12 research outputs found

    Toward <i>GW</i> Calculations on Thousands of Atoms

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    The <i>GW</i> approximation of many-body perturbation theory is an accurate method for computing electron addition and removal energies of molecules and solids. In a canonical implementation, however, its computational cost is O(N4) in the system size <i>N</i>, which prohibits its application to many systems of interest. We present a full-frequency <i>GW</i> algorithm in a Gaussian-type basis, whose computational cost scales with <i>N</i><sup>2</sup> to <i>N</i><sup>3</sup>. The implementation is optimized for massively parallel execution on state-of-the-art supercomputers and is suitable for nanostructures and molecules in the gas, liquid or condensed phase, using either pseudopotentials or all electrons. We validate the accuracy of the algorithm on the <i>GW</i>100 molecular test set, finding mean absolute deviations of 35 meV for ionization potentials and 27 meV for electron affinities. Furthermore, we study the length-dependence of quasiparticle energies in armchair graphene nanoribbons of up to 1734 atoms in size, and compute the local density of states across a nanoscale heterojunction

    Molecules–Oligomers–Nanowires–Graphene Nanoribbons: A Bottom-Up Stepwise On-Surface Covalent Synthesis Preserving Long-Range Order

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    We report on a stepwise on-surface polymerization reaction leading to oriented graphene nanoribbons on Au(111) as the final product. Starting from the precursor 4,4″-dibromo-<i>p</i>-terphenyl and using the Ullmann coupling reaction followed by dehydrogenation and C–C coupling, we have developed a fine-tuned, annealing-triggered on-surface polymerization that allows us to obtain an oriented nanomesh of graphene nanoribbons via two well-defined intermediate products, namely, <i>p</i>-phenylene oligomers with reduced length dispersion and ordered submicrometric molecular wires of poly­(<i>p</i>-phenylene). A fine balance involving gold catalytic activity in the Ullmann coupling, appropriate on-surface molecular mobility, and favorable topochemical conditions provided by the used precursor leads to a high degree of long-range order that characterizes each step of the synthesis and is rarely observed for surface organic frameworks obtained via Ullmann coupling

    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-metal–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

    Intraribbon Heterojunction Formation in Ultranarrow Graphene Nanoribbons

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    Graphene nanoribbonssemiconducting quasi-one-dimensional graphene structureshave great potential for the realization of novel electronic devices. Recently, graphene nanoribbon heterojunctionsinterfaces between nanoribbons with unequal band gapshave 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 ∼5 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 3‑Fold PdGa Surfaces

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    The atomic structure and composition of a catalyst’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 (−1–1–1) 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

    On-Surface Synthesis of Indenofluorene Polymers by Oxidative Five-Membered Ring Formation

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    On-surface synthesis is a successful approach to the creation of carbon-based nanostructures that cannot be obtained via standard solution chemistry. In this framework, we have established a novel synthetic pathway to one-dimensional conjugated polymers composed of indenofluorene units. Our concept is based on the use of <i>ortho-</i>methyl groups on a poly­(<i>para</i>-phenylene) backbone. In this situation, surface-assisted oxidative ring closure between a methyl and the neighboring aryl moiety gives rise to a five-membered ring. The atomically precise structures and electronic properties of the obtained indenofluorene polymers have been unambiguously characterized by STM, nc-AFM, and STS, supported by theoretical calculations. This unprecedented synthetic protocol can potentially be extended to other polyphenylenes and eventually graphene nanoribbons, to incorporate five-membered rings at desired positions for the fine-tuning of electronic properties

    On-Surface Synthesis of Heptacene Organometallic Complexes

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    We report the on-surface formation of Au-directed heptacene organometallic complexes on a Au(111) template in an ultrahigh vacuum environment. Successive thermal annealing steps investigated by means of scanning tunneling microscopy, noncontact atomic force microscopy, temperature-programmed desorption and density functional theory reveal the formation of heptacene organometallic complexes via a selective two-step activation of an Îą-diketone-protected heptacene precursor. Furthermore, we demonstrate the efficiency of tip-induced deprotection experiments as a complementary strategy in the complex formation. Our results provide perspectives for the on-surface synthesis of larger acenes featuring potential use in the fields of organic electronics, spintronics and nonlinear optics

    Electronic Structure of Atomically Precise Graphene Nanoribbons

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    Some of the most intriguing properties of graphene are predicted for specifically designed nanostructures such as nanoribbons. Functionalities far beyond those known from extended graphene systems include electronic band gap variations related to quantum confinement and edge effects, as well as localized spin-polarized edge states for specific edge geometries. The inability to produce graphene nanostructures with the needed precision, however, has so far hampered the verification of the predicted electronic properties. Here, we report on the electronic band gap and dispersion of the occupied electronic bands of atomically precise graphene nanoribbons fabricated <i>via</i> on-surface synthesis. Angle-resolved photoelectron spectroscopy and scanning tunneling spectroscopy data from armchair graphene nanoribbons of width <i>N</i> = 7 supported on Au(111) reveal a band gap of 2.3 eV, an effective mass of 0.21 <i>m</i><sub>0</sub> at the top of the valence band, and an energy-dependent charge carrier velocity reaching 8.2 × 10<sup>5</sup> m/s in the linear part of the valence band. These results are in quantitative agreement with theoretical predictions that include image charge corrections accounting for screening by the metal substrate and confirm the importance of electron–electron interactions in graphene nanoribbons

    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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