The Shape Transition of Gold Nanorods

We report a revised synthetic procedure based on an electrochemical method for preparing an aqueous solution containing suspended Au nanorods. The mean aspect ratios of the Au nanorods can be experimentally adjusted between 1 and 7. The evolution of the longitudinal surface plasmon bands shows an eminently sensitive dependence on the aspect ratios of the nanorods. Their dependence is accordingly described by classical-electrostatic-model predictions. The shape transition of the nanorod particles has been studied by varying some key influencing factors such as the wavelength, the laser fluence, and matrix effects. The nanorods were exposed to laser lines at 532 and 1064 nm, frequencies which correspond closely to the short- and long-axis plasmon resonances, respectively. A photon-induced shape transition process was evidenced, and the corresponding rod-to-sphere conversion contributed by a photoannealing process was observed in both cases. Meanwhile, we observed a new type of “φ-shaped” Au nanostructure in the case of 1064-nm irradiation, which possibly represents the early stage of the shape transition and indicates that the starting location of the atomic-scale restructuring is at the centroid of the Au nanorod. The results of laser fluence-dependence measurements state that an efficient shape transition occurs via a multiphoton process. We also demonstrate the fabrication of the Au nanorod@silica nanostructures for preliminary studies of the matrix effects. As a result of the higher rigidity of the thin-silica-coating layer, the associated shape transition requires higher energy and proceeds less efficiently as compared with the cases for the micelle-stabilized Au nanorods.