Publications by authors named "Jong Keum"

Phonon dynamics are a critical factor to control the optical properties of excited states in light-emitting materials. Here, we report an extremely slow relaxation of photoexcited lattice vibrations enabled by assembling the donor-acceptor (D-A) molecules [2-(9,9-dimethylacridin-10(9H)-yl)-9,9-dimethyl-9H-thioxanthene 10,10-dioxide], namely AC molecules, into dipolar crystal. By using photoexcitation-modulated Raman spectroscopy, we find that the crystalline-lattice vibrations monitored by Raman-scattering laser beam of 785 nm demonstrate an un-usual slow relaxation in the time scale of seconds after ceasing photoexcitation beam of 343 nm in such dipolar crystal.

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  • A new liquid crystalline epoxy network (LCEN) with shape memory properties was created to explore its suitability for 3D printing.
  • Researchers analyzed the curing process and shape memory qualities using parallel plate rheology and dynamic mechanical analysis, respectively.
  • The addition of fumed silica improved the uncured material's rheological properties, enhancing its printability while maintaining the shape memory features of the LCEN, although it affected the liquid crystalline alignment.
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  • The study examines how ultrathin films made of polyzwitterions respond to electric fields when thermally annealed and placed on silicon substrates, using specular neutron reflectometry (NR) and X-ray reflectivity (XRR).
  • Researchers applied a high voltage under vacuum and observed changes in film thickness, finding that the thickness decreased by up to 8% depending on the type of added salt, attributed to the loss of water and ionic liquids rather than electrostrictive effects.
  • The combination of NR and XRR techniques offered insights into the films' hygroscopic nature and structural changes in response to electric fields, highlighting their potential applications in charged polymer technologies.
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  • - This research focuses on how the surface properties of copper oxide-decorated carbon nanofiber membranes affect their efficiency in membrane distillation (MD) for desalination.
  • - The study created membranes using a phase inversion method, finding that adding 1 wt % of copper oxide and carbon nanofibers significantly improved water vapor flux by 64% and achieved over 99.8% salt rejection due to changes in chemical structure and porosity.
  • - To analyze and predict the membranes' performance, the researchers applied machine learning techniques on electron microscopy images to map pore distribution and used a statistical model to forecast performance over time based on MD test data.
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Mg alloy corrosion susceptibility is a major issue that limits its wide industrial application in transport, energy and medical sectors. A corrosion-resistant layer containing crystalline MgCO was formed on the surface of AZ91D Mg alloy by Li salt loading and thermal CO treatment. Compared to the uncoated AZ91D surface, the surface layer exhibited up to a ∼15-fold increase in corrosion resistance according to the electrochemical results in 3.

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We present a streamlined method to covalently bond hydroxylated carbon nanotubes (CNOH) within a polyphenol matrix, all achieved through a direct, solvent-free process. Employing an extremely small concentration of CNOH (0.01% w/w) along with topologically contrasting linkers led to a maximum of 5-fold increase in modulus and a 25% enhancement in tensile strength compared to the unaltered matrix, an order of magnitude greater reinforcement (w/w) compared to state-of-the-art melt-processed nanocomposites.

