1,721,079 research outputs found
From Chiral Islands to Smectic Layers: A Computational Journey Across Sexithiophene Morphologies on C-60
No full textA theoretical investigation of the molecular organization at a sexithiophene (T6)-C-60 fullerene planar heterojunction, based on atomistic molecular dynamics, is presented, in which the effect of two different sample preparation processes on the resulting interface morphology is explored. First, the landing of T6 on C-60(001) substrate is considered, which leads to crystalline layers of standing and tilted molecules, in accordance with experiments. The observation and the quantitative characterization of the nucleation and growth provide detailed insights on this out-of-equilibrium process, including the establishment of an epitaxial relationship between the substrate and the interfacial T6 layer, and the spontaneous formation of defective islands, characterized by chiral edges, during the growth of the second and third layers. It is then shown that molecular orientations can be radically changed upon annealing at 600 K, at which T6 forms a smectic phase with planar alignment, whose layers are perpendicular to the interface. The interfacial T6 morphologies are then analyzed in detail at room temperature and compared to the known bulk polymorphs
Charge separation energetics at organic heterojunctions: On the role of structural and electrostatic disorder
No full textImproving the performance of organic photovoltaic cells requires the individuation of the specific factors limiting their efficiency, by rationalizing the relationship between the chemical nature of the materials, their morphology, and the electronic processes taking place at their interface. In this contribution, we present recent theoretical advances regarding the determination of the energetics and dynamics of charge carriers at organic-organic interfaces, highlighting the role of structural and electrostatic disorder in the separation of electron-hole pairs. The influence of interfacial electrostatic interactions on charge carrier energetics is first illustrated in model aggregates. Then, we review some of our recent theoretical studies in which we combined molecular dynamics, quantum-chemical and classical micro-electrostatic methods to evaluate the energy landscape explored by the mobile charges in the vicinity of donor-acceptor interfaces with realistic morphologies. Finally, we describe the theoretical challenges that still need to be overcome in order to gain a complete overview of the charge separation processes at the molecular level
Second-order nonlinear optical properties of Stenhouse photoswitches:Insights from density functional theory
No full textWe report the first investigation of the second-order nonlinear optical (NLO) properties of donor-acceptor Stenhouse adducts (DASAs), an emerging class of colored photochromes that undergo photoswitching with visible light to a colorless form. By using time-dependent density functional theory, we provide insights into the relationships linking the nature of the chemical substituents to the amplitude and contrasts of the NLO response. Solvent and frequency dispersion effects are also analyzed. The calculations predict that DASAs behave as high contrast NLO switches, a finding that extends their potential applications to photo-responsive NLO materials and devices.</p
Cocrystal Growth in Organic Semiconductor Thin Films: Simulation of Pentacene, Perfluoropentacene, and Their 1:1 Blend Deposited On Graphite
Full text linkThe understanding of crystal formation in thin films and the precise knowledge of the relation between structure and surface diffusion are two important requirements for the efficient (nano)fabrication of organic electronic devices. Here a computational approach for simulating vapor-phase deposition is employed to obtain and investigate three types of crystalline thin films on graphite. All systems, namely pentacene, perfluoropentacene, and their 1:1 blend, which forms an alternate cocrystal, are constituted by recumbent molecules in accordance with experimental findings. The contributions of intermolecular interactions and of molecular rearrangements occurring during the deposition are analyzed to rationalize the final morphologies. Then, the generated structures are employed to evaluate the energy barriers that prevent molecular diffusion at terraces and step-edges, and to study the reorganization of the films upon high-temperature annealing. The broad agreement with experimental observations and the possibility of evaluating the potential energy surface at the molecular detail render the proposed approach a promising tool to make predictions for other systems
Predicting the Conditions for Homeotropic Anchoring of Liquid Crystals at a Soft Surface. 4-n-Pentyl-4′-cyanobiphenyl on Alkylsilane Self-Assembled Monolayers
No full textWe have studied, using atomistic molecular dynamics simulations, the alignment of the nematic liquid-crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) on self-assembled monolayers (SAMs) formed from octadecyl- and/or hexyltrichlorosilane (OTS and HTS) attached to glassy silica. We find a planar alignment on OTS at full coverage and an intermediate situation at partial OTS coverage because of the penetration of 5CB molecules into the monolayer, which also removes the tilt of the OTS SAM. Binary mixtures of HTS and OTS SAMs instead induce homeotropic (i.e., perpendicular) alignment. A comparison with the existing experimental literature is provided
From rod-like to disc-like Gay–Berne biaxial nematics and back
No full textBy means of Monte Carlo simulations at constant pressure and temperature, the phase behaviour of pure systems of soft biaxial Gay–Berne ellipsoids, prototypical models for thermotropic mesogens, was determined as a function of temperature and three different pressures. To reduce the number of free shape and energy parameters to only one, the medium ellipsoid axis σx, particles possessing identical total volume and length σz were studied, with interactions parameterised in terms of products of axes lengths. The resulting phase diagrams present a variety of crystalline and liquid crystalline phases characterised by different biaxiality and preferential axis alignment, with GB particles behaving like rods for more elongated shapes, and like discs for more squashed ones. In agreement with previous simulation and theoretical studies using simpler models, the transition between rod to disc behaviour (and back) occurs approximately when the so-called dual-shape condition is reached, i.e. when σx = √σyσz. This shape ideally identifies also the position of a Landau point, in which at a specific temperature, isotropic, calamitic nematic, discotic nematic, and biaxial nematic may coexist, and hence a direct isotropic to biaxial nematic transition could in principle occur. We did not observe such event in our simulations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Is the alignment of nematics on a polymer slab always along the rubbing direction? A molecular dynamics study
No full textWe have studied the alignment and molecular organisation within a thin film of the popular nematic 5CB sandwiched between two flat polymer slabs, examining the effect of polymer chemical nature and morphology with atomistic molecular dynamics simulations. We have chosen two common polymers: polystyrene (PS) and polymethylmethacrylate (PMMA), either with their chains in random coil conformation (disordered) or with chains unidirectionally stretched (ordered). We found that, independently on the arrangement of the chains, both surfaces align planarly the liquid crystal, in accord with experimental observation. Moreover, while 5CB molecules align along the chains stretching direction of the PMMA ordered surface, on the combed PS surface they arrange on average perpendicularly to the stretching direction. This behaviour is attributed to the chemically specific interactions between the respective aromatic moieties of PS and 5CB
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