2.1. Nano-rod preparation
CTAB-stablized gold nano-rods of aspect ratios 4 and 5 were
synthesized using a seed-mediated synthesis [5].Toremoveexcess
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-Q
water. 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.