Matteo Masino

Raman phonon spectra of pentacene polymorphs

We report for the first time lattice phonon Raman spectra of pentacene measured by means of a Raman microprobe technique. We experimentally prove the existence of two polymorphs, as expected from recent structural studies. A comparison with Quasi Harmonic Lattice Dynamics calculations, previously performed starting from the available X-ray data, help us in identifying the phase to which each crystal belongs.

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.

Interaction of charge carriers with lattice and molecular phonons in crystalline pentacene.

The computational protocol we have developed for the calculation of local (Holstein) and non-local (Peierls) carrier-phonon coupling in molecular organic semiconductors is applied to both the low temperature and high temperature bulk crystalline phases of pentacene. The electronic structure is calculated by the semimpirical INDO/S (Intermediate Neglect of Differential Overlap with Spectroscopic parametrization) method. In the phonon description, the rigid molecule approximation is removed, allowing mixing of low-frequency intra-molecular modes with inter-molecular (lattice) phonons. A clear distinction remains between the low-frequency phonons, which essentially modulate the transfer integral from a molecule to another (Peierls coupling), and the high-frequency intra-molecular phonons, which modulate the on-site energy (Holstein coupling). The results of calculation agree well with the values extracted from experiment. The comparison with similar calculations made for rubrene allows us to discuss the implications for the current models of mobility.

Lattice dynamics and electron-phonon coupling in the β − ( BEDT − TTF ) 2 I 3 organic superconductor

The crystal structure and lattice phonons of (BEDT-TTF)_2I_3 superconducting \beta-phase are computed and analyzed by the Quasi Harmonic Lattice Dynamics (QHLD) method. Whereas the crystal structure and its temperature and pressure dependence are properly reproduced within a rigid molecule approximation, this has to be removed to account for the specific heat data. Such a mixing between lattice and low-frequency intramolecular vibrations also yields good agreement with the observed Raman and infrared frequencies. From the eigenvectors of the low-frequency phonons we calculate the electron-phonon coupling constants due to the modulation of charge transfer (hopping) integrals. The hopping integrals are evaluated by the extended Hueckel method applied to all nearest-neighbor BEDT-TTF pairs in the ab crystal plane. From the averaged electron-phonon coupling constants and the QHLD phonon density of states we derive the Eliashberg coupling function, which compares well with that experimentally obtained from point-contact spectroscopy. The corresponding dimensionless coupling constant \lambda is found to be around 0.4 .

Evidence for a soft mode in the temperature induced neutral-ionic transition of TTF-CA

Abstract We report a parallel infrared and Raman investigation of the temperature induced neutral-ionic phase transition (NIT) of the mixed-stack charge-transfer (CT) crystal tetrathiafulvalene-chloranil (TTF-CA). We show that before the phase transition the vibronic bands present in the infrared spectra are due to sum and difference combinations, involving the lattice mode which gives rise to the Peierls distortion at the transition. We can then follow the temperature evolution of this Peierls mode, which shows a clear softening (from 70 to 20 cm −1 ) before the first order transition to the ionic ferroelectric state takes place.

BEDT-TTF organic superconductors: The role of phonons

We calculate the lattice phonons and the electron-phonon coupling of the organic superconductor κ-(BEDT-TTF) 2 I 3 , reproducing all available experimental data connected to phonon dynamics. Low-frequency intramolecular vibrations are strongly mixed to lattice phonons (LP). 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 K- and β*-(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) couplings are essential to reproduce the critical temperatures. The e-LP coupling is stronger, but e-MV is instrumental in increasing the average phonon frequency.

Pressure-driven neutral-ionic transition in ClMePD-DMeDCNQI

Application of about 0.8 GPa pressure is sufficient to induce the neutral-ionic transition in the mixed stack charge-transfer crystal 2-chloro-5-methyl-p-phenylenediamine\char21{}2,5-dimethyl-dicyanoquinonediimine (ClMePD-DMeDCNQI). The ionicity increases continuously from

Lattice dynamics and electron-phonon coupling in \beta-(BEDT-TTF)_2I_3 organic superconductor

\ensuremath{\sim}0.35

Intermediate regime in pressure-induced neutral-ionic transition in tetrathiafulvalene-chloranil

at ambient conditions to

Organic semiconductors: polymorphism, phonon dynamics and carrier-phonon coupling in pentacene.

\ensuremath{\sim}0.65