Publications by authors named "Byung Hee Hong"

130 Publications

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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.0c12939DOI Listing
February 2021

Graphene Quantum Dots Alleviate Impaired Functions in Niemann-Pick Disease Type C in Vivo.

Nano Lett 2021 03 20;21(5):2339-2346. Epub 2021 Jan 20.

Adult Stem Cell Research Center, College of Veterinary Medicine, Seoul National University, Seoul 08826, Korea.

While the neuropathological characteristics of Niemann-Pick disease type C (NPC) result in a fatal diagnosis, the development of clinically available therapeutic agent remains a challenge. Here we propose graphene quantum dots (GQDs) as a potential candidate for the impaired functions in NPC in vivo. In addition to the previous findings that GQDs exhibit negligible long-term toxicity and are capable of penetrating the blood-brain barrier, GQD treatment reduces the aggregation of cholesterol in the lysosome through expressed physical interactions. GQDs also promote autophagy and restore defective autophagic flux, which, in turn, decreases the atypical accumulation of autophagic vacuoles. More importantly, the injection of GQDs inhibits the loss of Purkinje cells in the cerebellum while also demonstrating reduced activation of microglia. The ability of GQDs to alleviate impaired functions in NPC proves the promise and potential of the use of GQDs toward resolving NPC and other related disorders.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.nanolett.0c03741DOI Listing
March 2021

Tailored Graphene Micropatterns by Wafer-Scale Direct Transfer for Flexible Chemical Sensor Platform.

Adv Mater 2021 Jan 20;33(2):e2004827. Epub 2020 Nov 20.

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

2D materials, such as graphene, exhibit great potential as functional materials for numerous novel applications due to their excellent properties. The grafting of conventional micropatterning techniques on new types of electronic devices is required to fully utilize the unique nature of graphene. However, the conventional lithography and polymer-supported transfer methods often induce the contamination and damage of the graphene surface due to polymer residues and harsh wet-transfer conditions. Herein, a novel strategy to obtain micropatterned graphene on polymer substrates using a direct curing process is demonstrated. Employing this method, entirely flexible, transparent, well-defined self-activated graphene sensor arrays, capable of gas discrimination without external heating, are fabricated on 4 in. wafer-scale substrates. Finite element method simulations show the potential of this patterning technique to maximize the performance of the sensor devices when the active channels of the 2D material are suspended and nanoscaled. This study contributes considerably to the development of flexible functional electronic devices based on 2D materials.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/adma.202004827DOI Listing
January 2021

Layer-Selective Synthesis of MoS and WS Structures under Ambient Conditions for Customized Electronics.

ACS Nano 2020 Jul 29;14(7):8485-8494. Epub 2020 Jun 29.

Functional Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea.

Transition metal dichalcogenides (TMDs) have attracted significant interest as one of the key materials in future electronics such as logic devices, optoelectrical devices, and wearable electronics. However, a complicated synthesis method and multistep processes for device fabrication pose major hurdles for their practical applications. Here, we introduce a direct and rapid method for layer-selective synthesis of MoS and WS structures in wafer-scale using a pulsed laser annealing system (λ = 1.06 μm, pulse duration ∼100 ps) in ambient conditions. The precursor layer of each TMD, which has at least 3 orders of magnitude higher absorption coefficient than those of neighboring layers, rigorously absorbed the incoming energy of the laser pulse and rapidly pyrolyzed in a few nanoseconds, enabling the generation of a MoS or WS layer without damaging the adjacent layers of SiO or polymer substrate. Through experimental and theoretical studies, we establish the underlying principles of selective synthesis and optimize the laser annealing conditions, such as laser wavelength, output power, and scribing speed, under ambient condition. As a result, individual homostructures of patterned MoS and WS layers were directly synthesized on a 4 in. wafer. Moreover, a consecutive synthesis of the second layer on top of the first synthesized layer realized a vertically stacked WS/MoS heterojunction structure, which can be treated as a cornerstone of electronic devices. As a proof of concept, we demonstrated the behavior of a MoS-based field-effect transistor, a skin-attachable motion sensor, and a MoS/WS-based heterojunction diode in this study. The ultrafast and selective synthesis of the TMDs suggests an approach to the large-area/mass production of functional heterostructure-based electronics.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsnano.0c02745DOI Listing
July 2020

Graphene quantum dots as anti-inflammatory therapy for colitis.

Sci Adv 2020 May 29;6(18):eaaz2630. Epub 2020 Apr 29.

