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Lee, Byoung Se,Yi, Mihye,Chu, So Young,Lee, Ja Young,Kwon, Hye Rim,Lee, Kyu Reon,Kang, Donghyeon,Kim, Wan Seop,Lim, Heung Bin,Lee, Jouhahn,Youn, Hyung-Joong,Chi, Dae Yoon,Hur, Nam Hwi Royal Society of Chemistry 2010 Chemical communications Vol.46 No.22
<P>Copper nitride nanoparticles supported on a mesoporous superparamagnetic silica microsphere exhibit superior activity toward the Huisgen cycloaddition of azides and alkynes. The nitride catalyst offers significant advantages over homogeneous Cu catalysts.</P> <P>Graphic Abstract</P><P>Cu<SUB>3</SUB>N nanoparticles supported on a superparamagnetic sphere exhibit superior activity toward the cycloaddition of azides and alkynes. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c001255f'> </P>
Park, So Young,Lee, Hyun Uk,Park, Eun Sik,Lee, Soon Chang,Lee, Jae-Won,Jeong, Soon Woo,Kim, Chi Hyun,Lee, Young-Chul,Huh, Yun Suk,Lee, Jouhahn American Chemical Society 2014 ACS APPLIED MATERIALS & INTERFACES Vol.6 No.5
<P>We have developed a simple approach for the large-scale synthesis of water-soluble green carbon nanodots (G-dots) from many kinds of large food waste-derived sources. About 120 g of G-dots per 100 kg of food waste can be synthesized using our simple and environmentally friendly synthesis approach. The G-dots exhibit a high degree of solubility in water because of the abundant oxygen-containing functional groups around their surface. The narrow band of photoluminescence emission (400–470 nm) confirms that the size of the G-dots (∼4 nm) is small because of a similar quantum effects and emission traps on the surfaces. The G-dots have excellent photostability; their photoluminescence intensity decreases slowly (∼8%) under continuous excitation with a Xe lamp for 10 days. We carried out cell viability assay to assess the effect of cytotoxicity by introducing G-dots in cells such as Chinese hamster ovary cells (CHO-K1), mouse muscle cells (C2C12), and African green monkey kidney cells (COS-7), up to a concentration of 2 mg mL<SUP>–1</SUP> for 24 h. Due to their high photostability and low cytotoxicity, these G-dots are excellent probes for in vitro bioimaging. Moreover, the byproducts (not including G-dots) of G-dot synthesis from large food-waste derived sources promoted the growth and development of seedlings germinated on 3DW-supplemented gauze. Because of the combined advantages of green synthesis, high aqueous stability, high photostability, and low cytotoxicity, the G-dots show considerable promise in various areas, including biomedical imaging, solution state optoelectronics, and plant seed germination and/or growth.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2014/aamick.2014.6.issue-5/am500159p/production/images/medium/am-2014-00159p_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am500159p'>ACS Electronic Supporting Info</A></P>
Hydrogen incorporation induced the octahedral symmetry variation in VO<sub>2</sub> films
Lee, Dooyong,Kim, Hyegyeong,Kim, Ji Woong,Lee, Ik Jae,Kim, Yooseok,Yun, Hyung-Joong,Lee, Jouhahn,Park, Sungkyun Elsevier 2017 APPLIED SURFACE SCIENCE - Vol.396 No.-
<P><B>Abstract</B></P> <P>This study examined the microscopic aspects of macroscopic physical property variations of hydrogen annealed VO<SUB>2</SUB> films, deposited on Al<SUB>2</SUB>O<SUB>3</SUB>(0001) substrates by RF magnetron sputtering. The temperature-dependent electrical resistivity showed that the as-grown film exhibited a metal-insulator-transition (MIT) at 55.20°C and 49.26°C during heating and cooling, respectively. On the other hand, no MIT was observed for the film annealed under a hydrogen environment. Spectroscopic measurements during the <I>in-situ</I> hydrogenation process showed that hydrogen annealing (∼0.3mbar, up to 300°C) promoted the V<SUP>3+</SUP> state above 100°C. Raman spectroscopy and X-ray diffraction confirmed that the as-grown film changed from a monoclinic to rutile structure during hydrogen annealing. In addition, the shift of the (020) diffraction peak position of the hydrogen-annealed film to a lower angle compare to that of the known rutile VO<SUB>2</SUB> film was attributed to the expansion of the unit cell. In addition, local structure analysis showed that an increase in octahedral symmetry after hydrogen annealing is one of the main explanations for the metallic characteristics of the hydrogen-annealed film.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Metal-insulator-transition disappeared after hydrogenation. </LI> <LI> Hydrogen incorporated into VO2 occurred at the low temperature (∼100°C). </LI> <LI> Hydrogen incorporation causes structural change from monoclinic to rutile. </LI> <LI> Reduced octahedron symmetry with hydrogen incorporation is related to the vanishing MIT characteristics. </LI> </UL> </P>
Jouhahn Lee,Yongsup Park 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.1
We have studied the interface electronic structures of Ca/tris-(8-hydorxyquinoline)aluminum (Alq3), Ca/NaF/Alq3, and Ca/MgF2/Alq3 by using X-ray and UV photoelectron spectroscopy (XPS and UPS). The UPS revealed that the Ca deposition on MgF2/Alq3 created weak gap states similar to Ca/Alq3 case, while the deposition of Ca on NaF/Alq3 showed relatively strong gap states. The N 1s core level peak measured with XPS exhibited a large satellite peak when Ca was deposited on NaF/Alq3, which was not seen in Ca/MgF2/Alq3. These changes indicate that both Na and Ca contribute to the charge transfer to the Alq3 molecules in Ca/NaF/Alq3, similar to Ca on LiF/Alq3. In contrast, only Ca or Mg plays a role in charge transfer in Ca/MgF2/Alq3, much like in Ca/Alq3. These observations suggest that charge transfer and chemical reactions at these interfaces could be important factors in energy-level alignments that determine the electron-injection barrier height.
