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Title: Strongly bound excitons in anatase TiO 2 single crystals and nanoparticles

Anatase TiO 2 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 to establish whether light absorption creates uncorrelated electron-hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier-Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. Furthermore, the universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.
Authors:
 [1] ; ORCiD logo [2] ;  [3] ;  [4] ;  [1] ;  [5] ;  [1] ;  [5] ;  [1] ;  [1] ;  [5] ;  [1] ; ORCiD logo [6] ;  [1]
  1. Ecole Polytechnique Federale de Lausanne, Lausanne (Switzlerland)
  2. Univ. Campus Bio-Medico di Roma, Rome (Italy); Istituto Italiano di Tecnologia, Rome (Italy)
  3. Max Planck Institute for the Structure and Dynamics Matter, Hamburg (Germany)
  4. Univ. Tor Vergata, Rome (Italy)
  5. Univ. of Fribourg, Fribourg (Switzerland)
  6. Max Planck Institute for the Structure and Dynamics of Matter, Hamburg (Germany); Univ. del Pais Vasco, San Sebastian (Spain)
Publication Date:
Grant/Contract Number:
AC02-05CH11231
Type:
Accepted Manuscript
Journal Name:
Nature Communications
Additional Journal Information:
Journal Volume: 8; Journal Issue: 1; Journal ID: ISSN 2041-1723
Publisher:
Nature Publishing Group
Research Org:
École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; electronic properties and materials; semiconductors
OSTI Identifier:
1355910