The highly promising optical, catalytic and electronic properties of gold nanoparticles, and in particular Au nanorods, have made them a major area of research in recent years. Gold nanorods are typically synthesised via a seed-mediated approach, in which silver ions and halides are used as surfactants. Despite intense interest, there is currently little agreement on a mechanism for anisotropic growth, and few insights into the fundamental symmetry breaking event that is a prerequisite for shape anisotropy. The question remains as to what causes an essentially spherical seed particle, with a cubic lattice, to develop a preferential growth direction?
Here, we present direct atomic-scale observations of the nanocrystal structure at the embryonic stages of gold nanorod growth. The onset of asymmetry of the nascent crystals is observed to occur only for single crystal particles that have reached diameters of 4-6 nm and only in the presence of silver ions. In this size range, small, asymmetric truncating surfaces with an open atomic structure become apparent. Furthermore, {111} twin planes are observed in some immature nanorods within 1-3 monolayers of the surface. Nano-dumbbells are formed after several minutes, corresponding to a maximum redshift in the longitudinal LSPR frequency. A blue shift is then observed as the nanoparticles transition from a dumbbell to rod morphology. These results provide the first direct observation of the structural changes that break the symmetry of isotropic nascent nanocrystals and ultimately enable the growth of asymmetric nanocrystals.
Acknowledgements: This work was supported by the Australian Research Council (ARC) grant DP120101573 and used microscopes at the Monash Centre for Electron Microscopy funded by ARC Grant LE0454166