exzitonen

Exzitonen (exciton) are bound states of an electron and a hole in a dielectric or semiconductor created when the material absorbs light with energy above the bandgap. They are neutral quasi-particles that can transport energy without carrying net electric charge, and they significantly influence the optical response of solids.

Formation and types: In inorganic semiconductors, two main classes are recognized. Wannier–Mott excitons have a large spatial extent and relatively small binding energy, described by a hydrogenic model using the material’s dielectric constant and effective masses. Frenkel excitons are more localized, with a radius on the order of a lattice constant, and are common in organic molecular crystals and some insulators. At interfaces, charge-transfer excitons can form, with the electron and hole residing on adjacent molecules or sites.

Properties: Binding energies range from a few millielectronvolts to hundreds of millielectronvolts, depending on the material and dimensionality. The exciton size, characterized by the Bohr radius, reflects whether the pair is Wannier–Mott-like or Frenkel-like. In two-dimensional materials such as transition metal dichalcogenides, excitons exhibit unusually large binding energies and strong optical signatures, enabling robust excitonic effects at room temperature.

Dynamics and applications: Exzitonen can diffuse or hop through a material and may recombine radiatively, emitting light, or non-radiatively. Their behavior shapes absorption spectra, luminescence, and energy transfer processes, with applications in light-emitting diodes, solar cells, and exciton-based devices. In 2D materials, strong excitonic effects drive advances in optoelectronics and valleytronics.

Observations: Exzitonen are detected by absorption and emission spectroscopy, time-resolved photoluminescence, and ultrafast spectroscopic techniques that reveal their formation, dynamics, and decay pathways.