From 2011.igem.org

Quantum dots are semiconducting nanoscale crystals with unique optical properties. They are usually made of cadmium and selenium (CdSe). They absorb a wide spectra of electromagnetic radiation and can emit visible photons of a narrow frequency range corresponding to the size of the crystal. Smaller particles appear bluer, and larger ones appear redder. Quantum dots are also bright, stable, and resistant to photobleaching. Because of these properties, quantum dots have many interesting applications, including medical imaging and labeling, enhanced LEDs, novel solar cells, and solid state quantum computation.