Ligand-Induced Chirality in Zig-Zag Topography of Two-Dimensional Iron Hydroxide for Spin-Polarized Photon Absorption Control

Chiromagnetic nanomaterials capable of combining magnetism and chirality are of the cutting-edge engineering for spin-polarized light-matter interaction because they can modulate circularly polarized light through a magnetic field or conversely modulate the excited state and spin state of a material with circularly polarized light. However, most chiral nanomaterials, including noble metals, semiconductors, and ceramics, have a strong chiroptical response but a weak net magnetic moment, limiting to induce strong coupling with the magnetic field of photons in polarized light-matter interaction. Transition metal oxides such as Co, Fe, Ni, and Mn can offer a strong magnetic dipole moment from the exchange coupling of 3d electron spins but rarely have optical activity in the UV-Vis range. Antiferromagnet has been marginalized in spin-polarized light modulation because the net magnetic moment is canceled out by antiparallel spin sublattice leading to negligible Zeeman effect. However, unlike ferromagnetism, which is usually limited to metals, antiferromagnets offer a much greater degree of freedom to control atomic topography and spin configuration.<br/>Here, we identified that previously unknown two-dimensional (2D) topography of iron oxyhydroxide enables the arrangement of low-dimensional antiferromagnet spin chains, which can be leveraged in a magnetically tunable spin-polarized photon absorption. We found a new type of nanoscale iron oxyhydroxide, layered ferric rust (LFR), with a monoclinic structure forming in the crystallization in the presence of acetate anion. The acid ligands coordinating iron atoms induce an unexpected zig-zag stacking of dimerized FeO octahedra, leading to a layered crystal structure with extended interlayer spacing. Topotactic intercalation of enantiomeric amino acids between the sheets breaks mirror symmetry in the crystal lattice and introduces screw axis symmetry, enabling transfer chirality of amino acid to LFR crystallization, producing chiroptical active LFRs. Achiral and chiral LFRs reserve low dimensional antiferromagnet spin chains where some portion of spins can be excited into parrell spin arrangement even under low external magnetic fields at room temperature. Therefore, the chiral LFR enables spin-polarized photon absorption in the UV-Vis wavelength range by the Zeeman effect in addition to the chiroptical activity contributed by the ligand-induced mirror symmetry breaking, as manifested by magnetic circular dichroism measurement. We believe our results would be a cornerstone for designing inorganic-organic hybrid two-dimensional nanomaterials for the various chiral spintronics, optoelectronics, and catalyst fields.