Grupo de Procesado por Láser del Instituto de Óptica “Daza de Valdés”

Author: Rocío Ariza García
Thesis submitted in: April 2022
Under the supervision of: Prof. Paloma Fernández Sánchez (UCM), Dr. Ana Urbieta Quiroga (UCM) and Prof. Javier Solís Céspedes (IO-CSIC)
Universidad Complutense de Madrid

Conformación de haces láser de pulsos ultracortos para la optimización de la escritura de guías de onda en dieléctricos
This research work is focused on the synthesis of micro- and nanostructures of ZnO doped with different metallic elements with the aim of assessing their properties and potential applications in optoelectronic and photocatalytic devices.

To this end, the possibility of spatially controlling their selective growth on various types of substrates previously structured with ultrashort laser pulses has also been analysed. ZnO is a broadband semiconductor belonging to the group of transparent conductive oxides (TCO ́s) whose properties are of special interest for different applications. It is known that the use of suitable dopants makes it possible to very significantly modify and enhance some of its properties, especially electrical, optical and luminescent ones.

This is the underlying reason for analysing how doping with different alkali (Li, K) and transition (Zr) metals influences the properties of micro- and nanostructures synthesized by the vapor-solid (VS) method. This growth technique allows to obtain structures with very good crystalline quality and does not use catalysts, which avoids the incorporation of unwanted impurities in the generated structures. Different tests have been carried out aimed at optimizing the growth process for each dopant, varying both the conditions of the thermal treatments, through the use of different precursors, and the use of different levels of doping. These tests have been fed back through the different characterization techniques used, of a morphological, compositional, structural and optical nature. In general, once the synthesis conditions have been optimized, a high density of elongated micro- and nanostructures with needle- and wire-like morphologies has been obtained.

It has been analysed how the morphologies vary as a consequence of the perturbations induced by the dopants when they are incorporated into the material lattice in different positions, so that structures in the form of ribbons, plates, triangles, or balls can be found. In the case of zirconium, in addition, a nanocomposite formed by separate phases (ZnO/ZrO2) has been obtained. Doping with alkaline elements (Li and K) gives rise to a very similar behaviour. In both cases, the position of the dopant in the ZnO crystal lattice correlates with the amount of dopant introduced. For low amounts, the dopant is interstitially incorporated and as doping increases, it moves to zinc substitutional positions. This behaviour is reflected in the structure of defects observed, as well as in the distortions of the crystalline structure that are observed in the samples and that are associated with both types of dopant positions. The appearance of compounds related to the precursors used and whose presence is manifested in the formation of spherical structures of Li2SO4 or K2SO4 has also been analysed. These phases are observed only in very local regions under very specific conditions. It has been verified that they do not affect the properties of the majority structures of ZnO doped with Li or K that are generated in the process. In fact, one of the most remarkable characteristics of the alkali-doped material is that it shows very good optical properties, which has allowed a detailed analysis of lithium and potassium doped micro- and nanostructures for potential photonic applications. The structures produced show good performance as light guides. Additionally, the morphologies obtained, together with the high refractive index of ZnO, facilitate the formation of optical resonators with Whispering Gallery modes that, in the case of lithium, show an excellent quality factor. Doping with zirconium gives rise to very different results compared to alkali metals due to the low solubility of Zr in ZnO, despite Zr having an ionic radius similar to that of Zn. This gives rise to the formation of a nanocomposite with segregated phases of ZnO and ZrO2.

The observed luminescent emission points, in turn, to the existence of a small cross – doping between both oxides (ZnO:Zr and ZrO2:Zn), which improves the stability of the structures and would facilitate its integration in devices where the working conditions require a high chemical and thermal stability. Along the previous results, the possibility of growing ordered micro- and nanostructures on the surface, without using catalysts, has been analysed through the use of substrates pre-structured with a pulsed laser source. This procedure has been carried out on a semiconductor (silicon) and on a metal (Zn) substrate, both processed with fs laser pulses in order to generate surface periodic structures (LIPSS). Unlike the case of Si, the formation of LIPSS in metal Zn has not been analysed so thoroughly and so a detailed analysis of the irradiation conditions and derived morphologies has been carried out. In both cases, the selective growth of the material on the laser pre-structured areas has been achieved, and the selective growth mechanisms that take place for each type of substrate have been analysed in detail. In the first case (Si), the local amorphization of the material in the LIPSS provides a local region where the nucleation energy decreases, favouring the growth of acicular ZnO structures. In the case of Zn, the oxidation of the surface upon laser irradiation and the formation of oxygen interstitials (in a process far from equilibrium) play a fundamental role in the spatial selectivity of the subsequent growth of ZnO nanowires by thermal treatment.

These nanowires allow to further increase the surface to volume ratio of the structures produced by the laser, so that they are promising for the development of reusable photocatalysts based on ZnO nanostructures. The preliminary studies carried out show a very promising photo-degradation capacity in the structures produced by this mixed technique of laser structuring and thermal growth at low temperature.