Using a representative model system, we describe here electronic and structural properties of aromatic self-assembled monolayers (SAMs) that contain an embedded, dipolar group. As polar unit we use pyrimidine, varying its orientation in the molecular backbone and, consequently, the direction of the embedded dipole moment. 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.We show 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 energy levels 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.

Self-assembled monolayers (SAMs) containing embedded dipolar groups offer the particular advantage of changing the electronic properties of a surface without affecting the SAM–ambient interface. Here we show that such systems can also be used for continuously tuning metal work functions by growing mixed monolayers consisting of molecules with different orientations of the embedded dipolar groups. To avoid injection hot-spots when using the SAM-modified electrodes in devices, a homogeneous mixing of the two components is crucial. We show that a combination of high-resolution X-ray photoelectron spectroscopy with state-of-the-art simulations is an ideal tool for probing the electrostatic homogeneity of the layers and thus for determining phase separation processes in polar adsorbate assemblies down to inhomogeneities at the molecular level.

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 non-substituted SAMs. We systematically varied chain lengths, 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), highresolution 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 like 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.orientation of this group in the backbone, the work function of the system can be changed by either +0.57 or −0.42 eV relative to a reference oligophenylene SAM. This variation is achieved without changing the chemistry for docking to the substrate or the chemical composition of the SAMambient interface. 28 Tuning the work function is, however, not the only effect of the embedded pyrimidine group in these systems, since it also induces a potential discontinuity inside the monolayer. This effect significantly changes the transport properties of the SAM, shifting the transition voltage and resulting in current rectification. 30,31 The potential discontinuity also shifts the core-level energies in the regions above and below the embedded dipoles relative to each other. These shifts can be observed directly by X-ray photoelectron spectroscopy (XPS), reflected as different binding energies (BEs) for the photoemission peaks associated with both regions. This observation, along with others, [32][33][34][35] lead us to question the generally accepted chemical shift model that assumes that shifts in the core-level BEs in monomolecular films are solely a consequence of different chemical environments of the respective atoms, 36 with the energy referenced to the Fermi level of the substrate. In contrast, it suggests that electrostatic shifts not related to the immediate chemical environment of an atom are similarly important. Generally, such electrostatic shifts are superimposed on the chemical ones and can under certain circumstances even play a dominant role. 37 The respective electrostatic effects in photoemiss...

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.

We studied here the effect of humidity on the electrical conductivity of pristine and nanoparticle-loaded hydrogel nanomembranes. The membranes were fabricated by the thermally activated cross-linking of amine-and epoxy-terminated, starbranched poly(ethylene glycol) (PEG) oligomers. The resistance of the pristine membrane changed by ∼5.5 orders of magnitude upon relative humidity (RH) variation from 0% to 100%, which is an unprecedented response for homogeneous materials. The dependence of the resistance on the moisture uptake into the membrane could be coarsely described by an exponential function. The loading of the membranes with gold and silver nanoparticles (NPs) resulted in a noticeable improvement of their conductance at low RH but in a small improvement or even a negative effect on the conductance at high RH. Both pristine and NP-loaded PEG hydrogel membranes have significant potential as highly sensitive elements in humidity sensors and moistureresponsive nanoelectronic devices. ■ INTRODUCTIONThere is a continuous demand for light, small, and flexible assemblies based on advanced inorganic and organic materials for different applications as sensors and stimuli-responsive systems. A particularly important area is humidity-sensitive systems, which are of significance in various fields, such as medicine, agriculture, industry, goods storage, and environmental monitoring. In this context, different materials including but not limited to metal oxides, ceramics, and polymers have been used, relying on different kinds of transduction techniques, such as capacitive, resistive, hygrometric, and gravimetric ones. 1,2 Since recently, nanostructured materials and organic/inorganic hybrid systems are utilized because of their superior performance in comparison to macroscopic onecomponent systems. Among nanostructured materials, monoclinic VO 2 nanostructures 3 and VS 2 ultrathin nanosheets 4 can be mentioned, showing particularly high sensitivity at a relative humidity (RH) above 50%. 4 As representative organic/ inorganic hybrid systems, tin oxide nanoparticle (NP) loaded cellulose, 5 Li-loaded nanoporous polymers, 6 multilayer graphene oxide/polyelectrolyte nanocomposite films, 7 multiwalled carbon nanotubes dispersed in cross-linked polyelectrolyte, 8 and LiCl−polymer composite nanofibers 9 can be listed, showing quite promising performance at high humidity 5,7 but also in the entire RH range. 7−9Among organic materials used in humidity-responsive hybrid assemblies, hydrogels are in particular attractive because of their intrinsic ability to absorb moisture, affecting their mechanical, optical, and electrical properties. 10−12 Some of the recently developed humidity-sensitive systems include superabsorbent poly(acrylamide)−montmorillonite composite hydrogels, 13 fluophore-loaded acrylamide, 12 hydrogel-actuated nanorod assembly, 11 and conductive polymer hydrogel cross-linked by phytic acid in poly(N-isopropylacrylamide) matrix. 14 Most of the above assemblies are, however, complex, requiring in some cases also a ...

