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Quantum Dots

Quantum dots are semiconductor nanoparticles that glow a particular color after being illuminated by light. The color they glow depends on the size of the nanoparticle. When the quantum dots are illuminated by UV light, some of the electrons receive enough energy to break free from the atoms. This capability allows them to move around the nanoparticle, creating a conductance band in which electrons are free to move through a material and conduct electricity. When these electrons drop back into the outer orbit around the atom (the valence band), as illustrated in the following figure, they emit light. The color of that light depends on the energy difference between the conductance band and the valence band.

Electrons in a quantum dot generating light

Electrons in a quantum dot generating light.

The smaller the nanoparticle, the higher the energy difference between the valence band and conductance band, which results in a deeper blue color. For a larger nanoparticle, the energy difference between the valence band and the conductance band is lower, which shifts the glow toward red.

Many semiconductor substances can be used as quantum dots, such as cadmium selenide, cadmium sulfide, or indium arsenide. Nanoparticles of these, or any other semiconductor substance, have the properties of a quantum dot. The gap between the valence band and the conductance band, which is present for all semiconductor materials, causes quantum dots to fluoresce.

Quantum dots may be able to increase the efficiency of solar cells. In normal solar cells, a photon of light generates one electron. Experiments with both silicon quantum dots and lead sulfide quantum dots can generate two electrons for a single photon of light. Therefore, using quantum dots in solar cells could significantly increase their efficiency in producing electric power.

Researchers are also working on the use of quantum dots in displays for applications ranging from your cell phone to large screen televisions that would consume less power than current displays. By placing different size quantum dots in each pixel of a display screen, the red, green and blue colors used to generate the full spectrum of colors would be available.

 

Excerpted from Nanotechnology For Dummies (2nd edition), from Wiley Publishing

 

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