Unraveling vacancies and interfaces of zinc oxysulfide nanocrystals using DNP and NMR spectroscopy – IVAN

Semiconductor nanocrystals (NCs) also called quantum dots, made of zinc oxide (ZnO) or zinc sulfide (ZnS), are promising materials for applications, such as opto-electronics, optical bioimaging, solar cells and photocatalysis. However, their application is limited by their wide band energy gap that can be reduced by controlling the formation of defects, such as oxygen or sulfur vacancies. In particular, the sulfidation of ZnO NCs by reaction with H2S to form zinc oxysulfide, Zn(O,S) NCs, is a promising approach for the introduction of defects (O/S substitution and vacancies) and hence, band gap energy reduction. Nevertheless, the rational improvement of the optoelectronic properties of these Zn(O,S) NCs is limited by the lack of information about the structure of the defects and the mechanism of their formation.
IVAN project aims at determining the structure and the quantity of defects in oxysulfide NCs and to better understand their formation and their influence on the optoelectronic properties of these materials. The defects will be characterized 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 use of dynamic nuclear polarization (DNP), very low-temperature (down to 30 K), ultra-high magnetic field (up to 28 T) and spin labelling strategies. The obtained NMR data for Zn(O,S) NCs in combination with DFT calculations will provide unique information on the structure of their defects (vacancies, O/S substitution), their amount and the mechanism of their formation. These structural data will be correlated to their synthesis conditions (sulfidation rate, nature of ligands) as well as their measured optical and electrical properties. These synthesis-structure-properties relationships will be useful to improve the opto-electronic properties of Zn(O,S) NCs in a rational way.