A New Approach to Polymorphism in Molecular Crystals: Substrate-Mediated Structures Revealed by Lattice Phonon Dynamics

The issue of polymorphism in molecular crystals is discussed, taking into account the substrate-mediated structures, that is, structures grown at the interface of different substrates. Bulk and thin films of a compound both share the potentiality to display different crystal forms. However, unlike bulk polymorphs, whose structures are determined by their different molecular packing, thin film structures depend very much on the molecular organization of the organic layers on the substrate, which may, or may not, lead to an ordered structure, depending on the nature of the interface and on the growth conditions. Based on large part in some of the authors' recent works, these thin film structures are classified as distorted bulk, substrate-selected and substrate-stabilized polymorphs, with some subtle differences which may yield a polymorph to belong not exclusively to a single one of these categories. Some experiments are then focused upon, involving charge transport at the interface, as well as how far the effect of the surface goes. Furthermore, the authors comment on how the surface-mediated structures evolve to the single crystal phase in the cases of pentacene and alpha-sexithiophene. Finally, the transition from a 3- to a 2D regime of growth is shortly discussed in terms of low-dimensional disorder

Terahertz Raman scattering as a probe for electron-phonon coupling, disorder and correlation length in molecular materials

Terahertz (or low-frequency) Raman spectroscopy has been shown to be a quite useful tool to infer important information on some key properties of molecular materials, like polymorphism, phase purity and phase transitions. Based on some of our recent studies, we present promising new directions and possible development of the technique for the characterization of electron-lattice phonon coupling, disorder and correlation length in systems of low-dimensionality. The relative strength of electron-lattice phonon coupling can be extracted from the intensities of the Raman bands in the pre-resonance Raman regime, as exemplified in the charge-transfer (CT) crystal N,N-dimethylphenazine-tetracyanoquinodimethane (M2P-TCNQ). Disorder is instead reflected in the Raman bandwidth, which we analyze with polarized light for systems of reduced dimensionality. The sample system studied for the one-dimensional case is the tetramethylbenzidine-tetrafluoroTCNQ CT crystal. As an example of a quasi two-dimensional (2D) system we address pentacene, the classical case of a monomolecular material widely studied for its application in organic electronics. Here the discussion is mostly related to the dispersion of the phonon branches, eventually leading to peculiar spectral profiles depending on the 2D or 3D regime of the films grown under different deposition conditions

BEDT-TTF organic superconductors: the entangled role of phonons

We calculate the lattice phonons and the electron-phonon coupling of the organic superconductor \kappa-(BEDT-TTF)_2 I_3, reproducing all available experimental data connected to phonon dynamics. Low-frequency intra-molecular vibrations are strongly mixed to lattice phonons. Both acoustic and optical phonons are appreciably coupled to electrons through the modulation of the hopping integrals (e-LP coupling). By comparing the results relevant to superconducting \kappa- and \beta-(BEDT-TTF)_2 I_3, we show that electron-phonon coupling is fundamental to the pairing mechanism. Both e-LP and electron-molecular vibration (e-MV) coupling are essential to reproduce the critical temperatures. The e-LP coupling is stronger, but e-MV is instrumental to increase the average phonon frequency.Comment: 4 pages, including 4 figures. Published version, with Ref. 17 corrected after publicatio

Direct evidence of overdamped Peierls-coupled modes in TTF-CA temperature-induced phase transition

In this paper we elucidate the optical response resulting from the interplay of charge distribution (ionicity) and Peierls instability (dimerization) in the neutral-ionic, ferroelectric phase transition of tetrathiafulvalene-chloranil (TTF-CA), a mixed-stack quasi-one-dimensional charge-transfer crystal. We present far-infrared reflectivity measurements down to 5 cm-1 as a function of temperature above the phase transition (300 - 82 K). The coupling between electrons and lattice phonons in the pre-transitional regime is analyzed on the basis of phonon eigenvectors and polarizability calculations of the one-dimensional Peierls-Hubbard model. We find a multi-phonon Peierls coupling, but on approaching the transition the spectral weight and the coupling shift progressively towards the phonons at lower frequencies, resulting in a soft-mode behavior only for the lowest frequency phonon near the transition temperature. Moreover, in the proximity of the phase transition, the lowest-frequency phonon becomes overdamped, due to anharmonicity induced by its coupling to electrons. The implications of these findings for the neutral-ionic transition mechanism is shortly discussed.Comment: 11 pages, 13 figure

Quasi-degenerate self-trapping in one-dimensional charge transfer exciton

The self-trapping by the nondiagonal particle-phonon interaction between two quasi-degenerate energy levels of excitonic system, is studied. We propose this is realized in charge transfer exciton, where the directions of the polarization give the quasi-degeneracy. It is shown that this mechanism, unlike the conventional diagonal one, allows a coexistence and resonance of the free and self-trapped states even in one-dimensional systems and a quantitative theory for the optical properties (light absorption and time-resolved luminescence) of the resonating states is presented. This theory gives a consistent resolution for the long-standing puzzles in quasi-one-dimensional compound A-PMDA.Comment: accepted to Phys. Rev. Letter

