UC San Diego

A thesis describing the construction and evaluation of optical sensors based on porous silicon rugate photonic crystals for the detection of explosive gases such as C₂ hydrocarbons and hydrogen. Useful properties of porous silicon for gas sensor applications include tunable pore size, large surface area, convenient surface chemistries, optical transduction of the sensing signal, and the potential for miniaturization. Chapter One introduces the fundamentals of porous silicon, including fabrication, sensing mechanisms, and sensor challenges. Additionally, the advantages and limitations of current point sensor technologies are briefly discussed. Chapter Two focuses on the evaluation of a carbon/porous silicon composite film for the optical detection of C₂ hydrocarbons. Porous silicon rugate photonic crystals are used as templates for the fabrication of glassy carbon nanofibers. The resulting carbon/porous silicon composites combine the strong adsorption characteristics of carbon and the optical signal transduction of photonic crystals to increase sensitivity. Chapter Three presents a new method for the fabrication of palladium/porous silicon composite films for the optical detection of hydrogen gas. The two-step synthesis involves seeding of palladium by thermal reduction of palladium ions, followed by growth of palladium nanoparticles in an electroless plating solution. Compared to the previous method of immersion plating, this new method is able to deposit palladium nanoparticles deeper into the porous matrix, thereby increasing the sensitivity