Understanding and controlling nanoscale spin coupling in molecular spintronic materials
Final Report Abstract
The Emmy Noether group was successfully established in Münster as an independent research group (i.e. not associated to or dependent on other groups) with three PhD students and several Bachelor and Master students over the course of the four years in Münster. Since May 2014, the group has relocated to Radboud University in Nijmegen where it has become a pillar of the SPM department. The main task of the group has been to create and characterize magnetic molecules and single molecule magnets (SMMs) in contact with metallic surfaces. To realize an atomic-scale controlled environment he main tool is low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) as well as STM manipulation. We want to understand fundamentally the interplay between structural, electronic, and magnetic properties of these molecular spin structures. A strong focus is on the role of the substrate. Such understanding can form the basis for the realization and directed improvement of prototype molecular spintronic devices. We successfully applied the pulse-valve and rapid-heating techniques to deposit large SMMs (synthesized by T. Glaser, Bielefeld) onto clean surfaces of Au(111), Ag(111) and NaCl/Au(111) under UHV conditions. By combining STM and STS, we performed an energy-dependent spectroscopic mapping of molecular frontier orbitals with sub-nm spatial resolution. The analysis revealed heteroradialene character for triplesalen complexes, which is not present for triplesalalen complexes. The latter exhibit a 3x higher spin coupling. Future experiments will aim at directly resolving the intramolecular spin texture via SP-STM. For this, we already proved that our STM can resolve spin contrast using a magnetic STM tip, and in the future we will be able to perform experiments in fields up to 9 Tesla and at temperatures far below 4 Kelvin. In the search for flexible ligands necessary for bottom-up created complex spin structures, we found and studied the exciting properties of cyclooctatetraene (COT) on various noble metals. Different substrates lead to different hybrid conformational states of the molecule. We also connected COT ligands with 3d transition-metal atoms and studied hybridization effects. This will be very useful in forthcoming experiments, where we will react COT with rare-eath atoms to form metallocenelike stacked structures with very large magnetic anisotropy energies to be expected. As an interesting by-product of our ligand studies, we found that TCNE forms surprisingly complex monolayer patterns. By employing a novel technique based on measuring the molecular vibrations via inelastic electron tunneling spectroscopy (IETS), we were able to determine the adsorption sites of each molecule, thus elucidating the monolayer structure with atomic resolution. A main objective of the research plan is to better understand the interplay of molecular magnets supported on substrates (as would be the case in a device environment). For this reason we extended our studies beyond noble-metal substrates, e.g. insulating films (NaCl, Cu2N). We also consider more complex substrate systems, e.g. with strong spin-orbit coupling such as Rashba systems or topological insulators. For this, we performed experiments on the BiCu2/Cu(111) surface alloy, where we found standing-wave patterns caused by multiple intraband and interband scattering channels, which can be used recover the full surface-band structure including the Rashba splitting. We have also learned how to grow graphene and Fe on Ir(111) surfaces, and we recently acquired spin contrast on magnetic substrates using magnetic tips for SP-STM. This is a strong basis for further experiments focusing on molecule-surface interactions of adsorbed SMMs. Finally, we initiated a new research project, studying Pt(III) triplet-emitter molecules that exhibit exciting properties for applications in organic light-emitting diodes (OLEDs). The gained knowledge on intramolecular tuning capabilities and understanding molecule-substrate interactions through targeted ligand modification is expected to also impact our molecular magnetism research efforts. We already achieved six publications in high-impact journals (e.g. Nano Lett., Phys. Rev. Lett., Chem. Commun.), another paper is currently in review with Nano Letters, and the remaining unpublished results are worth at least six further publications in the near future. Atomic-scale magnetism has found increasing interest and is still a hot research topic. The molecular approach offers promising advantages and therefore is of high relevance. I expect to be able to find new funding to continue and progress what was started with this Emmy Noether group.
Publications
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Tuning Molecule-Mediated Spin Coupling in Bottom-Up Fabricated Vanadium-TCNE Nanostructures, Physical Review Letters 103, 087205 (2009)
D. Wegner, R. Yamachika, X. Zhang, Y. Wang, T. Baruah, M. R. Pederson, B. M. Bartlett, J. R. Long, and M. F. Crommie
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Adsorption site determination of a molecular monolayer via inelastic tunneling, Nano Letters 13, 2346 (2013)
D. Wegner, R. Yamachika, X. Zhang, Y. Wang, M. F. Crommie, and N. Lorente
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Finding the Rashba-type spinsplitting from interband scattering in quasiparticle interference maps, Physical Review B 87, 245436 (2013)
M. Steinbrecher, H. Harutyunyan, C. R. Ast, and D. Wegner
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Hybridisation at the organic-metal interface: a surface-scientific analogue of Hückel’s rule?, Chemical Communications 49, 5993 (2013)
H. Harutyunyan, M. Callsen, T. Allmers, V. Caciuc, S. Blügel, N. Atodiresei, and D. Wegner
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Unraveling orbital Hybridization of Triplet Emitters at the Metal-Organic Interface, Physical Review Letters 111, 267401 (2013)
P. R. Ewen, J. Sanning, N. L. Doltsinis, M. Mauro, C. A. Strassert, and D. Wegner
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Adsorption and STM imaging of TCNE on Ag(001): An ab inito study, Physical Review B 89, 045405 (2014)
T. Deilmann, P. Krüger, M. Rohlfing, D. Wegner
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Deposition and Self-Assembly of Large Magnetic Molecules, Journal of Physical Chemistry C, 119, 28660 (2015)
J. Donner, J.-P. Broschinski, T. Glaser, and D. Wegner
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Visualizing improved spin coupling in molecular magnets
J. Donner, J.-P. Broschinski, B. Feldscher, A. Stammler, H. Bögge, T. Glaser, and D. Wegner
