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.

In this study, TiN submicrometer spherical particles were fabricated via pulsed laser melting in liquid using picosecond and nanosecond lasers applied to colloidal nanoparticles. The sizes of the obtained submicrometer spherical particles decreased as the pulse width decreased from nanoseconds to picoseconds. Further, the laser fluence required for fabricating submicrometer spherical particles by irradiation with a picosecond laser was lower than that with a nanosecond laser. This result suggests that the heat loss from the particles during pulsed laser heating is lower with shorter laser pulse durations. Therefore, picosecond laser irradiation is an energy-efficient method for fabricating submicrometer spherical particles.

Pulsed laser melting in liquids (LMLs), a convenient method of submicrometer-sized spherical particle (SMP) preparation, induces nanoparticle (NP) melting and fusion using laser irradiation at a moderate fluence for colloidal NPs. Our earlier study, conducted to produce gold SMPs (AuSMPs) from gold NPs (AuNPs) stabilized by sodium carbonate, demonstrated that the AuSMP size increased concomitantly with decreasing stabilizer concentration, although it has been suggested that laser fluence fundamentally determines the SMP size. This study elucidates an explanation of this phenomenon. Results obtained through experimentation showed the same relation when LML was conducted using AuNPs stabilized by sodium citrate. The narrower size distribution (65 ± 4 nm) of AuNPs used in this study than that of AuNPs used in earlier studies enabled us to ascertain that AuNPs smaller than the source AuNPs (approximately 10 nm diameter) are generated during laser irradiation via AuNP evaporation that occurs simultaneously with the AuNP agglomeration. A theoretical calculation predicts that the temperature increased by laser heating of AuNP depends strongly on the AuNP size, suggesting that efficiencies of stabilizer removal and agglomeration of the 10 nm AuNPs are lower than those of the source 60 nm AuNPs. For that reason, 10 nm AuNPs are likely to remain at high stabilizer concentration, leading to the formation of smaller AuSMPs because of insufficient growth. We also confirmed that such a mechanism (AuNP-size-dependent agglomeration efficiency) is applicable to improve the growth efficiency of AuSMPs on LML using AuNPs prepared by laser ablation in liquids with a wide size distribution. When smaller AuNPs were removed from the colloids by centrifugation, the amount of AuNPs remaining after LML decreased and larger AuSMPs were formed.

Submicrometer spherical particles (SMSPs) are reported to be fabricated by pulsed laser irradiation with a frequency of 10 or 30 Hz onto raw nanoparticles dispersed in liquid. Here, the effect of the pulse frequency on particles obtained by laser irradiation onto the suspension in a vessel, especially at higher pulse frequencies up to 800 Hz, is investigated. At 200 Hz or lower, SMSPs of similar size can be fabricated, as at 10 or 30 Hz, by the same number of pulses. This indicates that the time required for particle fabrication can be greatly reduced and production efficiency can be improved using a high-frequency laser. In contrast, at 400 Hz or above, nanospherical particles (NSPs) are formed in addition to SMSPs, and the mass fraction of SMSPs is drastically decreased. This result suggests that consecutive laser pulse irradiation induces heat accumulation in particles and suspensions, resulting in a temperature increase and partial evaporation of the particles at 400 Hz or above. From the temperature increase of the suspension, the local temperature of the liquid surrounding the particles is believed to be increased by heat dissipation from the heated particles. Calculations suggest that an increase in the local liquid temperature would cause further heating of the particles.

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