My thesis mostly focusses on the systems of porphyrin molecules adsorbed on single-crystalline metallic surfaces. Cyclic tetrapyrrole porphyrins play key roles in many important chemical and biological processes, such as oxygen transport in heme (iron porphyrin), electron transfer and oxidation reactions in photosynthetic chlorophyll (magnesium porphyrin). Comprehensive understanding of the magnetic and conformational properties of single porphyrin molecules adsorbed on metallic substrates attracts intensive research interest. In my thesis, I have studied the structural and electronics properties of porphyrin molecules by Low-temperature scanning tunneling spectroscopy (LT-STM) and scanning tunneling spectroscopy (STS) and theoretical methods. Owing to the high resolution of LT-STM, both geometric and electronic properties at the atomic level were probed. Moreover, the experimental results were understood by comprehensive theoretical methods, i.e. density functional theory, molecular dynamics, tight-binding and plane-wave expansion calculations.
This thesis is divided into five parts, which are: (1) Introduction of the research area. (2) Principles of experimental setups and theoretical methods. (3) Switching molecular Kondo effect via supramolecular interaction. (4) Single-molecule observation of surface-anchored porphyrins in saddle, dome and ruffled conformations. (5) Manipulation and characterization of the electronic properties in artificial graphene nano-flakes.
In chapter 3, I study the switching molecular Kondo effect via supramolecular interaction. We apply supramolecular assembly to control the adsorption configuration of Co-porphyrin molecules on Au(111) and Cu(111) surfaces. By means of cryogenic STM, we reveal that the Kondo effect associated with the Co center is absent or present in different supramolecular systems. We perform first-principles calculations to obtain spin-polarized electronic structures and compute the Kondo temperatures using the Anderson impurity model. The switching behavior is traced to varied molecular adsorption heights in different supramolecular structures. These findings unravel that a competition between intermolecular interactions and molecule–substrate interactions subtly regulates the molecular Kondo effect in supramolecular systems. In chapter 4, we investigated the conformation relaxation and stabilization processes of two porphyrin derivatives (5,15-dibromophenyl-10,20- diphenylporphyrin, Br2TPP, and 5,15-diphenylporphyrin, DPP) adsorbed on Au(111) and Pb(111) surfaces. We found that Br2TPP adopts either dome or saddle conformations on Au(111), but only the saddle conformation on Pb(111); whereas DPP deforms to a ruffled conformation on Au(111). We also resolved the structural transformation pathway of Br2TPP from the free-space conformations to the surface-anchored conformations. These findings provide unprecedented insights revealing the conformation adaptation process. We anticipate that our results may be useful for controlling the conformation of surface-anchored porphyrin molecules. In chapter 5, I employed the low-temperature STM manipulation to study the electronic properties of artificial graphene nano-flakes. Our major focus is the zero-energy edge state in different types of graphene nano-flakes. Due to the boundary conditions involved in the Dirac equations, the energy spectrum of the graphene system can be influenced by the edges types. This behavior is particularly true in the case of zigzag edges, which contribute unique magnetic properties to the 2D systems. Moreover, I built the hexagonal graphene nano-flakes with large deformation. The energy levels and spacial distribution of first several orders of pseudo Landau levels (pLLs) are studied in detail. To summary, the findings in my research works demonstrated that organic molecular systems exhibit interesting electronic, conformational and magnetic properties.