Consortium

INCDFM,    National Institute of Materials Physics, Bucharest
UPB,     UNIVERSITY ”POLYTEHNICA” BUCAREST
ICF,  INSTITUTE OF PHYSICAL CHEMISTRY OF THE ROMANIAN ACADEMY

National Institute of Materials Physics, Bucharest
NCDFM   http://www.infim.ro
Project Director:  Dr.Mihaela Baibarac  <barac@infim.ro>

UNIVERSITY ”POLYTEHNICA” BUCAREST
UPB   http://www.pub.ro/
Responsible: Prof. Dr. C.Cincu <c_cincu@yahoo.com>

INSTITUTE OF PHYSICAL CHEMISTRY OF THE ROMANIAN ACADEMY
ICF   http://www.icf.ro/
Responsible: Dr. V. Fruth <vfruth@icf.ro>

The results of this project were obtained in the frame of the bilateral scientific cooperation established between National Institute of Materials Physics (NIMP) Bucharest, Romania and Institut des Materiaux “Jean Rouxel” (IMN), Nantes, France

Methods & Laboratory Technologies

Poly para-phenylene vinylene (PPV) belongs to the class of π-conjugated polymers (also named as conducting polymers). The properties of conducting polymers, in particular those of PPV, are in forefront of many applications as light emission diode (LEDs), light emitting electrochemical cells, photovoltaic cells, rows and areas of photo-detectors, optical couplers, optically pumped lasers, field effect transistors. The insertion of some organic or inorganic nanoparticles in the polymer matrix can lead not only to the improvement of operating parameters for such optoelectronic devices but also to new electrical and optical properties in the composite material.
In this project one proposed: i) the synthesis of poly para-phenylene-vinylene/carbon nanotubes (PPV/NTC) and poly para-phenylene-vinylene/ oxide nanoparticles (PPV/NPO); ii) the investigation by photoconduction (PC), photoluminescence (PL) and transient emission spectroscopy of the particularities of the processes of photogeneration and charge transport as well as of radiative recombination of excitons in PPV based composites.
The PPV and composite films, on quartz substrate of optical quality were generated following the route of soluble precursor [Wessling and Zimmermann]. The PPV/NTC composites were prepared using carbon nanotubes of commercial provenience. In generating the PPV/NPO were used both oxide nanoparticles (TiO2 and SiO2) of commercial provenience and chemically synthesized particles by sol-gel method.
The problems approached in the frame of the project, were chronologically:
i) the investigation of the radiative recombination processes of excitons in PPV films;
ii) the evidence, by means of measurements of photoconduction in steady state regime, of the particularities of the processes of photogeneration and charge transport in PPV/NTC and PPV/TiO2 type composites;
iii) the investigation, by means of photoluminescence measurements performed under continuous excitation, of the particularities of the radiative recombination processes in PPV/NTC and PPV/NPO type composites;
iv) the investigation, by means of photoluminescence measurements performed under high energy pulse excitation, of the spectral, temporal and directionality particularities of the emission of the PPV/NPO type composites;
v) the revelation, by means of measurements of transient emission, of the dynamics of the excited states in PPV/NTC and PPV/NPO type composites.
The research objectives mentioned above are fully justified not only for a better understanding of the physics of conducting polymers and of composites based on such materials but also due to their main optoelectronic applications (photovoltaic cells and LEDs).
According to the excitonic model, in conducting polymers with non-degenerate fundamental state (as PPV, in fact), the absorption of the photons with energy over the π-π* bandgap does not lead directly to the formation of free charges, the electron and the hole remaining on the same polymeric chain, tied to one another by their electrostatic attraction; the electron-hole pair thus formed, with the electron and the hole having opposite spins, is known under the name intra-chain singlet exciton. Once generated, the exciton can migrate through the polymeric film towards lower energies sites. Being a neutral electrical system, the displacement of the exciton equalizes to a migration of the energy. The exciton ends by spontaneous recombination (geminate recombination), or by dissociation in free charge carriers (polarons). The exciton recombination may be radiative, with a photon emission, or nonradiative, with phonon emission. The radiative recombination of the exciton is crucial concerning the photoluminescence (PL), while the exciton dissociation is the key element in the photoconduction (PC) process. The dissociation mechanisms of excitons include thermal excitation, external electrical fields, defects and impurities, or collisions with other excitons.
The investigation of the radiative recombination processes of excitons in PPV films, depending on temperature, polymer film thickness and conversion thermal regime of the precursor polymer used in obtaining the PPV led to the following conclusions: i) at room temperature, the PL spectrum of thermal converted PPV (at 300 0C) reveals three well resolved 4
