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.

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.

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.

Electronic structure of pyridine-based SAMs on flat Au(111) surfaces: extended charge rearrangements and Fermi level pinning.

Density functional theory calculations are used to investigate the electronic structure of pyridine-based self-assembled monolayers (SAMs) on an Au(111) surface. We find that, when using pyridine docking groups, the bonding-induced charge rearrangements are frequently found to extend well onto the molecular backbone. This is in contrast to previous observations for the chemisorption of other SAMs, e.g., organic thiolates on gold, and can be explained by a pinning of the lowest unoccupied states of the SAM at the metal Fermi-level. The details of the pinning process, especially the parts of the molecules most affected by the charge rearrangements, strongly depend on the length of the molecular backbone and the tail-group substituent. We also mention methodological shortcomings of conventional density functional theory that can impact the quantitative details regarding the circumstances under which pinning occurs and highlight a number of peculiarities associated with bond dipoles that arise from Fermi-level pinning.

The electronic structure of mixed self-assembled monolayers.

The electronic structure of mixed self-assembled monolayers (SAMs) on Au(111) surfaces is modeled using slab-type density-functional theory calculations. The studied molecules have a dipolar character induced by polar and electron donating or accepting tail-group substituents. The resulting electronic structure of mixed layers is found to differ qualitatively from a simple superposition of those of the respective pure layers. Specifically, the positions of the frontier electronic states are shifted relative to the metal Fermi level, with the sign and magnitude of that shift depending on the dipole moment of the molecules and the mixing ratio in the film. This appears counterintuitive considering previous investigations, in which it has been shown that, for densely packed layers, tail-group substituents have no impact on the interfacial energy-level alignment. The seeming contradiction can be lifted by considering the local electrostatic interactions within the films in both mixed and homogeneous monolayers. Beyond that, we show that mixed SAMs provide an efficient tool for continuously tuning substrate work functions over a range that far exceeds that accessible by merely changing the coverage of homogeneous layers, with the net effect depending linearly on the mixing ratio in agreement with recent experimental findings.

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

Quantum-mechanical calculations are performed to investigate the interface between Au(111) surfaces and self-assembled monolayers (SAMs) of organic thiols. Dipolar pyrimidine units act as building blocks to systematically tune the molecular dipole moments via the number of repeat units. The resulting work-function modifications and the energetic alignment of the frontier electronic states in the SAM with the Fermi level are analyzed. Compared to SAMs where strong dipole moments are realized only by end-group substitutions on otherwise non-polar molecules, an entirely different evolution with backbone length is found for the present systems, where dipoles are built directly into the backbone. In particular, the achievable work-function modifications depend on peculiarities in the relative alignment of the energy levels in the SAM and in the metal. We thus introduce an additional degree of freedom for tuning surface and interface electronic properties with functional self-assembled monolayers.

Modeling the electronic properties of pi-conjugated self-assembled monolayers.

The modification of electrode surfaces by depositing self-assembled monolayers (SAMs) provides the possibility for controlled adjustment of various key parameters in organic and molecular electronic devices. Most important among them are the work function of the electrode and the relative alignment of its Fermi level with the conducting states in the SAM itself and with those in a subsequently deposited organic semiconductor. For the efficient application of such interface modifications it is crucial to reach a proper understanding of the relation between the chemical structure of a molecule, its molecular electronic characteristics, and the properties of the SAM formed by such molecules. Over the past years, quantum-mechanical calculations have proven to be a valuable tool for reaching a fundamental understanding of the relevant structure-property relations. Here, we provide a review over the field and report on recent progress in the modeling of the interfacial electronic properties of pi-conjugated SAMs. In addition to the insight that can be gained from simple electrostatic considerations, we focus on the quantum-mechanical description of the roles played by substituents, molecular backbones, chemical anchoring groups, and the packing density of molecules on the surface. Furthermore, we explicitly address the energy-level alignment at the interface between a prototypical organic semiconductor and a SAM-covered metal electrode and describe an approach suitable for extending the metallic character of the substrate onto the monolayer.

Understanding the electronic structure of metal/SAM/organic-semiconductor heterojunctions.

Computational modeling is used to describe the mechanisms governing energy level alignment between an organic semiconductor (OSC) and a metal covered by various self-assembled monolayers (SAMs). In particular, we address the question to what extent and under what circumstances SAM-induced work-function modifications lead to an actual change of the barriers for electron and hole injection from the metal into the OSC layer. Depending on the nature of the SAM, we observe clear transitions between Fermi level pinning and vacuum-level alignment regimes. Surprisingly, although in most cases the pinning occurs only when the metal is present, it is not related to charge transfer between the electrode and the organic layer. Instead, charge rearrangements at the interface between the SAM and the OSC are observed, accompanied by a polarization of the SAM.