Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

Nature Communications 8, (2017)

Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

E.Baldini,L.Chiodo,A.Dominguez,M. Palummo, S. Moser,M.Yazdi,G.Auböck,B.P. P. Mallett, H.Berger, A. Magrez, C. Bernhard, M.Grioni, A. Rubio, M. Chergui

Bound electronic excitations play a major role in the electrodynamics of insulators and are typically described by the concept of Wannier-Mott and Frenkel excitons. The former represent hydrogenic electron-hole pairs delocalized over several unit cells of a crystal and they occur in materials with high dielectric constant; the latter correspond to a correlated electron-hole pair localized on a single lattice site and they mostly prevail in molecular solids. Between these two extremes, an intermediate type of excitons exists, typically referred to as charge-transfer excitons. A prototypical system in which these rare quasiparticles have been theoretically predicted but never experimentally confirmed is the anatase polymorph of TiO2, which is one of the most promising material for light-energy conversion applications. Here, we combine angle-resolved photoemission and optical spectroscopies, along with ab initio state-of-the-art theoretical calculations, to demonstrate that the direct optical gap of anatase TiO2 is dominated by a charge-transfer exciton band rising over the continuum of indirect interband transitions. In particular, we find that the lowest exciton possesses a two-dimensional nature and is characterized by a giant binding energy of ∼ 300 meV. The universality of these findings is proven in highly defective samples used in light-energy conversion applications, by interrogating these systems out-of-equilibrium via ultrafast two-dimensional UV spectroscopy.

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