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JAPAN NANONET BULLETIN
-- 54th Issue -- September 29, 2005
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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IN THIS ISSUE
Nanonet Interview:
"What can be observed on a surface?
- Unlocking surface physics with computational science -"
Masaru TSUKADA, Professor, Graduate School of Nanoscience and
Nanotechnology, Waseda University
Young Researchers' Introduction:
"Diffusion of air molecules in antarctic ice-sheet"
Tomoko IKEDA-FUKAZAWA, Associate Professor, Department of Industrial
Chemistry, Meiji University
-- NANO CALENDAR --
For information on nanotechnology related symposiums and conferences
held in the world,
http://www.nanonet.go.jp/english/calendar/
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NANONET INTERVIEW
What can be observed on a surface?
- Unlocking surface physics with computational science -
(Issued in Japanese: February 24, 2004)
Masaru TSUKADA, Professor, Graduate School of Nanoscience and
Nanotechnology, Waseda University
Tremendous progress has been made in surface physics since the
invention of the scanning tunneling microscope (STM) in 1982. Even
though atomic images were obtained, how exactly they were taken was
unknown. "In order to determine the mechanisms, I developed a method
to do theoretical simulations using first principle computations. From
the simulation, I found that the tunneling current concentrated on the
atom of the probe closest to the sample surface, and this was the key
to STM with atomic scale resolution," said Prof. Tsukada.
Prof. Tsukada is also interested in non-contact atomic force
microscopy (ncAFM). Using an ncAFM slight changes in vibrational
frequency that occur when a probe vibrating at its resonant frequency
scans a sample surface can be measured. He says, "The implementation
of theoretical studies led us to be able to determine what is seen in
ncAFM images." Between the probe and the sample surface, there are
short-range chemical interaction forces and long-range van der Waals
forces. Van der Waals forces work over long ranges, and they are not
sensitive to changes in the position of the probe on the atomic scale.
On the other hand, chemical forces operate in the range between 0.2
and 0.3 nm, and they are sensitive to changes in the position of the
probe on the atomic scale. Thus, ncAFM images show variations in the
chemical forces with changes in the position of the probe. The
chemical forces are affected not only by the structure of the surface
but also by combinations of the constituent elements on the sample
surface and the probe, and therefore, it is possible to identify each
element. Prof. Tsukada developed a method for theoretically simulating
an ncAFM experiment in ultra high vacuum utilizing first principles.
However, it can also be used as a tool to observe biomolecules in air
or in solution. He thinks that the observation of biomolecules under
these conditions is theoretically very interesting and challenging.
Prof. Tsukada has also been trying to determine the electrical
conductivity of individual molecules, especially fullerenes and carbon
nanotubes with a cage-like surface. "Carbon nanotubes with a cage-like
surface are made by inserting five-membered rings or seven-membered
rings between six-membered rings. There are no defects in the
structure of five-membered rings, but there are phase defects that
change the electric wave properties when a wave goes around the ring.
It is interesting to determine how the phase defects influence the
wave functions," says Prof. Tsukada. He found that when two metallic
carbon nanotubes with different radii are connected with five-membered
rings and seven-membered rings, the electric current between these
carbon nanotubes depends only on the radius ratios because of the
scaling rule for any structures of the connected area.
He also found that, when a magnetic field is applied to the inside of
a carbon nanotube torus, a coherent permanent current could be
generated. Because of the unique band structure of carbon nanotubes, a
slight difference in energy is generated by a magnetic field between a
clockwise current and a counterclockwise current in the torus. "To
great amusement, the current flows in the direction that increases the
applied magnetic field, and therefore, the torus is magnetized even
with a low magnetic field. This could be used for several different
kinds of magnetic devices," says Prof. Tsukada. Calculations based on
first principles are used to derive the structures and the properties
of a system by determining the electronic states within the system
using fundamental quantum mechanics without any experimental
parameters and by obtaining the interatomic forces. This method is
more accurate than theoretical analysis using a model, but it requires
a large amount of computational time. Therefore, it is limited to
systems which have only a few hundred atoms.
Prof. Tsukada said, "No one tries to solve the mysteries of the
universe using individual stars. With a galaxy, you deal with millions
of stars as a unit. This concept also applies to materials. A group of
basic units, which form a periodic structure, can be considered as one
unit. If the unit should be more detailed, it can be simulated by
using the hierarchical structure of materials." Today, various kinds
of calculations can be done using ready-made software due to progress
in simulation software. He says, "Sometimes, computational science may
not seem so important, but I do not want young researchers to think
that it is less important." Theoretical scientists have to figure out
what should be computed and what it means. True theoretical scientists
can come up with the answers. He adds, "In order to truly understand
what is behind phenomenon, a theoretical scientist needs insights into
physics."
(Interviewer: Kuniko Ishiguro, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/054a.html
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YOUNG RESEARCHERS' INTRODUCTION
Diffusion of air molecules in antarctic ice-sheet
(Issued in Japanese: March 23, 2004)
Tomoko IKEDA-FUKAZAWA, Associate Professor, Department of Industrial
Chemistry, Meiji University
Diffusion of air molecules in ice was found from Raman spectroscopic
study of natural ice from the Antarctic ice-sheet (T. Ikeda-Fukazawa
et al., Geophys. Res. Lett. 26 (1999)). The results have important
implications for the reconstruction of the paleo-atmosphere from polar
ice cores. In order to investigate the diffusion of air molecules in
Antarctic ice-sheets in periods of tens of thousands years, I have
been studying the dynamics of water and air molecules in ice crystals.
I have performed molecular dynamics simulations involving the
diffusion of air molecules (e.g., N2, O2, and CO2) in ice crystals and
observed the diffusion hops for these molecules from a stable site to
the adjacent site. The results showed that the diffusion mechanism
for the air molecules significantly differs from small atoms, such as
He. The air molecules diffuse by distorting the ice lattice (see Fig.
2), whereas He atom hops from a stable interstitial site to the
adjacent site without distorting the lattice (see Fig. 1). The
diffusion velocity for this mechanism is a few orders of magnitude
larger than the value estimated from the interstitial mechanism. In
order to reconstruct accurate records of the paleo-atmosphere from
polar ice sheets, I have developed a model for the variation process
of the distribution of air molecules in the ice sheets.
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/054b.html
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