Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 1999, 38 (24), 3692 - 3694
Hard shell, glowing core
Nanocrystals made of two semiconductors
have interesting optical properties
Tiny 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 well-known laser dye.