Publications by authors named "Tae-Woo Lee"

128 Publications

Chiral polymer hosts for circularly polarized electroluminescence devices.

Chem Sci 2021 Jul 21;12(25):8668-8681. Epub 2021 May 21.

Division of Chemical Engineering and Materials Science, Graduate Program in System Health Science and Engineering, Ewha Womans University Seoul 03760 Republic of Korea

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral -alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred ()-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The ()-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent ()- and ()-dopants. The magnitude of the Kuhn dissymmetry factor ( ) for the ()-helically-enriched polymer film doped with the ()-dopant was found to be one order of magnitude higher than that of the film doped with the ()-dopant. Photoluminescence dissymmetry factors ( ) of the order of 10 were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of . The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the ()- and ()-dopants exhibit electroluminescence dissymmetry factors ( ) of 1.09 × 10 and -1.02 × 10 at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.
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http://dx.doi.org/10.1039/d1sc02095aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8246120PMC
July 2021

Recent Progress in Development of Wearable Pressure Sensors Derived from Biological Materials.

Adv Healthc Mater 2021 09 29;10(17):e2100460. Epub 2021 May 29.

Department of Materials Science and Engineering, Seoul National University, Seoul, 08826, Republic of Korea.

This review summarizes recent progress in the use of biological materials (biomaterials) in wearable pressure sensors. Biomaterials are abundant, sustainable, biocompatible, and biodegradable. Especially, many have sophisticated hierarchical structure and biological characteristics, which are attractive candidates for facile and ecologically-benign fabrication of wearable pressure sensors that are biocompatible, biodegradable, and highly sensitivity. The biomaterials and structures that use them in wearable pressure sensors that exploit sensing mechanisms such as piezoelectric, triboelectric, piezoresistive and capacitive effects are present. Finally, remaining impediments are discussed to use of biomaterials in wearable pressure sensors.
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http://dx.doi.org/10.1002/adhm.202100460DOI Listing
September 2021

Supra-Binary Polarization in a Ferroelectric Nanowire.

Adv Mater 2021 Jul 24;33(26):e2101981. Epub 2021 May 24.

Department of Chemistry, School of Natural Science, Center for Superfunctional Materials, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919, Republic of Korea.

The prediction and observation of supra-binary polarization in a ferroelectric nanowire (FNW) covered with a semicylindrical gate that provides an anisotropic electric field in the FNW are reported. There are gate-voltage-driven transitions between four polarization states in the FNW's cross-section, dubbed vertical-up, vertical-down, radial-in, and radial-out. They are determined by the interplay between the spatial depolarization energy and the free energy induced by an anisotropic external electric field, in clear distinction from the conventional film-based binary ferroelectricity. When the FNW is mounted on a biased graphene nanoribbon (GNR), these transitions induce exotic current-voltage hysteresis in the FNW-GNR transistor. This discovery suggests new operating mechanisms of ferroelectric devices. In particular, it enables intrinsic quaternary-digit information manipulation in parallel to the bit manipulation employed in conventional data storage.
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http://dx.doi.org/10.1002/adma.202101981DOI Listing
July 2021

Quantum-confinement effect on the linewidth broadening of metal halide perovskite-based quantum dots.

J Phys Condens Matter 2021 Jul 6;33(35). Epub 2021 Jul 6.

Research Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea.

The linewidth broadening caused by various physicochemical effects does limit the well-known advantage of ultrahigh color purity of metal halide perovskites (MHPs) for use in next-generation light-emitting diodes (LEDs). We have theoretically examined the quantum- and dielectric-confinement effects of a quantum dot (QD) on the degree of photoluminescence linewidth broadening. It is predicted that the linewidth (Δ) is mainly contributed by the two opposing effects: (i) the linewidth broadening due to the repulsive kinetic energy of confined excitons (ΔλQCKE) and (ii) the overall linewidth narrowing caused by the attractive Coulomb interaction (ΔλQCCoul). It is shown that the relative contribution essentially remains at a constant value and is evaluated asΔλQCCoul/ΔλQCKE=0.42, which is independent of the QD size and the chemical nature of semiconducting emitter. We have computed Δfor various QD sizes of the prototypical MHP emitter, MAPbBr, where MA denotes a methylammonium (CHNH) organic cation. The calculated results show that the linewidth broadening due to the quantum confinement (Δ) increases rapidly beginning at the QD radius approximately equal to 6.5 nm but Δis less than 2 nm even at= 1.5 nm. Thus, Δis much narrower than the linewidth caused by the exciton-LO phonon Fröhlich coupling (∼23.4 nm) which is known as the predominant mechanism of linewidth broadening in hybrid MHPs. Thus, the linewidth broadening due to the quantum confinement (Δ) is not a risk factor in the realization of MHP-based ultrahigh-quality next-generation LEDs.
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http://dx.doi.org/10.1088/1361-648X/ac00dbDOI Listing
July 2021

Extremely Stable Luminescent Crosslinked Perovskite Nanoparticles under Harsh Environments over 1.5 Years.

