Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

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.

Strongly Bound Excitons in Anatase TiO$_2$ Single Crystals and Nanoparticles

This paper investigates the nature of excitonic states in anatase TiO$_2$, a material pivotal for light-energy conversion applications, yet whose fundamental charge excitations remain largely unexplored. The research combines experimental techniques such as angle-resolved photoemission spectroscopy (ARPES) and spectroscopic ellipsometry (SE) with advanced ab initio calculations to present a comprehensive assessment of the excitonic behavior in this material.

The study reveals that anatase TiO$_2$ single crystals possess a strongly bound exciton, arising above the continuum of indirect interband transitions. This exciton exhibits a hybrid character, situated between the Wannier-Mott and Frenkel exciton regimes, and is characterized by an unusual two-dimensional wavefunction within the three-dimensional lattice. The paper further identifies the nature of higher-energy excitations, which contribute to the optical properties of this material.

Key numerical data from the study include measurements of the direct optical gap at 3.79 eV and an exciton binding energy ($E_\mathrm{B}$) of 180 meV for a bound exciton, derived via empirical methods for the first time. These findings are consistent across various doping levels and temperatures, demonstrating robustness of the elementary excitations up to room temperature. Ab initio calculations support the experimental data and provide insight into the stability and spatial properties of the bound excitons.

Implications for practical applications are considerable, as the discovery of stable bound excitons in anatase TiO$_2$ could enhance the material's capabilities in devices aimed at light-energy conversion, such as solar cells or optical switches. The excitons' two-dimensional nature may also lead to novel ways of guiding energy at the nanoscale, particularly through the design of engineered nanostructures.

The research lays the foundation for future explorations into exciton transport properties and their interaction with vibrational degrees of freedom, positing that further advances in many-body theory could enable technological breakthroughs in optoelectronic applications.