Growth and physical properties of crystalline rubrene - BOA Bicocca ...

3.1 Sample preparation 29
Figure 3.2: Vaporization rate as a function of the inert gas pressure (in this case
He) with the source at constant temperature. Only the three possible regimes for
a zero gas flux are shown here.
ing crystals coalesce, leading to the formation of larger crystals with a large
number of defects. If a non-zero flux of the inert gas is used, instead, the
convection due to the thermal gradients along the tube dominates over the
forced convection of the gas flux. In this case one can easily obtain larger
crystals (with a size up to few cm) with a good structural quality. Usually
crystals of oligoacenes grown by this technique are very wide and thin (with
an aspect ratio the order of 10 3 ). This is due to the peculiar characteris-
tics of the intermolecular bonds between conjugated molecules. Indeed, as
shown in chapter 1 the presence of the π orbitals, which are concentrated
around specific areas of the molecules, leads to intermolecular bonds which
are much stronger in specific directions with respect to others. Thus, the
approaching molecules easily attach to the borders of the growing crystal,
while the molecules landing on the surface of the crystal move along it until
they reach the crystal border.
3.1.2 Thin film growth
Epitaxial thin films of high crystalline quality and with controlled prop-
erties can be grown by Organic Molecular Beam Epitaxy (OMBE)[40, 77],
a technique based on Molecular Beam Epitaxy (MBE), which is nowadays
widely used for the epitaxial growth of inorganic materials[78].
This technique consists in slowly depositing the sublimated molecules