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264 results on '"Venkatesan, Koushik"'

1. Charge Transport and Conductance Switching of Redox-active Azulene Derivatives

Author

Schwarz, Florian, Koch, Michael, Kastlunger, Georg, Berke, Heinz, Stadler, Robert, Venkatesan, Koushik, and Loertscher, Emanuel

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Azulene (Az) is a non-alternating, aromatic hydrocarbon composed of a five-membered, electron-rich and a seven-membered, electron-poor ring; an electron distribution that provides intrinsic redox activity. By varying the attachment points of the two electrode-bridging substituents to the Az centre, the influence of the redox functionality on charge transport is evaluated. The conductance of the 1,3 Az derivative is at least one order of magnitude lower than those of the 2,6 Az and 4,7 Az derivatives, in agreement with density functional theory (DFT) calculations. In addition, only 1,3 Az exhibits pronounced nonlinear current-voltage characteristics with hysteresis, indicating a bias-dependent conductance switching. DFT identifies the LUMO to be nearest to the Fermi energy of the electrodes, but to be an active transport channel only in the case of the 2,6 and the 4,7 Az derivatives, whereas the 1,3 Az derivative uses the HOMO at low and the LUMO+1 at high bias. In return, the localized, weakly coupled LUMO of 1,3 Az creates a slow electron-hopping channel responsible for the voltage-induced switching due to the occupation of a single MO., Comment: 6 pages, 3 figures

Published
2017
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2. Field-induced Conductance Switching by Charge-state Alternation in Organometallic Single-Molecule Junctions

Author

Schwarz, Florian, Kastlunger, Georg, Lissel, Franziska, Egler-Lucas, Carolina, Semenov, Sergey N., Venkatesan, Koushik, Berke, Heinz, Stadler, Robert, and Loertscher, Emanuel

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Charge transport through single molecules can be influenced by the charge and spin states of redox-active metal centres placed in the transport pathway. These molecular intrinsic properties are usually addressed by varying the molecules electrochemical and magnetic environment, a procedure that requires complex setups with multiple terminals. Here we show that oxidation and reduction of organometallic compounds containing either Fe, Ru or Mo centres can solely be triggered by the electric field applied to a two-terminal molecular junction. Whereas all compounds exhibit bias-dependent hysteresis, the Mo-containing compound additionally shows an abrupt voltage-induced conductance switching, yielding high to low current ratios exceeding 1000 at voltage stimuli of less than 1.0 V. DFT calculations identify a localized, redox active molecular orbital that is weakly coupled to the electrodes and closely aligned with the Fermi energy of the leads because of the spin-polarised ground state unique to the Mo centre. This situation opens an additional slow and incoherent hopping channel for transport, triggering a transient charging effect of the entire molecule and a strong hysteresis with unprecedented high low-to-high current ratios., Comment: 9 pages, 4 figures

Published
2016
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3. High-Conductive Organometallic Molecular Wires with Delocalized Electron Systems Strongly Coupled to Metal Electrodes

Author

Schwarz, Florian, Kastlunger, Georg, Lissel, Franziska, Riel, Heike, Venkatesan, Koushik, Berke, Heinz, Stadler, Robert, and Lörtscher, Emanuel

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Besides active, functional molecular building blocks such as diodes or switches, passive components as, e.g., molecular wires, are required to realize molecular-scale electronics. Incorporating metal centers in the molecular backbone enables the molecular energy levels to be tuned in respect to the Fermi energy of the electrodes. Furthermore, by using more than one metal center and sp-bridging ligands, a strongly delocalized electron system is formed between these metallic "dopants", facilitating transport along the molecular backbone. Here, we study the influence of molecule--metal coupling on charge transport of dinuclear X(PP)$_2$FeC$_4$Fe(PP)$_2$X molecular wires (PP = Et$_2$PCH$_2$CH$_2$PEt$_2$); X = CN (1), NCS (2), NCSe (3), C$_4$SnMe$_3$ (4) and C$_2$SnMe$_3$ (5)) under ultra-high vacuum and variable temperature conditions. In contrast to 1 which showed unstable junctions at very low conductance ($8.1\cdot10^{-7}$ G$_0$), 4 formed a Au-C$_4$FeC$_4$FeC$_4$-Au junction 4' after SnMe$_3$ extrusion which revealed a conductance of $8.9\cdot10^{-3}$ G$_0$, three orders of magnitude higher than for 2 ($7.9\cdot10^{-6}$ G$_0$) and two orders of magnitude higher than for 3 ($3.8\cdot10^{-4}$ G$_0$). Density functional theory (DFT) confirmed the experimental trend in the conductance for the various anchoring motifs. The strong hybridization of molecular and metal states found in the C--Au coupling case enables the delocalized electronic system of the organometallic Fe$_2$ backbone to be extended over the molecule-metal interfaces to the metal electrodes to establish high-conductive molecular wires.

