PPV Precursor Research Articles

Optical probing of sample heating in scanning near-field experiments with apertured probes

We have used the inherent thermochromism of conjugated polymers to investigate substrate heating effects in scanning near-field experiments with metal-coated “apertured” probes. Chemically etched and pulled fibers were used to provide near-field excitation of fully converted films of poly(p-phenylene vinylene), PPV, and of poly(4,4′-diphenylene diphenylvinylene). We detect no significant blueshift of the photoluminescence spectra generated with near-field excitation, in comparison to those collected with far-field excitation. We conclude that polymer heating in the region contributing to the luminescence is less than 40K. We also demonstrate that thermolithography of the PPV precursor is not significant by comparing UV (325nm) and red (670nm) illumination.

FABRICATION AND CHARACTERIZATION OF SPIN SELF-ASSEMBLED PPV/MONTMORILLONITE MULTILAYER THIN FILMS

Multilayer nanostructural thin films through a layer-by-layer spin self-assembly method were prepared using poly(p-phenylene vinylene)/montmorillonite. Sodium montmorillonite particles exfoliated into single sheets and cationic PPV precursor and such anionic MMT plates were spin self-assembled by electrostatic attraction. Self-assembled MMT layers blocked the penetration of oxygen and moisture and they reduced the photo-oxidation of the emitting material. Spin self-assembled films showed higher environmental stability and luminescence, and their PL spectra were somewhat different from that of bulk PPV films.

Energy transfer in a multilayer film via spin self-assembly

Abstract We have fabricated ultrathin multilayer films via spin self-assembly quickly and easily, using cationic poly(p-phenylene vinylene) (PPV) precursor, and as its counterparts, anionic poly(styrene-4-sulfonate) (SPS) and dodecylbenzenesulfonate (DBS). The PPV precursor incorporated with DBS could be thermally converted to PPV rapidly at a relatively lower conversion temperature than the self-assembled (PPV precursor/SPS) film. When we thermally converted an alternating multilayer film of [(PPV precursor/SPS)5/(PPV precursor/DBS)5], at a low temperature (110 °C), more converted PPV with longer conjugation length (PPV/DBS) and less converted PPV with shorter conjugation length (PPV/SPS) coexisted. As a result, the multilayer film with the different energy band-gaps could be fabricated by thermal-converting an alternating multilayer film at a low conversion temperature in a relatively short time. UV/VIS absorption spectra and photoluminescent spectra demonstrated not only the structural regularity and reproducibility of this system but also the energy transfer between the layers with different conjugation lengths.

Photoconductivity and optical properties in composites of poly(paraphenylene vinylene) and single-walled carbon nanotubes

We present photoconductivity and Raman scattering data obtained from composite films of poly(paraphenylene vinylene) (PPV) and single-walled carbon nanotubes at different weight concentrations from 0% to 64%. It is found that the introduction of nanotubes in the PPV precursor polymer solution, heated at $300\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ to perform conversion into PPV, yields drastic modifications in both the structural features of the composite components and in the electronic properties of the composites. The PPV polymer matrix becomes more disordered due to the introduction of nanotubes which induce a shortening of the polymer conjugated segments as shown by Raman scattering spectra. In addition, these spectra yield information about the evolution from small bundles to thick bundles of single-walled nanotubes as function of their concentration $x$. Photoconductivity data show that the percolation regime begins at $x=2%$, indicating that a migration network for the photogenerated charges is established above this threshold. By using a model based on distributions of PPV conjugated lengths and their changes as function of $x$, we calculate the Raman scattering band shapes and their relative intensities. The theoretical results lead to a comprehensive interpretation of experimental data.

Investigation of heating effects in near-field experiments with luminescent organic semiconductors

We present a study of heating effects in apertured near-field gold-coated probes which makes use of the temperature dependence of the photoluminescence spectra of the polymer semiconductor poly(p-phenylene vinylene) (PPV). The small throughput of metal-coated probes used for aperture-scanning near-field optical microscopy (aperture-SNOM) can induce significant probe heating, even when operated with laser powers of only a few mW. According to recent literature the probe apex can reach several hundred degrees celsius, possibly producing local heating of the specimen. Such a possibility needs to be investigated carefully, since it could affect the results of both near-field spectroscopy or lithography, especially where the photosensitive material happens to be sensitive to high temperatures, as for the PPV precursor. Interestingly, the sensitivity of conjugated polymers optical properties to changes in temperature (blue-shift of either photoluminescence or absorption upon heating), can be used as a convenient tool for investigation of the sample heating. Here, we report a comparison between photoluminescence spectra collected in the far-field but with excitation in either the far- or near-field, which demonstrate the absence of any significant difference, and thus indicate a negligible heating of the polymers during SNOM experiments. In addition, we demonstrate that near-field illumination of PPV precursor films with red light (i.e. with a wavelength longer than the absorption edge of PPV precursor), does not result in any lithographic effect, confirming that thermolithography of the PPV precursor is not significant in our lithography experiments.

