Seonghoon Lee

Bright and Efficient Full-Color Colloidal Quantum Dot Light-Emitting Diodes Using an Inverted Device Structure

We report highly bright and efficient inverted structure quantum dot (QD) based light-emitting diodes (QLEDs) by using solution-processed ZnO nanoparticles as the electron injection/transport layer and by optimizing energy levels with the organic hole transport layer. We have successfully demonstrated highly bright red, green, and blue QLEDs showing maximum luminances up to 23,040, 218,800, and 2250 cd/m(2), and external quantum efficiencies of 7.3, 5.8, and 1.7%, respectively. It is also noticeable that they showed turn-on voltages as low as the bandgap energy of each QD and long operational lifetime, mainly attributed to the direct exciton recombination within QDs through the inverted device structure. These results signify a remarkable progress in QLEDs and offer a practicable platform for the realization of QD-based full-color displays and lightings.

Patterned selective growth of carbon nanotubes and large field emission from vertically well-aligned carbon nanotube field emitter arrays

We have grown well-aligned carbon nanotube arrays by thermal chemical vapor deposition at 800 °C on Fe nanoparticles deposited by a pulsed laser on a porous Si substrate. We also attain a selective growth of carbon nanotubes on a patterned Fe film on Si substrates in terms of pulsed laser deposition and a liftoff patterning method. Field emission measurement has been made on the carbon nanotube (CNT)-cathode diode device at room temperature and in a vacuum chamber below 10−6 Torr. The distance between the CNT cathode and the anode is 60 μm and is kept through an insulating spacer of polyvinyl film. The measured field emitting area is 4.0×10−5 cm2. Our vertically well-aligned carbon nanotube field emitter arrays on the Si-wafer substrate emit a large current density as high as 80 mA/cm2 at 3 V/μm. The transmission electron microscope image shows that they are multiwalled and bamboolike structures and that the tips of some of the carbon nanotube emitters are open. The open tip structure of our CNTs and thei...

Y 3Al5 O 12 : Ce0.05 Phosphor Coatings on Gallium Nitride for White Light Emitting Diodes

White light was obtained by mixing blue light from the emission of a gallium nitride (GaN) chip and yellow light from the fluorescence of a Y 3 Al 5 O 12 :Ce 0.05 yellow phosphor. A uniform coating and an optimized thickness of yellow phosphor layer on a GaN chip were necessary for achieving an efficient white light emitting diode. The phosphor particles were coated on a GaN chip or indium tin oxide by several methods including the slurry method, the settling method, and electrophoretic deposition (EPD). The properties of the phosphor layers prepared by these methods were examined using scanning electron microscope and photoluminescence. The chromaticity of white light was dependent upon the thickness of the phosphor layer. The properties of the phosphor layer prepared by EPD such as packing density, thickness, and uniformity could be more easily controlled than those by the slurry and settling methods. Further high packing density of the EPD could compensate for the typical thick phosphor layer, allowing the thin layer to be fabricated. To overcome the weak adhesion strength of phosphor particles by the EPD, an aqueous solution including poly(vinyl alcohol) + ammonium dichromate was coated on the phosphor layer and cured by exposure to ultraviolet light.

Multicolored Light-Emitting Diodes Based on All-Quantum-Dot Multilayer Films Using Layer-by-Layer Assembly Method

A systematic analysis of the exciton-recombination zone within all-quantum dot (QD) multilayer films prepared by a layer-by-layer assembly method was made, using sensing QD layers in QD-based light-emitting diodes (QLEDs). Large area practical multicolored colloidal QLEDs were also demonstrated by patterning and placing variously colored QDs (red, orange, yellow-green, and green) in the exciton-recombination zone.

Self-organized regular arrays of anodic TiO2 nanotubes.

