Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

TL;DR

This study reveals that anatase TiO2 exhibits strongly bound excitons with unique properties, confirmed through advanced spectroscopy and calculations, impacting understanding of its light-energy conversion capabilities.

Contribution

The paper combines experimental and theoretical methods to demonstrate the existence and nature of strongly bound excitons in anatase TiO2, a key insight for its optoelectronic applications.

Findings

Strongly bound exciton dominates the optical gap

Exciton has intermediate Wannier-Mott and Frenkel character

Excitations are consistent across different sample types and temperatures

Abstract

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. 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.