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      Synthesis of a Helical Bilayer Nanographene

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          Chiral Graphene Quantum Dots.

          Chiral nanostructures from metals and semiconductors attract wide interest as components for polarization-enabled optoelectronic devices. Similarly to other fields of nanotechnology, graphene-based materials can greatly enrich physical and chemical phenomena associated with optical and electronic properties of chiral nanostructures and facilitate their applications in biology as well as other areas. Here, we report that covalent attachment of l/d-cysteine moieties to the edges of graphene quantum dots (GQDs) leads to their helical buckling due to chiral interactions at the "crowded" edges. Circular dichroism (CD) spectra of the GQDs revealed bands at ca. 210-220 and 250-265 nm that changed their signs for different chirality of the cysteine edge ligands. The high-energy chiroptical peaks at 210-220 nm correspond to the hybridized molecular orbitals involving the chiral center of amino acids and atoms of graphene edges. Diverse experimental and modeling data, including density functional theory calculations of CD spectra with probabilistic distribution of GQD isomers, indicate that the band at 250-265 nm originates from the three-dimensional twisting of the graphene sheet and can be attributed to the chiral excitonic transitions. The positive and negative low-energy CD bands correspond to the left and right helicity of GQDs, respectively. Exposure of liver HepG2 cells to L/D-GQDs reveals their general biocompatibility and a noticeable difference in the toxicity of the stereoisomers. Molecular dynamics simulations demonstrated that d-GQDs have a stronger tendency to accumulate within the cellular membrane than L-GQDs. Emergence of nanoscale chirality in GQDs decorated with biomolecules is expected to be a general stereochemical phenomenon for flexible sheets of nanomaterials.
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            Formation of bilayer bernal graphene: layer-by-layer epitaxy via chemical vapor deposition.

            We report the epitaxial formation of bilayer Bernal graphene on copper foil via chemical vapor deposition. The self-limit effect of graphene growth on copper is broken through the introduction of a second growth process. The coverage of bilayer regions with Bernal stacking can be as high as 67% before further optimization. Facilitated with the transfer process to silicon/silicon oxide substrates, dual-gated graphene transistors of the as-grown bilayer Bernal graphene were fabricated, showing typical tunable transfer characteristics under varying gate voltages. The high-yield layer-by-layer epitaxy scheme will not only make this material easily accessible but reveal the fundamental mechanism of graphene growth on copper.
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              Is Open Access

              Graphene bilayer with a twist: electronic structure

              Electronic properties of bilayer and multilayer graphene have generally been interpreted in terms of AB or Bernal stacking. However, it is known that many types of stacking defects can occur in natural and synthetic graphite; rotation of the top layer is often seen in scanning tunneling microscopy (STM) studies of graphite. In this paper we consider a graphene bilayer with a relative small angle rotation between the layers and calculate the electronic structure near zero energy in a continuum approximation. Contrary to what happens in a AB stacked bilayer and in accord with observations in epitaxial graphene we find: (a) the low energy dispersion is linear, as in a single layer, but the Fermi velocity can be significantly smaller than the single layer value; (b) an external electric field, perpendicular to the layers, does not open an electronic gap
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                Author and article information

                Journal
                Angewandte Chemie International Edition
                Angew. Chem. Int. Ed.
                Wiley
                14337851
                June 04 2018
                June 04 2018
                March 13 2018
                : 57
                : 23
                : 6774-6779
                Affiliations
                [1 ]Departamento de Química Orgánica I; Facultad de Ciencias Químicas; Universidad Complutense de Madrid; Ciudad Universitaria s/n 28040 Madrid Spain
                [2 ]Institut des Sciences Chimiques de Rennes; UMR 6226 CNRS-; Univ. Rennes; Campus de Beaulieu 35042 Rennes Cedex France
                [3 ]Single Crystal X-ray Diffraction Laboratory; Interdepartmental Research Service (SIdI); Universidad Autónoma de Madrid; Cantoblanco 28049 Madrid Spain
                [4 ]IMDEA-Nanociencia; C/Faraday; 9, Campus de la Universidad Autónoma de Madrid 28049 Madrid Spain
                Article
                10.1002/anie.201800798
                29447436
                256ce1f2-7848-445b-a1db-f7884a11098f
                © 2018

                http://doi.wiley.com/10.1002/tdm_license_1.1

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