Semiconductors on the Nanoscale

As the size of the quantum dots decreases, does the wavelength of the emitted light increase or decrease  

A FIGURE 12.44 Photoluminescence. When illuminated with ultraviolet light, these solutions, each containing nanoparticles of the semiconductor CdSe, emit light that corresponds to their respective band gap energies. The light emitted depends on the size of the CdSe nanoparticles. 

Large crystals of ZhS can show photoluminescence, emitting ultraviolet photons with energies equal to the band gap energy and a wavelength of 340 nm. Is it possible to shift the luminescence so that the emitted photons are in the visible region of the spectrum by making appropriately sized nanocrystals  

Quantum dots are being explored for applications ranging from electronics to lasers to medical imaging because they are very bright, very stable, and small enough to be taken up by living cells even after being coated with a biocompatible surface layer. 

Semiconductors do not have to be shrunk to the nanoscale in all three dimensions in order to show new properties. They can be laid down in relatively large two-dimensional areas on a substrate but be only a few nanometers thick to make quantum wells. Quantum wires., in which the semiconductor wire diameter is only a few nanometers but its length is very long, have also been made by various chemical routes. In both quantum wells and quantum wires, measurements along the nanoscale dimension(s) show quantum behavior, but in the long dimension, the properties seem to be just like those of the bulk material. 

Making quantum dots is most easily accomplished using chemical reactions in solution. For example, to make CdS, you can mix Cd(N03)2 and Na2S in water. If you 

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