A comparison of the characteristics of nanosecond, picosecond and femtosecond lasers generated Ag, TiO₂ and Au nanoparticles in deionised water

Abstract

Although there have been large quantities of published work in laser generation of nanoparticles, it is still unclear on the comparative role of laser wavelengths and pulse widths in controlling the nanoparticle sizes, morphology and production rate. In this investigation, Ag, Au and TiO₂ nanoparticles were synthesised by nanosecond (λ = 532 nm, τ = 5 ns), picosecond (λ = 1064 nm, τ = 10 ps) and femtosecond (λ = 800 nm, τ = <100 fs) pulse lasers in deionised water. They are compared, in terms of their optical absorption spectra, morphology, size distribution and production rates, characterised by UV–Vis spectroscopy and transmission electron microscopy. The ablation rates of both Ag and Ti samples were shown as a function of laser pulse energy and water level above the samples. The average size of nanoparticles (10–50 nm) was found to be smaller for the shorter wavelength (532 nm) nanosecond pulsed laser compared with those of picosecond and femtosecond lasers, demonstrating a more dominating role of laser wavelength than pulse width in particle size control. The ps laser generated more spherical Ag nanoparticles than those with the ns and fs lasers. Under the same laser processing conditions, Au nanoparticles are smaller than Ag and TiO₂, with the latter, the largest. The nanoparticle production rate is relatively independent upon laser types, wavelengths and pulse lengths, but largely determined by the laser fluence and energy deposited.