Supramolecular self-assembly is a process by which individual molecular and atomic components are held together spontaneously via non-covalent interactions. The supramolecular networks can be constructed by exploiting van der Waals forces, hydrogen bonding, halogen bonding and coordination bonding, etc. Among them the coordination bonds have the advantages of strong bond strength, self-correcting property and better electron transfer capability. In this thesis, I focus on several metal-organic structures that are constructed on the metal substrates. In particular, the metal atoms coordinated in metal-organic structures exhibit interesting electronic and magnetic properties. In terms of electronic properties, these metal atoms can have a local effect on the orbital energy of bonded molec...[ Read more ]

Supramolecular self-assembly is a process by which individual molecular and atomic components are held together spontaneously via non-covalent interactions. The supramolecular networks can be constructed by exploiting van der Waals forces, hydrogen bonding, halogen bonding and coordination bonding, etc. Among them the coordination bonds have the advantages of strong bond strength, self-correcting property and better electron transfer capability. In this thesis, I focus on several metal-organic structures that are constructed on the metal substrates. In particular, the metal atoms coordinated in metal-organic structures exhibit interesting electronic and magnetic properties. In terms of electronic properties, these metal atoms can have a local effect on the orbital energy of bonded molecular ligands, and can also alter the band structure of the whole metal-organic structures. In terms of magnetism, the metal centers coordinated in the two-dimensional metal-organic networks can have magnetic order in some systems. Besides, the interaction of the localized magnetic metal centers with electrons may also lead to the formation of Kondo state and spin-flip excitations. This fundamental study provides insights for future applications of metal-organic materials.

This thesis consists of four projects as below:

In the first project, I use 1,3,5-tri(4-pyridyl)-benzene (TPyB) molecules and Cu atoms to fabricate porous metal-organic networks on Cu(111) surface. My collaborators and I study the interaction of Cu-coordinated TPyB metal-organic network with Cu(111) surface state electrons. We discover that the Cu adatoms exhibit weak potential barriers for the surface electrons and act like efficient transmission channels between adjacent pores that yields significant inter-pore coupling.

In the second project, I design and synthesize a two-dimensional metal-organic network, which comprises a Kagome lattice of Fe(II) ions. First-principles calculations reveal that the Fe(II) ions are at a high spin state of S = 2 and are coupled antiferromagnetically with nearest-neighboring(NN) exchange J₁ = 5.8 meV. The ground state comprises various degenerated spin configurations including the well-known q = 0 and q=√3×√3 phases. Also, we observe a global spin excitation, which likely indicates a spin gap.

In the third project, I synthesize a one-dimensional antiferroelastic coordination chain using Ni and tetrahydroxybenzene (THB) molecules on Au(111) substrate. The combined scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and Density Functional Theory (DFT) investigations reveal that the Ni atoms coordinated by deprotonated tetrahydroxybenzene linkers on Au(111) are at a low-spin (S = 0) or a high-spin (S = 1) state alternately along the chains. We demonstrate that the spin phase is stabilized by the combined effects of intrachain interactions and substrate commensurability. The stability of the antiferroelastic structure drives the collective spin-state switching of multiple Ni atoms in the same chain in response to electron/hole tunneling to a Ni atom via a domino-like magnetostructural relaxation process.

In the last project, I use 1,4,5,8,9,12-Hexaazatriphenylene (HAT) molecules and Fe atoms to synthesize a two-dimensional metal-organic network on Ag(111) surface. I use STS to examine the spin excitation of the coordinated Fe atoms in this network. The DFT calculation results suggest that the Fe atom has a spin state S = 2 and the Fe3(HAT)₂ metal-organic network has a ferromagnetic ground state.

In summary, I synthesize and investigate electronic and magnetic properties of four metal-organic structures. These studies may help people further investigate molecular magnetism in complex 2D systems and design novel nanoscale data storage devices.