UC San Diego

An ideal environmental sensor has zero baseline drift, a fast response time, is sensitive and selective to the analyte of interest, and has the ability to be miniaturized. Porous silicon is an attractive material for sensing applications due to its high surface area, readily modified surface chemistry, and optical signal transduction capability. This thesis describes the construction and chemical modification of porous silicon photonic crystals for use in chemical sensing. The specific aims of this work were to develop new methods to maximize sensor stability, remove background signal interference, and to induce chemical specificity into the sensor. This thesis begins with an introduction on porous silicon preparation methods and its sensing mechanisms, as well as different chemicals and biomolecules that have been detected and their detection limit. We show a multitude of chemical modifications of the porous silicon surface that produce long-term stability and induce analyte class specificity to the sensor. Next, a method to remove interfering effects of changing relative humidity from the response of porous silicon is developed. Two porous silicon films are separately etched and chemically modified into silicon, one on top of the other. The response of each film is measured simultaneously. Each film has a characteristic response towards water vapor. The effect of changing humidity can then be accounted for by calculating the weighted difference between the two layer responses. Thereby, building an internal reference into the sensor. A second type of internal spectral reference to eliminate artifacts associated with varying angle of incidence of an optical probing detector relative to a one-dimensional photonic crystal sensor was developed. The chemically non-responsive internal spectral reference was built into a photonic crystal sensor chemically modified to respond specifically to hydrofluoric acid. Lastly, a simple and inexpensive method to etch patterns into porous silicon was developed. A masking layer was imprinted onto a silicon surface through microcontact printing, followed by an electrochemical etch. The imprinted residue reduces the etching rate of the bulk silicon below, while unmasked silicon etches normally. The resulting inhomogeneous etching rate of silicon transfers the pattern into the bulk silicon