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spherulitic morphology with a low density of nucleation centres. The spherulitic
morphology is characterized by the radial growth of lamellar crystals around a
central nucleation point. Most of the knowledge on spherulitic growth arises pri-
marily from investigations performed on polymers. The usual method of obtaining
thin films of pure or of a mixture of polymers is the isothermal crystallization of
a supercooled melt, which is generally confined between two glasses. The same
behaviour has been reported for thin films of polymethine dye salts obtained by
spin coating. In this case the films remain amorphous below 30 nm in thickness but
crystallize with spherulitic morphology above 30 nm (Dahne, 1995).
Video microscopy with crossed polarizers permits the direct and non-invasive
observation of the nucleation and growth process for many substances, and thus
the study of the time evolution of the spherulite radius
R
(
t
). When the growth is
controlled by diffusion the radius of the spherulites increases as
R
(
t
)
t
1
/
2
, while
when the growth is determined by a nucleation-controlled process (incorporation
of atoms or molecules to the surface of the crystalline part) the radius increases
linearly with time,
R
(
t
)
∝
t
.
The thin films of
p
-NPNN reported here were grown on vacuum glass view-
ports by evaporation of the
p
-NPNN precursor in high vacuum (low 10
−
6
∝
mbar
0.5
µ
mh
−
1
and with the viewport held at RT. Under these con-
ditions the films crystallize in the monoclinic
range) with
D
t
∼
-phase and are (002)-oriented (
ab
-
planes parallel to the substrate surface) exhibiting a high degree of orientation (see
Fig. 3.19). Figure 1.23(a) shows the bulk molecular distribution of
α
-
p
-NPNN ori-
ented with the
ab
-planes parallel to the substrate. The real molecular arrangement
of the film/substrate interface is unknown and tentatively assumed to be the bulk
distribution. The
ab
-plane is the most densely packed plane and the stacking along
the
a
-direction might favour the radial in-plane growth.
Figure 5.7 shows an optical microscope image (crossed polarizers) of a thin
film of
p
-NPNN grown on a glass substrate exhibiting the characteristic Maltese
cross associated with 2D spherulitic morphology. Note the clearly defined bound-
aries between spherulites. The curvature of the boundaries between two spherulites
depends on the elapsed time between the formation of those two spherulites. Both
spherulites on the right-hand side of Fig. 5.7 are separated by a nearly linear bound-
ary because they formed roughly at the same time.
The time evolution of the formation of the film during evaporation is shown
in Fig. 5.8 as recorded with a CCD camera assembled to an optical microscope
with crossed polarizers. The 2D spherulites are labelled by order of formation. The
in-plane spherulitic crystallization is completed
α
300 s after the observation of the
first spherulite,
1
in Fig. 5.8(a). From Fig. 5.8 we observe that the nucleation cen-
tres do not appear simultaneously and that the density of spherulites is rather low
(
∼
1.6 mm
−
2
). This low density permits a detailed analysis of the time evolution of
∼
