Extremely sensitive optical sensing of ethanol using porous silicon

In this study, porous silicon was prepared by anodization of crystalline silicon wafer and was used as a large surface area matrix for sensing the concentration of ethanol solutions. Fabry–Perot reflectance fringes were observed from the samples prepared under the anodization conditions with duration of less than 3 min and current densities below 70 mA/cm2. Effective optical thickness was calculated from the fringe patterns obtained for extremely diluted ethanol solution concentrations in a range from 1×10−5 to 1×10−14 M. Using difference spectra obtained by subtracting the fringe patterns measured in ethanol solutions from those measured in pure deionized water, the effect of ethanol addition was analyzed on the change of fringe patterns. In this range of ethanol concentrations, the change in effective optical thickness obtained from wavelength shifts of fringe patterns did not show any noticeable relation to the ethanol concentration. Rather, standard deviations of the difference spectra were found to show an almost linear relationship to the logarithm of ethanol molar concentration in the range.