Showing papers by "Joel Nuno Pinto Borges published in 2015"

Evolution of the functional properties of titanium–silver thin films for biomedical applications: Influence of in-vacuum annealing

Abstract: In this work, the thermal stability of TiAgx thin films, deposited by magnetron sputtering, was evaluated, envisaging their application in biomedical devices, namely as electrodes for biosignal acquisition. Based on the composition and microstructural characterization, a set of four representative TiAgx thin films was selected in order to infer whether they are thermally stable in terms of functional properties. In order to achieve this purpose, the structural and morphological evolution of the films with annealing temperature was correlated with their electrical, mechanical and thermal properties. Two distinct zones were identified and two samples from each zone were extensively analysed. In the first zone (zone I), Ti was the main component (Ti-rich zone) while in the second, zone II, the Ag content was more significant. The selected samples were annealed in vacuum at four different temperatures up to 500 °C. For the samples produced within zone I, small microstructural changes were observed due to the recrystallization of the Ti structure and grain size increment. Also, no significant changes were observed with annealing temperature regarding the films' functional properties, being thermally stable up to 500 °C. For higher Ag contents (zone II) the energy supplied by thermal treatments was sufficient to activate the crystallization of Ti–Ag intermetallic phases. A strong increase of the grain size of these phases was also reported. The structural and morphological organization proved to be determinant for the physical responses of the TiAgx system. The hardness and Young's modulus were significantly improved with the formation of the intermetallic phases. The silver addition and annealing treatments also played an important role in the electrical conductivity of the films, which was once again improved by the formation of Ti–Ag phases. The thermal diffusivity of the films was practically unchanged with the heat-treatment. This set of results shows that this intermetallic-like thin film system has good thermal stability up to high temperatures (as high as 500 °C), which in case of the highest Ag content zone is particularly evident for electrical and mechanical properties, showing an important improvement. Hardness increases about three times, while resistivity values become half of those from the lowest Ag content zone. These sets of characteristics are consistent with the targeted applications, namely in terms of biomedical sensing devices.

Effect of surface plasmon resonance in TiO2/Au thin films on the fluorescence of self-assembled CdTe QDs structure

Abstract: The exceptional properties of localised surface plasmons (LSPs), such as local field enhancement and confinement effects, resonant behavior, make them ideal candidates to control the emission of luminescent nanoparticles. In the present work, we investigated the LSP effect on the steady-state and time-resolved emission properties of quantum dots (QDs) by organizing the dots into self-assembled dendrite structures deposited on plasmonic nanostructures. Self-assembled structures consisting of water-soluble CdTe mono-size QDs, were developed on the surface of co-sputtered TiO2 thin films doped with Au nanoparticles (NPs) annealed at different temperatures. Their steady-state fluorescence properties were probed by scanning the spatially resolved emission spectra and the energy transfer processes were investigated by the fluorescence lifetime imaging (FLIM) microscopy. Our results indicate that a resonant coupling between excitons confined in QDs and LSPs in Au NPs located beneath the self-assembled structure indeed takes place and results in (i) a shift of the ground state luminescence towards higher energies and onset of emission from excited states in QDs, and (ii) a decrease of the ground state exciton lifetime (fluorescence quenching).

Ag y :TiN x thin films for dry biopotential electrodes: the effect of composition and structural changes on the electrical and mechanical behaviours

Abstract: In the present work, Ag y :TiN x thin films, obtained by reactive DC magnetron sputtering, with decreasing [N]/[Ti] atomic ratios (from 1 to 0.1) and a fixed amount of Ag pellets placed in the erosion zone of a pure Ti target, were studied envisaging their application as biopotential electrodes. The strongly under-stoichiometric samples, [N]/[Ti] = 0.1 and 10 at.% Ag; [N]/[Ti] = 0.2 and 8 at.% Ag, were found to be composed of a N-doped hcp-Ti structure, with possible formation of TiAg or Ti2Ag intermetallics. These samples exhibit high electrical resistivity values and low hardness and reduced modulus. In the set of samples indexed to a transition zone, [N]/[Ti] = 0.3 and 15 at.% Ag; [N]/[Ti] = 0.7 and 32 at.% Ag, a hcp-Ti to fcc-TiN phase transformation took place, giving rise to a disaggregated N-deficient TiN matrix. It correlates with the high resistivity values as well as the higher hardness and reduced modulus values that were obtained. The last identified zone comprised the stoichiometric Ag:TiN x sample—[N]/[Ti] = 1 and 20 at.% Ag. Extensive metallic Ag segregation was detected, contributing to a significant decrease of the resistivity and hardness values.