[en] To investigate the thermal stabilities of CdSe and Cd-free quantum dots (QDs) in QD–silicone polymer nanocomposites, we synthesized both CdSe and CuInS₂ (CIS) QDs capped with different surface ligands (oleic acid (OA) and 1-dodecanethiol (1-DDT)). A comparison of the quantum yields of dried QDs after exposure to different temperatures revealed that the CIS QDs and 1-DDT ligands exhibited higher thermal stabilities than the CdSe QDs and OA ligands, respectively. We also prepared QD–silicone nanocomposites containing different types of QDs by varying the curing temperature and time and observed that nanocomposites containing CdSe QDs exhibited discoloration at high temperatures, whereas those containing CIS QDs did not. Moreover, the highest quantum efficiency (QE) of the QD–silicone nanocomposites was observed in the case of CIS QDs capped with 1-DDT and the lowest was observed in the case of CdSe QDs capped with OA. Because the dispersion states of QDs were not substantially affected at high temperatures, we attributed the high QE of nanocomposites containing the CIS QDs capped with 1-DDT to the superior thermal stability of CIS QDs and their surface ligand (i.e., 1-DDT).

[en] We present matrix-free methods for fabricating highly luminescent and transparent CdSe/ZnS quantum dot (QD)/polymer nanocomposites utilizing poly(methyl methacrylate) (PMMA)-grafted QDs with various molecular weights. We found that the QD-PMMA nanocomposites prepared by these matrix-free methods were superior to those prepared by a simple blending method in relation to their optical property, QD dispersion, and quantum efficiency (QE). In particular, a matrix-free nanocomposite containing PMMA with a molecular weight of 2000 had the highest QE (52.8%) and transmittance of all the samples studied even at a very high QD concentration (49 wt%). This finding was attributed to the enhanced passivation of the QD surface due to the higher grafting density of the PMMA ligands and reduced energy transfer due to more uniform dispersion of QDs. Finally, we applied the nanocomposites to LED devices, and found that the matrix-free nanocomposite exhibited a higher color conversion efficiency and smaller redshift in the peak emission wavelength than that prepared using a simple blending method. (paper)

[en] We have investigated the fluorescence properties of colloidal suspensions conntaining quantum dot (QD)/silica hybrid particles. First, we synthesized QD/silica hybrid particles with silica–QD–silica (SQS) core–shell–shell geometry, and monitored the quantum efficiencies of their suspensions at various particle concentrations. We found that the quantum efficiency (QE) of SQS particles in deionized (DI) water was much lower than that of the QDs even at low particle concentration, mainly due to the light scattering of emitted photons at the silica/water interface, followed by reabsorption by QDs. As the concentration of SQS particles was increased, both light scattering and reabsorption by QDs became more important, which further reduced the QE. Refractive index-matched solvent, however, reduced light scattering, yielding greater QE than DI water. Next, we induced aggregation of SQS particles, and found that QE increased as particles aggregated in DI water because of reduced light scattering and reabsorption, whereas it remained almost constant in the refractive index-matched solvent. Finally, we studied aggregation of highly concentrated silica particle suspensions containing a low concentration of SQS particles, and found that QE increased with aggregation because light scattering and reabsorption were reduced. (paper)