Colloidal semiconductor nanocrystals, also known as quantum dots (QDs), are vanishingly small “nano” particles of material. They are so tiny that it would take roughly 100,000 of them to span your fingernail. Owing to their small size, these materials are especially advantageous due to having remarkably high efficiency and size-tunable photoluminescence (PL, light emission) over a wide-range of colors. They are very effective at absorbing a broad spectrum of light and converting that energy into emitted light of a single color that we tune by controlling size. Tuning the size and sometimes composition of the dots is achieved by carefully adjusting manufacturing conditions.
Using “solution” processing techniques, liquids, solids, powders, and films of the material can be prepared. This allows for imparting the novel tunability of dots’ absorption/emission spectra into a wide variety of everyday products. In this way QDs can enhance some things or augment existing functionality. For example, making a pair of shoes exhibit fluorescence of any color across the full spectrum could create an attractive new look, but it could also be used to deter counterfeiters, or make a runner more visible at night.
As size increases, the absorption onset and PL spectrum shifts to redder wavelengths, while decreasing the diameter shifts the absorption and PL shift towards the blue. The size tunability of colloidal QDs make them an attractive alternative to fluorescent dyes or doped phosphors since many colors can be achieved from the same material. Compared with dyes, QDs are also typically more light-stable.
QDs typically have an onion-like structure of layers upon layers where subsequent shells protect the precious light-emitting “core” materials from the outside world. Their outer surface is terminated with a sort of “hair” composed of organic molecules.