Chemistry Reference
In-Depth Information
adjacent
ab
-planes. In the image, individual hollow cores are not found because the
dislocation density is rather high (
10
9
cm
−
2
), which induces the interaction
or coupling of spirals due to their close proximity. The origin of the high density of
dislocations is due to the way in which the films grow (rapid in-plane crystallization
as discussed in the previous section), under the given experimental conditions, rather
than to an intrinsic property of the molecular solid. In fact it is well established that
small distances between spirals of opposite sign, or equivalently a high density of
dislocations, can be obtained if stress is present at some stage of the formation of
the solid (Sutton & Balluffi, 1995).
According to F. C. Frank (Frank, 1949), growth from the vapour (or solution)
of crystals of highly non-equiaxed organic molecules proceeds through the forma-
tion of an adsorption film differing in molecular orientation from the bulk crystal
(liquid or liquid-crystalline) with a subsequent rearrangement. If the transition (or
rearrangement) to the solid phase occurs at an elevated rate, the density of defects
increases and the so-formed defects may act as spiral centres. Hollow cores have
also been observed for PVD-grown thin pentacene films (Nickel
et al.
, 2004). The
measured dislocation densities of 10-20 nm thick films grown on OTS-terminated
silicon wafers, a silicon oxide surface and on H-terminated silicon by PVD are
0
∼
3
×
10
11
,0
10
11
and 2
10
11
cm
−
2
, respectively. The films were prepared
.
5
×
.
9
×
.
1
×
10
−
3
nm s
−
1
keeping
T
sub
at RT and with a background pressure
with
D
t
=
1
.
25
×
10
−
7
mbar.
Two spirals can be coupled only if they have opposite signs, because in this case
they both define a common terrace. This is topologically not possible when two
spirals have the same sign, because in this case no common step can be formed.
In Fig. 5.12 a detailed example of two interacting spirals is illustrated. Here the
coupling of pairs of screw dislocations of opposite sign is clearly observed, thus
showing the out-of-plane mechanism of growth, following the BCF model referred
to at the beginning of this chapter (Burton
et al.
, 1951). Once
p
-NPNN molecules
reach the surface, they diffuse on it and those that do not re-evaporate interact with
a step. In this way the step advances by incorporation of material into the ledge
and the step tends to spiral around both static dislocations. This mechanism of
growth is known as the Frank-Read source (Frank & Read, 1950) and constitutes
a continuum way of growth, implying that no further surface nucleation is needed.
The measured step heights of the interacting spirals from the images shown
in Figs. 5.11 and 5.12 are 1.2 nm, as pointed out before. From both figures we
observe that either one step (single spiral) or two steps (double spiral) emerge from
the hollow cores but never more than two spirals. Single spirals are associated
with circular hollow cores with mean radii
R
hc
∼
of
∼
1
×
7-10 nm, whereas double spirals
emerge normally from asymmetrically shaped hollow cores (elliptic) and when they
appear circular (possibly due to insufficient lateral resolution), the measured radii
