Chemistry Reference
In-Depth Information
Fig. 8 RAS spectra of a 4T
film grown on KAP(010) at
300 K, with thickness
ranging from zero (clean
KAP) up to 10 nm. Each
spectrum has been collected
after a single deposition
step, corresponding to 1 nm
nominal film thickness. At
the
top
, the 4T molecular
structure is sketched (From
Sassella et al. [
26
])
ordered layer of
-sexithiophene (6 T) deposited onto KAP (which presents its main
optical absorption at that wavelength). At larger wavelength, the anisotropy spec-
trum presents evident artifacts due to the anisotropic optical properties of KAP.
Marked oscillations (due to the KAP birefringence) are superimposed onto the 6 T
signal, modulated by a Fabry-Perot term due to the multiple reflections inside the
sample. KAP rotates the plane of polarization of outcoming light with respect to the
original polarization plane. When the beam passes through the second polarizer of
the RAS apparatus (see next paragraph), a non-null signal is produced, whose
intensity continuously varies with wavelength. For a more detailed discussion, the
reader should read [
25
].
The importance of achieving a real-time monitoring of the growth is evident
when the studied system can undergo a modification with time or with temperature,
after
or even
during
deposition: this is the case when the deposition is performed at
low temperature (T) to get a better order of the layer and then the sample is
thermalized at room T, for example, after its extraction from the deposition
chamber for ex situ characterization. The spectra shown in Figs.
8
and
9
demon-
strate this issue with self-explaining evidence [
26
].
Quaterthiophene (4T) thin films were grown by OMBE under 5
ʱ
10
10
Torr
base pressure on substrates of (010)-oriented potassium acid phthalate (KAP) at
300 K (room temperature (RT)) or at 123 K (low temperature (LT)). A quartz
microbalance installed close to the substrate was used to measure the nominal film
