Transformation of a Thin Gold Film to Nanoparticles after Nanosecond – Laser Irradiation

Dynamics of nanoparticle formation were observed after nanosecond laser irradiation of thin gold films. Gold films of different thickness (3, 4, 5, 6, 10, 15, 20, 25 nm) were evaporated on the silicon (110) substrate and irradiated with the pulsed nanosecond laser using different pulse energies and the number of pulses in a burst. A single Gaussian beam as well as an interference of a few laser beams were used for irradiation. Morphological changes appeared in the films only when the pulse energy was high enough to initiate the phase transition. The threshold energy density for phase transitions in the films was estimated from the thermal model of the laser beam and sample interaction. With the pulse energy just above the threshold, it was possible to observe evolution of nanoparticle formation from a plane metal film by changing the number of pulses applied, as duration of the pulse burst represented the time the liquid phase existed. The final size of nanoparticles was a function of the film thickness and was found to be weakly dependent on the pulse energy and the number of pulses. The nanoparticle size distribution width was much narrower after irradiation with periodical intensity distribution produced by 3 interfering beams compared with the single beam treatment. Moreover, the nanoparticles tended to be periodically distributed on the substrate.