Public Thesis Defense of Julien BOUQUIAUX - IMCN

Vibronic Fingerprints of Inorganic Phosphor Materials from First Principles by Julien Bouquiaux

Vendredi 21 février 2025 à 16h00 – Auditoire SUD11, place Croix du Sud à 1348 Louvain-la-Neuve

This thesis explores the vibronic properties of Eu-doped phosphor materials, which are essential components in modern lighting technologies, particularly in phosphor-converted white light-emitting diodes. Using first-principles simulations, the work focuses on understanding and modeling the luminescence lineshapes of these materials through a generalized Huang-Rhys model, fed by density functional theory calculations. The computational methodology addresses key challenges such as describing long-range atomic displacements induced by electronic transitions in computationally tractable supercell sizes, and simultaneously capturing the coupling with long-wavelength phonons and defect-localized phonons using an embedding approach for the interatomic force constant matrix.
This research introduces the “Lumabi” Python package, a tool designed to automate the computation of phonon-resolved luminescence spectra in defect systems using the ABINIT density functional theory (DFT) software.
A significant portion of the study is dedicated to Eu-doped UCr4C4-type phosphors, such as Sr[Li2Al2O2N2]:Eu2+ and Sr[LiAl3N4]:Eu2+ . Detailed analyses of atomic displacements within the 1D cation channel provide insights into their emission spectra. The coupling of these displacements with low-frequency phonon modes was identified as a common cause of the characteristic narrow emission lineshapes observed in these materials.
Moreover, the theoretical methodology was applied to identify defect centers responsible for luminescence via their vibronic fingerprints. In the red-emitting phosphor Sr[LiAl3N4]:Eu2+, the two non-equivalent Sr sites were distinguished based on their respective luminescence spectra and comparisons with experimental data. Similarly, in the green-emitting phosphor -SiAlON: Eu2+, simulations confirmed the Eu interstitial site in the host structure to be the emitting one.
We hope to provide valuable insights and tools aimed at deepening the understanding of the material-property relationships in Eu-doped phosphors.
Jury members:
Prof. Xavier Gonze (UCLouvain), supervisor
Prof. Xavier Urbain (UCLouvain), chairperson
Prof. Gian-Marco Rignanese (UCLouvain), secretary
Prof. Clément Lauzin (UCLouvain)
Prof. Samuel Poncé (UCLouvain)
Dr. Stéphane Jobic (Institut des matériaux Nantes, France)
Dr. Masayoshi Mikami (Mitsubishi Chemical Corp., Japon)
Pay attention: the public defense of Julien Bouquiaux will also take place in the form of a videoconference.