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Hard shell, glowing core
December 09, 1999Tiny semiconductor crystals can be prompted to give off light - brighter
than conventional dyes and in wavelength ranges that these only produce
under certain conditions. Israeli chemist Uri Banin and his coworker
Yun-Wie Cao demonstrated this with several core/shell nanocrystals made of
the semiconductor indium arsenide.
It has been known for some time that tiny crystals of semiconducting
materials give off colored light when appropriately excited. The phenomenon
behind this effect is, in principle, the same mechanism that gives indigo
and other organic dyes their color. In these molecules, electrons can be
excited to higher energy levels, known as orbitals; after some time the
electrons fall back to their original energy state, giving off the energy
difference between the higher and lower levels in the form of colored light.
Whereas in organic dyes the electrons are strictly limited to individual
molecular regions, the electrons in indium arsenide can use all of the
atoms as their "stadium". This is possible because in semiconductors, the
electron orbitals of all of the atoms contained in a single crystal overlap
- so the bigger the morsel of indium arsenide, the more extensive the
orbitals available to the electrons it contains. Because the difference in
energy between the orbitals determines the wavelength of the light given
off, the color of the light shining from these groups of electrons depends
directly on the size of these nanocrystals. Semiconductor researchers are
thus no longer limited to the complicated design of dye molecules to obtain
a colored effect; they "only" have to see to it that nanometer-scale indium
arsenide particles of the required size are formed in their test tubes.
Until now, one problem with this approach was that the surface atoms of
these particles are easily attacked by oxygen in air. This siginificantly
reduces the brightness of the nanocrystals because in such a small
particle, a very large portion of the atoms are on its surface. Banin and
Cao wanted to protect the surface atoms by surrounding the semiconductor
crystals with a thin shell made of a few atomic layers of a second, more
stable semiconductor material, cadmium selenide. The cadmium selenide
capping serves as a protector for the inner indium arsenide core, and
causes the particles to glow even more brightly than a wellknown laser dye.
Contact:
Dr. U. Banin, Dr. Y.-W. Cao
Department of Physical Chemistry and
The Farkas Center for Light-Induced Processes
The Hebrew University of Jerusalem
Jerusalem 91904
Israel
Fax: (+972) 2-561-8033
E-mail: Banin@chem.ch.huji.ac.il
WILEY-VCH Verlag GmbH
Dr. U. Banin, Dr. Y.-W. Cao
Department of Physical Chemistry and
The Farkas Center for Light-Induced Processes
The Hebrew University of Jerusalem
Jerusalem 91904
Israel
Fax: (+972) 2-561-8033
E-mail: Banin@chem.ch.huji.ac.il
WILEY-VCH Verlag GmbH
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