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The concept of distributed dipoles in molecular self-assembly on solid substrates was tested for the example of thiolate self-assembled monolayers (SAMs) on Au(111) containing dipolar 2,5′-bipyrimidine units. These were attached to a thiol anchoring group either directly or via a phenylene–methylene spacer, with the spacer decoupling the dipolar moiety from the substrate and promoting layer formation. As expected, the SAMs containing the spacer groups exhibited a higher quality, including a higher packing density and nearly upright molecular orientation. The electrostatic effects of the dipolar bipyrimidine moieties were tested through C 1s and N 1s photoemission spectra, where electrostatic core-level shifts impact the shapes of the spectra. Additionally, changing the orientation of the dipoles allows a variation of the work function over a range of ∼1.35 eV. The experiments were complemented by density-functional theory calculations. The work function tuning range was reasonably high, but smaller than expected considering that for SAMs with a single embedded pyrimidine group per molecule work function changes already amounted to ∼1.0 eV. This behavior is rooted in an asymmetry of the studied SAMs: For dipoles pointing away from the substrate, the expected doubling of the work function change between monopyrimidine and bipyrimidine SAMs essentially occurs. Conversely, for the downward-oriented pyrimidine dipoles, the second polar ring has hardly any effect. Consistent observations were made for the core-level shifts. We discuss several factors, which are potentially responsible for this asymmetry, like disorder, depolarization, or Fermi-level pinning. Of these, the most likely explanation is the adsorption of airborne contaminants interacting with the nitrogen atoms in the immediate vicinity of the outer surface. These are present only in films with downward oriented dipoles. In spite of these complications, some of the introduced distributed dipole SAMs serve as important model systems for understanding electrostatic effects at interfaces. They are also of interest for controlling carrier-injection barriers in organic (opto) electronic devices.

The thermal stability of self-assembled monolayers (SAMs) is of fundamental importance for the majority of their applications. It strongly depends on the type of chemical group used for bonding the molecules forming the SAMs to the selected substrate. Here, we compare the impact of using S and Se bonding groups on the thermal stability of aromatic model SAMs based on naphthalene, containing a polar substituent, and formed on a Au(111) substrate. Using a combination of secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) while heating the samples, we show that the thermal stability of S-bonded SAMs is higher although the bonding between Se and the Au substrate is stronger. This seeming contradiction is found to result from a higher stability of the S–C compared to the Se–C bond. The latter forms the weakest link in the SAMs with Se anchor and, thus, controls its thermal stability. These conclusions are supported by state-of-the art dispersion-corrected density functional theory (DFT) calculations. Notably, full qualitative agreement between the experiments and simulations is obtained only when including Au adatoms in the setup of the unit cells, as these reinforce the bonding between the docking groups and the metal surface. This is an indication for the occurrence of such surface reconstructions also for SAMs consisting of comparably large aromatic molecules.

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.

In this article, we investigate the interface between shuttlecock-shaped chloro boron-subphthalocyanine molecules and the Cu(111) surface. We highlight how molecular planarization induced by van der Waals forces can fundamentally alter the interface properties and how it can enable a particularly strong hybridization between molecular and metal states. In our simulations, we start from a situation in which we disregard van der Waals forces and then introduce them gradually by rescaling the interaction parameter, thereby “pulling” the molecule toward the surface. This reveals two adsorption regimes with significantly different adsorption distances, molecular conformations, and adsorbate-induced changes of the work function. Notably, the above-mentioned massive hybridization of electronic states, also observed in photoelectron spectroscopy, is obtained solely for one of the regimes. We show that this regime is accessible only as a consequence of the planarization of the molecular backbone resulting from the van der Waals attraction between the molecule and the surface. The results of this study indicate that for certain metal–molecule combinations unusually strong interfacial electronic interactions can be triggered by van der Waals forces creating a situation that differs from the usually described cases of physisorptive and chemisorptive interactions.

