Strongly bound excitons in anatase TiO₂ single crystals and nanoparticles

Baldini, Eduardo; Chiodo, Letizia; Dominguez, A.; Palummo, M.; Moser, S.; Yazdi-Rizi, Meghdad; Auböck, G.; Mallett, Benjamin P. P.; Berger, Helmuth; Magrez, Arnaud; Bernhard, Christian; Grioni, M.; Rubio, Angel; Chergui, Majed

doi:10.1038/s41467-017-00016-6

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Journal article

Strongly bound excitons in anatase TiO₂ single crystals and nanoparticles

Baldini, Eduardo Laboratory of Ultrafast Spectroscopy, ISIC and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
Chiodo, Letizia Unit of Nonlinear Physics and Mathematical Modelling, Department of Engineering, Università Campus Bio-Medico di Roma, Rome, Italy - Center for Life Nano Science @ Sapienza, Istituto Italiano di Tecnologia, Rome, Italy
Dominguez, A. Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany
Palummo, M. Dipartimento di Fisica and INFN, Università TorVergata, Via della Ricerca Scientifica, Rome, Italy
Moser, S. Laboratory of Electron Spectroscopy, IPHYS and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
Yazdi-Rizi, Meghdad Department of Physics, University of Fribourg, Switzerland
Auböck, G. Laboratory of Ultrafast Spectroscopy, ISIC and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
Mallett, Benjamin P. P. Department of Physics, University of Fribourg, Switzerland
Berger, Helmuth Crystal Growth Facility, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland
Magrez, Arnaud Crystal Growth Facility, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland
Bernhard, Christian Department of Physics, University of Fribourg, Switzerland
Grioni, M. Laboratory of Electron Spectroscopy, IPHYS and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
Rubio, Angel Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany - Departamento Fisica de Materiales, Universidad del País Vasco, San Sebastian, Spain
Chergui, Majed Laboratory of Ultrafast Spectroscopy, ISIC and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

13.04.2017

Published in:

Nature Communications. - 2017, vol. 8, no. 1, p. 13

English 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.

Faculty

Faculté des sciences et de médecine

Department

Département de Physique

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