Chiral-Induced Spin Selectivity Effect in a 1 nm Thin 1,1'-Binaphthyl-2,2'-diyl Hydrogenphosphate Self-Assembled Monolayer on Nickel Oxide
The chiral-induced spin selectivity (CISS) effect describes an observed correlation between the orientation of an electron spin transported or transferred through a molecule and that molecule's chirality. Suitable molecules are usually arranged as self-assembled monolayers (SAMs), and the primary CISS systems are based on multiple nanometer-long biomolecules exhibiting helical chirality. Aside from these typically thiolate-anchored molecules, phosphonic and phosphoric acid SAMs may well become significant for those CISS applications that require a more robust molecular coupling to metal oxide surfaces. In this work, we report on our studies, employing the aromatic, low-molecular-mass, axially chiral organophosphoric acid derivative 1,1'-binaphthyl-2,2'-diyl hydrogenphosphate (BNP). Grown as a roughly 1 nm thin SAM on top of a NiOx/Ni substrate, a strong circular dichroism signal indicates that the thin films preserved chirality. The CISS response exhibits a high magnetoresistance with a spin polarization of 50-80% when measured using magnetic-conductive atomic force microscopy. For biases above 0.5 V, the magnetoresistance curves could be well fitted to the Fowler-Nordheim (FN) tunneling model. Using a minimal FN model, we determined that, depending on the magnetization direction and the handedness of the molecules, electrons of a certain spin direction face an effective tunneling barrier at high bias, which is either 80 % higher or 40 % lower compared to the barrier for electrons of the opposite spin direction. Due to the small size of the molecules, their compatibility with oxide materials, and their commercial availability, they are excellent candidates for the realization of novel (nanoscale) organic spintronic devices.