Geração localizada de nanopartículas de Au em matrizes poliméricas via laser

Abstract

The demand for portable electronic devices has increased substantially and driven the development of technologies that make it possible to create integrated devices at the micro and nano-scale. Thus, this work aims to investigate the generation of gold nanoparticles in polymeric matrices with micrometric localization for different technological applications. For this purpose, polymeric films of polymethylmethacrylate (PMMA) and poly(vinyl acetate) (PVA) containing chlorine auric acid (HAuCl4.H2O) were prepared via the casting technique. Initially, the PMMA film of around 100 μm containing 2 mM of HAuCl4.H2O was placed in a laser microfabrication system (pulsed laser with 1 ns duration, 2 kHz repetition rate, average power around 90 mW) and 10x microscope objectives (NA=0.25). After micromanufacturing the polymeric films, they were submitted to UV-vis spectroscopy and the plasmon band was observed around 550 nm. From this characterization it was possible to prove the production of metallic nanoparticles. Although we were able to modify the “color” of the plasmonic microstructures in PMMA, the grooves obtained were very wide, on the order of 100 µm. Thus, we set out to produce PVA films, which are soluble in water like HAuCl4 and have higher thermal conductivity than PMMA. In this step, we changed the excitation beam to a continuous laser at 405 nm and a 20x objective and microscopy (NA=0.4), in order to obtain a more efficient process in the generation of gold nanoparticles. Initially, we investigated the effect of laser scanning speed (from 50 µm/s to 250 µm/s) on the production of plasmonic microstructures. We observed an exponentially decreasing behavior of the absorption in the plasmon band as a function of the laser scanning speed, indicating that the generated nanoparticles decrease in size with the increase in the scanning speed. This behavior was observed for PVA samples containing 2 mM, 3.5 mM and 7 mM of HAuCl4. We also observed that increasing the concentration of chloroauric acid increases the width of grooves produced and the size of generated gold nanoparticles. For the width of the tracks an increase from 10 µm to 35 µm is observed with the change from 2 mM to 7 mM of chloroauric acid. In order to obtain quantitative data on the size of the generated nanoparticles, the absorption curves were modeled based on the Gans-Mie theory. The results suggest that the generated nanoparticles have the shape of an oblate spheroid, analogous to the shape generated through chemical methods. Furthermore, the results show that there are two regions of velocities that produce considerably different nanoparticle sizes. The first region between the velocities of 50 and 150 μm/s where the average value of the semi-major and minor axis were between (6.8 ± 1.9) nm and (4.4 ± 1.5) nm, respectively. And a second region for higher velocities (150 and 250 μm/s) and smaller diameters ((2.0 ± 0.6) nm for the semi-major axis and (1.3 ± 0.5) nm for the semi-minor axis). This analysis was performed only for samples containing PVA with 7 mM HAuCl4. Therefore, it was possible to control the size of the produced nanoparticles just by modifying the laser scanning speed. This manufacturing process using PVA and continuous laser at 405 nm proved to be much more effective in controlling nanoparticles than the first method. Our results open up the possibility of fabricating flexible plasmonic devices using the laser microfabrication technique.