Layer-by-Layer Doping of Few-Layer Graphene Film

Gü,nes&#x327,, Fethullah,Shin, Hyeon-Jin,Biswas, Chandan,Han, Gang Hee,Kim, Eun Sung,Chae, Seung Jin,Choi, Jae-Young,Lee, Young Hee American Chemical Society 2010 ACS NANO Vol.4 No.8

<P>We propose a new method of layer-by-layer (LbL) doping of thin graphene films. Large area monolayer graphene was synthesized on Cu foil by using the chemical vapor deposition method. Each layer was transferred on a polyethylene terephthalate substrate followed by a salt-solution casting, where the whole process was repeated several times to get LbL-doped thin layers. With this method, sheet resistance was significantly decreased up to ∼80% with little sacrifice in transmittance. Unlike samples fabricated by topmost layer doping, our sample shows better environmental stability due to the presence of dominant neutral Au atoms on the surface which was confirmed by angle-resolved X-ray photoelectron spectroscopy. The sheet resistance of the LbL-doped four-layer graphene (11 × 11 cm<SUP>2</SUP>) was 54 Ω/sq at 85% transmittance, which meets the technical target for industrial applications.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2010/ancac3.2010.4.issue-8/nn1008808/production/images/medium/nn-2010-008808_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn1008808'>ACS Electronic Supporting Info</A></P>

Redox-Dependent Spatially Resolved Electrochemistry at Graphene and Graphite Step Edges

Gü,ell, Aleix G.,Cuharuc, Anatolii S.,Kim, Yang-Rae,Zhang, Guohui,Tan, Sze-yin,Ebejer, Neil,Unwin, Patrick R. American Chemical Society 2015 ACS NANO Vol.9 No.4

<P>The electrochemical (EC) behavior of mechanically exfoliated graphene and highly oriented pyrolytic graphite (HOPG) is studied at high spatial resolution in aqueous solutions using Ru(NH<SUB>3</SUB>)<SUB>6</SUB><SUP>3+/2+</SUP> as a redox probe whose standard potential sits close to the intrinsic Fermi level of graphene and graphite. When scanning electrochemical cell microscopy (SECCM) data are coupled with that from complementary techniques (AFM, micro-Raman) applied to the same sample area, different time-dependent EC activity between the basal planes and step edges is revealed. In contrast, other redox couples (ferrocene derivatives) whose potential is further removed from the intrinsic Fermi level of graphene and graphite show uniform and high activity (close to diffusion-control). Macroscopic voltammetric measurements in different environments reveal that the time-dependent behavior after HOPG cleavage, peculiar to Ru(NH<SUB>3</SUB>)<SUB>6</SUB><SUP>3+/2+</SUP>, is not associated particularly with any surface contaminants but is reasonably attributed to the spontaneous delamination of the HOPG with time to create partially coupled graphene layers, further supported by conductive AFM measurements. This process has a major impact on the density of states of graphene and graphite edges, particularly at the intrinsic Fermi level to which Ru(NH<SUB>3</SUB>)<SUB>6</SUB><SUP>3+/2+</SUP> is most sensitive. Through the use of an improved voltammetric mode of SECCM, we produce movies of potential-resolved and spatially resolved HOPG activity, revealing how enhanced activity at step edges is a subtle effect for Ru(NH<SUB>3</SUB>)<SUB>6</SUB><SUP>3+/2+</SUP>. These latter studies allow us to propose a microscopic model to interpret the EC response of graphene (basal plane and edges) and aged HOPG considering the nontrivial electronic band structure.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-4/acsnano.5b00550/production/images/medium/nn-2015-00550c_0009.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5b00550'>ACS Electronic Supporting Info</A></P>

Negative and Positive Persistent Photoconductance in Graphene

Biswas, Chandan,Gü,nes&#x327,, Fethullah,Loc, Duong Dinh,Lim, Seong Chu,Jeong, Mun Seok,Pribat, Didier,Lee, Young Hee American Chemical Society 2011 NANO LETTERS Vol.11 No.11

<P>Persistent photoconductance, a prolonged light-induced conducting behavior that lasts several hundred seconds, has been observed in semiconductors. Here we report persistent negative photoconductance and consecutive prominent persistent positive photoconductance in graphene. Unusually large yields of negative PC (34%) and positive PC (1652%) and remarkably long negative transient response time (several hours) were observed. Such high yields were reduced in multilayer graphene and were quenched under vacuum conditions. Two-dimensional metallic graphene strongly interacts with environment and/or substrate, causing this phenomenon, which is markedly different from that in three-dimensional semiconductors and nanoparticles.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2011/nalefd.2011.11.issue-11/nl202266h/production/images/medium/nl-2011-02266h_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl202266h'>ACS Electronic Supporting Info</A></P>