peaks, located at approximately 522 nm, 552 nm and 596 nm, and a weak shoulder outlined in the low energy region. The emission spectrum of PPV is conventionally interpreted as the vibronic progression of a inhomogeneous broadened exciton, the first, second and third peaks being associated to 0-0, 0-1, respectively 0-2 vibronic transitions.
The fact that the first peak in the PL spectrum at room temperature (0-0 transition) is systematically found weaker than the second reveals an increased value of the Huang-Rhys factor, i.e. a strong coupling of the S1→S0 electronic transition to the stretching vibrations of the C-C bounds of the polymeric chain; ii) the excitation of the PPV films with the Ar+ laser lines (457.9, 488 and 514.5 nm) showed no variation of the PL spectrum at the modification of the incident photon energy, sustaining that the luminescence of standard PPV (converted at 3000C) has its origin exclusively in the longest conjugated segments; iii) the passing from the room temperature (RT) to liquid nitrogen temperature (LNT) is reflected in the PL spectrum by a red-shift of the emission band and a gradual increase of the I1/I2 ratio of the first two peak intensities; the effects induced by lowering the temperature are consistent with increasing the ordering degree in the molecular chain, and normally, with increasing the conjugation length as a result of the molecular torsion “freezing”, iv) the effect of polymer film thickness in PL spectra is reflected into an important change of the emission curve shape, the I1/I2 ratio increasing as the film thickness is reduced. A similar effect was observed in the case of PPV films converted at temperatures lesser than 300 0C; the changes induced in the PL spectra by film thickness and conversion temperature admonish about the role played in the emission process by the polymer film local morphology.
The study of photogeneration and charge transfer processes in PPV/NTC and PPV/TiO2 composites led to the following results: i) steady state photoconduction measurements on PPV/NTC at RT evidenced a dramatic increase of the photocurrent, with the increase of NTC concentration in composite; when NTC concentration exceeds 2%, the photocurrent increase follows a percolative regime; ii) the data fit with a Iphotocurrent = C ⎪f-fc⎪t type equation (assumed from the percolation theory) allowed the determination of the percolation threshold fc and the critical exponent t (fc =2 % and t = 2); the value determined for t is typical for a tri-dimensional system; iii) the temporal evolution of the photocurrent, at the interruption of the excitation, revealed that under percolation threshold, the lifetime of charge carriers increases substantially at the increase of NTC concentration, indicating an increased state of disorder onto the molecular chain.; iv) the photocurrent measurements depending on temperature, recorded on the PPV/NTC probe with 2%NTC concentration, allowed the determination of the thermal activation energy of the photocurrent; the found value of ≅0.2 eV is very close to the activation energy of neat PPV (0.25 eV). This result implies that the photoconductivity is due to the charges created in PPV and that the NTC fraction in the composite assures only the migration lattice of the photogenerated charges; v) the interpretation in a deductive manner of the data representing the photocurrent values for PPV/TiO2 composites (depending on the TiO2 concentration) allows us to conclude that at the level of polymer/TiO2 nanocrystallite heterojunctions in the composite film, a charge transfer process making the exciton disociation more efficient takes place; v) differently from the NTC lattice, the one formed by the TiO2 nanocrystallites in the PPV matrix seems to be a non-percolative-type one, at least for the TiO2 concentration domain the measurements were performed at (the maximum value of the concentration in the probe set did not exceed 32%); vii) the spectral response of the photoconductivity recorded at room temperature on a PPV film confirmed again the ~ 2.3 eV value for the photocurrent onset.
Incorporating NTC and NPO into PPV polymer matrix induces significant changes not only in the photoconduction properties, but also in photoluminescence ones. In this way: i) inserting NTC into the PPV matrix drastically modifies the relative ratio of the vibronic bands in the PL spectrum at RT, the peak towards high energies (peak I) increasing gradually with the increase of nanotube concentration; a similar phenomenon was observed in PPV/NPO case; ii) the shape of the PL spectra of the PPV/NTC and PPV/NPO composites was simulated using two models: one based on a bimodal distribution of the conjugation lengths and the other implying exclusively the electron-phonon coupling (Frank-Condon model); iii) comparing the calculated spectra of the PPV/NTC composites with the experimental ones , we reach with the first model at the following conclusions: the peak located into the high energy range (peak I) is due to the 0-0 electronic transition of the n=5 conjugated segments; the second peak (the dominant one in the case of low NTC concentration composites) has its origin mainly in the 0-0 electronic transition of long segments (n=6 and n=7…10); the third and the threshold in the low energy range may be explained as a result of the convolutions of the vibronic replicas (of first and second order) of the two pure electronic transitions; iv) the second model (Frank-Condon) applied for the PPV/SiO2 composite spectra reveals that the global emission is in fact the result of the superposition of two contributions originating in two distinct electronic states (separated by about 180 meV): the molecular exciton, responsible for the main emission and the aggregated exciton formed by dipole-dipol interaction, being at the origin of an emission at lower energies; the contribution of the aggregated exciton increases with the decrease of the masic NPO fraction into the polymeric matrix, the maximum being reached in PPV films with no SiO2 al all.
