Drastic surface plasmon mode shifts in gold nanorods due to electron charging

The color of small gold rods changes dramatically when electrons are injected by chemical reductants. The longitudinal and transverse plasmon modes are both found to blue-shift, and the shift is larger for rods with larger aspect ratios. The color changes are visible to the eye for rods with aspect ratios around 2-3. It is found that the surface plasmon band is damped when charging becomes high. The effects are in qualitative agreement with a model in which the gold plasma frequency increases due to an increase in electron density.