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DDC: 660 × Type: Text × Version: publishedVersion × Publisher: London : Nature Publishing Group × WGL Institute: PDI ×

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    Selective control of molecule charge state on graphene using tip-induced electric field and nitrogen doping
    (London : Nature Publishing Group, 2019) Pham, Van Dong; Ghosh, Sukanya; Joucken, Frédéric; Pelaez-Fernandez, Mario; Repain, Vincent; Chacon, Cyril; Bellec, Amandine; Girard, Yann; Sporken, Robert; Rousset, Sylvie; Dappe, Yannick J.; Narasimhan, Shobhana; Lagoute, Jérôme
    The combination of graphene with molecules offers promising opportunities to achieve new functionalities. In these hybrid structures, interfacial charge transfer plays a key role in the electronic properties and thus has to be understood and mastered. Using scanning tunneling microscopy and ab initio density functional theory calculations, we show that combining nitrogen doping of graphene with an electric field allows for a selective control of the charge state in a molecular layer on graphene. On pristine graphene, the local gating applied by the tip induces a shift of the molecular levels of adsorbed molecules and can be used to control their charge state. Ab initio calculations show that under the application of an electric field, the hybrid molecule/graphene system behaves like an electrostatic dipole with opposite charges in the molecule and graphene sub-units that are found to be proportional to the electric field amplitude, which thereby controls the charge transfer. When local gating is combined with nitrogen doping of graphene, the charging voltage of molecules on nitrogen is greatly lowered. Consequently, applying the proper electric field allows one to obtain a molecular layer with a mixed charge state, where a selective reduction is performed on single molecules at nitrogen sites. © 2019, The Author(s).
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    Spatially controlled epitaxial growth of 2D heterostructures via defect engineering using a focused He ion beam
    (London : Nature Publishing Group, 2021) Heilmann, Martin; Deinhart, Victor; Tahraoui, Abbes; Höflich, Katja; Lopes, J. Marcelo J.
    The combination of two-dimensional (2D) materials into heterostructures enables the formation of atomically thin devices with designed properties. To achieve a high-density, bottom-up integration, the growth of these 2D heterostructures via van der Waals epitaxy (vdWE) is an attractive alternative to the currently mostly employed mechanical transfer, which is problematic in terms of scaling and reproducibility. Controlling the location of the nuclei formation remains a key challenge in vdWE. Here, a focused He ion beam is used to deterministically place defects in graphene substrates, which serve as preferential nucleation sites for the growth of insulating, 2D hexagonal boron nitride (h-BN). Therewith a mask-free, selective-area vdWE (SAvdWE) is demonstrated, in which nucleation yield and crystal quality of h-BN are controlled by the ion beam parameters used for defect formation. Moreover, h-BN grown via SAvdWE is shown to exhibit electron tunneling characteristics comparable to those of mechanically transferred layers, thereby lying the foundation for a reliable, high-density array fabrication of 2D heterostructures for device integration via defect engineering in 2D substrates.