Quantum Dots Dielectric Shelling Optimization Exploiting a Design of Experiment Approach

Colloidal Quantum Dots (QDs) are a mature technology, currently exploited in consumer electronics products such as displays [1]. Some of the QDs light-emission properties are exploitable at the nanoscale too, via their integration in advanced photonic devices. Usually, to accomplish this integration, photonic components are added through microfabrication on the same substrate where QDs are deposited. A novel approach would be to combine photonic components directly on the QD surface, thus obtaining single particles presenting efficient and tunable photoluminescence together with a controlled emission. However, the difference in size between photonics components (&gt; 100 nm) and QDs (&lt; 20 nm) strongly hinders this approach. Dielectric shells such as SiO₂ or TiO₂ are promising candidates to increase the size of QDs to make their incorporation possible into larger structures with no or low impact on their emission properties. Other than increasing the particles’ size, these shells can also be exploited as a support for their combination with photonic components through nano fabrication, or as a template for building multi-layer and multi-materials structures [2]. Recently, our group exploited the growth of a SiO₂ shell on CdSe/CdS QDs to create ordered arrays of single photon emitters inside a poly(methyl methacrylate) pattern [3]. In most cases, however, the final particles size obtained through literature available procedures (either Reverse Microemulsion or Stöber) is modest. These procedures suffer from low reproducibility and tunability mainly because they were developed and optimized exploiting an OVAT (one-variable-at-a-time) approach [4]. Here we report a systematic optimization study on SiO₂shell growth on CdSe/CdS QDs, exploiting a full factorial “design of experiments” (DoE) [5]. Our approach allowed us to increase the SiO₂ shell thickness obtained with a single Reverse Microemulsion reaction from 15.5 nm to 22.75 nm, with a 47% improvement. The addition of a further Reverse Microemulsion step and of a Stöber process, made viable by the increased stability of the particles obtained through the optimized procedure, allowed to consistently grow a thicker SiO₂ shell, increasing the total diameter of QDs up to 95 nm. Finally, the influence of a SiO₂ shell having different structures (glass, SiO₂ and quartz) or the presence of multi-materials shells on the emission of a QD was investigated through a dedicated modelling. Our results showed that the presence of a large enough shell with high refractive index or the presence of multiple layers of different materials can enhance the emission of the quantum dots through photon outcoupling.<br/><b>References</b><br/>[1] Kim, J.; Roh, J.; Park, M.; Lee, C. “Recent Advances and Challenges of Colloidal Quantum Dot Light-Emitting Diodes for Display Applications” Adv. Mat. 2212220 (2023)<br/>[2] Ji, B.; Giovanelli, E.; Habert, B.; Spinicelli, P.; Nasilowski, M.; Xu, X.; Lequeux, N.; Hugonin, J.-P.; Marquier, F.; Greffet, J.-J.; Dubertret, B. “Non-Blinking Quantum Dot with a Plasmonic Nanoshell Resonator”, Nat. Nanotechnol<i>. </i><b>10</b>, 170 (2015)<br/>[3] Barelli, M.; Vidal, C.; Fiorito, S.; Myslovska, A.; Cielecki, D.; Aglieri, V.; Moreels, I.; Sapienza, R.; Di Stasio, F. “Single-Photon Emitting Arrays by Capillary Assembly of Colloidal Semiconductor CdSe/CdS/SiO ₂ Nanocrystals”, ACS Photonics <b>10</b>, 1662 (2023)<br/>[4] Koole, R., van Schooneveld, M. M., Hilhorst, J., de Donegal, C. M., ’T Hart, D. C., van Blaaderen, A., Vanmaekelbergh, D., & Meijerink, A. “On the incorporation mechanism of hydrophobic quantum dots in silica spheres by a reverse microemulsion method”, Chem. Mater. <b>20</b>, 2503 (2008)<br/>[5] Leardi, R. “Experimental design in chemistry: A tutorial”, Anal. Chim. Acta <b>652</b>, 161 (2009).