Self-assembled monolayers of polar molecules on Au(111) surfaces: distributing the dipoles

Electrostatically Designing Materials and Interfaces

Collective electrostatic effects arise from the superposition of electrostatic potentials of periodically arranged (di)polar entities and are known to crucially impact the electronic structures of hybrid interfaces. Here, it is discussed, how they can be used outside the beaten paths of materials design for realizing systems with advanced and sometimes unprecedented properties. The versatility of the approach is demonstrated by applying electrostatic design not only to metal-organic interfaces and adsorbed (complex) monolayers, but also to inter-layer interfaces in van der Waals heterostructures, to polar metal-organic frameworks (MOFs), and to the cylindrical pores of covalent organic frameworks (COFs). The presented design ideas are straightforward to simulate and especially for metal-organic interfaces also their experimental implementation has been amply demonstrated. For van der Waals heterostructures, the needed building blocks are available, while the required assembly approaches are just being developed. Conversely, for MOFs the necessary growth techniques exist, but more work on advanced linker molecules is required. Finally, COF structures exist that contain pores decorated with polar groups, but the electrostatic impact of these groups has been largely ignored so far. All this suggest that the dawn of the age of electrostatic design is currently experienced with potential breakthroughs lying ahead.

How far the chemistry of self-assembled monolayers on gold surfaces affects their work function?

Self-assembled monolayers composed of various long-chain aliphatic molecules and different tail functional groups have been synthesized on the Au(111) surface and characterized by Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Carboxy, amino, thio and methyl terminal groups have been considered in the design of self-assembled monolayers with different aliphatic chain lengths (from C6 to C16). Work function measurements by Kelvin probe force microscopy have been carried out under a controlled and room atmosphere. Remarkably, a reduction of the relative humidity from 40% to 3% has induced a work function shift of up to 0.3 eV. As expected, the changes of the chain length of the aliphatic moiety and of the tail group have a significant impact on the tuning of the measured work function (3.90 eV for dodecanethiol versus 4.57 eV for mercaptohexadecylamine). Surprisingly, the change of the net dipole moment of the tail group (sign and amplitude) does not dominate the work function variations. In contrast, the change of the chain length and the possibility of the tail group to form a complex hydrogen bond network between molecules lead to significant modulations of the work function. In order to interpret these original findings, density functional theory models of equivalent self-assembled monolayers adsorbed on the Au(111) surface have been developed at an unprecedented level of description with large supercells including simultaneously 27 co-adsorbed molecules and weak van der Waals interactions between them. Such large systems have allowed the theoretical modeling of complex hydrogen bond networks between molecules when possible (carboxy tail group). The comparison between computed and measured work functions shows a striking agreement, thus allowing the disentanglement of the previously mentioned competing effects. This consistency between experiment and theory will help in designing the electronic properties of self-assembled monolayers in the context of molecular electronics and organic transistors.

Site-selective functionalization of in-plane nanoelectrode-antennas

Stacked organic optoelectronic devices make use of electrode materials with different work functions, leading to efficient large area light emission. In contrast, lateral electrode arrangements offer the possibility to be shaped as resonant optical antennas, radiating light from subwavelength volumes. However, tailoring electronic interface properties of laterally arranged electrodes with nanoscale gaps - to e.g. optimize charge-carrier injection - is rather challenging, yet crucial for further development of highly efficient nanolight sources. Here, we demonstrate site-selective functionalization of laterally arranged micro- and nanoelectrodes by means of different self-assembled monolayers. Upon applying an electric potential across nanoscale gaps, surface-bound molecules are removed selectively from specific electrodes by oxidative desorption. Kelvin-probe force microscopy as well as photoluminescence measurements are employed to verify the success of our approach. Moreover, we obtain asymmetric current-voltage characteristics for metal-organic devices in which just one of the electrodes is coated with 1-octadecanethiol; further demonstrating the potential to tune interface properties of nanoscale objects. Our technique paves the way for laterally arranged optoelectronic devices based on selectively engineered nanoscale interfaces and in principle enables molecular assembly with defined orientation in metallic nano-gaps.

