Atomistic Simulation-Driven Design of STM Tips for NiCp₂ Adsorption and Spin-State Modulation

Despite the widespread use of scanning tunneling microscopy (STM) in atomic-scale investigations, the influence of the tip's atomic structure remains insufficiently characterized. This study addresses the issue by analyzing the electronic and magnetic properties of transition-metal-functionalized STM tips using both multireference wavefunction methods and density functional theory. The results demonstrate that strong electron correlations in transition-metal-based tips must be accounted for to accurately describe the structural and magnetic parameters involved-an essential requirement for the correct setup of inelastic and scanning tunneling spectroscopy experiments. By considering both minimal tip models and larger, more realistic pyramid structures, the approach balances computational efficiency with experimental relevance. The mechanism of spin-state reduction in NiCp₂-functionalized tips is clarified, revealing the central roles of charge transfer, molecular distortion, and metal-substrate hybridization. Furthermore, selective substitution of the Cu apex atom in Cu(111)-based tips with 3d transition metals allows controlled modulation of the NiCp₂ spin state. This provides a practical strategy for designing STM tips with tailored magnetic properties. Overall, the findings establish a robust theoretical framework for interpreting complex molecule-substrate interactions in spintronic systems and support the development of next-generation spin-polarized STM tips and molecular spintronic devices.