posted on 2018-11-26, 00:00authored byMichael Gärtner, Eric Sauter, Giulia Nascimbeni, Andreas Petritz, Adrian Wiesner, Martin Kind, Tarek Abu-Husein, Michael Bolte, Barbara Stadlober, Egbert Zojer, Andreas Terfort, Michael Zharnikov
Self-assembled
monolayers (SAMs) are frequently used for interfacial
dipole engineering in organic electronics and photovoltaics. This
is mostly done by the attachment of dipolar tail groups onto the molecular
backbone of the SAM precursors. The alternative concept of embedded
dipoles involves the incorporation of polar group(s) into the backbone.
This allows one to decouple the tuning of the electrostatic properties
of the SAM from the chemical identity of the SAM–ambient interface.
Here we present design and synthesis of particularly promising SAM
precursors utilizing this concept. These precursors feature the thiol-docking
group and a short heteroaromatic backbone, consisting of a nonpolar
phenyl ring and a polar pyrimidine group, embedded in two opposite
orientations. Packing density, molecular orientation, structure, and
wetting properties of the SAMs on Au substrates are found to be nearly
independent of their chemical structure, as shown by a variety of
complementary experimental techniques. A further important property
of the studied SAMs is their good electrical conductivity, enabling
their application as electrode modifiers for low-contact resistances
in organic electronic devices. Of particular interest are also the
electronic properties of the SAMs, which were monitored by Kelvin
probe and high-resolution X-ray photoelectron spectroscopy measurements.
To obtain a fundamental understanding of these properties at an atomistic
level, the experiments were combined with state-of-the-art band structure
calculations. These not only confirm the structural properties of
the films but also explain how the C 1s core-level binding energies
of the various atoms are controlled by their chemical environments
in conjunction with the local distribution of the electrostatic potential
within the monolayer.