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.

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.

Circular dichroism in the angular distribution (CDAD) is the effect that the angular intensity distribution of photoemitted electrons depends on the handedness of the incident circularly polarized light. The origin of CDAD can be manifold, including intrinsic properties of the system under study, such as chirality, spin-orbit interaction, or quantum-geometrical properties, but CDAD can also originate from final-state effects influenced by the experimental geometry. For example, CDAD has been reported for achiral organic molecules at the interface to metallic substrates. For this latter case, we investigate two prototypical π-conjugated molecules, namely, tetracene and pentacene, whose frontier orbitals have a similar shape but exhibit distinctly different symmetries. By comparing experimental CDAD momentum maps with simulations within time-dependent density functional theory, we show how the final state of the photoelectron must be regarded as the source of the CDAD in such otherwise achiral and quantum-geometrically trivial systems. We gain additional insight into the mechanism by employing a simple scattering model for the final state, which allows us to decompose the CDAD signal into partial wave contributions.

We present μ-ARPES spectra of the Mott-insulating van der Waals antiferromagnet NiPS_3. Signatures of strong correlations – such as the onset of atomic or atomic-ligand multiplets and spin-orbit-entangled exciton have been observed in this material by various two-particle spectroscopies, but not previously in photoemission. Our measurements reveal a weakly dispersive feature at the valence-band edge that is absent in DFT+U calculations and remains unchanged across the Néel transition. After critically examining and ruling out alternative interpretations, we show that an exact diagonalization of a NiS_6 cluster yields low-energy final-state configurations of mixed multiplet d^7 and d^8L character, whose energy differences are consistent with the observed additional feature. This implies that ARPES directly accesses local Ni-S multiplet physics in NiPS_3, revealing a many-body structure beyond mean-field theory. Our results confirm that NiPS_3 is an excellent model platform in which strong correlations, reduced dimensionality, and covalent metal-ligand bonding jointly shape both two- and single-particle spectroscopies, underscoring the need for a genuinely quantum many-body description of two-dimensional quantum materials.

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 O2 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.

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.

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 L3,2-edges spectroscopic fingerprints for nearly planar (Co2+, S = 1/2) and distorted tetrahedral (Co2+, 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.

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 TN. It is attributed to bands of mixed Ni and S character related to the superexchange interaction of the Ni 3t2g 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.

Magnetic 2D materials enable novel tuning options of magnetism. As an example, the van der Waals material FePS3, a zigzag-type intralayer antiferromagnet, exhibits very strong magnetoelastic coupling due to the different bond lengths along different ferromagnetic and antiferromagnetic coupling directions enabling elastic tuning of magnetic properties. The likely cause of the length change is the intricate competition between direct exchange of the Fe atoms and superexchange via the S and P atoms. To elucidate this interplay, we study the band structure of exfoliated FePS3 by mu m scale ARPES (Angular Resolved Photoelectron Spectroscopy), both, above and, for the first time, below the Neel temperature TN. We find three characteristic changes across TN. They involve S 3p-type bands, Fe 3d-type bands and P 3p-type bands, respectively, as attributed by comparison with density functional theory calculations (DFT+U). This highlights the involvement of all the atoms in the magnetic phase transition providing independent evidence for the intricate exchange paths.

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.

Chemical vapor deposition (CVD) stands as one of the most effective methods for obtaining large-area, highquality graphene sheets. While extensive research has focused on hexagonally symmetric substrates, there is a striking scarcity in the literature addressing graphene growth on quadratic surfaces of ferromagnetic transition metals. In this study, we devised a growth pathway of monolayer graphene on a thin Co(100) film and unraveled the electronic and magnetic properties using a combination of surface-sensitive spectro-microscopy techniques. X-ray photoemission (XPS) and angle-resolved photoemission spectroscopy (ARPES) reveal the formation of a free-standing graphene layer decoupled from the cobalt support due to the presence of a metal-carbide interlayer. Notably, high-temperature annealing transforms this decoupled graphene into a strongly interacting system, with the recrystallized cobalt assuming a hexagonal symmetry. Finally, oxygen intercalation at the Gr/C/Co(100) interface leads to the removal of the carbidic layer, forms an oxygen interlayer, which induces p-doping in the graphene layer and decouples it from the substrate.