Adv Mater 2021 Jan 4;33(3):e2005255. Epub 2020 Dec 4.

Department of Materials Science and Engineering, Institute of Engineering Research, Research Institute of Advanced Materials, Nano Systems Institute (NSI), BK21 PLUS SNU Materials Division for Educating Creative Global Leaders, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

Organic-inorganic hybrid perovskite nanoparticles (NPs) are a very strong candidate emitter that can meet the high luminescence efficiency and high color standard of Rec.2020. However, the instability of perovskite NPs is the most critical unsolved problem that limits their practical application. Here, an extremely stable crosslinked perovskite NP (CPN) is reported that maintains high photoluminescence quantum yield for 1.5 years (>600 d) in air and in harsher liquid environments (e.g., in water, acid, or base solutions, and in various polar solvents), and for more than 100 d under 85 °C and 85% relative humidity without additional encapsulation. Unsaturated hydrocarbons in both the acid and base ligands of NPs are chemically crosslinked with a methacrylate-functionalized matrix, which prevents decomposition of the perovskite crystals. Counterintuitively, water vapor permeating through the crosslinked matrix chemically passivates surface defects in the NPs and reduces nonradiative recombination. Green-emitting and white-emitting flexible large-area displays are demonstrated, which are stable for >400 d in air and in water. The high stability of the CPN in water enables biocompatible cell proliferation which is usually impossible when toxic Pb elements are present. The stable materials design strategies provide a breakthrough toward commercialization of perovskite NPs in displays and bio-related applications.
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http://dx.doi.org/10.1002/adma.202005255DOI Listing
January 2021

Chemically Robust Indium Tin Oxide/Graphene Anode for Efficient Perovskite Light-Emitting Diodes.

ACS Appl Mater Interfaces 2021 Feb 25;13(7):9074-9080. Epub 2021 Jan 25.

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea.

Graphene is an optimal material to be employed as an ionic diffusion barrier because of its outstanding impermeability and chemical robustness. Indium tin oxide (ITO) is often used in perovskite light-emitting diodes (PeLEDs), and it can release indium easily upon exposure to the acidic hole-injection layer so that luminescence can be quenched significantly. Here, we exploit the outstanding impermeability of graphene and use it as a chemical barrier to block the etching that can occur in ITO exposed to an acidic hole-injection layer in PeLEDs. This barrier reduced the luminescence quenching that these metallic species can cause, so the photoluminescence lifetime of perovskite film was substantially higher in devices with ITO and graphene layer (87.9 ns) than in devices that had only an ITO anode (22.1 ns). Luminous current efficiency was also higher in PeLEDs with a graphene barrier (16.4 cd/A) than in those without graphene (9.02 cd/A). Our work demonstrates that graphene can be used as a barrier to reduce the degradation of transparent electrodes by chemical etching in optoelectronic devices.
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http://dx.doi.org/10.1021/acsami.0c12939DOI Listing
February 2021

Controllable deposition of organic metal halide perovskite films with wafer-scale uniformity by single source flash evaporation.

Sci Rep 2020 Nov 2;10(1):18781. Epub 2020 Nov 2.

Department of Physics and Astronomy, and Institute of Applied Physics, Seoul National University, Seoul, 08826, Korea.

Conventional solution-processing techniques such as the spin-coating method have been used successfully to reveal excellent properties of organic-inorganic halide perovskites (OHPs) for optoelectronic devices such as solar cell and light-emitting diode, but it is essential to explore other deposition techniques compatible with large-scale production. Single-source flash evaporation technique, in which a single source of materials of interest is rapidly heated to be deposited in a few seconds, is one of the candidate techniques for large-scale thin film deposition of OHPs. In this work, we investigated the reliability and controllability of the single-source flash evaporation technique for methylammonium lead iodide (MAPbI) perovskite. In-depth statistical analysis was employed to demonstrate that the MAPbI films prepared via the flash evaporation have an ultrasmooth surface and uniform thickness throughout the 4-inch wafer scale. We also show that the thickness and grain size of the MAPbI film can be controlled by adjusting the amount of the source and number of deposition steps. Finally, the excellent large-area uniformity of the physical properties of the deposited thin films can be transferred to the uniformity in the device performance of MAPbI photodetectors prepared by flash evaporation which exhibited the responsivity of 0.2 A/W and detectivity of 3.82 × 10 Jones.
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http://dx.doi.org/10.1038/s41598-020-75764-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7608649PMC
November 2020

Association between urinary polycyclic aromatic hydrocarbons and hypertension in the Korean population: data from the Second Korean National Environmental Health Survey (2012-2014).

Sci Rep 2020 10 13;10(1):17142. Epub 2020 Oct 13.

Department of Occupational and Environmental Medicine, Inje University Haeundae Paik Hospital, 875 Haeundae-ro, Haeundae-gu, Busan, 48108, South Korea.

