For the first
time, Chinese scientists have shown that nanometer-sized dots of information
can be written on a thin film and erased. The work suggests that an
organic film, altered electrically to create such dots, could hold a
million times more data than a CD-ROM. Calculations by Karl Sohlberg,
a theoretical chemist in ORNL's Solid State Division, have enhanced
the understanding of the mechanism behind this discovery. The results
of the collaborative research, which also involved the University of
Chicago, were published in the February 21, 2000, issue of Physical
Review Letters.
In August 1997,
Hongjun Gao, then with the Beijing Laboratory for Vacuum Physics, came
to ORNL's Solid State Division (SSD) as a guest scientist. He wanted
to both use the division's state-of-the-art microscopes and tap SSD
expertise. Gao and his Chinese colleagues had discovered that by exposing
an organic film on a graphite substrate to voltage pulses from a scanning
tunneling microscope (STM), tiny regions, or "nano-dots," of the non-conductive
film become electrically conductive.
Gao told SSD's
Steve Pennycook and Sohlberg that when a voltage was applied to a perfect
crystalline film on graphite, virtually no electrical current was measured
because of the film's high resistivity. But after the film was exposed
to positive voltage pulses, it became conductive. Gao was interested
in determining the changes at the molecular level that altered the film's
electrical properties.
Sohlberg, who
was Gao's office mate at ORNL, made calculations and used infrared spectra
and other data from various experiments conducted on the organic films
at the Beijing lab to make this determination. "We were trying to test
hypotheses suggested by the experimental results and the scientific
literature," he says. "That way we hoped to arrive at the correct explanation."
The researchers
ruled out several suggested explanations for the conductivity of the
altered film, including the buildup of static electricity and the burning
of a hole through the film to the electrically conductive graphite base.
"Then we got an
insight from another experiment carried out in the Beijing lab," Sohlberg
says. "Occasionally, the Chinese scientists would deposit a film at
too fast a rate. When they characterized one such film, they found it
was amorphous rather than crystalline like the good film. They also
observed that the disordered film was as conductive as the crystalline
film altered by voltage pulses."
This correlation
suggested that the nanodots are actually tiny regions of "local disorder"
in the otherwise well-ordered film and that their amorphous nature makes
the film conductive. At Beijing, says Sohlberg, the "nail in the coffin"
experiment was done to verify this prediction and close the case on
why altered crystalline films become conductive.
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STM
images of an organic film on graphite. (a) An image of the film
surface showing crystalline order; (b) an array of nanodots formed
by positive voltage pulses; (c) an "A" pattern formed by voltage
pulses; (d) and (e) STM images after erasing marks one at a time
using negative voltage pulses; (f) resolution test using voltage
pulses (the distance between neighboring dots is 1.7 nanometers).
|
Pennycook suggested
that a thin-film data storage device would be more marketable if data
could be erased as well as written on it. So, the Beijing group did
some experiments and found that subjecting the nanodots to negative
voltage pulses restored them to the nonconductive state. This was the
first demonstration of writing and erasing information at or near the
single-molecule limit.
In March 2000,
Gao left ORNL to become a group leader in the Beijing Laboratory for
Vacuum Physics. Sohlberg says that Gao's laboratory will be trying to
meet the challenges of making a commercial high-data-density thin-film
storage device. In such a device, a conductive nanodot could represent
a "1" bit and nonconductive regions could be "0" bits.
"A massively parallel
device must be built to read so much stored information at an acceptable
speed," Sohlberg says. "In addition, the altered organic materials must
be made more stable and durable."
Gao thinks it may be possible
to connect the conductive dots, sandwich them in glass, and pack these
nanosized circuits in microchips to produce computers that are 10 times
smaller and faster. One way or another, because of the growing ability
to control their properties on the nanometer scale, organic films may
change the big picture for computing technologies.
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