Showing papers by "Anabela G. Rolo published in 2013"

Ferroelectric characterization of aligned barium titanate nanofibres

Abstract: We report the synthesis, structural and ferroelectric characterization of continuous well-aligned nanofibres of barium titanate produced by the electrospinning technique. The fibres with average diameter of 150–400 nm consist of connected nanoparticles of BaTiO3 stacked together to form the shape of a long filament. The tetragonal phase in the obtained nanofibres was revealed by the x-ray diffraction and Raman spectroscopy and has been also confirmed by the second harmonic generation (SHG) and piezoresponse force microscopy (PFM). The temperature dependence of the SHG in the vicinity of the paraelectric–ferroelectric phase transition suggests that barium titanate nanofibres are indeed ferroelectric with an apparent glass-like state caused by metastable polar nanoregions. The existence of domain structure and local switching studied by PFM present clear evidence of the polar phase at room temperature.

Copper(II)–imida‐salen Complexes Encapsulated into NaY Zeolite for Oxidations Reactions

Abstract: I. K. B. thanks the Fundacao para Ciencia e Tecnologia (FCT) for a contract under the program Ciencia 2007. The authors are grateful to Dr. A. S. Azevedo for collecting the powder diffraction data. The authors thank the FCT and the European Fund for Regional Development (FEDER, COMPETE-QREN-EU) for financial support to the Research Centre (CQ/UM PEst-C/QUI/UI0686/2011, FCOMP-01-0124-FEDER-022716) and to the financial support from the NMR Portuguese Network (PT NMR, Bruker Avance III 400, Univ. of Minho).

Charge trapping properties and retention time in amorphous SiGe/SiO 2 nanolayers

Abstract: In this paper, we report on the electrical properties of metal–oxide–semiconductor (MOS) capacitors containing a well-confined 8 nm-thick SiGe amorphous layer (a-SiGe) embedded in a SiO2 matrix grown by RF magnetron sputtering at a low temperature (350 ◦ C). Capacitance–voltage measurements show that the introduction of the SiGe layer leads to a significant enhancement of the charge trapping capabilities, with the memory effect and charge retention time larger for hole carriers. The presented results demonstrate that amorphous floating-gate SiGe layers embedded in SiO2 may constitute a suitable alternative for memory applications. (Some figures may appear in colour only in the online journal)

A shadowed off-axis production of Ge nanoparticles in Ar gas atmosphere by pulsed laser deposition

Abstract: In this work, we report on the production of Ge nanoparticles (NPs) in an inert Ar gas atmosphere by pulsed laser deposition (PLD) at room temperature (RT). The direct deposition of energetic particles/droplets resulting from the ablation process of the target material has been avoided by using an original and customized off-axis shadow mask (shadowed off-axis) deposition set-up where the NPs deposition on the substrate takes place by means of scattering between the NPs formed in the vapor phase and the background Ar atoms. It is found that the Ar gas pressure parameter has a relevant role in the crystallization process, with better crystallinity obtained as the background Ar pressure is raised for the given experimental conditions.

Charge storage behavior of nanostructures based on SiGe nanocrystals embedded in Al2O3 matrix

Abstract: The charge storage behavior of nanostructures based on Si1−x Ge x (0 ≤ x ≤ 1) nanocrystals (NCs) in an Al2O3 matrix was investigated. The structures have been grown by RF magnetron sputtering and subsequently annealed at temperatures ranging from 700 °C to 1000 °C for 30 min in nitrogen ambient. The stoichiometry of the SiGe NCs and the alumina crystalline structure were found to be significantly dependent on the RF power and the annealing temperature. The sizes of the SiGe NCs and their distribution were investigated by grazing incidence small angle X-ray scattering (GISAXS). The capacitance-voltage (C-V) and conductance-voltage (G-V) measurements were performed to investigate the charge trapping characteristics of the memory structures. The C-V hysteresis width depends on variations in the crystalline structure resulting from different annealing temperatures. It is also shown that charge injection is governed by the Fowler-Nordheim tunnel mechanism for higher electric fields.

Influence of RF-sputtering power on formation of vertically stacked Si1−xGex nanocrystals between ultra-thin amorphous Al2O3 layers: structural and photoluminescence properties

Abstract: In this work, we investigate the structural and photoluminescence (PL) properties of (SiGe+Al2O3)/Al2O3 multi-layer films with layer thicknesses in the range of a few nanometres. The films were prepared by magnetron sputtering deposition at room temperature followed by an annealing process to promote the formation of small SiGe nanocrystals (NCs) (∼3 to 5 nm) embedded between ultra-thin (∼6 nm thickness) Al2O3 layers. Our results show that the structural and compositional properties of the films can be tuned by changing the RF-power. It is found that nearly spherical and well confined isolated SiGe NCs (∼5 nm) are obtained for an RF-power value of 80 W. The PL properties of the films were studied and optical emission in the blue visible wavelength region was observed.