7 research outputs found

    Simulation of Adsorption Processes at Metallic Interfaces: An Image Charge Augmented QM/MM Approach

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    A novel method for including polarization effects within hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of adsorbate-metal systems is presented. The interactions between adsorbate (QM) and metallic substrate (MM) are described at the MM level of theory. Induction effects are additionally accounted for by applying the image charge formulation. The charge distribution induced within the metallic substrate is modeled by a set of Gaussian charges (image charges) centered at the metal atoms. The image charges and the electrostatic response of the QM potential are determined self-consistently by imposing the constant-potential condition within the metal. The implementation is embedded in a highly efficient Gaussian and plane wave framework and is naturally suited for periodic systems. Even though the electronic properties of the metallic substrate are not taken into account explicitly, the augmented QM/MM scheme can reproduce characteristic polarization effects of the adsorbate. The method is assessed through the investigation of structural and electronic properties of benzene, nitrobenzene, thymine, and guanine on Au(111). The study of small water clusters adsorbed on Pt(111) is also reported in order to demonstrate that the approach provides a sizable correction of the MM-based interactions between adsorbate and substrate. Large-scale molecular dynamics (MD) simulations of a water film in contact with a Pt(111) surface show that the method is suitable for simulations of liquid/metal interfaces at reduced computational cost

    Room Temperature Metalation of 2H-TPP Monolayer on Iron and Nickel Surfaces by Picking up Substrate Metal Atoms

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    Here, it is demonstrated, using high-resolution X-ray spectroscopy and density functional theory calculations, that 2<i>H</i>-tetraphenyl porphyrins metalate at room temperature by incorporating a surface metal atom when a (sub)monolayer is deposited on 3d magnetic substrates, such as Fe(110) and Ni(111). The calculations demonstrate that the redox metalation reaction would be exothermic when occurring on a Ni(111) substrate with an energy gain of 0.89 eV upon embedding a Ni adatom in the macrocycle. This is a novel way to form, <i>via</i> chemical modification and supramolecular engineering, 3d-metalā€“organic networks on magnetic substrates with an intimate bond between the macrocycle molecular metal ion and the substrate atoms. The achievement of a complete metalation by Fe and Ni can be regarded as a test case for successful preparation of spintronic devices by means of molecular-based magnets and inorganic magnetic substrates

    Adsorption of Small Hydrocarbons on the Three-Fold PdGa Surfaces: The Road to Selective Hydrogenation

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    Intermetallic compounds are a promising class of materials as stable and selective heterogeneous catalysts. Here, the (111) and (āˆ’1ā€“1ā€“1) single crystal surfaces of the PdGa intermetallic compound were studied as model catalysts with regard to the selective hydrogenation of acetylene (C<sub>2</sub>H<sub>2</sub>) to ethylene (C<sub>2</sub>H<sub>4</sub>). The distinct atomic surface structures exhibit isolated active centers of single atomic and three atomic Pd ensembles, respectively. For the two prototypal model catalyst surfaces, the adsorption sites and configurations for hydrogen (H<sub>2</sub>), acetylene, and ethylene were investigated by combining scanning tunneling microscopy, temperature-programmed desorption, and <i>ab initio</i> modeling. The topmost Pd surface atoms provide the preferred adsorption sites for all studied molecules. The structural difference of the Pd ensembles has a significant influence on the adsorption energy and configuration of C<sub>2</sub>H<sub>2</sub>, while the influence of the ensemble structure is weak for C<sub>2</sub>H<sub>4</sub> and H<sub>2</sub> adsorption. To approach the question of catalytic performance, we simulated the reaction pathways for the heterogeneous catalytic hydrogenation of acetylene on the two surfaces by means of density functional theory. Due to the geometrical separation of the Pd sites on the surfaces, the steric approach of the reactants (H and C<sub>2</sub>H<sub><i>x</i></sub>) was found to be of importance to the energetics of the reaction. The presented study gives a direct comparison of binding properties of catalytic Pd on-top sites vs three-fold Pd hollow sites and is therefore of major relevance to the knowledge-based design of highly selective hydrogenation catalysts

    Intraribbon Heterojunction Formation in Ultranarrow Graphene Nanoribbons

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    Graphene nanoribbonsī—øsemiconducting quasi-one-dimensional graphene structuresī—øhave great potential for the realization of novel electronic devices. Recently, graphene nanoribbon heterojunctionsī—øinterfaces between nanoribbons with unequal band gapsī—øhave been realized with lithographic etching techniques and <i>via</i> chemical routes to exploit quantum transport phenomena. However, standard fabrication techniques are not suitable for ribbons narrower than āˆ¼5 nm and do not allow to control the width and edge structure of a specific device with atomic precision. Here, we report the realization of graphene nanoribbon heterojunctions with lateral dimensions below 2 nm <i>via</i> controllable dehydrogenation of polyanthrylene oligomers self-assembled on a Au(111) surface from molecular precursors. Atomistic simulations reveal the microscopic mechanisms responsible for intraribbon heterojunction formation. We demonstrate the capability to selectively modify the heterojunctions by activating the dehydrogenation reaction on single units of the nanoribbons by electron injection from the tip of a scanning tunneling microscope

    Ensemble Effect Evidenced by CO Adsorption on the 3ā€‘Fold PdGa Surfaces

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    The atomic structure and composition of a catalystā€™s surface have a major influence on its performance regarding activity and selectivity. In this respect, intermetallic compounds are promising future catalyst materials, as their surfaces exhibit small and well-defined ensembles of active metal atoms. In this study, the active adsorption sites of the 3-fold-symmetric surfaces of the PdGa intermetallic compound were investigated in a combined experimental and computational approach using CO as a test molecule. The PdGa(111) and (āˆ’1ā€“1ā€“1) surfaces exhibit very similar electronic structures, but have Pd sites with very different, well-defined atomic coordination and separation. They thereby serve as prototypical model systems for studying ensemble effects on bimetallic catalytic surfaces. Scanning tunneling microscopy and Fourier transform infrared spectroscopy show that the CO adsorption on both surfaces is solely associated with the topmost Pd atoms and Ga acts only as an inactive spacer. The different local configurations of these Pd atoms dictate the CO adsorption sites as a function of coverage. The experimental results are corroborated by density functional theory and illustrate the site separation and ensemble effects for molecular adsorption on intermetallic single crystalline surfaces

    Identifying Photoreaction Products in Cinnamate-Based Photoalignment Materials

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    A novel joint computational and experimental strategy is developed and applied for the detection and the identification of photoreaction products in cinnamate-based photoalignment materials. Based on NEXAFS, IR, and NMR spectroscopies and supported by computer simulation tools, this structural analysis method allows distinguishing the typical signatures of products resulting from UV-induced photoreactions between isomers of cinnamate-based model compounds. Besides deepening the understanding of typical photoalignment reaction products, the proposed strategy acquires technological relevance in supporting the realization of next generation materials for the LCD panel industry

    Termini of Bottom-Up Fabricated Graphene Nanoribbons

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    Atomically precise graphene nanoribbons (GNRs) can be obtained via thermally induced polymerization of suitable precursor molecules on a metal surface. This communication discusses the atomic structure found at the termini of armchair GNRs obtained via this bottom-up approach. The short zigzag edge at the termini of the GNRs under study gives rise to a localized midgap state with a characteristic signature in scanning tunneling microscopy (STM). By combining STM experiments with large-scale density functional theory calculations, we demonstrate that the termini are passivated by hydrogen. Our results suggest that the length of nanoribbons grown by this protocol may be limited by hydrogen passivation during the polymerization step
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