77 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
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-
Work as a process of personal development / Il lavoro come processo di sviluppo personale
This volume introduces a conception of work as a process of personal growth and development. Three fundamental moments of passage are discussed, in which the life and work connection becomes evident: the transition from education to work, the daily relationship between work and extra-work life, the evaluations in the workplace upon which the career paths depend. Critical reflections about mainstream theories, methodologies and practices are suggested. Examples and case studies illustrate their limitations and negative consequences. Finally, alternative organizational choices aimed at improving the work situations and facilitating, in the workplace, the development and well-being of people, are proposed
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
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
Tempo, lavoro e regolazione: verso il processo d'azione a più livelli
International audienceVorremmo riprendere come filo conduttore il perimetro dell’azione, e scegliere il punto di vista di un suo ampliamento ai fini dell’analisi. Questa prospettiva vorrebbe giovarsi di altri autori, che permettono di pensare l’azione come un processo che si sviluppa attraverso scalarità e livelli, come Jean-Daniel Reynaud (1997; 1999) (teoria della regolazione sociale, TRS) e Gilbert de Terssac (2011) (teoria del lavoro d’organizzazione, TTO). L’obiettivo è continuare la riflessione proposta da Bruno Maggi (2011a) (teoria dell’agire organizzativo, TAO), seguendo Max Weber sull’azione in quanto mette a fuoco il processo d’azione e non il risultato. Uno degli ostacoli di questo passaggio dall’azione all’agire riguarda la chiusura dell’analisi su livelli particolari. Secondo il punto di vista che vorremmo sviluppare, la specializzazione su un livello d’azione – come ad esempio la situazione di lavoro per la negoziazione collettiva d’impresa – non dovrebbe impedire di guardare altri livelli che permettono di comprendere il processo d’azione
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
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