Characterising the shape, size and facet crystallography of metallic nanoparticles is extremely important for understanding their growth mechanism and their optical properties. However, nanoparticle surfaces are extremely difficult to characterise due to their miniscule size. The surface facets can be just a few atoms wide, so determining their crystallographic orientation is a challenge. Furthermore, there is no established method for measuring nanofacet surface energy, a critical parameter in understanding shape control.
Here we apply a quantitative scanning transmission electron microscopy (STEM) technique [1] to count the number of atoms in each atomic column, as identified in an atomic resolution STEM image of a single crystal gold nanorod, orientated in two different zone axes. Using this method we are able to determine the shape and facet crystallography of the nanorod. Furthermore, by applying this method at successive time intervals and comparing the images it is possible to determine the relative stability of the different crystallographic facets and the overall stability of the nanoparticle shape [2].
We determine that the nanorod side-facets comprise both high {0 1 1+√2} and low {110}, {100} index facets and that all 3 types of side-facets have comparable size,shape and stability. Despite observing significant movement of atoms across all facets, we find that the overall shape and size of the rod, facet crystallography and size remain the same. These findings shed new light on the growth kinetics of Au nanorods and suggest that this nanorod, with its’ high and low index facets, is in an equilibrium state.
Acknowledgement:
This research was supported under the Discovery Project funding scheme of the Australian Research Council (Project no DP120101573). We also thank the Monash Centre for Electron Microscopy for facilities and staff support