Abstract
Low-dimensional open-shell organic radicals embedded in surface-confined metal-organic frameworks (2D-MOFs) offer a unique platform for exploring quantum magnetism, manybody correlations, and spin-spin interactions at the single-molecule scale. In this thesis, lowtemperature scanning tunneling microscopy (LT-STM) and scanning tunneling spectroscopy (LT-STS) are employed to systematically investigate the electronic and magnetic properties of low-dimensional radical architectures self-assembled on Au(111) surfaces.
This thesis focuses on using on-surface reactions to synthesize organic radicals in several platforms of 2D MOFs and using LT-STM/LT-STS to characterize their quantum spin properties. The thesis consists of four projects as below:
In the first project, a Co-coordinated Porphyrin Co2Por3 metal-organic network with a hexagonal lattice is synthesized, wherein thermal cyclodehydrogenation of porphyrin yields stable radicals. Spectroscopic measurements reveal distinct spin states of these radicals: singleradical exhibiting a strong Kondo resonance with a Kondo temperature TK=99.6±0.49K, and double-radical displaying symmetric inelastic spin-excitation features at ±7meV corresponding to a triplet-singlet transition. Gaussian calculations indicate that metal coordination alters radical delocalization, strengthening Kondo screening while weakening intramolecular spinspin coupling.
In the second project, a bimolecular HAT-Cu-Por framework is constructed using hexaazatriphenylene (HAT) linkers to establish intermolecular π-d-π coupling. The porphyrin radical dimers connected through HAT display double-step inelastic spectroscopic features at 6.3meV and 31.4meV. Quantitative analysis using S=1 Heisenberg Hamiltonian obtains an intermolecular exchange interaction of 15.5 meV, revealing that delocalized π-electron channels facilitate intermolecular quantum spin coupling.
In the third project, STM tip-induced manipulation is utilized for dehydrogenation of triphenylmethane (TPM) precursors to yield radicals (dTPM) in metal-TPM coordination networks. In the Cu-TPM network, single dTPM radicals exhibit a sharp Kondo resonance with TK=99.4±0.52K, which is more than double that observed in Ni-TPM (40.8K). In contrast, the Kondo screening is suppressed in the Co-TPM network. Using the Anderson impurity model, the variation of the Kondo temperature of the three systems is quantitatively explained by the
DFT calculated Coulomb repulsion U and shifts in frontier orbital levels.
In the fourth project, site-selective tip manipulation is employed to fabricate precise, finite artificial spin lattices of different geometries (e.g., triangles and hexagons). Spatially resolved spectroscopic analysis reveals spin-wave excitations of ferromagnetic 2D Heisenberg lattices, which are quantitatively described by Linear Spin Wave Theory (LSWT).
In summary, this thesis demonstrates that in metal-organic networks, metal-ligand coordination effectively tailors the many-body spin properties of the radicals, including spin-substrate interaction, as revealed in the Kondo effect, spin-spin coupling, as revealed in spin excitations, and collective spin excitations, as revealed in spin waves. These results establish a solid foundation for engineering many-body quantum spin phases in metal-organic-radical platforms. Consider the rich metal-ligand chemistry, this direction is expected to greatly broaden the organic- based quantum spin physics and spintronics.