Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

Anatase TiO2 is among the most studied materials for light-energy conversion applications,but the nature of its fundamental charge excitations is still unknown. Yet it is crucial toestablish whether light absorption creates uncorrelated electron–hole pairs or bound excitonsand, in the latter case, to determine their character. Here, by combining steady-state angleresolvedphotoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art abinitio calculations, we demonstrate that the direct optical gap of single crystals is dominatedby a strongly bound exciton rising over the continuum of indirect interband transitions. Thisexciton possesses an intermediate character between the Wannier–Mott and Frenkel regimesand displays a peculiar two-dimensional wavefunction in the three-dimensional lattice.The nature of the higher-energy excitations is also identified. The universal validity of ourresults is confirmed up to room temperature by observing the same elementary excitations indefect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensionaldeep-ultraviolet spectroscopy.