Publications by authors named "Jang-Kun Song"

The self-assembly of nematic molecules in microcompartments with unambiguously defined surface anchoring is well predictable and is likely to have a single stable topological structure. Here, in contrast, a confined nematic system comprising an array of microcompartments interconnected by channels is demonstrated, exhibiting diverse molecular assembly pathways leading to the formation of four types of topological structures and twelve different patterns randomly distributed. Intercompartment communication via channels plays a crucial role in the diversity of patterns and distributions.

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Quantum-dot light-emitting diodes (QD-LEDs) have gained attention as potential display technologies. However, the solvents used to dissolve a polymeric hole transport layer (HTL) are hazardous to both humans and the environment. Additionally, intermixing the HTL and QD layers presents a significant challenge when fabricating inverted QD-LEDs.

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Quantum-dot (QDs) polymer composite films, which are key components in recent display applications, require improved photoluminescence (PL) intensity and color conversion efficiency for better display quality and low power consumption. In this study, we developed a novel approach to improve the photoluminescence (PL) of quantum dot (QDs)-polymer nanocomposite films. This was achieved by incorporating CO micropores and scattering particles into QD-embedded photopolymerizable polymer films.

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Processing of mesoscale structures of soft matter and liquid is of great importance in both science and engineering. In this work, we introduce the concept of laser-assisted micromachining to this field and inject a certain number of microdroplets into a preselected location on the surface of a liquid crystal drop through laser irradiation. The impact of laser energy on the triggered injection is discussed.

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We report waveform-induced rotation-time symmetry breaking in liquid crystal director motion. Homeotropic cells filled with a negative dielectric anisotropy chiral nematic exhibit persistent and visually observable waves of director orientation with a time period of at least 30 driving field cycles. Their existence in the space of driving waveform parameters is explored.

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Multiresponsive functional materials that respond to more than one external stimulus are promising for novel photonic, electronic, and biomedical applications. However, the design or synthesis of new multiresponsive materials is challenging. Here, this work reports a facile method to prepare a multiresponsive colloidal material by mixing a liquid-crystalline 2D nanocolloid and a functional polymer colloid.

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A lighting device with a wide color-tunable range is still a challenge for lighting based on either organic light-emitting diodes (OLEDs) or inorganic LEDs. In this work, we first proposed a novel hybrid device of organic LEDs and inorganic blue GaN LEDs to achieve full white and other colors. Organic LEDs were stacked with green and red emissive layers and connected with blue GaN LEDs in parallel but in opposite polarity voltage.

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Widely tunable color emission from a single pixel is a promising but challenging technology for quantum-dot light-emitting diodes (QD-LEDs). Even a QD-LED pixel with stacked multi-QD layers having different colors is likely to emit a monotonic color because the exciton recombination mostly occurs in 1 or 1.5 QD layers with better charge balance.

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Dielectrophoresis (DEP) is an excellent tool for manipulating small particles within a liquid or gas medium. However, when the size of the particles is too small, such as with quantum dots (QDs), it is difficult to manipulate the particles using DEP because the dielectrophoretic force () depends on the volume of the particles and is therefore too weak to achieve particle migration. Herein, we demonstrate a novel method for controlling nanoscale QD particles using DEP by introducing photopolymerized reactive mesogen (RM) bead vehicles.

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We demonstrate a novel structure for a quantum-dot light-emitting diode (QD-LED) with wide-range colour-tuneable pixels, fabricated via full solution processing. The proposed device has a symmetrical structure produced via stacking of an inverted-structure diode with a green QD emission layer (EML) and normal-structure diode with a red QD EML. It is an electron-only device; however, a charge generation layer in the middle of the device generates holes for the formation of excitons.

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Dielectrophoresis (DEP) in a medium with anisotropic dielectric susceptibility is very different from typical DEP in an isotropic medium: The direction of particle actuation can be switched depending on the direction of the susceptibility tensor of the medium. However, the understanding of switchable DEP (SDEP) in an anisotropic medium is still in its infant stage. Here, we investigate SDEP using heat-generated isotropic droplets in a nematic liquid crystal (LC) medium.

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The ordering of 2D biaxial graphene oxide (GO) particles is investigated under the application of orthogonal electric (E) and magnetic fields (B); nematic, antinematic, or biaxial nematic ordering of GO particles is selectively obtained depending on the field conditions. Particularly, a perfect biaxial nematic ordering with the highest birefringence is induced by the dual fields. Unexpectedly, the presence of B enhances the effective polarizability anisotropy, which may attribute to the enhanced steric interparticle interaction.

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Microfabrication of complex double emulsion droplets with controlled substructures, which resemble biological cells, is an important but a highly challenging subject. Here, a new approach is proposed based on laser-induced injection of water nanodroplets into a liquid crystal (LC) drop. In contrast to the conventional top-down microfluidic fabrication, this method employs a series of bottom-up strategies such as nanodroplet injection, spontaneous and assisted coalescence, elastically driven actuation, and self-assembly.

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The manipulation of a large number of nanoparticles (NPs) is an interesting but challenging task. Here, we demonstrate a new method to fabricate an NP cluster array, in which the shape and size of each NP cluster can be controlled. The method involves the use of the solubility contrast of NPs in the isotropic and nematic liquid crystal (LC) media, and the isotropic-preference difference depending on the types of the surfaces.

