makes nanotube memory
Technology Research News
Carbon nanotubes have been used to make
experimental transistors, chemical sensors and memory devices that are
far smaller than anything available today. But moving from experimental
prototypes to practical devices requires overcoming a large hurdle: controlling
the way nanotubes grow.
Nanotubes tend to form as mixes of two types -- semiconducting and metallic
-- with semiconducting the more technologically desirable. In April, 2001,
IBM researchers announced they could weed out metallic nanotubes by sending
enough current through a batch of nanotubes to burn up the metallic tubes
but not enough to damage the semiconducting ones.
Researchers at the Max Planck Institute in Germany have come up with an
alternative method of producing all-semiconducting bundles that, in addition,
prepares the microscopic tubes for use in memory devices. The technique
allows researchers to oxidize bundles of a few nanotubes or individual
nanotubes that measure as small as 2 nanometers in diameter. A nanometer
is one millionth of a millimeter, and a line of 20 hydrogen atoms spans
Oxidizing a bundle of nanotubes converts metallic ones to semiconducting,
said Marko Burghard, a scientist at the Max Planck Institute for Solid
State Research in Germany. Assemble the oxidized bundles into larger arrays
and they could be "key building blocks for low-cost memories with ultra-high
storage densities," he said.
The process could theoretically produce memory devices that hold one trillion
bits per square centimeter, said Burghard. A trillion bits is about 31
DVDs worth of data.
The researchers hit on the process after finding that about half of nanotube
bundles left in open air for several months had changed from metallic
to semiconducting. This happened because oxygen atoms in the air combined
with the carbon atoms in the metallic nanotubes to form a nonmetallic
The researchers were able to induce the effect by heating the nanotubes
in air or treating them with oxygen plasma. A plasma is a gas whose atoms
are ionized, meaning they have more or fewer electrons than normal and
so can conduct electricity.
The researchers took advantage of a consequence of the oxidation process
to make prototype memory devices from the oxidized tubes. The devices
use a single oxidized nanotube or bundle of oxidized nanotubes as the
semiconducting channel of a transistor. Data is represented by tiny electric
charges of one or a few electrons stored in a defect on the surface of
a nanotube produced by the oxidation. The defects are tiny clumps of amorphous,
or jumbled, carbon attached to the otherwise orderly, crystalline nanotubes.
By sending three volts of electricity through the nanotubes, the researchers
stored a charge in a surface defect. In electronic memory, the presence
of a charge generally represents a 1 and the absence of a charge a 0.
To read the 1s and 0s, the researchers sent a small current through the
nanotubes to measure their conductivity. Nanotubes that harbor a stored
charge are about 1,000 times more conductive than those without a charge.
Charge storage memory devices based on nanotubes were first developed
several years ago; research teams at the University of Maryland and the
University of Pennsylvania have recently developed experimental devices.
The Max Planck Institute memory device, however, is able to store charges
longer than the other devices, said Burghard. The Maryland researchers
reported a charge storage time of 1.4 hours, and the Pennsylvania researchers
16 hours. The Max Planck device is able to store charges for more than
12 days, Burghard said. Stored charge memory devices can be used as nonvolatile
computer memory, which retains its data when the power is off.
The research is important work, said Vincent Crespi, an assistant professor
of physics at Pennsylvania State University. "It enables a memory device
to be implemented within a single nanotube plus three contacts," he said.
"The charge trap seems to come along for free."
The researchers will need precise, reproducible control over the character
of the charge trap before the device can be used in practical applications,
The researchers plan to study further the oxidation process and the nature
of the charge storage defects, said Burghard. Another goal is to search
for better, more controllable chemical modifications of the nanotubes,
"for example, by electrochemically attaching appropriate chemical residues
or small metal clusters, which could then be used for charge storage,"
The researchers' nanotube memory element could be used in practical applications
in five to ten years, said Burghard.
Burghard's research colleagues were Jingbiao Cui, Roman Sordan and Klaus
Kern. They published the research in the October 21, 2002 issue of the
journal Applied Physics Letters. The research was funded by the Max Planck
Timeline: 5-10 years
TRN Categories: Nanotechnology; Data Storage Technology;
Materials Science and Engineering; Chemistry
Story Type: News
Related Elements: Technical paper, "Carbon nanotube memory
devices of high charge storage stability," Applied Physics Letters, October
27/December 4, 2002
Molecule stores picture
Fast quantum crypto demoed
Software system heals
Motifs distinguish networks
Oxygen makes nanotube
Research News Roundup
Research Watch blog
View from the High Ground Q&A
How It Works
News | Blog
Buy an ad link