Identifying defects in ZnO@ZnS core-shell nanostructures using ultra-high-field NMR, NQR and DNP

Semiconductor nanocrystals, also known as quantum dots (QDs), are promising materials for a wide range of applications, including optoelectronics (display devices), optical bioimaging, solar cells and photocatalysis. Nevertheless, many QDs, such as CdS, contain toxic elements. ZnO or ZnS are environmental-friendly alternatives but their use is limited by their wide band gap, which prevents the absorption of visible light. Heterostructures made of ZnO core with ZnS shell (ZnO@ZnS) represent a promising solution to reduce the band gap. They can be prepared by sulfidation of ZnO nanocrystals. Nevertheless, mastering their design requires to control their defects, and notably the O and S vacancies, which are assumed to facilitate the exchange of O2− and S2− anions during sulfidation reaction. The IDEA project aims at determining the structure and the concentration of defects in ZnO@ZnS heterostructures through the development of advanced solid-state NMR experiments. Since the isotopes found in Zn(O,S) nanocrystals (17O, 67Zn and 33S) are extremely challenging to detect by NMR (quadrupolar nuclei with low gyromagnetic ratio and low natural abundance), we will develop innovative solid-state NMR methods relying on ultra-high-field NMR (up to 28 T), dynamic nuclear polarization (DNP), low-temperature (down to 30 K), and spin labelling strategies. The obtained NMR data for ZnO@ZnS heterostructures in combination with DFT calculations will provide unique information on the structure of their defects (vacancies, O/S substitution), their amount and distribution between the core and surface regions. These novel insights will be crucial to optimize the optoelectronic properties of these quantum dots in a rational way. Furthermore, this project will equip with unique research and outstanding competences, which will lay the foundation of my future career as a highly qualified scientist capable of starting an independent career.