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- Influence of growth temperature and carrier flux on the structure and transport properties of highly oriented CrO2 on Al2O3 (0001)Publication . Sousa, Pedro M.; Dias, Sonia A.; Conde, Olinda; Silvestre, António Jorge; Branford, William R.; Morris, Benjamin; Yates, Karen; Cohen, Lesley F.In this work we report on the structure and magnetic and electrical transport properties of CrO2 films deposited onto (0001) sapphire by atmospheric pressure (AP)CVD from a CrO3 precursor. Films are grown within a broad range of deposition temperatures, from 320 to 410 degrees C, and oxygen carrier gas flow rates of 50-500 seem, showing that it is viable to grow highly oriented a-axis CrO2 films at temperatures as low as 330 degrees C i.e., 60-70 degrees C lower than is reported in published data for the same chemical system. Depending on the experimental conditions, growth kinetic regimes dominated either by surface reaction or by mass-transport mechanisms are identified. The growth of a Cr2O3 interfacial layer as an intrinsic feature of the deposition process is studied and discussed. Films synthesized at 330 degrees C keep the same high quality magnetic and transport properties as those deposited at higher temperatures.
- Influence of Re and Ru doping on the structural, optical and photocatalytic properties of nanocrystalline TiO2Publication . Barrocas, Beatriz; Monteiro, Olinda; Nunes, Manuel R.; Silvestre, António JorgeTiO2 and TM-doped TiO2 (TM = Re, Ru) anatase crystalline nanopowders were synthesized by a simple hydrothermal method. Samples with nominal TM/Ti ratio of 0.01 were prepared for this study. Their structural, microstructural and optical properties were studied. The lattice parameters of the different prepared samples were calculated and their mean crystallite sizes determined to be in the range 15–7 nm, the lower values being obtained for the doped crystallites. The samples’ specific surface areas were determined and correlated with their mean crystallite sizes. The incorporation of the dopant elements results in an increase of the optical absorption in the visible range. The samples’ optical bandgap and Urbach energies were calculated from UV–Vis diffuse reflectance spectra. The photocatalytic behavior of the synthesized samples was investigated for the rhodamine B and phenol degradation processes, the kinetics of the different photo-oxidation reactions being also studied. The results showed that doping either with Re or Ru can lead to enhancement of the TiO2 photocatalytic efficiency and that, among the synthesized samples Ru-doped TiO2 is the most efficient photocatalyst for both the Rhodamine B and phenol photodegradation reactions.
- The spin polarization of CrO2 revisitedPublication . Yates, Karen A.; Branford, W. R.; Magnus, F.; Miyoshi, Y.; Morris, B.; Cohen, L. F.; Sousa, P. M.; Conde, O.; Silvestre, António JorgeHere, we use Andreev reflection spectroscopy to study the spin polarization of high quality CrO2 films. We study the spin polarization as a function of growth temperature, resulting in grain size and electrical resistivity. In these films low temperature growth appears to be a necessary but not sufficient condition to guarantee the observation of high spin polarization, and this is only observed in conjunction with suppressed superconducting gap values and anomalously low interface properties. We suggest that this combination of observations is a manifestation of the long range spin triplet proximity effect. (C) 2007 American Institute of Physics.
- Surprises of the transformer as a coupled oscillator systemPublication . Silva, J. P.; Silvestre, António JorgeWe study a system of two RLC oscillators coupled through a variable mutual inductance. The system is interesting because it exhibits some peculiar features of coupled oscillators: (i) there are two natural frequencies; (ii) in general, the resonant frequencies do not coincide with the natural frequencies; (iii) the resonant frequencies of both oscillators differ; (iv) for certain choices of parameters, there is only one resonant frequency, instead of the two expected.
- Coexistence of Universal and Topological Anomalous Hall Effects in Metal CrO2 Thin Films in the Dirty LimitPublication . Branford, W. R.; Yates, Karen A.; Barkhoudarov, E.; Moore, J. D.; Morrison, K.; Magnus, F.; Miyoshi, Y.; Sousa, P. M.; Conde, O.; Silvestre, António Jorge; Cohen, L. F.The scaling exponent of 1.6 between anomalous Hall and longitudinal conductivity, characteristic of the universal Hall mechanism in dirty-metal ferromagnets, emerges from a series of CrO2 films as we systematically increase structural disorder. Magnetic disorder in CrO2 increases with temperature and this drives a separate topological Hall mechanism. We find that these terms are controlled discretely by structural and magnetic defect populations, and their coexistence leads to apparent divergence from exponent 1.6, suggesting that the universal term is more prevalent than previously realized.
