exciton

exciton [′ek·sə‚tän]

(solid-state physics)

An excited state of an insulator or semiconductor which allows energy to be transported without transport of electric charge; may be thought of as an electron and a hole in a bound state.

---

Exciton

A fundamental quantum of electronic excitation in condensed matter, consisting of a negatively charged electron and a positively charged hole bound to each other by electrostatic attraction. Excitons exist in all kinds of condensed matter, whenever it is possible for an electron to be excited from a filled energy level to an empty one, leaving behind a hole. Unlike an excitation in a single atom or molecule, an exciton can in general move through the solid like a particle. Excitons transport energy, not charge or mass. Typically, an exciton is created when a photon is absorbed in a solid; the exciton then moves through the crystal; and finally the electron and hole recombine, resulting in the emission of another photon, often at a wavelength different from that of the original photon. Excitons can also be created by injection of free electrons into excited states via an electric current. See Electron-hole recombination, Hole states in solids, Luminescence

Excitons fall into two broad classes, Wannier (or Wannier-Mott) excitons and Frenkel excitons, based on their size relative to the interatomic or intermolecular distances in the material. In Wannier excitons, typically observed in covalent semiconductors and insulators, the electron and hole are separated by a distance much larger than the atomic spacing, so that the effect of the crystal lattice on the exciton can be taken into account primarily via an average permittivity. In Frenkel excitons, typically seen in molecular or rare-gas crystals, the electron and hole are separated by a distance comparable to the atomic spacing, so that the exciton is localized to a single site at any given time. Wannier excitons move essentially like free particles, while motion of Frenkel excitons is envisioned as hopping from one site to another. See Electric insulator, Semiconductor