Quantum phenomena in molecular graphene devices and metal-organic coordination frameworks attract immense attention due to their fascinating physical properties and potential application in spintronics. However, the fabrication of a metal-organic networks on graphene to avoid the screen effect remains a significant challenge to researchers. In this regard, investigating a directly bottom-up synthesized metal-organic network on a weak coupling surface like graphene is highly desirable. On the other hand, directly grown metal-organic networks or self-assembled molecules on graphene to induce spin-orbital coupling and magnetism is a promising approach to realize the non-trivial topological phases in graphene. This thesis mainly focuses on the spin transport of molecular graphene devices, o...[ Read more ]

Quantum phenomena in molecular graphene devices and metal-organic coordination frameworks attract immense attention due to their fascinating physical properties and potential application in spintronics. However, the fabrication of a metal-organic networks on graphene to avoid the screen effect remains a significant challenge to researchers. In this regard, investigating a directly bottom-up synthesized metal-organic network on a weak coupling surface like graphene is highly desirable. On the other hand, directly grown metal-organic networks or self-assembled molecules on graphene to induce spin-orbital coupling and magnetism is a promising approach to realize the non-trivial topological phases in graphene. This thesis mainly focuses on the spin transport of molecular graphene devices, on-surface Cu-catalyzed dehydrogenation reaction to synthesize a single layer of a metal-organic network on graphene, and quantum phenomena in metal-organic networks. Scanning tunneling microscopy and quantum transport are used as experimental tools to study the structures and quantum phenomena of these systems.

The thesis is composed of four projects as below:

In the first project, I fabricated a molecular graphene device using Iron (II) Phthalocyanine (FePc) molecules epitaxial grown on high-quality graphene on h-BN device. The epitaxial FePc on graphene device plays as the spin hot spots and the spin-flip probability is conservative. According to the non-local quantum spin transport, the spin single and triple-state transitions happened depending on the gate voltage. On the other hand, the magnetic proximity effect emerges in this molecular graphene device caused by the Zeeman splitting field. A canted antiferromagnetic ground state is observed on the monolayer graphene, and a pronounced Zeeman Hall effect is realized on bilayer graphene.

In the second project, I synthesized a single-layer metal-organic network on a graphene surface using an on-surface Cu-catalyzed dehydrogenation reaction. The structure of the network is determined to be Cu₂(C₆O₆) using the techniques of scanning tunneling microscopy and density function theory calculation. The energy levels and band morphology of the coordination network adsorbed on graphene change slightly, indicating a weak interaction between the network and the graphene substrate. According to the DFT calculation, the free-standing structure is a conventional semiconductor with a narrow indirect band gap of 0.485 eV.

In the third project, I synthesized a single-layer metal-organic network of Fe₂(Fe − DPyP)₃ using DPyP molecules coordinated with Fe atoms. The experimental scanning tunneling spectroscopy (STS) results show different magnetic signatures at the two Fe sites in the coordination structure. One Fe features a spin-flip excitation with magnetic spin state of S=1, and the other Fe shows the Kondo effect. The magnetic ground state of Fe₂(Fe − DPyP)₃ is in-plane ferromagnetic, and the band structure of the free-standing structure features multiple Dirac cone bands and flat bands. This work presents a spin-mixed lattice composed of different magnetic sublattices in a metal-organic network system, which is important in statical and solid-state physics.

In the fourth project, I synthesized a two-component metal-organic network using organic linkers DPyP, and HAT coordinating with Fe atoms. The structure and the electronic properties are characterized by scanning tunneling microscopy, density functional theory, and tight binding modeling. This metal-organic network can be classified as a p-orbital structure with in-plane px/py hopping. The electronic properties of this two-dimensional metal-organic network demonstrate multiple spin-polarized Dirac bands and flat bands around the Fermi level. The spin-flip excitation is observed in the Fe of metalized Fe-DPyP. The local magnetic moments in Fe break the time-reversal symmetry and lift the spin degeneracy both on the molecular frame and Fe atoms. This work demonstrates a p-orbital honeycomb Kagome lattice in a metal-organic network system, which is important in studying orbital physics.

In summary, these studies have contributed to the fabrication and characterization of molecular graphene devices and metal-organic networks growing on graphene.