Structural and electronic characterization of molecules and their STM-based manipulation

Advances in single-atom/molecule manipulation using the scanning tunneling microscope (STM) tip have enabled scientists to construct atomic-scale structures and probe physical phenomena at the atomic level. These techniques have revealed many unique electronic and magnetic properties in two-dimensional materials, which have attracted great interest. Multiple methods have been developed to prepare two-dimensional materials, among which molecular self-assembly is a well-known approach for preparing low-dimensional functional nanostructures. As scientists increasingly investigate the adsorption configurations of single molecules and the self-assembled structures of multiple molecules on surfaces, chirality frequently becomes a crucial factor. In this thesis, two different systems formed by the adsorption of molecules on metallic and ferromagnetic surfaces are investigated by means of STM and scanning tunneling spectroscopy (STS). In the first system, utilizing 3,9-diboraperylene diborinic acid as precursors and a controlled on-surface non-covalent synthesis approach, I investigate the self-assembled chiral kagome supramolecular networks on the Ag(111) surface. These networks are stabilized by intermolecular hydrogen-bonding interactions at low temperatures. STM and STS measurements reveal a flat band at approximately 0.33 eV above the Fermi level, which is localized at the molecular center, in good agreement with tight-binding model calculations of flat bands characteristic of kagome lattices. These findings have broadened the knowledge of the fabrication of two-dimensional complex supramolecular networks on metal surfaces as well as their electronic properties, providing yet another template on which the nontrivial topology can be tuned. In the second part of this thesis, the initial plan was to investigate the chiral induced spin selectivity (CISS) effect by depositing [7]helicene onto Co/Cu(111). However, during subsequent preparations, contamination of cobalt nanoislands was observed, which was determined to be caused by hydrogen atoms through STS and atomic resolution STM topography. Parameters for STM-induced hydrogen desorption can be identified, and a potential mechanism is speculated. These findings provide a possible explanation for the underlying process of hydrogen desorption on the Co(0001) surface. After numerous tests, the potential source of hydrogen is traced to the cryostat wall because hydrogen condenses on the wall over time.