Abstract
Two-dimensional (2D) conjugated metal-organic frameworks (c-MOFs) that are flexible and feature unique electronic properties are promising candidates for fabricating multifunctional nano-devices. However, synthesizing and characterizing single-layer c-MOFs remains a significant challenge to researchers. This thesis is focused on synthesizing and structural characterizing of 2D c-MOF via an on-surface self-assembly method. Combining scanning tunneling microscopy (STM) with density functional theory (DFT) calculations, I fabricate and study the structures of 1,4,5,8,9,12-hexaazatriphenylene (HAT)-based 2D c-MOFs self-assembled on Au(111), Ag(111), Cu(111) and MoS2 surfaces at the singlemolecule level.
The thesis is composed of three projects as below:
In the first project, I demonstrate an effective route for fabricating 2D M3HAT2 (M=Fe, Ni, and Co) frameworks via an on-surface reaction, while they can’t be synthesized in soxvii lution due to the strong steric hindrance. The frameworks comprise a hexagonal lattice of HAT molecules and a Kagome lattice of metal atoms. It reveals that the frameworks appear non-periodical under specific scanning conditions, which originates from HAT molecules tilted out of the framework plane. The theoretical analyses reveal that Ni, Co and Fe possessing a magnetic moment of 1.1, 2.5, and 3.7 μB, respectively. Considering the neighboring metal atoms are linked by the highly conjugated HAT ligands with a short nearestneighboring distance of 6.9 ËšA, the presence of magnetic moments raises great interest to explore the magnetic phases of the M3HAT2 frameworks in the future.
In the second project, I explore the formation of Cu-HAT coordination structures on Cu(111), Au(111) and MoS2 surfaces. On Cu(111) substrate, (Cu4)3HAT2 coordination framework is formed due to the substrate template effect and excess Cu, which is not observed on Au(111) and MoS2 surfaces. Both annealing treatment and low-flux deposition of HAT molecules supply more Cu atoms, which favors the (Cu4)3HAT2 framework on Cu(111) whose surface atomic lattice matches the framework structure. Cu3HAT2 framework was observed on Au(111), which is explained by the substrate atomic lattice matching the Cu3HAT2 framework. That Cu3HAT2 framework synthesized on a weakly-interacting MoS2 surface indicates the Cu3HAT2 framework is the most energetically favored structure for free-standing Cu-HAT coordination structures. These results show that the selfassembly of 2D MOFs depends strongly on the substrate-template surfaces. The successful synthesis of Cu3HAT2 frameworks on MoS2 surface is an overture of synthesizing 2D c-MOFs on inert surfaces, which paves the way to decouple 2D c-MOFs from underlying metallic substrates to study their electronic and magnetic properties.
In the third project, I report 2D c-MOFs Ni3(HAT)2 and Co3(HAT)2 frameworks selfassembled on an inert MoS2 surface. They share the same structure with M3(HAT)2 frameworks formed on Au(111) and Ag(111) substrates, where HAT molecules constitute a honeycomb lattice and the metal atoms constitute a Kagome lattice. This work broadens the on-surface molecular self-assembly strategies on inert surfaces. It motivates exploring the synthesis and characterization of 2D c-MOFs on monolayer 2D inert substrates.
In summary, these studies establish robust methods for on-surface synthesis of lowdimensional conjugated metal-organic nanostructures. Incorporating these 2D c-MOFs into devices by engineering interfaces with 2D materials could yield optoelectronic, spintronic and sensing applications.