Chemistry Reference
In-Depth Information
Figure 5.7. Optical microscope image of a thin film (thickness
∼
2
m) of
α
-
p
-
µ
0mm
2
, crossed polarizers). Reprinted
from
Journal of Crystal Growth
, Vol. 209, J. Caro, J. Fraxedas and A. Figueras,
Thickness-dependent spherulitic growth observed in thin films of the molecular
organic radical p-nitrophenyl nitronyl nitroxide
, 146-158, Copyright (2000), with
permission from Elsevier.
NPNN grown on a glass substrate (1
.
6
×
1
.
the radii of the spherulites in a wide time period. We also observe that when two
spherulitic fronts meet, the boundaries have a well-defined curvature and remain
static. Another important point is that the vertical resolution improves with depo-
sition time:
8
and
9
in Fig. 5.8(f) are better resolved than
1
and
2
in Fig. 5.8(a)
despite having similar values for the radii. This implies that the thickness of the 2D
spherulites is independent of the radius and that it increases with deposition time.
Figure 5.9 shows the time evolution of the radii of selected 2D spherulites from
Fig. 5.8. We observe that the process is non-linear and accelerated, d
2
R
d
t
2
0.
It is also interesting to notice that, at a given time, the radial growth velocity
v
R
=
/
>
d
t
(slope) is nearly the same for all spherulites, which implies that it
depends on the deposition time and certainly not on the radius of the spherulites. In
the case discussed here the thickness of the film is increasing with time because of
continuous exposure to the molecular beam. The non-linearity is more pronounced
at the beginning of the experiment (roughly between 250 and 350 s) and the velocity
nearly tends towards an asymptotic value, so that 2D spherulites that are formed
last show almost linear growth.
The mechanism of growth of the films may be modelled by considering that the
vapour phase transported to the substrate initially condenses in an amorphous state
d
R
/
