Daniel Baranowski

ZUSAMMENFASSUNG: Die Stabilisierung von nullwertigen Ni(0)‐Zentren in ausgedehnten Koordinationsnetzwerken bleibt aufgrund ihrer ausgeprägten Reaktivität und Anfälligkeit für Oxidation eine große Herausforderung. Wir demonstrieren, dass die Stabilisierung von Ni(0)‐Atomen auf Ag(100) durch eine Eingrenzung auf eine lineare Koordinationsumgebung innerhalb einer oberflächengestützten metallorganischen Kette ermöglicht wird. Die Symmetrie von 9,10‐Dicyanoanthracen (DCA), kombiniert mit durch das Substrat induzierten Templateeffekten, steuert die Bildung eines geordneten linearen Ni‐DCA‐Koordinationsmotivs. Spektroskopische Messungen liefern eindeutige Belege für einen formalen Ni(0)‐Oxidationszustand und zeigen die seltene Realisierung eines nullwertigen Übergangsmetallzentrums innerhalb eines ausgedehnten, atomar definierten Systems. Die elektronische Struktur wird durch eine Hybridisierung zwischen den Ni 3d‐Orbitalen und dem π‐System von DCA bestimmt, was zu einer anisotropen elektronischen Dispersion mit überwiegend eindimensionalem Charakter führt. Diese Ergebnisse etablieren die lineare Koordination als Strategie zur Stabilisierung reaktiver, niedervalenter Metallzentren in oberflächengestützten metallorganischen Architekturen.

The stabilization of zero‐valent nickel Ni(0) centers in extended coordination networks remains a major challenge due to their pronounced reactivity and susceptibility to oxidation. Here, we demonstrate that linear coordination confinement within a surface‐supported metal–organic chain enables the stabilization of Ni(0) atoms on Ag(100). The symmetry of 9,10‐dicyanoanthracene (DCA), combined with substrate templating effects, directs the formation of an ordered linear Ni‐DCA coordination motif. Spectroscopic measurements provide unambiguous evidence for a formal Ni(0) oxidation state, representing a rare realization of a zero‐valent transition metal center within an extended, atomically defined system. The electronic structure is governed by hybridization between Ni 3d orbitals and the π‐system of DCA, resulting in an anisotropic electronic dispersion with predominantly one‐dimensional character. These results establish linear coordination as a strategy to stabilize reactive, low‐valent metal centers in surface‐supported metal–organic architectures.

The properties of single-atom catalysts are governed by their local environment, motivating efforts to control catalytic performance by tailoring their coordination sphere. In this context, we address the critical role of single ligands bound to individual catalytic centers in controlling their selectivity. Using temperature-programmed desorption and X-ray photoelectron spectroscopy, we demonstrate that Rh model single-atom catalysts on Fe3O4(001) activate the low-temperature oxidation of ethanol to acetaldehyde. However, the subsequent decomposition of acetaldehyde to methane and carbon monoxide can further occur. The formed carbon monoxide remains strongly bound to the Rh center, while the ligand regulates the Rh sites, ultimately catalyzing the selective oxidation of ethanol to acetaldehyde. This behavior can be reproduced by exposing the model system to carbon monoxide, which highlights that single-atom catalysts are truly dynamic in their response to the reaction conditions. Their performance may be impacted by the co-adsorption of ligands that strongly coordinate to the functional catalytic centers. These selectivity-regulating ligands may be formed from intermediates or products, be present as impurities, or intentionally added. Our surface science approach allows us to vary the coordination sphere one ligand at a time, introducing the precision characteristic of coordination chemistry into heterogeneous catalysis.

Understanding how charge transfer and ligand activation processes govern metal-organic coordination at surfaces is crucial for controlling on-surface synthesis and the formation of low-dimensional architectures. Here, we show that Ni promotes the deprotonation of the carboxyl groups of terephthalic acid (TPA) on Ag(100), leading to the formation of linear metal-organic coordination chains. Scanning tunneling microscopy reveals that these chains emerge from preassembled hydrogen-bonded TPA stripes. X-ray photoelectron spectroscopy identifies stabilization of the Ni centers in the Ni(I) oxidation state through a single-electron charge transfer process, accompanied by the formation of deprotonated carboxylate species. Valence band spectroscopy reveals a coordination-induced electronic reorganization between Ni and TPA through the emergence of hybrid states, in agreement with complementary theoretical modeling. Together, these findings identify charge-transfer-driven deprotonation as the central mechanism governing the formation of linear metal-organic chains.

