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JAPAN NANONET BULLETIN
-- 65th Issue -- March 2, 2006
Nanotechnology Researchers Network Center of Japan
Ministry of Education, Culture, Sports, Science and Technology (MEXT)
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IN THIS ISSUE
Nanonet Interview:
"Quantum dots studied by optical spectroscopy
-- Quantum dots behaving like molecules --"
Yasuaki MASUMOTO, Professor, Institute of Physics, University of
Tsukuba
Young Researchers' Introduction:
"Materials science using low energy synchrotron radiation"
Shin-ichi KIMURA, Associate Professor, UVSOR Facility, Institute for
Molecular Science, National Institutes for National Sciences
-- 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
Quantum dots studied by optical spectroscopy
-- Quantum dots behaving like molecules --
(Issued in Japanese: March 30, 2004)
Yasuaki MASUMOTO, Professor, Institute of Physics, University of
Tsukuba
When electrons are confined in a quantum dot, their energy levels are
quantized depending on the size of the quantum dot. However, it is
very difficult to control precisely the size of individual quantum
dots. Prof. Masumoto has investigated how ensembles of quantum dots or
individual quantum dots behave.
Ensembles of molecules and ions dispersed in crystals, or glass, show
a broad absorption spectrum, caused by the inhomogeneity. When the
crystals, or glass, are exposed to laser light with a very narrow
linewidth, molecules and ions are selectively excited. This is called
site-selective excitation. Then spectral holes in the absorption
spectrum are observed. When the spectral hole is observed for longer
time than the excited state lifetime, the phenomenon is called as
"persistent spectral hole burning". It was considered to be inherent
to molecules and ions and not to bulk crystals. However, Prof.
Masumoto discovered that it also occurs in a quantum dot, which is
much larger than a molecule. Electrons and holes are generated in a
quantum dot that has been excited by a laser beam, and the electron-
hole pairs act as excitons. When electrons, or holes, are trapped in
the region surrounding the quantum dot, the quantum dot energy changes
due to their Coulombic interactions. If this state is kept for long
time at low temperatures, persistent holes are formed in the
absorption spectrum. Persistent spectral hole burning can be applied
to optical multiple memory because many persistent holes can be made
in an absorption spectrum. He said, "Current CD-Rs and CD-RWs hold
only a single bit of information in one area on the disk. If many
spectral holes in one area can be made with various laser wavelengths,
more information can be stored in that area. Therefore, the
multiplicity of the memory devices will be drastically increased."
Site-selective spectroscopy can be used to study the averaged
properties of several quantum dots with the same energy. In contrast,
single quantum dot spectroscopy can be used to study the properties of
a single quantum dot. Prof. Masumoto determined the mechanism for
"intermittent light emission," where a quantum dot excited by a laser
beam blinks, using single quantum dot spectroscopy. When InP quantum
dots are excited by laser light, some of them blink. He has confirmed
that quantum dots around scratches blink, and the off-state
luminescence spectrum of a quantum dot is consistent with the spectrum
of a quantum dot under an applied electric field. In other words, when
there are defects with shallow energy levels near quantum dots,
carriers generated by photoexcitation are trapped in the scratches,
and thus, an electric field is applied to the quantum dots. As a
result, an off-state of the fluorescence intermittency is induced due
to the electric field. This intermittent light emission phenomenon,
like a persistent spectral hole burning, is also observed with
molecules embedded in solids. "A quantum dot is composed of 1,000 to
100,000 atoms, and its size is in between a molecule/atom and bulk. It
is largely affected by external fields due to its large surface-to-
volume ratio. Therefore, I was able to discover that quantum dots
behave like molecules," said Prof. Masumoto.
Prof. Masumoto started his research in the 70's. At the time, with
laser spectroscopy, only static optical analysis of matter was
possible. He said, "In static spectroscopy, temporal parameters are
only hypothetical. I want to see the dynamical changes occurring in
matter, like the changes in the positions of the subjects in
stroboscopic photography. It is technically hard to see them, but I
think direct observation of dynamic phenomena is indispensable for the
future." He studied femtosecond and picosecond spectroscopy using a
ultrahigh-speed laser spectroscope, of which only a few existed in the
world at that time, and determined the dynamic phenomena that occurs
in a semiconductor quantum structure by analyzing phenomena from the
viewpoint of time and energy. He talked about the pleasures of not
only determining physical phenomena but also developing cutting-edge
measuring methods. "If measuring objects, or amounts, smaller than
ever before becomes possible by increasing the sensitivity to its
maximum, my research will expand drastically, and I can conduct top-
rate research. I get excited just thinking about it," said Prof.
Masumoto.
(Interviewer: Yumiko Honda, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2006/065a.html
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YOUNG RESEARCHERS' INTRODUCTION
Materials science using low energy synchrotron radiation
(Issued in Japanese: October 13, 2004)
Shin-ichi KIMURA, Associate Professor, UVSOR Facility, Institute for
Molecular Science, National Institutes for National Sciences
Strongly correlated electron systems have recently attracted attention
because they have a variety of physical properties, including
superconductivity, colossal magneto-resistance, non-Fermi liquid and
so on. The origin of these systems is the energy balance between the
kinetic energy of carriers and the Coulomb interactions, and their
physical properties appear in the vicinity of the critical point.
These materials are expected to be used in future devices.
The physical properties originate from the electronic structures near
the Fermi level. Optical reflection and photoemission spectroscopies
can directly detect the electronic structure. Then we design a method
for probing the electronic structure using synchrotron radiation (SR)
and investigate the electronic structure of such materials. SR is
a broadband light source from the terahertz (far-infrared) to x-ray
regions. Since SR has its own properties of high brilliance and good
linear/circular polarization, experimental methods, which have never
been realized by using conventional light sources are available.
Recently, we have been studying the following:
1. Local electronic structure and its spatial distribution using
infrared magneto-optical imaging.
2. Element specific electronic structure using resonant angle-resolved
high-energy-resolution photoemission.
3. Photoemission and infrared reflection spectroscopies of electronic
structure of strongly correlated thin films.
For more information,
http://www.nanonet.go.jp/english/mailmag/2006/065b.html
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