Porous Silicon Structures as Optical Gas Sensors

Abstract

We present a short review of recent progress in the field of optical gas sensors based on porous silicon (PSi) and PSi composites, which are separate from PSi optochemical and biological sensors for a liquid medium. Different periodical and nonperiodical PSi photonic structures (bares, modified by functional groups or infiltrated with sensory polymers) are described for gas sensing with an emphasis on the device specificity, sensitivity and stability to the environment. Special attention is paid to multiparametric sensing and sensor array platforms as effective trends for the improvement of analyte classification and quantification. Mechanisms of gas physical and chemical sorption inside PSi mesopores and pores of PSi functional composites are discussed.

Keywords: gas sensors; multiparametric; optical; porous silicon; sensor arrays.

Figure 1

Major components of PSi optical gas sensors.

Figure 2

The 3D plot showing the wavelength shift of the photonic stop band of the bottom layer (Δλ bottom) and the retention time (Δτ) as a function of concentration for the seven analytes indicated. Reprinted with permission from Reference [46]. Copyright 2011 American Chemical Society.

Figure 3

Reflectance (black) and fluorescence (red) of PSi MC infiltrated with Poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) with a top (a) low porosity layer and (b) high porosity layer. Insets depict (a) deep and (b) shallow infiltration resulting in a resonance fluorescence peak and spectral “hole,” respectively (blue arrow—excitation light; yellow arrows—fluorescence; green color—infiltrated polymer). Reprinted with permission from Reference [21]. Copyright 2013 American Chemical Society.

Figure 4

Time traces recorded at λ₁ and λ₂ wavelengths of the fluorescence intensity for (a) PSi MC infiltrated with MEH-PPV and (b) MEH-PPV film deposited on the flat Si substrate upon exposure to saturated TNT vapors (~7 ppb). Insets show λ₁ and λ₂ wavelengths on corresponding spectra with colors matching time traces. Reprinted with permission from Reference [21]. Copyright 2013 American Chemical Society.

Figure 5

Optical response curve of the ammonia sensor to four kinds of gases. Reprinted with permission from Reference [52]. Copyright 2011 Elsevier B.V.

Figure 6

Vapor induced splitting of the Zener tunneling peak. (a) Experimental spectra of the sample exposed to ethanol vapors, (b) transfer-matrix calculations. The original double resonance peak splits into two separate modes when the built-in tilting is modified by the external “field”, mimicked by the refractive index gradient induced by the ethanol vapor flow. Reprinted with permission from Reference [66]. Copyright 2006 American Physical Society.

Figure 7

Schematic diagram of a typical 2D photonic crystal gas sensor. The light impinging from the left side is absorbed by the gas molecules inside the photonic crystal. Due to the reduced group velocity, the interaction path is effectively reduced. Reprinted with permission from Reference [71]. Copyright 2011 AIP Publishing, LLC.

Figure 8

Sensing volatile organic compounds using ‘‘smart dust’’ photonic crystals. An array of microscopic porous Si photonic crystals exposed to toluene vapor is shown in these images. The top left image is the collection held in air. Bottom left is the sample after introduction of toluene vapor. The images on the right are difference maps, showing the difference between the red (top right) and blue (bottom right) channels. The size of these particles is of the order of 300 nm, and their surfaces are modified with dodecyl functionalities. Adapted with permission from Reference [79]. Copyright 2005 the Royal Society of Chemistry.

Figure 9

Comparison of the response signals measured by three different detection methods. This graph shows the response signals on the color-difference (a), current (b), and PL intensity (c). Reprinted with permission from Reference [80]. Copyright 2010 Elsevier B.V.

Figure 10

The temporal response of PSi MC infiltrated with MEH-PPV (Figure 2) on the pulse of TNT vapors (7 ppb, dotted line); black—the intensity of the fluorescent resonance peak; red—its spectral position; blue—resistance (R); green—capacitance (C); magenta—inductance (L). Three electrical signals recorded by the LCR meter (100 Hz) were synchronized with two optical signals (peak intensity and spectral position) through data acquisition system and LabView software [89].

Figure 11

PCA-score plots obtained with the PSi/IL sensor arrays responding to six VOCs. The plot demonstrates that each VOC can be well-distinguished. Reprinted with permission from Reference [91]. Copyright 2011 Wiley-VCH.