- The effect of ionic Co presence on the structural, optical and photocatalytic properties of modified cobalt–titanate nanotubesPublication . Barrocas, B.; Silvestre, António Jorge; Rolo, A. G.; Monteiro, O. C.With the aim of producing materials with enhanced optical and photocatalytic properties, titanate nanotubes (TNTs) modified by cobalt doping (Co-TNT) and by Na+ -> Co ion-exchange (TNT/Co) were successfully prepared by a hydrothermal method. The influence of the doping level and of the cobalt position in the TNT crystalline structure was studied. Although no perceptible influence of the cobalt ion position on the morphology of the prepared titanate nanotubes was observed, the optical behaviour of the cobalt modified samples is clearly dependent on the cobalt ions either substituting the Ti4+ ions in the TiO6 octahedra building blocks of the TNT structure (doped samples) or replacing the Na+ ions between the TiO6 interlayers (ion-exchange samples). The catalytic ability of these materials on pollutant photodegradation was investigated. First, the evaluation of hydroxyl radical formation using the terephthalic acid as a probe was performed. Afterwards, phenol, naphthol yellow S and brilliant green were used as model pollutants. Anticipating real world situations, photocatalytic experiments were performed using solutions combining these pollutants. The results show that the Co modified TNT materials (Co-TNT and TNT/Co) are good catalysts, the photocatalytic performance being dependent on the Co/Ti ratio and on the structural metal location. The Co(1%)-TNT doped sample was the best photocatalyst for all the degradation processes studied.
- Structural and Microanalytical Studies of CrO2 Thin Films on c-Sapphire by High Resolution Electron Microscopy MethodsPublication . Ortiz, M. I.; Sousa, P. M.; Ballesteros, C.; Jorge Silvestre, António; Cohen, L. F.; Conde, O.Chromium dioxide (CrO2) has been extensively used in the magnetic recording industry. However, it is its ferromagnetic half-metallic nature that has more recently attracted much attention, primarily for the development of spintronic devices. CrO2 is the only stoichiometric binary oxide theoretically predicted to be fully spin polarized at the Fermi level. It presents a Curie temperature of ∼ 396 K, i.e. well above room temperature, and a magnetic moment of 2 mB per formula unit. However an antiferromagnetic native insulating layer of Cr2O3 is always present on the CrO2 surface which enhances the CrO2 magnetoresistance and might be used as a barrier in magnetic tunnel junctions.
- Synthesis and properties of Co-doped titanate nanotubes and their optical sensitization with methylene bluePublication . Ferreira, V. C.; Nunes, M. R.; Silvestre, António Jorge; Monteiro, O. C.Here we report on a novel chemical route to synthesize homogenous cobalt-doped titanate nanotubes (CoTNT), using an amorphous Co-doped precursor. The influence of the synthesis temperature, autoclave dwell time and metal doping on the structural and microstructural as well as on the optical properties of the synthesized titanate nanotubes is studied and discussed. The optical band gaps of the CoTNT samples are red shifted in comparison with the values determined for the undoped samples, such red shifts bringing the absorption edge of the CoTNT samples into the visible region. CoTNT materials also demonstrate particular high adsorption ability for methylene blue, the amount of the adsorbed dye being higher than the one predictable for a monolayer formation. This suggests the possibility of intercalation of the dye molecule between the TiO6 layers of the TNT structure. It is also shown that the methylene blue sensitized Co-doped nanostructures are highly stable under UV radiation and present a strong and broad absorption in the visible region. (C) 2013 Elsevier B.V. All rights reserved.
- Física: uma introduçãoPublication . Silvestre, António Jorge; Teixeira, PauloO presente livro destina-se ao ensino de uma primeira disciplina de Física de cursos de ciências ou de engenharia, compreendendo o cânone da Física Clássica - Mecânica, Electromagnetismo e Termodinâmica — que um futuro cientista ou engenheiro deve dominar, e que poderá, eventualmente, aprofundar em disciplinas mais avançadas. Tem a sua génese na unidade curricular de “Física” que temos vindo a leccionar há vários anos no segundo semestre do primeiro ano da Licenciatura em Matemática Aplicada à Tecnologia e à Empresa do Instituto Superior de Engenharia de Lisboa - ISEL. É, portanto, um livro marcadamente pedagógico, em que procurámos, sem prejuízo do rigor, que a exposição teórica fosse sempre acompanhada de exemplos que permitam uma mais fácil assimilação das diferentes matérias.
- Magnetic properties of Co-doped TiO(2) anatase nanopowdersPublication . Pereira, L. C. J.; Nunes, M. R.; Monteiro, O. C.; Silvestre, António JorgeThis letter reports on the magnetic properties of Ti(1-x)Co(x)O(2) anatase phase nanopowders with different Co contents. It is shown that oxygen vacancies play an important role in promoting long-range ferromagnetic order in the material studied in addition to the transition-metal doping. Furthermore, the results allow ruling out the premise of a strict connection between Co clustering and the ferromagnetism observed in the Co:TiO(2) anatase system.