Chirality Induced Odd-Even Spin Selectivity in Self-Assembled Peptide-Based Helical Nanofibers

Chiral organic molecules are emerging as key materials in the field of spintronics, primarily because of the chirality-induced spin selectivity (CISS) effect. This phenomenon has provided valuable insights into how nonmagnetic chiral structures can exhibit significant spin selectivity exclusively as a result of their inherent chirality. Previous studies on CISS in organic molecules have primarily focused on cases where spin polarization arises from the intrinsic chirality of monomers or from achiral monomers influenced by a chiral solvent. However, there has been no experimental evidence demonstrating that an achiral segment within a molecule, even in the presence of a chiral group, can drive the CISS effect. Here, we investigate the experimental observation of the CISS effect in peptide-based helical nanofibers, where spin selectivity can be switched by altering the length of achiral spacers between the rigid chromophore and the chiral terminal residue. Our findings reveal a distinct odd-even effect: molecules with an even number of methylene spacers exhibit one type of spin selectivity, while those with an odd number of methylene spacers display the opposite spin preference. We have utilized magnetic-field-dependent atomic force microscopy (mc-AFM) and Kelvin probe force microscopy (KPFM) techniques to demonstrate a robust relationship between chirality and spin transfer. Additionally, to obtain more insight into the spin-dependent transport process, we fabricated a prototype device of spin-valve configuration. It is important to note that the device results not only validate the occurrence of the CISS effect in these studied materials but also correlate well with the findings from spin-dependent processes observed in both the mc-AFM and KPFM measurements. Furthermore, we have successfully demonstrated an innovative approach by investigating the role of the achiral solvent ratio on the spin-dependent transport properties through helical nanofibers.