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International audience; We present molecular dynamics simulations of self-assembled monolayers (SAMs) chemisorbed on an atomically flat amorphous silicon dioxide substrate. We model two prototypical SAM-forming alkylsilanes, octadecyltrichlorosilane (OTS) and 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), that find widespread use in organic electronic applications. Crucially, our model does not rely on an explicit bonding between the alkylsilane and the substrate, thus allowing for the spontaneous organization of molecules into regular structures, which we studied as a function of coverage. By comparing the calculated tilt angle, film thickness, and lattice parameters with experiments, we conclude that the simulated morphologies are quantitatively consistent with the experimental evidence, demonstrating the accuracy of the simulation methodology. We take advantage of the atomistic resolution of the calculations for carrying out a detailed one-to-one comparison between the structure and the electronic properties of the two SAMs. In particular, we find that OTS molecules show a coverage-dependent tilt, while FDTS molecules are always vertically oriented, regardless of the coverage. More importantly for organic electronic applications, we observe that OTS SAMs do not alter the electrostatic potential of silica, while FDTS SAMs induce a negative voltage shift which increases with coverage and saturates at about -2 V

We present a computational approach to model hole transport in an amorphous semiconducting fluorene-triphenylamine copolymer (TFB), which is based on the combination of molecular dynamics to predict the morphology of the oligomeric system and Kinetic Monte Carlo (KMC), parameterized with quantum chemistry calculations, to simulate hole transport. Carrying out a systematic comparison with available experimental results, we discuss the role that different transport parameters play in the KMC simulation and in particular the dynamic nature of positional and energetic disorder on the temperature and electric field dependence of charge mobility. It emerges that a semi-quantitative agreement with experiments is found only when the dynamic nature of the disorder is taken into account. This study establishes a clear link between microscopic quantities and macroscopic hole mobility for TFB and provides substantial evidence of the importance of incorporating fluctuations, at the molecular level, to obtain results that are in good agreement with temperature and electric field-dependent experimental mobilities. Our work makes a step forward towards the application of nanoscale theoretical schemes as a tool for predictive material screening.

International audience; We present an atomistic molecular dynamics simulation of freely suspended films of the smectic liquid crystal 8CB formed by n(l) = 2, 3,...,10, 20 theoretical monolayers, determining their orientational and positional order as a function of the film thickness. We find that films are always composed by bilayers of antiparallel molecules, and that in the case of odd n(l), the system prefers to self-assemble in (n(l) + 1)/2 bilayers, with an increase of surface tension with respect to even n(l) samples. We also show that external layers have higher positional and orientational order, and that upon heating the disordering of the system proceeds from the inside, with the central layers progressively losing their smectic character, while the external ones are more resistant to temperature changes and keep the film from breaking.

The NMR spectrum of n-hexane orientationally ordered in the nematic liquid crystal ZLI-1132 is analysed using covariance matrix adaptation evolution strategy (CMA-ES). The spectrum contains over 150 000 transitions, with many sharp features appearing above a broad, underlying background signal that results from the plethora of overlapping transitions from the n-hexane as well as from the liquid crystal. The CMA-ES requires initial search ranges for NMR spectral parameters, notably the direct dipolar couplings. Several sets of such ranges were utilized, including three from MD simulations and others from the modified chord model that is specifically designed to predict hydrocarbon-chain dipolar couplings. In the end, only inaccurate dipolar couplings from an earlier study utilizing proton-proton double quantum 2D-NMR techniques on partially deuterated n-hexane provided the necessary estimates. The precise set of dipolar couplings obtained can now be used to investigate conformational averaging of n-hexane in a nematic environment. (C) 2015 AIP Publishing LLC.

The report provides the results of Monte Carlo computer simulation studies on systems of rod-like mesogens doped with nanoparticles. In particular, we have focused on the effects of nanoparticle shape (spherical, rod and disk like), strength of the specific interactions between nanoparticles and between nanoparticles and mesogens (solvent affinity), polarity of the nanoparticles on phase behavior, long-range positional and orientational order and overall organization of these mixture systems. The results clearly show that even a simple model based on a multi-site Gay Berne potential and dipolar interaction can help to figure out the features which favors the enhancement of the LC order as well as the formation of nanoparticle aggregates. In particular we found that doping a mesogenic system with nanoparticles of any shape has the overall effect of reducing both the orientational and positional order, with the most disordering effects observed for the embedded spherical nanoparticles. The specific nanoparticle-solvent interaction has a significant influence in determining the aggregation/dispersion state of the dopant NP, anyway all the mixtures evidence a shift of the TNI temperature towards lower values, if compared with the pure system. The only exception is given by the mesogenic system doped with polar rod-shaped NP with very low solubility features, which essentially shows invariance of the ordering properties and of the relevant transition temperatures. This behavior is related to the formation of nanoparticles aggregates, each aggregate containing a large number ( 20) of nanoparticles, overall oriented along the mesophase director.