Variable π-d orbital hybridization in 2D transition metal-organic frameworks.

Two-dimensional metal-organic frameworks (2D MOFs) have emerged as promising platforms for exploring novel quantum phenomena and tunable electronic functionalities. Here, we investigate π-d orbital hybridization in monolayer M(HAT) (M = Ni, Co, Fe; HAT = 1,4,5,8,9,12-hexaazatriphenylene) frameworks by combining density functional theory (DFT) calculations and scanning tunneling microscopy/spectroscopy (STM/STS) characterization. Despite identical lattice geometries, the Ni-HAT framework exhibits a dispersive, gapless band structure, while the Co- and Fe-HAT frameworks display localized electronic states and semiconducting bandgaps. Projected density of states (PDOS) analysis attributes these differences to different degrees of π-d orbital hybridization involving out-of-plane orbitals between the metal nodes and HAT ligands. STM confirms the formation of isostructural honeycomb-kagome lattices synthesized on Au(111) surfaces, and STS measurements validate their distinct electronic behaviors. Our findings highlight the critical role of π-d coupling in band structure engineering in 2D MOFs, offering a rational pathway to design 2D framework materials with tailored electronic, magnetic, and catalytic properties.