Observation of a quadrupole surface plasmon mode for Au nanorods: effects of surface roughness and crystal facets

This paper describes how the surface roughness and synthetic methods of Au nanorods affect the optical properties that are often associated with localized surface plasmon resonances. We synthesized Au nanorods with different aspect ratios and surface roughness by using two different synthetic strategies to observe surface plasmon resonance bands. One set of nanorods was prepared in high yield by using a seed-mediated dropwise-addition method with a growth-directing surfactant in aqueous solution (Au nanorods in aqueous phase, GNRA). The other set of Au nanorods were synthesized by the electrochemical deposition of Au onto an anodized aluminum oxide (AAO) template (Au nanorods by AAO template, GNRT). The length of the nanorods was controlled by changing the total charge that was passed through the cell and their diameter was monitored by changing the diameter of the template channel. The as-prepared Au nanorods were optimized to observe a quadrupole mode, which is one of the higher-order surface plasmon bands. Our results showed differences between the optical properties of GNRA and GNRT. The roughness and crystal structure of the Au nanorods affected their optical properties. Smooth and single-crystal surface on GNRA had larger and sharper peaks than GNRT. The discrete dipole approximation (DDA) method was used to calculate the optical properties of the Au nanorods and these results were in good agreement with our experimental results.