Multiscale modeling of self-assembled monolayers of thiophenes on electronic material surfaces

Computer simulation programs, spanning different time and length scales, are used to describe the fundamentals of thin film growth morphology in organic self-assembled monolayers using thiophenes on gold as representative systems. Ab initio calculations created a catalog of the energetics between two N-[4-(thien-2ylethynyl)phenyl] hydroxyl ("1P" molecules) in vacuum and interactions in three orthogonal orientations (parallel, perpendicular, and gamma-phase) to a Au (111) surface. This energetic dataset was supplied as the input for kinetic Monte Carlo simulations of dimer and trimer representations of small organic molecules to describe both sub-monolayer and multilayer growth on a series of hypothetical model substrates. On strongly binding metallic-like substrates, sub-monolayers of the model organic molecules formed ordered phases in the x and y directions at high temperatures and a disordered polycrystalline structure at low temperatures with the molecules lying down. Only at high temperatures was a "phase inversion" observed from a completely flat to an upright structure, suggesting the upright phase to be kinetically limited. Results for multilayer deposition of 1P molecules on three substrates which differ in their binding energy to the molecule (from non-interacting to strongly binding substrates) provided a rich view of the polymorphism that can result from differing choices of temperature and flux conditions. Irrespective of the binding energy of the molecule to the substrate, on highly corrugated surfaces we always observed 3D-island growth of multiple layers of the thiophenes, in contrast to Stranski-Krastanov or Frank-van der Merwe growth on more uniform substrates. The qualitative picture we obtained agrees with the growth habits of other small organic molecule systems like the acene series. Finally, molecular dynamics studies were used to understand the packing structures of stable polymorphs of thiophene SAMs. Different deposition conditions and substrate-molecule binding captured different regimes of growth morphology, some of which have already been observed experimentally.     

Systematic Study of Quaternary Ammonium Cations for Bromine Sequestering Application in High Energy Density Electrolytes for Hydrogen Bromine Redox Flow Batteries

Bromine complexing agents (BCAs) are used to reduce the vapor pressure of bromine in the aqueous electrolytes of bromine flow batteries. BCAs bind hazardous, volatile bromine by forming a second, heavy liquid fused salt. The properties of BCAs in a strongly acidic bromine electrolyte are largely unexplored. A total of 38 different quaternary ammonium halides are investigated ex situ regarding their properties and applicability in bromine electrolytes as BCAs. The focus is on the development of safe and performant HBr/Br₂/H₂O electrolytes with a theoretical capacity of 180 Ah L⁻¹ for hydrogen bromine redox flow batteries (H₂/Br₂-RFB). Stable liquid fused salts, moderate bromine complexation, large conductivities and large redox potentials in the aqueous phase of the electrolytes are investigated in order to determine the most applicable BCA for this kind of electrolyte. A detailed study on the properties of BCA cations in these parameters is provided for the first time, as well as for electrolyte mixtures at different states of charge of the electrolyte. 1-ethylpyridin-1-ium bromide [C2Py]Br is selected from 38 BCAs based on its properties as a BCA that should be focused on for application in electrolytes for H₂/Br₂-RFB in the future.     

Computationally derived rules for persistence of C60 nanowires on recumbent pentacene bilayers

The tendency for C(60) nanowires to persist on two monolayers of recumbent pentacene is studied using molecular dynamics (MD) simulations. A review of existing experimental literature for the tilt angle adopted by pentacene on noble metal surfaces shows that studies cover a limited range from 55° to 90°, motivating simulation studies of essentially the entire range of tilt angles (10°-90°) to predict the optimum surface tilt angle for C(60) nanowire formation. The persistence of a 1D nanowire depends sensitively on this tilt angle, the amount of initial tensile strain, and the presence of surface step edges. At room temperature, C(60) nanowires oriented along the pentacene short axes persist for several nanoseconds and are more likely to occur if they reside between, or within, pentacene rows for ? ≤ ～60°. The likelihood of this persistence increases the smaller the tilt angle. Nanowires oriented along the long axes of pentacene molecules are unlikely to form. The limit of stability of nanowires was tested by raising the temperature to 400 K. Nanowires located between pentacene rows survived this temperature rise, but those located initially within pentacene rows are only stable in the range ?(1) = 30°-50°. Flatter pentacene surfaces, that is, tilt angles above about 60°, are subject to disorder caused by C(60) molecules "burrowing" into the pentacene surface. An initial strain of 5% applied to the C(60) nanowires significantly decreases the likelihood of nanowire persistence. In contrast, any appreciable surface roughness, even by half a monolayer in height of a third pentacene monolayer, strongly enhances the likelihood of nanowire formation due to the strong binding energy of C(60) molecules to step edges.     