Published
2015
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6. Metallic nanoparticle contacts for high-yield, ambient-stable molecular-monolayer devices

Author

Puebla-Hellmann, Gabriel, Venkatesan, Koushik, Mayor, Marcel, and Lörtscher, Emanuel

Subjects
Nanoparticles, Resveratrol, Generic drugs, Metals (Materials), Polystyrene, Clathrate compounds, Technology application, Environmental issues, Science and technology, Zoology and wildlife conservation
Abstract

Accessing the intrinsic functionality of molecules for electronic applications.sup.1-3, light emission.sup.4 or sensing.sup.5 requires reliable electrical contacts to those molecules. A self-assembled monolayer (SAM) sandwich architecture.sup.6 is advantageous for technological applications, but requires a non-destructive, top-contact fabrication method. Various approaches ranging from direct metal evaporation.sup.6 over poly(3,4-ethylenedioxythiophene) polystyrene sulfonate.sup.7 (PEDOT:PSS) or graphene.sup.8 interlayers to metal transfer printing.sup.9 have been proposed. Nevertheless, it has not yet been possible to fabricate SAM-based devices without compromising film integrity, intrinsic functionality or mass-fabrication compatibility. Here we develop a top-contact approach to SAM-based devices that simultaneously addresses all these issues, by exploiting the fact that a metallic nanoparticle can provide a reliable electrical contact to individual molecules.sup.10. Our fabrication route involves first the conformal and non-destructive deposition of a layer of metallic nanoparticles directly onto the SAM (itself laterally constrained within circular pores in a dielectric matrix, with diameters ranging from 60 nanometres to 70 micrometres), and then the reinforcement of this top contact by direct metal evaporation. This approach enables the fabrication of thousands of identical, ambient-stable metal-molecule-metal devices. Systematic variation of the composition of the SAM demonstrates that the intrinsic molecular properties are not affected by the nanoparticle layer and subsequent top metallization. Our concept is generic to densely packed layers of molecules equipped with two anchor groups, and provides a route to the large-scale integration of molecular compounds into solid-state devices that can be scaled down to the single-molecule level. A top-contacting method for the fabrication of molecular devices uses metallic nanoparticles to electrically contact self-assembled monolayers, enabling the preparation of thousands of identical, ambient-stable metal-molecule-metal devices., Author(s): Gabriel Puebla-Hellmann [sup.1] [sup.2] , Koushik Venkatesan [sup.3] [sup.4] , Marcel Mayor [sup.2] [sup.5] [sup.6] , Emanuel Lörtscher [sup.1] Author Affiliations: (1) IBM Research - Zurich, Rüschlikon, Switzerland (2) [...]

Published
2018
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10. Tunable Light‐Emission Properties of Solution‐Processable N‐Heterocyclic Carbene Cyclometalated Gold(III) Complexes for Organic Light‐Emitting Diodes

Author

Malmberg, Robert, von Arx, Tobias, Hasan, Monirul, Blacque, Olivier, Shukla, Atul, McGregor, Sarah K M, Lo, Shih‐Chun, Namdas, Ebinazar B, Venkatesan, Koushik, University of Zurich, and Venkatesan, Koushik

Subjects
10120 Department of Chemistry, 1503 Catalysis, 540 Chemistry, Organic Chemistry, General Chemistry, Catalysis, 1605 Organic Chemistry
Published
2021
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12. Benzalkonium chloride antagonises aminoglycoside antibiotics and promotes evolution of resistance