Polycyclic aromatic hydrocarbons (PAHs) are environmental and occupational pollutants derived from incomplete combustion of organic materials, including wood and fossil fuels. Epidemiological studies have evaluated the association between PAH exposure and hypertension or cardiovascular disease in the general population, but the evidence is limited. In this study, we evaluated the association between urinary PAH metabolites and hypertension in the Korean adult population. A total of 6478 adults who participated in the Second Korean National Environmental Health Survey (2012-2014) were included. The differences in urinary concentrations of four PAH metabolites, including 1-hydroxypyrene, 2-hydroxyfluorene, 1-hydroxyphenanthrene, and 2-naphthol, were compared according to hypertension status using a general linear model. Adjusted odds ratios (aORs) for hypertension were calculated according to the quartile groups of urinary PAH metabolites after adjusting for age, sex, body mass index (BMI), smoking, and alcohol consumption in multiple logistic regression analyses. The estimated mean concentrations of urinary 1-hydroxyphenanthrene were significantly higher in the hypertension group than in the non-hypertension group. In 1-hydroxyphenanthrene, the OR for hypertension was significantly higher in the third and fourth quartile groups than in the first quartile group (third: OR 1.707, 95% CI 1.203-2.423, fourth: OR 1.604, 95% CI 1.158-2.223). No significant associations were detected for the other metabolites. Our results suggest an association between exposure to PAHs and hypertension in a Korean adult population. Further studies are required to evaluate the effects of low-dose long-term exposure to PAHs on hypertension and cardiovascular disease.
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http://dx.doi.org/10.1038/s41598-020-74353-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7555493PMC
October 2020

Aromatic nonpolar organogels for efficient and stable perovskite green emitters.

Nat Commun 2020 Sep 15;11(1):4638. Epub 2020 Sep 15.

Department of Materials Science and Engineering, Seoul National University, Seoul, 08826, Republic of Korea.

Existing gels are mostly polar, whose nature limits their role in soft devices. The intermolecular interactions of nonpolar polymer-liquid system are typically weak, which makes the gel brittle. Here we report highly soft and transparent nonpolar organogels. Even though their elements are only carbon and hydrogen, their elastic modulus, transparency, and stretchability are comparable to common soft hydrogels. A key strategy is introducing aromatic interaction into the polymer-solvent system, resulting in a high swelling ratio that enables efficient plasticization of the polymer networks. As a proof of applicability, soft perovskite nanocomposites are synthesized, where the nonpolar environment of organogels enables stable formation and preservation of highly concentrated perovskite nanocrystals, showing high photoluminescence efficiency (~99.8%) after water-exposure and environmental stabilities against air, water, acid, base, heat, light, and mechanical deformation. Their superb properties enable the demonstration of soft electroluminescent devices that stably emit bright and pure green light under diverse deformations.
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http://dx.doi.org/10.1038/s41467-020-18383-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7493929PMC
September 2020

Electroplated Silver-Nickel Core-Shell Nanowire Network Electrodes for Highly Efficient Perovskite Nanoparticle Light-Emitting Diodes.

ACS Appl Mater Interfaces 2020 Sep 18;12(35):39479-39486. Epub 2020 Aug 18.

Department of Materials Science and Engineering, School of Chemical and Biological Engineering, Research Institute of Advanced Materials, Institute of Engineering Research, Nano Systems Institute (NSI), BK21 PLUS SNU Materials Division for Educating Creative Global Leaders, Seoul National University, Seoul 08826, Republic of Korea.

The low sheet resistance and high optical transparency of silver nanowires (AgNWs) make them a promising candidate for use as the flexible transparent electrode of light-emitting diodes (LEDs). In a perovskite LED (PeLED), however, the AgNW electrode can react with the overlying perovskite material by redox reactions, which limit the electroluminescence efficiency of the PeLED by causing the degradation of and generating defect states in the perovskite material. In this study, we prepared Ag-Ni core-shell NW electrodes using the solution-electroplating technique to realize highly efficient PeLEDs based on colloidal formamidinium lead bromide (FAPbBr) nanoparticles (NPs). Solvated Ni ions from the NiSO source were deposited onto the surface of AgNW networks in three steps: (i) cathodic cleaning, (ii) adsorption of the Ni-ion complex onto the AgNW surface, and (iii) uniform electrodeposition of Ni. An ultrathin (∼3.5 nm) Ni layer was uniformly deposited onto the AgNW surface, which exhibited a sheet resistance of 16.7 Ω/sq and an optical transmittance of 90.2%. The Ag-Ni core-shell NWs not only increased the work function of the AgNW electrode, which facilitated hole injection into the emitting layer, but also suppressed the redox reaction between Ag and FAPbBr NPs, which prevented the degradation of the emitting layer and the generation of defect states in it. The resulting PeLEDs based on FAPbBr NPs with the Ag-Ni core-shell NWs showed high current efficiency of 44.01 cd/A, power efficiency of 35.45 lm/W, and external quantum efficiency of 9.67%.
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http://dx.doi.org/10.1021/acsami.0c10386DOI Listing
September 2020

Water Passivation of Perovskite Nanocrystals Enables Air-Stable Intrinsically Stretchable Color-Conversion Layers for Stretchable Displays.

