Elettra | ALOISA, NanoESCA From localized states to dispersive bands in hierarchically grown 2D metal-organic frameworks Two-Dimensional Metal-Organic Frameworks (2D-MOFs) are atomically thin materials that combine the structural tunability of molecular systems with the long-range order characteristic of crystalline solids. The strong bonding between organic linkers and transition metal centers is expected to give rise to delocalized electronic states. However, the microscopic mechanism by which electronic band structures emerge in 2D-MOFs through coupling of the electronic states of the individual units remains largely unexplored. In this work, we demonstrate the on-surface synthesis of a 2D-MOF exhibiting pronounced π-conjugation. By combining experimental and theoretical methods, we provide direct evidence for the formation of electronic band structure during the hierarchical selfassembly process, from isolated metalorganic complexes to a fully conjugated two-dimensional framework. To construct these metal-organic systems on a metal substrate, we deposited 1,2,4,5-tetracyanobenzene (TCNB) and Ni atoms on an Au(111) substrate. Depending on the Ni coverage, two distinct Ni(TCNB)X phases (X = 2, 4) are resolved in STM images (Fig. 1 a). At low Ni concentrations, a Ni(TCNB)4 phase (“Ni-Complex”) dominates, while with increasing Ni content, the Ni(TCNB)2 phase gradually emerges (“Ni-MOF”). In both phases, the Ni centers adopt a square-planar coordination geometry, indicating a directional bond formation between the Ni atoms and the cyano (-CN) groups of the TCNB ligands. To complement the STM characterization, we conducted NEXAFS measurements at the ALOISA beamline to investigate the on-surface orientation of the TCNB constituents. Fig. 1 (b) shows the NEXAFS spectra at the C and N K-edges for TCNB, Ni-Complex, and NiMOF phases on Au(111). At the C K-edge, a feature at ≈ 285 eV (highlighted in orange in Fig. 1 (b) corresponds to π*- resonances localized on the benzene core. These resonances are significantly suppressed in s-polarization and only observed with p-polarized light, confirming that the benzene rings lie flat on the surface. Similarly, at the N K-edge, a π*-resonance at ≈401 eV, associated with the -CN groups, is also only visible in p-polarization (Fig. 1 b), indicating a planar adsorption geometry for the entire TCNB unit. Importantly, the on-surface orientation of the benzene and cyano groups remains unchanged across all phases. Beyond geometrical insight, the NEXAFS data also reveal changes in the electronic structure. As the Ni coverage increases, the relative intensity of the π*-resonances compared to the σ*- resonances decreases. Given that the TCNB surface density is constant in all phases, and the π*-resonances remain unchanged, this reduction cannot be attributed to altered molecular adsorption or substrate charge transfer. Instead, the quenching of π*- resonances is attributed to hybridization of TCNB π*-orbitals with Ni states, suggesting a π-symmetric Ni-TCNB interaction. This hybridization involves both the benzene and cyano moieties, pointing to a delocalized interaction across the ligand. To further investigate the Ni-TCNB interaction, we analyzed angleintegrated valence band photoemission spectra in the range from the Fermi level to a Binding Energy (BE) of 1.8 eV. These experiments were carried out using the momentum microscope available at the NanoESCA beamline. Upon formation of the Ni-Complex, a peak appears at a BE ≈ 1.30 eV, which shifts by about 50 meV towards higher BE in the Ni-MOF. This shift reflects a change in the electronic environment and supports the formation of a hybrid metal-organic state in both phases, with further evolution upon network formation. To interpret these findings, we performed DFT calculations using structural models guided by STM and NEXAFS data. The calculated spinresolved projected Density Of States (pDOS) for Ni 3d and TCNB 2pz orbitals (Fig. 1 c) shows that in both Ni-Complex and Ni-MOF systems, new states with strong Ni 3d-character appear at BE ≈ 1.5 eV, between the HOMO and LUMO of isolated TCNB. These states are accompanied by a substantial TCNB 2pz contribution, confirming orbital hybridization between Ni and the ligand. A detailed orbital analysis reveals that the dominant hybridization occurs between TCNB 2pz and Ni 3dxz/yz orbitals. Additionally, a clear energy broadening of these states is observed when transitioning from isolated complexes to the extended MOF, characteristic of the emergence of delocalized electronic states. We measured complete momentum maps for k|| < 2.20 Å-1 over the BE range of interest, resulting in a three-dimensional (3D) I (kx, ky, BE) data set. Experimental and calculated band maps along the Γ - M directions of the Au(111) substrate confirm the formation of extended states (Fig. 1 d). The Ni-Complex shows a discrete, localized state (marked with a star), whereas the Ni-MOF exhibits a dispersive feature (indicated by arrows), visible in both measured and simulated data. In conclusion, we have characterized the electronic structure characteristic of the coordinative interaction between a transition metal center and its organic linker with ultimate precision, and further examined to what extent it is affected by structural adaptations within metal-organic nanostructures. At low Ni coverage, a periodic layer of electronically isolated Ni(TCNB)4 complexes forms on Au(111). As the Ni coverage increases, an extended Ni(TCNB)2 metal-organic framework emerges. Notably, in the Ni(TCNB)2 MOF, π-delocalization across the network leads to the formation of dispersive electronic features. These findings demonstrate that controlled on-surface synthesis allows tuning of the electronic structure in 2D-MOFs, offering a pathway to design materials with tailored band structures for applications in molecular electronics and optoelectronics. Elettra Highlights 2024–2025 104
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