Titanium dioxide (TiO2) in the form of pellets (pressed powder) and thin films are investigated, revealing the presence of distinct phases: mainly anatase and rutile. Characterization of optical, structural and electrical properties were carried out on samples submitted to different sort of thermal annealing (TA), at distinct temperatures, 500 and 1000ºC, due to their influence on the obtained phases. TA temperature along with pressure application for sample conformation, determine the bands present in the photoluminescence (PL) spectra, being about 550nm, characteristic of anatase phase and 800nm, related to the presence of rutile phase. The bandgap of thin films is determined from optical absorbance data, yielding 3.4eV for anatase phase (indirect transition), and 2.9eV for rutile phase (direct transition). Besides, irradiation with monochromatic light strongly affects the thin film conductivity, but the energy range (above or below the bandgap energy) does not seem to affect the behavior, which is associated with the excitation of intrabandgap states or crystallites belonging to phases with distinct bandgaps.

Alternative materials for use in electronic devices have grown interest in the past recent years. In this paper, the heterojunction SnO2/Al2O3 is tested concerning its use as a transparent insulating layer for use in FETs. The alumina layer is obtained by thermal annealing of metallic Al layer, deposited by resistive evaporation technique. Combination of undoped SnO2, deposited by sol-gel-dip-coating technique, and Al thermally annealed in O2-rich atmosphere, leads to fair insulation when the number of aluminum oxide layers is 4, with 0.3% of the current lost through the gate terminal as leakage current. This insulation is not obtained for devices with alumina layer treated for long time, under room atmosphere, due to degradation of the insulating film and interfusion with the conduction channel even using Sb-doped SnO2. The annealing of Al deposited on soda-lime glass substrate leads also to the formation of a Si layer, crystallized at Substrate/Al2O3 interface. The conclusion is that for an efficient insulation the thermal annealing must be short and then, O2-rich atmospheres are preferred.

Rare-earth doped (Eu3+ or Ce3+) thin layers of tin dioxide (SnO2) are deposited by the sol-gel-dip-coating technique, along with gallium arsenide (GaAs) films, deposited by the resistive evaporation technique. The as-built heterojunction has potential application in optoelectronic devices, because it may combine the emission from the rare-earth-doped transparent oxide, with a high mobility semiconductor. Trivalent rare-earth-doped SnO2 presents very efficient emission in a wide wavelength range, including red (in the case of Eu3+) or blue (Ce3+). The advantage of this structure is the possibility of separation of the rare-earth emission centers, from the electron scattering, leading to an indicated combination for electroluminescence. Electrical characterization of the heterojunction SnO2:Eu/GaAs shows a significant conductivity increase when compared to the conductivity of the individual films. Monochromatic light excitation shows up the role of the most external layer, which may act as a shield (top GaAs), or an ultraviolet light absorber sink (top RE-doped SnO2). The observed improvement on the electrical transport properties is probably related to the formation of short conduction channels in the semiconductors junction with two-dimensional electron gas (2DEG) behavior, which are evaluated by excitation with distinct monochromatic light sources, where the samples are deposited by varying the order of layer deposition.

Thermally stimulated depolarization current (TSDC) measurements were carried out for SiO2 in the amethyst form, aiming to investigate the relationship of observed current with relaxation phenomena related to quartz impurities. In addition to TSDC conventional dark procedure, photo-induced TSDC was also carried out, where the exciting light came from an Ar+ laser, tuned either at 488 nm or at 541 nm. X-ray diffraction and optical absorption measurements were used as complement for the interpretation of TSDC data. Optical absorption data, mainly in the range 400-700 nm, allow identifying the characteristic bands of amethyst as well as to relate them with TSDC and photo-induced TSDC data, leading to a relationship between absorption bands and light irradiation with selected wavelengths. These results allow determining how the formation of a TSDC band in the range 220-260 K, is affected by the light absorption, modifying the formation and the dipole orientation distribution in the samples. Results also help the verification of defects formed by Fe3+ or Fe4+ ions in the amethyst structure, as well as suggest that these defects, besides the participation in the amethyst structure as color centers, also play a role in the formation of TSDC bands, contributing for the observed effect of monochromatic light irradiation on these bands.

