Fatema Yahya Mohamed

Project title: Excitons in novel one-dimensional materials

Host Institution: École Polytechnique (l’X)

Host Supervisor: Dr. Matteo Gatti

Co-host Institution: École Polytechnique Fédérale de Lausanne (EPFL)

Co-host Supervisor: Prof. Nicola Marzari

Summary project: The purpose of this project is to characterise, understand, and tailor exciton properties of novel one-dimensional (1D) materials. Excitons are quasiparticle excitations consisting of
electron-hole pairs bound by the Coulomb interaction. They play a key role in the optical response of materials as they mediate the absorption and emission of light. In solar cells,
excitons need to dissociate in order to convert their energy into electricity. Conversely, in excitonic optoelectronic devices, excitons have to remain bound in order to carry
information or to be converted back into photons. Therefore, mastering the nature of the excitons is crucial, as their binding energy and spatial localisation dictate which transport regime is possible. Low-dimensional compounds pose a great challenge since the large exciton binding energy in optical spectra cannot be used alone to identify the exciton character. The project combines forefront computational materials design and theoretical spectroscopy approaches to turn this challenge into an unprecedented opportunity to manipulate excitons at the nanoscale. Aiming at a more ecological approach, it will integrate efficient computational algorithms into accurate many-body theoretical methods. The project will identify novel 1D materials by data mining a materials database. It will uncover unconventional screening and exciton properties in 1D materials, establishing the best measurable quantity to determine the exciton character in nanosystems. Finally, it will achieve a predictive understanding of the relation between the exciton character and intuitive descriptors of materials properties (e.g. the size of band gaps and the nature of chemical bonds), drawing a powerful map of the 1D exciton landscape.