Author

Short, Francesca L, primary, Lee, Victor, additional, Mamun, Rafa, additional, Malmberg, Robert, additional, Li, Liping, additional, Espinosa, Monica I, additional, Venkatesan, Koushik, additional, Paulsen, Ian T, additional, Abbu, Kristel-Ione, additional, Algie, Jarrod, additional, Callaghan, Amy, additional, Cenderawasih-Nere, Priscilla, additional, Cullen, Victoria, additional, Gooden, Meredith, additional, Kanoun, Mayes, additional, Lawrie, Angus, additional, Maher, Justin, additional, Malek, Vanessa, additional, Safdari, Zahra, additional, Shehadie, Benjamin, additional, Shifa, Shaila-Yasmin, additional, Simpson, Irene, additional, Towns, Madeleine, additional, and Valter, Stefan, additional

Published
2021
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21. Cover Feature: Tunable Light‐Emission Properties of Solution‐Processable N‐Heterocyclic Carbene Cyclometalated Gold(III) Complexes for Organic Light‐Emitting Diodes (Chem. Eur. J. 25/2021)

Author

Malmberg, Robert, primary, Arx, Tobias, additional, Hasan, Monirul, additional, Blacque, Olivier, additional, Shukla, Atul, additional, McGregor, Sarah K. M., additional, Lo, Shih‐Chun, additional, Namdas, Ebinazar B., additional, and Venkatesan, Koushik, additional

Published
2021
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26. Monocyclometalated (C N) Gold(III) Metallacycles: Tunable Emission and Singlet Oxygen (1O2) Generation Properties.

Author

Malmberg, Robert, Suter, Dominik, Blacque, Olivier, and Venkatesan, Koushik

Subjects
GOLD, REACTIVE oxygen species, METALLACYCLES, VISIBLE spectra, EXCITED states, THIN films
Abstract

The synthesis, characterization and photoluminescent properties of four cyclometalated (C N)‐type gold(III) complexes bearing a bidentate diacetylide ligand, tolan‐2,2'‐diacetylide (tda), are reported. The complexes exhibit highly tunable excited state properties and show photoluminescence (PL) across the entire visible spectrum from sky‐blue (λPL=493 nm) to red (λPL=675 nm) with absolute PL quantum yields (PLQY) of up to 75 % in solution, the highest PLQY found for any monocyclometalated Au(III) complex in solution. As a consequence of the use of the strongly rigidifying diacetylide bidentate ligand, a significant increase in the excited state lifetimes (τ0=16–258 μs) was found in solution and in thin films. The complexes showed remarkable singlet oxygen generation in aerated solution with absolute singlet oxygen quantum yield (ϕ1Δ) values reaching up to 7.5×10−5 and singlet oxygen lifetimes (τ01Δ) in the range of 66–95 μs. Furthermore, the radiative and non‐radiative rates of singlet oxygen were determined using the ϕ1Δ and τ01Δ values and correlations are drawn between the formation of singlet oxygen and its interaction with cyclometalated (C N) gold(III) complexes. [ABSTRACT FROM AUTHOR]

Published
2021
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33. Stable and tunable phosphorescent neutral cyclometalated Au(III) diaryl complexes

Author

Garg, Jai Anand, Blacque, Olivier, Fox, Thomas, and Venkatesan, Koushik

Subjects
Ring formation (Chemistry) -- Analysis, Density functionals -- Usage, Excited state chemistry -- Analysis, Gold -- Chemical properties, Gold -- Optical properties, Organometallic compounds -- Structure, Organometallic compounds -- Chemical properties, Organometallic compounds -- Optical properties, Phosphorescence -- Analysis, Chemistry
Published
2010

34. An iron-capped metal-organic polyyne: {[Fe][(C triple bond C).sub.2][W]triple bond C C triple bond C C triple bond [W][(C triple bond C).sub.2][Fe]}

Author

Semenov, Sergey N., Taghipourian, Shiva F., Blacque, Olivier, Fox, Thomas, Venkatesan, Koushik, and Berke, Heinz

Subjects
Electron transport -- Analysis, Iron -- Chemical properties, Iron -- Atomic properties, Organometallic compounds -- Structure, Organometallic compounds -- Chemical properties, Chemistry
Abstract

The article presents a new approach for the synthesis of an iron-capped metal-organic polyyne, {[Fe][(C triple bond C).sub.2][W]triple bond C C triple bond C C triple bond [W][(C triple bond C).sub.2][Fe]}.