Laser spectroscopic investigation of a new precursor-type polyparaphenylenevinylene

Optical, structural and morphological properties of thin films of polyparaphenylenevinylene (PPV) formed by an alkyl sulfinyl precursor route have been studied. Thin films were fabricated on an optical glass and on quartz glass either by spin-coating of the precursor solution or by layer-by-layer deposition using Langmuir–Blodgett technique. PPV precursor films were also spin-coated on gold-coated glass in order to study thin-film optical parameters by surface plasmon spectroscopy. We have been successful in forming about 40 precursor mono layers on quartz glass by Langmuir–Blodgett technique using optimized surface pressure and dipping conditions. After thermal conversion of the precursor layers good quality fluorescent PPV films of yellow colour have been obtained. Optical characterization of the films was carried out by linear absorption and emission spectroscopy, ellipsometry, and surface plasmon spectroscopy. Structural and morphological studies on the thin films were carried out by using X-ray scattering and atomic force microscopy. Wave-guided travelling-wave laser action has been achieved in a PPV film on quartz glass. The sample was transversally pumped with picosecond laser pulses (wavelength 347.15 nm, duration 35 ps). Laser emission occurred at 550 nm for pump pulse energy densities above 1 × 10 - 4 J cm - 2 .

Size effect of nanoparticles on the conjugated polymer in PPV/SiO2 nanocomposites

The size effect of nanoparticles on the conjugated polymer was investigated in terms of the optical and electronic properties of poly(p-phenylenevinylene) (PPV) and PPV/SiO2 nanocomposites. PPV/SiO2 nanocomposites were prepared by mixing PPV precursor and SiO2 sols with 5 and 19nm. The conjugation length of the nanocomposite prepared using nanoparticles with 5nm is much shorter than that of PPV. The nanoparticles with the similar size (5nm) to the conjugation length of PPV result in an increase of morphological disorder of PPV, while the nanocomposite prepared using nanoparticle with 20nm shows the decrease of disorder. Therefore, the current density of the device using nanoparticle with 5nm was lower than that of PPV, while the nanocomposite prepared using nanoparticle with 20nm shows enhanced current density.

Fabrication and characterization of polymer/nanoclay hybrid ultrathin multilayer film by spin self-assembly method

We have prepared ultrathin multilayer nanostructural films by a layer-by-layer spin self-assembly method using poly (p-phenylene vinylene) (PPV)/layered silicate and characterized them by contact angle measurement, surface dying technique, UV/Vis spectroscopy, photoluminescent (PL) spectroscopy, X-ray reflectivity (XRR), and model-fitting. The hybrid ultrathin multilayer film was stepwisely deposited using the electrostatic forces between the cationic PPV precursor and the negatively charged surface of layered silicate, and finally thermally converted to (PPV/Laponite RD) n film. The surface coverage of the PPV precursor onto layered silicate and vice versa could be clearly observed using the contact angle measurement and surface dying technique. The continuous increase of UV/Vis absorbance and PL intensity of the films with each bilayer demonstrated the regular and reproducible deposition of this system, and the Kiessig fringes and Bragg peaks in XRR spectra indicated the well-ordered internal structure.

Fabrication and characterization of polymer/nanoclay hybrid ultrathin multilayer film by spin self-assembly method

We have prepared ultrathin multilayer nanostructural films by a layer-by-layer spin self-assembly method using poly (p-phenylene vinylene) (PPV)/layered silicate and characterized them by contact angle measurement, surface dying technique, UV/Vis spectroscopy, photoluminescent (PL) spectroscopy, X-ray reflectivity (XRR), and model-fitting. The hybrid ultrathin multilayer film was stepwisely deposited using the electrostatic forces between the cationic PPV precursor and the negatively charged surface of layered silicate, and finally thermally converted to (PPV/Laponite RD) n film. The surface coverage of the PPV precursor onto layered silicate and vice versa could be clearly observed using the contact angle measurement and surface dying technique. The continuous increase of UV/Vis absorbance and PL intensity of the films with each bilayer demonstrated the regular and reproducible deposition of this system, and the Kiessig fringes and Bragg peaks in XRR spectra indicated the well-ordered internal structure.

