Spontaneous Chirality Evolved at the Au-Ag Interface in Plasmonic Nanorods

Chiral ligands are considered a requiredingredient during thesynthesis of dissymmetric plasmonic metal nanocrystals. The mechanismbehind the generation of chiral structures involves the formationof high Miller index chiral facets, induced by the adsorption of suchchiral ligands. We found, however, that chirality can also evolvespontaneously, without the involvement of any chiral ligands, duringthe co-deposition of Au and Ag on Au nanorods. When using a specificAu/Ag ratio, phase segregation of the two metals leads to an interfacewithin the obtained AuAg shell, which can be exposed by removing theAg component via oxidative etching. Although a close-to-racemic mixtureof chiral Au nanorods with right- and left-handedness is found insolution, electron tomography analysis evidences left- and right-handedhelicities, both at the Au-Ag interface and at the exposedsurface of Au NRs after Ag etching. The helicity profile of the NRsindicates dominating inclination angles in the range from 30 & DEG;to 60 & DEG;. Single-particle optical characterization also revealsrandom handedness in the plasmonic response of individual nanorods.We hypothesize that the origin of chirality is related to symmetrybreaking during the co-deposition of Au and Ag, through an initialperturbation at a small region on the Au-Ag interface, whicheventually leads to chiral segregation throughout the nanocrystal.

Automated summary

Chiral ligands are necessary for the synthesis of dissymmetric plasmonic metal nanocrystals. However, chirality can also spontaneously evolve during the co-deposition of Au and Ag on Au nanorods. The presence of a specific Au/Ag ratio leads to phase segregation, forming an interface within the AuAg shell. Chirality is observed both at the Au-Ag interface and at the exposed surface of Au nanorods after Ag etching.