Multiscale Mass Transport in Porous Silicon Gas Sensors

Abstract

Porous silicon (PS) is a material that has garnered considerable research attention over the past 15 years. It is formed by the dissolution of single crystalline silicon. The resulting material's morphology depends upon the silicon doping and the dissolution process. The dissolution process can be varied by changing the applied current and illumination, solvent conditions, and etching time, producing a diverse range of pore diameters (1–12) which can be made to vary from the 1 to 10 nm2–6 range (nanoporous silicon) to sizes in the 1–3 μm range (9) (microporous silicon). Interestingly, different dissolution processes lead to very different pore sizes. One can fabricate a range of hybrid structures between two limiting well-defined PS morphologies: (1) PS fabricated from aqueous electrolytes which consists of highly nanoporous, structures, and (2) PS fabricated from nonaqueous electrolytes, which is comprised of open and accessible microporous structures with deep, wide, well-ordered channels that display a crystalline Si (100) influenced pyramidal termination. The ability to control the interplay of these two regimes of porosity provides a means to exploit both the bulk and surface properties of the resulting porous membrane. In fact, the hybrid microporous/nanoporous structure etched into a silicon framework as depicted in Fig. 1, representing an extrapolation of the Probst and Kohl study (10), provides a useful platform for the construction of a conductometric PS-based sensor. All dissolution processes seem to result in mono- or bidisperse pore size distributions (13), with the typical diameters for the two sizes of pores being of the order ∼ 1 μm and <20 nm. In this chapter, the larger (∼ 1 μm pores) will be called micropores, and the smaller (<20 nm) pores will be called nanopores. This terminology is not universal! For monodisperse pore diameter porous silicon, either micro or nanopores may be present. Because the synthesis conditions that lead to a given morphology have been much perfected, reproducible PS production is now possible, a feature that is necessary for practical utility.