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| Author(s): |
Paulo Henrique Dias Ferreira
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Carlos. |
| Institution: | Universidade de São Paulo (USP). Instituto de Física de São Carlos (IFSC/BT) |
| Defense date: | 2011-10-27 |
| Examining board members: |
Cleber Renato Mendonça;
Ricardo Rego Bordalo Correia;
Edilson Lucena Falcão Filho;
Luis Gustavo Marcassa;
Ricardo Elgul Samad
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| Advisor: | Cleber Renato Mendonça |
| Abstract | |
In this work we have studied the synthesis of Au nanoparticles using shaped ultrashort pulses (800 nm, 30 fs, 1 kHz and 2 mJ), induced by the ionization of the chitosan. Initially we developed a pulse shaping setup that uses a spatial light modulator (liquid crystals array), with which we are able to impose distinct phase mask to the laser pulse. In order to monitor the nanoparticles production process, we used a pump-probe system, consisting of femtosecond pulses (pump) and white light (probe), which allows the observation of the plasmon band enhancement and hence the nanoparticles formation dynamics. The results obtained by Fourier Transform limited pulses have shown that the nanoparticles formation is due to the nonlinear ionization of chitosan, which is related to hydroxyl group oxidation to the carbonyl group. Transmission electron microscopy measurements provided the sizes (20-100 nm) and shapes (spheres, pyramids, hexagons, rods, etc.) of the produced nanoparticles. Moreover, our results revealed that ionization is initiated by multiphoton absorption, more specifically by four photons absorption. Using pulses shaped with constant, step and cossenoidal (with different frequencies) spectral phase masks, we investigated their influence in the nanoparticles formation. We conclude that longer pulses are more favorable to the ionization process and, consequently, to the gold ions reduction for the synthesis of the metallic nanoparticles. This behavior is probably due to the redistribution of the absorbed energy to the vibrational modes, which is more likely for longer pulses. Therefore, the approach presented here can open new ways to produce metallic nanoparticles, which can be further explored from applied and fundamental points of view. (AU) | |