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The high reactivity of magnetic substrates toward molecular overlayers has so far inhibited the realization of more sophisticated on-surface reactions, thereby depriving these interfaces of a significant class of chemically tailored organics such as graphene nanoribbons (GNRs), oligonuclear spin-chains, and metal−organic networks. In the first part of this talk, I will present a multi-technique characterization of the polymerization of 4,4″-dibromo-p-terphenyl precursors into ordered poly(p-phenylene)[1] arrays on top of an emerging class of two-dimensional magnetic surface alloys[2,3]. The chemical reaction was followed by temperature-dependent X-ray photoelectron spectroscopy (XPS) and the structural characterizations of supramolecular and polymeric phases were performed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Taking advantage of the high homogeneity of the samples, the electronic structure of the valence band was determined with angle-resolved photoemission spectroscopy (ARPES). Moreover, ferromagnetic ordering in the two-dimensional GdAu2 alloy was demonstrated for all phases by X-ray absorption spectroscopy (XAS)[4]. In the second part, I will briefly show that GdAu2 is not only a suitable template for the surface-confined Ullmann-like coupling reaction but is also thermally sufficiently stable to sustain a cyclodehydrogenation reaction for the on-surface synthesis of more complex GNRs. Thus, the system is demonstrated to close a longstanding experimental gap in the study of GNRs as it enables spin-polarized STM characterizations[5] of e.g. the spin-polarization of the elusive GNR edge-states, which were theoretically predicted for chiral GNRs[6] and experimentally, albeit lacking spin-sensitivity, observed on Au(111)[7]. However, successive theoretical studies questioned [8] and refined [9], respectively, the experimental prerequisites under which magnetic ordering of edge-states in GNRs may be observed, thereby highlighting the need for direct experimental access of the spin-polarization of these states. References: [1] Vasseur, G.; Abadía, M.; Miccio, L. A.; Brede, J.; Garcia-Lekue, A.; de Oteyza, D. G.; Rogero, C.; Lobo-Checa, J. & Ortega, J. E. Pi Band Dispersion along Conjugated Organic Nanowires Synthesized on a Metal Oxide Semiconductor J. Am. Chem. Soc. 138, 5685-5692 (2016) [2] Corso, M.; Fernández, L.; Schiller, F. & Ortega, J. E. Au(111)-Based Nanotemplates by Gd Alloying ACS Nano 4, 1603-1611 (2004) [3] Ormaza, M.; Fernández, L.; Lafuente, S.; Corso, M.; Schiller, F.; Xu, B.; Diakhate, M.; Verstraete, M. J. & Ortega, J. E. LaAu2 and CeAu2 surface intermetallic compounds grown by high-temperature deposition on Au(111) Phys. Rev. B 88, 125405 (2013) [4] Abadía, M.; Ilyn, M.; Piquero-Zulaica, I.; Gargiani, P.; Rogero, C.; Ortega, J. E. & Brede, J. Polymerization of Well-Aligned Organic Nanowires on a Ferromagnetic Rare-Earth Surface Alloy, ACS Nano 11, 12392-12401 (2017) [5] Brede, J. & Wiesendanger, R. Spin-resolved imaging and spectroscopy of individual molecules with sub-molecular spatial resolution MRS Bulletin 39, 608-613 (2014) [6] Yazyev, O. V.; Capaz, R. B. & Louie, S. G. Theory of magnetic edge states in chiral graphene nanoribbons Phys. Rev. B 84, 115406 (2011) [7] Tao, C.; Jiao, L.; Yazyev, O. V.; Chen, Y.-C.; Feng, J.; Zhang, X.; Capaz, R. B.; Tour, J. M.; Zettl, A.; Louie, S. G.; Dai, H. & Crommie, M. F. Spatially resolving edge states of chiral graphene nanoribbons, Nature Physics 7, 616-620 (2011) [8] Kunstmann, J.; Özdoğan, C.; Quandt, A. & Fehske, H. Stability of edge states and edge magnetism in graphene nanoribbons Phys. Rev. B, 83, 045414 (2011) [9] Golor, M.; Lang, T. C. & Wessel, S. Quantum Monte Carlo studies of edge magnetism in chiral graphene nanoribbons Phys. Rev. B, 87, 155441 (2013)

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Jens Brede , Centro de Física de Materiales, UPV (Spain)
Seminar room Institute of Physics 2
Contact: N. Atodiresei