Crystal lattices with broken inversion symmetry can induce chirality in spin configurations and phonon vibrations, opening new avenues for topological and quantum properties in light-matter interactions. These features have attracted significant attention for next-generation applications in optics, electronics, and energy. However, controlling chirality in long-range ordered metal oxides or metal chalcogenides is particularly challenging, especially when extended beyond atomic layers into the nanoscale regime. In this presentation, I will share our recent discoveries made with Prof. Nicholas Kotov, focusing on how to realize crystallographically chiral nanocrystals and reveal their emergent chiral interactions with light through spin and phonon modes. First, we demonstrate a previously unknown two-dimensional (2D) iron oxyhydroxide (FeOOH) that can host a low-dimensional quantum spin system, enabled by chiral hybridization between organic ligands and Fe–O octahedra. The material is synthesized as uniform nanoplatelets (~40 nm in thickness) and can be scaled up to 100 grams. The surface ligand incorporation process is universal across various ligand types and induces a transition from achiral to chiral crystallographic symmetry upon the intercalation of enantiomeric amino acids. This chiral transformation also significantly affects the quantum magnetic state. The resulting quantum spin state, capable of site-specific spin polarization, manifests not only in magnetic circular dichroism in the visible range but also in enhanced electrochemical catalytic activity. Second, we revealed that chiral phonon excitation can also lead to strong polarization of terahertz (THz) light. Using the surface ligand penicillamine, we directed the chirality of mercury sulfide (α-HgS) nanoparticles to adopt specific handedness among the enantiomeric space group (P3₁21 or P3₂21). Despite the significant length-scale mismatch between the nanomaterials and the wavelength of THz light, the restructured HgS nanoparticle assemblies exhibited exceptionally strong optical rotation with a high asymmetry factor (g-factor), driven by strong interactions between chiral phonons referred to as collective chiral phonon modes. In conclusion, these results provide a versatile strategy for exploring diverse families of chiral crystal lattices with novel quantum and topological functionalities.