Adv Mater 2020 Sep 26;32(37):e2001989. Epub 2020 Jul 26.

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

Conventional organic light-emitting devices without an encapsulation layer are susceptible to degradation when exposed to air, so realization of air-stable intrinsically-stretchable display is a great challenge because the protection of the devices against penetration of moisture and oxygen is even more difficult under stretching. An air-stable intrinsically-stretchable display that is composed of an intrinsically-stretchable electroluminescent device (SELD) integrated with a stretchable color-conversion layer (SCCL) that contains perovskite nanocrystals (PeNCs) is proposed. PeNCs normally decay when exposed to air, but they become resistant to this decay when dispersed in a stretchable elastomer matrix; this change is a result of a compatibility between capping ligands and the elastomer matrix. Counterintuitively, the moisture can efficiently passivate surface defects of PeNCs, to yield significant increases in both photoluminescence intensity and lifetime. A display that can be stretched up to 180% is demonstrated; it is composed of an air-stable SCCL that down-converts the SELD's blue emission and reemits it as green. The work elucidates the basis of moisture-assisted surface passivation of PeNCs and provides a promising strategy to improve the quantum efficiency of PeNCs with the aid of moisture, which allows PeNCs to be applied for air-stable stretchable displays.
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http://dx.doi.org/10.1002/adma.202001989DOI Listing
September 2020

Ultrashort laser pulse doubling by metal-halide perovskite multiple quantum wells.

Nat Commun 2020 Jul 17;11(1):3361. Epub 2020 Jul 17.

Joint Key Laboratory of the Ministry of Education, Institute of Applied Physics and Materials Engineering, University of Macau, 999078, Macao, China.

Multiple ultrashort laser pulses are widely used in optical spectroscopy, optoelectronic manipulation, optical imaging and optical signal processing etc. The laser pulse multiplication, so far, is solely realized by using the optical setups or devices to modify the output laser pulse from the optical gain medium. The employment of these external techniques is because the gain medium itself is incapable of modifying or multiplying the generated laser pulse. Herein, with single femtosecond laser pulse excitation, we achieve the double-pulsed stimulated emission with pulse duration of around 40 ps and pulse interval of around 70 ps from metal-halide perovskite multiple quantum wells. These unique stimulated emissions originate from one fast vertical and the other slow lateral high-efficiency carrier funneling from low-dimensional to high-dimensional quantum wells. Furthermore, such gain medium surprisingly possesses nearly Auger-free stimulated emission. These insights enable us a fresh approach to multiple the ultrashort laser pulse by gain medium.
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http://dx.doi.org/10.1038/s41467-020-17096-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7368017PMC
July 2020

Proton-transfer-induced 3D/2D hybrid perovskites suppress ion migration and reduce luminance overshoot.

Nat Commun 2020 Jul 6;11(1):3378. Epub 2020 Jul 6.

Department of Materials Science and Engineering, Seoul National University, Seoul, 08826, Republic of Korea.

Perovskite light-emitting diodes (PeLEDs) based on three-dimensional (3D) polycrystalline perovskites suffer from ion migration, which causes overshoot of luminance over time during operation and reduces its operational lifetime. Here, we demonstrate 3D/2D hybrid PeLEDs with extremely reduced luminance overshoot and 21 times longer operational lifetime than 3D PeLEDs. The luminance overshoot ratio of 3D/2D hybrid PeLED is only 7.4% which is greatly lower than that of 3D PeLED (150.4%). The 3D/2D hybrid perovskite is obtained by adding a small amount of neutral benzylamine to methylammonium lead bromide, which induces a proton transfer from methylammonium to benzylamine and enables crystallization of 2D perovskite without destroying the 3D phase. Benzylammonium in the perovskite lattice suppresses formation of deep-trap states and ion migration, thereby enhances both operating stability and luminous efficiency based on its retardation effect in reorientation.
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http://dx.doi.org/10.1038/s41467-020-17072-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7338442PMC
July 2020

A 2D Titanium Carbide MXene Flexible Electrode for High-Efficiency Light-Emitting Diodes.

Adv Mater 2020 Jun 30;32(23):e2000919. Epub 2020 Apr 30.

