Metal–organic coordination structures are materials comprising reticular metal centers and organic linkers in which the two constituents bind with each other via metal–ligand coordination interaction. 2D metal-organic (or porous coordination) frameworks have attracted tremendous attention in the last two decades owing to their unique electronic, topological, magnetic properties. These properties of 2D metal-organic frameworks suggest many potential applications of the materials, for instance, they might be promising candidates to build functional molecular devices. However, the fabrication of single layers of 2D metal-organic frameworks remains a great challenge. In this regard, synthesizing conjugated organic monolayer with unique properties is highly desirable. This thesis foc...[ Read more ]

Metal–organic coordination structures are materials comprising reticular metal centers and organic linkers in which the two constituents bind with each other via metal–ligand coordination interaction. 2D metal-organic (or porous coordination) frameworks have attracted tremendous attention in the last two decades owing to their unique electronic, topological, magnetic properties. These properties of 2D metal-organic frameworks suggest many potential applications of the materials, for instance, they might be promising candidates to build functional molecular devices. However, the fabrication of single layers of 2D metal-organic frameworks remains a great challenge. In this regard, synthesizing conjugated organic monolayer with unique properties is highly desirable. This thesis focuses on the fabrication of 2D metal–organic coordination structures through on-surface synthesis on Au(111), Cu(111) and Ag(111) substrates. We used scanning tunneling microscopy (STM) as an experimental tool and density-functional theory (DFT) as a theoretical tool to characterize the electronic, topological, magnetic properties of the networks at a single-molecular level.

This thesis consists of four projects as below:

In the first project, we synthesize single-layer Ni₃(HITP)₂ on a Au(111) substrate. We resolve its structure at sub-molecular resolution using STM. The DFT calculations show that upon adsorption on Au(111), the single-layer Ni₃(HITP)₂ interacts weakly with the substrate and retains its planar structure. Interestingly, the non-trivial topological gap of the free-standing layer is preserved in the surface-adsorbed layer. These results demonstrate that on-surface self-assembly is a viable route to realize 2D-MOFs exhibiting exotic quantum phases.

In the second project, we design and synthesize a two-dimensional metal-organic network [Fe₃(HITP)₂], which comprises a Kagome lattice of Fe atoms. DFT calculation indicates that there is a ferromagnetic ground state and a non-trivial 15 meV gap between the Dirac bands and the flat band. Experimentally, we synthesize this structure on a Au(111) surface. We study this structure at a single-molecule resolution and confirm that the on-surface structure is nearly identical to the free-standing framework. We also use scanning tunneling spectroscopy (STS) to reveal the presence of a magnetic moment on Fe atoms in the framework.

In the third project, we study the coordination behavior of Ni, Pt, Pd metal with 2,3,6,7,10,11-Hexaaminotriphenylene (HATP) molecules on a Ag(111) surface. We confirm that the coordination reaction can happen with all three metals and also resolve the metal-organic coordination structures at an atomic resolution.

In the last project, we synthesize single-layer 2D-MOF structures of Ni₃(HAT)₂ and Fe₃(HAT)₂ networks on a Ag(111) surface, and Cu₃(HAT)₂ networks on a Cu(111) surface. The high resolution STM images show that the Ni₃(HAT)₂ and Fe₃(HAT)₂ networks are not flat but titled out of plane on the Ag(111) surface, while the Cu₃(HAT)₂ network can grow into larger and flat networks on the Cu(111) surface. Moreover, we use STS to study the coordinated Fe in Fe₃(HAT)₂ coordination network. The double-step structure in the STS spectra indicates that the coordinated Fe atoms undergo spin-flip processs.

In summary, we design and synthesize five 2D metal-organic frameworks which comprises a Kagome lattice of coordinated metal atoms. These studies may contribute to the development of low-dimensional conjugated metal-organic materials.