When modeling inorganic/organic interfaces with density functional theory (DFT), the outcome often depends on the chosen functional. Hybrid functionals, which employ a fraction of Hartree–Fock exchange α, tend to give better results than the more commonly applied semilocal functionals, because they remove or at least mitigate the unphysical electron self-interaction. However, the choice of α is not straightforward, as its effect on observables depends on the physical properties of the investigated system, such as the size of the molecule and the polarizability of the substrate. In this contribution, we demonstrate this impact exemplarily for tetrafluoro-1,4-benzoquinone on semiconducting (copper-I-oxide Cu2O) and metallic (Cu) substrates and explore how the simulated charge transfer depends on α. We determine the value α* that marks the transition point between spurious over-localization and over-delocalization of charges. This allows us to shed light on the interplay between the value of α* and the physical properties of the interface. We find that on the inert semiconducting substrate, α* strongly depends on surface screening. Furthermore, α has a significant impact on the amount of charge transfer and, in particular, the charge localization. Conversely, for the adsorption on Cu, α affects only the amount of transferred charge, but not its localization, which is a consequence of strong hybridization. Finally, we discuss limitations to the predictive power of DFT for modeling charge transfer at inorganic/organic interfaces and explain why the choice of a “correct” amount of Hartree–Fock exchange is difficult, if not impossible. However, we argue why simulations still provide valuable insights into the charge-transfer mechanism at organic/inorganic interfaces and describe how α can be chosen sensibly to simulate any given system.

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.

In this combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study, we investigate self-assembly of the dipolar nonplanar organic semiconductor chloro boron-subphthalocyanine (ClB-SubPc) on Cu(111). We observe multiple distinct adsorption configurations and demonstrate that these can only be understood by taking surface-catalyzed dechlorination into account. A detailed investigation of possible adsorption configurations and the comparison of experimental and computational STM images demonstrates that the configurations correspond to “Cl-up” molecules with the B–Cl moiety pointing toward the vacuum side of the interface, and dechlorinated molecules. In contrast to the standard interpretation of adsorption of nonplanar molecules in the phthalocyanine family, we find no evidence for “Cl-down” molecules where the B–Cl moiety would be pointing toward the Cu surface. We show computationally that such a configuration is unstable and thus is highly unlikely to occur for ClB-SubPc on Cu(111). Using these assignments, we discuss the different self-assembly motifs in the submonolayer coverage regime. The combination of DFT and STM is essential to gain a full atomistic understanding of the surface–molecule interactions, and our findings imply that phthalocyanines may undergo surface-catalyzed reactions hitherto not considered. Our results also indicate that care has to be taken when analyzing possible adsorption configurations of polar members of the phthalocyanine family, especially when they are adsorbed on comparably reactive surfaces like Cu(111).

Using density functional theory augmented with state-of-the-art van der Waals corrections, we studied the geometric and electronic properties of nonplanar chlorogallium-phthalocyanine GaClPc molecules adsorbed on Cu(111). Comparing these results with published experimental data for adsorption heights, we found indications for breaking of the metal–halogen bond when the molecule is heated during or after the deposition process. Interestingly, the work-function change induced by this dissociated geometry is the same as that computed for an intact adsorbate layer in the “Cl-down” configuration, with both agreeing well with the experimental photoemission data. This is unexpected, as the chemical natures of the adsorbates and the adsorption distances are markedly different in the two cases. The observation is explained as a consequence of Fermi-level pinning due to fractional charge transfer at the interface. Our results show that rationalizing the adsorption configurations on the basis of electronic interface properties alone can be ambiguous and that additional insight from dispersion-corrected DFT simulations is desirable.

The focus of the present article is on understanding the insight that X-ray photoelectron spectroscopy (XPS) measurements can provide when studying self-assembled monolayers. Comparing density functional theory calculations to experimental data on deliberately chosen model systems, we show that both the chemical environment and electrostatic effects arising from a superposition of molecular dipoles influence the measured core-level binding energies to a significant degree. The crucial role of the often overlooked electrostatic effects in polar self-assembled monolayers (SAMs) is unambiguously demonstrated by changing the dipole density through varying the SAM coverage. As a consequence of this effect, care has to be taken when extracting chemical information from the XP spectra of ordered organic adsorbate layers. Our results, furthermore, imply that XPS is a powerful tool for probing local variations in the electrostatic energy in nanoscopic systems, especially in SAMs.

