Color of porous silicon

Abstract

The visual color of a material is often not important for many applications but can be crucial for those that involve consumer acceptance and branded products. Solid silicon is gray, but porous silicon can have varied colors depending on its physical form and pore contents. Silicon chip-based layers can exhibit vivid colors, tunable across the visible spectrum through their lowered refractive index and optical interference with the underlying bulk silicon. Highly columnar morphologies, referred to as "black silicon," include highly porous forms. Even white silicon is possible via photonic crystals. Polydisperse mesoporous silicon microparticle powders are typically dark brown through light tan, depending on bandgap widening, particle size, and the level of oxidation, which is useful for matching skin tone in cosmetic products, but disadvantageous with various foodstuffs, beverages, and oral care products. The color of such powders can be better tuned chemically by the impregnation of specific food nutrients that themselves have vivid colors. Some such natural pigments can themselves benefit with improved fading resistance as a result of UV protection via oxidized porous silicon impregnation.