Adult Stem Cell Research Center and Research Institute for Veterinary Science, College of Veterinary Medicine, Seoul National University, Seoul 08826, Republic of Korea.

While graphene and its derivatives have been suggested as a potential nanomedicine in several biomimetic models, their specific roles in immunological disorders still remain elusive. Graphene quantum dots (GQDs) may be suitable for treating intestinal bowel diseases (IBDs) because of their low toxicity in vivo and ease of clearance. Here, GQDs are intraperitoneally injected to dextran sulfate sodium (DSS)-induced chronic and acute colitis model, and its efficacy has been confirmed. In particular, GQDs effectively prevent tissue degeneration and ameliorate intestinal inflammation by inhibiting T1/T17 polarization. Moreover, GQDs switch the polarization of macrophages from classically activated M1 to M2 and enhance intestinal infiltration of regulatory T cells (T). Therefore, GQDs effectively attenuate excessive inflammation by regulating immune cells, indicating that they can be used as promising alternative therapeutic agents for the treatment of autoimmune disorders, including IBDs.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1126/sciadv.aaz2630DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7190325PMC
May 2020

pH-Triggered Silk Fibroin/Alginate Structures Fabricated in Aqueous Two-Phase System.

ACS Biomater Sci Eng 2019 Nov 9;5(11):5897-5905. Epub 2019 Oct 9.

MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, Collaborative Innovation Center of Chemistry for Energy Materials, Key Laboratory for Chemical Biology of Fujian Province, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.

An aqueous two-phase system (ATPS) is a water-in-water biphasic system, which is generally formed by two incompatible polymers. Recently, considerable effort has been dedicated to search for new ATPS polymer pairs to further expand ATPS's applications. In this paper, a new ATPS system based on silk fibroin (SF) and alginate is introduced. A phase diagram was established to show the critical concentrations for the formation of an SF/alginate ATPS. The present system is sensitive to pH stimulus and transformed from an ATPS into a single-phasic system as pH increases above ∼9.5. Circular dichroism, fluorescence emission spectra, hydrodynamic diameter, and ζ-potential data together indicate that the SF chains undergo a dramatic extension as pH is increased, which is the reason underlying the pH-triggered phase transition. As feasible applications of this biphasic system, compartmentalized multiplex immunoassay, controlled encapsulation and release, and hierarchical fiber fabrication were demonstrated using the SF/alginate ATPS.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsbiomaterials.9b01216DOI Listing
November 2019

Graphene-Enhanced Raman Spectroscopy Reveals the Controlled Photoreduction of Nitroaromatic Compound on Oxidized Graphene Surface.

ACS Omega 2018 Sep 13;3(9):11084-11087. Epub 2018 Sep 13.

Department of Chemistry, Seoul National University, Seoul 151-742, Korea.

Although graphene-enhanced Raman spectroscopy has been investigated for several years, there have been no studies that have applied it to real-time observations of chemical catalytic reactions. Here, we report that UV/ozone-treated oxidized graphene was used to both control and monitor the photoreduction of an adsorbed nitroaromatic dye compound. Graphene-enhanced Raman spectroscopy studies show that more oxidized graphene surface leads to faster photoreduction. This is due to the lowering of the Fermi level in the oxidized graphene, which is in agreement with the highest occupied molecular orbital level of the adsorbed dye molecule, leading to a rapid electron transfer from graphene to the dye. Our findings will be useful in understanding and exploiting the photocatalytic properties of oxidized graphene on adsorbed molecular species.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsomega.8b01285DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6645006PMC
September 2018

Multifunctional reduced graphene oxide-CVD graphene core-shell fibers.

Nanoscale 2019 Jul;11(26):12637-12642

Department of Chemistry, Seoul National University, Gwanak_599, Gwanak-ro, Gwanak-gu, Seoul 151-747, Republic of Korea.

The insufficient electrical conductivity and mechanical stretchability of conventional graphene fibers based on reduced graphene oxide liquid crystals (rGO-LCs) has limited their applications to numerous textile devices. Here, we report a simple method to fabricate multifunctional fibers with mechanically strong rGO cores and highly conductive CVD graphene shells (rGO@Gr fibers), which show an outstanding electrical conductivity as high as ∼137 S cm-1 and a failure strain value of 21%, which are believed to be the highest values among polymer-free graphene fibers. We also demonstrate the use of the rGO@Gr fibers for high power density supercapacitors with enhanced mechanical stability and durability, which would enable their practical applications in various smart wearable devices in the future.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8nr07527aDOI Listing
July 2019

Au decoration of a graphene microchannel for self-activated chemoresistive flexible gas sensors with substantially enhanced response to hydrogen.

