Pulse-Width Dependence of the Cooling Effect on Sub-Micrometer ZnO Spherical Particle Formation by Pulsed-Laser Melting in a Liquid.

Sub-micrometer spherical particles can be synthesized by irradiating particles in a liquid with a pulsed laser (pulse width: 10 ns). In this method, all of the laser energy is supposed to be spent on particle heating because nanosecond heating is far faster than particle cooling. To study the cooling effect, sub-micrometer spherical particles are fabricated by using a pulsed laser with longer pulse widths (50 and 70 ns). From the increase in the laser-fluence threshold for sub-micrometer spherical particle formation with increasing pulse width, it is concluded that the particles dissipate heat to the surrounding liquid, even during several tens of nanoseconds of heating. A particle heating-cooling model considering the cooling effect is developed to estimate the particle temperature during laser irradiation. This model suggests that the liquid surrounding the particles evaporates, and the generated vapor films suppress heat dissipation from the particles, resulting in efficient heating and melting of the particles in the liquid. In the case of small particle sizes and large pulse widths, the particles dissipate heat to the liquid without forming such vapor films.