The report in the literature of an emission with LASER characteristics in dyes containing TiO2 particles in suspension, triggered a great interest in the scientific community for the study of light amplification in random dispersing media with gain (also named as "random lasers"). A priori, the PPV/NPO composites may be regarded as “random lasers” too, the PPV playing the role of the amplifier medium, while the TiO2 and SiO2 particles those of multiple light scattering centers. The generation of an emission having LASER characteristics in a polymer film claims the fulfillment of two essential conditions namely: i) an excitation energy high enough to lead to an important density of excitons ii) the exciton recombination to take place in a greater extent on radiative ways. In “random lasers” experiments performed in the frame of the present project on PPV/NPO composites did not succeed putting in evidence of a narrow spectral emission as a first indication of radiation amplification by stimulated emission, despite the fact that for excitation high energy pulses provided by a nitrogen laser (2.5mJ/pulse) were used. This result might be once explained as the consequence of a great defect extent in the investigated films, favoring the non-radiative exciton recombination paths, and on the other side by the presence of certain inhomogeneities inside the film, leading at the increase of the losses (and, an essential condition for lasing is that the gain should exceed the losses). The PL spectra were recorded for two different collecting geometries of emission: a quasi-frontal one, and a lateral one, using for detection a boxcar integrator. The modest narrowing of the spectral distribution observed in the case of lateral gathering was interpreted rather as a re-absorption effect than the result of an incipient amplification of the spontaneous radiation in polymer medium.
The study of the exciton recombination process kinetics in PPV films and in PPV/NTC and PPV/NPO composites was made by time-correlated single-photon counting (TCSPC) method, using for excitation short pulses generated by a laser diode with emission in blue (the width of the optical pulses <200ps); a part of the temporal resolution experiments were carried out using as excitation ultra-short pulses generated by a laser system including a Ti:sapphire oscillator (width of optical pulse <130fs), and for detection-a streak camera. The main conclusions of the study are the followings: i) the recombination processes of excitons in PPV
and composites PPV/NTC respectively PPV/NPO are fast processes, taking place on a time scale of hundreds of picoseconds; the recombination is realized through a combination of radiative and non-radiative ways; ii) the luminescence decay of the PPV films at LNT implies longer decay times than those determined for films at RT; because the nonradiative phenomena are generally thermal activated, the increase of the decay time at low temperature may be qualitatively explained as a result of the restrain in the nonradiative recombination routes of excitons (the decay time at LNT, determined by iterative reconvolution was τ ≅ 230 ps); iii) the PL decay curves recorded on the vibonic maxima from the emission spectrum of PPV films reveal the increase of decay times in the red region of the spectrum; this increase may be understood qualitatively by admitting that the emission at large wavelengths is mainly due to certain "deep" excitonic levels, for this type of levels the probability that some nonradiative recombination ways, thermally activated to exist being smaller; iv) the data of luminescence decay on composite films show that the incorporation of NTC in PPV leads to a shortening of the decay time, while NPO (TiO2 and SiO2) leads to its lengthening; the explanation for the mentioned shortening of the decay time of luminescence in PPV/NTC composites must be searched for in a fast exciton dissociation process, as a concurrent phenomenon for the radiative recombination (for the existence of such a process in PPV/NTC composite pleading the increase of the photoconduction current in percolative regime and the photoluminescence quenching at the increase of nanotube concentration). The longer decay time observed in PPV/NPO composites, as compared with that determined in PPV films might be explained admitting that the presence into the polymer matrix of the oxide nanoparticles leads, through an isolation effect of polymeric chains to the increase of the pound of radiative recombination paths for the intrachain excitons.

