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We studied the shape transformation (by use of TEM and optical absorption spectroscopy) of gold nanorods in micellar solution by exposure to laser pulses of different pulse width (100 fs and 7 ns) and different energies (μJ to mJ) at 800 nm, where the longitudinal surface plasmon oscillation of the nanorods absorb. At moderate energies, the femtosecond irradiation melts the nanorods to near spherical particles of comparable volumes while the nanosecond pulses fragment them to smaller near-spherical particles. At high energies, fragmentation is also observed for the femtosecond irradiation. A mechanism involving the rate of energy deposition as compared to the rate of electron−phonon and phonon−phonon relaxation processes is proposed to determine the final fate of the laser-exposed nanorods, i.e., melting or fragmentation.

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Gold nanorods are prepared in aqueous solution by an electrochemical method and are shape controlled by using capping micelles as described by Yu et al. (J. Phys. Chem. B 1997, 101 (34), 6661). Transmission electron microscopic (TEM) images are determined for these gold nanorods, taken from solutions heated to various temperatures in the range of 25−160 °C. Size and shape analysis of their TEM images showed that the mean aspect ratio of the nanorods in solution decreases with increasing temperature, mostly as a result of reduction in their length. Heating the dried nanorods themselves on a TEM slide to much higher temperatures does not produce any shape changes. This suggests that the observed relative instability of the longer nanorods in the micellar solutions is attributed to the relative instability of the micelles capping the longer gold nanorods. Following the change with time of the longitudinal surface plasmon absorption band of the gold nanorods at five different temperatures, the activation energy for the thermal decomposition of the micelles is found to be 21.0 ± 1.0 K cal mol-1. The most stable rod-shaped micelle is found to have an aspect ratio of 2.0. The craft decomposition (dissolution) temperature of these micelles is found to be ∼155 °C. This study offers a new method in which the thermal stability of micelles of various shapes is used to change the size distribution of metal nanorods in solution. On the other hand, TEM or the absorption maximum of the longitudinal plasmon resonance of gold nanorods can be used as a sensitive monitor in studying the physical properties of the host micelles themselves.