Phonons and structures of tetracene polymorphs at low temperature and high pressure

Crystals of tetracene have been studied by means of lattice phonon Raman spectroscopy as a function of temperature and pressure. Two different phases (polymorphs I and II) have been obtained, depending on sample preparation and history. Polymorph I is the most frequently grown phase, stable at ambient conditions. A pressure induced phase transition, observed above 1 GPa, leads to polymorph II, which is also obtained at temperatures below 140 K. Polymorph II can also be maintained at ambient conditions. We have calculated the crystallographic structures and phonon frequencies as a function of temperature, starting from the configurations of the energy minima found by exploring the potential energy surface of crystalline tetracene. The spectra calculated for the first and second deepest minima match satisfactorily those measured for polymorphs I and II, respectively. All published x-ray structures, once assigned to the appropriate polymorph, are also reproduced.Comment: 8 pages, 5 figures, RevTeX4, update after referees report

Impacts Environnementaux De La Gestion Des Déchets Solides Ménagers Dans La Cite Lacustre De Ladji Au Sud Du Benin

La recherche de meilleures conditions d’hygiène et d’assainissement, doit s’inscrire au premier rang des préoccupations de nos municipalités, car l’insalubrité à laquelle nous assistons dans nos villes, continue de dégrader l’environnement et la santé des populations, freinant ainsi le développement urbain.L’évaluation de la gestion des déchets solides ménagers dans la cité lacustre de Ladji a révélé l’inexistence d’un mode de gestion adéquat. Les DSM sont en grande partie rejetés directement dans le lac causant une pollution aussi bien organique, chimique que microbienne. Ainsi, le Lac, ressource vitale, se trouve transformer à la fois en un réservoir sans aucun traitement pour les fèces et en même temps comme poubelle pour les DSM. "Qui sème les déchets récolte la pollution" dit-on, ainsi, les maladies liées à un défaut d’assainissement, surtout, celles d’origine  hydro-fécales, sont au rendez-vous, signe de risque pour la santé publique. Les principales maladies sont : le paludisme, le choléra, la dysenterie et la fièvre typhoïde. De façon générale l’environnement de Ladji n’est pas sain, la pollution y est visible. D’importants efforts sont à faire sur le plan information, éducation et communication avec la population.Pour lutter contre la pollution du lac et améliorer les conditions de vie de la population de Ladji, les suggestions suivantes sont formulées à l’endroit des autorités et institutions compétentes qui interviennent dans la gestion des déchets solides ménagers et excrétas. Il s’agit de :-          Mettre en place des latrines publiques et privées ECOSAN afin de lutter contre la pollution par la défécation du lac ;-          Mettre en place une structure de gestion des déchets, afin de lutter contre la pollution du lac par le rejet des DSM ;-          Faire de vastes campagnes de sensibilisation de la population sur les règles d’hygiène et les bonnes pratiques ;-          Informer, Eduquer et Communiquer avec la population sur la gestion des déchets solides ménagers

Effect of Benzoic Acids on Barite and Calcite Precipitation

The effect of various benzoic acids on the precipitation of barite (BaSO4) and calcite (CaCO3) was investigated. The acids varied in the number of carboxylate groups, from dibenzoic acids (phthalic, isophthalic, and terephthalic) through to the hexabenzoic acid (mellitic acid). It was found that the stereochemistry of the dibenzoic acids was important, as was the pH of the solution (trimesic acid was used as a test case and showed that greatest inhibition was achieved with all carboxylate groups deprotonated). Interestingly, for both the calcite and barite systems, mellitic acid was found to be both a potent inhibitor and a significant crystal growth modifier. In the case of barite, the presence of mellitic acid produced nanoparticles that agglomerated. The nanoparticles were found to be 20 nm in size from X-ray diffraction (XRD) line width analysis and 20-50 nm from transmission electron microscopy (TEM). Humic acid was also tested and found to form bundled fibers of barium sulfate

Structure and dynamics of pentacene on SiO2: From monolayer to bulk structure

We have used confocal micro Raman spectroscopy, atomic force microscopy (AFM), and x-ray diffraction (XRD) to investigate pentacene films obtained by vacuum deposition on SiO2 substrates. These methods allow us to follow the evolution of lattice structure, vibrational dynamics, and crystal morphology during the growth from monolayer, to TF, and, finally, to bulk crystal. The Raman measurements, supported by the AFM and XRD data, indicate that the film morphology depends on the deposition rate. High deposition rates yield two-dimensional nucleation and quasi-layer-by-layer growth of the T-F form only. Low rates yield three-dimensional nucleation and growth, with phase mixing occurring in sufficiently thick films, where the T-F form is accompanied by the "high-temperature" bulk phase. Our general findings are consistent with those of previous work. However, the Raman measurements, supported by lattice dynamics calculations, provide additional insight into the nature of the TFs, showing that their characteristic spectra originate from a loss of dynamical correlation between adjacent layers