Chemistry Reference
In-Depth Information
presence of two phases: the well-known monoclinic one and an additional phase
closely related to it. This new phase is welcome considering the fact that TTF
exhibits at least two polymorphs as we saw in Section 1.6.
XRD patterns of TTF-TCNQ films grown by CVD on Si(100) substrates also
show this kind of extra reflections (de Caro
et al.
, 2000a). This is perhaps the first
evidence, albeit incomplete and thus questionable, of a new crystallographic phase
of TTF-TCNQ. The conclusive observation of a new phase of TTF-TCNQ, e.g.,
with mixed-stacked structure as for the red phase of TMTSF-TCNQ, would be ex-
tremely interesting. The known and newly observed structures for both TMTTF and
TTF-TCNQ might be tentatively ascribed to the thermodynamical and kinematical
phases, respectively.
If we use different substrates under identical experimental conditions (samples
grown in the same experimental run) we find interesting results that are summarized
in Fig. 5.16. The figure shows the
XRD patterns for TMTTF films grown on
three different substrates: (a) OTS films prepared on silicon wafers, (b) LB BaAA
films and (c) thermally grown SiO
2
. For OTS substrates the
θ/
2
θ
202
202
*
families are clearly resolved and in addition some microcrystals are oriented with the
(200) and (204) planes parallel to the substrate. What is relevant is that using either
BaAA or SiO
2
substrates either the
{
}
and
{
}
202
202
{
}
{
}
* families are predominantly
selected, respectively. The conclusion is that substrates can not only determine the
orientation of the molecules but discriminate polymorphs.
Moving to different materials, for instance pentacene, two different crystallo-
graphic phases characterized by different interlayer distances
d
(100)
have been iden-
tified for thin films. As mentioned above, the pentacene (100) plane exhibits the
lowest-energy surface and therefore the films tend to be oriented with these planes
parallel to the substrate surface. The deposition of pentacene on SiO
2
initially leads
to the formation of dendritic domains with a substrate-induced structure (so-called
thin film phase or
or the
-phase), which is characterized by a layer separation of 1.55 nm
(Dimitrakopoulos
et al.
, 1996). When exceeding a critical coverage, the XRD pat-
tern points towards the formation of a second phase (
α
45 nm,
which corresponds to the S-phase discussed in Section 1.6 (the V-phase is also
known as the
β
-phase) with
d
(001)
=
1
.
-phase). A comparison of the interlayer separation in these two
phases,
d
(100)
, with the length of the pentacene molecule,
γ
∼
1.6 nm, suggests that
the molecules are inclined by different angles with respect to the layer planes.
This critical thickness is
T
sub
-dependent, ranging from about 100 nm at RT down
to 30 nm for
T
sub
360 K (Bouchoms
et al.
, 1999). Films deposited at RT con-
sist completely of the
α
-phase, while the
β
-phase appears for increasing substrate
temperatures.
Pentacene and tetracene films can be oriented by using friction-transferred PTFE
polymer substrates (see Section 3.3). The films consist of both crystallographic