Conferences & Workshops

1. Characterization of mesoporous structure of SiO2
V. Fruth, F. Papa, L. Sarbu, E. Tenea, G. Voicu, A. Barau, C. Andronescu, C. Nistor, A. Sarbu
SIZEMAT, 25-27 mai, 2006, Varna, Bulgaria

2. Monocomponent powders and thin films in the binary system SiO2 – TiO2
M. Gartner, M. Crisan, L. Predoana, M. Zaharescu, A. Barau, S. Preda
Scoala de vara SICMAC, 11-17 iunie 2006, Mahon, Spania

3. Evaluation and Characterization of mesoporous structure of SiO2
V. Fruth, F. Papa, L. Sarbu, E. Tenea, G. Voicu, C. Andronescu, C. Nistor, A. Sarbu
Electroceramics X, 18-22 iunie 2006, Toledo, Spania

4. Radiative recombination of excitons in poly para-phenylenevinylene films
M. Baibarac, N. Preda, I. Baltog, L. Mihut, J. Wery, S. Lefrant
The International Conference of Physical Chemistry ROMPHYSCHEM-12, 6-8 Septembrie 2006, Bucuresti, Romania

Published Papers

1. SERS, FTIR and photoluminescence studies on single-walled carbon nanotubes/conducting polymers composites",
S. Lefrant, M. Baibarac, I. Baltog, C. Godon, J.Y. Mevellec, J.Wery, E. Faulques, L. Mihut, H. Arab, and O. Chauvet,
Synth. Metals, 155, (3), 666-669 (2005)

2. Optical properties of carbon nanotube-PPV composites: influence of the PPV conversion temperature and nanotube concentration
E. Mulazzi, R. Perego, H. Aarab, L. Mihut, E. Faulques, S. Lefrant and J. Wery, L.Mihut, S. Lefrant, E. Faulques,
Synthetic Metals 154, 221, 2005

3. Evidence of temperature dependence charge migration on conjugated segments in poly-p- phenylene vinylene and single-walled carbon nanotubes composite film
E. Mulazzi, R. Perego, J. Wery, L.Mihut, S. Lefrant, E. Faulques,
J. Chem. Phys. 125, 014703 (2006)

4. Optical properties of poly (para-phenylene vinylene) and single-walled carbon nanotube composite films: effects of conversion temperature, precursor dilution and nanotube concentration
F. Massuyesu, H. Aarab, L. Mihut, S. Lefrant, E. Faulques, J. Wery, E. Mulazzi,
Journal of Physical Chemistry C 111, 15111 (2007)

Activities

Resources