Encapsulation of Cadmium-Free InP-based Quantum Dots Into Polymer Microparticles: Electrospraying and Microfluidic Approaches

Promovendus/a: Iurii Babkin

Promotor(en): Prof. dr. Christian Clasen, Prof. dr. Guy Van den Mooter

Quantum dots (QDs) are semiconductor nanocrystals utilized in various optoelectronic applications, including light-emitting diodes (LEDs) and displays. The majority of currently available QDs are based on hazardous and highly toxic metals such as cadmium (Cd), lead (Pb), and mercury (Hg), which not only pose environmental concerns but also fail to comply with the European Restriction of Hazardous Substances (RoHS) regulation of the European Union. Recent advancements in the field have focused on safer QD alternatives, employing elements from the III-V group of the periodic table, notably indium (In). However, these novel developments still lag behind the performance quality of the first generation of Cd-based QDs and are only beginning to catch up. One significant challenge that arises is the low overall photostability of QDs under environmental influences, particularly in oxygen-rich atmospheres, due to photooxidative degradation processes. The conventional core/shell design of colloidal QDs is often insufficient to maintain the desired level of stability on its own, thus additional protective measures are required -- through encapsulation in suitable host materials.

In light of this, the focus of this dissertation is to present pathways for the production of photostable InP-based QDs by encapsulation in mono-disperse polymer microparticles of tunable dimensions via microfluidic and electrospraying approaches.

As a starting point, the encapsulation strategy for QDs is presented in the thesis, along with a detailed properties overview of both pristine and encapsulated InP/ZnSe/ZnS QDs in poly(lauryl methacrylate-co-ethylene glycol dimethacrylate) for a better understanding of the consequences of the incorporation into the polymer. The strategy is a new approach that combines the dispersion of colloidal InP-based core-shell QDs with modified ligands within a suitable monomer to prevent aggregation, and the subsequent creation of a cross-linking network to kinetically trap QDs in parallel with a copolymerization via direct covalent bonding of QDs with the growing polymer chains via their ligands. The possibility of covalent bond formation between the ligands of the QDs and the polymer was verified.

After that, a successful embedding of the QDs into polymer microparticles via a droplet-based microfluidic approach is discussed. Monomer droplets containing the QDs were generated by an oil-in-water (O/W) co-flow setup and were exposed to UV irradiation, which crosslinked them into solid particles in-flow in a delay channel. The inclusion of the nanoparticles in the polymer precursors increased the viscosity of the dispersed phase and affected the flow conditions, thus the transition from dripping to jetting and droplet sizes. The addition of QDs postponed the start of the polymerization reaction but did not affect the rheological polymerization profile. Besides this, a photobrightening effect was observed during stability tests. The oxygen influence on the QDs demonstrated to have a greater impact in relation to the decreased thickness of the polymer protection layer.

Finally, the QDs were encapsulated by coaxial electrospraying and the results are discussed. A liquid-core/solid-shell particle structure was first formed due to the required cross-linking polymerization step from initial liquid monomers. During the flight of droplets, the monomer core with QDs was captured by the shell formed due to solvent evaporation. After that, the monomers were later polymerized via a thermal initiation of the reaction, resulting in particles with embedded InP-based QDs. The monomer solution, with and without QDs or the initiators, was possible to electrospray without the use of any solvents. The addition of QDs almost did not affect the thermal polymerization reaction times and did not affect rheological profiles. The photobrightening effect was also observed during stability tests and was more pronounced in particles with the monomer presence. Overall, this approach showcased how coaxial electrospraying could be used not only for polymers with linear chains that can be completely soluble in solvents but also for cross-linked polymers that are of the main interest for the encapsulation of QDs.

In conclusion, the overall result of both approaches demonstrated a significantly improved photostability of the embedded InP-based QDs in comparison to the pristine QDs, representing two unique pathways to achieving stable and environmentally friendly QDs.