Variable π–d Orbital Hybridization in 2D Transition Metal–Organic Frameworks

Abstract

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 M3(HAT)2 (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) characterizations. 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 for band structure engineering in 2D MOFs, offering a rational pathway to design 2D framework materials with tailored electronic, magnetic, and catalytic properties.