Multishell Silver Indium Selenide-Based Quantum Dots and Their Poly(methyl methacrylate) Composites for Application in Red-Light-Emitting Diodes

In this work, the production of novel multishell silver indium selenide quantum dots (QDs) shelled with zinc selenide and zinc sulfide through a multistep synthesis precisely designed to develop high-quality red-emitting QDs is explored. The formation of the multishell nanoheterostructure significantly improves the photoluminescence quantum yield of the nanocrystals from 3% observed for the silver indium selenide core to 27 and 46% after the deposition of the zinc selenide and zinc sulfide layers, respectively. Moreover, the incorporation of the multishelled QDs in a poly(methyl methacrylate) (PMMA) matrix via in situ radical polymerization is investigated, and the role of thiol ligand passivation is proven to be fundamental for the stabilization of the QDs during the polymerization step, preventing their decomposition and the relative luminescence quenching. In particular, the role of interface chemistry is investigated by considering both surface passivation by inorganic zinc chalcogenide layers, which allows us to improve the optical properties, and organic thiol ligand passivation, which is fundamental to ensuring the chemical stability of the nanocrystals during in situ radical polymerization. In this way, it is possible to produce silver-indium selenide QD-PMMA composites that exhibit bright red luminescence and high transparency, making them promising for potential applications in photonics. Finally, it is demonstrated that the new silver indium selenide QD-PMMA composites can serve as an efficient color conversion layer for the production of red light-emitting diodes.


𝑠𝑡
Where,  represents the photoluminescent quantum yield,  is the integrated intensity of the photoluminescent spectra,  is the absorption factor at the excitation wavelength and  is the diffraction index of the solvent.Finally, the subscripts  and  refer to the sample and the standard respectively.
Thick shell AISe/Zn(SeS) QDs, synthesis and characterisations: thick shell AISe/Zn(SeS) are produced using the synthetic scheme represented in (Figure S4).In this approach, a dispersion of zinc stearate in the presence of selenium and 1-dodecanthiol as the selenium and sulfur sources respectively is slowly injected into the reaction mixture containing AISe QDs core at 180 °C.
Synthesis: 12.0 ml of reaction mixture containing AISe QDs was degassed at 80 °C for 30 minutes to remove any volatile species formed during the synthesis.The temperature was raised to 180 °C and a mixture of zinc stearate (2.0 mmol) and selenium (2.0 mmol) dispersed in 1-octadecene (10.0 ml), 1-oleylamine (4.0 ml), 1-dodecanthiol (1.0 ml) and oleic acid (5.0 ml) was injected at the speed of 2.0 ml/h for 10 h.Then, the reaction mixture was maintained at 180 °C for further 8 h after the injection.When the reaction finished, the reaction mixture was cooled in a water bath.The product was transferred to a centrifuge tube, precipitated with methanol and ethanol, and then collected by centrifugation at 9000 rpm for 5 minutes.The pellet was dispersed in toluene.This purification step was repeated three times.The purified AISe/Zn(SeS) QDs were then dispersed in a 5.0 ml toluene solution for further use.
Characterisations: the addition of the zinc chalcogenide precursor solution causes the gradual blue shift of the UV-Vis absorption (Figure S5a) as expected for the diffusion of zinc in the QDs core, inducing the increasing of the band gap. 3,4Similar behavior is observed in the PL spectra (Figure S5b), the emission peak shifts with the increasing addition of the zinc chalcogenides precursor from 660 nm observed after the first hour of addition to 635, 628 and finally 610 nm for 3, 6 and 18 h respectively.A significant reduction of the FWHM from 141 to 122 nm and an increasing of the PLQY from 3 to 53% is observed.TEM (Figure S5c) shows the increasing of the nanocrystal size from 2.4 ± 0.5 nm to 4.0 ± 0.7 nm (Figure S5d) due to the deposition of the zinc chalcogenide shell.The XRD pattern (Figure S5e) shows a shifting of the diffraction of AISe QDs chalcopyrite core to higher angles in line with the deposition of the zinc chalcogenide shell. 5In particular, the (112) diffraction is observed at 26.7 °2θ, while the diffraction from the (201, 220) and (312,116) planes are observed at 44.6 and 52.7 °2θ respectively.Moreover, the diffractions at 64.2 and 71.5 °2θ can be related to (400) and (331) planes of sphalerite phase of ZnSe. 4,6The EDS analysis (Figure S5f) confirms that the final AISe/ZnSeS QDs retain the original silver to indium ratio of 1:2:91.Moreover, a S7 sulfur to selenium ratio of 1:1.8 and silver to zinc ratio of 1:6.8 is observed, considerably higher than that observed for the AISe/ZnSe/ZnS QDs sample characterised by thin shells.

Figure S4 :
Figure S4: representation of the synthetic scheme used for the production of AISe/Zn(SeS) QDs.

Figure S9 :
Figure S9: Photographs of AISe/Zn(SeS) PMMA composites produced without (pristine) and in the presence of MPTMS under ambient a) and UV b) light.c) UV-Vis absorption and transmittance spectra of AISe/Zn(SeS) PMMA composites.d) PL spectra of AISe/Zn(SeS) 3.0% w (solid line) and AISe/Zn(SeS) 1.5% w (dashed line) PMMA composite produced in the presence of MPTMS.

Figure S8 :
Figure S8: a) UV-Vis absorption and b) PL spectra of AISe/ZnSe/ZnS QDs un-passivated (dispersed without addition of further ligand after cleaning) and passivated by different thiol ligands in toluene.

Table S1 :
results of the analysis of the decay data =        2 