Work function shifts of a zinc oxide surface upon deposition of self-assembled monolayers: a theoretical insight.

We have investigated at the theoretical Density Functional Theory level the way the work function of zinc oxide layers is affected upon deposition of self-assembled monolayers (SAMs). 4-tert-Butylpyridine (4TBP) and various benzoic acids (BA) were adsorbed on the apolar (101[combining macron]0) ZnO and used as probe systems to assess the influence of several molecular parameters. For the benzoid acids, we have investigated the impact of changing the nature of the terminal group (H, CN, OCH3) and the binding mode of the carboxylic acid (monodentate versus bidentate) on the apolar (101[combining macron]0) surface. For each system, we have quantified the contribution from the molecular core and the anchoring group as well as of the degree of surface reconstruction on the work function shift. For the benzoic acids, the structural reorganization of the surface induces a negative shift of the work function by about 0.3 ± 0.15 eV depending on the nature of the binding mode, irrespective of the nature of the terminal function. The bond-dipole potential strongly contributes to the modification of the work function, with values in the range +1.2 to +2.0 eV. In the case of 4TBP, we further characterized the influence of the degree of coverage and of co-adsorbed species (H, OH, and water molecules) on the ZnO/SAM electronic properties as well as the influence of the ZnO surface polarity by considering several models of the polar (0001) ZnO surface. The introduction of water molecules in the (un)dissociated form at full coverage on the non-polar surface only reduces the work function by 0.3-0.4 eV compared to a reference system without co-adsorbed species. Regarding the polar surface, the work function is also significantly reduced upon deposition of a single 4BTP molecule (from -1.44 eV to -1.73 eV for our model structures), with a shift similar in direction and magnitude compared to the non-polar surfaces.

Exploring the energy landscape of the charge transport levels in organic semiconductors at the molecular scale.

The extraordinary semiconducting properties of conjugated organic materials continue to attract attention across disciplines including materials science, engineering, chemistry, and physics, particularly with application to organic electronics. Such materials are used as active components in light-emitting diodes, field-effect transistors, or photovoltaic cells, as a substitute for (mostly Si-based) inorganic semiconducting materials. Many strategies developed for inorganic semiconductor device building (doping, p-n junctions, etc.) have been attempted, often successfully, with organics, even though the key electronic and photophysical properties of organic thin films are fundamentally different from those of their bulk inorganic counterparts. In particular, organic materials consist of individual units (molecules or conjugated segments) that are coupled by weak intermolecular forces. The flexibility of organic synthesis has allowed the development of more efficient opto-electronic devices including impressive improvements in quantum yields for charge generation in organic solar cells and in light emission in electroluminescent displays. Nonetheless, a number of fundamental questions regarding the working principles of these devices remain that preclude their full optimization. For example, the role of intermolecular interactions in driving the geometric and electronic structures of solid-state conjugated materials, though ubiquitous in organic electronic devices, has long been overlooked, especially when it comes to these interfaces with other (in)organic materials or metals. Because they are soft and in most cases disordered, conjugated organic materials support localized electrons or holes associated with local geometric distortions, also known as polarons, as primary charge carriers. The spatial localization of excess charges in organics together with low dielectric constant (ε) entails very large electrostatic effects. It is therefore not obvious how these strongly interacting electron-hole pairs can potentially escape from their Coulomb well, a process that is at the heart of photoconversion or molecular doping. Yet they do, with near-quantitative yield in some cases. Limited screening by the low dielectric medium in organic materials leads to subtle static and dynamic electronic polarization effects that strongly impact the energy landscape for charges, which offers a rationale for this apparent inconsistency. In this Account, we use different theoretical approaches to predict the energy landscape of charge carriers at the molecular level and review a few case studies highlighting the role of electrostatic interactions in conjugated organic molecules. We describe the pros and cons of different theoretical approaches that provide access to the energy landscape defining the motion of charge carriers. We illustrate the applications of these approaches through selected examples involving OFETs, OLEDs, and solar cells. The three selected examples collectively show that energetic disorder governs device performances and highlights the relevance of theoretical tools to probe energy landscapes in molecular assemblies.

Methodological aspects of the quantum-chemical description of interface dipoles at tetrathiafulvalene∕tetracyanoquinodimethane interfaces.

The formation of dipoles at interfaces between organic semiconductors is expected to play a significant role in the operation of organic-based devices, though the electronic processes at their origin have still to be clearly elucidated. Quantum-chemical calculations can prove very useful to shed light on such electronic interfacial phenomena provided that a suitable theoretical approach is used. In this context, we have performed calculations on small vertical stacks of TTF-TCNQ molecules, first at the CAS-MRCI level to validate the use of single-determinantal approaches, then at the MP2 level set as a benchmark. Various density functional theory (DFT) functionals have then been applied to larger stacks, showing that long-range corrected functionals are required to reproduce MP2 results taken as benchmark. Finally, the use of periodic boundary conditions at the DFT level points to the huge impact of depolarization effects between adjacent stacks.

Effect of substitutional N on important chemical vapor deposition diamond growth steps.

This study analyses theoretically the effects of substitutional N on three different chemical vapor deposition diamond growth steps. The investigation is based on density functional theory, using both cluster and periodic models. The reaction steps, assumed to be predominantly occurring during diamond growth, are (i) CH(2) insertion within a carbon dimer, (ii) H transfer from a neighboring surface carbon to an adsorbed CH(2), and (iii) surface migration of CH(2). Carbon atoms at various lateral positions are substituted by N within the second, third, and fourth carbon layers beneath the surface. Both reaction energies and barrier energies were for all reaction steps carefully calculated. For the CH(2) insertion into a carbon dimer, the reaction energy was found to be in principle unaffected by substitutional N. However, the activation energy for the CH(2) insertion reaction was with one exception observed to be significantly increased by the presence of substitutional N. The H migration reaction was only found to be sensitive to the lateral position of N in the carbon layers. The reaction is observed to be favored or disfavored depending on this lateral position. For the CH(2) migration reaction, the substitutional N was observed to increase the activation barriers and thereby negatively affect the reaction kinetics.

Effect of coadsorbed dopants on diamond initial growth processes: CH3 adsorption.

An investigation based on an ultrasoft pseudopotential density functional theory (DFT) method, using the generalized gradient approximation (GGA) under periodic boundary conditions, has been performed in order to investigate how the presence of a neighboring dopant is affecting the CH 3 adsorption reaction (regarded to be an initial growth process). For this study, both the (100) and (111) diamond surface orientations have been considered, and various dopants in two different hydrogenated forms AH X (A = N, B, S, P, or C; X = 0 or 1 for S, X = 1 or 2 for N, B, and P, and X = 2 or 3 for C) were especially scrutinized. For most of the cases studied, the presence of a coadsorbed dopant was found to disfavor CH 3 adsorption with an efficiency that depends on the surface orientation as well as dopant type and position. The NH 2, PH 2, and SH species have the strongest effect in counteracting the CH 3 adsorption to the diamond (111) surface. This is also the situation with the dopants adsorbed on either of two specific surface sites (out of three positions studied) on the diamond (100)-2 x 1 surface. The main reasons for these observations are induced steric hindrances between the two coadsorbates. The BH 2 species, adsorbed to the third type of surface site on diamond (100), has been found to affect the adsorption reaction by formation of a C surf-B bond prior to CH 3 adsorption. The dopants in their radical forms are generally shown to always strongly disfavor the CH 3 adsorption reaction by formation of a C surf-X bond prior to adsorption. However, the NH radical will only form this new bond with the radical surface C site when it is adsorbed to position 3 on the surface.