Dr. Peter William de Oliveira

We characterize 10-nm outer diameter multi-walled carbon nanotubes (powder and dispersion), which will be used for the preparation of conductive layers on polycarbonate (PC) substrates. The optical, electrical, and mechanical characterization of the spin-coated deposited layers is shown and compared with results obtained for layers deposited in borosilicate glass substrates. In both glass and plastic, the layers have shown transmittance higher than 78% in the visible range and have passed the tape and pencil standard tests for adherence and hardness, respectively. However, the sheet resistance presented by layers deposited on PC is still much superior to that of layers deposited on glass and sintered at higher temperatures. Nevertheless, the results obtained still allow the use of such layers in antistatic applications.

Nanocomposites of TiO2 and multi-walled carbon nanotubes (MWNTs) were produced by the addition of different concentrations of MWNTs to a TiO2 sol matrix. Conductive coatings were prepared by spin coating the nanocomposite sols on glass substrates and sintering the samples at 300 ° C for 15 min. No crystalline phase of TiO2 was formed at this temperature. The sheet resistance of the coatings was decreased from some hundreds of M Ω/sq to just a few k Ω/sq by the addition of MWNTs to the TiO2 matrix. Moreover, sintering of the coatings in a reducing atmosphere has lead to a lower sheet resistance than that presented by coatings sintered in air. The lowest resistivity of 2.0 x 10-1 Ω.cm was obtained by coatings prepared with 5.0 mg/ml MWNTs in the composite sol. The optical, structural and electrical properties of the coatings were correlated and demonstrated.

ITO (tin doped indium oxide) coatings were produced by gravure printing process on PET and PEN foils. The printing paste consists of ITO nanoparticles, which are dispersed in a solvent and mixed with a binder. By modification of the printing paste, the sheet resistance (R/sq) of the ITO coatings after hardening under UV-irradiation at low temperatures (< 130 °C) could be decreased to 1 kΩ/sq. R/sq could be further reduced down to 0.5 kΩ/sq by heat treatment under forming gas atmosphere (N2/H2), the transmission of the ITO coated foils still being more than 80% in the visible range. The application of these ITO films as a bottom electrode in organic photodiodes (OPDs) is shown, and the current density-voltage characteristics of the OPDs are presented.

The fabrication of organic photodiodes on solution cast indium tin oxide (ITO) bottom electrodes on flexible substrates is described. ITO coatings with a sheet resistance of 2 to 3 kΩ/sq are produced by gravure printing process on PEN (polyethylene 2,6-naphthalate) films. The ITO films are directly used as transparent bottom electrodes for organic photodiodes (OPD). Furthermore, first experiments to integrate one of these OPDs in an all-organic opto-chemical sensor are successfully demonstrated. The implementation of OPDs as detectors in applications such as integrated sensors demands a fabrication of these devices on flexible substrates. For these applications the OPDs on printed ITO on PEN are especially suitable.

Multi-walled carbon nanotubes (MWNTs) dispersed in surfactants were investigated as additional conductive elements in an antimony-doped tin oxide (ATO) matrix. Dispersion of MWNTs in a cationic surfactant was more efficient than in anionic or non-ionic surfactants. Precursor solutions as well as coatings of ATO and ATO/MWNT composites were characterized. The effect of the addition of MWNTs on the optical, electrical and mechanical properties of the original ATO coatings was demonstrated. Low concentrations of MWNTs are sufficient to decrease the resistivity of conducting ATO films by a factor of 16, with preserved transparency (90%) in the visible range. Fewer ATO nanoparticles and lower sintering temperatures than those presented in the literature lead to films with comparable resistivities and even higher transparency, with reduced manufacturing costs. The addition of MWNTs has also increased the hardness of the ATO coatings from HB to 2H-4H, depending on the MWNT concentration employed.

Ionic liquid was used as additive in photosensitive photopolymerizable materials. Ionic liquid as additive in photopolymerizable hologram material increases the sensitivity, the resolution, and the refractive index modulation of the material efficiently, which present strong diffusion after stopping the exposure due to the diffusion-controlled polymerization in ionic liquid. The diffraction efficiency was raised up to the theoretical maximum of 34% in the thin hologram.

Nanocomposites of titanium dioxide (TiO2) and multi-walled carbon nanotubes (MWNTs) were prepared and deposited by sol-gel spin coating on borosilicate substrates and sintered in air at 300 °C for 15 min. Further irradiation of the films with different CO2 laser intensities (4.3-17 W m-2) was carried out in order to crystallize TiO2 in the anatase form while preserving the MWNT's structure. The laser irradiation changed the crystal structure of the coatings and also affected the wettability and photocatalytic activity of the films. The anatase phase was only observed when a minimum laser intensity of 12.5 W m-2 was used. The contact angle decreased with the enhancement of the laser intensity. The photocatalytic activity of the films was determined from the degradation of a stearic acid layer deposited on the films. It was observed that the addition of carbon nanotubes themselves increases the photocatalytic activity of TiO2 films. This efficiency is even improved when high CO2 laser intensities are used during the sintering of the coatings.

Conductive coatings made from very thin functionalized (-NH2) multi-walled carbon nanotubes dispersed in a TiO2-based sol were irradiated with CO2 laser using different intensities. It was observed that the electrical and optical properties of the film have changed. Laser-treated films present lower resistivity than the original non-irradiated coating. We attribute this behavior to a better densification of the nanotubes with the assist of the laser. Films irradiated with CO2 presented a higher refractive index than non-irradiated coatings, showing the susceptibility of the films in optic-electronic devices.

A novel sol-gel synthesis route for the preparation of a transparent organic-inorganic nanocomposite was developed by combining methacrylic acid (MA) stabilized, amorphous ZrO2 nanoparticles, which were synthesized by the sol-gel process, with an organic-inorganic dodecandioldimethacrylate (DDDMA)/3-methacryloxypropyl trimethoxysilane (MPTS) hybrid matrix. The average hydrodynamic particle size was determined to be approximately 6 nm by photon correlation spectroscopy. HR-TEM micrographs present irregular shaped zirconia particles with diameters up to 3 nm. Nearly solvent-free nanocomposites with zirconium (Zr) contents up to 15.2 mol% were synthesized and photochemically cured to transparent crack-free bulks. The surface charged nanoparticles in 1-propanol had an electrophoretic mobility of 0.017 (μm cm)/(V s), measured by Laser Doppler Anemometry (LDA) and a refractive index ne of 1.648 ± 0.007 determined by spectroscopic ellipsometry. After filling the nanocomposite into a linear electrophoresis cell (1 × 1.6 × 0.8 cm3), positively charged high refractive nanoparticles migrated through the low refractive hybrid matrix toward the cathode by the application of an electric potential difference of 2 kV/cm for 96 h. A 67% increase in Zr over a distance of 8 mm between the cathode and anode was observed by high-resolution scanning electron microscopy (HR-SEM) and energy dispersive X-ray spectroscopy (EDXS).