Department of Materials Science and Engineering, Institute of Engineering Research, Research Institute of Advanced Materials, BK21 PLUS SNU Materials Division for Educating Creative Global Leaders, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

Although several transparent conducting materials such as carbon nanotubes, graphene, and conducting polymers have been intensively explored as flexible electrodes in optoelectronic devices, their insufficient electrical conductivity, low work function, and complicated electrode fabrication processes have limited their practical use. Herein, a 2D titanium carbide (Ti C ) MXene film with transparent conducting electrode (TCE) properties, including high electrical conductivity (≈11 670 S cm ) and high work function (≈5.1 eV), which are achieved by combining a simple solution processing with modulation of surface composition, is described. A chemical neutralization strategy of a conducting-polymer hole-injection layer is used to prevent detrimental surface oxidation and resulting degradation of the electrode film. Use of the MXene electrode in an organic light-emitting diode leads to a current efficiency of ≈102.0 cd A and an external quantum efficiency of ≈28.5% ph/el, which agree well with the theoretical maximum values from optical simulations. The results demonstrate the strong potential of MXene as a solution-processable electrode in optoelectronic devices and provide a guideline for use of MXenes as TCEs in low-cost flexible optoelectronic devices.
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http://dx.doi.org/10.1002/adma.202000919DOI Listing
June 2020

Retina-Inspired Carbon Nitride-Based Photonic Synapses for Selective Detection of UV Light.

Adv Mater 2020 Mar 27;32(11):e1906899. Epub 2020 Jan 27.

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

Photonic synapses combine sensing and processing in a single device, so they are promising candidates to emulate visual perception of a biological retina. However, photonic synapses with wavelength selectivity, which is a key property for visual perception, have not been developed so far. Herein, organic photonic synapses that selectively detect UV rays and process various optical stimuli are presented. The photonic synapses use carbon nitride (C N ) as an UV-responsive floating-gate layer in transistor geometry. C N nanodots dominantly absorb UV light; this trait is the basis of UV selectivity in these photonic synapses. The presented devices consume only 18.06 fJ per synaptic event, which is comparable to the energy consumption of biological synapses. Furthermore, in situ modulation of exposure to UV light is demonstrated by integrating the devices with UV transmittance modulators. These smart systems can be further developed to combine detection and dose-calculation to determine how and when to decrease UV transmittance for preventive health care.
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http://dx.doi.org/10.1002/adma.201906899DOI Listing
March 2020

Emerging Halide Perovskite Materials and Devices for Optoelectronics.

Authors:
Tae-Woo Lee

Adv Mater 2019 Nov;31(47):e1905077

Department of Materials Science and Engineering, Institute of Engineering Research, Research Institute of Advanced Materials, Nano Systems Institute (NSI), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

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http://dx.doi.org/10.1002/adma.201905077DOI Listing
November 2019

The effects of exposure to lead, cadmium and mercury on follicle-stimulating hormone levels in men and postmenopausal women: data from the Second Korean National Environmental Health Survey (2012-2014).

Ann Occup Environ Med 2019 28;31:e21. Epub 2019 Aug 28.

Department of Occupational and Environmental Medicine, Inje University Haeundae Paik Hospital, Busan, Korea.

Background: Follicle-stimulating hormone (FSH), a gonadotropin secreted by the pituitary gland, is a representative secondary sex hormone and an important indicator of reproductive function. The effects of heavy metals such as lead, cadmium, and mercury on humans have been studied, but reports on their effects on sex hormone levels are lacking. Therefore, we investigated the relationship between heavy metal exposure and FSH levels in Korean men and postmenopausal women.

Methods: A total of 4,689 adults (2,763 men and 1,926 postmenopausal women aged 50 years or over) who participated in the Second Korean National Environmental Health Survey (2012-2014) were included. We compared differences in serum FSH levels by demographic characteristics using the t-test and analysis of variance. Multiple linear regression analysis was used to determine the relationship between the blood levels of lead and mercury and the urine cadmium level, and serum FSH levels.

Results: On multiple linear regression analysis, lead exposure was positively associated with serum FSH concentrations in postmenopausal women (β = 2.929, = 0.019). However, we found no significant association between serum FSH concentration and blood lead and mercury levels, or urine cadmium level, in men.

Conclusions: This study suggests that lead exposure can affect the FSH level in postmenopausal women. Further studies are needed to evaluate the effects of low-dose long-term exposure to heavy metals on sex hormones.
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http://dx.doi.org/10.35371/aoem.2019.31.e21DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6779926PMC
August 2019

Flexible Neuromorphic Electronics for Computing, Soft Robotics, and Neuroprosthetics.

Adv Mater 2020 Apr 26;32(15):e1903558. Epub 2019 Sep 26.

