Single crystal gold nanorods, among all the metal nanoparticles, are arguably the most studied anisotropic morphology. Yet, much remains unknown about the mechanism and driving force for the formation of an anisotropic nanorod from a highly symmetric cuboctahedral seed (ie, the symmetry breaking step). Here we report our investigation into aspects of the symmetry breaking step of nanocrystalline seeds during the rod growth. Aberration-corrected phase contrast TEM allows us to understand the size, shape, and crystal structure of nanocrystals during the symmetry breaking step under different silver nitrate concentration.
The gold seed particles are slightly overgrown to the size range at which symmetry breaking occurs (4-6 nm). This overgrowth is performed in the presence of varying concentrations of silver nitrate, with the Au/Ag ratio adjusted to the typical seed-mediated gold nanorod growth, specifically, with Au/Ag ratios ranging from 5 to 25. The overgrown seeds are studied by a combination of aberration-corrected phase contrast TEM and UV-Vis extinction measurements. The broadening of resonance peaks and appearance of additional lower energy resonance peaks in extinction spectra suggest the formation of nascent nanorods. Aberration-corrected phase contrast TEM confirms the existence of the nascent nanorods. The overgrown seeds are predominantly single crystalline, with a small proportion of twinned and multiply twinned structures. Further size analysis shows that the symmetry breaking occurs at discrete sizes between 4 nm to 6 nm. These size limits as well as the dependence of the size on the solution silver concentration will be discussed. The effect of the symmetry breaking on further rod growth is studied by sudden step-changes to the AgNO3 concentration throughout the symmetry-breaking period. The results show that the nanocrystal size at which symmetry breaking occurs has a profound influence on the width, length and aspect ratio of the mature nanorods.