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Dielectrophoresis (DEP) is widely used in nanoscience and biology to control small particles but its applicability is significantly limited by its one-way impetus characteristics along the square field gradient (∇E2) direction, that is, DEP force, FDEP ∼ ∇E2. Here, switchable DEP (SDEP) using the anisotropic property of a nematic medium is demonstrated; FDEP does not need to be parallel to ∇E2 but is arbitrarily changeable depending on the permittivity tensor orientation of the medium. To effectively demonstrate the SDEP phenomenon, isotropic droplets with infinitesimal surface anchoring in a nematic medium are introduced, in which topological defects of the nematic medium around dispersed objects are effectively eliminated.

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Epitaxial alignment of organic liquid crystal (LC) molecules on single-crystal graphene (SCG), an effective epitaxial molecular assembly template, can be used in alignment-layer-free liquid crystal displays. However, selectivity among the threefold symmetric easy axes of LCs on graphene is not well understood, which limits its application. Here, sixfold symmetric radial LC domains are demonstrated by dropping an LC droplet on clean SCG, which reveals that the graphene surface does not have an intrinsic preferential direction.

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Two-dimensional (2D) nanoparticles in an oligomer-tethered alpha zirconium phosphate (αZrP) colloid self-assemble to form a cofacial lamellar structure with regular spacing parallel to the surface and exhibit high reflectance and vivid structural colors within the visible frequency spectrum. Here, we demonstrate electrical switching of the structural color reflection by electrical control of the liquid crystalline phase of the αZrP colloid. At low frequency (less than 15 Hz, optimally at 1 Hz), electrohydrodynamic flow in the colloid destroys the photonic crystalline lamellar phase and creates an apparently disordered dynamic state with local nematic orientation.

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Even though a graphene-oxide (GO) dispersion is attractive for electro-optical switching devices because of its high Kerr coefficient, it has several limitations such as chemical instability and optical loss due to absorption at visible wavelengths. Here we introduce the use of tetrabutylammonium-tethered α-zirconium phosphate (TBA-ZrP) colloid in various solvents for electro-optical switching devices; the TBA-ZrP colloid is chemically stable and optically transparent. We find that the electrical switching behavior of TBA-ZrP is sensitively dependent on the type of solvent.

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Functional polymer films are key components in the display industry, and the theoretical prediction of the optical properties of stretched polymer films is important. In this study, we try to establish the theoretical calculation process without an empirical database to predict the refractive index, including wavelength dispersions and optical retardation of stretched polymer films using several commercial simulation tools. The polarizability tensor and molecular volume for periodic units of polymers are accurately simulated, resulting in the accurate prediction of the mean refractive index and its dispersion for raw polymer materials.

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The mechanism of the iridescent color reflection from dried thin graphene oxide (GO) film on Si wafer is clarified. Dissimilarly to the photonic crystalline reflection in aqueous GO dispersion, the color reflection in dried GO film originates from the thin film interference. The peak reflection can reach 23% by optimizing the GO thickness and the substrate.

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The temperature dependence of the orientational order parameters 〈P_{2}(cosβ)〉 and 〈P_{4}(cosβ)〉 in the nematic (N) and twist-bend nematic (N_{tb}) phases of the liquid crystal dimer CB7CB have been measured using x-ray and polarized Raman scattering. The 〈P_{2}(cosβ)〉 obtained from both techniques are the same, while 〈P_{4}(cosβ)〉, determined by Raman scattering is, as expected, systematically larger than its x-ray value. Both order parameters increase in the N phase with decreasing temperature, drop across the N-N_{tb} transition, and continue to decrease.

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Although the large Kerr coefficient of aqueous graphene oxide (GO) dispersions is quite attractive for electro-optical applications with low power consumption, the maximum birefringence of GO dispersions is not sufficiently high for actual display applications. Here we report that adding a small amount of larger GO particles (about 4 μm) into a high-concentration dispersion of small GO (about 0.2 μm) can improve the electro-optical sensitivity to an electric field and also the maximum birefringence.

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We report a cost effective and easy chemical reduction method for exfoliated individual graphene oxide (GO) and GO paper using p-toluene sulfonic acid (PTSA) under mild conditions. Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), X-ray photon spectroscopy (XPS), thermo gravimetric analysis (TGA) and transmission electron microscopy (TEM) analysis were performed to investigate the quality of GO reduction. Data resulting from the spectral analysis suggest that the reduction method using PTSA is an efficient method to remove oxygen functionalities in the GO and also as an alternative to commonly used reducing agents.

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Electrowetting (EW) enables facile manipulation of a liquid droplet on a hydrophobic surface. In this study, manipulation of an electrolyte droplet having a small floating object on it was investigated on a solid hydrophobic substrate under the EW process. Herein, the floating object exhibited a vertical motion under an applied electric field owing to the spreading and contraction of the droplet on its connecting substrates.

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Chiral nematic droplets exhibit abundant topological defect structures, which have been intensively studied, both theoretically and experimentally. However, to observe and reconstruct the exact shape of three-dimensional (3D) defect structures has been a challenging task. In this study, we successfully reconstruct the 3D defect structures within a CLC microsphere with long helical pitches by combining polarized optical microscopy (POM) and laser scanning type fluorescence confocal polarizing microscopy (FCPM).

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