Elettra | NanoESCA, ALOISA Dual charge transfer process stabilizes the electronic properties of a 2D metal-organic framework The development of functional materials is in high demand across a wide range of applications and, consequently, constitutes a highly active research field. A particularly intriguing class of materials is that of two-dimensional metal-organic frameworks (2D MOFs), extended regular networks constituted by alternating molecular coordinating ligands and metal centers, deposited on supporting substrates. The wide variety of ligandmetal combinations, each with distinct characteristics, allows for significant control over the material properties, resulting in a high application versatility. 2D MOFs are typically formed via a bottom-up approach. Ligands are deposited first, followed by transition metal (TM) atoms, which bind to the molecular functional groups, rearranging the structure in the final MOF product. The process often involves transitions between multiple structural arrangements, leading eventually to the coexistence of different phases in largescale preparations. Since these phases could differently influence the global properties of the system and hinder consistent performances, a detailed investigation of each phase behavior is fundamental for real-world applications. A representative model system is the combination of 7,7,8,8-tetracyanoquinoimethane (TCNQ) and nickel (Ni) deposited on Ag(100). TCNQ can form up to four bonds with surrounding Ni atoms through its cyano (-CN) end groups. Depending on the Ni coverage of the surface, different TCNQ saturation degrees can be achieved, resulting in multiple MOF phases. These phases share the same local coordination environment at the Ni sites but differ in TCNQ orientation and saturation, leading to diverse arrangements. We focus on three such phases: the self-assembled monolayer (SAM), the low-coverage (LC) and the high-coverage (HC) phases. The SAM is composed solely of TCNQ molecules, arranged on the substrate through weak intermolecular interactions and substrate templating effects. In the LC phase (Fig. 1 a, left), the Ni coverage is sufficient to bind only two out of the four CN groups of each TCNQ molecule in a trans-pattern, reaching half-saturation. In contrast, in the HC phase (Fig. 1 a, right) all four CN groups are bound, and full saturation is reached. These phases were characterized via Near-Edge X-ray Fine Structure (NEXAFS) spectroscopy across the Ni L3,2- and N K-edges. These experiments were performed at the ALOISA beamline. The Ni L3,2-edges spectra (Fig. 1 b) exhibit peaks centered at 852.3 and 869.5 eV, corresponding to the L3 and L2 edges, respectively. Their energy positions and accompanying satellite features are consistent with a Ni oxidation state of +1 in both MOF phases, in agreement with similar systems. The N K-edge spectra (Fig. 1 c) display common features across all phases. Notably, the absence of signal at 397.5 eV, associated with the lowest unoccupied molecular orbital (LUMO) of TCNQ, implies that this orbital is filled in all phases, including the SAM. This finding indicates a significant substratemediated charge transfer to TCNQ, even in the absence of metal coordination, favored by the high electron affinity of TCNQ. Two additional peaks, centered at 399.2 and 400.0 eV (labeled 2a and 2b), are attributed to π-symmetric molecular orbitals oriented perpendicular to the CN groups. Their invariant energy positions at increasing Ni coverages suggest that Ni-induced charge transfer, following Ni oxidation to +1, does not populate these orbitals. Since direct charge transfer from Ni to the substrate is forbidden for symmetry reasons, the charge is likely directed again into the TCNQ LUMO, supporting the coexistence of distinct yet balanced charge transfer pathways. Valence band (VB) behavior was further analyzed using the momentum microscope available at the NanoESCA beamline. The resulting VB profiles (Fig. 1 d) confirm once more the filling of LUMO (labeled “L”) across all phases. Notably, new states with π-symmetry character, labeled mixed states (m), arise in the MOF phases, but are absent in the SAM. Simulated density of states (DOS) profiles (Fig. 1 e) confirm the π-symmetry character of these states, indicating that they originate from the overlap between the highest-occupied molecular orbital (HOMO) and LUMO of TCNQ and the 3dxz/yz states of Ni. This overlap provides evidence for the formation of coordinative bonds between Ni and TCNQ. A comparison between the experimental and simulated momentum maps further confirms this mixed character and supports the formation of networks stabilized by strong lateral interactions between molecules and TM centers. Collectively, the results reveal that the electronic properties remain constant in all phases. This stability is attributed to the coexistence of two charge transfer mechanisms that contribute to the TCNQ LUMO filling: a substratemediated mechanism and a redoxdriven pathway involving Ni oxidation. Consistent with experimental work function measurements performed at the NanoESCA beamline, further simulations reveal that the Ag(100) substrate donates +1.1 e- to the monolayer in the LC phase, but only +0.3 e- in the HC phase, indicating an evident threefold reduction in charge donation. This trend suggests that substrate-to-TCNQ charge transfer is tunable and decreases with increasing Ni coverage. In conclusion, the LUMO filling in the SAM phase is governed by the substrate-to-TCNQ charge transfer from the Ag(100) substrate. Upon Ni coordination and oxidation, an additional charge transfer pathway opens, gradually becoming the dominant contributor in the MOF phases. Thus, the substrate plays a supporting yet crucial role by stabilizing the electronic properties across all phases through a complementary and adaptable charge transfer mechanism. Elettra Highlights 2024–2025 118
RkJQdWJsaXNoZXIy MjQ4NzI=