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

Abstract

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 degrees C, which is much lower than their melting point, ca. 1060 degrees C. The shape transformation of gold nanoparticles induced by a single pulse of a Nd:YAG laser (lambda = 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 degrees 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 degrees 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.

Sharpening the shape distribution of gold nanoparticles by laser irradiation

“…A similar shape conversion is reported after irradiation of much larger NPs ͑38 nm of average major axis͒ at shorter wavelength ͑355 nm͒ and picosecond laser pulses. 10 These results further support the idea that the morphological transformations depend neither on the irradiation wavelength nor on the starting dimensions of the NPs. This conclusion is also in agreement with results achieved after irradiation of colloidal Au nanorods with both nanosecond and femtosecond laser pulses.…”

“…7,10 In the case of Au and Ag islands produced by sputtering on a substrate, 7 thus having similar features to the ones studied in this work, the conversion of irregularly shaped NPs into spherical ones upon laser irradiation at 532 nm occurs in a narrow fluence interval ͑our fluence regime 3͒ that has been related to laserinduced melting. A similar shape conversion is reported after irradiation of much larger NPs ͑38 nm of average major axis͒ at shorter wavelength ͑355 nm͒ and picosecond laser pulses.…”

“…1,4 Whereas laser irradiation of NPs dissolved in liquids often leads to a reduction of dimensions 5 or fragmentation, 6 the most widely reported result when irradiating NPs on substrates is the conversion of irregular NPs or islands into spherical NPs. [7][8][9][10] Furthermore, colloidal gold nanorods are converted into spherical NPs after irradiation with laser pulses, the efficiency of the process being higher for femtosecond than for nanosecond laser pulses and independent of the excitation wavelength. 1,4 These results are in contrast to the control of the aspect ratio by changing the wavelength reported for silver NPs.…”

“…This is different for Ag, for which the SPR can also be tuned but in a range shifted to the near UV. Most of the work reported on laser irradiation of Au NPs on surfaces [8][9][10] uses a single wavelength much shorter ͑248 and 355 nm͒ than that of the SPR of spherical Au NPs ͑typically around 530 nm͒. 2 At those short irradiation wavelengths, the absorption is dominated by interband transitions that may screen the possible size selectivity effect from resonant plasmon excitation reported elsewhere for the case of Ag NPs during growth.…”

“…2 At those short irradiation wavelengths, the absorption is dominated by interband transitions that may screen the possible size selectivity effect from resonant plasmon excitation reported elsewhere for the case of Ag NPs during growth. 11 In addition, a single laser fluence is used in most cases, [7][8][9][10] and the results relate to large NPs ͑Ͼ35 nm͒ 7,8,10 for which the melting temperature is expected to be close to that of the bulk material. 12 The aim of this work is to determine the role of laser fluence and wavelength in sharpening the dimension and shape distributions of irregularly shaped NPs on substrates by laser irradiation.…”