A. Mosquera

Exciton and core-level electron confinement effects in transparent ZnO thin films

The excitonic light emission of ZnO films have been investigated by means of photoluminescence measurements in ultraviolet-visible region. Exciton confinement effects have been observed in thin ZnO coatings with thickness below 20 nm. This is enhanced by a rise of the intensity and a blue shift of the photoluminescence peak after extraction of the adsorbed species upon annealing in air. It is found experimentally that the free exciton energy (determined by the photoluminescence peak) is inversely proportional to the square of the thickness while core-level binding energy is inversely proportional to the thickness. These findings correlate very well with the theory of kinetic and potential confinements.

Design and rapid prototyping of DLC coated fractal surfaces for tissue engineering applications

Several medical devices (both implantable and for in vitro diagnosis) benefit greatly from having microtextured surfaces that help to improve and promote phenomena such as osteointegracion and cell / tissue growth on the surface of a device. Normally, the use of abrasives or chemical attacks are employed for obtaining such surface microtextures, however, it is sometimes difficult to precisely control the final surface characteristics (porosity, roughness, among others) and consequently the related biological aspects. In this work, we propose an alternative process based on the use of fractal surface models for designing special surfaces, which helps controlling the desired contact properties (from the design stage) in multiple applications within biomedical engineering, especially regarding tissue engineering tasks. Manufacturing can be directly accomplished by means of rapid prototyping technologies. This method supposes a focus change from a conventional top-down to a more versatile bottom-up approach. Finally, in order to improve the possible biological response, the surfaces of the designed devices were coated with hydrogen-free amorphous carbon (a-C) thin films, known to be highly biocompatible materials. The films were deposited at room temperature using the vacuum filter cathodic arc technique. Our first prototypes have helped verify the viability of the approach and to validate the design, manufacturing and coating processes.

Effect of silver on the phase transition and wettability of titanium oxide films

The effect of silver on the phase transition and microstructure of titanium oxide films grown by pulsed cathodic arc had been investigated by XRD, SEM and Raman spectroscopy. Following successive thermal annealing up to 1000 °C, microstructural analysis of annealed Ag-TiO2 films reveals that the incorporation of Ag nanoparticles strongly affects the transition temperature from the initial metastable amorphous phase to anatase and stable rutile phase. An increase of silver content into TiO2 matrix inhibits the amorphous to anatase phase transition, raising its temperature boundary and, simultaneously reduces the transition temperature to promote rutile structure at lower value of 600 °C. The results are interpreted in terms of the steric effects produced by agglomeration of Ag atoms into larger clusters following annealing which hinders diffusion of Ti and O ions for anatase formation and constrains the volume available for the anatase lattice, thus disrupting its structure to form rutile phase. The effect of silver on the optical and wetting properties of TiO2 was evaluated to demonstrate its improved photocatalytic performance.

XANES observations of the inhibition and promotion of anatase and rutile phases in silver containing films

X-ray absorption near-edge structure (XANES) spectroscopy has been applied to characterize the phase evolution of Ag–TiO2 films subjected to annealing treatments up to 1000 °C. Silver has been theoretically predicted as a rutile promoter in anatase powders lowering the transition temperature, however, no systematic studies on films have been reported yet. In this work, the Ti L2,3- and O K-edge in XANES spectra were used as fingerprints of the phase structure of the films. The results show that silver atoms strongly affect the transition of the initially amorphous films to the anatase structure, delaying the crystallization temperature up to 400 °C and, at the same time, promoting the subsequent phase transformation from anatase to rutile at temperatures as low as 600 °C. These results have been confirmed by X-Ray Diffraction (XRD) analysis as well as by High Resolution TEM images (HRTEM) of films.