Abstract
Self-assembled two-dimensional (2D) organic systems refer to networks constructed from organic building blocks connected by various bonding forces, such as van der Waals forces, hydrogen bonding, halogen bonding, and coordination bonding. When these 2D systems are synthesized on a coinage metal surface, the electrons in the organic 2D structures interact with the electrons in the metal surface. This interaction leads to intriguing electronic and magnetic properties. This thesis focuses on two organic systems that are subjected to molecule-metal electronic coupling.
The first organic system is a porous macrocycle array grown on an Ag(111) surface, which behaves as coupled 2D quantum dots (QDs). In this project, I utilized 4,4'-(2,2-diphenylethene-1,1-diyl)bis(bromobenzene) (Br2-TPE) as the precursor to realize this macrocycle array. Scanning tunneling microscopy (STM) reveals that the 2D macrocycle array exhibits unique intra- and intermolecular nanocavities, which are arranged as a Lieb lattice. Different nanocavities are of different sizes and possess different energy levels. By employing both plane-wave (PW) calculations and scanning tunneling spectroscopy (STS) characterization, we discover that this structure gives rise to an isolated flat-band in the electronic band structure.
The second organic system is a metal-organic framework (MOF) consisting of 4-[bis(4-cyanophenyl)methyl]benzene-1-carbonitrile (TPM) molecules and Ni atoms on an Au(111) surface. This system hosts artificially created spin structures. I have completed three projects.
In the first project, I utilized tip manipulation to induce a dehydrogenation reaction on the TPM molecule within the 2D MOF, resulting in the formation of a radical monomer with a magnetic moment. The Kondo resonance is detected by employing the STS method. Furthermore, I conducted STS measurements at various temperatures and observed that the results agree with the predictions of the Fermi liquid theory. This provides compelling evidence for the existence of the Kondo effect. Through DFT calculations, we discovered that the Kondo screening is driven by the interaction between the π electrons in the dehydrogenated TPM molecule and the electrons of the metal substrate through the Ni atom.
In the second project, I extended the spin structures from the radical monomer to dimers, linear trimers, and a six-membered ring, through precise tip manipulation. By utilizing STS, I conducted a comprehensive characterization and analysis of the spectral properties of these structures. I observed that the radicals within these structures interact with each other through Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. The STS spectra of the dimer agree well with the theoretical models reported in literature. However, the STS spectra at each site of the linear trimer and six-membered Kondo ring do not exhibit uniform features following their geometric symmetry. Based on the distribution of JRKKY in dimers, we propose that this lack of uniformity arises from a non-uniform in JRKKY values between the radicals.
In the third project, I successfully generated 2D finite radical arrays including triangle-shaped, rhombus-shaped, and David star structures. To investigate the spectral properties of these 2D systems, I utilized site-specific STS methods. I observed complicated behaviors that are highly desirable for future investigations.
In conclusion, I experimentally realized the 2D organic system exhibits isolated flat bands and constructed the spin structures from monomers to linear and 2D finite structures. I expect that the work presented in this thesis will provide a platform for the investigation of heavy-fermion systems.
