Control of Symmetry Breaking Size and Aspect Ratio in Gold Nanorods: Underlying Role of Silver Nitrate

Abstract

Single crystal gold nanorods remain one of the most important and intensively studied anisotropic nanocrystals. The aspect ratio of the nanorods is controlled during the colloidal synthesis using silver nitrate; however, the mechanisms for the underlying control are not well understood. Here, we investigate the growth of gold nanocrystals at the stage where they break symmetry and begin anisotropic growth into nanorods. Using high resolution electron microscopy, we determine directly the size and atomic structure of the nanocrystals at the symmetry breaking point. We find that silver nitrate controls the size of the crystal at which symmetry breaking occurs. The seed crystal undergoes a symmetry breaking event at a critical diameter between 4 and 6 nm that depends upon the [HAuCl4]:[AgNO3] ratio. The smallest diameter for symmetry breaking, ∼4 nm, is observed at the lowest [HAuCl4]:[AgNO3] ratio (i.e., the highest AgNO3 concentration) corresponding to the minimum size at which a “truncation” can form, a p...