of the resulting intra-molecular voltage
inverter. Open red circles are raw data for five different measurements
on the same device (V=±2V). The blue line is the average of these
five measurements. The thin straight line corresponds to an output/input
gain of one.
· Chip
Evolution: IBM Scientists Develop Breakthrough Transistor Technology
with Carbon Nanotubes
· Carbon
Nanotube Publications
· Nano
Letters (American Chemical Society)
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