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  • - This study focuses on designing flexible, solvent-free polymer electrolytes for solid-state batteries by understanding how ion transport is influenced by the material's structure and dynamics of the polymers involved.
  • - Researchers found that modifying the polymer/ceramic interface can enhance the ion dissociation and lithium ion (Li) conductivity, crucial for optimizing the electrolyte's performance and stability.
  • - The research utilized polyethylene oxide (PEO) with lithium salts in combination with garnet-type ceramics (Al-LLZO) to investigate the movement of Li ions, employing techniques like dielectric relaxation spectroscopy and X-ray scattering for deeper insights.
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  • - Despite advancements in sustainable elastomers, creating elastic vitrimers with better mechanical properties and recyclability is still a challenge.
  • - The study introduces a new design principle for highly durable and recyclable elastic vitrimers using crosslinked disulfide-containing PDMS chains and tetra-arm PEG, resulting in superior performance compared to traditional methods.
  • - The new vitrimers, known as PDMS-disulfide-D, not only maintain their mechanical strength after recycling but also perform well in applications like electromyography sensors, highlighting their potential for practical use.
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Solvent-based CO capture consumes significant amounts of energy for solvent regeneration. To improve energy efficiency, this study investigates CO fixation in a solid form through solvation, followed by ionic self-assembly-aided precipitation. Based on the hypothesis that CO ions may bind with monovalent metal ions, we introduced Na into an aqueous hexane-1,6-diamine solution where CO forms carbamate and bicarbonate.

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  • Interfacial strain in oxide thin films is important for enhancing material properties and has the potential to improve ion conduction, particularly in multilayer thin films.
  • The study reveals that while compressively strained Gd-doped CeO (GDC) thin films with Yttrium oxide (YO) show lower ion conductivity, tensile strained GDC with Samarium oxide (SmO) exhibit improved conductivity due to higher oxygen vacancy concentration.
  • The research confirms that simply increasing the number of interfaces in multilayer films doesn't necessarily enhance ionic conductivity and highlights how interfacial strain can be strategically used to design better ion conductors.
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  • Polyzwitterions (PZs) mimic intrinsically disordered proteins and can change between globular and random coil shapes in dilute aqueous solutions; these conformations are expected to be influenced by added salt.
  • This study tests these expectations by examining how potassium bromide (KBr) affects the size and structure of poly(sulfobetaine methacrylate) using advanced scattering techniques.
  • Results show that while PZs develop a net positive charge in low-salt conditions, the addition of KBr causes complex changes in their size, revealing both antipolyelectrolyte and polyelectrolyte effects that are crucial for understanding how salt impacts their behavior.
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Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine CrTe thin films manifests a unique temperature-dependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional CrTe epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry.

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Oppositely charged polyelectrolytes often form polyelectrolyte complexes (PECs) due to the association through electrostatic interactions. Obtaining PECs using natural, biocompatible polyelectrolytes is of interest in the food, pharmaceutical, and biomedical industries. In this work, PECs were prepared from two biopolymers, positively charged chitosan and negatively charged alginate.

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Inspired by spider silk's hierarchical diversity, we leveraged peptide motifs with the capability to tune structural arrangement for controlling the mechanical properties of a conventional polymer framework. The addition of nanofiller with hydrogen bonding sites was used as another pathway towards hierarchical tuning matrix-filler interactions. Specifically, peptide-polyurea hybrids (PPUs) were combined with cellulose nanocrystals (CNCs) to develop mechanically-tunable nanocomposites tailored matrix-filler interactions (or peptide-cellulose interactions).

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The success of the lead halide perovskites in diverse optoelectronics has motivated considerable interest in their fundamental photocarrier dynamics. Here we report the discovery of photocarrier-induced persistent structural polarization and local ferroelectricity in lead halide perovskites. Photoconductance studies of thin-film single-crystal CsPbBr at 10 K reveal long-lasting persistent photoconductance with an ultralong photocarrier lifetime beyond 10 s.