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

Flexible neuromorphic electronics that emulate biological neuronal systems constitute a promising candidate for next-generation wearable computing, soft robotics, and neuroprosthetics. For realization, with the achievement of simple synaptic behaviors in a single device, the construction of artificial synapses with various functions of sensing and responding and integrated systems to mimic complicated computing, sensing, and responding in biological systems is a prerequisite. Artificial synapses that have learning ability can perceive and react to events in the real world; these abilities expand the neuromorphic applications toward health monitoring and cybernetic devices in the future Internet of Things. To demonstrate the flexible neuromorphic systems successfully, it is essential to develop artificial synapses and nerves replicating the functionalities of the biological counterparts and satisfying the requirements for constructing the elements and the integrated systems such as flexibility, low power consumption, high-density integration, and biocompatibility. Here, the progress of flexible neuromorphic electronics is addressed, from basic backgrounds including synaptic characteristics, device structures, and mechanisms of artificial synapses and nerves, to applications for computing, soft robotics, and neuroprosthetics. Finally, future research directions toward wearable artificial neuromorphic systems are suggested for this emerging area.
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http://dx.doi.org/10.1002/adma.201903558DOI Listing
April 2020

Low-dimensional iodide perovskite nanocrystals enable efficient red emission.

Nanoscale 2019 Jul 1;11(27):12793-12797. Epub 2019 Jul 1.

Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, Catedrático José Beltrán, 2, 46980 Paterna, Spain.

We report herein a simple ligand-assisted reprecipitation method at room temperature to synthesize mixed-cation hybrid organic-inorganic perovskite nanocrystals with low structural dimensionality. The emission wavelength of iodide-based perovskites is thus tuned from the near-infrared to the red part of the visible spectrum. While this is mostly achieved in the literature by addition of bromide, we demonstrate here a controllable blueshift of the band gap by varying the chain length of the alkylammonium ligands. Furthermore, an antisolvent washing step was found to be crucial to purify the samples and obtain single-peak photoluminescence with a narrow linewidth. The so-formed nanocrystals exhibit high and stable photoluminescence quantum yields exceeding 90% over 500 hours, making these materials ideal for light-emitting applications.
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http://dx.doi.org/10.1039/c9nr04520aDOI Listing
July 2019

Perovskites for Next-Generation Optical Sources.

Chem Rev 2019 Jun 25;119(12):7444-7477. Epub 2019 Apr 25.

Department of Electrical and Computer Engineering, University of Toronto, 10 King's College Road, Toronto, Ontario M5S 3G4, Canada.

Next-generation displays and lighting technologies require efficient optical sources that combine brightness, color purity, stability, substrate flexibility. Metal halide perovskites have potential use in a wide range of applications, for they possess excellent charge transport, bandgap tunability and, in the most promising recent optical source materials, intense and efficient luminescence. This review links metal halide perovskites' performance as efficient light emitters with their underlying materials electronic and photophysical attributes.
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http://dx.doi.org/10.1021/acs.chemrev.9b00107DOI Listing
June 2019

Organic Synapses for Neuromorphic Electronics: From Brain-Inspired Computing to Sensorimotor Nervetronics.

Acc Chem Res 2019 04 21;52(4):964-974. Epub 2019 Mar 21.

Living organisms have a long evolutionary history that has provided them with functions and structures that enable them to survive in their environment. The goal of biomimetic technology is to emulate these traits of living things. Research in bioinspired electronics develops electronic sensors and motor systems that mimic biological sensory organs and motor systems and that are intended to be used in bioinspired applications such as humanoid robots, exoskeletons, and other devices that combine a living body and an electronic device. To develop bioinspired robotic and electronic devices that are compatible with the living body at the neuronal level and that are operated by mechanisms similar to those in a living body, researchers must develop biomimetic electronic sensors, motor systems, brains, and nerves. Artificial organic synapses have emulated the brain's plasticity with much simpler structures and lower fabrication cost than neurons based on silicon circuits, and with smaller energy consumption than traditional von Neumann computing methods. Organic synapses are promising components of future neuromorphic systems. In this Account, we review recent research trends of neuromorphic systems based on organic synapses, then suggest research directions. We introduce the device structures and working mechanisms of reported organic synapses and the brain's plasticity, which are mainly imitated to demonstrate the learning and memory function of the organic synapses. We also introduce recent reports on sensory synapses and sensorimotor nervetronics that mimic biological sensory and motor nervous systems. Sensory nervetronics can be used to augment the sensory functions of the living body and to comprise the sensory systems of biomimetic robots. Organic synapses can also be used to control biological muscles and artificial muscles that have the same working mechanism as biological muscle. Motor nervetronics would impart life-like motion to bioinspired robots. Chemical approaches may provide insights to guide development of new organic materials, device structures, and working mechanisms to improve synaptic responses of organic neuromorphic systems. For example, organic synapses can be applied to electronic and robotic skins and bioimplantable medical devices that use mechanically stable, self-healing, and biocompatible organic materials. Biochemical approaches may expand the plasticity of the brain and nervous system. We expect that organic neuromorphic systems will be vital components in bioinspired robotic and electronic applications, including biocompatible neural prosthetics, exoskeletons, humanoid soft robots, and cybernetics devices that are integrated with biological and artificial organs.
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http://dx.doi.org/10.1021/acs.accounts.8b00553DOI Listing
April 2019

Wearable Bioelectronics: Opportunities for Chemistry.

Acc Chem Res 2019 03;52(3):521-522

Seoul National University.

