Laser-Induced Shape Transformation of Gold Nanoparticles below the Melting Point:  The Effect of Surface Melting

Relatively large gold nanoparticles (mean diameter of major axis 38.2 nm, mean aspect ratio 1.29) in aqueous solution were found to undergo shape transformations from ellipsoids to spheres at ca. 940 °C, which is much lower than their melting point, ca. 1060 °C. The shape transformation of gold nanoparticles induced by a single pulse of a Nd:YAG laser (λ = 355 nm, pulse width = 30 ps) was directly observed by a transmission electron microscope (TEM). Analysis of the experimental data showed that the threshold energy for photothermally induced shape transformation was on the order of 40 fJ for a particle, which is smaller than the energy, 67 fJ, required for its complete melting. Estimations based on the heat balance and surface melting model revealed that the temperature which particles reach after a single laser pulse was about 940 °C, with the thickness of the liquid layer on the surface of the solid core being 1.4 nm. We also examined thermally induced shape transformation of gold nanoparticles on Si substrates; above 950 °C they changed their shapes to spheres, which supported our estimation. Due to the surface melting of particles, their shape transformation occurs at a temperature much lower than their melting point.