Nanoscale 2019 Feb;11(6):2966-2973

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

Graphene is one of the most promising materials for high-performance gas sensors due to its unique properties such as high sensitivity at room temperature, transparency, and flexibility. However, the low selectivity and irreversible behavior of graphene-based gas sensors are major problems. Here, we present unprecedented room temperature hydrogen detection by Au nanoclusters supported on self-activated graphene. Compared to pristine graphene sensors, the Au-decorated graphene sensors exhibit highly improved gas-sensing properties upon exposure to various gases. In particular, an unexpected substantial enhancement in H2 detection is found, which has never been reported for Au decoration on any type of chemoresistive material. Density functional theory calculations reveal that Au nanoclusters on graphene contribute to the adsorption of H atoms, whereas the surfaces of Au and graphene do not bind with H atoms individually. The discovery of such a new functionality in the existing material platform holds the key to diverse research areas based on metal nanocluster/graphene heterostructures.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8nr09076aDOI Listing
February 2019

Threshold Voltage Control of Multilayered MoS Field-Effect Transistors via Octadecyltrichlorosilane and their Applications to Active Matrixed Quantum Dot Displays Driven by Enhancement-Mode Logic Gates.

Small 2019 Feb 13;15(7):e1803852. Epub 2019 Jan 13.

Department of Electrical and Computer Engineering, Inter-University Semiconductor Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.

In recent past, for next-generation device opportunities such as sub-10 nm channel field-effect transistors (FETs), tunneling FETs, and high-end display backplanes, tremendous research on multilayered molybdenum disulfide (MoS ) among transition metal dichalcogenides has been actively performed. However, nonavailability on a matured threshold voltage control scheme, like a substitutional doping in Si technology, has been plagued for the prosperity of 2D materials in electronics. Herein, an adjustment scheme for threshold voltage of MoS FETs by using self-assembled monolayer treatment via octadecyltrichlorosilane is proposed and demonstrated to show MoS FETs in an enhancement mode with preservation of electrical parameters such as field-effect mobility, subthreshold swing, and current on-off ratio. Furthermore, the mechanisms for threshold voltage adjustment are systematically studied by using atomic force microscopy, Raman, temperature-dependent electrical characterization, etc. For validation of effects of threshold voltage engineering on MoS FETs, full swing inverters, comprising enhancement mode drivers and depletion mode loads are perfectly demonstrated with a maximum gain of 18.2 and a noise margin of ≈45% of 1/2 V . More impressively, quantum dot light-emitting diodes, driven by enhancement mode MoS FETs, stably demonstrate 120 cd m at the gate-to-source voltage of 5 V, exhibiting promising opportunities for future display application.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/smll.201803852DOI Listing
February 2019

Silicon germanium photo-blocking layers for a-IGZO based industrial display.

Sci Rep 2018 Dec 3;8(1):17533. Epub 2018 Dec 3.

Department of Chemistry, College of Natural Science, Seoul National University, Seoul, 08826, Korea.

Amorphous indium- gallium-zinc oxide (a-IGZO) has been intensively studied for the application to active matrix flat-panel display because of its superior electrical and optical properties. However, the characteristics of a-IGZO were found to be very sensitive to external circumstance such as light illumination, which dramatically degrades the device performance and stability practically required for display applications. Here, we suggest the use for silicon-germanium (Si-Ge) films grown plasma-enhanced chemical vapour deposition (PECVD) as photo-blocking layers in the a-IGZO thin film transistors (TFTs). The charge mobility and threshold voltage (V) of the TFTs depend on the thickness of the Si-Ge films and dielectric buffer layers (SiN), which were carefully optimized to be ~200 nm and ~300 nm, respectively. As a result, even after 1,000 s illumination time, the V and electron mobility of the TFTs remain unchanged, which was enabled by the photo-blocking effect of the Si-Ge layers for a-IGZO films. Considering the simple fabrication process by PECVD with outstanding scalability, we expect that this method can be widely applied to TFT devices that are sensitive to light illumination.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41598-018-35222-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6277433PMC
December 2018

Large-scale transfer-free growth of thin graphite films at low temperature for solid diffusion barriers.