Published
2010

35. Electron communication in dinuclear [C.sub.4]-bridged tungsten complexes

Author

Semenov, Sergey N., Blacque, Olivier, Fox, Thomas, Venkatesan, Koushik, and Berke, Heinz

Subjects
Carbonyl compounds -- Structure, Carbonyl compounds -- Chemical properties, Oxidation-reduction reaction -- Analysis, Photolysis -- Analysis, Tungsten -- Chemical properties, Tungsten -- Electric properties, Chemistry
Abstract

The dinuclear tungsten carbyne complexes are prepared from the bisacetylide precursor through oxidative replacement of one CO group at each tungsten center with a halide substituent. The studies have showed that the [W][triple bond]C-C[triple bond]-C[triple bond][W] canonical form of the bridged system with strong tungsten-carbon interaction has contributed to the electronic coupling in the mixed-valent species and to the strong antiferromagnetic coupling in the dicationic complex.

Published
2010

36. Metal-free triplet phosphors with high emission efficiency and high tunability

Author

Koch, Michael, Perumal, Karthikeyan, Blacque, Olivier, Garg, Jai Anand, Saiganesh, Ramanathan, Kabilan, Senthamaraikannan, Balasubramanian, Kallupattu Kuppusamy, Venkatesan, Koushik, University of Zurich, and Venkatesan, Koushik

Subjects
10120 Department of Chemistry, 1503 Catalysis, 540 Chemistry, 1600 General Chemistry, General Chemistry, Catalysis
Published
2014

37. Smectic A mesophases from luminescent sandic platinum(II) mesogens

Author

Venkatesan, Koushik, Kouwer, Paul H. J., Swager, Timothy M., Krikorian, Markrete, Voll, Constantin-Chri Alexander, Yoon, Maryam A., Swager, Timothy M, University of Zurich, Swager, Timothy M, Massachusetts Institute of Technology. Department of Chemistry, Krikorian, Markrete, Voll, Constantin-Chri Alexander, and Yoon, Maryam A.

Subjects
10120 Department of Chemistry, Materials science, 3104 Condensed Matter Physics, chemistry.chemical_element, 1600 General Chemistry, 02 engineering and technology, 010402 general chemistry, 01 natural sciences, Liquid crystal, 540 Chemistry, Organic chemistry, General Materials Science, Lamellar structure, Alkyl, chemistry.chemical_classification, Molecular Materials, Mesophase, General Chemistry, 021001 nanoscience & nanotechnology, Condensed Matter Physics, 2500 General Materials Science, 0104 chemical sciences, Crystallography, chemistry, 0210 nano-technology, Luminescence, Platinum
Abstract

Square planar platinum(II) thienyl pyridyl complexes with board-shaped structures assemble into lamellar (SmA) liquid crystal phases at elevated temperatures. Liquid crystals of this type are expected to have stronger biaxial correlations than typical calamitic mesogens. The mesophase stability improves with decreasing alkyl chain lengths with C₈H₁₇ having the widest range of stability. All complexes are luminescent in solution., National Science Foundation (U.S.) (Grant DMR-1410718)

Published
2016

44. Triptycene based luminescent metal-organic gels for chemosensing

Author

Barman, Samir, Garg, Jai Anand, Blacque, Olivier, Venkatesan, Koushik, Berke, Heinz, University of Zurich, and Venkatesan, Koushik

Subjects
10120 Department of Chemistry, 1503 Catalysis, 540 Chemistry, 2508 Surfaces, Coatings and Films, 2506 Metals and Alloys, 2504 Electronic, Optical and Magnetic Materials, 2503 Ceramics and Composites, 1600 General Chemistry, 2505 Materials Chemistry
Published
2012

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