THESIS
2017
132, 1 pages : illustrations ; 30 cm
Abstract
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-...[
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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, Br
2TPP, and 5,15-diphenylporphyrin, DPP) adsorbed on Au(111) and Pb(111) surfaces. We found that Br
2TPP
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 Br
2TPP 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.
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