Optical and Spectroscopic Studies of Ppv/Pu Blends

The poly(p-phenylenevinylene)/polyurethane(PPV/PU)composite films obtained from water-borne PU system with PPV precursor(PrecPPV)were investigated. UV-VIS absorption, photoluminescence(PL), thermoluminescence(TL)spectra and dynamic mechanical analysis(DMA)results of pure PPV and its composite are reported and analysed. The conversion of the PrecPPV/PU into PPV/PU composite was confirmed also by mean of dielectric spectroscopy. It was concluded from the UV-VIS absorption and PL spectra at room temperature that the conjugation length of the PPV chains in this composite and in pure PPV is similar. In the PL spectra at low temperatures positions of the peaks are the same in the pure PPV and in the composite. Upon cooling all peaks shift towards longer wavelengths and change their relative intensities. The TL spectra show that in these materials relatively shallow traps dominate, yielding TL maximum at ca. 70 K. The composite films cast from water suspension are flexible in contrast to pure PPV which forms rigid film. In DMA it is reflected by lower values of the storage modulus E' and of the amplitude of tanδin the composites compared to pure PPV film.

Effect of TiO 2 and SiO 2 nanoparticles on the stability of poly( p-phenylene vinylene) precursor

The effects of inorganic nanoparticles on the optical and chemical properties of poly( p-phenylene vinylene) (PPV) precursor were investigated. There was an increase in the conjugated length of the PPV precursor after long time storage and ultraviolet (UV) irradiation, while the PPV/TiO 2 nanocomposite precursor showed little increase in conjugated length. This phenomenon was more pronounced when TiO 2 was used to prepare the nanocomposite precursor than when SiO 2 was used. It is suggested that the TiOH group on the surface of TiO 2 nanoparticle interact with tetrahydrothiophene that results in the impediment of the elimination reaction of tetrahydrothiophene. Nanoparticles are known to delay the preliminary elimination of PPV precursor by blocking the penetration of light into PPV.

Nanorheological approach for characterization of electroluminescent polymer thin films

Shear-modulated scanning force microscopy (SM-SFM) is introduced as a nanorheological characterization method for the determination of optoelectronic properties of semiconducting polymer thin films (<100 nm). In this letter, the photoluminescence quantum efficiency of poly(p-phenylenevinylene) (PPV) was directly correlated to the glass transition property by SM-SFM. Conformational changes and chain packing were discussed as a function of the conversion temperature of the soluble PPV precursor. Compared to the bulk, very low glass transition temperature values in the range of 65 to 85 °C were found, which imply an increased molecular mobility in thin films of conjugated polymers.

Electrosynthesis and characterisation of Poly(paraphenylene vinylene C60) films

The electrochemical reductive polymerization of a co-polymer of phenylene vinylene units and C 60 was studied. The fullerene is introduced into the polymer synthesis as a film on the electrode substrate. During reduction the film is dissolved and the reduced fullerene species interact with the PPV precursor in solution and as a result a film is formed on the substrate. The coupling reaction between the reduced species should lead to a material where the fullerene units are incorporated into the main polymer chain. The films are characterised by cyclic voltammetry and by FTIR and Raman spectroscopy.

Formação e caracterização óptica de filmes automontados de POMA/PPV

A study is presented of the formation and optical properties of polymeric heterostructures from poly(p-phenylene vinylene) (PPV) and poly(o-methoxyaniline) (POMA) produced via the self-assembly technique. POMA layers were obtained in a non self-limiting process from its emeraldine salt, semiconducting form in HCl solution. Thermal conversion of PPV was performed at low temperatures with the substitution of the counter-ion Cl in the PPV precursor by a long sulfonic chain, the dodecylbenzenesulfonate (DBS) ion. The optical properties of PPV films converted in this way are not affected by POMA, which can be used as transparent electrode of PPV luminescent devices.

Enhanced Optical Properties of Layer-by-Layer Films of Poly(p-phenylenevinylene) Alternated with a Long Chain Counterion and Converted at Low Temperatures

Layer-by-layer (LBL) thin films of a poly(p-phenylenevinylene) (PPV) precursor have been processed using the long chain dodecylbenzenesulfonate anion (DBS). We adopted the alternative approach with controlled substitution of the chloride counterion of the water-soluble PPV precursor by DBS. The internal layer structure of these films is likely to be different from conventional LBL films, since DBS is incorporated into the PPV precursor chain. In addition to the fast adsorption of the sulfonic salt, introducing DBS allows the films to be converted at 110 °C within 12 min with a high conjugation length. By varying the amount of DBS in the solution, the conjugation length of the PPV segments can be controlled. The luminescence and absorption spectra show intense vibronic structures that are better resolved than for cast and Langmuir−Blodgett films of PPV. LBL films converted at lower temperatures display higher quantum efficiency, owing to the absence of carbonyl groups arising from high-temperature conversi...

