Porous silicon and polymer materials for optical chemical sensors

Abstract

The optical characteristics of porous materials depend on their structural properties (porosity, pore size, and pore distribution). Specifically in materials with air-filled pores, the effective refractive index is a weighted average of the refractive indices of the relevant material and air, which is thus directly related to the material’s porosity. Mesoporous materials, such as porous silicon and porous polymers, have therefore been exploited to create photonic devices, from simple interference filters to exotic photonic-bandgap (PBG) structures.1, 2 In addition, the high internal surface area of porous materials provides an excellent host medium to immobilize analyte-specific recognition elements through sequestration.3 Interactions of the analyte molecules with the sequestered recognition elements often alter the effective index or other optical properties. These optical variations can be employed as a simple and straightforward transduction approach in optical sensing. Typically, porous silicon is produced by electrochemical etching in a hydrogen fluoride-based liquid etchant. A major drawback of this approach is the drying process, because the pressure gradient across the gas/liquid interface during evaporation produces high capillary stress in the porous structure that can lead to systematic cracking and destruction of the pores. Supercritical drying is often employed to avoid this problem. We recently developed a dry-etching method that overcomes the significant challenges of producing porous silicon by wet-etching chemistry. Our method relies on gaseous xenondifluoride to etch and produce a thin layer of macroporous silicon on the surface of bulk silicon. This layer produces an optical interference pattern whose reflectance spectra can be Figure 1. (a) Cross-sectional scanning-electron-microscope (SEM) image and (b) reflectance of a dry-etched porous-silicon thin film at different angles of incidence ( i ). The differences in the reflectance spectra are caused by optical interference.