Sharpening the shape distribution of gold nanoparticles by laser irradiation

Abstract

This work reports on the optical response and morphological changes of gold nanoparticles (NPs) induced by laser irradiation with single nanosecond laser pulses of different fluences and wavelengths. The as-grown specimens consist of irregularly shaped NPs (10–13 nm in average dimensions) produced by pulsed laser deposition on a substrate. They exhibit a broad optical absorption band related to the surface plasmon resonance (SPR) peaking in the range of 634–685 nm. After irradiation with fluences above a threshold value, a blueshift, a decrease of the overall absorption, and a narrowing of the SPR band are observed. The SPR peak wavelength and amplitude after irradiation at increasing fluences reach an approximately constant value, irrespective of the irradiation wavelength. This value is consistent with a homogeneous distribution of spherical gold NPs with a mean diameter close to the average initial dimensions as determined by transmission electron microscopy. The results demonstrate that the conversion of irregularly shaped NPs into spherical NPs with reduced dimension dispersion is a thermally driven process and occurs within a fluence interval defined by the melting threshold of all NPs and the ablation threshold. Whereas the useful fluence interval is controlled by the effective fluence absorbed, that is dependent on the laser wavelength and thermal properties of the substrate, the final shape and dimensions of the NPs are independent of the irradiation wavelength provided the fluence is within this useful interval.