Carbon layer supported nickel catalyst for sodium borohydride (NaBH<sub>4</sub>) dehydrogenation
Lee, Jaeyeong,Shin, Hojun,Choi, Kyoung Soon,Lee, Jouhahn,Choi, Jae-Young,Yu, Hak Ki Elsevier 2019 International journal of hydrogen energy Vol.44 No.5
<P><B>Abstract</B></P> <P>In this paper, a nickel (Ni) catalyst with large surface area supported by a graphitic layer is proposed. The graphitic layer is formed on a Ni foil by chemical vapor deposition (CVD) using CH<SUB>4</SUB> as a source. The Ni foil is partially etched by a chemical etchant to increase the surface to volume ratio. The H<SUB>2</SUB> generation rates of the catalysts for different Ni etching times are measured and their surfaces are observed using an optical microscope. The optimized Ni etching time is 6 h, and the catalyst prepared by 6 h of etching shows an H<SUB>2</SUB> generation rate of about 600 ml/min⋅(g metal). The 6-h etched Ni/graphitic layer (Ni/G) is reused five times to confirm durability; it shows a high performance without any degradation. The H<SUB>2</SUB> generation rate for different concentrations of NaBH<SUB>4</SUB> is measured. The reaction follows first order kinetics, which means that the higher the concentration of NaBH<SUB>4</SUB> is, the faster the H<SUB>2</SUB> generation is.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Nickel foil with increased surface area showed excellent catalytic performance. </LI> <LI> A graphitic layer prevents nickel from tearing or damaging. </LI> <LI> The rate of hydrogen generation of sodium borohydride is about 600 ml/min·(g metal). </LI> <LI> The catalyst showed excellent stability even after 5 repetitions. </LI> </UL> </P>
Lee, Dooyong,Kim, Jiwoong,Cho, Chang-Woo,Bae, Jong-Seong,Won, Jonghan,Lee, Jouhahn,Park, Sungkyun Elsevier 2018 APPLIED SURFACE SCIENCE - Vol.447 No.-
<P><B>Abstract</B></P> <P>The asymmetric response of the cation distribution on [001] oriented CoFe<SUB>2</SUB>O<SUB>4</SUB> films were examined. Depending on the deposition temperature and substrates (MgO(0 0 1) or SrTiO<SUB>3</SUB>(0 0 1)), the films exhibited a range of compressive or tensile strain states along the out-of-plane. With increases in deposition temperature, the initial compressive (tensile) strain for the films grown on MgO(0 0 1) (SrTiO<SUB>3</SUB>(0 0 1)) was reduced. Furthermore, the out-of-plane (in-plane) magnetic anisotropy of the film grown on MgO(0 0 1) (SrTiO<SUB>3</SUB>(0 0 1)) decreased with increasing deposition temperature. The uniaxial magnetic anisotropy energy calculation based on the magnetic anisotropy (crystalline and shape) and strain of films confirmed that the strain reduction with increasing deposition temperature is the main source of the uniaxial magnetic anisotropy reduction, regardless of the sign of the strain. X-ray photoelectron spectroscopy explained the saturation magnetization variation by the cation distribution depending on the strain state. Furthermore, the calculated magnetic moment based on the cation distribution were well matched for the high crystalline films.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Epitaxial strain is reduced as the growth temperature is increased. </LI> <LI> Out-of-plane tensile trained films exhibit in-plane magnetic anisotropy. </LI> <LI> Uniaxial magnetic anisotropy energy decreases with decreasing out-of-plane strain. </LI> <LI> Depending on the strain state the opposite behavior of the cation distribution occurred. </LI> <LI> Variation of the saturation magnetization is related to the cation distribution. </LI> </UL> </P>
Position-selective metal oxide nano-structures using graphene catalyst for gas sensors
Lee, Aram,Park, Jinheon,Choi, Kyoung Soon,Lee, Jouhahn,Yoo, Ilhan,Cho, In Sun,Ahn, Byungmin,Seo, Hyungtak,Choi, Jae-Young,Yu, Hak Ki Elsevier 2017 Carbon Vol.125 No.-
<P>The thermal transport growth of various metal oxide (MOx) nanostructures using graphene as a catalytic layer was studied. Graphene was synthesized by Cu-catalyzed chemical vapor deposition and transferred onto a SiO2-covered Si substrate using bubble transfer methods. Due to the catalytic activity of the atomic-thick carbon layer, control of the position of the MOx nanostructures as well as the growth parameters, such as nucleation density and growth rate, could be achieved. The position-selective and density-controlled MOx nanostructures were evaluated for hydrogen gas sensor applications, where different responses were obtained with hydrogen molecules. (C) 2017 Elsevier Ltd. All rights reserved.</P>