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There has been a lot of press lately about Quantum Dots,
but just what are they? You can think of a Quantum Dot as a tiny box full of
electrons. They are created from semiconductor materials using modern crystal
growth techniques, and have typical dimensions between a few nanometers (10-9
meters) to a few microns. (How big is a nanometer? A penny is about 20 million
nanometers across.)
Quantum Dots contain a
small controllable number of electrons, usually from 1 to 1,000. The droplet of
charge is trapped in a three dimensional container and, depending on the
geometry and fabrication techniques, the droplet can be a three-dimensional
sphere, a two-dimensional disc, or a one-dimensional rod. Further, there are
various ways to control the shape of the dots. In the case of two-dimensional
dots, they can be made circular or rectangular. Below is an image from a
Transmission Electron Microscope of three Quantum Dot shapes (courtesy of Delft
University of Technology).
What’s interesting about Quantum Dots (and hence the name) is
that if they are made small enough, quantum mechanical effects can be observed.
The effect is due to the electrons being trapped in a quantum wave form within
the Quantum Dot. These properties can then be observed more easily by
scientists since, while exhibiting the quantum traits of a single atom, they
are much larger and easily examined macroscopically. Unlike atoms, Quantum Dots
can be easily connected to electrodes and thus make the study of their
atom-like properties much simpler.
 Artificial Atoms
Two quantum dots can be combined in such a way that the electrons move between the dots and form a Quantum Dot 'Molecule'. While not a true molecule, the Quantum Dot molecule displays the same electron shell behaviors as molecules. In fact, the electron shells (1s, 2s, 3s, 3p…) in two dimensional Quantum Dot molecules are identical to real molecules. (A discussion on Electron Shells can be found here.)
But since they have much larger dimensions they are suitable for experiments
that cannot be carried out in atomic physics. By studying the behaviors of
these artificial molecules, a periodic table of artificial 2-D molecules has
been developed to help better understand these characteristics.
Conclusion
Quantum Dots are by far the most important discovery in the last
decade, as they enable scientists to better examine and understand the quantum
behaviors of atoms. In fact, Science Magazine recognized Quantum Dot research
as one of the top ten scientific breakthroughs of 2003. But beyond the
implications for understanding the nature of Quantum Mechanics, Quantum Dots
have a wide range of ‘real-world’ applications, including Bio-imaging and
Quantum Computing.
The NASA Glenn
Research Center
has been investigating the synthesis of Quantum Dots for use in solar cells.
Using quantum dots in a solar cell to create an intermediate band will allow
the harvesting of a much larger portion of the available solar spectrum.
Theoretical studies predict a potential efficiency of 63.2 percent, which is
approximately a factor of 2 better than any state-of-the-art devices available
today.
Additional Reading
Quantum Dots
by Leo Kouwenhoven and Charles Marcus
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