High-performance molecular spin filters based on a square-planar four-coordinate Fe complex and covalent pyrazine anchoring groups

Abstract

Achieving a high degree of spin polarization at the nanoscale is essential in the field of molecular spintronics. In order to design an efficient molecular spin filter generating highly spin-polarized currents, three requirements need to be met: (1) a large spin polarization (SP), implying a substantial difference between the transmission coefficients for the two spin channels at the Fermi energy, E_F; (2) a sufficiently high transmission for the favored spin channel, and (3) an appreciable difference in transmission between the two spin channels over a broad energy range around E_F. Considering that for single-molecule devices, frontier molecular orbitals, especially the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the central molecule, generally play a dominant role in the device performance, here we propose a promising method for meeting all the three requirements. Our concept is based on a magnetic molecule, whose HOMO and LUMO of one spin type are entirely localized, making little contribution to the transport around E_F, in stark contrast to the delocalized HOMO and LUMO of the other spin type. A high electric current for the favored spin channel can then be obtained, thanks to the small energy gap between the delocalized HOMO and LUMO, as well as an appropriate interfacial charge transfer that moves the delocalized HOMO or LUMO close to the electrodes’ E_F. High-performance molecular spin filters are thus effectively realized by a square-planar four-coordinate Fe complex (FeN₄) sandwiched between two armchair single-walled carbon nanotube (SWCNT) electrodes with covalent pyrazine anchors. This demonstrates the validity of our proposed concept and offers a new and tantalizing route towards the design of future high-performance molecular spintronic devices.