Polymerization Behavior of Xanthate-Containing Monomers toward PPV Precursor Polymers: Study of the Elimination Behavior of Precursor Polymers and Oligomers with in-Situ FT-IR and UV−Vis Analytical Techniques

We have prepared two xanthate precursor polymers (6) and one sulfinyl precursor oligomer (7). In this way the polymerization behavior of three xanthate-containing monomers (3, 4, and 5) was studied. The chemical modifications on the monomer stage allowed to evaluate the outcome of the polymerization in relation to the chemical structure of the monomer. Polymerizations were performed in dry THF at different temperatures. The thermal conversion to the conjugated PPV structure, as well as its stability, was studied with different analytical techniques such as in-situ FT-IR spectroscopy, in-situ UV−vis spectroscopy, thermogravimetric analysis (TGA), and direct insert probe mass spectroscopy (DIP-MS). By combining the information obtained from these techniques, we were able to obtain a very detailed picture about the elimination process. These measurements also confirmed the formation of cis double bonds in xanthate precursor polymers on conversion. However, subsequent cis−trans isomerization in the end leads ...

Determination of Molecular Orientational Order in Cold-Stretched Poly( p-phenylene vinylene) Thin Films by DECODER 13C NMR

The properties of poly( p-phenylene vinylene) (PPV) films depend on the degree of orientational order present in the films. Recently, Dermaut et al. reported a novel cold-stretching technique ( Macromolecules 33, 5634–5637 (2000)) in which chain alignment can be introduced into PPV precursor films by uniaxially stretching them prior to the thermal elimination reaction that forms PPV. The two-dimensional direction exchange with correlation for orientation-distribution evaluation and reconstruction (DECODER) 13C NMR technique was applied to both unstretched PPV films and PPV films that were uniaxially cold stretched to a draw ratio λ=l/l 0=5. The unstretched films were found to be moderately ordered, comprised of a component present at 80% with a Gaussian distribution of 60° fwhm, while the remaining 20% is isotropically distributed. A distribution of 9°±3° fwhm was measured by NMR in good agreement with IR dichroism measurements for the uniaxially cold-stretched films, establishing that a high degree of orientational order can be introduced by cold stretching PPV films.

Study of the conversion process of sulfinyl-OC 1C 10–PPV precursor polymers with different analytical techniques

The use of sulfinyl groups as thermally eliminable groups in precursor routes towards conjugated systems, like OC 1C 10-PPV, is evaluated by different techniques: Fourier Transform Infrared Spectroscopy (FT-IR), Ultraviolet Visible Spectroscopy (UV-Vis), X-ray Photoelectron Spectroscopy, Electron Spin Resonance (ESR) and Raman Spectroscopy. In-situ FT-IR and UV-Vis, allows determination of the temperature range where elimination takes place. XPS was performed on the surfaces of the polymer films.

Change in electrical characteristics of poly(p-phenylene vinylene)-based self-assembled devices by addition of ionic salt to poly(sodium 4-styrenesulfonate)

A systematic study was performed on the effect of added salt on the effective conjugation lengths poly(p-phenylene vinylene)s (PPVs) formed by heat-treatment of PPV precursors in self-assembled films based on the layer-by-layer deposition between PPV and poly(sodium 4-styrenesulfonate) (PSS). It was demonstrated that the increased adsorbed amount of negatively charged PSS layer by adding NaCl salt to the PSS solution caused a blue spectral shift in electroluminescence and at the same time yielded a high turn-on voltage. The changes of electrical properties of PPV in the self-assembled films were mainly attributed to the restricted growth of conjugation bond length of PPVs induced by the increase of binding sites with the adsorbing PSS and the subsequent formation of sulfonic ester group during the thermal elimination reaction. Consequently, it is shown that the electrical properties of PPV can be simply tuned by the variation of physical parameters for the fabrication of self-assembled films without a change of PPV chemistry.

Self-assembly of poly( p-phenylene vinylene) using long chain counter-ion: a new process for fabrication of multilayer thin films heterostructures

Layer-by-layer thin films of sulfoniun poly(p-phenylene vinylene) (PPV) precursor have been processed using the long chain dodecylbenzenesulfonate (DBS) anion. Here we adopted the alternative approach consisting of the controlled substitution of the chloride counterion of a water-soluble PPV precursor by DBS. The internal layer structure of these films is therefore likely to be different from conventional layer-by-layer films, since DBS ions are expected to be incorporated into the PPV precursor chain. The main advantage of this novel procedure is that thermal conversion through the elimination of DBS may be realized at considerably lower temperatures (80-100 °C) in a few minutes, with enhanced optical properties. Using this new conversion procedure, the conjugation length can also be varied along the deposition direction by controlling the immersion time or DBS concentration in the film. This opens up a number of new possibilities to realize molecularly band-gap engineered structures such as asymmetric ladder type or V-type structures along the film.