Diffusion Mechanism of Lithium Ion through Basal Plane of Layered Graphene

Yao, Fei,Gü,nes&#x327,, Fethullah,Ta, Huy Quang,Lee, Seung Mi,Chae, Seung Jin,Sheem, Kyeu Yoon,Cojocaru, Costel Sorin,Xie, Si Shen,Lee, Young Hee American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.20

<P>Coexistence of both edge plane and basal plane in graphite often hinders the understanding of lithium ion diffusion mechanism. In this report, two types of graphene samples were prepared by chemical vapor deposition (CVD): (i) well-defined basal plane graphene grown on Cu foil and (ii) edge plane-enriched graphene layers grown on Ni film. Electrochemical performance of the graphene electrode can be split into two regimes depending on the number of graphene layers: (i) the corrosion-dominant regime and (ii) the lithiation-dominant regime. Li ion diffusion perpendicular to the basal plane of graphene is facilitated by defects, whereas diffusion parallel to the plane is limited by the steric hindrance that originates from aggregated Li ions adsorbed on the abundant defect sites. The critical layer thickness (<I>l</I><SUB>c</SUB>) to effectively prohibit substrate reaction using CVD-grown graphene layers was predicted to be ∼6 layers, independent of defect population. Our density functional theory calculations demonstrate that divacancies and higher order defects have reasonable diffusion barrier heights allowing lithium diffusion through the basal plane but neither monovacancies nor Stone-Wales defect.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-20/ja301586m/production/images/medium/ja-2012-01586m_0003.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja301586m'>ACS Electronic Supporting Info</A></P>

Influence of Copper Morphology in Forming Nucleation Seeds for Graphene Growth

Han, Gang Hee,Gü,nes&#x327,, Fethullah,Bae, Jung Jun,Kim, Eun Sung,Chae, Seung Jin,Shin, Hyeon-Jin,Choi, Jae-Young,Pribat, Didier,Lee, Young Hee American Chemical Society 2011 NANO LETTERS Vol.11 No.10

<P>We report that highly crystalline graphene can be obtained from well-controlled surface morphology of the copper substrate. Flat copper surface was prepared by using a chemical mechanical polishing method. At early growth stage, the density of graphene nucleation seeds from polished Cu film was much lower and the domain sizes of graphene flakes were larger than those from unpolished Cu film. At later growth stage, these domains were stitched together to form monolayer graphene, where the orientation of each domain crystal was unexpectedly not much different from each other. We also found that grain boundaries and intentionally formed scratched area play an important role for nucleation seeds. Although the best monolayer graphene was grown from polished Cu with a low sheet resistance of 260 Ω/sq, a small portion of multilayers were also formed near the impurity particles or locally protruded parts.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2011/nalefd.2011.11.issue-10/nl201980p/production/images/medium/nl-2011-01980p_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl201980p'>ACS Electronic Supporting Info</A></P>

Enhancing Charge Transfer Kinetics by Nanoscale Catalytic Cermet Interlayer

An, Jihwan,Kim, Young-Beom,Gü,r, Turgut M.,Prinz, Fritz B. American Chemical Society 2012 ACS APPLIED MATERIALS & INTERFACES Vol.4 No.12

<P>Enhancing the density of catalytic sites is crucial for improving the performance of energy conversion devices. This work demonstrates the kinetic role of 2 nm thin YSZ/Pt cermet layers on enhancing the oxygen reduction kinetics for low temperature solid oxide fuel cells. Cermet layers were deposited between the porous Pt cathode and the dense YSZ electrolyte wafer using atomic layer deposition (ALD). Not only the catalytic role of the cermet layer itself but the mixing effect in the cermet was explored. For cells with unmixed and fully mixed cermet interlayers, the maximum power density was enhanced by a factor of 1.5 and 1.8 at 400 °C, and by 2.3 and 2.7 at 450 °C, respectively, when compared to control cells with no cermet interlayer. The observed enhancement in cell performance is believed to be due to the increased triple phase boundary (TPB) density in the cermet interlayer. We also believe that the sustained kinetics for the fully mixed cermet layer sample stems from better thermal stability of Pt islands separated by the ALD YSZ matrix, which helped to maintain the high-density TPBs even at elevated temperature.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2012/aamick.2012.4.issue-12/am3019788/production/images/medium/am-2012-019788_0009.gif'></P>

Epitaxial and Polycrystalline Gadolinia-Doped Ceria Cathode Interlayers for Low Temperature Solid Oxide Fuel Cells

Kim, Young Beom,Shim, Joon Hyung,Gü,r, Turgut M.,Prinz, Fritz B. The Electrochemical Society 2011 Journal of the Electrochemical Society Vol.158 No.11

