Highly Luminescent and Photostable Quantum Dot–Silica Monolith and Its Application to Light-Emitting Diodes

A highly luminescent and photostable quantum dot–silica monolith (QD–SM) substance was prepared by preliminary surface exchange of the QDs and base-catalyzed sol–gel condensation of silica. The SM was heavily doped with 6-mercaptohexanol exchanged QDs up to 12 vol % (26 wt %) without particle aggregation. Propylamine catalyst was important in maintaining the original luminescence of the QDs in the SM during sol–gel condensation. The silica layer was a good barrier against oxygen and moisture, so that the QD–SM maintained its initial luminescence after high-power UV radiation (∼1 W) for 200 h and through the 150 °C LED encapsulant curing process. Green and red light-emitting QD–SMs were applied as color-converting layers on blue LEDs, and the external quantum efficiency reached up to 89% for the green QD–SM and 63% for the red one. A white LED made with a mixture of green and red QDs in the SM, in which the color coordinate was adjusted at (0.23, 0.21) in CIE1931 color space for a backlight application, showed an efficacy of 47 lm/W, the highest value yet reported

Determination of the Energy Band Gap Depending on the Oxidized Structures of Quantum Dots

Theoretical and experimental studies on the changes of the optical properties of CdSe/CdS/ZnS (core/double-shell) quantum dots (QDs) during the oxidation process were first performed. An effective medium approach using the modified Khon–Sham equation presents a new method to predict the effects of the oxidation and to determine the oxidized ratio of nanoscale materials by a quantitative comparison with the experimental photoluminescence (PL) changes. As the oxidation progressed from the CdSe/CdS/ZnS nanocrystal surface, the PL peak shifted to longer wavelength and the quantum efficiency (QE) continuously decreased. It was also found that such changes were accelerated when the thickness of the outermost ZnS shell became thinner than a monolayer. The radial wave functions showed that the electron carriers rapidly extended into the shell region while the hole carriers spread very little into the core region. This indicates that the electrons are the key carriers to induce the changes in the energy band gap and the QE