Chemistry Reference
In-Depth Information
the alloy formed by the metal and the substrate. The result is the formation of an
array of nanoscaled islands whose size can be controlled by varying the procedure
parameters such as temperature and dewetting time or by starting from a different
film thickness.
Surface energy minimization is the driving force of this process, since the
surface to volume ratio of a nanoparticles array is smaller than that of a thin
film (Thompson
2012
).
A said before, the size of the islands demonstrated to be determinant for the final
diameter of the wires (Gudiksen and Lieber
2000
) and it is strictly correlated to the
metal layer thickness (Kojima and Kato
2008
) so, by varying this in the deposition
stage, it is possible to obtain a certain control over the size of the islands and,
consequently, over wires diameter.
According to equilibrium thermodynamics it is possible to calculate the minimum
radius of a liquid metal cluster as
2
γ
LV
V
L
RT
ln
σ
r
min
=
(5.4)
where
γ
LV
is the liquid vapour surface free energy, V
L
is the molar volume, R is
the ideal gas constant, T is temperature and
is the vapour phase supersaturation
(Hu et al.
1999
). Nevertheless droplets diameters smaller than those predicted by
this formula have been frequently experimentally found (Kojima and Kato
2008
)
suggesting that this constraint from thermodynamics can be overcome out of equi-
librium. Still this can be used as a general rule of thumb to know the dependence of
droplets size from the thermodynamic conditions.
A scheme of a VLS growth mechanism with the dewetting of a solid-state film is
represented in Fig.
5.2
.
σ
Dewetting Procedure for Au, Fe, Ni Thin Films
The dewetting stage occurs in
hydrogen atmosphere without the precursors at different temperatures for the various
metals. The choice of the dewetting temperature is made basing on metal-catalyst
phase diagram. The primary feature, is the eutectic temperature of the alloy because,
for a liquid phase dewetting to occur it is necessary to exceed it but, if the film is
thin enough, a solid-state dewetting can occur via surface diffusion even below the
melting temperature (Thompson
2012
).
The best temperatures for dewetting of Ni, Fe and Au films was found to be 1100
◦
,
1250
◦
and 600
◦
C respectively, while for nanoparticles no dewetting is needed. A
summary of the eutectic temperatures of the alloys and the dewetting temperature
used is reported in Table
5.1
.
In Fig.
5.3
the dewetting of a nickel layer is reported, the size of the islands is
mainly in between 10 nm and 1
μ
m and the shape is generally squared.
