Application of picosecond laser ablation to the production of colloidal gold nanoparticles

In the nanoscience, nanotechnology, the main unifying theme is the control of matter on a scale smaller than l micrometre, normally approximately l to 100 nanometers, as well as the fabrication of devices of this size. The nanoscience is a highly multidisciplinary field, drawing from fields such as applied physics, materials science, colloidal science, device physics, supramolecular chemistry, and even mechanical and electrical engineering. Nanotechnology can be seen as an extension of existing sciences into the nanoscale, or as a recasting of existing sciences using a newer, more modern term. Among all nanomaterials, the nanoparticles are very attractive due to their physical and chemical properties and to their applications in a wide range of fields. Metal nanoparticles are of great interest because of their size and shape-dependent properties, the most important being the plasmon resonance. The noble metals nanoparticles started to be intensively studied in the last decades, because of their unique properties which make them useful for applications in several rapidly developing fields like photonics, information technology, cancer treatment, in vivo spectroscopy, biomacromolecules labeling, etc. Many different techniques have been developed for the synthesis of metal nanoparticles, the most widely used being based on chemical reactions in solutions that yield colloids of metal nanoparticles. These techniques usually employ a chemical agent to stop the growth of the particles at the nanoscale, and capping materials, such as surfactants, polymers or dendrimers, are used to prevent aggregation and precipitation of the metal nanoparticles. Chemically produced gold and silver nanoparticles are commercially available, but the samples vary from batch to batch and could be contaminated from chemicals used in the synthetic procedure. Moreover, the choice of the solvent and surface chemistry often reduces the number of possible synthetic techniques for the desired process. The use of pulsed laser ablation could be an alternative method sice it has a great flexibility in the use of materials and solvents, it is less time consuming, and above all has the advantage of producing nanoparticles free from by-products of chemical reactions. The results presented in literature for the production of nanoparticles by pulsed laser ablation of a solid target in liquids have demonstrated that this method is a promising alternative to the chemical synthesis of nanoparticles. The previous works were done mainly with nanosecond and femtosecond laser pulses and have observed a dependence of the ablation mechanism on the length of the laser pulse. Since only few works report results obtained by using picosecond laser ablation, the primary purpose of this thesis was to investigate the application of the picosecond laser pulse to the production of colloidal nanoparticles. In this thesis was studied the production of colloidal gold nanoparticles by laser ablation of a solid gold target in liquids using the fundamental (1064nm) or second harmonic (532nm) of a modelocked Nd-YAG laser (EKSPLA PL2l43A: repetition rate 10Hz, pulse width 25ps at 1064nm and 20ps at 532nm). The liquids employed were doubly deionized water, aqueous solution of sodium dodecyl sulphate and aqueous solutions of poly(amidoamine) dendrimer, PAMAM G5. Wes also initiated the study of' the production of gold nanoparticles in organic solvents. Our investigation was founded on the combined use of several experimental techniques, mainly on-line monitoring of the optical transmission of a low power beam at 5 l4.5nm from an Ar laser, UV-Vis spectroscopy and TEM microscopy. The results show the that by picosecond laser ablation spherical shape gold nanoparticles were produced in all experimented liquids. The mean diameter of the resulting nanoparticles turned out to be dependent on the laser wavelength employed for the target ablation, on the nature of the liquid environment, and on the stabilizing agent. Its value was rather small: between l.7nm in toluene and 4.51nm in aqueous solution of PAMAM G5, with the exception of diethyl ether in wich the mean diameter of the nanoparticles was increased to l6.8nm. The experiments confirmed the instability of the free gold nanoparticles in water and their tendency to form large aggregates with dendritic structures and revealed the existence of two different mechanisms of production of the nanoparticles depending on the laser wave length. Stable gold nanoparticles were produced and metal-dendrimer nanocomposites (DNC) were found to be formed in aqueous solutions of PAMAM G5. The known size reduction effect of the 532nm radiation has been investigated end the photofragmentation of PAMAM GS-capped-gold nanoparticles has been showed to be clue to the multiphoton absorption of the 532mm radiation.