Work Function Tuning at Interfaces by Monomolecular Films

The control over the work function of surfaces and interfaces is one of the most important issues of modern surface science and nanotechnology, e.g. in context of organic electronics and photovoltaics. The goal of this work was to look for new ways to control the work function of metal substrates by using molecular self-assembly. Two different strategies were used. The first strategy was to use aliphatic and aromatic molecules which contain an embedded dipolar group (midchain functionalization). Such self-assembled monolayers (SAMs) allow for tuning the substrate work function in a controlled manner, independent of the docking chemistry and, most importantly, without modifying the SAM-ambient interface. In the case of aliphatic films, we used alkanethiols functionalized with an embedded ester dipole, with the length of both top and bottom segments as well as the direction of the embedded dipole being varied. In the case of aromatic systems, we used terphenyl based thiols functionalized with an embedded pyrimidine dipolar group, with the direction of the dipole being varied. The electronic and structural properties of these embedded-dipole SAMs were thoroughly analyzed using a number of complementary characterization techniques combined with quantummechanical modeling. It is shown that such mid-chain-substituted monolayers are highly interesting from both fundamental and application viewpoints, as the dipolar groups are found to induce a potential discontinuity inside the monolayer, electrostatically shifting the core-level energies in the regions above and below the dipoles relative to one another. Particularly imptortant, in context of the present work, is the fact that the mid-chain functionalized films are indeed well suited to adjust the work function of metal substrates. This could be e.g. done by varying the orientation of the dipolar group but also by mixing the molecules with differently oriented dipoles as was demonstrated in the present work. Within the second strategy, we used photoresponsive systems, viz. azobenzene substituted alkanethiols, having a specially designed architecture to control the packing density and carrying an additional dipolar tail group. These novel SAMs were studied in detail by using spectroscopic and microscopic techniques. Performing photoisomerization experiments we obtained a reproducible, stimuli-responsive change in the work function which was, however, limited to some extent due to the strong steric hindrance effects. In order to reduce these effects, we diluted the azobenzene molecules with short spacer molecules, which resulted in an improvement in the photoswitching behavior

The Effects of Embedded Dipoles in Aromatic Self-Assembled Monolayers

Using a representative model system, here electronic and structural properties of aromatic self-assembled monolayers (SAMs) are described that contain an embedded, dipolar group. As polar unit, pyrimidine is used, with its orientation in the molecular backbone and, consequently, the direction of the embedded dipole moment being varied. The electronic and structural properties of these embedded-dipole SAMs are thoroughly analyzed using a number of complementary characterization techniques combined with quantum-mechanical modeling. It is shown that such mid-chain-substituted monolayers are highly interesting from both fundamental and application viewpoints, as the dipolar groups are found to induce a potential discontinuity inside the monolayer, electrostatically shifting the core-level energies in the regions above and below the dipoles relative to one another. These SAMs also allow for tuning the substrate work function in a controlled manner independent of the docking chemistry and, most importantly, without modifying the SAM-ambient interface

Nanoscale electrical investigation of layer-by-layer grown molecular wires

Nanoscopic metal-molecule-metal junctions consisting of Fe-bis(terpyridine)-based ordered nanostructures are grown in layer-by-layer fashion on a solid support. Hopping is demonstrated as the main charge-transport mechanism both experimentally and theoretically

Self-Assembled Monolayers of Pseudo‑<i>C</i>_2<i>v</i>-Symmetric, Low-Band-Gap Areneoxazolethiolates on Gold Surfaces

A series of three homologous arene­[2,3-<i>d</i>]-oxazole-2-thiols (benzoxazole-2-thiol (BOxSH), naphthaleneoxazole-2-thiol (NOxSH), and anthraceneoxazole-2-thiol (AOxSH)) were deposited onto Au(111) to obtain surfaces suitable as injection layers for organic electronics. The guiding idea was that the increasingly extended conjugated system would lower the band gap of the films while the introduction of the annulated heteroaromatic ring would provide the opportunity for pseudosymmetric attachment of the sulfur anchor, what should lower the conformational freedom of the system. In fact, the annulation of the oxazole ring lowers the optical band gaps of the parent compounds to 3.1–4.0 eV, depending on the number of benzene rings. To characterize the respective monolayers, a variety of spectroscopic techniques such as ellipsometry, infrared reflection–absorption spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy have been utilized. The monolayers of BOxS exhibit a lower film quality than those of NOxS and AOxS, with enhanced molecular density and more upright molecular orientation with increasing molecular length. Infrared spectroscopy suggests that the nitrogen atoms of the oxazole rings are located more closely to the Au(111) surface than the oxygen atoms, although no hints for an electronic interaction between the N atoms and the gold surface could be found. This preferred orientation could be tentatively traced to packing effects, solving a conundrum of the literature

Effects of Embedded Dipole Layers on Electrostatic Properties of Alkanethiolate Self-Assembled Monolayers

Alkanethiolates (ATs) forming self-assembled monolayers (SAMs) on coinage metal and semiconductor substrates have been used successfully for decades for tailoring the properties of these surfaces. Here, we provide a detailed analysis of a highly promising class of AT-based systems, which are modified by one or more dipolar carboxylic acid ester groups embedded into the alkyl backbone. To obtain comprehensive insight, we study nine different embedded-dipole monolayers and five reference nonsubstituted SAMs. We systematically varied lengths of the alkyl segments, ester group orientations, and number of ester groups contained in the chain. To understand the structural and electronic properties of the SAMs, we employ a variety of complementary experimental techniques, namely, infrared reflection absorption spectroscopy (IRS), high-resolution X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM), and Kelvin probe (KP) AFM. These experiments are complemented with state-of-the-art electronic band-structure calculations. We find intriguing electronic properties such as large and variable SAM-induced work function modifications and dipole-induced shifts of the electrostatic potential within the layers. These observations are analyzed in detail by joining the results of the different experimental techniques with the atomistic insight provided by the quantum-mechanical simulations