The diameter of an ion beam is only
20 nm and its aiming accuracy is 60 nm. "Single Nanoscale
structures with unique properties are a treasure trove
for developing novel nanoelectronic devices of the next
generation.
However, understanding the properties of such nanoscale
structures is harder than creating the nanostructures.
Prof. Aono, who has been creating various nanostructures
of interest by manipulating atoms and molecules using
a probe tip of a scanning tunneling microscope (STM),
has expanded his research field to the property measurement
of nanostructures.
In 1989, Prof. Aono launched the "Atomcraft" Project
under the sponsorship of the ERATO Program of the Research
Development Corporation of Japan (JRDC) (currently, Japan
Science and Technology Agency (JST)) in 1989. In this project,
he demonstrated through many examples that it was possible
not only to extract single atoms from any designated positions
on a sample surface but also to supply single atoms at
any designated positions on the sample surface even at
room temperature by using the probe tip of a scanning tunneling
microscope.
The method of manipulating atoms at room temperature
developed in the project is the basis for today's bottom-up
nanoscale structure fabrication.
"A nanowire made of a chain of atoms, for example," Prof.
Aono says, "cannot be used for nanoscale electronic
circuits unless we know how electrically conductive they
are." He began to develop a double-probe STM, in which
two probe tips could be operated independently. The two
probe tips can be positioned at any two different points
on a given nanostructure for electrical conductivity measurements.
Although the development work took longer than expected,
the first STM with two probe tips or the "nanotester" was
finally completed in 1998. The "nanotester" enabled
him to make a direct measurement of the electrical conductivity
of a one-dimensional electrical conductor with a thickness
of about 1 nm for the first time in the world. "We
succeeded to measure the electrical conductivity of a nanowire
of metallic ErSi2 as thin as about 1 nm by placing one
probe tip of an STM at a designated position on the nanowire
and changing the position of the other probe tip on the
nanowire over a length of several hundred nanometers," Prof.
Aono says. "Nobody would have ever imagined this kind
of sophisticated measurement before." Although he
stresses that the development of a multiple probe STM is
still in the early stage, his group has already developed
one with four probe tips. The group has also developed
an atomic force microscope (AFM) with four probe tips designed
to measure electrical conductivities of nanostructures
created on insulator substrates. By making use of these
microscopes, Prof. Aono's group has been trying to measure
electrical conductivities of DNA, carbon nanotubes, polymer
nanowires, and other materials of interest, in addition
to metal nanowires.
As one of his remarkable researches in recent years, Prof.
Aono discovered an unexpected phenomenon of importance
in his research on building nanostructures with silver
atoms supplied from a STM tip made of silver sulfide. "I
found that when a tunneling current flowed from a STM probe
tip made of silver sulfide (a solid electrolyte) to a sample,
silver atoms were deposited at the apex of the probe tip," he
says. He also found that when the direction of the electric
current was reversed, the deposited silver atoms were re-dissolved
into the probe tip. "I thought intuitively that this
phenomenon could be used for novel switches because this
mechanism would work as a two-terminal switch that turns
on when the deposited silver atoms contact the opposite
electrode and turns off when the silver atoms re-dissolve
into the probe."
It was found later that these switches were capable of
working repeatedly and that their switching speeds were
high. Prof. Aono believes that he will be able to develop
a new scientific field, "atomic electronics," based
on these atomic switches. An atomic switch is made at the
crosspoint of a silver sulfide-coated silver wire and a
platinum wire. His group is conducting joint research with
NEC Corporation to commercialize a new computer architecture
based on this type of crossbar structure.
Prof. Aono tells younger researchers not to follow others.
"Researchers should do attractive and unique studies
which other people want to follow. They should not aim to
be the best but to be the only ones. If the results of your
unique research are appraised highly by others, you will
not be able to forget such an exciting feeling." Now,
he wants to study an unexplored area of biotechnology.
Prof. Aono says, "I want to study thoroughly the slick
exchange mechanisms of signals among biological materials
using a STM with at least 1,000 probe-tips. My ultimate
goal is to create a new paradigm in the computation field
based on my research in such an area."
(Interviewer: Aya Araoka, Nanonet)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/048a.html
