Electrode-dependent spin and thermoelectric transport in M@C80 (M=Fe, Co, Ni) molecular junctions

The ability to engineer molecular junctions with tunable spin and thermoelectric properties is central to the development of high-performance nanoscale devices. In this work, we investigate endohedral fullerenes M@C80 (M=Fe, Co, Ni) contacted by gold and graphene electrodes using density functional theory combined with the non-equilibrium Green’s function formalism. Encapsulation of transition-metal atoms inside the C80 cage induces pronounced charge transfer, orbital hybridization, and spin splitting, leading to strongly modulated and energy-selective transmission spectra. The Au–Fe@C80–Au junction exhibits the highest spin polarization and near-unity transmission at the Fermi level, making it particularly suitable for spintronic applications. In contrast, the Gr–Co@C80–Gr junction displays an exceptionally large thermoelectric response. The thermoelectric figure of merit is reaching ZT > 1000 which originates from sharp, asymmetric transmission resonances near the Fermi energy and enhanced π–π coupling with graphene. These values represent upper theoretical limits within a coherent electronic transport regime and do not imply a violation of thermodynamic constraints, as the thermoelectric efficiency remains bounded by the Carnot limit. The Ni@C80 junction shows broader transmission features, which are favorable for energy-selective transport. A comparative analysis of electrode materials reveals that gold provides stronger and more stable molecule–electrode coupling with higher electrical conductance, whereas graphene enables superior energy filtering and an enhanced Seebeck response.