The Shape Transition of Gold Nanorods

Abstract

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.

Laser-Induced Shape Changes of Colloidal Gold Nanorods Using Femtosecond and Nanosecond Laser Pulses

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Time-resolved transient absorption spectroscopy allows one to follow the electronphonon relaxation dynamics in thin metal films [21][22][23][24] and small metal particles. [1][2][3][4][5][6][7][8][9][10][11][12][13] In particular, silver 1,2,9 and gold [2][3][4][5][6][7][8][10][11][12][13] nanoparticles have been studied intensively as they show a strong absorption band in the visible region, which is due to the excitation of the surface plasmon resonance.…”

“…However, if the pump power is increased significantly, the temperature of the metal nanoparticle lattice can be raised to values well above its melting temperature within a few picoseconds (when using ultrashort laser pulses) while the temperature of the environment initially remains constant. This can then lead to size [14][15][16][17] and shape [18][19][20] changes of the nanoparticles before the deposited laser energy can be released to the surrounding medium by phonon-phonon interactions which are usually on the order of 100 ps. 4,7 It was recently reported 29 that gold nanorods prepared by an electrochemical method 30 and encapsulated in micelles in aqueous solution undergo a rod-to-sphere shape transformation within about 30 ps due to melting of the nanorods.…”

“…More information about the structural dynamics of metal nanoparticles after laser excitation can be obtained when nanoparticles having different shapes than spheres are used. 18,19 Gold nanorods were found to undergo a shape transformation from a rod to a sphere. 18,19 While femtosecond laser pulses present an easy, clean, and gentle way to induce this type of shape transformation, nanosecond pulses can cause fragmentation of the gold nanorods into smaller spherical particles.…”

Controllable Cobalt Oxide/Au Hierarchically Nanostructured Electrode for Nonenzymatic Glucose Sensing

“…Previous work studying the effect of pulsed laser on nanoparticles showed that exposure to irradiation resulted in fragmentation of particles and the generation of new extinction peaks associated with the new population [161,169,[171][172][173]. In this work, particles with diameters ranging from 20 to 50 nm with SPRs ranging from 550 to 750 nm were subjected to NIR pulsed light for 10 min at two different powers (350 and 700 J) [170].…”

Synthesis of Gold Nanorods from Metallic Gold by a Sonoelectrochemical Method

“…Second, most metals crystallize in highly symmetric cubic lattices. Therefore, soft and rigid templates had been used to achieve rod-shaped metal nanostructures, so the seeded method [2,3] and sonoelectrochemical [6,7] is often used.…”