Eunjoo Jang

White‐Light‐Emitting Diodes with Quantum Dot Color Converters for Display Backlights

Quantum dots (QDs) have attracted great attention as good candidate for the next generation displays due to their narrow emission, and high luminescence efficiency, and tunable emission covering all visible range. However, QDs easily lost their initial optical properties during the process for a device fabrication and practical operation. We synthesized well passivated green and red light emitting QDs that show almost 100% of QE. When the highly luminescent green and red light emitting QDs were applied as color converters in InGaN blue LEDs, resulting cool white QD-LEDs showed 41lm/W and more than 100% of color reproducibility compared to NTSC standard in CIE1931 and maintained their optical properties for a long time operation. We also demonstrated a 46-inch LCD panel using the white QDLED backlight was successfully demonstrated (Figure).

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.

Bright and Stable Alloy Core/Multishell Quantum Dots

Color conversion: a quantum dot (QD) structure consisting of an alloy core (CdSe//ZnS) and multishells (CdSZnS) was prepared. The photoluminescence of the QDs could be tuned especially in the green-light region by controlling the thickness of the inner CdS shell. The alloy core/multishell (AC/MS) QDs showed a quantum efficiency of 100 % and a narrow spectrum width.

Pd-nanocrystal-based nonvolatile memory structures with asymmetric SiO2∕HfO2 tunnel barrier

Pd nanocrystals (NCs) on asymmetric tunnel barrier (ATB) composed of stacked SiO2 and HfO2 layers have been employed for nonvolatile memory devices. The Pd-NC layers are formed by electrostatic self-assembly of negatively charged colloidal Pd NCs. The presence of isolated Pd NCs of ∼5nm embedded in HfO2 is confirmed by scanning and transmission electron microscopy images. Outstanding program∕erase (P∕E) properties from C‐V curves are observed with a memory window of 6V under ±17V. Extrapolation of the data up to ten years shows that the flatband voltage drops at the P∕E levels are maintained within only 1.0∕0.5V, respectively, resulting from the efficient data retention based on the ATB. These results are promising enough for the memory structure to be utilized for the multilevel charge storage.

High quality CdSeS nanocrystals synthesized by facile single injection process and their electroluminescence.

Highly luminescent CdSeS nanocrystals (quantum efficiency up to 85%), showing tunable luminescence properties from red to blue region with narrow band edge (FWHM = 34 nm), were synthesized by one-step addition of Se and S source mixture into the Cd precursor solution at elevated temperature, and the resulting nanocrystals were successfully embedded in a traditional OLED structure to give spectrally clean and narrow electroluminescence emission at identical positions of the photoluminescence spectrum.

Synthesis of multi-shell nanocrystals by a single step coating process

We synthesized CdSe/CdS/ZnS core/multi-shell nanocrystals through a single coating step by using different growth rates of CdS and ZnS crystalline layers on CdSe cores. The resulting nanocrystals exhibited a controlled redshift from the initial photoluminescent (PL) emission of CdSe cores with much improved quantum yield (QY above 70%).

Alkyl thiols as a sulfur precursor for the preparation of monodisperse metal sulfide nanostructures

We demonstrate a new method for preparing metal sulfide nanocrystals using alkyl thiols as a sulfur precursor. Alkyl thiols have many advantages for practical synthesis because they are miscible with most organic solvents and very stable under an air atmosphere. CdS nanocrystals were made with CdO and thiols with different alkyl chains such as n-octanethiol and octadecanethiol. They exhibited uniform size, highly crystalline structure and a sharp photoluminescence spectrum. Also, CdSe/CdS core/shell nanocrystals can be prepared by single injection of a mixture consisting of alkyl thiol and Se in trioctylphosphine to a Cd precursor. A reaction scheme is proposed as alkyl thiols react with the metal precursor to form stable metal thiolate intermediates during the initial period of reaction, and the thiolate decomposes slowly to form homogeneous nuclei.

Interfused semiconductor nanocrystals: brilliant blue photoluminescence and electroluminescence

We describe a method for producing blue light-emitting interfused CdSe//ZnS (QE up to 60%) nanocrystals and report the good performance of an electroluminescent device which uses them (external quantum efficiency approximately 1.5 cd A(-1)).

Shape-controlled synthesis of silver sulfide nanocrystals by understanding the origin of mixed-shape evolution

The shape of silver sulfide nanomaterials was successfully controlled by understanding the origin of the mixed-shape problem.

Trap Passivation in Indium-Based Quantum Dots through Surface Fluorination: Mechanism and Applications

Treatment of InP colloidal quantum dots (QDs) with hydrofluoric acid (HF) has been an effective method to improve their photoluminescence quantum yield (PLQY) without growing a shell. Previous work has shown that this can occur through the dissolution of the fluorinated phosphorus and subsequent passivation of indium on the reconstructed surface by excess ligands. In this article, we demonstrate that very significant luminescence enhancements occur at lower HF exposure though a different mechanism. At lower exposure to HF, the main role of the fluoride ions is to directly passivate the surface indium dangling bonds in the form of atomic ligands. The PLQY enhancement in this case is accompanied by red shifts of the emission and absorption peaks rather than blue shifts caused by etching as seen at higher exposures. Density functional theory shows that the surface fluorination is thermodynamically preferred and that the observed spectral characteristics might be due to greater exciton delocalization over the outermost surface layer of the InP QDs as well as alteration of the optical oscillator strength by the highly electronegative fluoride layer. Passivation of surface indium with fluorides can be applied to other indium-based QDs. PLQY of InAs QDs could also be increased by an order of magnitude via fluorination. We fabricated fluorinated InAs QD-based electrical devices exhibiting improved switching and higher mobility than those of 1,2-ethanedithiol cross-linked QD devices. The effective surface passivation eliminates persistent photoconductivity usually found in InAs QD-based solid films.