Origin of Chirality in Nanobipyramids

Chirality at the nanoscale is vital for advances in emerging applications in biosensing, catalysis, and photonics, yet the atomistic mechanisms governing its emergence during nanoparticle growth remain poorly understood. Here, we investigate the origin of chirality in nanobipyramids formed through Au and Ag overgrowth on gold bipyramid seeds in the presence of three chiral encoders, L-cysteine, N-acetyl-L-cysteine, and D-penicillamine combining molecular dynamics simulations with experimental measurements. Our simulation results show that without chiral encoders, overgrowth is tip-dominated, yielding symmetric bipyramids or rod-like morphologies consistent with experimental observations. In the presence of cysteine, tip growth is suppressed and lateral deposition is promoted on stepped facets, producing anisotropic, gnocchi-like Au and truncated bipyramidal Ag morphologies. Our simulation results also show that chirality originates from the formation of chiral kink atoms at intersections of (111), (100), and (110) facets. The overall chiral kink densities depend on the chiral encoders and directly correlate with the experimentally measured circular dichroism responses across different chiral encoders. Furthermore, chirality transfer is enantioselective: L-cysteine stabilizes S-type kink configurations, whereas D-cysteine gives R-type configurations, explaining mirror-image circular dichroism spectra obtained with these enantiomers. These results establish a predictive atomistic framework for rationally designing chiral plasmonic nanostructures.