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aggregates from solution onto the substrate is therefore likely for
DMF and THF. However, the laminar structures on the substrate
observed for ethanol solution cannot be explained by a simple crystal
transfer. A new self-assembly mechanism, illustrated in Fig. 12.5, is
therefore proposed: Before spin coating, interactions between the
PYPA molecules are prevented owing to a strong screening action
of the solvent. When the spinning process is started, excess fluid is
spun away, significantly reducing the number of PYPA molecules on
the substrate. During the spin-coating process, the solvent starts
to evaporate resulting in increasing concentrations of PYPA in the
solution. At a critical concentration, the molecules start to interact
with each other resulting in nucleation of crystals. The pyrene
moieties form hydrophobic
stacks pointing toward the air at the
air/ethanol interface. In contrast, utilizing hydrogen bonding, the
hydrophilic phosphonic acid groups develop 2D networks pointing
into the solution. Finally, a Langmuir
π−π
Blodgett (LB) like structure
is formed, which continues to grow into the laminated bilayer
structure. After complete evaporation of the solvent, the crystals
are deposited on the substrate as polycrystalline film. Washing of
the sample surface with ethanol removes physisorbed molecules,
leaving behind a monolayer of PYPA [4]. Similar experiments with
derivatives of PYPA (PYPDEt, ODPA, and PhPA) did not result in the
formation of polycrystalline films, which points to nucleation and
growth of the laminar structures to be possible only for molecules
with both strong
π−π
stacking of their aromatic rings and hydrogen-
bonding capabilities. The importance of the air/alcohol interface for
the assembly process was demonstrated via control experiments in
environments saturated with ethanol where laminate crystal growth
could not be achieved [3].
between interdigitated
Au electrodes was used to carry out electrical measurements under
vacuum to characterize their charge conduction characteristics. The
three-terminal device structure consisted of two Au electrodes and
a Si back gate. At low temperatures (
Self-assembly of PYPA multilayers on SiO
2
240 K), a modulation of the
drain current (
) was recorded, indicating
p-type semiconducting behavior of the stacked PYPA film with on/
of ratios of up to 1000 (Fig. 12.6). Carrier mobility was observed
to depend heavily on the gate voltage, which reveals the presence
of trap states in the polycrystalline layer. Trap-filled space charge
limited conduction dominates. At room temperature, Poole
I
) with gate voltage (
V
DS
GS
Frenkel
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