Nanoscale 2018 Aug;10(31):14819-14823

Department of Chemistry, College of Natural Science, Seoul National University, Gwanakro-1, Seoul 151-747, Republic of Korea.

Amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) have been under intense investigation as one of the promising candidates for active matrix flat-panel displays. However, solid diffusion of a-IGZO to other layers during TFT device fabrication highly degrades their electrical and optical properties. It is expected that the diffusion-impenetrable properties of graphitic materials can be utilized as diffusion barriers. A conventional transfer method and direct growth on TFTs with high temperature are limited due to wet transfer conditions and low Tg (∼540 °C) of the glass substrates, respectively. Here we report the large-scale transfer-free growth of thin graphite films at low temperature (∼350 °C) for solid diffusion barriers in the a-IGZO TFTs using plasma enhanced chemical vapor deposition (PECVD), which can be widely used to protect solid-diffusion for sustainable and scalable future industrial technology.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8nr03842bDOI Listing
August 2018

Hierarchical carbon-silicon nanowire heterostructures for the hydrogen evolution reaction.

Nanoscale 2018 Aug 18;10(29):13936-13941. Epub 2018 Jul 18.

Advanced Institute of Convergence Technology and Department of Chemistry, Graphene Research Center, Seoul National University, Seoul 08826, Republic of Korea. and Graduate School of Convergence Science and Technology & Graphene Square Inc., Seoul National University, Seoul 08826, Republic of Korea.

Silicon nanowires (SiNWs) opened up exciting possibilities in a variety of research fields due to their unique anisotropic morphologies, facile tuning capabilities, and accessible fabrication methods. The SiNW-based photoelectrochemical (PEC) conversion has recently been known to provide an efficiency superior to that of various photo-responsive semiconductor heterostructures. However, a challenge still remains in designing optimum structures to minimize photo-oxidation and photo-corrosion of the Si surface in a liquid electrolyte. Here, we report a simple method to synthesize hierarchically branched carbon nanowires (CNWs) on SiNWs utilizing copper vapor as the catalyst in a chemical vapor deposition (CVD) process, which exhibits outstanding photocatalytic activities for hydrogen generation along with excellent chemical stability against oxidation and corrosion. Thus, we believe that the CNW-SiNW photoelectrodes would provide a new route to developing high-performing cost-effective catalysts essential for advanced energy conversion and storage technologies.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8nr02262cDOI Listing
August 2018

Graphene quantum dots prevent α-synucleinopathy in Parkinson's disease.

Nat Nanotechnol 2018 09 9;13(9):812-818. Epub 2018 Jul 9.

Neuroregeneration and Stem Cell Programs, Institute for Cell Engineering, The Johns Hopkins University School of Medicine, Baltimore, MD, USA.

Though emerging evidence indicates that the pathogenesis of Parkinson's disease is strongly correlated to the accumulation and transmission of α-synuclein (α-syn) aggregates in the midbrain, no anti-aggregation agents have been successful at treating the disease in the clinic. Here, we show that graphene quantum dots (GQDs) inhibit fibrillization of α-syn and interact directly with mature fibrils, triggering their disaggregation. Moreover, GQDs can rescue neuronal death and synaptic loss, reduce Lewy body and Lewy neurite formation, ameliorate mitochondrial dysfunctions, and prevent neuron-to-neuron transmission of α-syn pathology provoked by α-syn preformed fibrils. We observe, in vivo, that GQDs penetrate the blood-brain barrier and protect against dopamine neuron loss induced by α-syn preformed fibrils, Lewy body/Lewy neurite pathology and behavioural deficits.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41565-018-0179-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6351226PMC
September 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41467-018-04385-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5966423PMC
May 2018

Catalytic degradation of phenols by recyclable CVD graphene films.

Nanoscale 2018 Mar;10(13):5840-5844

Department of Chemistry, Seoul National University, Gwanak_599, Gwanak-ro, Gwanak-gu, Seoul 151-747, Korea.

Ferrous ion-based catalysts have been widely employed to oxidatively destruct the major industrial pollutants such as phenolic compounds through advanced oxidation processes (AOPs). These agents, however, inevitably show several drawbacks including the need for pH adjustment and further purification steps to remove residual salts. Here we report the use of a chemical vapour deposition (CVD) graphene film as a novel metal-free catalyst for the AOP-based degradation of phenols in aqueous solution, which does not require additional steps for salt removal nor external energy to activate the process. We have also verified that the catalytic activity is strongly dependent on the surface area of the graphene film and the degradation efficiency can be markedly improved by exploiting an array of multiple graphene films. Finally, the recyclability of the graphene film has been validated by performing repetitive degradation tests to ensure its practical use.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c8nr00045jDOI Listing
March 2018

Ultrastrong Graphene-Copper Core-Shell Wires for High-Performance Electrical Cables.

