Dr. Peter William de Oliveira

Transparent semiconducting ITO:Ti thin films, prepared by a sol-gel process, has been deposited by spin-coating technique onto alkali-free glass substrates. The as-coated films were annealed in ambient air at 550 °C for 1 h and further annealed in a reducing atmosphere. The influences of the Ti content in the sol on the surface morphology, microstructure, optical properties and electrical resistivity have been investigated. These properties were found to depend on the Ti content in the coating sol. Ti addition led to dense smooth layers with larger crystallite size (20–30 nm). Double layers synthesized with Ti:ITO = 0.53 wt% and submitted to reducing treatment in forming gas exhibited the lowest sheet resistance R□ = 60 Ω□ with an average transmittance of 87% at 550 nm.

We describe the fabrication and characterization of organic photodiodes on solution cast ITO (tin doped indium oxide) bottom electrodes. ITO coatings were produced by gravure printing process on PET and PEN substrates. The sheet resistance could be decreased by heat treatment at 120°C under forming gas atmosphere (N2/H2) to 1.5 kΩ. The transmission of the ITO coated PET and PEN substrates is more than 80% in the visible range. The printed films were hardened under UV-irradiation at low temperatures (< 130°C) and used as the bottom electrode of an organic photodiode (OPD), consisting of a stacked layer of copper phthalocyanine (p-type material), perylene tetracarboxylic bisbenzimidazole (n-type material) and Aluminium tris(8-hydroxyquinoline). The performance of the photodiodes with printed ITO on plastic substrates could be improved by adding a smoothing layer of PEDOT/PSS (Baytron® P) on the ITO coated films and was then similar to the performance of photodiodes with semi-transparent gold as anode. These results demonstrate the suitability of the printed ITO layers as bottom electrode for organic photodiodes. Furthermore the influence of different treatments (forming gas and oxygen plasma treatment) of the ITO bottom electrode on the current-voltage characteristics of the OPDs was studied.

The influence of ionic liquids in photopolymerizable holographic materials was investigated extensively. The structures of ionic liquids have important effect on the properties of the materials. Although not all tested ionic liquids can improve the properties of the materials, the ionic liquids based on imidazolium, pyridium, or phosphonium with appropriate counter anions can be used as additives to increase the sensitivity, the diffraction efficiency, and the resolution of the materials in the thin hologram. Polymerizable ionic liquids have also been used as additives. Higher sensitivity, higher diffraction efficiency and higher resolution were obtained as well. These ionic liquids can carry out the photopolymerization during exposure to UV light to recording the hologram. They may assist to form a more stable hologram.

Aluminum nitride (AlN) is a direct bandgap semiconductor with a bandgap about 6.1 eV at room temperature, the largest among semiconductors. This paper emphasizes experimental results of the growth and optical properties of AlN nanostructures by direct nitridation. The nitridation process was performed by chemical vapor deposition method with nitrogen (N2) gas flow. AlN nanostructures were analyzed by scanning electron microscope (SEM) equipped with energy-dispersive X-ray (EDX) spectroscope and photoluminescence (PL) spectroscopy. AlN nanowires with different widths from ultrathin to thick were synthesized with this method. All of the samples had high purity without presence of any other material in EDX spectrum. The PL spectra were obtained by a 325-nm helium-cadmium (He-Cd) laser as the excitation source showing high-intensity light emitting visible wavelengths for these structures at room temperature.

SnO2 nanostructures were directly synthesised by chemical vapour transport on different substrates in a horizontal furnace. The influence of substrate on the morphology of these nanostructures was investigated by changing the substrate type, coating, and temperature. The SnO2 nanowires and nanorods were one dimensional (1D) structures with widths and lengths of 50-200 nm and several micrometers respectively. Scanning electron microscope (SEM) images show formation of short nanorods with lengths of less than 1 µm on indium-tin oxide (ITO) substrates. The effect of substrate temperature on growth was studied. SnO2 nanowires were obtained using silicon substrate, and the effect of Au coating on the size and morphology of these structures was proposed. By coating the Si wafer with a thin layer of Au, the size of the nanostructure was reduced and the length increased. The differences in size and morphology are shown by transmission electron microscopy (TEM). X-ray diffraction (XRD) spectra show tetragonal structures for both substrates.

This paper provides a selective description of the development of nanostructured materials and the fabrication of the devices for optical applications. Examples are interference coatings, refractive and diffractive lenses, and macro- and micro-GRIN (graded refractive index) optical elements. Hybrid materials containing nanoparticles are of particular interest for the production of optical elements because, by exploiting the intrinsic solid state properties of the nanoparticles, nanocomposites can be tailored to exhibit the desired properties. A particular advantage of wet chemical processing lies in its great flexibility for depositing functional coatings.

The solvothermal reactions of Ti(OiPr)4 in alcohol using ionic liquid as additive were investigated. In the presence of [BMIM][Cl], [BMIM][Br], [BMIM][NTf2], [BMIM][SO3Me], [BMIM][SO4Me], or [BMIM][OTf] (BMIM = 1-Butyl-3-methylimidazolium), pure anatase nanoparticles were obtained. The controlled hydrolysis of Ti(OiPr)4 in the presence of ionic liquids to form titanium oxo clusters plays a key role in the formation of anatase nanostructures, and ionic liquids can be repeatedly used to synthesise anatase nanoparticles. However, in the presence of [BMIM][PF6], [BMIM]2[Ti(OH)6] was obtained by an anion exchange reaction.

The hydrolysis reactions of Ti(OiPr)4 in the presence of different ionic liquids using ethanol as solvent were investigated. The reaction in the presence of [BMIM][BF4] gave rise to the anion exchange reaction product [BMIM]2[Ti(OH)6] in moderate yield. On the contrary, Ti7O4(OEt)20 was obtained in ionic liquids with other anions. The anion exchange reaction has a broad capacity for group functionalization. It can be used to synthesize both imidazolium and pyridium [Ti(OH)6]2− salts. In contrast, under the similar reaction conditions, the reactions of zirconium alcoholate gave rise to [Zr(OH)5]− or [Zr(OH)6]2− intermediates depending on the counter cations. Under the solvothermal reaction conditions, [PF6]− based ionic liquids can also undertake the similar anion exchange reactions.

Symmetric and asymmetric diffusers with a directional diffusion property were both fabricated based on a photopolymerizable hologram material using an ionic liquid as an additive. The diffusion property can be regulated by changing the concentration of the ionic liquid. The fiber structure, the surface-relief structure, and the formation of nanoparticles led to the directional diffusion property of the diffuser.