Impregnation of porous silicon matrices for technological applications

Porous silicon (PSi) is a very versatile material, whose main feature is the large developed surface. This inner surface, generated by the presence of pores, makes PSi particularly suitable for applications needing pores impreg- nation. In this way, many new different systems can be formed, and their characteristics can be exploited for several technological applications. PSi, beneath its complex nature, is very attractive in many different technological fields, from energy storage and production, sensors and optoelectronic devices and biomedical applications. Many research efforts have been done for the optimization of these kind of devices, including the optimization of pores impregnation mechanisms, aiming at the improvement of the performances of the final structures. In this PhD thesis I will report a study on the impregnation of PSi with different materials for a variety of technological applications. In particular, the impregnation process has been studied for organic and inorganic materials, with the aim of optimizing the process and, as a consequence, the samples properties. In the first part of this study, the chemical impregnation of the PSi matrix with melanin will be presented. After a brief introduction on the past discoveries on the hybrid junction, I will present the results in the understanding of the mechanisms governing the penetration of melanin starting monomers and their polymerization into melanin, together with the improvement we achieved in increasing the lifetime of the hybrid structure in terms of production of photogenerated current. I will also present the results obtained using a different PSi matrix, that is a porous structure obtained by using metal- assisted chemical etching (MACE). The MACE approach has been used to limit the high Si reflectivity by a suitable surface structurations. The impregnation of MACE-formed structures with melanin is then aimed at an increase of the photovoltaic properties lead by the increased light power entering the MACE-based structures. v The second part of the project regards the PSi impregnation with inorganic materials using an electrochemical approach for the impregnation of the porous matrix with erbium and nickel. Erbium has been chosen because of its demonstrated photoluminescence (PL) properties when hosted in a silicon matrix; the interest on this topic has started to decrease when it has been found that erbium clustering limits the PL emission. In this thesis I will show by a wide multidisciplinar study that pores filling, instead of the standard pores doping approach, can be a promising route to overcome the erbium clusters formation and to enhance the PL intensity. The impregnation of PSi with nickel has a different goal and is aimed at the fabrication of a multiphase material that can be used to define a valid protocol for the analysis and accurate reconstruction of nanoporous materials with atom probe tomography (APT), a technique lacking of a reliable approach for the definition of accurate reconstruction parameters. The main characteristic of this particular pair of materials is that silicon and nickel have similar evaporation fields, which is essential to perform the analysis of porous composite materials with APT. Despite the different materials and the various technological applications of the analyzed samples, the common feature of the work is the study of the impregnation processes. A good understanding of the impregnation process is the base for the optimization of the final device. The understanding and control of the parameters governing the filling of meso- and nano-pores is in fact a very complex matter that is unavoidably influenced by many environmental parameters (temperature, humidity...) that can be difficult to control properly. For these reasons, my PhD work has been aimed at the understanding of what parameters are fundamental for a successful pores impregnation and why.