Increasing the Structural Chirality of Metal Nanocrystals Created by Circularly Polarized Light via Surface Ligand Engineering

Plasmon-mediated synthesis enables isotropic metal nanocrystal growth with linearly polarized light. This limits the effect of the polarization of incident light during synthesis, and thus restricts the structural chirality of nanocrystals produced with circularly polarized light (CPL). This study here demonstrates that surface engineering of initial achiral silver nanorods (AgNRs) can enhance the structural chirality of the resulting nanostructures produced with CPL. Specifically, the surface ligand hexadecyltrimethylammonium bromide (CTAB) stabilizes the lateral (100) facet-terminated sides of achiral AgNRs and inhibits lateral growth. This surface engineering with achiral ligands results in increased dissymmetry of the nanostructures during the early stages of photo-growth and leads to the formation of "hook" structures, where silver preferentially deposits near the tips of the nanorods. Upon further CPL illumination, these "hook" structures exhibit a significantly larger dissymmetry in the local electric field enhancement distribution compared to the initial achiral AgNRs. This highly dissymmetric electric field enhancement profile influences subsequent growth, resulting in AgNRs with enhanced structural chirality. Notably, the optical dissymmetry of these chiral nanostructures with g-factor ≈0.05 is an order of magnitude greater than that reported in previous studies conducted under similar chemical conditions but without surface engineering.