Structural considerations on multistopband mesoporous silicon rugate filters prepared for gas sensing purposes
ABSTRACT Different designs for producing multiple stopband mesoporous silicon rugate filters via electrochemical anodization are compared. The effects of light absorption and dispersion to visible range filter design are investigated. Thermal oxidation is applied for passivating the chemically reactive porous silicon surface, and the response of the passivated structures to ethanol vapor is examined. Differences in gas sensing properties for the various designs are evaluated and possible reasons for the observed differences are discussed. Methods for sidelobe suppression in multipeak filters are discussed and demonstrated, and their effects in gas sensing applications are estimated.
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ABSTRACT: We prepared macro-porous silicon (MPS) by electrochemical corrosion in a double-tank cell on the surface of single-crystalline P-type silicon. Then, nano-WO3 films were deposited on MPS layers by DC facing target reactive magnetron sputtering. The morphologies of the MPS and WO3/MPS samples were investigated by using a field emission scanning electron microscope. The crystallization of WO3 and the valence of the W in the WO3/MPS sample were characterized by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The gas sensing properties of MPS and WO3/MPS gas sensors were thoroughly measured at room temperature. It can be concluded that: the WO3/MPS gas sensor shows the gas sensing properties of a P-type semiconductor gas sensor. The WO3/MPS gas sensor exhibits good recovery characteristics and repeatability to 1 ppm NO2. The addition of WO3 can enhance the sensitivity of MPS to NO2. The long-term stability of a WO3/MPS gas sensor is better than that of an MPS gas sensor. The sensitivity of the WO3/MPS gas sensor to NO2 is higher than that to NH3 and C2H5OH. The selectivity of the MPS to NO2 is modified by deposited nano-WO3 film.Journal of Semiconductors 05/2012; 33(5). DOI:10.1088/1674-4926/33/5/054012
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ABSTRACT: Preparation of graded-index optical microcavities based on porous silicon is demonstrated, and chemical modifications for obtaining improved stability under alkaline conditions are studied. Four surface modification methods for stabilizing the samples are examined, and the effects on the optical properties are verified. Two different thermal carbonization treatments resulting in hydrophilic and hydrophobic surfaces are employed. In addition, modification with undecylenic acid is performed on as-prepared and thermally hydrocarbonized porous silicon surfaces. Stability and sensing capabilities of the modified samples are examined by subjecting them to different concentrations of methylamine and trimethylamine vapors. Vapor induced changes in the reflectance spectra are used for evaluating sensitivity and stability. Sensitivity towards ethanol vapor is also measured in order to compare the sensitivity to a normal organic solvent. The results show that the two carbonization treatments and the undecylenic acid functionalization of the hydrocarbonized surface result in greatly improved stability. In contrast, derivatization of as-prepared porous silicon with undecylenic acid does not protect the surface sufficiently against oxidation under the highly basic conditions produced by the amine vapors. Surface chemistry is also shown to have a large effect on sensitivity towards the examined vapors. X-ray photoelectron spectroscopy was used to assess changes in elemental composition of sample surface. The results suggest that thermally promoted addition of undecylenic acid on hydrocarbonized porous silicon is an effective method for producing highly stable optical structures with a carboxyl group functionalization.Advanced Functional Materials 09/2012; 22(18). DOI:10.1002/adfm.201200386 · 10.44 Impact Factor
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ABSTRACT: Porous silicon has been established as an excellent sensing platform for the optical detection of hazardous chemicals and biomolecular interactions such as DNA hybridization, antigen/antibody binding, and enzymatic reactions. Its porous nature provides a high surface area within a small volume, which can be easily controlled by changing the pore sizes. As the porosity and consequently the refractive index of an etched porous silicon layer depends on the electrochemial etching conditions photonic crystals composed of multilayered porous silicon films with well-resolved and narrow optical reflectivity features can easily be obtained. The prominent optical response of the photonic crystal decreases the detection limit and therefore increases the sensitivity of porous silicon sensors in comparison to sensors utilizing Fabry-Pérot based optical transduction. Development of porous silicon photonic crystal sensors which allow for the detection of analytes by the naked eye using a simple color change or the fabrication of stacked porous silicon photonic crystals showing two distinct optical features which can be utilized for the discrimination of analytes emphasize its high application potential.Sensors 04/2013; 13(4):4694-713. DOI:10.3390/s130404694 · 2.05 Impact Factor