<p>Polymer-based organic light-emitting diodes (OLEDs) with the structure ITO / PEDOT:PSS / MDMO-PPV / Metal were prepared by spin coating. It is known that electroluminescence of these devices is strongly dependent on the material used as cathode and on the deposition parameters of the polymer electroluminescent layer MDMO-PPV. <strong>Objective.</strong> In this work the effect of i) the frequency of the spin coater (1000-8000 rpm), ii) the concentration of the MDMO-PPV: Toluene solution, and iii) the material used as cathode (Aluminium or Silver) on the electrical response of the devices, was evaluated through current-voltage (I-V) measurements. <strong>Materials and methods</strong>. PEDOT:PPS and MDMO-PPV organic layers were deposited by spin coating on ITO substrates, and the OLED structure was completed with cathodes of aluminium and silver. The electric response of the devices was evaluated based on the I-V characteristics. <strong>Results.</strong> Diodes prepared with thinner organic films allow higher currents at lower voltages; this can be achieved either by increasing the frequency of the spin coater or by using concentrations of MDMO-PPV: Toluene lower than 2% weight. A fit of the experimental data showed that the diodes have two contributions to the current. The first one is attributed to parasitic currents between anode and cathode, and the other one is a parallel current through the organic layer, in which the carrier injection mechanism is mediated by thermionic emission. <strong>Conclusions.</strong> The results fitting and the energy level alignment through the whole structure show that PPV-based OLEDs are unipolar devices, with current mainly attributed to hole transport.</p> <p><strong>Key words:</strong> organic semiconductors, OLEDs, electroluminescent polymers, MDMO-PPV, PEDOT:PSS, Spin coating, HOMO, LUMO, carrier injection, thermionic emission.</p><br />

<p><strong>Objective:</strong> Fabrication and optical characterization of close-packed 225 nm SiO₂ -based colloidal crystals<strong>. Materials and methods:</strong> The vertical convective self-assembly method is used to grow high-quality 225 nm close-packed SiO₂-based colloidal crystals. An annealing process (550°C) is made in order to improve the mechanical stability of the sample. Optical characterization is done by angle-resolved transmission spectroscopy (A-RTS) and structural characterization by Scanning Electron Microscopy (SEM). <strong>Results:</strong> Both, A-RTS and SEM, show that with the vertical convective self-assembly method, with the appropriate parameters of temperature of evaporation (60°C), volume fraction of the colloidal suspension (0.2% w/w) and acidity (pH=6), highly ordered close packed face centered cubic (fcc) SiO₂ based colloidal crystals are obtained. <strong>Conclusions:</strong> The growth of high-quality (long range order and defect-free) face centered cubic opal-based photonic crystals is reported.</p> <p><strong>Key words:</strong><em> </em>Photonic crystals, colloidal crystals, artificial opals, vertical convective deposition method, Bragg diffraction</p><br /><br />

Objectives: The aim of this study was to assess the potential of a smear-layer removing agent (citric acid) vs. an accepted gutta-percha-softening agent (Xylol) as an alternative substance for removing the root canal filling materials, while investigating the potential for associated demineralization effects. Materials and Methods: Seventy healthy, recently extracted premolars were used, ten as control, with sixty with their canals enlarged, shaped and cleaned and obturated using lateral compaction. Teeth were distributed into 3 groups as follows: 1) no solvent and mechanical removal of the filling materials; 2) 1ml of Xylol for 1 minute followed by mechanical removal; and 3) 10% citric acid for 1 minute followed by mechanical removal. Two sections of the root were used, one for Raman spectroscopy analysis to evaluate morphological changes in dentine surface and the other for micro-hardness testing (Vickers). Results: The use of 10% citric acid in in the removal of gutta-percha and sealer was more effective than the Xylol and mechanical group (p<0.05), presented less remnants of filling material debris and with non-observable demineralizing effects. Conclusion: Citric acid might be considered as a viable alternative in the removal of gutta-percha and sealer during root canal retreatment.Clinical Relevance: This research showed how citric acid can be used as an alternative in endodontic retreatment; how efficiently removed the filling material without damaging the dental tissue.