Understanding the Properties of Tailor-Made Self-Assembled Monolayers with Embedded Dipole Moments for Interface Engineering

Abstract

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.

A dithiocarbamate anchoring group as a flexible platform for interface engineering

“…XP spectra were acquired using either a Scienta R3000 electron energy analyzer (BESSY II) or a SCIENTA SES200 spectrometer (Max IV) having established performance and reliability. 23,24,40,42 Spectra acquisition was carried out in normal emission geometry with an energy resolution of either B0.1 eV (Max IV) or B0.3 eV, B0.6 eV, and 0.7 eV (BESSY II) at excitation energies of 350 eV, 580 eV, and 720 eV, respectively. The binding energy (BE) scale of the XP spectra was referenced to the Au 4f 7/2 peak of the underlying substrate at a BE of 84.0 eV, at each particular excitation energy.…”

“…Therefore, for the most basic PPd and PPz SAMs, we rely on theoretical simulations (Section 3.4), deriving b and g from the optimized molecular structures, calculating the respective a, and comparing them with the experimental values (such an approach was recently implemented in ref. 24).…”

“…a docking group that provides anchoring to the substrate, a tail group comprising the ''outer'' SAM-ambient interface, and a spacer that separates the docking and tail groups and drives the self-assembly. 17,18 While all these building blocks can contribute to the joint dipole moment of the molecules constituting the film, it is mostly a dipolar tail group 2,3,5,10,19 or a polar mid-chain moiety [20][21][22][23][24] which is selected to specifically adjust the interfacial dipole. In contrast, the docking group is predominantly chosen based on its affinity to a particular substrate serving as the electrode or, in the context of organic photovoltaics, also as the buffer layer.…”

X-Ray Photoelectron Spectroscopy for Determining Interface Dipoles of Self-Assembled Monolayers

“…The superposition of the electric fields due to the interface dipoles of all adsorbed molecules causes a step in the electrostatic energy. [31,40,42,45,46] This not only triggers a shift in the position of the vacuum level above the surface and, consequently, changes the sample work function; [8,11,[14][15][16][17]19,21,22,27,28,31,32,35,41,43,44,47,39,25,[48][49][50][51][52] it also modifies the interfacial energy-level alignment, i.e., the positions of the electronic states of the adsorbed monolayer relative to those of the substrate. [27,28,31,39,40,42,[45][46][47][53][54][55][56][57][58][59][60]…”

“…[27,28,31,39,40,42,[45][46][47][53][54][55][56][57][58][59][60][61][62] This has an immediate impact on chargetransport through the adsorbed layer [53][54][55][56][57][58][59][60][61][62] and also on the positions of the core levels of the adsorbate. [20][21][22]42,48,49,[63][64][65] These changes in core-level binding energies can be probed by X-ray photoelectron spectroscopy (XPS), [66][67][68][69] which is one of the default techniques for characterizing SAMs, used, for example, for analyzing the quality of self-assembled monolayers and to verify bonding. [70][71][72][73][74][75]…”