Quantum Dot/Siloxane Composite Film Exceptionally Stable against Oxidation under Heat and Moisture

Abstract

We report on the fabrication of a siloxane-encapsulated quantum dot (QD) film (QD-silox film), which exhibits stable emission intensity for over 1 month even at elevated temperature and humidity. QD-silox films are solidified via free radical addition reaction between oligosiloxane resin and ligand molecules on QDs. We prepare the QD-oligosiloxane resin by sol-gel condensation reaction of silane precursors with QDs blended in the precursor solution, forgoing ligand-exchange of QDs. The resulting QD-oligosiloxane resin remains optically clear after 40 days of storage, in contrast to other QD-containing resins which turn turbid and ultimately form sediments. QDs also disperse uniformly in the QD-silox film, whose photoluminescence (PL) quantum yield (QY) remains nearly unaltered under harsh conditions; for example, 85 °C/5% relative humidity (RH), 85 °C/85% RH, strongly acidic, and strongly basic environments for 40 days. The QD-silox film appears to remain equally emissive even after being immersed into boiling water (100 °C). Interestingly, the PL QY of the QD-silox film noticeably increases when the film is exposed to a moist environment, which opens a new, facile avenue to curing dimmed QD-containing films. Given its excellent stability, we envision that the QD-silox film is best suited in display applications, particularly as a PL-type down-conversion layer.

Heat- and water-proof quantum dot/siloxane composite film: Effect of quantum dot-siloxane linkage

“…Enhanced PL QY of CdSe/ZnS QDs by passivating trap states with the help of adsorbed water molecules has been reported in previous studies . From this standpoint, both increased radiative (from 34.4 to 43.2 ns) and disappeared delayed recombination components of the QD/siloxane (methacryl) film indicate reduced trap states after aging which correspond with its enhanced PL QY.…”

“…Both films are fabricated via three steps. First, QDs are dispersed in the mixture of silane precursors, which are MPTS/DPSD and ECTS/DPSD, separately, forgoing ligand‐exchange of QDs . During this step, hydrophobic interaction between oleic acid (OA, surface ligand on QDs) and functional groups of the silane precursors triggers encapsulation of the QDs .…”

“…As shown in Fig. b, the free radical addition reaction of the QD/oligosiloxane (methacryl) resin results in the formation of carbon single bonds by two cross‐linking reaction pathways: the one is the reaction between C═C bonds of methacryl groups of siloxane matrix (i.e., cross‐linked bonds between siloxane matrix), and another is the reaction between C═C bonds of the methacryl group and OA (i.e., cross‐linked bonds between QD and siloxane matrix) . While, the epoxy ring opening cationic reaction of the QD/oligosiloxane (epoxy) resin results in the formation of only ether bonds of between epoxy groups of siloxane matrix .…”

“…A siloxane‐based film, which is synthesized via sol–gel condensation reaction of silane precursors, has been widely studied for encapsulating molecules, e.g., catalysts, enzymes, and fluorescent dyes, by formation of chemical linkages between molecules and siloxane matrices . Recently, we reported a QD–siloxane composite film which shows long‐term PL QY stability under heat or moisture conditions with the aid of thermally stable Si―O bonds and cross‐linked bonds between ligand of QD and siloxane matrix, compared with a hydrocarbon‐based QD composite film …”

“…[18][19][20] Recently, we reported a QD-siloxane composite film which shows long-term PL QY stability under heat or moisture conditions with the aid of thermally stable Si-O bonds and cross-linked bonds between ligand of QD and siloxane matrix, compared with a hydrocarbon-based QD composite film. 21 In this study, we investigate stability of QD composite films using CdSe/ZnS core/shell QDs surrounded by oleic acid embedded in two different siloxane films. One is QD/siloxane (methacryl) film, which has chemical linkages between QD and siloxane matrix, and the other is QD/siloxane (epoxy) film, which has no linkages between QD and siloxane matrix.…”