Multilayer Transfer Printing for Pixelated, Multicolor Quantum Dot Light-Emitting Diodes.

Here, we report multilayer stacking of films of quantum dots (QDs) for the purpose of tailoring the energy band alignment between charge transport layers and light emitting layers of different color in quantum dot light-emitting diodes (QD LED) for maximum efficiency in full color operation. The performance of QD LEDs formed by transfer printing compares favorably to that of conventional devices fabricated by spin-casting. Results indicate that zinc oxide (ZnO) and titanium dioxide (TiO2) can serve effectively as electron transport layers (ETLs) for red and green/blue QD LEDs, respectively. Optimized selections for each QD layer can be assembled at high yields by transfer printing with sacrificial fluoropolymer thin films to provide low energy surfaces for release, thereby allowing shared common layers for hole injection (HIL) and hole transport (HTL), along with customized ETLs. This strategy allows cointegration of devices with heterogeneous energy band diagrams, in a parallelized scheme that offers potential for high throughput and practical use.

High temperature continuous flow synthesis of CdSe/CdS/ZnS, CdS/ZnS, and CdSeS/ZnS nanocrystals.

Continuous flow reactors show great promise for large-scale synthesis of quantum dots. Here, we discuss results for the synthesis of multi-layered Cd-based hybrid nanocrystals - CdSe/CdS/ZnS, CdS/ZnS, and CdSeS/ZnS - in a continuous flow reactor. The simple reactor design and liquid-phase chemistry obviate the need for preheating or in-line mixing, and the chosen reactor dimensions and operating conditions allow for high flow rates (∼10 mL min(-1)). Additionally, the simple reactor design is well suited for scale-up. The CdSe/CdS/ZnS particles synthesized at elevated temperatures in the reactor exhibit quantum yields of over 60% at longer wavelengths (red region). The shell growth for these particles is conducted without the need for complex dropwise addition or SILAR shell growth procedures used in batch reactors. CdS-based particles were shown to have a higher performance when using octadecene-S instead of TOP-S, which improved the quality of shell growth. In addition, stoichiometric synthesis of the alternate CdSeS/ZnS alloy particles was conducted, removing the need for a large excess of S to offset the lower S reactivity. CdSeS/ZnS alloy nanoparticles exhibit quantum yields of about 50% in the intermediate wavelength range (500-600 nm).

Region specific enhancement of quantum dot emission using interleaved two-dimensional photonic crystals.

The power efficiency, spectral characteristics, and output directionality of light emitting diodes (LEDs) used for lighting and video display may be tailored by integrating nanostructures that interact with photon emitters. In this work, we demonstrate an approach in which visible-wavelength-emitting quantum dots (QDs) are integrated within a polymer-based photonic crystal (PC) and excited by an ultraviolet-emitting LED. The PC design incorporates two interleaved regions, each with distinct periods in orthogonal directions. The structure enables simultaneous resonant coupling of ultraviolet excitation photons to the QDs and visible QD emission at two different wavelengths to efficiently extract photons normal to the PC surface. The combined excitation and extraction enhancements result in a 5.8X increase in the QD output intensity. Further, we demonstrate multiple QD-doped PCs combined on a single surface to optimally couple with distinct populations of QDs, offering a means for blending color output and directionality of multiple wavelengths. Devices are fabricated upon flexible plastic surfaces by a manufacturable replica molding approach.

Double-heterojunction nanorods.

As semiconductor heterostructures play critical roles in today's electronics and optoelectronics, the introduction of active heterojunctions can impart new and improved capabilities that will enable the use of solution-processable colloidal quantum dots in future devices. Such heterojunctions incorporated into colloidal nanorods may be especially promising, since the inherent shape anisotropy can provide additional benefits of directionality and accessibility in band structure engineering and assembly. Here we develop double-heterojunction nanorods where two distinct semiconductor materials with type II staggered band offset are both in contact with one smaller band gap material. The double heterojunction can provide independent control over the electron and hole injection/extraction processes while maintaining high photoluminescence yields. Light-emitting diodes utilizing double-heterojunction nanorods as the electroluminescent layer are demonstrated with low threshold voltage, narrow bandwidth and high efficiencies.

Serum retinol, alpha-tocopherol, and lipid peroxide concentrations in low-income indian pregnant mothers.

Serum alpha-tocopherol, retinol, and malondialdehyde concentrations were measured at 7(th) month of pregnancy in 122 women from low socio-economic background. Maternal anthropometric measurements, 24-hour nutrient intakes, and pregnancy outcome were recorded. One-third (34.3 %) of mothers had low birth weight babies and 10 % delivered preterm. Maternal diets were inadequate in all the nutrients, the most limiting being vitamin A. Three-fourths of mothers had intakes less than 25 % of the Indian recommended daily intake (RDI) of 600 microg retinol equivalents/day. Serum alpha-tocopherol concentrations of 98 % were indicative of deficiency and almost half the mothers had low serum retinol concentrations (< or =10 microg/dL). Mean malondialdehyde level was 7.0 +/- 1.4 nmoles/mL. These values are higher than reports in the literature and were attributable to poor intake of most dietary antioxidants. Malondialdehyde concentrations were negatively correlated with serum alpha-tocopherol. Birth weight was positively correlated with maternal weight and biceps skinfold thickness, macronutrient intakes, serum retinol, and alpha-tocopherol concentrations. Preterm delivery was associated with low anthropometric measurements and low nutrient intakes especially vitamin A. Mothers who delivered preterm had significantly lower malondialdehyde concentrations than those who delivered at term. The role of antioxidant nutrients, especially vitamin A and oxidative stress in relation to fetal growth and pregnancy outcome among mothers from low socioeconomic settings requires attention.