Concept of Embedded Dipoles as a Versatile Tool for Surface Engineering

ConspectusControlling the physical and chemical properties of surfaces and interfaces is of fundamental relevance in various areas of physical chemistry and a key issue of modern nanotechnology. A highly promising strategy for achieving that control is the use of self-assembled monolayers (SAMs), which are ordered arrays of rodlike molecules bound to the substrate by a suitable anchoring group and carrying a functional tail group at the other end of the molecular backbone. Besides various other applications, SAMs are frequently used in organic electronics for the electrostatic engineering of interfaces by controlling the interfacial level alignment. This is usually achieved by introducing a dipolar tail group at the SAM-semiconductor interface. Such an approach, however, also changes the chemical character of that interface, for example, affecting the growth of subsequent layers. A strategy for avoiding this complication is to embed polar groups into the backbones of the SAM-forming molecules. This allows disentangling electronic interface engineering and the nucleation of further layers, such that both can be optimized independently. This novel concept was successfully demonstrated for both aliphatic and aromatic SAMs on different application-relevant substrates, such as gold, silver, and indium tin oxide. Embedding, for example, ester and pyrimidine groups in different orientations into the backbones of the SAM-forming molecules results in significant work-function changes. These can then be fine-tuned over a wide energy range by growing mixed monolayers consisting of molecules with oppositely oriented polar groups. In such systems, the variation of the work function is accompanied by pronounced shifts of the peaks in X-ray photoelectron spectra, which demonstrates that electrostatically triggered core-level shifts can be as important as the well-established chemical shifts. This illustrates the potential of X-ray photoelectron spectroscopy (XPS) as a tool for probing the local electrostatic energy within monolayers and, in systems like the ones studied here, makes XPS a powerful tool for studying the composition and morphology of binary SAMs. All these experimental observations can be rationalized through simulations, which show that the assemblies of embedded dipolar groups introduce a potential discontinuity within the monolayer, shifting the energy levels above and below the dipoles relative to each other. In molecular and monolayer electronics, embedded-dipole SAMs can be used to control transition voltages and current rectification. In devices based on organic and 2D semiconductors, such as MoS₂, they can reduce contact resistances by several orders of magnitude without adversely affecting film growth even on flexible substrates. By varying the orientation of the embedded dipolar moieties, it is also possible to build p- and n-type organic transistors using the same electrode materials (Au). The extensions of the embedded-dipole concept from hybrid interfaces to systems such as metal-organic frameworks is currently underway, which further underlines the high potential of this approach.

Understanding Chemical versus Electrostatic Shifts in X-ray Photoelectron Spectra of Organic Self-Assembled Monolayers

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.

Interface electronic structures of reversible double-docking self-assembled monolayers on an Au(111) surface

Double-docking self-assembled monolayers (DDSAMs), namely self-assembled monolayers (SAMs) formed by molecules possessing two docking groups, provide great flexibility to tune the work function of metal electrodes and the tunnelling barrier between metal electrodes and the SAMs, and thus offer promising applications in both organic and molecular electronics. Based on the dispersion-corrected density functional theory (DFT) in comparison with conventional DFT, we carry out a systematic investigation on the dual configurations of a series of DDSAMs on an Au(111) surface. Through analysing the interface electronic structures, we obtain the relationship between single molecular properties and the SAM-induced work-function modification as well as the level alignment between the metal Fermi level and molecular frontier states. The two possible conformations of one type of DDSAM on a metal surface reveal a strong difference in the work-function modification and the electron/hole tunnelling barriers. Fermi-level pinning is found to be a key factor to understand the interface electronic properties.

Outer-valence Electron Spectra of Prototypical Aromatic Heterocycles from an Optimally Tuned Range-Separated Hybrid Functional

Density functional theory with optimally tuned range-separated hybrid (OT-RSH) functionals has been recently suggested [Refaely-Abramson et al. Phys. Rev. Lett.2012, 109, 226405] as a nonempirical approach to predict the outer-valence electronic structure of molecules with the same accuracy as many-body perturbation theory. Here, we provide a quantitative evaluation of the OT-RSH approach by examining its performance in predicting the outer-valence electron spectra of several prototypical gas-phase molecules, from aromatic rings (benzene, pyridine, and pyrimidine) to more complex organic systems (terpyrimidinethiol and copper phthalocyanine). For a range up to several electronvolts away from the frontier orbital energies, we find that the outer-valence electronic structure obtained from the OT-RSH method agrees very well (typically within ∼0.1–0.2 eV) with both experimental photoemission and theoretical many-body perturbation theory data in the GW approximation. In particular, we find that with new strategies for an optimal choice of the short-range fraction of Fock exchange, the OT-RSH approach offers a balanced description of localized and delocalized states. We discuss in detail the sole exception found—a high-symmetry orbital, particular to small aromatic rings, which is relatively deep inside the valence state manifold. Overall, the OT-RSH method is an accurate DFT-based method for outer-valence electronic structure prediction for such systems and is of essentially the same level of accuracy as contemporary GW approaches, at a reduced computational cost.

Anticorrelation between the Evolution of Molecular Dipole Moments and Induced Work Function Modifications

We explore the limits of modifying metal work functions with large molecular dipoles by systematically increasing the dipole moment of archetype donor-acceptor molecules in self-assembled monolayers on gold. Contrary to intuition, we find that enhancing the dipoles leads to a reduction of the adsorption-induced change of the work function. Using atomistic simulations, we show that large dipoles imply electronic localization and level shifts that drive the interface into a thermodynamically unstable situation and trigger compensating charge reorganizations working against the molecular dipoles. Under certain circumstances, these are even found to overcompensate the effect that increasing the dipoles has for the work function.

