Electric Birefringence of Gold Nanorods: Effect of Surfactant Coating

The optical and electrical properties of gold nanorods, together with the possibility to control their orientation, have proven crucial for their use in applications such as metamaterials, nanoelectronics, molecular devices, or particle traps. When the rods are immersed in liquid media, their orientation can be controlled by electric fields and monitored by electro-optical techniques. We demonstrate that both the charged surfactant layer that surrounds the particle and the conducting core affect electro-orientation. An interpretation in light of a soft particle polarization model is provided: below 1 MHz, ions in the surfactant layer produce a large dipole accounting for the response of the system. With this model, we obtain the charge density of the coating and the conductivity of the metallic core. The results are contrasted with electrophoresis data. We also demonstrate that the presence of surfactant micelles in the medium slows down the dynamics of the system.