ACS Nano 2018 03 14;12(3):2803-2808. Epub 2018 Mar 14.

Applied Quantum Composites Research Center, Institute of Advanced Composite Materials , Korea Institute of Science and Technology , Jeollabuk-do 55324 , Republic of Korea.

Recent development in mobile electronic devices and electric vehicles requires electrical wires with reduced weight as well as enhanced stability. In addition, since electric energy is mostly generated from power plants located far from its consuming places, mechanically stronger and higher electric power transmission cables are strongly demanded. However, there has been no alternative materials that can practically replace copper materials. Here, we report a method to prepare ultrastrong graphene fibers (GFs)-Cu core-shell wires with significantly enhanced electrical and mechanical properties. The core GFs are synthesized by chemical vapor deposition, followed by electroplating of Cu shells, where the large surface area of GFs in contact with Cu maximizes the mechanical toughness of the core-shell wires. At the same time, the unique electrical and thermal characteristics of graphene allow a ∼10 times higher current density limit, providing more efficient and reliable delivery of electrical energies through the GFs-Cu wires. We believe that our results would be useful to overcome the current limit in electrical wires and cables for lightweight, energy-saving, and high-power applications.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsnano.8b00043DOI Listing
March 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.7b15307DOI Listing
February 2018

Roll-to-Roll Laser-Printed Graphene-Graphitic Carbon Electrodes for High-Performance Supercapacitors.

ACS Appl Mater Interfaces 2018 Jan 26;10(1):1033-1038. Epub 2017 Dec 26.

Department of Chemistry, College of Natural Science, Seoul National University , Seoul 440-746, Republic of Korea.

Carbon electrodes including graphene and thin graphite films have been utilized for various energy and sensor applications, where the patterning of electrodes is essentially included. Laser scribing in a DVD writer and inkjet printing were used to pattern the graphene-like materials, but the size and speed of fabrication has been limited for practical applications. In this work, we devise a simple strategy to use conventional laser-printer toner materials as precursors for graphitic carbon electrodes. The toner was laser-printed on metal foils, followed by thermal annealing in hydrogen environment, finally resulting in the patterned thin graphitic carbon or graphene electrodes for supercapacitors. The electrochemical cells made of the graphene-graphitic carbon electrodes show remarkably higher energy and power performance compared to conventional supercapacitors. Furthermore, considering the simplicity and scalability of roll-to-roll (R2R) electrode patterning processes, the proposed method would enable cheaper and larger-scale synthesis and patterning of graphene-graphitic carbon electrodes for various energy applications in the future.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.7b13741DOI Listing
January 2018

Multiscale Modulation of Nanocrystalline Cellulose Hydrogel via Nanocarbon Hybridization for 3D Neuronal Bilayer Formation.

Small 2017 07 22;13(26). Epub 2017 May 22.

School of Materials Science and Engineering, Gwangju Institute of Science and Technology, 123 Cheomdan-gwagiro, Buk-gu, Gwangju, 65001, Republic of Korea.

Bacterial biopolymers have drawn much attention owing to their unconventional three-dimensional structures and interesting functions, which are closely integrated with bacterial physiology. The nongenetic modulation of bacterial (Acetobacter xylinum) cellulose synthesis via nanocarbon hybridization, and its application to the emulation of layered neuronal tissue, is reported. The controlled dispersion of graphene oxide (GO) nanoflakes into bacterial cellulose (BC) culture media not only induces structural changes within a crystalline cellulose nanofibril, but also modulates their 3D collective association, leading to substantial reduction in Young's modulus (≈50%) and clear definition of water-hydrogel interfaces. Furthermore, real-time investigation of 3D neuronal networks constructed in this GO-incorporated BC hydrogel with broken chiral nematic ordering revealed the vertical locomotion of growth cones, the accelerated neurite outgrowth (≈100 µm per day) with reduced backward travel length, and the efficient formation of synaptic connectivity with distinct axonal bifurcation abundancy at the ≈750 µm outgrowth from a cell body. In comparison with the pristine BC, GO-BC supports the formation of well-defined neuronal bilayer networks with flattened interfacial profiles and vertical axonal outgrowth, apparently emulating the neuronal development in vivo. We envisioned that our findings may contribute to various applications of engineered BC hydrogel to fundamental neurobiology studies and neural engineering.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/smll.201700331DOI Listing
July 2017

Mapping of Bernal and non-Bernal stacking domains in bilayer graphene using infrared nanoscopy.

