First proposed by Yakov Frenkel in 1931, an exciton forms when a photon promotes a valence electron to the conduction band and Coulomb attraction binds it to the positively charged hole left behind.
Tightly bound Frenkel excitons in insulators and organic crystals have binding energies of 0.1 to 1 eV, whereas screened Wannier-Mott excitons in semiconductors span many unit cells (e.g. 4.2 meV and 13 nm Bohr radius in GaAs).
In atomically thin 2D transition-metal dichalcogenide monolayers, reduced dielectric screening boosts exciton binding energies to roughly 0.5 eV, making excitonic optical peaks prominent even at room temperature.