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  • Concentrated ionic solutions, like those involving lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI) and acetonitrile, can benefit from cosolvation to enhance conductivity, although high viscosities typically limit this improvement.* -
  • The study uses experiments and molecular dynamics simulations to analyze how different cosolvents—such as toluene, dichloromethane, acetone, methanol, and water—impact the structure and charge transport properties in these solutions.* -
  • While some cosolvents can reduce conductivity by affecting Li-TFSI interactions, others may enhance specific interactions despite ultimately leading to lower conductivities than expected, highlighting the complex relationships between
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  • * This stretching process involves heating the SPE films above the melting point of PEO in an inert atmosphere and allowing crystallization while the material is still under strain.
  • * Experimental techniques like wide-angle and small-angle X-ray scattering reveal that this alignment creates a more efficient pathway for ion movement, resulting in a significant increase in ionic conductivity (1.4 to 3.5 times better).
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  • * This study focuses on compatibilization, testing three commercial ethylene copolymer compatibilizers (EAA, PTW, and Surlyn) on a blend of PET and HDPE, common packaging materials.
  • * Researchers assess how the location of these compatibilizers within the polymer blend impacts compatibility, which helps in understanding their effectiveness and improving strategies for upcycling waste plastics.
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  • Single-ion conducting polymer electrolytes (SICPEs) are seen as a promising advanced electrolyte system for batteries due to their high cationic transference number, but their low ionic conductivity compared to liquid electrolytes remains a major limitation.
  • Polyethylene oxide (PEO) is highlighted as a key component in SICPEs due to its excellent salt solubility, good interaction with lithium ions, and fast charge transport properties.
  • The study suggests that the copolymer's structure, particularly the polarity of side chains and the design of polyanions with delocalized charges, plays a crucial role in enhancing lithium ionic conductivity in SICPEs.
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Understanding the feasibility to couple semiconducting and magnetic properties in metal halide perovskites through interface design opens new opportunities for creating the next generation spin-related optoelectronics. In this work, a fundamentally new phenomenon of optically induced magnetization achieved by coupling photoexcited orbital magnetic dipoles with magnetic spins at perovskite/ferromagnetic interface is discovered. The depth-sensitive polarized neutron reflectometry combined with in situ photoexcitation setup, constitutes key evidence of this novel effect.

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  • A novel quantum disordered state of magnetic charges was demonstrated in a specially designed nanoengineered honeycomb lattice, despite the absence of observed quantum magnetic states in practice.
  • The research utilized spin-resolved neutron scattering and revealed a complex ground state with low and high integer magnetic charges, displaying cooperative paramagnetism at low temperatures.
  • The findings highlight the significant impact of exchange energy at the nanoscale, suggesting a new platform for exploring quantum mechanical phenomena associated with magnetic charges.
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Solid polymer electrolytes are promising in fulfilling the requirements for a stable lithium metal anode toward higher energy and power densities. In this work, we investigate the segmental dynamics, ionic conductivity, and crystallinity of a polymer electrolyte consisting of poly(ethylene oxide) (PEO) and lithium triflate salt, in the semi-crystalline state. Using quasi-elastic neutron scattering, the segmental dynamics of PEO chains confined between the crystalline lamellae is quantified, using Cole-Cole analysis.

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Chloroquine and its derivative hydroxychloroquine are primarily known as antimalaria drugs. Here, we investigate the influence of hydration water on the molecular dynamics in hydroxychloroquine sulfate, a commonly used solubilized drug form. When hydration, even at a low level, results in a disordered structure, as opposed to the highly ordered structure of dry hydroxychloroquine sulfate, the activation barriers for the rotation of methyl groups in the drug molecules become randomized and, on average, significantly reduced.

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The mechanical and thermal conductivity properties of two composite elastomers were studied. Styrene-butadiene rubber (SBR) filled with functionalized graphene oxide (GO) and silica nanofibers, and styrene-butadiene-styrene (SBS) block copolymers filled with graphene oxide. For the SBR composites, GO fillers with two different surface functionalities were synthesized (cysteamine and dodecylamine) and dispersed in the SBR using mechanical and liquid mixing techniques.

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Polydopamine (PDA) is an emerging nature-inspired biopolymer material that possesses many interesting properties including self-assembly and universal adhesion. PDA is also able to form coordination bonds with various metal ions, which can be reduced to metal nanoparticles (NPs) as a result of thermal annealing under protective environment. In this study, PDA has been utilized as a support material to synthesize Pt NPs in an aqueous solution at room temperature.

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