Chemistry Reference
In-Depth Information
and
T
sub
(Servet
et al
., 1994). At
T
sub
=
RT the long interlayer spacing of films
obtained under low
D
t
(0.1-0.5 nm s
−
1
) is about 2.37 nm, while for fast
D
t
this
distance increases to 2.44 nm, corresponding to the most stable form (
α
-phase) and
to the metastable kinetically favoured (
463 K
the interlayer spacing is 2.24 nm, indicating a different structure, which has been
designated as the
β
-phase), respectively. When
T
sub
-phase. At temperatures near the bulk melting point (about
560 K) additional reflections suggest the appearance of a new phase, the
γ
δ
-phase,
with a long spacing of 3.63 nm.
The deposition of DIP on atomically smooth SiO
2
surfaces at
T
sub
418 K leads
to the formation of organic thin films with an exceptionally high degree of molecular
ordering (Durr
et al
., 2003a). Films 40 nm thick exhibit a layered structure with
a characteristic layer spacing of 1.65 nm. The value of this distance suggests an
upright orientation of the DIP molecules within the layers. A comparison with the
length of the molecules of 1.84 nm yields an estimation of the tilt angle of 15-20
◦
relative to the substrate normal. This thin-film phase is referred to as the
-phase.
When grown on polycrystalline gold surfaces the DIP films crystallize in a second
phase (
σ
-phase). This polymorph is characterized by an interlayer distance of about
1.6 nm. In this case the tilt angle between the DIP molecular axis and the crystal
planes is found to be 17
◦
. In contrast to the situation for the SiO
2
substrate discussed
above, the DIP molecules are oriented with their long axes almost parallel to the
gold surface.
p
-6P films deposited on isotropic substrates such as glass or ITO-coated glass
reveal the existence of different growth regimes. When the deposition is carried out
at
T
sub
<
λ
433 K, the known bulk-like
β
-phase is found, whereas at
T
sub
>
433 K
a new phase, denoted as the
γ
-phase, is observed. This monoclinic
γ
-phase is
characterized by the lattice parameters
a
=
0
.
798 nm,
b
=
0
.
554 nm,
c
=
2
.
724 nm
8
◦
(Resel, 2003).
Concerning Pc-based films, new crystalline phases have been suggested for H
2
Pc
(Heutz & Jones, 2002) and TiOPc (Brinkmann
et al
., 2002a). In this last case the
phase is triclinic, with space group
P
1, and with lattice parameters
a
and
β
=
99
.
=
1
.
40 nm,
90
◦
. Recall here the
case of CuPc thin films grown in microgravity briefly described in Section 3.2.
When grown in the Space Shuttle Orbiter in low Earth orbit, CuPc crystallizes in
the M-phase, a phase that has never been obtained under unity gravity (Debe &
Kam, 1990).
Although the detailed information on the polymorphic phases has to be obtained
with e.g., diffraction and scanning probe methods, our eyes are attracted by the
beauty of optical microscope images often encountered with polymorphs under
transformation. Figure 5.17 shows an optical microscopy image of a
p
-NPNN
thin film (thickness
2
◦
,
9
◦
and
b
=
1
.
40 nm,
c
=
1
.
06 nm,
α
=
109
.
β
=
133
.
γ
=
∼
2
µ
m) on a glass substrate, exhibiting a transformation at
