Estudo sobre a cinética e os mecanismos de crescimento “Bottom-up” de nanopartículas de ouro via pulsos laser altamente focalizados

Abstract

Plasmonics is a field of science that has aroused great interest in the scientific community due to its wide range of technological applications. For example, this area involves the synthesis of metallic nanoparticles (NPs), the development of optical sensors, the use of metallic nanostructures to improve the development of experimental techniques such as high-resolution microscopy, among others. In this master's work, the kinetics and growth mechanisms of gold nanoparticles via laser was investigated by the bottom-up method that consists in the production of nanostructures built atom by atom, or molecule by molecule, or even through clusters of atoms or molecules [1] .Although there are already several works in the literature reporting the use of laser for the generation of metallic nanoparticles, these works are always conducted with high energy per pulse (mJ) laser. Here the synthesis of metallic NPs by the bottom-up method using low energy (µJ) but highly focused laser pulses has been proposed. Moreover, the mechanisms for the generation of metallic NPs via laser are not yet fully understood. In this work, the growth kinetics of NPs using the excitation and proof technique was investigated in real time. Here, a 532 nm pulsed laser with a time duration of 1 ns and laser repetition rate of 2 kHz was employed as the excitation laser while the proof beam was used a highly stabilized white light of (400-900 nm). The effect of HAuCl4 concentration on the growth kinetics of gold NPs was also evaluated. The results show that the generated nanoparticles are stable, do not need any chemical reducing agent or surfactant to avoid agglomeration. Therefore, this is an environmentally friendly method. It is worth noting that the synthesis of metal nanoparticles occurs by chemical and biological methods, which are known to receive mostly the "bottom-up" approach, moreover, the "purple" phase that generates large agglomerates of nanoparticles during chemical synthesis is not present in this physical method. Gans theory was also used to obtain the size and shape of the NPs. The results indicate that the generated nanoparticles have an oblate spheroid shape with sizes of the smaller half-axis ranging from 3 nm to 10 nm, while the larger half-axis from 5 nm to 17 nm. Finally, a model was proposed to calculate the concentration of gold NPs during the synthesis process and the growth and nucleation kinetics were evaluated using a two-step model.