trions

A trion is a charged exciton, a bound state of three charge carriers: two electrons and one hole (negative trion) or two holes and one electron (positive trion).

Trions form in semiconductors when an exciton captures an extra charge carrier. Binding energies are typically a few meV in bulk materials, but in two-dimensional systems such as monolayer transition metal dichalcogenides they reach tens of meV.

In optically active materials, trions show characteristic photoluminescence and absorption features at energies below the neutral exciton. They have finite lifetimes and their properties depend on carrier density, temperature, and the surrounding dielectric environment.

Trions have been observed in a range of materials, including quantum wells, carbon nanotubes, and quantum dots, with particular prominence in two-dimensional semiconductors such as MoS2, WS2, and WSe2. They interact with valley and spin degrees of freedom in these systems.

Theoretical descriptions treat trions as three-body Coulomb bound states, using effective mass approximations, variational methods, or many-body formalisms such as the Bethe–Salpeter equation. Material parameters determine binding energies and the spatial structure of the trion wavefunction.

Research on trions informs the understanding of exciton physics, carrier dynamics, and the design of optoelectronic and valleytronic devices in low-dimensional semiconductors, where charged excitons influence light emission and charge transport.