84 research outputs found
A new type of neutral-ionic interface in mixed-stack organic charge-tranfer crystals: Temperature induced ionicity change in ClMePD-DMeDCNQI
Raman and polarized infrared spectra of the mixed stack charge transfer
crystal 2-chloro-5methyl-p-phenylendiamine- -2,5-dimethyl-dicyanoquinonediimine
(ClMePD-DMeDCNQI) are reported as a function of temperature. A detailed
spectral interpretation allows us to gain new insight into the temperature
induced neutral-ionic transition in this compound. In particular, the crossing
of the neutral-ionic borderline appears to be quite different from that of the
few known temperature induced neutral-ionic phase transitions. First of all,
the ionicity change is continuous. Furthermore, the onset of stack dimerization
precedes, rather than accompanies, the neutral-ionic crossing. The (second
order) phase transition is then driven by the dimerization, but the extent of
dimerization is in turn affected by the ionicity change.Comment: LaTex (revTeX), 6 figures. Yields 10 pages postscript (including
figures
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--phenylenediamine--2,5-dimethyl-dicyanoquinonediimine
({\CD}). The ionicity increases continuously from 0.35 at ambient
conditions to 0.65 when the pressure is raised up to 2 GPa. Moreover,
stack dimerization begins well before the crossing of the neutral-ionic
interface. The evolution of the transition is similar to what observed in the
temperature induced phase change in the same compound. (cond-mat/0101179) A
distinguishing feature is represented by the simultaneous presence of domains
of molecules with slightly different ionicities across the transition pressure.
A comparison of the present example of pressure driven neutral-ionic transition
with the well studied cases of tetrathiafulvalene--chloranil and of
tetrathiafulvalene--2,5-dichloro-p-benzoquinone puts in evidence the remarkably
different evolution of the three transitions.Comment: 6 pages in *.PS format, 5 figure
A new type of neutral-ionic interface in mixed-stack organic charge-tranfer crystals: Temperature induced ionicity change in ClMePD-DMeDCNQI
Raman and polarized infrared spectra of the mixed stack charge transfer
crystal 2-chloro-5methyl-p-phenylendiamine- -2,5-dimethyl-dicyanoquinonediimine
(ClMePD-DMeDCNQI) are reported as a function of temperature. A detailed
spectral interpretation allows us to gain new insight into the temperature
induced neutral-ionic transition in this compound. In particular, the crossing
of the neutral-ionic borderline appears to be quite different from that of the
few known temperature induced neutral-ionic phase transitions. First of all,
the ionicity change is continuous. Furthermore, the onset of stack dimerization
precedes, rather than accompanies, the neutral-ionic crossing. The (second
order) phase transition is then driven by the dimerization, but the extent of
dimerization is in turn affected by the ionicity change.Comment: LaTex (revTeX), 6 figures. Yields 10 pages postscript (including
figures
Polymorphism, phonon dynamics and carrier-phonon coupling in pentacene
The crystal structure and phonon dynamics of pentacene is computed with the
Quasi Harmonic Lattice Dynamics (QHLD) method, based on atom-atom potential. We
show that two crystalline phases of pentacene exist, rather similar in
thermodynamic stability and in molecular density. The two phases can be easily
distinguished by Raman spectroscopy in the 10-100 cm-1 spectral region. We have
not found any temperature induced phase transition, whereas a sluggish phase
change to the denser phase is induced by pressure. The bandwidths of the two
phases are slightly different. The charge carrier coupling to low-frequency
phonons is calculated.Comment: 6 pages, 3 figures. Presented at ICFPAM-
Raman Identification of Polymorphs in Pentacene Films
We use Raman spectroscopy to characterize thin films of pentacene grown on Si/SiO x by Supersonic Molecular Beam Deposition (SuMBD). We find that films up to a thickness of about 781 A (∼ 52 monolayers) all belong to the so-called thin-film (TF) phase. The appearance with strong intensity of some lattice phonons suggests that the films are characterized by good intra-layer order. A comparison of the Raman spectra in the lattice and CH bending spectral regions of the TF polymorph with the corresponding ones of the high-temperature (HT) and low-temperature (LT) bulk pentacene polymorphs provides a quick and nondestructive method to identify the different phases
Bulk and Surface-Mediated Polymorphs of Bio-Inspired Dyes Organic Semiconductors: The Role of Lattice Phonons in their Investigation
AbstractRaman 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
Solution equilibrium between two structures of Perylene-F <sub>2</sub> TCNQ charge transfer co-crystals
We report on the solution growth of the two known structures of Perylene-F(2)TCNQ charge transfer complexes. The transformation accompanied by a marked morphological change from needle 1:1 to platelet 3:2 crystal structure is observed in the mother liquor. Lattice phonon Raman spectroscopy is used for an easy structure identification of the different morphologies before and after the process. X-ray and lattice phonons spectra of reference samples obtained by physical vapor transport is used to identify the two complexes. A fully spectroscopic analysis of the intramolecular Raman modes is presented to estimate the degree of ionicity, which is found to agree with the value previously reported
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
(Perylene)3-(TCNQF1)2: Yet Another Member in the Series of Perylene–TCNQFx Polymorphic Charge Transfer Crystals
The 3:2 Charge Transfer (CT) co-crystal (Perylene)3(TCNQF1)2 is grown by the Physical Vapor Transport (PVT) method, and characterized structurally and spectroscopically. Infrared analysis of the charge sensitive modes reveals a low degree of charge transfer (less than 0.1) between donor and acceptor molecules. The crystal is isostructural to the other 3:2 CT crystals formed by Perylene with TCNQF2 and TCNQF4, whereas such stoichiometry and packing is not known for the CT crystals with non-fluorinated TCNQ. The analysis of the isostructural family of 3:2 Perylene–TCNQFx (x = 1,2,4) co-crystal put in evidence the role of weak F…HC bonding in stabilizing this type of structure</jats:p
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
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