A method for the characterisation of gold nano-rods

Interest in gold nanorods derives from their unique optical properties, which strongly depend on both the particle size and shape. Many questions remain as to the relationship between the shape and optical properties of the nanorods. In addition, the anisotropic growth mechanism, which drives a crystal structure with cubic symmetry to grow into a rod, remains a mystery. In order to address these issues, there is a need to characterize the shape of the nano-rods in three dimensions (3D). A challenge for transmission electron microscopy (TEM) is to determine 3D shapes of particles from 2D images. Established tomography methods for reconstructing the 3D morphology of materials require a large number of images which is a time-consuming task. In the specific case of Au nanorods, there is a need for a more rapid method that can survey the 3D shape of a large number of particles to obtain good statistics in the comparison with their properties. This work will describe the development of a novel method to measure the thickness of a nanorod in the direction parallel to the incident beam from high angle annular dark field scanning transmission electron microscope (HAADF-STEM) image intensities, providing a map of thickness versus position across the nanorod – a “thickness projection image”. This method requires measurement of absolute intensity scattered to a fully characterised HAADF detector, so that an appropriately normalized HAADF image is obtained by correlation with the incident beam intensity. The relationship between intensity and specimen thickness is provided by HAADF image simulations, where dynamical elastic scattering, followed by single thermal diffuse scattering is taken into account. The rapidity of this method makes it an attractive new tool for the characterization of many nano-scale structures. Furthermore, in principle a 3D reconstruction can be achieved from only two thickness projection images at different orientations. The algorithm required to achieve this, and the practical issues affecting its experimental execution, are described here. This novel method was applied here to the characterization of gold nanorods. The detailed shape and crystallographic orientation of the surface facets of these nanorods was determined and found to differ from popular hypotheses. This may provide new insights into the growth mechanism of these nanorods.