Quantum dots make nearly ideal photonic devices. And colloidal chemistry can make nearly ideal quantum dots. Measuring some 1–6 nm across, such QDs are semiconductor crystals in which the potential-energy barriers at the dot’s boundaries strongly confine the electron wavefunctions in three dimensions. Owing to that confinement, a QD’s electronic response to a photon is much like that of an atom, producing a discrete energy spectrum that arises from the excitation of electron–hole pairs. The electron and hole that make up the pair, called an exciton, attract each other electrostatically and can recombine to create a photon extremely efficiently, a property that makes the dots strong light emitters.
What’s more, the wavelength of that light emission can be tuned over a wide range simply by tailoring the size of QDs grown in solution. And because such QDs can be chemically manipulated like large molecules, they can be painted onto surfaces,...