Tin dioxide (SnO2) thin films are deposited by the dip-coating technique from colloidal suspensions prepared with distinct pH through the sol-gel method. The decrease of the pH contributes to the destruction of an electrical layer adjacent to particles in solution, leading to a high degree of aggregation among these particles due to the generation of cross-linked bonds (Sn-O-Sn) between them. The aggregation affects the electrical properties of films, because the pH variation produces particle with distinct sizes in the film. Undoped samples prepared from pH 6 leads to the highest conductivity among the investigated undoped samples, in agreement with X-ray diffractograms, which indicate higher crystallinity for lower pH. Arrhenius plot evaluated from temperature dependent conductivity data leads to activation energies of the deepest level between 67 to 140 meV, for the films prepared from suspensions with pH 6 to 11. The most probable explanation for this variation in the conductivity and activation energy is related to distinct potential barriers between grains, due to distinct packing caused by cross-linked bonds formed during suspension phase. Characterization of samples lightly doped with Er3+ confirms that acid pH leads to higher conductivity, but the highest conduction takes place at even lower pH when compared to undoped thin films.

The absorption edge and the bandgap transition of sol-gel-dip-coating SnO2 thin films, deposited on quartz substrates, are evaluated from optical absorption data and temperature dependent photoconductivity spectra. Structural properties of these films help the interpretation of bandgap transition nature, since the obtained nanosized dimensions of crystallites are determinant on dominant growth direction and, thus, absorption energy. Electronic properties of the bulk and (110) and (101) surfaces are also presented, calculated by means of density functional theory applied to periodic calculations at B3LYP hybrid functional level. Experimentally obtained absorption edge is compared to the calculated energy band diagrams of bulk and (110) and (101) surfaces. The overall calculated electronic properties in conjunction with structural and electro-optical experimental data suggest that the nature of the bandgap transition is related to a combined effect of bulk and (101) surface, which presents direct bandgap transition.

Er-doped SnO2 thin films and xerogels are obtained by sol-gel technique. In order to understand the Er³+ rare-earth ion activity in SnO2 matrix, a characterization of Er incorporation is done through emission and excitation spectra of xerogels, besides thin film electrical characterization. Effects to grain dimensions are also analyzed based on X-ray diffraction data showing the particle growth with annealing temperature and inhibition of this growth by Er doping. Electrical characterization results suggest that Er³+ has an acceptor-like character in SnO2, and that codoping with Yb³+ allows an energy transfer process Yb³+ -> Er³+.

Transparent electrically conducting antimony-doped SnO2 thin films have been prepared by sol-gel dip-coating process from colloidal aqueous suspension. The effect of doping content on the structural, optical and electrical properties is analyzed. Results from infrared optical transmission and reflection have shown that the higher the Sb concentration the lower the transmission intensity and the higher the reflection signal. Absorption intensity increases as well. Results of X-ray reflectometry and electron microscopy have shown that the density of films fired at 400 °C after each dip is higher than that of multi-dipped films prepared with a single annealing. Both the electrical characteristics in the dark and the increase in conductivity as function of illumination through different filters, at 190 K, evidence that the transport properties of these films are dominated by the presence of defects, including the trapping at grain boundary due to excess of oxygen.

Resumo: As limitações postas pela deficiência visual aos seus portadores não devem constituir obstáculos intransponíveis para que aprendam conteúdos científicos de física. Procedimentos educacionais a serem adotados, devem objetivar a efetivação de um processo de continuidade com os conhecimentos que o sujeito já possui acerca de sua realidade e de ruptura entre essa realidade e o senso comum, em direção ao conhecimento científico. Discutiremos neste texto, uma análise das concepções alternativas de repouso e movimento, encontradas junto a um grupo de seis sujeitos cegos (Camargo, 2000). Pretendemos que tais concepções, bem como sua análise, forneçam subsídios à elaboração de atividades de ensino desse conceito, atividades estas que não sejam fundamentadas em referenciais observacionais visuais e que colabore com o ensino de física do aluno cego, visto que, todos são capazes e competentes para aprender desde que sejam proporcionadas condições adequadas, considerando-se características típicas do aprendiz e do conteúdo a ser ensinado.