T.P. Nguyen

Characterization of singlewalled carbon nanotubes-PMMA composites

Thin films of poly(methyl methacrylate)-singlewalled nanotubes (PMMA-SWNTs) composite were produced by spin coating using different nanotubes concentrations. Characterization of these new materials was performed by scanning electron microscopy (SEM) and Raman spectroscopy in order to obtain information on the possible interactions between these two materials and especially, on the modifications of the nanotubes and their organization. It is found that in the composite films, the distance between the nanotubes in bundles increases because of the intercalation of polymer. For low nanotube concentrations, amorphous carbon is dispersed in the polymer matrix giving more uniform thin films.

Red organic light emitting device made from triphenylene hexaester and perylene tetraester

Saturated red light emission from organic light emitting diodes is less common than emission in the green or the blue. Most organic red light emitting devices are based on rare earth complexes, mainly europium, which are known to exhibit stability problems. The present article describes new diodes made of indium tin oxide-coated glass/triphenylene hexaether/perylene tetraester/aluminum. The band diagram was determined by ultraviolet photoemission spectroscopy, cyclic voltammetry, scanning tunneling microscopy, and absorbance measurements. The interfaces between electrodes and organic layers were investigated by x-ray photoelectron spectroscopy. The current–voltage and luminance–voltage characteristics are very reproducible from device to device, with an emission peak at 620 nm and a full width at half maximum of 80 nm, a current rectification ratio of about 30, I∼V2 at low voltages and I∼Lum∼V6 at higher voltages.

An investigation into the effect of chemical and thermal treatments on the structural changes of poly(3,4-ethylenedioxythiophene)/ polystyrenesulfonate and consequences on its use on indium tin oxide substrates

The effect of thermal and chemical treatments on the structural change of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS) have been investigated using X-ray photoelectron spectroscopy (XPS), optical spectroscopy and conductivity measurements. Surface analysis and optical measurements of the samples showed that the thermal treatments have little effects on the structure of the film. A hydrochloric acid (HCl) etching modified the composition of the dopant, in particular, by dissolving the sodium salt in PSS. The resistivity of thermally treated polymer films is slightly increased as compared to pristine films while acid treated films shows a drastic decrease in resistivity, probably by a protonic doping of the polymer. In ITO/PEDOT:PSS systems, it has been proved that indium from the oxide has diffused through the polymer film, probably due to the decomposition of the ITO film in contact with the polymer suspension, which contains water. A thermal treatment of the system increased the diffusion process. With acid etching, indium impurities were removed, and the process may be used to improve the stability of the light emitting diodes.

Characterization of multiwalled carbon nanotubes-PMMA composites

We report, for the first time, preliminary results obtained from analysis of poly(methyl methacrylate) -multiwalled carbon nanotubes composites thin films. These films were prepared by mixing the polymer with different nanotube concentrations and were deposited by spin coating on glass substrates. The composites were characterized by Raman spectroscopy and scanning electron microscopy. The evolution of the conductivity versus the nanotube concentration was carried out in the order to determine the transport process in these materials. Such a composite is promising for use as transporting layer in multilayer diodes.

Thermal and chemical treatment of ITO substrates for improvement of OLED performance

Abstract We have investigated thin film of Indium Tin Oxide (ITO) deposited on glass substrates by thermal annealing and by H 3 PO 4 chemical etching in order to improve their quality for use in polymer based light-emitting diodes. The morphology of the film surface was significantly changed by both treatments as observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The sheet resistivity of the annealed sample was found to be dependent on the annealing temperature, while it was practically not affected by the chemical treatment. On the chemically treated samples, analysis of the film surface by X-ray photoelectron spectroscopy (XPS) revealed that a thin ionic phosphorus layer was formed on the ITO surface. The electrical and optical characteristics of light-emitting diodes with poly(2-methoxy,5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) or MEH-PPV as an active layer, deposited on treated and untreated ITO substrates were determined and compared. Subsequent enhancement of performance of the diodes was obtained with treated ITO substrates, proving the efficiency of the performed treatments.

Raman characterization of singlewalled carbon nanotubes and PMMA-nanotubes composites

The Raman spectroscopy have allowed us to perform studies on singlewalled nanotubes (SWNTs) produced by following methods: electric arc, laser ablation and solar energy. As this characterization method provides a great deal of informations, we will present a comparison between the nanotubes produced by all these processes and the influence of some synthesis parameters. By using spin casting, we have produced thin films of PMMA-SWNTs for different concentrations. Then, we have characterized these new materials by Raman spectroscopy. The aim of these investigations is to get information on the possible interactions between these two materials. In particular, we have studied the evolution of the composites films spectra as a function of the nanotubes concentration in the polymer.

XPS analysis of thermal and plasma treated polyparaphenylene-vinylene thin films and their interface formed with aluminum layer

Abstract X-ray photoelectron spectroscopy (XPS) was used to investigate the interface formed between thermally and plasma treated polyparaphenylene-vinylene thin films and an aluminium layer deposited on them. Enhancement in adhesion of the metal on thermally treated polymer films is explained by the structural change of aluminum. In contrast, plasma treatments induce morphological modifications of the polymer surface and favor the formation of compounds between the polymer and the metal which in turn improve the adhesive strength.

Studies of the polyparaphenylene/aluminium interface

Abstract The dielectric properties of thin film Al/polyparaphenylene/Al structures have been investigated in the frequency range from 10−2 to 5 × 105 Hz. Experimental data suggest the existence of interfacial layers at the polymer/metal contacts. An equivalent circuit analysis, taking into account the dielectric response of these elements, is presented. To clarify the nature of the contact, X-ray photoelectron spectroscopy (XPS) has been performed on polyparaphenylene (PPP) films covered with Al layers. Displacements of the C(1s), O(1s) and Al(2p) lines lead to the conclusion that a chemical reaction between carbon, oxygen and aluminium occurs at the interface to form a metal-oxygen-polymer complex which is found to be responsible for the electrical a.c. behaviour of Al/PPP/Al structures.

Thermal stabilisation of PVC via the polaron mechanism Part III: Stabilisation with organotin compounds

Abstract By the polaron theory, it has been shown that the stabilisation of poly(vinyl chloride) (PVC) with organotin compounds is due to many different reactions: (1) substitution of the activated hydrogen in PVC by the stabiliser molecule or by-products of the stabiliser, this reaction being one of the most important because it takes place throughout the stabilisation; (2) substitution by the stabiliser of labile chlorine atoms, which occurs only in the early stages of the induction period; (3) saturation of the degraded PVC by by-products of tin compounds or HCl; and (4) stabilisation due to the interaction of organo groups with the degraded PVC.

Studies of the polymer-metal interface in metal-PPV-metal devices

Abstract We report the results obtained from electrical measurements and surface analysis by photoelectron spectroscopy (XPS) performed in polyparaphenylene-vinylene (PPV) thin films sandwiched between metallic electrodes. Current-voltage characteristics of metal-polymer-metal devices are dissymetrical with rectifying contacts at top electrodes (Cr or Al) and blocking one at bottom electrodes (Cr). Analysis of the core level spectra of carbon, oxygen and metals in the interfacial layers leads to the conclusion that the rectifying behaviour of polymer-metal contact is in close relation with the formation of metallic oxide-carbon complexes at the interfaces.