Cheolsang Yoon

Highly luminescent and stable white light-emitting diodes created by direct incorporation of Cd-free quantum dots in silicone resins using the thiol group

We have fabricated highly luminescent and stable white light-emitting diodes (WLEDs) by directly incorporating CIS/ZnS quantum dots (QDs) into silicone resins using the thiol group. First, QD-containing phenyl-vinyl-oligosiloxane base resins were prepared by an in situ condensation reaction of sol–gel precursors and QDs with and without 3-(mercaptopropyl)trimethoxysilane. Next, base resins were cured with the crosslinker resin through hydrosilylation to fabricate QD-containing phenyl hybrimer (QDPH) resins. We found that the QDPH resin containing a thiol group exhibited higher transmittance and quantum efficiency than that without the thiol group because of more uniform dispersion of QDs. Finally, fabrication of WLEDs with the thiol-containing resin exhibited enhancements over that without the thiol group with regard to photo and thermal stabilities, color conversion and luminous efficiencies, and color accuracy. These enhancements were attributed to the stronger interaction between QDs and the polymer network through the thiol-anchoring group.

Fabrication of high quantum yield quantum dot/polymer films by enhancing dispersion of quantum dots using silica particles

We have fabricated quantum dot (QD)/polymer films of high quantum yield by coating silica particles with quantum dots. When particles were dispersed in tetrahydrofuran, free QD suspension exhibited higher quantum yield than QD-coated silica particles. Scattering is a most likely reason for the drop in quantum yield for the QD-coated silica particles, as supported by results of silica particles with varying morphologies: for example, QD-coated hollow silica particles showed higher quantum yield than filled silica particles, as the hollowness gave rise to reduced scattering. In the QD/polymer films, however, QD-coated filled/hollow silica particles showed significant enhancement in quantum yield (i.e., up to 2.4 times higher than that of free QDs). Confocal microscopy revealed that the enhanced quantum yield likely results from improved dispersion of QD-coated silica particles. In addition, the quantum yield of QD-coated hollow silica particles in films was lower than that of filled particles because of lower structural stability. Introducing silica (either filled or hollow) particles prevents spectral redshift of emission peak when prepared in the form of film, as opposed to QD-only sample. Our findings point to the possibility that QD-coated filled/hollow silica particles exhibit superior stability, quantum efficiency, and color accuracy, which render them potentially useful for the next-generation light-emitting devices and photovoltaics.

Enhancing performance of quantum dot-based light emitting diodes by using poly(methyl methacrylate)@quantum dot hybrid particles

Quantum dots (QDs) are attractive alternatives for organic phosphors in light emitting diodes (LEDs) due to their high quantum yield and photostability. Various methods have been developed for fabrication of LEDs using QDs, yet the reduction in quantum yield during film formation still limits their practical applications. We prepared hybrid particles by coating spherical poly(methyl methacrylate) (PMMA) particles with the CdSe/ZnS QDs, and dispersed them in the PMMA matrix. The PMMA particles were derived from the same material as the PMMA matrix, so that they could not only act as a spacer but also match the refractive index between the polymer particles and matrix. The PMMA@QD hybrid particles exhibited higher quantum yield in both suspension and film states than the pristine QDs. In addition, the dispersion state of QDs in PMMA matrix was significantly improved by using the hybrid particles. Finally, it was demonstrated that the QD-based LED device containing the PMMA@QD hybrid particles exhibited enhancement in both color conversion and luminous efficiencies.

Quantum efficiency of colloidal suspensions containing quantum dot/silica hybrid particles.

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.

Wafer-level detection of organic contamination by ZnO-rGO hybrid-assisted laser desorption/ionization time-of-flight mass spectrometry

A technique for wafer-level detection of organic contaminations via surface-assisted laser desorption/ionization time-of-flight mass spectrometry was developed. To replace the organic matrix in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, zinc oxide-reduced graphene oxide (ZnO-rGO) hybrid was prepared by a hydrothermal reaction and used as the matrix in the detection of benzo[a]pyrene (B[a]P). By varying the rGO content and the amount of hybrid, the optimal rGO content in the hybrid for the detection of B[a]P was determined to be 4 wt% and the optimal amount of hybrid was 20 ng. The limit of detection of this method was found to be 1.6 × 1014 C atoms cm-2, which is lower than the concentration of residual organic contamination at which serious failure occurs during semiconductor fabrication. This method was also successfully used to detect other aromatic and aliphatic species on a semiconductor wafer. This approach is fast, accurate, simple, and inexpensive compared to other conventional methods, and can be used to identify localized micro-contamination in the semiconductor industry.

Fabrication of highly luminescent and concentrated quantum dot/poly(methyl methacrylate) nanocomposites by matrix-free methods

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.