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Sandwich-molecular nanowires: on-surface synthesis, structure and magnetism
DFG research grant

Compounds containing rare-earth elements are of interest in organometallic research due to the robust magnetic moment of the 4f ion and their potential application in molecular spintronics. Until now the most studied systems have been either zero-dimensional, e.g. single-molecule magnets, or two-dimensional, such as metal-organic networks. Within this project, we investigate one-dimensional (1D) systems, namely sandwich-molecular wires. In the Figure, wires consisting of rare earth di-cations, such as Eu2+, and the 8-fold symmetric cyclooctatetraene di-anion (C8H82-, Cot) are depicted. Because of organometallic hybridization between the metal-atomic states and the extended p-orbitals of the Cot, the metal ions in the wire couple magnetically. With proper choice of rare earth element and ligand molecule, these systems could be more stable magnetic units than single-molecule magnets and could display larger magnetic anisotropy with correspondingly higher blocking temperatures.

Based on the ultrahigh vacuum on-surface synthesis method developed in Cologne [more] [more = link to Huttmann et al, JACS 139 (2017) 9895] we investigate the growth, structure, and magnetism of such sandwich-molecular nanowire carpets on graphene. Besides scanning tunneling microscopy (STM) and low-energy electron diffraction, X-ray magnetic circular dichroism is used as a method of investigation. It provides element-specific information on magnetic coupling and magnetic moments. Thereby, we demonstrated ferromagnetic coupling in such wires as shown in the Figure. We explore the electronic structure of such wires, how modified ligands affect their magnetism, how their magnetism can be tuned using different rare-earth cations like Dy, Tm or Tb, and how wire carpets can be aligned using low-symmetry substrates.

Left: STM topograph of EuCot nanowire carpets on Gr/Ir(111). Image size 260 nm x 260 nm. The inset displays a nanowire carpet in molecular resolution. Large scale hexagonal undulation is due to the graphene corrugation underneath the carpet. Image size 10 nm x 10 nm. Center: top and side view of a ball-and-stick model of a single EuCot nanowire. Right: hysteresis curve of an EuCot nanowire carpet at 5 K obtained with XMCD.  Inset: molecular resolution nanowire carpet as in left inset overlaid with structure model of the nanowires [more].