A.D. Rodrigues

Structural characterization of indium oxide nanostructures: a Raman analysis

In this work we report on structural and Raman spectroscopy measurements of pure and Sn-doped In2O3 nanowires. Both samples were found to be cubic and high quality single crystals. Raman analysis was performed to obtain the phonon modes of the nanowires and to confirm the compositional and structural information given by structural characterization. Cubic-like phonon modes were detected in both samples and their distinct phase was evidenced by the presence of tin doping. As a consequence, disorder effects were detected evidenced by the break of the Raman selection rules.

Aspects of cellular planning for emergency and safety services in mobile WiMax networks

This work addresses cellular planning aspects for a WiMax (worldwide interoperability for microwave access) point-to-multipoint wireless network to be developed in the region of Beira Interior. It is concentrated in more detail in the district of Covilha, and, particularly, in the urban area of the city, in order to guarantee communications from the Health Science Faculty of University of Beira Interior (HSF/UBI)/ Hospital Pero da Covilha to all central urban area of the city. By having aspects related with the carrier-to-noise and carrier-to-interference ratios into account, one concluded that it is necessary to consider a cellular structure with sectorisation, and cell coverage distances around 3 km. Because this zone is very hilly, the optimization of the choice of the location for base stations is essential; the potentialities of the GIS (geographic information systems) tools were explored to optimise the existence of line of sight coverage in coverage. Future research directions regarding architectures for interoperability and mobile IP, security, and the impact of multimedia applications are also addressed.

Bioprospecting of yeasts for amylase production in solid state fermentation and evaluation of the catalytic properties of enzymatic extracts

Profiling microorganisms with potential for amylase production in low cost culture media has been widely recognized due to its broad applicability. The aim of this study was to select yeast strains with potential to produce amylolytic enzymes by solid state fermentation. Fifty-four (54) strains were assessed and three exhibited ability to produce amylases: Candida parapsilosis with 14.68 U/mL (146.8 U/g substrate); Rhodotorula mucilaginosa with 25.0 U/mL (250 U/g substrate), and Candida glabrata with 25.39 U/mL (253.9 U/g substrate), in solid state fermentation, for 120 h at 28°C, using wheat bran with 70% moisture. The enzymes exhibited maximum activity at a pH of 7.0 and at 60°C. Amylases demonstrated satisfactory structural stability, maintaining their catalytic activity after 1 h at 50°C. All enzymes were ethanol tolerant and retained more than 70% of their original activities in 15% ethanol solution. Corn starch was efficiently hydrolyzed by enzymes and the extracts produced by C. parapsilosis and C. glabrata exhibited dextrinizing activity, while those produced by R. mucilaginosa exhibited saccharifying activity. Key words : Candida parapsilosis, Candida glabrata, Rhodotorula mucilaginosa , dextrinizing and saccharifying activity

Espectroscopias de infravermelho, Raman e de fotoluminescência: potencialidades e complementaridades

It is quite common that the student engaged in the analysis of properties of materials using spectroscopic techniques based on the effects of infrared absorption, luminescence and Raman scattering, faces difficulties such as the interpretation of the information that can be accessed in each case, the choice of the best technique in order to study a specific phenomenon and, especially, the cross-checking of the information obtained in each one of the three techniques. Trying to make easier the apprenticeship of the rational use of these techniques, we summarize in this paper the fundamental physical principles related to each one of them, and how these principles determine their employment - individually or in a complementary way - to explore their potentiality towards better characterizing the desired properties.

Planning of an IEEE 802.16e network for emergency and safety services

An overlaid Wimax cellular structure is proposed for emergency and safety services, with micro-cells covering urban hotspots over a macro-cellular structure. In the region of Beira Interior the process of cellular planning has to account for carrier-to-noise-interference ratio. Sectorisation is used in order to reduce reuse patterns and increase system capacity, while cell overlay is proposed to overcome coverage problems in large towns and cities. Geographic information systems tools are considered for line-of-sight discovery, and ~75% of LoS coverage is guaranteed in the city of Covilha. The point-to-point component is also being incorporated in the planning tool. By comparing the 3.5 GHz and the 5.8 GHz bands one concludes that it is possible to use lower values of the antenna gains in the first case.