The work function of metal substrates can be easily tuned, for instance, by adsorbing layers of molecular electron donors and acceptors. In this work, we discuss the possibility of changing the donor/acceptor mixing ratio reversibly afteradsorption by choosing a donor/acceptor pair that is coupled via a redox reaction and that is in equilibrium with a surrounding gas phase. We discuss such a situation for the example of tetrafluoro-1,4-benzenediol (TFBD)/tetrafluoro-1,4-benzoquinone (TFBQ), adsorbed on Cu(111) and Ag(111) surfaces. We use density functional theory and ab initiothermodynamics to show that arbitrary TFBD/TFBQ mixing ratios can be set using hydrogen pressures attainable in low to ultrahigh vacuum. Adjusting the mixing ratio allows modifying the work function over a range of about 1 eV. Finally, we contrast single-species submonolayers with mixed layers to discuss why the resulting inhomogeneities in the electrostatic energy above the surface have different impacts on the interfacial level alignment and the work function.

Tuning the transport properties of molecular junctions by chemically modifying the molecular structure is one of the key challenges for advancing the field of molecular electronics. In the present contribution, we investigate current–voltage characteristics of differently linked metal–molecule–metal systems that comprise either a single molecule or a molecular assembly. This is achieved by employing density functional theory in conjunction with a Green’s function approach. We show that the conductance of a molecular system with a specific anchoring group is fundamentally different depending on whether a single molecule or a continuous monolayer forms the junction. This is a consequence of collective electrostatic effects that arise from dipolar elements contained in the monolayer and from interfacial charge rearrangements. As a consequence of these collective effects, the “ideal” choice for an anchoring group is clearly different for monolayer and single molecule devices. A particularly striking effect is observed for pyridine-docked systems. These are subject to Fermi-level pinning at high molecular packing densities, causing an abrupt increase of the junction current already at small voltages.

We study the impact of coverage on the electronic structure of substituted biphenylthiolate-based self-assembled monolayers (SAMs) on Au(111) surfaces with a particular focus on SAM-induced work-function changes, ΔΦ. This is done using density functional theory accounting also for van der Waals interactions. We find that the tilt angle of the molecules increases significantly when reducing the coverage, which results in a marked decrease of the perpendicular component of the molecular dipole moment. However, ΔΦ does not follow the trend that one would expect on purely geometrical grounds. While for donor-substituted SAMs, ΔΦ decreases much more slowly than anticipated, for acceptor-substituted SAMs the coverage-induced reduction of ΔΦ is clearly more pronounced than expected. In fact, in that case ΔΦ already vanishes around half coverage. This is in part associated with the (coverage-dependent) bond dipole originating from the “Au–S–C” bond. Especially for low coverages, however, the relevance of the “Au–S–C” dipole diminishes, and we observe a significant contribution of Pauli-Pushback (also known as “cushion effect”) to the interfacial charge rearrangements, an effect that hitherto received only minor attention in the discussion of covalently bonded SAMs.

A microscopic understanding of charge transport through molecule-based systems is essential for advancing the field of molecular electronics. In the present paper we highlight fundamental differences between devices with an individual molecule or a homogeneous monolayer as the active element. These differences arise from collective electrostatic effects that govern the electronic level alignment in monolayer-based devices, but are absent in the case of individual molecules. Employing density functional theory in conjunction with a Green’s function approach, we show that depending on the chemical nature of the employed molecules collective electrostatic effects can result in either a significant increase or a significant decrease of the current per molecule as a function of the packing density, in certain cases even resulting in a change in the transport polarity. Understanding the underlying principles also allows for designing molecules in which dipolar effects cancel, and thus, the transport characteristics of individual molecules and monolayers remain similar.

For organic and hybrid electronic devices, the physicochemical properties of the contained interfaces play a dominant role. To disentangle the various interactions occurring at such heterointerfaces, we here model a complex, yet prototypical, three-component system consisting of a Cu–phthalocyanine (CuPc) film on a 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) monolayer adsorbed on Ag(111). The two encountered interfaces are similar, as in both cases there would be no bonding without van der Waals interactions. Still, they are also distinctly different, as only at the Ag(111)–PTCDA interface do massive charge-rearrangements occur. Using recently developed theoretical tools, we show that it has become possible to provide atomistic insight into the physical and chemical processes in this comparatively complex nanostructure distinguishing between interactions involving local rearrangements of the charge density and long-range van der Waals attraction.