Nanoscale 2017 Mar;9(12):4191-4195

Department of Chemistry, Seoul National University, Seoul 08826, Korea.

Bilayer graphene (BLG) shows great potential as a new material for opto-electronic devices because its bandgap can be controlled by varying the stacking orders, as well as by applying an external electric field. An imaging technique that can visualize and characterize various stacking domains in BLG may greatly help in fully utilizing such properties of BLG. Here we demonstrate that infrared (IR) scattering-type scanning near-field optical microscopy (sSNOM) can visualize Bernal and non-Bernal stacking domains of BLG, based on the stacking-specific inter- and intra-band optical conductivities. The method enables nanometric mapping of stacking domains in BLG on dielectric substrates, augmenting current limitations of Raman spectroscopy and electron microscopy techniques for the structural characterization of BLG.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c7nr00713bDOI Listing
March 2017

High-Density Single-Layer Coating of Gold Nanoparticles onto Multiple Substrates by Using an Intrinsically Disordered Protein of α-Synuclein for Nanoapplications.

ACS Appl Mater Interfaces 2017 Mar 1;9(10):8519-8532. Epub 2017 Mar 1.

School of Materials Science and Engineering, Gwangju Institute of Science and Technology , Gwangju 500-712, Korea.

Functional graffiti of nanoparticles onto target surface is an important issue in the development of nanodevices. A general strategy has been introduced here to decorate chemically diverse substrates with gold nanoparticles (AuNPs) in the form of a close-packed single layer by using an omni-adhesive protein of α-synuclein (αS) as conjugated with the particles. Since the adsorption was highly sensitive to pH, the amino acid sequence of αS exposed from the conjugates and its conformationally disordered state capable of exhibiting structural plasticity are considered to be responsible for the single-layer coating over diverse surfaces. Merited by the simple solution-based adsorption procedure, the particles have been imprinted to various geometric shapes in 2-D and physically inaccessible surfaces of 3-D objects. The αS-encapsulated AuNPs to form a high-density single-layer coat has been employed in the development of nonvolatile memory, fule-cell, solar-cell, and cell-culture platform, where the outlying αS has played versatile roles such as a dielectric layer for charge retention, a sacrificial layer to expose AuNPs for chemical catalysis, a reaction center for silicification, and biointerface for cell attachment, respectively. Multiple utilizations of the αS-based hybrid NPs, therefore, could offer great versatility to fabricate a variety of NP-integrated advanced materials which would serve as an indispensable component for widespread applications of high-performance nanodevices.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.6b16411DOI Listing
March 2017

Smart Contact Lenses with Graphene Coating for Electromagnetic Interference Shielding and Dehydration Protection.

ACS Nano 2017 06 21;11(6):5318-5324. Epub 2017 Feb 21.

Graphene Research Center, Advanced Institute of Convergence Technology & Department of Chemistry, Seoul National University , Gwanakro-1, Seoul 08826, Republic of Korea.

Recently, smart contact lenses with electronic circuits have been proposed for various sensor and display applications where the use of flexible and biologically stable electrode materials is essential. Graphene is an atomically thin carbon material with a two-dimensional hexagonal lattice that shows outstanding electrical and mechanical properties as well as excellent biocompatibility. In addition, graphene is capable of protecting eyes from electromagnectic (EM) waves that may cause eye diseases such as cataracts. Here, we report a graphene-based highly conducting contact lens platform that reduces the exposure to EM waves and dehydration. The sheet resistance of the graphene on the contact lens is as low as 593 Ω/sq (±9.3%), which persists in an wet environment. The EM wave shielding function of the graphene-coated contact lens was tested on egg whites exposed to strong EM waves inside a microwave oven. The results show that the EM energy is absorbed by graphene and dissipated in the form of thermal radiation so that the damage on the egg whites can be minimized. We also demonstrated the enhanced dehydration protection effect of the graphene-coated lens by monitoring the change in water evaporation rate from the vial capped with the contact lens. Thus, we believe that the graphene-coated contact lens would provide a healthcare and bionic platform for wearable technologies in the future.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsnano.7b00370DOI Listing
June 2017

Continuous Films of Self-Assembled Graphene Quantum Dots for n-Type Doping of Graphene by UV-Triggered Charge Transfer.

