Electronic and magnetic properties of molecule-metal interfaces: transition metal phthalocyanines adsorbed on Ag(100) (1203.2747v1)

Abstract: We present a systematic investigation of molecule-metal interactions for transition-metal phthalocyanines (TMPc, with TM = Fe, Co, Ni, Cu) adsorbed on Ag(100). Scanning tunneling spectroscopy and density functional theory provide insight into the charge transfer and hybridization mechanisms of TMPc as a function of increasing occupancy of the 3d metal states. We show that all four TMPc receive approximately one electron from the substrate. Charge transfer occurs from the substrate to the molecules, inducing a charge reorganization in FePc and CoPc, while adding one electron to ligand \pi-orbitals in NiPc and CuPc. This has opposite consequences on the molecular magnetic moment: in FePc and CoPc the interaction with the substrate tends to reduce the TM spin, whereas in NiPc and CuPc an additional spin is induced on the aromatic Pc ligand, leaving the TM spin unperturbed. In CuPc, the presence of both TM and ligand spins leads to a triplet ground state arising from intramolecular exchange coupling between d and \pi electrons. In FePc and CoPc the magnetic moment of C and N atoms is antiparallel to that of the TM. The different character and symmetry of the frontier orbitals in the TMPc series leads to varying degrees of hybridization and correlation effects, ranging from the mixed-valence (FePc, CoPc) to the Kondo regime (NiPc, CuPc). Coherent coupling between Kondo and inelastic excitations induces finite-bias Kondo resonances involving vibrational transitions in both NiPc and CuPc and triplet-singlet transitions in CuPc.

Electronic and Magnetic Properties of Transition Metal Phthalocyanines on Ag(100)

The paper entitled "Electronic and magnetic properties of molecule-metal interfaces: transition metal phthalocyanines adsorbed on Ag(100)" presents a comprehensive paper of the interactions between transition-metal phthalocyanines (TMPc), specifically those containing Fe, Co, Ni, and Cu, and the Ag(100) surface. The investigation employs a combination of scanning tunneling spectroscopy (STS), scanning tunneling microscopy (STM), and density functional theory (DFT) to elucidate the electronic and magnetic modifications induced by adsorption.

Key Findings

1. Adsorption Geometry and Electronic Chirality: The paper reveals that TMPc adsorb with their aromatic planes parallel to the Ag(100) surface with a +/-30° azimuthal rotation. This geometry, consistent across all TMPc, indicates a predominant role of van der Waals forces, as confirmed by various DFT approximations (LDA, GGA+vdW). Notably, the paper highlights the emergence of electronic chirality in NiPc and CuPc due to nodal planes, a phenomenon not significantly influenced by structural conformation but rather by the symmetry mismatch between molecular orbitals (MOs) and surface electronic states.
2. Electronic Structure and Charge Transfer: Using STS and DFT, the paper details the impact of approximately one electron transferred from the Ag substrate to the TMPc. This charge transfer significantly alters the electronic structure, with NiPc and CuPc showing occupancy changes in the ligand 2e_g orbital, compared to FePc and CoPc, where charge redistributes among multiple MOs due to hybridization.
3. Magnetic Moments and Kondo Effect: The magnetic characterization shows contrasting behaviors among the TMPc. FePc and CoPc exhibit quenching of magnetic moments due to strong hybridization and charge reorganization, with STM revealing no observable Kondo effect, suggestive of mixed-valence states. Conversely, the spin multiplicity in NiPc and CuPc is enhanced through ligand-based spins. NiPc's zero Kondo resonance and CuPc's robust triple state ground reconfirm magnetic interaction between ligand and TM spins.
4. Hybrid Interactions and Vibrational Coupling: The interactions in NiPc and CuPc include coherent coupling between Kondo and inelastic molecular vibrations, leading to observable side peaks in STS, affirming the presence of intricate spin and vibrational excitations likely impacting potential molecular spintronic applications.

Implications and Future Directions

The paper provides insights into the complex interplay of electronic and magnetic properties at the TMPc/Ag(100) interface, emphasizing how subtle changes in electronic configurations, hybridization, and orbital alignment can drastically affect the physical properties. The findings have significant implications for the design of molecular electronic devices, where controlling magnetic and electronic states at the molecule-metal interface is paramount.

Future research could explore lateral coupling within organized TMPc arrays on surfaces to explore extended magnetic interactions and conductance changes, potentially realizing novel spintronic devices. Additionally, theoretical models might extend beyond DFT to incorporate dynamic correlation effects, offering improved predictability of TMPc interface behavior across different metallic surfaces.

This paper serves as a vital resource for understanding molecule-metal interfaces, contributing to both fundamental surface science and the prospective development of nanoscale electronics and magnetism-centered applications.