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http://dx.doi.org/10.1021/acs.accounts.9b00048DOI Listing
March 2019

Strategies to Improve Luminescence Efficiency of Metal-Halide Perovskites and Light-Emitting Diodes.

Adv Mater 2019 Nov 17;31(47):e1804595. Epub 2018 Dec 17.

Department of Materials Science and Engineering, Institute of Engineering Research, Research Institute of Advanced Materials, Nano Systems Institute (NSI), BK21 PLUS SNU Materials Division for Educating Creative Global Leaders, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

Metal-halide perovskites (MHPs) are well suited to be vivid natural color emitters due to their superior optical and electrical properties, such as narrow emission linewidths, easily and widely tunable emission wavelengths, low material cost, and high charge carrier mobility. Since the first development of MHP light-emitting diodes (PeLEDs) in 2014, many researchers have tried to understand the properties of MHP emitters and the limitations to luminescence efficiency (LE) of PeLEDs, and have devoted efforts to increase the LE of MHP emitters and PeLEDs. Within three and half years, PeLEDs have shown rapidly increased LE from external quantum efficiency ≈0.1% to ≈14.36%. Herein, the factors that limit the LE of PeLEDs are reviewed; the factors are characterized into the following groups: i) photophysical properties of MHP crystals, ii) morphological factors of MHP layers, and iii) problems caused by device architectures. Then, the strategies to overcome those luminescence-limiting factors in MHP emitters and PeLEDs are critically evaluated. Finally, research directions to further increase the LE of MHP emitters and the potential of MHPs as a core component in next-generation displays and solid-state lightings are suggested.
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http://dx.doi.org/10.1002/adma.201804595DOI Listing
November 2019

Stretchable organic optoelectronic sensorimotor synapse.

Sci Adv 2018 11 23;4(11):eaat7387. Epub 2018 Nov 23.

Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea.

Emulation of human sensory and motor functions becomes a core technology in bioinspired electronics for next-generation electronic prosthetics and neurologically inspired robotics. An electronic synapse functionalized with an artificial sensory receptor and an artificial motor unit can be a fundamental element of bioinspired soft electronics. Here, we report an organic optoelectronic sensorimotor synapse that uses an organic optoelectronic synapse and a neuromuscular system based on a stretchable organic nanowire synaptic transistor (s-ONWST). The voltage pulses of a self-powered photodetector triggered by optical signals drive the s-ONWST, and resultant informative synaptic outputs are used not only for optical wireless communication of human-machine interfaces but also for light-interactive actuation of an artificial muscle actuator in the same way that a biological muscle fiber contracts. Our organic optoelectronic sensorimotor synapse suggests a promising strategy toward developing bioinspired soft electronics, neurologically inspired robotics, and electronic prostheses.
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http://dx.doi.org/10.1126/sciadv.aat7387DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6251720PMC
November 2018

A bioinspired flexible organic artificial afferent nerve.

Science 2018 06;360(6392):998-1003

Department of Materials Science and Engineering, Seoul National University, Seoul, South Korea.

The distributed network of receptors, neurons, and synapses in the somatosensory system efficiently processes complex tactile information. We used flexible organic electronics to mimic the functions of a sensory nerve. Our artificial afferent nerve collects pressure information (1 to 80 kilopascals) from clusters of pressure sensors, converts the pressure information into action potentials (0 to 100 hertz) by using ring oscillators, and integrates the action potentials from multiple ring oscillators with a synaptic transistor. Biomimetic hierarchical structures can detect movement of an object, combine simultaneous pressure inputs, and distinguish braille characters. Furthermore, we connected our artificial afferent nerve to motor nerves to construct a hybrid bioelectronic reflex arc to actuate muscles. Our system has potential applications in neurorobotics and neuroprosthetics.
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http://dx.doi.org/10.1126/science.aao0098DOI Listing
June 2018

Extremely stable graphene electrodes doped with macromolecular acid.

Nat Commun 2018 05 23;9(1):2037. Epub 2018 May 23.

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro Gwanak-gu, Seoul, 08826, Republic of Korea.

Although conventional p-type doping using small molecules on graphene decreases its sheet resistance (R), it increases after exposure to ambient conditions, and this problem has been considered as the biggest impediment to practical application of graphene electrodes. Here, we report an extremely stable graphene electrode doped with macromolecular acid (perfluorinated polymeric sulfonic acid (PFSA)) as a p-type dopant. The PFSA doping on graphene provides not only ultra-high ambient stability for a very long time (> 64 days) but also high chemical/thermal stability, which have been unattainable by doping with conventional small-molecules. PFSA doping also greatly increases the surface potential (~0.8 eV) of graphene, and reduces its R by ~56%, which is very important for practical applications. High-efficiency phosphorescent organic light-emitting diodes are fabricated with the PFSA-doped graphene anode (~98.5 cd A without out-coupling structures). This work lays a solid platform for practical application of thermally-/chemically-/air-stable graphene electrodes in various optoelectronic devices.
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http://dx.doi.org/10.1038/s41467-018-04385-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5966423PMC
May 2018

High-Efficiency Polycrystalline Perovskite Light-Emitting Diodes Based on Mixed Cations.

