Embedding Quantum Dots with High Quantum Yield in Inorganic Matrix By Sol-Gel Method
Authors: Nishimura Daiki , Uematsu Taro , Torimoto Tsukasa , Kuwabata Susumu 
Published: December 2020

Colloidal quantum dots (QDs) are semiconductor nanoparticles, size of which is 10 nm or less. Some of QDs are known to show a wide spectral range of absorption and emit a highly monochromatic photoluminescence (PL) from the band-edge transition as one of their properties of direct semiconductors. However, cadmium selenide (CdSe) QDs, one of the most investigated QDs exhibit PL with relatively low quantum yield (QY) of ~10% due to intrinsic surface defects resulting in the quenching of excitons. Typically, a practical PL QY of ~80% is achieved by coating the core-only CdSe QDs with CdS and/or ZnS shells to passivate their surface. Thus, the QDs have gained the position of next-generation fluorescent materials and applied to display, electroluminescence (EL) device, and bioimaging. In addition to the shells, QDs have an awkward complexity arising from surface-adsorbed ligands that are organic molecules coordinating to outermost layer of QDs, compared to inorganic fluorescent materials. Along with the inorganic shells, recent studies have revealed that organic ligands also provide insulating characteristics to the QDs and support to prevent excitons leaking out from the QDs. In other words, their role is not only to keep QDs dispersed but also to play an essential role for the emission even if the QDs have the core/shell structure. On the other hand, their own weakness due to their organic nature and coordination bond with QDs sometimes impair long term stability of the materials due to degradation and/or desorption, leading to the formation of carrier trap states. In this study, we developed a way to keep band-edge emission without using organic ligands when embedding QDs in inorganic matrix prepared by sol-gel method. Two types of QDs, CdSe/CdS/ZnS core/multishell QDs and AgInS2/GaSx core/shell QDs are embedded in ZnS and GaSx matrix, respectively, by growing the matrices from the surface of the QDs.
To prepare CdSe/CdS/ZnS QDs embedded in ZnS matrix, colloidal CdSe/CdS/ZnS QDs were firstly synthesized following previous report,1 and they were transferred to 1-propanol solution by ligand exchange to 4-tert-butylpyridine. Then, they were mixed with ZnS precursor solution containing zinc acetate, thioacetamide and 4-amino-1-buthanol2, were reacted for several hours at room temperature. Pale orange powders showing intense photoluminescence were successfully obtained. To prepare AgInS2/GaSx QDs embedded in GaSx matrix, AgInS2/GaSx QDs were synthesized following previous report3 and were embedded in GaSx by the similar sol-gel method by using a gallium source instead of the zinc source.
Fig.1a and 1b show photographs of CdSe/CdS/ZnS QDs embedded in ZnS matrix under room light and UV light, the latter of which gave a PL QY over 60%. This value was even higher than that obtained in colloidal state, as shown in Fig.2. These results indicated that ZnS matrix could confine the excitons inside the QDs more strongly than organic ligands (4-tert-butylpyridine) and promote radiative recombination in the QDs. In terms of stability over time, PL QY of embedded QDs that were stocked in air at room temperature were maintained their luminescence property over 9 months. In contrast, the original QDs in colloidal state decreased their PL QY due to the deterioration of the surface condition. AgInS2/GaSx QDs embedded in GaSx matrix also showed PL QY of over 50% and were observed the improvement of durability over time as same as CdSe/CdS/ZnS QDs.

Figure 1