Abstract
Molecular spin systems on surfaces provide atomically precise platforms for exploring correlated magnetism and many-body electronic phenomena. Among them, π-radicals are particularly attractive because their unpaired π-electrons generate localized magnetic moments that can be chemically designed, spatially arranged, and directly probed by scanning tunneling microscopy and spectroscopy. When such radicals are arranged artificially and coupled to metallic substrates, Kondo screening, inter-spin exchange interactions, and finite-size effects can coexist, giving rise to rich low-energy electronic behavior. The ability to construct these spin units into programmable lattices further opens a route to studying Kondo-lattice-related physics and artificial quantum matter at the single-molecule level.
This thesis reports the construction and characterization of programmable molecular spin systems based on tip-induced π-radicals in two-dimensional metal–organic frameworks on Au(111). Low-temperature scanning tunneling microscopy and spectroscopy, together with density functional theory and model Hamiltonian calculations, are used to investigate the structural, electronic, and magnetic properties of these systems. Site-selective dehydrogenation of tris(4-carbonitrile)-triphenylmethyl (TPM) molecules generates radical molecular spin units, which exhibit low-energy spectroscopic features associated with Kondo correlations. By arranging these radicals into finite one-dimensional chains, systematic changes in the local spectra are observed as a function of chain length and site position, revealing the competition between local Kondo screening and inter-radical magnetic coupling.
The same platform is further extended to finite two-dimensional molecular spin arrays with controlled geometries. Triangular, rhombic, and larger radical lattices show increasingly complex spectral responses near the Fermi level, including peak-like, dip-like, and gap-like features. Their size- and geometry-dependent evolution suggests the development of collective interactions among molecular spins, while also emphasizing the roles of finite-size effects, local coordination, and non-uniform coupling. These results provide a molecular-scale view of how isolated Kondo impurities evolve toward interacting spin arrays and possible finite-size precursors of Kondo-lattice-like behavior.
In addition to probing correlated spin physics, this thesis develops an artificial-intelligence-assisted STM strategy for scalable molecular spin construction. Reinforcement learning is used to optimize tip-manipulation parameters for site-selective dehydrogenation, enabling autonomous fabrication of designed radical patterns and extended molecular lattices with improved efficiency and reproducibility. This approach establishes AI-assisted single-molecule lithography as a powerful route toward programmable artificial quantum structures.
Together, these studies demonstrate a chemically tunable and spatially programmable platform for investigating correlated magnetism in molecular spin systems. By integrating on-surface synthesis, STM tip manipulation, local spectroscopy, theoretical modeling, and AI-assisted fabrication, this work provides a foundation for constructing and studying artificial molecular spin lattices and a metal-organic framework platform with single-molecule precision.