Micro and nano structure formation produced by fs laser ablation in reactive gas or liquid

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Micro and nano structure formation produced by fs laser ablation in reactive gas or liquid

Μπαρμπέρογλου, Μάριος
Barberoglou, Marios

Στρατάκης, Μανώλης

Femtosecond laser pulses open new research possibilities, allowing not only the observation of ultrafast phenomena, like chemical reactions, but also offering high energies for studying non-linear phenomena. Femtosecond laser pulses interact with mater allowing minimal thermal effects, producing micro/nano structures with desired properties. This thesis consists of two parts. In the first and main part of this work, we will investigate the microstructuring on Silicon via laser ablation followed by a study of the wetting properties on these structured surfaces is made. In the second part a brief discussion is made on aluminum nanostructures and nanoparticles produced by fs ablation of Al in liquid media. Silicon is widely used in microelectronic devices mainly due to its well defined characteristics and low cost. However,commercial silicon has its limitations. .For example it is transparent at wavelengths close to the NIR, which are mainly used in telecommunications (1,33μm and 1,55μm), making the detection of these frequencies impossible. By laser structuring of the silicon we are able to overcome these difficulties and the produced micro roughness has new electrical and optoelectronic properties. In addition it shows enhanced wetting properties when water drops contact the structured surface. Besides the electronic and optoelectronic properties, microstructuring of silicon gives rise to other important properties, such as wetting properties. We demonstrate the ability to change the surface from hydrophilic to hydrophobic, by measuring the corresponding contact angle of water drops sitting on surfaces with different roughness. A comparison with the Lotus leaf, one of the most famous water repellent plant surfaces, is also investigated. The main part of the thesis is dedicated on the microstructuring of silicon via femtosecond laser irradiation. A systematic study has been made by irradiating flat crystalline silicon inside a vacuum chamber in the presence of 500 Torr SF6 etching gas at different laser fluencies, varying from 0.33 - 2.25 J/cm2 producing structures with different roughness. They exhibit a conical shape and their height, density and geometry were calculated by corresponding scanning electron microscope (SEM) images. By letting a nanopure water drop sit or roll on the silicon structured surfaces, we were able to measure the static contact and sliding angles and study the change in the wetting properties due to different surface roughness. The increasing surface roughness has a direct impact on the contact angle of the water droplet so that the surface changes from hydrophilic to hydrophobic. Following the ablation process the modified surfaces were covered by a hydrophobic coating rendering the surface superhydrophobic. Accordingly, a surface tilt of only 4° was enough to initiate the water drops motion for the roughest surface. The dynamic behavior of water droplets free falling on flat or patterned surfaces as well as on those of the Lotus leaf was examined using a high-speed camera. The velocities before and after each shock were calculated either from the distance traveled between successive snapshots (at high impact speeds), or from the corresponding maximum heights attained (at low impact speeds). Self cleaning experiment have also been carried out, by covering the structured surface with carbon dust particles and letting a water drop roll or bounce on it. The dynamic optical control/response of the wetting behavior of liquids was studied on hierarchically structured ZnO surfaces produced by depositing ZnO on the structured silicon surfaces by pulsed laser deposition (PLD). The final surface exhibits roughness at two-length scales comprising micrometer-sized Si spikes decorated by ZnO nano-protrusions. It is shown that a liquid droplet on these surfaces can be rapidly and reversibly switched between hydrophobicity and superhydrophilicity by alternation of UV illumination and dark storage or thermal heating. By studying the magnitude and the rate of the photoinduced transitions, the contribution of roughness at different scales is investigated in the framework of two theoretical wettability models. In the second part on the thesis, the formation of self-organized nanostructures (NS) on bulk Al were studied when ablated by fs pulses either in air, or immersed in liquids. In the later case, the NS are regularly formed on the Al surface, independently of the laser polarization. Additionally, a dispersion of Al nanoparticles (NPs) into the liquid occurs and irregular nano-bumps are produced when the irradiation takes place in air. NP dispersions as well as NS formed on Al surface show a characteristic absorption peak in the near UV which has been attributed to plasmon oscillation of electrons. The wings of this peak extending to the visible, lead to a distinct yellow coloration of the processed Al surface as well as the liquid dispersions. Ultrafast laser processing of bulk Al in liquids may be potentially a promising technique for efficient production of nanosized aluminum. (EN)

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