Ph. D. THESIS 2009

Our objective was to fabricate silver nanostructures using a two-photon
absorption photochemical process that occurs at the laser beam spot. Due to its
nonlinear character, the light absorption is highly localized in a small volume
near the focal point and allows the laser writing fabrication in three-dimensions
through a deep penetration within the material.
The general redox process of silver ions reduction at the laser focal point can be
written:
Silver salt (water soluble) + Fe(III)citrate complex Silver(water
insoluble) + Fe(II)citrate complex
Preliminary experiments performed by laser irradiation of an aqueous solution
of silver salt and sensitizer generated within solution a large amount of colloids
instead of a solid structure.
Figure 3. Optical image of silver colloids recorded in dark-field with x20 objective
The dark-field image from Figure18 displays the light scattering from dispersed
silver colloids using white-light illumination.
The key point of our experiment was to increase the solution viscosity which
allows to control the rate of reduction and to keep the metallic deposition
localized in the small volume at the focal point during the laser action and after
it stops.
In this respect we chose the poly (4-styrenesulfonic acid), denoted PSS that
embodies all these properties, as a system stabilizer. Thus, we created a
chemical system that contains silver nitrate as active specie and ammonium
ferric citrate as sensitiser, dispersed into a PSS matrix. It offers the possibility
to fabricate continuous and well defined silver nanostructures by two-photon
absorption.
3.3. TPA fabrication of silver wires
Networks of continuous silver lines were fabricated using the chemical system
containing the active species included in the PSS polymer host.
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