Chemistry Reference
In-Depth Information
working with
T
sub
≤
RT OMBD results in the formation of amorphous films, highly
ordered pentacene films have been grown at low substrate temperatures (200 K)
when pentacene molecules with high kinetic energies of a few eV were deposited
on a silver substrate.
By varying the nature and/or the pressure of the carrier gas and/or the temperature
of the organic material, the translational kinetic energy of the molecules can be
controlled. In particular, given a carrier gas pressure of about 300Torr and the vapour
pressure of pentacene being 1
10
−
3
Torr at 493 K, the translational kinetic
energy is estimated to vary between 5 and 0.4 eV by changing the carrier gas from
He to Kr. UHV is still needed in order to keep substrate surfaces atomically clean
prior to deposition. The growth of good quality pentacene thin films at relatively
low substrate temperature points to the fact that the local annealing induced by the
impact of the hyperthermal molecules is a more efficient route to order than using
either a global annealing of the film or higher substrate temperatures.
If we allow for a relaxation of the stringent experimental conditions inherent
to OMBD by working in a HV environment, high-quality thin films can also be
obtained. As case studies we consider TTF-TCNQ and
p
-NPNN, and the charac-
terization of their physical properties will be discussed in detail in Chapter 6.
High-quality thin TTF-TCNQ films can be obtained by thermal sublimation
in HV (
.
2
×
10
−
6
mbar) of recrystallized TTF-TCNQ powder.
Ex situ
cleaved al-
kali halide substrates such as NaCl(100), KCl(100) and KBr(100) are held at
RT and the evaporation temperature set to 433 K. TMAFM images of such
films are depicted in Figs. 3.18(a) and (b). The films consist of highly oriented
and strongly textured rectangular-shaped microcrystals, which are oriented with
their
a
- and
b
-axis parallel to the [110] and [110] substrate directions, respec-
tively, due to the cubic symmetry of the substrates. FTIR spectra of the films,
grown on KBr(100), confirm the existence of CT. This is shown by the ob-
served shift of the CN stretching mode shown in Fig. 3.18(c), which is associated
with
∼
from the
π
-donor molecule TTF to the acceptor TCNQ (Chappell
et al.
,
1981).
The second example corresponds to thin films of
p
-NPNN, grown on glass,
on
ex situ
cleaved alkali halides and on mica (muscovite) substrates, obtained
by sublimation of the recrystallized powder
p
-NPNN precursor at
c
.10
−
6
mbar,
sublimation temperature 413 K and
T
sub
=
RT, which crystallize in the monoclinic
α
-phase (see Fig. 1.23(a)). The films are (002)-oriented (
ab
-planes parallel to the
substrate surface) as illustrated in Fig. 3.19 for a glass substrate with a high degree of
orientation, indicated by a small FWHM of the rocking curve:
1
◦
. In the case
of highly oriented pyrolytic graphite (HOPG) substrates, the films crystallize in the
monoclinic
δ
-phase (see Fig. 1.23(d)) and are (020)-oriented (
ac
-planes parallel
to the substrate surface). Note that the HV-grown films never crystallize in the
