Abstract
InP/ZnS core/shell quantum dot (QD) is suited to application to white LEDs, but the photoluminescence (PL) intensity decreases under continuous photo-excitation. This QD degradation is attributed to photo-oxidation, so it is needed to avoid contact between QD and air. In this study, we embedded InP/ZnS core/shell QD into tetramethylammonium silicate (TMAS)-derived silica monolith to improve its photostability. Hydrophilization of InP/ZnS QD capped with 1-dodecanethiol was needed for embedding them into silica. For ligand exchange, 3-mercaptopropionic acid (MPA) was injected into a colloidal solution of InP/ZnS QD capped with 1-dodecanethiol at 240 °C. Then, InP/ZnS QD capped with MPA was dispersed in TMAS aqueous solution. Methyl L-(−)-lactate was added to the solution for gelation. Monolithic InP/ZnS@TMAS-derived silica composite was obtained after drying. InP/ZnS@polymethylmethacrylate (PMMA) composite was also prepared to compare with InP/ZnS@TMAS-derived silica composite in terms of photostability. QD-silica composite exhibited high transparency due to homogeneous dispersion of InP/ZnS QD capped with MPA in TMAS aqueous solution. Carboxyl group of MPA is deprotonated under basic conditions, so InP/ZnS QD capped with MPA possesses negative charge in the basic TMAS aqueous solution. This causes electrostatic repulsion between QD and silicate anion, and hence high dispersibility of InP/ZnS QD capped with MPA in TMAS aqueous solution. The emission peak of InP/ZnS@TMAS-derived silica composite under 400 nm excitation was observed at 543 nm. To evaluate photostability, we examined PL intensity after continuous excitation for 180 min. PL intensity of InP/ZnS QD capped with MPA after continuous excitation was only 25% of the initial PL intensity. On the other hand, InP/ZnS@TMAS-derived silica composite retained 90% of the initial PL intensity. This result means protecting QD against air achieves high photostability. Furthermore, InP/ZnS@TMAS-derived silica composite was more photostable than InP/ZnS@PMMA composite, which retained 68% of the initial PL intensity. This indicates that the gas-barrier property of TMAS-derived silica is higher than that of PMMA.
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