Explicit all-atom modeling of realistically sized ligand-capped nanocrystals

We present a study of an explicit all-atom representation of nanocrystals of experimentally relevant sizes (up to 6 nm), "capped" with alkyl chain ligands, in vacuum. We employ all-atom molecular dynamics simulation methods in concert with a well-tested intermolecular potential model, MM3 (molecular mechanics 3), for the studies presented here. These studies include determining the preferred conformation of an isolated single nanocrystal (NC), pairs of isolated NCs, and (presaging studies of superlattice arrays) unit cells of NC superlattices. We observe that very small NCs (3 nm) behave differently in a superlattice as compared to larger NCs (6 nm and above) due to the conformations adopted by the capping ligands on the NC surface. Short ligands adopt a uniform distribution of orientational preferences, including some that lie against the face of the nanocrystal. In contrast, longer ligands prefer to interdigitate. We also study the effect of changing ligand length and ligand coverage on the NCs on the preferred ligand configurations. Since explicit all-atom modeling constrains the maximum system size that can be studied, we discuss issues related to coarse-graining the representation of the ligands, including a comparison of two commonly used coarse-grained models. We find that care has to be exercised in the choice of coarse-grained model. The data provided by these realistically sized ligand-capped NCs, determined using explicit all-atom models, should serve as a reference standard for future models of coarse-graining ligands using united atom models, especially for self-assembly processes.     

Sur quelques exemples de structures pfaffiennes et presque cosymplectiques

Résumé On donne quelques exemples de structures pfaffiennes on cosymplectiques obtenues par plongement régulier. Cas particulier des sphéres et des “quadriques” réelles. Exemple de variété à la foi presque complexe non intêgrable et complexe. Propriétés du groupe G′₂, et de certains espaces homogènes non compacts. à M. Enrico Bompiani pour son Jubilé scientifique.     

Catalyse micellaire—I: Etude de la réactivite en milieu micellaire a l'aide de reactions compétitives

The limitations of macroscopic models of micellar solutions are revealed by thermodynamic and kinetic studies of micellar catalysis and in fact the properties of such media are essentially dependent on the microscopic organization. The micellar effects of CTAB [C₁₆H₃₃N⁺(CH₃)₃Br^?] and SDS (C₁₂H₂₅SO₄^?Na⁺) on the competitive reactions (S_N1, S_N2 and E2) of 1-bromo-2-phenyl propane in basic medium and on the alkaline and neutral hydrolysis of α-phénylallylic esters has been studied. The specific effect of the micellar microenvironment is shown. The results are interpreted in terms of an enhancement of the nucleophilicity of the hydroxide ion and a diminution of the nucleophilic and electrophilic properties of water. The variation of the catalytic effect of cationic micelles with the concentration of the nucleophilic reagent is discussed.     

Melting and freezing characteristics and structural properties of supported and unsupported gold nanoclusters

Molecular dynamics simulations in conjunction with MEAM potential models have been used to study the melting and freezing behavior and structural properties of both supported and unsupported Au nanoclusters within a size range of 2 to 5 nm. In contrast to results from previous simulations regarding the melting of free Au nanoclusters, we observed a structural transformation from the initial FCC configuration to an icosahedral structure at elevated temperatures followed by a transition to a quasimolten state in the vicinity of the melting point. During the freezing of Au liquid clusters, the quasimolten state reappeared in the vicinity of the freezing point, playing the role of a transitional region between the liquid and solid phases. In essence, the melting and freezing processes involved the same structural changes which may suggest that the formation of icosahedral structures at high temperatures is intrinsic to the thermodynamics of the clusters, rather than reflecting a kinetic phenomenon. When Au nanoclusters were deposited on a silica surface, they transformed into icosahedral structures at high temperatures, slightly deformed due to stress arising from the Au-silica interface. Unlike free Au nanoclusters, an icosahedral solid-liquid coexistence state was found in the vicinity of the melting point, where the cluster consisted of coexisting solid and liquid fractions but retained an icosahedral shape at all times. These results demonstrated that the structural stability in the structures of small Au nanoclusters can be enhanced through interaction with the substrate. Supported Au nanoclusters demonstrated a structural transformation from decahedral to icosahedral motifs during Au island growth, in contrast to the predictions of the minimum-energy growth sequence: icosahedral structures appear first at very small cluster sizes, followed by decahedral structures, and finally FCC structures recovered at very large cluster sizes. The simulations also showed that island shapes are strongly influenced by the substrate, more specifically, the structural characteristic of a Au island is not only a function of size, but also depends on the contact area with the surface, which is controlled by the wetting of the cluster to the substrate.