97 research outputs found
The quest for electronic ferroelectricity in organic charge-transfer crystals
Organic ferroelectric materials are in demand in the growing field of
environmentally friendly, lightweight electronics. Donor-Acceptor charge
transfer crystals have been recently proposed as a new class of organic
ferroelectrics, which may possess a new kind of ferroelectricity, the so-called
electronic ferroelectricity, larger and with faster polarity switching in
comparison with conventional, inorganic or organic, ferroelectrics. The current
research aimed at achieving ambient conditions electronic ferroelectricity in
organic charge transfer crystals is shortly reviewed, in such a way to evidence
the emerging criteria that have to be fulfilled to reach this challenging goal.Comment: 6 pages, 7 figures. Proceedings of "2018 SUSTAINABLE INDUSTRIAL
PROCESSING SUMMIT AND EXHIBITION
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
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
DMTTF-CA revisited: temperature-induced valence and structural instability
We report a detailed spectroscopic investigation of temperature-induced
valence and structural instability of the mixed-stack organic charge-transfer
(CT) crystal 4,4'-dimethyltetrathiafulvalene-chloranil (DMTTF-CA). DMTTF-CA is
a derivative of tetrathiafulvalene-chloranil (TTF-CA), the first CT crystal
exhibiting the neutral-ionic transition by lowering temperature. We confirm
that DMTTF-CA undergoes a continuous variation of the ionicity on going from
room temperature down to 20 K, but remains on the neutral side
throughout. The stack dimerization and cell doubling, occurring at 65 K, appear
to be the driving forces of the transition and of the valence instability. In a
small temperature interval just below the phase transition we detect the
coexistence of molecular species with slightly different ionicities. The
Peierls mode(s) precursors of the stack dimerization are identified.Comment: 8 pages, 6 figures, Phys. Rev. B forma
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
Anomalous dispersion of optical phonons at the neutral-ionic transition: Evidence from diffuse X-ray scattering
Diffuse X-ray data for mixed stack organic charge-transfer crystals
approaching the neutral-ionic phase transition can be quantitatively explained
as due to the softening of the optical phonon branch. The interpretation is
fully consistent with vibrational spectra, and underlines the importance of
electron-phonon coupling in low-dimensional systems with delocalized electrons.Comment: 4 pages, 4 figure
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
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