A detailed analysis is presented of the X-ray Photoelectron Spectroscopy (XPS) of thin films of three azines: pyrazine, pyridine, and pyrimidine. This includes not only the binding energies of the various core ionizations but also their intensities. A major focus is to compare our theoretical predictions with our measured XPS for N(1s) and C(1s) as a basis for assigning the features and for justifying the broadening parameters that must be applied to the theoretical results. The C(1s) XPS of pyridine and pyrimidine are significantly broadened because of unresolved XPS for their inequivalent C atoms. The extent of the binding energy (BE) shifts and the XPS intensities for the unique C atoms, which are responsible for this broadening, are obtained from the theory. The additional broadening parameters to be applied to the theory to enable comparison with measured XPS are discussed in terms of lifetime broadening, experimental resolution, and BE shifts in different layers of the thin film. A novel feature of our analysis is that we have investigated and justified the broadenings necessary to make of the calculated BEs in order to fit the observed XPS spectra. The results presented have general value for extracting chemical and physical information from XPS.

Atomic‐scale control of oxide–ferromagnet interfaces is crucial for optimizing spintronic heterostructures, yet interfacial oxygen remains difficult to control and verify. Here, we deterministically tune the prototypical MgO/Fe(100) interface from oxygen‐free terminations to fully intercalated oxygen layers by reactive growth under controlled O ₂ exposure, while preserving epitaxy. Momentum‐resolved photoemission identifies oxygen‐dependent fingerprints in k ‐space that originate from the buried interface and persist up to a thickness of 8 layers of MgO. Insights from complementary spectroscopic methods link these k ‐space signatures to interfacial chemistry, structural order, work‐function shifts, and an oxygen‐induced interface resonance within the MgO gap that alters the tunneling response. The combined results define a calibrated growth protocol that allows reproducibly preparing and identifying three distinct terminations — oxygen‐free, partially oxidized, and oxygen‐intercalated — and enables post‐growth conversion even in thicker films. Complementary spin‐resolved experiments reveal that oxygen‐free interfaces exhibit pronounced suppression of minority‐spin spectral weight at the Fermi level, consistent with coherent spin filtering across crystalline MgO, whereas oxygen intercalation reduces the spin contrast at E _F . By turning interfacial oxygen from an uncontrolled variable into a measurable, adjustable parameter, our approach establishes MgO/Fe(100) as a benchmark platform for optimizing spintronic functionality in oxide/metal junctions.

NiPS₃ is an exfoliable van der Waals intralayer antiferromagnet with a zigzag-type spin arrangement. It is distinct from other TMPS₃ (TM: transition metal) materials by optical excitations into a strongly correlated state that is tied to the magnetic properties. However, the related fundamental band structure across the antiferromagnetic phase transition has not been probed yet. Here, we use angular-resolved photoelectron spectroscopy with μm resolution in combination with DFT + U calculations for that purpose. We identify a characteristic band shift across the T_N. It is attributed to bands of mixed Ni and S character related to the superexchange interaction of the Ni 3t_2g orbitals. Moreover, we find a structure above the valence band maximum with little angular dispersion that could not be reproduced by the calculations. The discrepancy suggests the influence of many-body interactions beyond the DFT + U approximations, in striking contrast to the results on MnPS₃ and FePS₃, where these calculations were sufficient for an adequate description.

Determining the local geometry of metal‐organic architecture on substrates is challenging, as substrate interactions can alter the metal coordination relative to the free‐standing structure. Here, combining density functional theory (DFT) and restricted open‐shell configuration interaction with singles (ROCIS) calculations on isolated cobalt‐7,7,8,8‐tetracyanoquinodimethane (Co‐TCNQ) complexes, together with X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD), directly reveals the coordination motifs of Co centers in a 2D Co‐TCNQ framework on graphene. The calculated Co L _3,2 ‐edges spectroscopic fingerprints for nearly planar (Co ²⁺ , S = 1/2) and distorted tetrahedral (Co ²⁺ , S = 3/2) structures exhibit distinct features, allowing unambiguous assignment of spin and oxidation states of the metal centers, as well as confirmation of the local geometry. Comparison with experimental spectra confirms that the high‐spin tetrahedral geometry is realized in the supported framework, demonstrating how spectroscopic fingerprints can directly link coordination geometry to spin and oxidation states in low‐dimensional metal‐organic systems.

Structurally defined interfaces between magnetic substrates and molecular layers, so-called spinterfaces, represent a critical frontier in the design of spin-functional devices. Here, we show that long-range molecular order enables coherent Umklapp scattering of spin-polarized substrate electrons, modifying the spin-resolved electronic structure at the interface. Using spin-resolved momentum microscopy and photoemission tomography, we compare iron (FePc) and metal-free phthalocyanine (H₂Pc) monolayers assembled on an oxygen-passivated iron surface. We find that both molecular lattices give rise to distinct Umklapp replicas of the substrate valence bands. By selectively probing the momentum space, we demonstrate that spin polarization near normal emission is predominantly governed by scattering rather than direct contributions from possibly spin-polarized molecular orbitals. This work underscores the intricate relationship between structural order and spin functionality, providing valuable insights into engineering spinterfaces.