Synthesis, Processing and Morphological Properties of Highly Porous Supercritically Dried Mesoporous Silicon Nanostructures for Drug Delivery Applications

Over the last few decades, the interest in porous semiconductors increased dramatically. Porous silicon was discovered accidentally in the 1950s by Uhlir at the Bell Laboratories. Since then, pSi has been of focus for multiple applications, such as drug delivery, photonics, gas sensing, cosmetics industry, solar cells etc. This thesis focuses on the fabrication, processing and characterisation of highly porous silicon nanostructures with a possible application in drug delivery. The thesis is ordered in such a manner that after introducing the motivation and all the background topics (chapter 1) , the reader is taken through the common sequence of material fabrication (chapter 2), processing (chapters 2& 3) and then characterization (chapter 4). Chapter 1 gives an overview and background information about the topic. It is divided into separate sections named Fabrication, characterisation, properties of porous silicon, and applications. Chapter 2 provides the results of Fabrication, processing and characterisation of the highly porous silicon membranes. This chapter is based on the author's three publications. The main achievements were the highest reported pore volume of free-standing porous silicon membranes reported up-to-date via optimised electrochemical anodisation of silicon wafers and supercritical drying; the first direct comparison of two processing techniques that generate particles (milling and sonication); and the first demonstration of how sedimentation can be used to tune microparticle size distribution. Chapter 3 focuses on the importance of drying and supercritical fluid drying as the main processing technique utilized in this doctoral thesis. This specific type of drying avoids the capillary forces that can cause the pore collapse. Instead, the material is brought to a critical point (pressure and temperate) avoiding the evaporation of ethanol and further flushes using supercritical CO2. Chapter 4 describes in more detail the characterisation techniques used to investigate the fabricated structures. Fabricated porous silicon structures were characterised using several techniques, such as gas adsorption analysis, scanning electron microscope, particle sizing employing laser diffraction, gravimetric method. Chapter 5 summarizes all the previous chapters as well as providing an outlook of possible future work. The proposed work includes electrochemical anodisation and supercritical drying of n-wafers as well as the pore impregnation using polymers in order to preserve the pore volume/porosity and surface area during the processing techniques. Chapter 6 consists of Appendices regards standard operating procedures (SOPs). Guidelines for the reader: Please note that some details on characterisation techniques and experimental methods in Chapter 4 would be helpful to read before or whilst reading Chapter 2 in order to provide a complete understanding of relevant topics.