Laser photothermal melting and fragmentation of gold nanorods: Energy and laser pulse-width dependence

We studied the shape transformation (by use of TEM and optical absorption s pectroscopy) of gold nanorods in micellar solution by exposure to laser pul ses of different pulse width (100 fs and 7 ns) and different energies (mu J to mJ) at 800 nm, where the longitudinal surface plasmon oscillation of th e nanorods absorb. At moderate energies, the femtosecond irradiation melts the nanorods to near spherical particles of comparable volumes while the na nosecond 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 de termine the final fate of the laser-exposed nanorods, i.e., melting or frag mentation.