Scientists from the North West Laser Engineering Consortium at the Universities of Liverpool and Manchester, UK, say they have developed a more efficient way of producing nanoparticles using a continuous wave (CW) fibre laser.
By applying the CW laser ablation in liquid, the team was able to produce titanium-oxide nanoparticles with a mean diameter of 30-40nm at a rate of around two grammes/hour. Traditional pulsed laser sources operate at rates of 4.4 milligrammes/hour.
‘It is the first time to our knowledge that a CW laser has been used [to generate] nanoparticles via laser ablation of a metallic target in liquids,' says Dr Amin Abdolvand of The University of Manchester. ‘Fibre lasers were proposed for their high average power and brightness, a feature that is difficult to achieve in bulk lasers.' The output beam quality is determined by the guiding properties of the doped core, so the beam is focused onto a spot of less than 20µm. It has a relatively large depth of focus, providing high and uniform irradience. ‘Thus the material processing/ablation can be tailored as desired,' adds Abdolvand.
In an experiment, the group submerged a one-millimetre high-purity titanium plate in eight millilitres of an aqueous surfactant solution. Using a ytterbium-doped high-power, high-brightness CW fibre laser, the team focused 250W on a spot 40nm in diameter, with a power density of 20MWcm-2. ‘Even a short exposure time of one second was enough to remove up to 0.4mg of the material,' says Abdolvand. The team has also applied this technique to nickel.
While the CW laser consumes more energy than pulsed laser ablation, ‘the efficiency of CW fibre lasers [is] higher,' says Dr Martin Sharp of the University of Liverpool. ‘It is a clean, one-step process. It may be possible to build a "nanoparticle-on-demand" machine, capable of delivering nanoparticles of a wide range of materials.'
‘We hope to achieve faster production rates,' adds Abdolvand. ‘More importantly, we would like to have a more controllable process to gain control over the size distribution of the particles'. Preliminary results indicate that the size, distribution and composition of the nanoparticles is dependant on laser parameters, such as spot size and the type of environment.