Self-assembled monolayers (SAMs) of π-conjugated organic molecules have attracted significant interest in the field of molecular and organic electronics because of their ability to change electrode work functions combined with a considerable conductivity. Studies simultaneously addressing both their geometrical and morphological structure as well as their electronic properties are, however, scarce. Here, we provide a detailed description of layers consisting of anthracene-2-selenolate on Au(111), which display extraordinarily well and long-range ordered structures. Combining experimental data with the results of slab-type band-structure calculations, we are able to unambiguously determine the alignment of the molecules on the surface. The electronic structure of the SAMs is then determined by ultraviolet photoelectron spectroscopy (UPS) and by density functional theory (DFT) based simulations. For the SAM-induced work-function modification a particularly close agreement between the experimental value of −1.3 eV and the calculated −1.37 eV is found. This supports the notion that the currently available modeling approaches have the potential to quantitatively predict important aspects of the electronic structure of SAMs as long as truly well-ordered monolayers are investigated.

Application of (sub)monolayers of organic molecules on metal electrodes in order to tune the effective work function has become a field of significant interest. Due to its low ionization potential and quinoidal structure, viologen (1H,1′H-[4,4′]bipyridinylidene) is proposed as a particularly potent work function reducing molecule. In the present contribution, its interaction with Au(111), Ag(111), and Cu(111) is compared to that of the prototypical electron donor tetrathiafulvalene (TTF) using density functional theory based band-structure calculations. The work function modification in both systems is found to be determined by a subtle interplay between effects due to adsorption induced geometric distortions and the donation of electrons from the respective molecular HOMO to the metal. The interfacial charge transfer is investigated in real space as well as in terms of changes in the occupation of the molecular orbitals. Overall, viologen is found to be an excellent choice for decreasing the substrate work function. On gold, a reduction by up to −1.6 eV is predicted, resulting in the viologen covered Au surface having a work function equivalent to that of pristine magnesium.

Thin layers of organosilanes on oxidic surfaces are of great interest for various applications including the control of surface properties and immobilization of catalysts and biomolecules and in nanotechnology. In the present work, the reaction of thin layers on silicon oxide consisting of 2-4-(chlorosulfonyl)phenylethyltrichlorosilane (CSTS) with ammonia is studied by means of contact angle (CA) measurements, -potential measurements, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FT-IR). CA measurements are used for a fast monitoring of film quality and as a first probe for the reaction of the CSTS sample with ammonia. The results from XPS measurements show that the chlorine peak of the CSTS molecule disappears completely after the reaction with ammonia and instead a well-resolved nitrogen peak appears. FT-IR spectroscopy in conjunction with theoretical calculations is used to gain a more in-depth understanding of the chemical reaction. The shift of the asymmetric SO vibrational mode during the surface reaction is in line with the theoretical predictions and serves as a proof for the surface reaction to happen. By FT-IR, an additional side reaction of sulfonic acid groups, which are present in the commercial CSTS, to the ammonium sulfonate salt is observed.

Metal–organic frameworks (MOFs) are a particularly intriguing class of self-assembled materials, whose elastic properties crucially impact many of their envisioned applications. Thus, we here present an in-depth, “nanoscale” discussion of these properties for the prototypical class of MOF-74 derivatives. These provide a particular wealth of insights due to their pronounced anisotropy with fundamentally different building blocks connecting the structures parallel and perpendicular to the pore direction. To go beyond solely reporting macroscopic parameters, we trace their values back to atomistic displacements under stress employing state-of-the-art dispersion-corrected density functional theory. Interestingly, all of the studied MOFs exhibit qualitatively different responses to either unidirectional or isotropic stress, which can be ascribed to distinctly different atomic rearrangements for stress parallel or perpendicular to the channel direction. In the former case, one primarily observes a lateral expansion and rotation of the nodes, which can be impeded, e.g., by an exoskeleton formed by an adsorbed water layer. Conversely, for stress perpendicular to the channel, the MOFs comply with a deformation of the hexagonal pores, which causes a significant expansion perpendicular to the stress direction. We also show that the details of these atomistic motions impact the structure-to-property relationships for a variety of MOF-74 variants beyond the expectations based on bonding strengths and the degree of porosity.