Small 2017 09 16;13(35). Epub 2017 Jan 16.

Department of Chemistry, Seoul National University, Seoul, 151-744, Republic of Korea.

The demands to examine components serving as one of the active layers in heterostructures of 2D materials have been recently increasing. Nanomaterials synthesized from a solution process and their self-assembly can provide a promising route to build a new type of mixed dimensional heterostructures, and several methodologies have been reported previously to construct 2D assemblies from colloidal nanostructures in solution. Graphene quantum dots (GQDs), receiving much interest due to the tunable optical band gap and the capability of chemical functionalization, are considered as emerging nanomaterials for various optoelectronic and biological applications. This study fabricates a closely packed GQDs film (GQDF) from colloidal solutions using a solvent-assisted Langmuir Blodgett method, and investigates the optical and electrical characteristics of the heterostacked graphene/GQD film (G/GQDF) structures. It is observed that the GQDF plays a role not only as a buffer layer that isolates Chemical Vapor Deposited graphene (CVD graphene) from undesired p-doping but also as a photoactive layer that triggers n-doping of the heterostacked CVD graphene film. The n-doping density of the G/GQDF device is proportional to UV irradiation time, but its carrier mobility remains constant regardless of doping densities, which are unique characteristics that have not been observed in other doping methods.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/smll.201603142DOI Listing
September 2017

Double-Layer Graphene Outperforming Monolayer as Catalyst on Silicon Photocathode for Hydrogen Production.

ACS Appl Mater Interfaces 2017 Feb 20;9(4):3570-3580. Epub 2017 Jan 20.

Department of Materials Science and Engineering, Seoul National University , Seoul 08 826, Republic of Korea.

Photoelectrochemical cells are used to split hydrogen and oxygen from water molecules to generate chemical fuels to satisfy our ever-increasing energy demands. However, it is a major challenge to design efficient catalysts to use in the photoelectochemical process. Recently, research has focused on carbon-based catalysts, as they are nonprecious and environmentally benign. Interesting advances have also been made in controlling nanostructure interfaces and in introducing new materials as catalysts in the photoelectrochemical cell. However, these catalysts have as yet unresolved issues involving kinetics and light-transmittance. In this work, we introduce high-transmittance graphene onto a planar p-Si photocathode to produce a hydrogen evolution reaction to dramatically enhance photon-to-current efficiency. Interestingly, double-layer graphene/Si exhibits noticeably improved photon-to-current efficiency and modifies the band structure of the graphene/Si photocathode. On the basis of in-depth electrochemical and electrical analyses, the band structure of graphene/Si was shown to result in a much lower work function than Si, accelerating the electron-to-hydrogen production potential. Specifically, plasma-treated double-layer graphene exhibited the best performance and the lowest work function. We electrochemically analyzed the mechanism at work in the graphene-assisted photoelectrode. Atomistic calculations based on the density functional theory were also carried out to more fully understand our experimental observations. We believe that investigation of the underlying mechanism in this high-performance electrode is an important contribution to efforts to develop high-efficiency metal-free carbon-based catalysts for photoelectrochemical cell hydrogen production.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.6b11750DOI Listing
February 2017

Nanoscale Direct Mapping of Noise Source Activities on Graphene Domains.

ACS Nano 2016 11 11;10(11):10135-10142. Epub 2016 Nov 11.

Center for Electricity & Magnetism, Korea Research Institute of Standards and Science , Daejeon 305-340, Korea.

An electrical noise is one of the key parameters determining the performance of modern electronic devices. However, it has been extremely difficult, if not impossible, to image localized noise sources or their activities in such devices. We report a "noise spectral imaging" strategy to map the activities of localized noise sources in graphene domains. Using this method, we could quantitatively estimate sheet resistances and noise source densities inside graphene domains, on domain boundaries and on the edge of graphene. The results show high activities of noise sources and large sheet resistance values at the domain boundary and edge of graphene. Additionally, we showed that the top layer in double-layer graphene had lower noises than single-layer graphene. This work provides valuable insights about the electrical noises of graphene. Furthermore, the capability to directly map noise sources in electronic channels can be a major breakthrough in electrical noise research in general.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsnano.6b05288DOI Listing
November 2016

Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor.