Structural properties and stability of the selfassembled monolayers (SAMs) of two prototypical azobenzene-based alkanethiols (C 6 H 5 −NN−C 6 H 4 −(CH 2 ) n −SH) on Au(111) and Ag(111) substrates were studied in detail using a combination of complementary experimental techniques. The azobenzene moiety in these films was linked to the thiol headgroup via short aliphatic spacers of variable length, i.e., (CH 2 ) 3 or (CH 2 ) 4 , corresponding to a different parity of n. For both Au(111) and Ag(111) substrates, a pronounced dependence of the packing density and molecular inclination on the parity of n was observed, with a higher packing density (by ∼14%) and smaller inclination (by ∼17°) of the azobenzene moieties for n = odd as compared to n = even on Au(111) and reversed, but somewhat reduced, behavior on Ag(111). This dependence was related to the well-known odd− even effects in molecular assembly on noble metal substrates, reported previously for a variety of oligophenyl-substituted alkanethiolate SAMs and observed now for the azobenzene-substituted monolayers as well, underlining their generality. The structural odd−even behavior was accompanied by odd−even effects in the stability of the substrate−S bond, with the latter effects being directly correlated to the respective structure variation. The above results are of general importance for the design of functional monomolecular films and of a particular significance as a basis for dedicated photoisomerization experiments.

Poly(ethylene glycol) based hydrogel nanomembranes (PHMs) are demonstrated to be able to host protein-specific receptors, providing, at the same time, stable, protein-repelling matrices with a characteristic mesh size up to 7-8 nm. The membranes were prepared by crosslinking of amino- and epoxy-terminated STAR-PEG precursors and maintained their hydrogel and protein-repelling properties even at a deviation of the precursor composition from the equilibrium value (1 : 1). The grafting density of the test avidin protein, specifically attached to the biotin moieties coupled to the free amine groups in the PHMs, varied from 0.45 × 10(12) to 1.3 × 10(12) proteins per cm(2) within the sampling depth of the experiments (∼11.5 nm), depending on the precursor composition, whereas the analogous values for the non-specifically adsorbed proteins were lower by a factor of 4-5. The engineering of PHMs with biomolecule-specific receptors and their loading with biomolecules are of potential interest for sensor fabrication and biomedical applications, including tissue engineering and regenerative therapy.

As a material with relatively small band gap and low lying valence orbitals, perfluoroanthracene (PFA) is of interest for the modification of electrode surfaces, for example, as charge injection layers for n-type organic semiconductors. To covalently attach PFA in the form of self-assembled monolayers (SAMs), we developed a synthesis of derivatives with a sulfur termination, linked to the 2-position of the PFA moieties by an -NH- group and a short alkane chain with two and three methylene groups, respectively. Spectroscopic characterization of the SAMs reveals that the molecules adopt an almost upright orientation on the gold surface, with the packing density mostly determined by the steric demands of the PFA units. The number of the methylene groups in the -NH-alkyl linker has only a minor impact on the SAM structure because of the nonsymmetric attachment of the PFA units, which permits the compensation of the orientational constraints imposed by the bending potential. The investigated SAMs alter the work function of gold by +(0.59-0.64) eV, suggesting comparably strong depolarization effects, affecting the extent of the work function modification.