The formation of self-organized regular arrays of oxide nanotubes lies in a delicate balance between the oxide growth rate and the oxide etching rate and a lattice mismatch between the grown metal oxide and the underlying valve metal. The requisites for their fabrication are the electropolishing and a two-step anodization. The most uniform and self-organized regular arrays of anodic TiO2 nanotubes among those known so far are reported as another example of valve metal oxide nanotube arrays since regular arrays of anodic aluminum oxide nanochannels were produced. Our findings can be generalized to fabricate self-organized regular arrays of other valve metal oxides.

Low-resistance Pt/Ni/Au ohmic contacts to p-type GaN

We report on a Pt (20 nm) Ni (30 nm)/Au (80 nm) metallization scheme for low-resistance ohmic contacts to the moderately doped p-type GaN:Mg (3×1017 cm−3). Both as-deposited and annealed Pt/Ni/Au contacts on p-GaN exhibit linear current–voltage characteristics, showing that a high-quality ohmic contact is formed. The Pt/Ni/Au scheme shows the specific contact resistance of 5.1×10−4 Ω cm2 when annealed at 350 °C for 1 min in a flowing N2 atmosphere.

Highly efficient cadmium-free quantum dot light-emitting diodes enabled by the direct formation of excitons within InP@ZnSeS quantum dots.

We demonstrate bright, efficient, and environmentally benign InP quantum dot (QD)-based light-emitting diodes (QLEDs) through the direct charge carrier injection into QDs and the efficient radiative exciton recombination within QDs. The direct exciton formation within QDs is facilitated by an adoption of a solution-processed, thin conjugated polyelectrolyte layer, which reduces the electron injection barrier between cathode and QDs via vacuum level shift and promotes the charge carrier balance within QDs. The efficient radiative recombination of these excitons is enabled in structurally engineered InP@ZnSeS heterostructured QDs, in which excitons in the InP domain are effectively passivated by thick ZnSeS composition-gradient shells. The resulting QLEDs record 3.46% of external quantum efficiency and 3900 cd m(-2) of maximum brightness, which represent 10-fold increase in device efficiency and 5-fold increase in brightness compared with previous reports. We believe that such a comprehensive scheme in designing device architecture and the structural formulation of QDs provides a reasonable guideline for practical realization of environmentally benign, high-performance QLEDs in the future.

Using Patent Information for Designing New Product and Technology: Keyword Based Technology Roadmapping

With the rapid change in markets and technologies, it is becoming essential for firms to develop new products constantly. This can most successfully be achieved by using technology roadmaps (TRMs), which are effective tools for connecting product and technology planning. However, TRMs generally tend to overstate the qualitative and expert-dependent knowledge rather than incorporating quantitative and objective information. This paper proposes a new approach where patent data are used in a quantitative methodology to support reliable decision-making in roadmapping processes. In this study, text-mining techniques were utilized to extract the relevant information on which portfolio, co-word, and network analyses were carried out. The results were three types of product-technology maps that can be applied to specific roadmapping steps. The suggested approach is expected to yield useful information about roadmapping, and help improve the overall effectiveness and quality of the technique.

R/G/B/Natural White Light Thin Colloidal Quantum Dot‐Based Light‐Emitting Devices

Bright, low-voltage driven colloidal quantum dot (QD)-based white light-emitting devices (LEDs) with practicable device performances are enabled by the direct exciton formation within quantum-dot active layers in a hybrid device structure. Detailed device characterization reveals that white-QLEDs can be rationalized as a parallel circuit, in which different QDs are connected through the same set of electrically common organic and inorganic charge transport layers.

Perspective on synthesis, device structures, and printing processes for quantum dot displays

Quantum dot-based light emitting diodes have extensively been investigated over the past two decades in order to utilize high color purity and photophysical stability of quantum dots. In this review, progresses on the preparation of quantum dots, structural design of electroluminescence devices using quantum dots, and printing processes for full-color quantum dot display will be discussed. The obstacles originating from the use of heavy metals, large hole injection barrier, and imperfect printing processes for pixilation have limited the practical applications of quantum dot-based devices. It is expected that recent complementary approaches on materials, device structures, and new printing processes would accelerate the realization of quantum dot displays.