ACS Nano 2018 03 6;12(3):2883-2892. Epub 2018 Mar 6.

Cavendish Laboratory , University of Cambridge , JJ Thomson Avenue , Cambridge CB3 0HE , United Kingdom.

We have achieved high-efficiency polycrystalline perovskite light-emitting diodes (PeLEDs) based on formamidinium (FA) and cesium (Cs) mixed cations without quantum dot synthesis. Uniform single-phase FACs PbBr polycrystalline films were fabricated by one-step formation with various FA:Cs molar proportions; then the influences of chemical composition on film morphology, crystal structure, photoluminescence (PL), and electroluminescence (EL) were systematically investigated. Incorporation of Cs cations in FAPbBr significantly reduced the average grain size (to 199 nm for FA:Cs = 90:10) and trap density; these changes consequently increased PL quantum efficiency (PLQE) and PL lifetime of FACs PbBr films and current efficiency (CE) of PeLEDs. Further increase in Cs molar proportion from 10 mol % decreased crystallinity and purity, increased trap density, and correspondingly decreased PLQE, PL lifetime, and CE. Incorporation of Cs also increased photostability of FACs PbBr films, possibly due to suppressed formation of light-induced metastable states. FACs PbBr PeLEDs show the maximum CE = 14.5 cd A at FA:Cs = 90:10 with very narrow EL spectral width (21-24 nm); this is the highest CE among FA-Cs-based PeLEDs reported to date. This work provides an understanding of the influences of Cs incorporation on the chemical, structural, and luminescent properties of FAPbBr polycrystalline films and a breakthrough to increase the efficiency of FACs PbBr PeLEDs.
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http://dx.doi.org/10.1021/acsnano.8b00409DOI Listing
March 2018

Improving the Stability of Metal Halide Perovskite Materials and Light-Emitting Diodes.

Adv Mater 2018 Oct 25;30(42):e1704587. Epub 2018 Jan 25.

Department of Materials Science and Engineering, Research Institute of Advanced Materials, BK21 PLUS SNU Materials Division for Educating Creative Global Leaders, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

Metal halide perovskites (MHPs) have numerous advantages as light emitters such as high photoluminescence quantum efficiency with a direct bandgap, very narrow emission linewidth, high charge-carrier mobility, low energetic disorder, solution processability, simple color tuning, and low material cost. Based on these advantages, MHPs have recently shown unprecedented radical progress (maximum current efficiency from 0.3 to 42.9 cd A ) in the field of light-emitting diodes. However, perovskite light-emitting diodes (PeLEDs) suffer from intrinsic instability of MHP materials and instability arising from the operation of the PeLEDs. Recently, many researchers have devoted efforts to overcome these instabilities. Here, the origins of the instability in PeLEDs are reviewed by categorizing it into two types: instability of (i) the MHP materials and (ii) the constituent layers and interfaces in PeLED devices. Then, the strategies to improve the stability of MHP materials and PeLEDs are critically reviewed, such as A-site cation engineering, Ruddlesden-Popper phase, suppression of ion migration with additives and blocking layers, fabrication of uniform bulk polycrystalline MHP layers, and fabrication of stable MHP nanoparticles. Based on this review of recent advances, future research directions and an outlook of PeLEDs for display applications are suggested.
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http://dx.doi.org/10.1002/adma.201704587DOI Listing
October 2018

Solution-Processed n-Type Graphene Doping for Cathode in Inverted Polymer Light-Emitting Diodes.

ACS Appl Mater Interfaces 2018 Feb 26;10(5):4874-4881. Epub 2018 Jan 26.

Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH) , Pohang, Gyungbuk 790-784, Republic of Korea.

n-Type doping with (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl) dimethylamine (N-DMBI) reduces a work function (WF) of graphene by ∼0.45 eV without significant reduction of optical transmittance. Solution process of N-DMBI on graphene provides effective n-type doping effect and air-stability at the same time. Although neutral N-DMBI act as an electron receptor leaving the graphene p-doped, radical N-DMBI acts as an electron donator leaving the graphene n-doped, which is demonstrated by density functional theory. We also verify the suitability of N-DMBI-doped n-type graphene for use as a cathode in inverted polymer light-emitting diodes (PLEDs) by using various analytical methods. Inverted PLEDs using a graphene cathode doped with N-DMBI radical showed dramatically improved device efficiency (∼13.8 cd/A) than did inverted PLEDs with pristine graphene (∼2.74 cd/A). N-DMBI-doped graphene can provide a practical way to produce graphene cathodes with low WF in various organic optoelectronics.
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http://dx.doi.org/10.1021/acsami.7b15307DOI Listing
February 2018
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