Physicists shine light on solid way to extend excitons' life

Optics researchers at The University of Texas at Dallas have shown for the first time that a new method for manufacturing ultrathin semiconductors yields material in which excitons survive up to 100 times longer than in materials ...

Atomically thin semiconductors for nanophotonics

Atomically thin semiconductors such as molybdenum disulfide and tungsten disulfide are promising materials for nanoscale photonic devices. These approximately 2D semiconductors support so-called excitons, which are bound ...

Tunable quantum traps for excitons

Researchers at ETH Zurich have succeeded for the first time in trapping excitons—quasiparticles consisting of negatively charged electrons and positively charged holes—in a semiconductor material using controllable electric ...

Developing inorganic lead-free perovskite for broadband emission

Artificial lighting accounts for one-fifth of global electricity consumption, and developing efficient and stable luminescence materials is critical to avoid unnecessary waste of electric energy. The single emitters with ...

Strong light-matter coupling in organic crystals

Organic semiconductors are an emerging class of materials for opto-electronic devices such as solar cells and organic light emitting diodes. As a result, it's important to tune materials properties for specific requirements ...

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Exciton

An exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge.

An exciton forms when a photon is absorbed by a semiconductor. This excites an electron from the valence band into the conduction band. In turn, this leaves behind a localized positively-charged hole (holes actually don't exist, the term is an abstraction for the location an electron moved from; they have no charge in and of themselves). The electron in the conduction band is then attracted to this localized hole by the Coulomb force. This attraction provides a stabilizing energy balance. Consequently, the exciton has slightly less energy than the unbound electron and hole. The wavefunction of the bound state is said to be hydrogenic, an exotic atom state akin to that of a hydrogen atom. However, the binding energy is much smaller and the particle's size much larger than a hydrogen atom. This is because of both the screening of the Coulomb force by other electrons in the semiconductor ( i.e., its dielectric constant), and the small effective masses of the excited electron and hole. The recombination of the electron and hole, i.e. the decay of the exciton, is limited by resonance stabilization due to the overlap of the electron and hole wave functions, resulting in an extended lifetime for the exciton.

The electron and hole may have either parallel or anti-parallel spins. The spins are coupled by the exchange interaction, giving rise to exciton fine structure. In periodic lattices, the properties of exciton show momentum (k-vector) dependence.

The concept of excitons was first proposed by Yakov Frenkel in 1931, when he described the excitation of atoms in a lattice of insulators. He proposed that this excited state would be able to travel in a particle-like fashion through the lattice without the net transfer of charge.

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