Dimensionality effects in the electronic structure of organic semiconductors consisting of polar repeat units

In conjugated organic molecules, excitation gaps typically decrease reciprocally with increasing the number of repeat units, n. This usually holds for individual molecules as well as for the corresponding bulk materials. Here, we show using density-functional theory calculations that a qualitatively different evolution is found for layers built from molecules consisting of polar repeat units. Whereas a 1/n-dependence is still observed in the case of isolated polar molecules, the global gap decreases essentially linearly with n in the corresponding 2D-periodic systems and vanishes beyond a certain molecular length, with the frontier states being localized at opposite ends of the layer. The latter is accompanied by a saturation of the dipole moment per molecule, an effect not observed in the isolated polar molecules. Interestingly, in both cases the limit of the gap for long (but finite) molecules differs qualitatively from that of infinite length obtained in 1D-periodic and 3D-periodic calculations, the latter serving as models for polymers and the bulk. We rationalize these dimensionality effects as a consequence of the potential gradient within the finite-length layers. They arise from the collective action of intra-molecular dipoles in the 2D periodic layers and can be traced back to surface effects.

Mixed self-assembled monolayers of azobenzene photoswitches with trifluoromethyl and cyano end groups

Mixed self-assembled monolayers (SAMs) of alkanethiolates carrying azobenzene chromophores with either a trifluoromethyl or a cyano substituent have been studied. High-resolution x-ray photoelectron spectroscopy proves that the ratio of adsorbed molecules can be arbitrarily adjusted via the molar fractions in solution. As a function of these molar fractions core level shifts are observed which are attributed to local work-function changes. By simulating the electric dipole field distribution, the continuous core level shifts are ascribed to a homogeneous mixture of molecules with different end groups adsorbed on adjacent lattice sites. Near-edge x-ray absorption fine structure measurements reveal formation of well-ordered SAMs. Despite the difference in dipole moment of the end groups, the molecular tilt and twist angles are identical for both single-component SAMs and a 1:1 mixed SAM.

Understanding Interfacial Electronic Structure and Charge Transfer: An Electrostatic Perspective

The challenge of understanding electronic structure and dynamics at organic semiconductor interfaces arises from the richness and importance of weak interactions in thin films of extended π-conjugated molecules. In this Perspective, I discuss a conceptually simple electrostatic approach toward a molecular-level description of the electronic structure and dynamics at a subset of such interfaces. Self-assembled monolayers of oriented dipolar molecules physisorbed on metal surfaces generate sizable collective electric fields, and electrostatics determines the key factors for energy level alignment and molecular electronic structure. A rigorous quantum mechanical treatment of such interfaces supports this conclusion and sheds light on the subtle interplay of the different interfacial interactions. The electrostatic model of the interface has the potential to offer also insights into the role of strong collective electric fields on interfacial charge-transfer dynamics.

Polarity switching of charge transport and thermoelectricity in self-assembled monolayer devices

Self-assembled monolayer devices can exhibit drastically different charge-transport characteristics and thermoelectric properties despite being composed of isomeric molecules with essentially identical frontier-orbital energies. This is rationalized by the cooperative electrostatic action of local intramolecular dipoles in otherwise nonpolar species, thus revealing new challenges but also new opportunities for the targeted design of functional building blocks in future nanoelectronics.

Collectively induced quantum-confined Stark effect in monolayers of molecules consisting of polar repeating units

The electronic structure of terpyrimidinethiols is investigated by means of density-functional theory calculations for isolated molecules and monolayers. In the transition from molecule to self-assembled monolayer (SAM), we observe that the band gap is substantially reduced, frontier states increasingly localize on opposite sides of the SAM, and this polarization in several instances is in the direction opposite to the polarization of the overall charge density. This behavior can be analyzed by analogy to inorganic semiconductor quantum-wells, which, as the SAMs studied here, can be regarded as semiperiodic systems. There, similar observations are made under the influence of a, typically external, electric field and are known as the quantum-confined Stark effect. Without any external perturbation, in oligopyrimidine SAMs one encounters an energy gradient that is generated by the dipole moments of the pyrimidine repeat units. It is particularly strong, reaching values of about 1.6 eV/nm, which corresponds to a substantial electric field of 1.6 × 10(7) V/cm. Close-lying σ- and π-states turn out to be a particular complication for a reliable description of the present systems, as their order is influenced not only by the docking groups and bonding to the metal, but also by the chosen computational approach. In the latter context we demonstrate that deliberately picking a hybrid functional allows avoiding pitfalls due to the infamous self-interaction error. Our results show that when aiming to build a monolayer with a specific electronic structure one can not only resort to the traditional technique of modifying the molecular structure of the constituents, but also try to exploit collective electronic effects.

Work-function modification beyond pinning: when do molecular dipoles count?

Deposition of monolayers of strong electron donors or acceptors on metal surfaces in many cases results in a metal-independent work function as a consequence of Fermi-level pinning. This raises the question whether in such a situation molecular dipoles, which are also frequently used to tune the interface energetics, still can have any impact. We use density functional theory to show that the spatial position of the dipoles is the determining factor and that only dipoles outside the immediate metal-molecule interface allow work-function changes beyond the pinning limit.