Size dependent optical effects of semiconductor nanoparticles. Semiconductor nanoparticles contain size dependent electronic and optical properties. A series of five different sized ZnS-capped CdSe semiconductor nanoparticles called quantum dots is used to demonstrate this principle. When these quantum dots are 2 nm, they emit in the blue but at 6 nm, they emit in the red. The size is inversely related the bandgap energy of the nanoparticle. This bandgap energy dictates the fluorescence emission.