Figure 1. Research connectivity in ONL.
Our research group focuses on a realization of novel, high performance optoelectronic nanomaterials and their applications, for instance, light emitting diodes, solar cells, photo-detectors, and so forth. Our research efforts span across three research areas from wet chemistry of semiconductor materials (Thrust 1), surface/interface chemistry (Thrust 2), to fabrication and characterization of optoelectronic devices (Thrust 3). Not only in-depth research on each thrust, we also combine them in multilateral way to reach our ultimate goal, the high performance optoelectronic devices.
Please find following paragraphs for more information on our research capabilities.
Thrust 1. Wet chemistry of semiconductor nanocrystals (NCs)
Making nanocrystal is simple. For example, increasing temperature of mixture comprising of cation precursors (e.g., Cd), anion precursors (e.g., Se) and surfactants (e.g., primary alkylamines, alkylcarboxylic acids, alkylphosphonic acids) produces CdSe nanocrystals through pyrolysis processes. A size of resulting nanocrystal reaches from ~2 to 20 nm depending on the reaction condition. Elaborated reaction schemes even realize various shapes (e.g., sphere, rod, or tetrapods) and heterostructures (e.g., core/shell, dot-in-rod, and so forth) that determine their phophysical properties. Mostly, these nanomaterials are dispersed in solution and needed to be purified and modified for device fabrication processes.
When the size of semiconductor NCs is reduced below a certain level (e.g., exciton Bohr radius), physical phenomena that are not observed in bulk materials are observed. Most well-known and unique phenomenon is the quantum confinement effect, the increment of band gap (energy difference between valence band and conduction ban) with decreasing size. Namely, we can create NCs with various emitting colors without changing complex chemical structure. Based on this advantage, academia and industry have actively researched on next generation displays or lightings using spherical semiconductor NCs (or quantum dots, QDs). Our research group will explore the next generation emitters featuring high efficiency, high color purity, and environmental safety.
Figure 3. Schematic on quantum confinement effect (top), spherical NCs with different diameters (middle), and size-controlled spherical NCs under ultraviolet irradiation (bottom).
Figure 2. Typical experiment setup for NC synthesis (top) and shape-controlled NCs (bottom)
Thrust 2. Surface/interface chemistry
For better functionality, we combine the semiconductor NCs with other functional materials whether they are organic or inorganic. By combining polar/nonpolar or conductive/insulating molecules or polymers , we can freely control solvents and their viscosity for following fabrication process. And we can tune optical and electrical properties of these hybrid materials, too.
Figure 4. A representative combination of organic conducting polymers and QDs (top) and morphology control of tetrapod NCs - polymer hybrids by 'grafting-to' approach (bottom).
Thrust 3. Device fabrication and characterization
Figure 5. A schematic of typical QD-based electroluminescence (EL) devices (top), a comparison or EL spectra from QDs and OLEDs (middle), and various operation modes of white QLEDs (bottom).
We can apply the semiconductor NCs to various fields that requires photon-to-electron or electron-to-photon inter-conversion, for example, absorbing light and generating electricity or vice versa. Most striking and popular example must be QD-based light emitting diodes (QLEDs). In this technology, we fabricate QDs in core/shell heterostructures to improve photoluminescence quantum yield (PL QY) and adapt them as an emissive layer in light emitting diodes. In this device, electrons and holes are transferred through charge transport layers and generate exciton (electron-hole pair) in QDs, and consequently, photons with an energy equivalent to the band gap of QDs are generated from the exciton. Precise and delicate engineering on QD size and chemical composition is able to produce very pure red, green and blue colors that have hardly been made in previous technologies. Using these primary colors to micro pixels in displays or lighting panels, we can realize next generation displays, high quality & smart white lightings, and even ultraviolet lamps for sensitization or nail polish.
What is the advantage of QD-based displays? Colors in display are produced by mixing three primary colors (red, green, blue) and the more pure primary colors, the greater number of combination is possible. QDs have excellent color purity compared to conventional dyes. Thus, more vibrant and realistic displays can be possible if we use QDs.
Figure 6. CIE 1931 chromaticity diagram, coordinates of primary colors in conventional CRT display (grey dot), OLEDs (blue triangle) and QLEDs (red square), and corresponding color gamuts (left). Conceptual picture produced by LCDs, OLEDs, and QLEDs. For readers' convenience, hue and saturation are adjusted (bottom).
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