Recent years have witnessed a steady progress towards blending two-dimensional quantum materials into technology, with future applications often rooted in the electronic structure. Since crossings and inversions of electronic bands with different orbital characters determine intrinsic quantum transport properties, knowledge of the orbital character is essential. Here, we benchmark angle-resolved photoelectron emission spectroscopy (ARPES) as a tool to experimentally derive orbital characters. For this purpose we study the valence electronic structure of two technologically relevant quantum materials, graphene and WSe2, and focus on circular dichroism that is believed to provide sensitivity to the orbital angular momentum. We analyze the contributions related to angular atomic photoionization profiles, interatomic interference, and multiple scattering. Regimes in which initial-state properties could be disentangled from the ARPES maps are critically discussed and the potential of using circular dichroic ARPES as a tool to investigate the spin polarization of initial bands is explored. For the purpose of generalization, results from two additional materials, GdMn6Sn6 and PtTe2, are presented in addition. This research demonstrates rich complexity of the underlying physics of circular dichroic ARPES, providing insights that will shape the interpretation of both past and future circular-dichroic ARPES studies.

The Frontier electronic structure of tetraphenylporphyrinato (TPP2-) and phthalocyaninato (Pc2-) square planar transition metal complexes (MTPP and MPc; M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) has been revisited through DFT calculations. The different ground state symmetry and spin multiplicity between MPc and MTPP of the same M is shown to originate from the different Pc2- and TPP2- ligand field, stronger in the former ligand than in the latter. The corresponding spatial localization and symmetry of the unoccupied molecular orbitals postulate unescapable geometric constraints to their overlap with the electron cloud of a crystalline metal surface. From comparison with literature experimental evidence, we show that the adsorption geometry (atomic site and azimuthal orientation) of MTPPs and MPcs on the low index crystal planes of coinage metals (CM = Au, Ag, Cu) may be predicted when two conditions are satisfied: (i) evidence of a surface → adsorbate charge transfer, (ii) absence of significant distortion of the macrocycle upon adsorption. In this regard, the overall susceptibility to charge transfer is determined by the strength of the molecular ligand field (i.e., charge transfer to MPc is more favoured than to MTPP) and inversely linked to the electronegativity of the surface atoms (being Au the most inert CM substrate thanks to its highest electronegativity).

Understanding chemical bonding at molecule–metal interfaces is essential for advancing applications in catalysis, spintronics, and organic electronics. While the Newns–Anderson and d ‐band models have provided key insights, their applicability remains limited in systems involving large organic adsorbates and correlated metallic substrates. This work investigates the interaction between pentacene (5A) and an oxygen‐passivated Fe(100) surface (Fe–O), where oxygen chemisorption gives rise to strong electronic correlations. A combination of photoemission orbital tomography, scanning tunneling spectroscopy, and electronic structure calculations reveals pronounced hybridization between 5A frontier orbitals and the Fe d ‐states. A tailored DFT+U approach with a negative effective on‐site interaction ( U eff = −3.1 eV) captures the experimentally observed reduction in d ‐band spin splitting and narrowing, consistent with dynamical mean‐field theory. These correlation‐induced modifications enhance the energetic overlap between metal d ‐states and molecular orbitals, driving a transition from physisorption to strong chemisorption. Building on these insights, the d ‐band model is extended to include spatially modulated adsorbate–substrate coupling, successfully reproducing the experimentally observed orbital substructures. These findings offer a tractable route for incorporating many‐body effects into simplified chemisorption models, enabling predictive insights into molecule–metal bonding at correlated surfaces and guiding the design of 3 d ‐metal catalysts and organic spintronic interfaces.

