A quantum dot (QD), or semiconducting nanocrystal (NC), is a single crystal of a semiconducting material measuring only a few nanometres in diameter. When excited, the small size of the crystal acts a ‘quantum box’ and confines electrons and holes in an volume smaller than the corresponding exciton Bohr radius. The smaller the dot, the greater the confinement energy and the higher the energy of photons that are absorbed or emitted.
The most well-studied quantum dots are metal chalcogenide quantum dots based on semiconductors such as cadmium selenide, indium phosphide or Lead(II)sulfide. The bandgap of such quantum dots can be tuned throughout the entire visible spectrum simply by changing their size during chemical synthesis.
For the highest photoluminescence quantum yields (PLQYs), a core/shell structure is usually required. In this arrangement, a second semiconductor is used to encapsulate the nanocrystal (e.g. CdSe/CdS, InP/ZnS). This material passivates surface defects of the emissive core which would otherwise act as non-radiative recombination sites for excitons.
Due to their high PLQY, relative ease of fabrication and wide emission-colour tunability, quantum dots having this type of structure are especially suitable for application in display and imaging technologies - and are already appearing in commercial products such as televisions.