L. A. Cury

Temperature effects on the vibronic spectra of BEH–PPV conjugated polymer films

Good quality thin films of poly(2,5-bis(2′-ethyl-hexyl)–1,4-phenylenevinylene) (BEH–PPV) were optically characterized by photoluminescence, absorption, and Raman scattering techniques. The temperature evolution of the vibronic structures in the photoluminescence and absorption spectra was analyzed. At low temperatures it was possible to identify the different phonon modes that contribute to the vibronic spectra. The correlation of the Raman and photoluminescence spectra enabled us to conclude that the main vibrational modes are the two most intense Raman bands at 1310 and 1579 cm−1. The emission efficiency highly increases and the absorption spectra become much more resolved with decreasing temperature. The temperature dependence of the zero-phonon line in the absorption and photoluminescence measurements is attributed to an increase of the effective conjugation length at low temperatures. The results from the polarization-resolved photoluminescence demonstrated the high degree of the in-plane structural ...

One pot synthesis of fluorescent π-conjugated materials: immobilization of phenylene–ethynylene polyelectrolytes in silica confined ionogels

Phenylene–ethynylene monomer and copolymer bearing pendant imidazolium groups were synthesized in order to obtain luminescent entities highly soluble in ionic liquids (ILs). Their stabilities under non-aqueous sol–gel conditions (i.e. using formic acid) allowed their incorporation into silica in the form of ionogels. This was performed in a one step synthesis and the resulting materials were found to be easily processable as monoliths and showed interesting luminescent properties. Given the high solubility of 2,4-dinitrotoluene in the investigated IL, (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), the relevance of these ionogels as luminescent sensors was tested.

Long range energy transfer in conjugated polymer sequential bilayers

Steady-state and time-resolved photoluminescence have been used to investigate the optical properties of bilayer and blend films made from poly(9,9-dioctyl-fluorene-2,7-diyl) (PFO) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH PPV). Energy transfer has been observed in both systems. From steady-state photoluminescence measurements, the energy transfer was characterized by the effective enhancement of the MEH PPV emission intensity after exciting the donor states. Relatively faster decays for the PFO donor emission have been observed in the blends as well as in the bilayer structures, confirming effective energy transfer in both structures. In contrast to the bilayers, the time decay of the acceptor emission in the blends presents a long decay component, which was assigned to the exciplex formation in these samples. For the blends the acceptor emission is in fact a composition of exciplex and MEH PPV emissions, the later being due to Förster energy transfer from PFO. In the bilayers, the exciplex is not observed and temperature dependence photoluminescence measurements show that exciton migration has no significant contribution to the energy transfer. The efficiency and very long range of the energy transfer in the bilayers is explained assuming a surface-surface interaction geometry where the donor/acceptor distances involved are much longer than the common Förster radius.

Large blue shift in the absorption spectra of BEH-PPV films containing gold nanoparticles

Abstract We report on a relatively large spectral blue shift observed in the absorption of conjugated polymer films of poly(2,5-bis(2′-ethyl-hexyl)-1,4-phenylenevinylene) containing gold colloidal nanoparticles (BEH-PPV/Au). The range of the photoluminescence (PL) emission energy, contrarily, does not change when compared to that of the pure BEH-PPV films. A large broadening effect in the PL peaks was observed for the BEH-PPV/Au film, which was attributed to a larger structural disorder induced in the polymer matrix by the colloidal nanoparticles. Micro-Raman spectroscopy revealed a broadening in the most intense phonon modes, which corroborates the disorder effects in the BEH-PPV/Au film structure. At low temperatures and also at higher excitation intensities, however, the PL vibronic peaks recover their relatively sharp characteristics, indicating a relative suppression of the disorder effects on the optical properties of the BEH-PPV/Au film. Our experimental results support the statement that gold colloidal nanoparticles, interspersed in the polymer matrix, can effectively lower the barrier for a crossover to a structurally disordered phase, resulting in a decrease in the effective conjugation length and consequently leading to a blue shift in the absorption.

