Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

E. Baldini (), L. Chiodo, A. Dominguez, M. Palummo, S. Moser, M. Yazdi-Rizi, G. Auböck, B.P.P. Mallett, H. Berger, A. Magrez, C. Bernhard, M. Grioni, A. Rubio and M. Chergui ()
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E. Baldini: École Polytechnique Fédérale de Lausanne (EPFL)
L. Chiodo: Università Campus Bio-Medico di Roma
A. Dominguez: Max Planck Institute for the Structure and Dynamics of Matter
M. Palummo: Università Tor Vergata
S. Moser: École Polytechnique Fédérale de Lausanne (EPFL)
M. Yazdi-Rizi: University of Fribourg
G. Auböck: École Polytechnique Fédérale de Lausanne (EPFL)
B.P.P. Mallett: University of Fribourg
H. Berger: École Polytechnique Fédérale de Lausanne (EPFL)
A. Magrez: École Polytechnique Fédérale de Lausanne (EPFL)
C. Bernhard: University of Fribourg
M. Grioni: École Polytechnique Fédérale de Lausanne (EPFL)
A. Rubio: Max Planck Institute for the Structure and Dynamics of Matter
M. Chergui: École Polytechnique Fédérale de Lausanne (EPFL)

Nature Communications, 2017, vol. 8, issue 1, 1-11

Abstract: Abstract 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 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. 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.

Date: 2017
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DOI: 10.1038/s41467-017-00016-6

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