Two-dimensional bismuth (Bi) is a promising platform for quantum and energy technologies due to strong spin-orbit coupling, high thermoelectric efficiency, and magnetoresistance. However, scalable and flexible synthesis of high-quality Bi with fast research turnaround remains challenging. We report a controlled colloidal synthesis of Bi nanosheets with tunable lateral sizes (0.6 - 4.1 um), hexagonal shape, and a layered single-crystalline structure along the 00l planes. The nanosheets exhibit excellent oxidation resistance and ambient stability. ARPES measurements on individual nanosheets reveal a band structure in excellent agreement with DFT calculations, confirming high crystal quality and uniformity. Our findings enable fast production and characterization of two-dimensional Bi, paving the way for fundamental studies and integration into next-generation quantum and energy devices.

Two-dimensional metal-organic frameworks (2D MOFs) are atomically thin materials that combine the properties of organic molecules with the structural characteristics of crystalline inorganic solids. Their unique magnetic and electronic properties...

The on-surface coordination of transition metals with pi-extended organic molecules offers a versatile strategy to tailor electronic states in low-dimensional systems. However, the relationship between structural order and coordination-mediated electronic properties remains underexplored. Here, we investigate the self-assembly of two nitrile-functionalized polyphenyl ligands, [1,1 ':4 ',1 ''-terphenyl]-4,4 ''-dicarbonitrile and biphenyl-4,4 '-dicarbonitrile, on Ag(111), and examine the effects of subsequent Ni coordination. Low-energy electron diffraction confirms the formation of ordered self-assembled monolayers prior to metal coordination, while valence band spectroscopy indicates weak interaction with the Ag(111) substrate. Upon Ni coordination, the long-range order is disrupted, and new electronic states appear in the valence band region. Momentum-resolved photoemission reveals that these coordination-induced states exhibit weak dispersive character, consistent with metal-ligand orbital coupling and indicative of electronic delocalization. These findings demonstrate that coordination can induce robust electronic functionality even in structurally disordered two-dimensional metal-organic architectures.

Chirality is ubiquitous in nature and manifests in a wide range of phenomena including chemical reactions, biological processes, and quantum transport of electrons. In quantum materials, the chirality of fermions, given by the relative directions between the electron spin and momentum, is connected to the band topology of electronic states. This study shows that in structurally chiral materials like CoSi, the orbital angular momentum (OAM) serves as the main driver of a nontrivial band topology in this new class of unconventional topological semimetals, even when spin-orbit coupling is negligible. A nontrivial orbital-momentum locking of multifold chiral fermions in the bulk leads to a pronounced OAM texture of the helicoid Fermi arcs at the surface. The study highlights the pivotal role of the orbital degree of freedom for the chirality and topology of electron states, in general, and paves the way towards the application of topological chiral semimetals in orbitronic devices.

2D metal-organic frameworks (2D MOFs) attract considerable attention because of their versatile properties and as potential candidates for single-atom catalysis, high-density information storage media or molecular electronics and spintronics devices. Their unique characteristics arise from an intricate interplay between the metal center, the surrounding ligands and the underlying substrate. Here, the intrinsic magnetic and electronic properties of a single-layer MOF on graphene is investigated with a combination of spectroscopic techniques and theoretical modeling. Taking advantage of the weak interaction between the MOF and graphene substrate, it is specifically focused on the influence of the coordination environment on these properties. Notably, two distinct coordination configurations are observed for the transition metal centers within the 2D MOF, and clarify how axial distortions in the ligand field affect the hybridization between the Ni 3d states and the pi-symmetric molecular orbitals of 7,7,8,8-tetracyanoquinodimethane ligands, leading to the coexistence of two Ni redox states with different spin configurations. Furthermore, the transition from a nearly free-standing MOF is examined to metal-supported frameworks, elucidating the impact of substrate interactions on the electronic and magnetic properties. The findings advance the understanding of MOFs and offer insights into developing functional materials with tailored magnetic and electronic properties.