2.1. Nano-rod preparation
CTAB-stablized gold nano-rods of aspect ratios 4 and 5 were synthesized using a seed-mediated synthesis [5].To remove excess CTAB from the suspensions, they were centrifuged at 15,000 rpm for 10 min. The supernatant was carefully removed, leaving the sedi-mented nano-rods in the bottom of the centrifuge tube. Finally, the nano-particles were resuspended in the same amount of Milli-Qwater. This procedure was performed twice. In addition, the suspen-sions were centrifuged at 5600 rpm for 5 min to eliminate most spheres from the suspension. Ultraviolet-visible (UV-VIS) spectro-scopy was used to identify typical resonances in the as-prepared nano-particles consisting of rods and some remaining spheres. The longitudinal peaks were situated at 800 nm and 860 nm for nano-rods of aspect ratios 4 and 5, respectively. The corresponding rod
lengths amount to 45 nm75 nm for aspect ratio 4 and 55 nm75nm for aspect ratio 5. The width of all rods is between 10 nm and 12 nm.Samples were prepared for HIM and SEM analysis, by drop-casting 30 μl of each suspension onto a clean SiO2 substrate. Within 2 h the liquid has completely evaporated, leaving a coffee-stain ring of gold nano-particles. No further sample conditioning was necessary for the subsequent SEM and HIM imaging.