An LED’s color depends on the semiconductor material it’s made of. When current flows through an LED, an excited electron crosses the semiconductor bandgap. It can then relax and recombine with a hole in the valence band, resulting in the emission of a photon. The light’s color, therefore, varies with the energy required to cross the bandgap. (To learn more about blue LEDs, see Physics Today, December 2014, page 14.)

But new behavior emerges when a semiconductor shrinks to a few nanometers in size. When electrons in nanocrystals get excited, the emitted photons can have only certain allowed energies. And as the space that an electron can occupy shrinks, so too do the allowed values of its wavelength, according to the particle-in-a-box model in quantum mechanics, and the bandgap of the semiconductor widens.

Semiconductor nanocrystals are often called quantum dots, and the wavelengths they shine at depend only...

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