Abstract
Two-dimensional metal-organic frameworks (MOFs) refer to a class of porous materials composed of metal ions coordinated to organic ligands. MOFs can be designed with peculiar physical, chemical, and structural properties by taking advantage of their tunability. These properties can be applied in a wide range of fields, including energy storage, photocatalysis, heterogeneous catalysis, sensing and proton conductors, and optical and electronic devices. In this thesis, I focus on several metal-organic structures that are constructed on both metal and Van der Waals substrates. On metal substrates, the local electronic and magnetic properties of 2D MOFs are studied by STM and STS. On HOPG substrate, the 2D MOFs are effectively decoupled and the intrinsic electronic properties corroborate freestanding DFT calculated PDOS. This fundamental study paves the way for future applications of metal-organic materials.
This thesis consists of three projects as below:
In the first project, I synthesize and characterize two new 2D-cMOFs, namely ð‘ð‘–3(ðµð»ð‘‚−6ð») and Co-BHO using benzene-1,2,3,4,5,6-hexaol (BHO) molecules as organic ligands. In ð‘ð‘–3(ðµð»ð‘‚−6ð»), each Ni atom is coordinated with four oxygen atoms, and each oxygen atom binds with two Ni atoms, comprising a Kagome lattice of Ni atoms. The Co-BHO system exhibits long-range lattice periodicity but displays disordered electronic states and magnetic features.
In the second project, I design and synthesize a series of 2D-MOFs ð‘€3(ð»ð´ð‘‡)2 (M= Fe, Co, Ni, Cu), on the Au (111) and Ag (111) surface. These MOFs are all constructed by a honeycomb lattice of HAT molecules and a Kagome lattice of M atoms. The symmetric double-step like features observed in the vicinity of the Fermi level in ð‘€3(ð»ð´ð‘‡)2 (M= Fe, Co, Ni) are believed to originate from spin excitations resulting from spin-spin coupling. This work is, to our knowledge, the first experimental demonstration of spin excitations due to spin-spin coupling in 2D MOFs systems.
In the third project, I synthesize and characterize ð‘ð‘–3(ð»ð´ð‘‡)2 and ð¶ð‘œ3(ð»ð´ð‘‡)2 MOFs on both Au (111) and HOPG substrates. Their structures are nearly identical, indicating the synthesis of ð‘€3(ð»ð´ð‘‡)2 does not rely on the substrate. Meanwhile, the interaction between the MOFs with HOPG is much weaker than with Au (111), resulting in effective electronic decoupling of the MOFs from HOPG. In addition, the STS data confirm that ð‘ð‘–3(ð»ð´ð‘‡)2 is a narrow bandgap MOF with bandgap below 0.2V, and ð¶ð‘œ3(ð»ð´ð‘‡)2 inherently possess an extremely narrow bandgap below 15meV. Both systems have great potential to be used as highly conductive 2D materials. This work is, to our knowledge, the first experimental demonstration of 2D MOFs directly grown on bulk Van der Waals substrate.
In summary, I synthesized and characterized the structural, electronic, and magnetic properties of two series of 2D MOFs on both metal and HOPG substrates. These studies may help researchers investigate coupled magnetism in complex 2D systems and further study the intrinsic properties of 2D MOFs on Van der Waals substrates.