Excitons are electron-hole pairs in semiconductors that are electrostatically bound by strong Coulombic interactions. In ultrathin 2D semiconductors that are a few atoms thick (typically ~0.7 nm), the decreased z-dimension presents strong quantum confinement and reduced dielectric effects that feature a plethora of stable excitonic species (neutral excitons, trions, biexcitons, defect-bound excitons).