IL-4 abrogates T_H17 cell-mediated inflammation by selective silencing of IL-23 in antigen-presenting cells

Guenova, Emmanuella,Skabytska, Yuliya,Hoetzenecker, Wolfram,Weindl, Gü,nther,Sauer, Karin,Tham, Manuela,Kim, Kyu-Won,Park, Ji-Hyeon,Seo, Ji Hae,Ignatova, Desislava,Cozzio, Antonio,Levesque, Mitc National Academy of Sciences 2015 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.112 No.7

<P>Interleukin 4 (IL-4) can suppress delayed-type hypersensitivity reactions (DTHRs), including organ-specific autoimmune diseases in mice and humans. Despite the broadly documented antiinflammatory effect of IL-4, the underlying mode of action remains incompletely understood, as IL-4 also promotes IL-12 production by dendritic cells (DCs) and IFN-gamma-producing T(H)1 cells in vivo. Studying the impact of IL-4 on the polarization of human and mouse DCs, we found that IL-4 exerts opposing effects on the production of either IL-12 or IL-23. While promoting IL-12-producing capacity of DCs, IL-4 completely abrogates IL-23. Bone marrow chimeras proved that IL-4-mediated suppression of DTHRs relies on the signal transducer and activator of transcription 6 (STAT6)-dependent abrogation of IL-23 in antigen-presenting cells. Moreover, IL-4 therapy attenuated DTHRs by STAT6-and activating transcription factor 3 (ATF3)-dependent suppression of the IL-23/T(H)17 responses despite simultaneous enhancement of IL-12/T(H)1 responses. As IL-4 therapy also improves psoriasis in humans and suppresses IL-23/ T(H)17 responses without blocking IL-12/T(H)1, selective IL-4-mediated IL-23/T(H)17 silencing is promising as treatment against harmful inflammation, while sparing the IL-12-dependent T(H)1 responses.</P>

Coordinated Molecule-Modulated Magnetic Phase with Metamagnetism in Metal-Organic Frameworks

Son, Kwanghyo,Kim, Jin Yeong,Schu&#x308,tz, Gisela,Kang, Sung Gu,Moon, Hoi Ri,Oh, Hyunchul ACS AMERICAN CHEMICAL SOCIETY 2019 Inorganic Chemistry Vol.58 No.14

<P>Most well-known metal-organic frameworks (MOFs) possessing the magnetic Ni<SUB>2</SUB>O<SUB>2</SUB>(CO<SUB>2</SUB>)<SUB>2</SUB> chains, called Ni-MOF-74, have been investigated with regard to magnetic properties at open-metal sites. We present the modulation of their magnetic phase and metamagnetism via imidazole molecule coordination.</P><P>The magnetic properties in metal−organic frameworks (MOFs), possessing Ni<SUP>II</SUP> ions in a honeycomb structure, are experimentally and theoretically investigated. The magnetically ordered state at low temperatures can be altered via imidazole molecule (IM) coordination. IM coordination influences the spin state of Ni<SUP>II</SUP> ions with orbital geometries, resulting in observed shifts in the critical temperature and field. The magnetic behavior in MOF-74-IMs reveals the coexistence of spin canting, metamagnetism, and magnetic phase transition.</P> [FIG OMISSION]</BR>

Laser Thinning for Monolayer Graphene Formation: Heat Sink and Interference Effect

Han, Gang Hee,Chae, Seung Jin,Kim, Eun Sung,Gü,nes&#x327,, Fethullah,Lee, Il Ha,Lee, Sang Won,Lee, Si Young,Lim, Seong Chu,Jeong, Hae Kyung,Jeong, Mun Seok,Lee, Young Hee American Chemical Society 2011 ACS NANO Vol.5 No.1

<P>Despite the availability of large-area graphene synthesized by chemical vapor deposition (CVD), the control of a uniform monolayer graphene remained challenging. Here, we report a method of acquiring monolayer graphene by laser irradiation. The accumulation of heat on graphene by absorbing light, followed by oxidative burning of upper graphene layers, which strongly relies on the wavelength of light and optical parameters of the substrate, was <I>in situ</I> measured by the G-band shift in Raman spectroscopy. The substrate plays a crucial role as a heat sink for the bottom monolayer graphene, resulting in no burning or etching. Oscillatory thinning behavior dependent on the substrate oxide thickness was evaluated by adopting a simple Fresnel’s equation. This paves the way for future research in utilizing monolayer graphene for high-speed electronic devices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2011/ancac3.2011.5.issue-1/nn1026438/production/images/medium/nn-2010-026438_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn1026438'>ACS Electronic Supporting Info</A></P>