Nat Commun 2016 11 10;7:13261. Epub 2016 Nov 10.

Department of Materials Science and Engineering, Yonsei University, 50 Yonsei, Seodaemun, Seoul 03722, Korea.

Graphene is currently at the forefront of cutting-edge science and technology due to exceptional electronic, optical, mechanical, and thermal properties. However, the absence of a sizeable band gap in graphene has been a major obstacle for application. To open and control a band gap in functionalized graphene, several gapping strategies have been developed. In particular, hydrogen plasma treatment has triggered a great scientific interest, because it has been known to be an efficient way to modify the surface of single-layered graphene and to apply for standard wafer-scale fabrication. Here we show a monolayer chemical-vapour-deposited graphene hydrogenated by indirect hydrogen plasma without structural defect and we demonstrate that a band gap can be tuned as wide as 3.9 eV by varying hydrogen coverage. We also show a hydrogenated graphene field-effect transistor, showing that on/off ratio changes over three orders of magnitude at room temperature.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/ncomms13261DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5109464PMC
November 2016

Distortion in Two-Dimensional Shapes of Merging Nanobubbles: Evidence for Anisotropic Gas Flow Mechanism.

Langmuir 2016 11 7;32(43):11303-11308. Epub 2016 Sep 7.

Graduate School of Convergence Science and Technology, Seoul National University , Suwon 443-270, Korea.

Two nanobubbles that merge in a graphene liquid cell take elliptical shapes rather than the ideal circular shapes. This phenomenon was investigated in detail by using in situ transmission electron microscopy (TEM). The results show that the distortion in the two-dimensional shapes of the merging nanobubbles is attributed to the anisotropic gas transport flux between the nanobubbles. We also predicted and confirmed the same phenomenon in a three-nanobubble system, indicating that the relative size difference is important in determining the shape of merging nanobubbles.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.langmuir.6b01672DOI Listing
November 2016

Strain Relaxation of Graphene Layers by Cu Surface Roughening.

Nano Lett 2016 Oct 3;16(10):5993-5998. Epub 2016 Oct 3.

Department of Chemistry, Seoul National University , Seoul 151-747, Republic of Korea.

The surface morphology of copper (Cu) often changes after the synthesis of graphene by chemical vapor deposition (CVD) on a Cu foil, which affects the electrical properties of graphene, as the Cu step bunches induce the periodic ripples on graphene that significantly disturb electrical conduction. However, the origin of the Cu surface reconstruction has not been completely understood yet. Here, we show that the compressive strain on graphene induced by the mismatch of thermal expansion coefficient with Cu surface can be released by forming periodic Cu step bunching that depends on graphene layers. Atomic force microscopy (AFM) images and the Raman analysis show the noticeably longer and higher step bunching of Cu surface under multilayer graphene and the weaker biaxial compressive strain on multilayer graphene compared to monolayer. We found that the surface areas of Cu step bunches under multilayer and monolayer graphene are increased by ∼1.41% and ∼0.77% compared to a flat surface, respectively, indicating that the compressive strain on multilayer graphene can be more effectively released by forming the Cu step bunching with larger area and longer periodicity. We believe that our finding on the strain relaxation of graphene layers by Cu step bunching formation would provide a crucial idea to enhance the electrical performance of graphene electrodes by controlling the ripple density of graphene.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.nanolett.6b01578DOI Listing
October 2016

Controlling the ripple density and heights: a new way to improve the electrical performance of CVD-grown graphene.

Nanoscale 2016 May 27;8(18):9822-7. Epub 2016 Apr 27.

Department of Physics, Sogang University, Seoul 04107, South Korea.

We report a new way to enhance the electrical performances of large area CVD-grown graphene through controlling the ripple density and heights after transfer onto SiO2/Si substrates by employing different cooling rates during fabrication. We find that graphene films prepared with a high cooling rate have reduced ripple density and heights and improved electrical characteristics such as higher electron/hole mobilities as well as reduced sheet resistance. The corresponding Raman analysis also shows a significant decrease of the defects when a higher cooling rate is employed. We suggest a model that explains the improved morphology of the graphene film obtained with higher cooling rates. From these points of view, we can suggest a new pathway toward a relatively lower density and heights of ripples in order to reduce the flexural phonon-electron scattering effect, leading to higher lateral carrier mobilities.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c6nr00706fDOI Listing
May 2016