We performed first-principle calculations to study the electronic and optical properties of TiO2, with special emphasis in the interplay between the charge excitations and the coherent vibration of the lattice (phonons). Especially, we have sought to improve the theoretical description of the interaction between electrons and phonons. This is a very challenging task when done at a high level of theoretical accuracy, but necessary to correctly characterise the optical properties of titania and their dependence with the temperature. Furthermore, we have studied the mobility and transport of the charge carriers in TiO2, and compared the properties for the migration in different directions inside the crystal.
We found the deformation potential coupling as the dominant mechanism for the generation and detection of coherent acoustic phonons in TiO2 nanoparticles and single crystals. These coherent acoustic phonons are responsible for a huge modulation of the exciton peak amplitude as well as a giant exciton energy shift in bulk TiO2. The detection and generation of these phonons are observed close to an exciton resonance at room temperature. Our calculations reveal that this exciton resonance exhibits extraordinarily large photo elastic coefficients, comparable to those found for quantum confined nanoparticles in the visible spectrum. These findings pave the way for the design of exciton control schemes using strain pulses.
For the proper description of the electron-phonon interaction in TiO2, we have joined efforts with theoreticians working on a framework that can describe the effects of multiple phonons using a single arrangement of the atomic positions in a supercell. The combination of this scheme with many body perturbation theory has resulted so far in the accurate account of the zero point renormalisation of the electronic gap of TiO2. This had never been accomplished before.
The results obtained within this project have been presented in the following conferences and seminars:
-2017 Seminar at Duke University, USA, “Excitonic quasiparticles in TiO2”
-2018 Seminar at UPV/EHU, Basque Country, Spain “Excitonic quasiparticles in TiO2”
-2018 Seminar at University of Oxford, UK “Excitonic quasiparticles in TiO2”
-2018 9th International Conference on Spontaneous Coherence in Excitonic Systems, Montreal, Canada “Excitonic quasiparticles in TiO2”
-2019 DESY NanoLab annual kickoff meeting, Zeuthen, Germany “Bulk and 2028surface properties of TiO2 photocatalyst”
-2019 World Congress on Laser, Optics and Photonics, Barcelona, Spain “Excitonic quasiparticles in TiO2”
All results and internal reports have been brought multiple time to discussion with our collaborators for their further exploitations. Some of our findings have been published in high impact international journals with open access. The rest will be published in the near future, always making them accessible free of charge.
