**1. Introduction**

Porous silicon, solid silicon with voids therein, is one of the most important porous materials with a wide range of applications from batteries and fuel cells to drug delivery and diagnostics [1–3]. Although optoelectronics, especially light emission, has been porous silicon's primary area of interest for the last 25 years, the material has recently found its way to cosmetics, consumer care, nutrition, and food industry. The preparation of porous silicon is rather simple and inexpensive. It can be performed by several methods depending on the desired structure and properties. It shows highly tunable structural, mechanical, optical, electrical, thermal, emissive, and physiochemical properties. Some of its properties like luminescence and medical biodegradability are direct consequences of nanoscale porosification and are not observed in bulk silicon.
