Towards temperature dependent exciton properties in titania using properly screened density functional approximations

• Applicant: Professor Dr. Michael Sentef, since 9/2023
• Subject Area: Theoretical Condensed Matter Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496750092

Titania is doubtlessly one of the most studied materials in the presenty due to is potential in far-reaching fields like photo-catalysis, light-energy conversion and air cleaning. However, there is still a huge knowledge vacuum to be filled, especially on the nature of its fundamental optical ex-citations. The reason why a deep comprehension of the behaviour of excitons in this semicon-ductor has remained evasive is in great part due to the large computational effort entailed in its study. In this project we address specifically the temperature dependence of the optical spectra in tita-nia. Recent experimental studies indicate that temperature induced changes in reflectivity de-pend strongly on the polarization of the incident light and lead to blue shifts of some excitonic features, while others shift to the red spectral region. Such a highly anisotropic and state specific response cannot simply be explained by thermal lattice expansion. Instead, it calls for atomistic simulations that take the normal mode specific electron-phonon interactions into account. In addi-tion, an accurate determination of the titania band structure and screened electron-hole interac-tion in this material is prerequisite. To reach these requirements, we propose to extend the time-dependent density functional based tight binding method (TD-DFTB) to periodic systems, incorporating XC functionals with a proper screening of the electron-electron interaction and accurate long-range behavior. TD-DFTB is an approximate version of time-dependent DFT (TD-DFT). Given the accurate predictions of TD-DFTB for finite molecular systems (including the difficult class of charge-transfer states), we expect to arrive at a generally applicable method to predict exciton energies, absorption strengths and electron-hole pair localization. This development makes the large class of crystalline materials amenable to a simplified theoretical treatment, that takes key as-pects of the electron-electron interaction into full account. The special interest in TD-DFTB in the context of temperature dependent optical properties stems from the highly reduced computational cost with respect to conventional TD-DFT and many body perturbation theory. In a first principles treatment, temperature dependent spectra can be obtained by sampling the nuclear wavefunction at various geometries and computing the dielectric function for each of the configurations. For TD-DFTB, this sampling can be converged without an ad-hoc selection of certain normal modes that are deemed more important than others. In this way we attempt to provide our collaborating experimentalists with an unbiased microscopic picture of the temperature dependent photo-response of titania in its various poly-morphs. This novel approach will also be applied to other oxides.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Professor Dr. Edoardo Baldini
• Ehemalige Antragsteller: Professor Dr. Thomas Frauenheim, until 10/2022; Professor Dr. Frank Jahnke, from 10/2022 until 8/2023