CARACTERIZAÇÃO DE NANOFIOS DE GERMÂNIO SINTETIZADOS UTILIZANDO COBRE COMO CATALISADOR

This paper presents reliable synthesis of germanium nanowires by the vapor-liquid-solid method using copper as an alternative catalyst to gold, the most commonly used metal. The morphological study showed long range single-crystalline germanium nanowires with diamond structure and diameters ranging from 20 nm to 80 nm and lengths in tenths of a micrometer, which was much larger than the cross-section. In an investigation of electronic properties, devices built from a thin film of nanowires were studied and the semiconductor behavior of the samples confirmed, with variable range hopping identified as the main transport mechanism. Additionally, metal-nanowire contact behavior - ohmic or with Schottky barrier - was found to depend on the metal used in the device. Therefore, the possibility of controlling current-voltage behavior combined with the good crystalline quality of the nanowires renders these nanostructures an attractive option for future electronic and optoelectronic applications.

Optical and spin properties of localized and free excitons in GaBixAs1-x/GaAs multiple quantum wells

Raman spectroscopy and magneto-photoluminescence measurements under high magnetic fields were used to investigate the optical and spin properties of GaBiAs/GaAs multiple quantum wells (MQWs). An anomalous negative diamagnetic energy shift was observed at higher temperatures and higher laser intensities, which was associated to a sign inversion of hole effective mass in these structures. In addition, an enhancement of the polarization degree with decreasing of laser intensity was observed (experimental condition where the emission is dominated by localized excitons). This effect was explained by changes of spin relaxation and exciton recombination times due to exciton localization by disorder.

Growth and characterization of Mn-doped ZnO thin films

This work shows an investigation of Zn1-xMnxO thin films at concentrations up to x=0.10 as well two reference samples: Mn3O4 and ZnO. The samples were grown by Spray Pyrolysis on top of glass substrates using as precursors zinc acetate dihydrate and manganese acetate tetrahydrate in aqueous solution. The samples were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Optical Absorption measurements. Zn1-xMnxO and Mn3O4 were identified as wurtzite and hausmannites phases respectively. No significant change in the optical gap was observed for concentrations up to 7% Mn. After an annealing at 500°C during one hour, there was a phase transition from Mn3O4 to -Mn2O3. Our results show that the bandgap discrepancies between as-grown and annealed samples are due to presence of both Mn2+ and Mn3+ in as-grown sample as compared to only Mn3+ after annealing.

Optical and transport properties correlation driven by amorphous/crystalline disorder in InP nanowires

Indium phosphide nanowires with a single crystalline zinc-blend core and polycrystalline/amorphous shell were grown from a reliable route without the use of hazardous precursors. The nanowires are composed by a crystalline core covered by a polycrystalline shell, presenting typical lengths larger than 10 μm and diameters of 80-90 nm. Raman spectra taken from as-grown nanowires exhibited asymmetric line shapes with broadening towards higher wave numbers which can be attributed to phonon localization effects. It was found that optical phonons in the nanowires are localized in regions with average size of 3 nm, which seems to have the same order of magnitude of grain sizes in the polycrystalline shell. Regardless of the fact that the nanowires exhibit a crystalline core, any considerable degree of disorder can lead to a localized behaviour of carriers. In consequence, the variable range hopping was observed as the main transport instead of the usual thermal excitation mechanisms. Furthermore the hopping length was ten times smaller than nanowire cross-sections, confirming that the nanostructures do behave as a 3D system. Accordingly, the V-shape observed in PL spectra clearly demonstrates a very strong influence of the potential fluctuations on the exciton optical recombination. Such fluctuations can still be observed at low temperature regime, confirming that the amorphous/polycrystalline shell of the nanowires affects the exciton recombination in every laser power regime tested.

Magnetic and structural properties of Fe-implanted cubic GaN

In this article, we report on structural and magnetic properties of cubic GaN epitaxial layers implanted with Fe ions and subsequently subjected to thermal annealing. The epitaxial quality of the layers was studied by X-ray diffraction rocking curves (ω-scans) and Raman spectroscopy. The results show that the implantation damages the crystal structure producing an expansion of the lattice parameter in the implanted region. These damages are partially removed by the thermal treatment. Room temperature ferromagnetism is observed for the sample implanted with a dose of 1.2 × 1016 cm−2, while samples implanted with 2.4 × 1016 cm−2 show a coexistence of ferromagnetism and paramagnetism due to disperse Fe3+. Thermal annealing changes these magnetic properties. For the low dose sample, the ferromagnetism is converted into paramagnetism while for the high dose we observed an enhancement of the ferromagnetic contribution characterized by a superparamagnetism behavior attributed to Fe-based particles.