1,721,005 research outputs found
Terahertz Raman scattering as a probe for electron-phonon coupling, disorder and correlation length in molecular materials
Full text linkTerahertz (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
Organic Field-Effect Transistors Based on Ternary Blends Including a Fluorinated Polymer for Achieving Enhanced Device Stability
Full text linkThe stability of organic semiconductors (OSCs) is strongly hampered by the presence of water molecules. One approach that has been proved to lead to organic field-effect transistors with an enhanced performance is the use of blends of OSCs with insulating binding polymers. In this work, the fabrication of OSC thin films based on polymeric ternary blends including a hydrophobic fluorinated polymer is reported as a novel route to engineer long-term reliable organic field-effect transistors (OFET) devices. In particular, OFETs based on blends of bis(triisopropylsilylethynyl)pentacene (TIPS) with polystyrene (PS) and poly(pentafluorostyrene) (PFS) are explored. The PS:PFS ratio is tuned in order to find the optimum formulation. It is shown that films including 20% of PFS in the polymeric blend exhibit an improved device performance, which is reflected by a low bias stress and an exceptional environmental stability, without significantly hampering the OFET mobility. This work advocates that adding a small percentage of fluorinated polymers in OSC blends is a promising route to realize more reliable and stable devices without importantly compromising the device mobility
A New Approach to Polymorphism in Molecular Crystals: Substrate-Mediated Structures Revealed by Lattice Phonon Dynamics
Full text linkThe 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
Bulk and Surface-Mediated Polymorphs of Bio-Inspired Dyes Organic Semiconductors: The Role of Lattice Phonons in their Investigation
Full text linkRaman spectroscopy of organic molecular materials in the low-wavenumber region gives access to lattice vibrational modes and to the wealth of information on solid state properties that these can provide. In the field of organic electronics a useful application concerns the discrimination of the crystalline forms i. e. polymorphism of the semiconductor. The capability of characterizing and identifying the polymorphs of a compound is in fact the prerequisite for an exhaustive study of the charge transport characteristics which arise from the relationship between molecular, electronic, and crystal structures. Thus, the need is felt of a non-invasive, non-destructive tool such as Raman, which probes the crystal phase by detecting the lattice modes which are sensitive even to subtle variations of the packing. Here we review the contribution of the technique to the study of organic pigments displaying promising semiconducting properties and characterized by polymorphism both in their bulk and thin film phases
What Changes in Topochemistry when Going from Small Molecule Dimerizations to Polymerizations in Single Crystals?
No full textThis publication promotes the increased necessity for strain management in topochemical reactions with an enlarged structural extension of the molecular products formed, i. e., when going from small molecule dimerizations, through linear polymerizations to the formation of 2D polymers. Further, it promotes to combine the trap model for photon absorption with concrete molecular scale consequences of this absorption on topochemical transformations and briefly discusses the expected consequences topological dimensionality of the forming (macro)molecular products has on trap location. The time appears ripe for going in this direction because local information concerning structural changes within single crystals is now accessible by the 3D-ΔPDF method. This method greatly facilitates the analysis of diffuse X-ray scattering providing access to concrete values of pair distribution functions and, thus, factual information on which and how distances change near a reaction site. Although only based on a first case where distance changes could be quantified in a lateral polymerization, the thoughts put forward may ignite more research towards a full understanding of all the action that occurs when a photochemically triggered topochemical reaction takes place. The 3D-ΔPDF method is so attractive for this purpose because it provides otherwise inaccessible local information in pair correlation functions rather than average structure information, which is used through the ubiquitous Bragg scattering
Thorough investigation on the high-temperature polymorphism of dipentyl-perylenediimide: thermal expansion vs. polymorphic transition
Full text linkN,N′-Dipentyl-3,4,9,10-perylendiimide (PDI-C5) is an organic semiconducting material which has been extensively investigated as model compound for its optoelectronic properties. It is known to be highly thermally stable, that it exhibits solid-state transitions with temperature and that thermal treatments lead to an improvement in its performance in devices. Here we report a full thermal characterization of PDI-C5 by combination of differential scanning calorimetry, variable temperature X-ray diffraction, hot stage microscopy, and variable temperature Raman spectroscopy. We identified two high temperature polymorphs, form II and form III, which form respectively at 112 °C and at 221 °C and we determined their crystal structure from powder data. Form II is completely reversible upon cooling with low hysteresis, while form III revealed a different thermal behaviour upon cooling depending on the technique and crystal size. The crystal structure features of the different polymorphs are discussed and compared, and we looked into the role of the perylene core and alkyl chains during solid-state transitions. The thermal expansion principal axis of PDI-C5 crystal forms is reported showing that all the reported forms possess negative thermal expansion (X1) and large positive thermal expansion (X3) which are correlated to thermal behaviour observed
Revisiting the Disorder–Order Transition in 1-X-Adamantane Plastic Crystals: Rayleigh Wing, Boson Peak, and Lattice Phonons
Full text linkIn this paper, we revisit the disorder-order transition in 1-chloro-adamantane and present new data on 1-bromo-adamantane. Rayleigh scattering and Raman-active optical lattice phonons are used to easily characterize the phase change. Unlike Cl-derivative, the Br-derivative shows a semiordered structure in the vicinity of ambient P,T conditions, which is peculiar of this compound and seems to be due to a more reduced rotational disorder of the Br-derivative with respect to the other halogen-adamantanes. The semiordered phase is briefly discussed in terms of its Rayleigh Wing and Boson peak
Commenting on the photoreactions of anthracene derivatives in the solid state
No full textWe review a series of photochemical reactions related to the renowned family of anthracene derivatives. We first classify the photodimerizations of 9-methyl-anthracene, 9-cyano-anthracene and 9-anthracene-carboxylic acid as topochemical, non-topochemical and reversible, respectively. We then introduce a chemical reaction where the lattice change involves two different molecular species. In the latter case, the compatibility of the reactive species occurs at the level of symmetry matching of the unit cells rather than via a correct registry of molecular orientations. We then describe the relationship between incoming photons and mechanical motion, a common occurrence in crystal-to-crystal photoreactions, which shows stunning effects when 9,10-dinitro-anthracene is transformed to anthraquinone. Finally, the kinetics of these systems is shortly discussed. By means of a micro-Raman technique, we are capable of monitoring, at the same time and on the very same spot, the molecular (intramolecular vibrations or internal motions) and the lattice (intermolecular vibrations or lattice phonons) transformations. It turns out that molecular changes start first, producing, in a following time, the symmetry modifications of the crystal lattice. The extent of the delay of the structural change may considerably vary depending upon the system studied.Peer reviewe
Experimental Estimate of the Holstein Electron–Phonon Coupling Constants in Perylene
No full textA careful analysis of the vibrational spectra of two isostructural charge transfer systems with potential applications in optoelectronics, (perylene)3-(TCNQF4)2, and (perylene)3-(TCNQF2)2, highlights the presence of three strong infrared bands, polarized along the (perylene-TCNQFx-perylene) trimeric units present in the crystal. These bands are recognized as due to the Holstein, or electron–molecular vibration (e–mv) interaction. Use of appropriate modeling, based on the analysis of the frequency shifts due to the e–mv coupling, allows to extract the relevant Holstein coupling constants for the HOMO of the perylene molecule. The so-obtained experimental values agree very well with density functional calculations, thus validating the results. The perylene relaxation energy, or small polaron binding energy, is about 70 meV, of the same order of magnitude as that of pentacene
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