LASER-INDUCED NANOPARTICLE FORMATION IN LIQUIDS: INVOLVED MECHANISMS AND ROLE OF THE EXPERIMENTAL PARAMETERS
Résumé
Laser ablation in liquids is a versatile and promising method for synthesis of colloidal nanoparticles (NPs) that are particularly attractive for bio-sensing, medical application, as well as for catalysis [1]. In the experiments, NPs of different materials are successfully produced by using laser systems with various pulse durations, shape, wavelengths, and fluence. The major mechanisms involved in the formation of these NPs are, however, still under discussion
Here, we focus our attention at the better understanding of the physical processes involved in NPs formation by laser ablation in liquids. For this, a series of both experimental and numerical studies are performed. In particular, by using different laser systems with pulse duration from femtosecond to CW range, metallic NPs are obtained with different mean size and size distributions. Furthermore, the role of the solution concentration, liquid properties and of laser-induced fragmentation is demonstrated [1]. To analyze these results, multi-physical numerical modeling is performed. First, laser ablation is modeled for various laser fluences [2,3]. Simulation results show the beginning of the formation of the ablation plume, shock wave, a void/bubble in front of the target. The bubble is formed only above a well-defined both target and liquid-dependent threshold. It then collapses and the target is reheated, so that more NPs originate from this effect. Later, diffusion-driven nucleation and collision growth enter into play.
In addition, laser-induced fragmentation and growth of nanoparticles are examined in liquids [4]. The resulted nanoparticles are particularly attractive for bio-sensing, medical application, as well as for catalysis. First, laser energy absorption is considered. The role of laser wavelength is underlined. Then, ultra-short laser-induced fragmentation is considered. The obtained results help us to elucidate the roles of thermal, mechanical and electrostatic effects in the particle decomposition. The absorption cross sections are calculated based on the generalized Mie theory. In addition, combined FDTD-rate equation calculations are carried out revealing a considerable field enhancement around the particle [5]. These results agree with the experiments.
Finally, the obtained results demonstrate that the major decomposition mechanism mostly depends on the particle size, laser wavelength and both particle and liquid properties. It is found that as a result of a quite moderate ultra-short laser irradiation, only melting and partial evaporation occurs. With the increase of laser fluence, particle decomposition is observed resulting from both thermal and mechanical effects. The role of thermionic emission [6] and the possibility of Coulomb explosion are also discussed. The results may help in the analysis of the final NP's size distributions.
Support from France-Russia collaborative grant PICS 6106 is gratefully acknowledged.
[1] K. Maksimova, Synthesis of novel nanomaterials by ultra-short laser ablation in liquids for biomedical applications, PhD thesis, Aix-Marseille University (2014)
[2] M.E. Povarnitsyn, T.E. Itina, P.R. Levashov, K.V. Khishchenko, Phys Chem Chem Phys 15(9) 3108-3114, (2013)
[3] T. E. Itina, J. Chem. Phys. C 115 (12) 5044-5048 (2010)
[4] L. Delfour and T. E. Itina, Proceedings of ALT 2014, Cassis, France (2014)
[5]A. Rudenko , JPh. Colombier T. E. Itina, PIERS 2015, (2015)