The effect of potential fluctuations on the optical properties of InGaAs∕InAlAs superlattices

Photoluminescence (PL) measurements as a function of temperature and excitation intensity were carried out in a sample containing two InGaAs∕InAlAs superlattices, grown on the same InP substrate, with quantum wells and barriers of different widths. The fluctuations in the confinement potential for excitons in both structures are investigated by following the blueshift of the PL peaks with increasing temperature as well as with rising excitation intensity, at low temperatures. A decrease in the full width at half maximum of the PL peaks with increasing excitation power was also observed. The change in the PL linewidth with excitation power is interpreted in terms of the variation of the relative contribution of the excitons localized at the excitonic band tail (due to the potential fluctuations) and of the nonlocalized excitons. Moreover, the activation energies of the nonradiative channels responsible for the thermal quenching of the photoluminescence peaks are deduced from an Arrhenius plot of the integr...

Interfacial exciplex formation in bilayers of conjugated polymers

The donor-acceptor interactions in sequential bilayer and blend films are investigated. Steady-state and time-resolved photoluminescence (PL) were measured to characterize the samples at different geometries of photoluminescence collection. At standard excitation, with the laser incidence at 45° of the normal direction of the sample surface, a band related to the aggregate states of donor molecules appears for both blend and bilayer at around 540 nm. For the PL spectra acquired from the edge of the bilayer, with the laser incidence made at normal direction of the sample surface (90° geometry), a new featureless band emission, red-shifted from donor and acceptor emission regions was observed and assigned as the emission from interfacial exciplex states. The conformational complexity coming from donor/acceptor interactions at the heterojunction interface of the bilayer is at the origin of this interfacial exciplex emission.

Arrhenius analysis optical transitions in strained InGaAsP quantum wells

Strained InGaAsP/InP single quantum wells grown by low pressure metalorganic vapor phase epitaxy are studied by photoluminescence. We demonstrate that the analysis of the Arrhenius plot, specially modified to fit the temperature dependence of the integrated photoluminescence intensity, can be used as a complementary technique in order to identify different optical transitions that take place in more complex photoluminescence spectra.

Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) conjugated polymer domains in a thermoplastic polyurethane matrix

Self-sustained and spin-casting films formed from blends of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) conjugated polymer and thermoplastic poly-urethane (TPU) at different concentrations have been investigated. Scanning electron microscopy images of self-sustained films show the formation of circularlike domains of MEH-PPV in the TPU matrix, characterizing the blend material as heterogeneous. The circularlike domains in the spin-casting films were also observed by electric force microscopy (EFM) and by atomic force microscopy. The EFM, in particular, has allowed the differentiation of the MEH-PPV domains from the TPU matrix due to the electrical properties of the conjugated polymer. The MEH-PPV domains in the spin-casting films are much smaller in size than in the case of self-sustained films. The glass transition temperatures for the TPU and for the self-sustained blended films were obtained by temperature modulated differential scanning calorimetry. A very small variation in ...

Dependence of the vibronic emission on self-absorption and reemission processes in conjugated polymers

Photoluminescence spectra from the edge of thin films of a conjugated polymer were obtained by varying the self-absorption level at different temperatures. This technique together with the use of a Franck-Condon analysis enabled a clear identification of the vibrational relaxation processes, providing a way to recognize the participating vibrational modes. The higher order vibronic bands were observed to shift to lower energies than those predicted theoretically. This was interpreted as an effect of self-absorption processes, which lead to the loss of energy by intramolecular interactions due to the increase of thermal disorder.

Blends of poly[2-(2′,5′-bis(2′-ethyl-hexyloxy)phenyl)-1,4-phenylenevinylene] and poly(3-hexylthiophene) as base materials for broad band light emission devices

Photoluminescence (PL) and electroluminescence (EL) emission from blended films composed by poly[2-(2′,5′-bis(2′-ethyl-hexyloxy)phenyl)-1,4-phenylenevinylene] (BEHP-PPV) and poly(3- hexylthiophene) (P3HT) conjugated polymers with different relative concentrations are investigated. The main changes observed on the PL spectra of the blended films with temperature and excitation power are associated to an increase of the emission efficiency of the P3HT constituent. The EL spectrum of a light emission device (LED) fabricated with the BEHP-PPV:P3HT blend presents a similar broad emission as in the PL measurement but shifted to higher wavelengths. The blended LEDs show a significantly higher emission efficiency than the LEDs made with the pure constituents. The apparent color of the blended LED shifts to a purer red as the applied voltage is increased. This tuning capacity was interpreted as due to a change in the effective gap of the blend caused by the redistribution of injected carriers in the BEHP-PPV:P3HT ...