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 Ni
3(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 Ni
3(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
[Fe
3(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 Ni
3(HAT)
2 and
Fe
3(HAT)
2 networks on a Ag(111) surface, and Cu
3(HAT)
2 networks on a Cu(111) surface. The
high resolution STM images show that the Ni
3(HAT)
2 and Fe
3(HAT)
2 networks are not flat but
titled out of plane on the Ag(111) surface, while the Cu
3(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
Fe
3(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.
Post a Comment