2023 Publications

We report the successful preparation and characterization of active silicon oxynitride thin films with controlled europium (Eu) doping by alternated pulsed laser deposition. The successful Eu doping with a nanostructured dopant distribution, as well as the stoichiometry of the oxynitride film, have been determined by ion beam analysis (RBS). The oxidation state of the incorporated Eu ions has been determined by in-depth x-ray photoelectron spectroscopy (XPS) and it is shown that the Eu ions are in the 2+ state, in contrast to the usual results found in oxide matrices for which the 3+ oxidation state prevails. The Eu-doped films show an intense broadband emission (FWHM >210 nm) associated to the optical transition 4f65d1 → 8S7/2 of the Eu2+ ions within the amorphous matrix. As expected, the intensity of the emission band increases as the Eu concentration increases, and it is remarkable that the emission shifts towards longer wavelengths. In terms of the chromatic coordinates (CIE) this implies a color tuning from a bluish to orange that enables color tunable emission, and potential white like emission by combining layers with different Eu2+ ions concentration. Therefore, the developed oxynitride films with controlled Eu2+ ions concentration achieved by PLD are promising for the development of color -tailored LED’s.

In this work, Nb₂O₅ layers with highly improved electrical properties respect to pristine material have been produced by ultrafast-laser powder bed fusion process. The conditions required for producing uniform and compact layers of Nb₂O₅ from powder material have been studied and optimized. It has been established that ultrafast-laser irradiation, performed in air at room temperature, leads to the formation of dense Nb₂O₅ layers with the high temperature monoclinic crystal structure (H-Nb₂O₅) but oxygen deficient. The layers show a preferential crystal orientation with the short axis of the monoclinic structure lying in the structure plane. This preferential orientation can be controlled by the laser irradiation conditions. Anisotropic resistivity has been observed as a consequence of the induced microstructure, while the overall material resistivity is decreased by more than eight orders of magnitude due to the oxygen deficiency. These results indicate that it is feasible to use ultrafast laser processing to promote high-temperature non-stoichiometric niobium oxide phases in a few seconds and with low energy consumption. The highly improved electrical properties of the laser irradiated Nb₂O₅ layers are extremely interesting for different electronic and sensing applications.

Periodic arrays of sub-wavelength-scale holes in plasmonic metal films are known to provide resonant transmission peaks via the extraordinary optical transmission (EOT) effect. Active control of the spectral position of such transmission peaks can be obtained by adding a layer of phase-change material (PCM) to the EOT device. Switching the PCM layer between its amorphous and crystalline states can shift the spectral position of the resonance, enabling potential applications in the fields of active filtering and sensing (e.g., multispectral sensing), and for signal modulation. Here, the design, fabrication, and characterization of active EOT devices are targeted at various important regions of the optical spectrum.

ZnO nano- and microstructures doped with K were grown by the Vapor–Solid method. Wires and needles are the main morphology observed, although some structures in the form of ribbons and triangular plates were also obtained. Besides these, ball-shaped structures which grow around a central wire were also detected. Raman and cathodoluminescence investigations suggest that variations in morphology, crystalline quality and luminescence emissions are related to the different lattice positions that K occupies depending on its concentration in the structures. When the amount is low, K ions mainly incorporate as interstitials (Ki), whereas K occupies substitutional positions of Zn (KZn) when the amount of K is increased. Electron Backscattered Diffraction shows that ribbons and triangular plates are oriented in the (0001) direction, which indicates that the growth of this type of morphologies is related to distortions introduced by the Ki since this position favors the growth in the (0001) plane. In the case of the ball-shaped structures, the compositional analysis and Raman spectra show that they consist of K2SO4. Finally, the capability of the elongated structures to act as waveguides and optical resonators was investigated. Due to the size of the K ion, practically double that of the Zn, and the different positions it can adopt within the ZnO lattice (Ki or KZn), high distortions are introduced that compromise the resonators performance. Despite this, quality factor (Q) and fineness (F) show acceptable values (80 and 10 at 544 nm, respectively), although smaller than those reported for doping with smaller size alkali, such as Li.