Porous silicon (PS) is a nanostructured material generated by electrochemically etching silicon in electrolytes containing hydrofluoric acid (HF) with many potential application areas such as optoelectronics and biosensing. PS retains the advantages of silicon technology while adding the ability of controlling optical properties. Fabry-Pérot interferometers, Fabry-Pérot filters or distributed Bragg reflectors are some of the 1D structures that have been fabricated in PS under different etching conditions, e.g. changing anodization current and concentration of HF.
Tuning pore diameter is essential for some applications in which substances must be flown through the pores, so that a size-based filtering of the molecules can be done. However, macropore (>50nm) formation on p-type silicon is still poorly known due to the strong dependence with resistivity . Electrochemically etching heavily doped p-type silicon usually forms micropores (<10nm) but it has been found that bigger sizes can be achieved by adding a solvent to the electrolyte (aqueous or organic).
In this work we present the results of using dimethylformamide (DMF), dimethylsulfoxide (DMSO), potasium hydroxide (KOH) and sodium hydroxide (NaOH) for macropore formation in p-type silicon with resistivities between 0.001 and 9 Ω∙cm, achieving pore size range from 5 to 100nm.
 G.X. Zhang, Porous silicon: morphology and formation mechanisms, Modern Aspects of Electrochemistry, number 39, edited by C. Vayenas et al., Springer, New York, 2005.
Track-etched polycarbonate membranes (TEPM) are commercially available membranes typically used in particle filtration due to the pores present in their surfaces. This porous structure reminds of that of a Fabry-Pérot interferometer made on porous silicon, an optical structure long employed in optical chemical sensing. Because of this morphological similarity, we hypothesized that TEPM could exhibit a similar optical response and thus being useful for creating simpler to fabricate, easily available and low cost chemical sensors. To asses this hypothesis, we investigated the optical response of TEPM in the infrared range, improved it by chemically attaching the membranes to a silicon flat surface and then, performed reflectivity measurements in presence of different concentrations of ethanol.
When exposing a TEPM to a change of the refractive index of the medium it is surrounded by (air) by placing a drop of ethanol, with a higher refractive index, we can observe a shift of its spectrum towards higher wavelengths. This indicates the presence of the solvent, and we could check that the bigger the concentration, the bigger the magnitude of that shift. Furthermore, when the solvent is evaporated the spectrum returns to its initial position, which allowed us to perform different concentration sensing steps using the same sample. These promising results, although early, could indicate the utility of these membranes to easily fabricate cheap chemical sensors and, probably, optical biosensors as their surface can be chemically modified.
Porous silicon (PS) is a good host for fabricating high sensitivity sensors due to the high aspect ratio between surface and volume that can be achieved. The anodization in hydrofluoric acid solutions forms nano-size pores with a few microns of thickness that can be functionalized for the detection of analytes. However, closed-ended porous silicon films have some drawbacks, like air entrapment and bad flow diffusion that can lower the sensitivity.
Porous silicon membranes appeared as a solution to those effects by flowing the substances through open-ended films. This reduces the time of detection, optimises the sensitivity and avoid mixture of different substances .
Lift-off of a porous silicon film is the easiest method for obtaining self-standing porous silicon membranes. The layer is detached from the substrate in a single step by electrochemically etching with a current close to electropolishing.
In this work, we present the experimental results of sensing with a PS sensor based on the lift-off method. We have measured the reflectance spectrum each 30 seconds and followed the shift while flowing through the pores. Experimental sensitivity values are in good agreement with the theoretical simulations performed.
 Y. Zhao, G. Gaur, R.L. Mernaugh, P.E. Laibinis, S.M, Weiss, Comparative kinetics analysis of closed-ended and open-ended porous silicon, Nanoscale Research Letters, 11:395, 2016.