Strongly bound excitons in anatase TiO₂ single crystals and nanoparticles

Area of Science:

• Materials Science
• Condensed Matter Physics
• Photochemistry

Background:

• Anatase titanium dioxide (TiO2) is a key material for light-energy conversion.
• Understanding fundamental charge excitations in anatase TiO2 is crucial but remains unknown.
• Distinguishing between uncorrelated electron-hole pairs and bound excitons is essential.

Purpose of the Study:

• To elucidate the nature of fundamental charge excitations in anatase TiO2.
• To determine if light absorption generates uncorrelated electron-hole pairs or bound excitons.
• To characterize the nature of these excitations, particularly in single crystals.

Main Methods:

• Combined steady-state angle-resolved photoemission spectroscopy (ARPES) and spectroscopic ellipsometry.
• Employed state-of-the-art ab initio calculations.
• Utilized ultrafast two-dimensional deep-ultraviolet (2D-DUV) spectroscopy on various samples.

Main Results:

• Demonstrated that the direct optical gap of single-crystal anatase TiO2 is dominated by a strongly bound exciton.
• This exciton exhibits an intermediate character (Wannier-Mott/Frenkel) with a 2D wavefunction in a 3D lattice.
• Identified higher-energy excitations and confirmed results up to room temperature in defect-rich samples.

Conclusions:

• Light absorption in anatase TiO2 primarily forms strongly bound excitons, not uncorrelated pairs.
• The identified excitons possess unique intermediate and quasi-2D characteristics.
• These findings are universally valid across different sample types and temperatures.