Quantum confinement

Quantum confinement is when electrons and holes in a semiconductor are confined by a potential well in 1D (quantum well), 2D (quantum wire), or 3D (quantum dot). That is, quantum confinement occurs when one or more of the dimensions of a nanocrystal is made very small so that it approaches the size of an exciton in bulk crystal, called the Bohr exciton radius. A quantum well is a structure where the height is about the Bohr exciton radius while the length and breadth can be large. A quantum wire is a structure where the height and breadth is made small while the length can be long. A quantum dot is a structure where all dimension are near the Bohr exciton radius, typically a small sphere.

Light emission from bulk (macroscopic) semiconductors such as LEDs results from exciting the semiconductor either electrically or by shining light on it, creating electron-hole pairs which, when they recombine, emit light. The energy, and therefore the wavelength, of the emitted light is governed by the composition of the semiconductor material. If, however, the physical size of the semiconductor is considerably reduced to be much smaller than the natural radius of the electron-hole pair (Bohr radius), additional energy is required to "confine" this excitation within the nanoscopic semiconductor structure leading to a shift in the emission to shorter wavelengths.