Ms. Hsuan Chang Profile

Hsuan Chang

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Proceedings Article | 3 September 2019 Paper

Surface modification of mesoporous silicon optical rugate filters for low-concentration vapor detection

Anushree Chaudhuri, Riddhi Ananth, Lorenzo Calvano, Hsuan Chang, Michael Sailor

Proceedings Volume 11089, 110892Q (2019) https://doi.org/10.1117/12.2534963

KEYWORDS: Optical gas detection, Toxic gases, Nanotechnology, Nanosensors, Photonic crystals, Silicon, Sensors, Optical coatings, Optical filters, Refractive index, Ozone, Chemical detection, Water, Spectroscopy, Chemical analysis

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Current chemical detection techniques are not feasible in rugged environments where speed of detection, autonomous operation, and compactness are crucial factors. In this study, we investigate the promising photonic crystal structure of porous silicon (pSi) for portable sensing. HF-ethanol electrolyte with Pt/Al electrodes and a sinusoidal current oscillating between 15 mA and 108 mA was used to electrochemically etch p++-type porous silicon optical rugate filters with a 50% porosity. The samples were hydroxylated in ozone gas and functionalized through a ring-opening living polymerization reaction with a heterocyclic silane to retain thermal and alkaline stability. ATR-FTIR spectra and SEM confirmed the ring-opening reaction with grafting via O-Si bonds to the pSi surface. Hydrophobicity was demonstrated by water contact angles of 120-130 degrees. A stable stopband was maintained after varying thermal and alkaline conditions. After confirming pSi stability, a broad range of VOCs were cycled through a pressurized flow chamber and a bifurcated optical fiber CCD spectrometer continuously monitored for changes in the reflected stopband of the sensor. Target library VOCs underwent microcapillary condensation in pSi cavities, changing the composite refractive index and producing predictable optical stopband shifts with a precision within 0.03 nm for pure VOCs and 0.79 nm for complex mixtures and a threshold detection concentration of 1 microgram per cubic meter. With a precision which parallels state-of-theart chemical detection methods, as well as high stability and portability, this nanosensor is viable in chemical detection systems for real-time air quality monitoring, military defense, and forensic analysis.

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