Ph. D. THESIS 2009

Figure 15. a Optical image of gold wires network in polymer matrix, b)
SEM image of two double wires after removing the unreacted matrix
There are two main aspects which allow a clear description of a “double-wire”
characteristic: the size of the wire, denoted wire width, and the double-wire
separation distance, denoted distance between lines.
For comparison, atomic force microscopy (AFM) measurements were
performed in order to establish the 3D aspect of the metallic wires (Figure 16).
2.0µm
Z[nm]
z [nm]
600
500
400
300
200
100
0
0 0.5 1 1.5 2 2.5 3 3.5 4 5
x[m]
X[µm]
11
Distance
between lines
Wire width
Figure 16. AFM measurements of a typical gold double-wire: a) topographic
view, b) cross-section along the double wire, c) 3D view
A topographic AFM view is shown in the left images of Figure 16a) and a
cross section along them is plotted at the centre, in Figure 16b). Each metallic
wire has a full width at half maximum of 600 nm with a height of 500 nm. A
3D view is shown on the right images. As it can be seen, regular and
continuous metallic wires can be fabricate, their walls being almost vertical
[10].
At the first estimation it can be assumed that the metal deposition does not
occur at the center of the laser beam, but on its outer diameter. In fact, there is
about more complex phenomena that occur during the gold wires fabrication.
How can we control the cluster generation, and further the growth of
metallic structures, when the energy provided by laser irradiation is
considerably lower than the UV one but highly localized within the gold salt-
doped matrix?
The fabricated lines are very sensitive to the writing conditions.
At low laser power the lines are thin and show granular structure, as shown in
Figure 17(a). On the contrary, at higher power the lines are regular, but exhibit
a double-wire shape due to the ablation process, which occurs immediately
after laser interaction with gold nanoparticles previously formed – Figure 17(b).
We may admit that the size of nanoparticles that act as nucleation centres is
another parameter which influences the evolution of fabrication process. The
morphology of patterns undergoes a spectacular change if the laser power and
the critical exposure time are oversized, due to a novel ablation process.