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Optical sensing of flammable substances using porous silicon microcavities
Materials Science and Engineering B100 (2003) 271 /274 www.elsevier.com/locate/mseb
Optical sensing of flammable substances using porous silicon microcavities Luca De Stefano a,*, Ivo Rendina a, Luigi Moretti b, Andrea Mario Rossi c a
IMM-CNR-Sezione di Napoli, Via P. Castellino 111, 80131 Napoli, Italy b DEIS-University of Calabria, 87036 Rende (CS), Italy c Istituto Elettrotecnico Nazionale ‘G. Ferraris’, Strada delle Cacce 91, Torino, Italy Received 22 October 2002; received in revised form 10 March 2003; accepted 31 March 2003
Abstract A porous silicon multilayer, constituted by a Fabry /Pe`rot cavity between two distributed Bragg reflectors, is exposed to vapor of several organic species. Different resonant peak shifts in the reflectivity spectra, ascribed to capillary condensation of the vapor in the silicon pores, have been observed. Starting from experimental data, the layer liquid volume fractions condensed in the sensing stack have been numerically estimated. Values ranging between 0.27 (for ethanol) and 0.33 (for iso-propanol) have been found. Time-resolved measurements show that the solvent identification occurs in less then 10 s. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Optical sensors; Microcavities; Low-dimensional silicon structure
1. Introduction In presence of harsh environment, optical sensing is by far the best technique for in situ monitoring of hazardous compounds: in fact, it does not require electric contacts that may cause explosions or fire, nor any sample preparation. Moreover, response times can be very fast. Many optical sensors of organic chemicals and solvents, based on Porous Silicon (PSi), have recently been presented, due to its very peculiar structural properties, in particular its large internal surface. In the last few years, PSi has generated deep interest in photonics and sensing, but if the great reactivity of this material could affect the photonic device characteristics, on the other hand, its a great advantage in sensing applications. The specific surface area, on the order of 500 m2 cm 3, assures a rapid and effective interaction between the organic substances and the PSi. This way, evident changes in several physical quantities such as reflectivity [1 /3], photoluminescence [4], electrical con-
* Corresponding author. Tel.: /39-081-613-2375; fax: 39-081-6132598. E-mail address: [email protected] (L.D. Stefano).
ductivity [5], and optical waveguiding [6,7], can be used for sensing purposes. Thus, liquid, gas, vapor and biological sensors based on PSi technology have been proposed. Furthermore, PSi offers many other advantages: its layers can be used to produce compact and low cost sensor systems on a chip, where both the sensing element and the read-out electronics can be effectively integrated on the same wafer. In this work, we present some very interesting results about the detection of several flammable chemical compounds by using porous silicon microcavities (PSMs). We have systematically studied the sensor response on exposure to each substance both from experimental and theoretical point of view.
2. Theory The microcavity is made of two Distributed Bragg Reflectors (DBRs) with a Fabry/Pe`rot cavity of l /2thickness in the middle. Several alternating pairs of PSilayers, having different refractive indexes, obtained modulating the porosity, constitute the DBRs. The optical thickness (nd ) of each single-layer is l/4, where
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(03)00114-4
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L.D. Stefano et al. / Materials Science and Engineering B100 (2003) 271 /274
d is the physical thickness of the layer, n its refractive index and l is the Bragg wavelength, thus the whole stack is a resonating structure at the Bragg wavelength l. The optical characteristic is a high reflectivity stop band having a narrow peak of transmittance approximately in the centre. Its width is controlled by a properly design of the layer stack. Fig. 1 shows the experimental measured and simulated reflectivity at a fixed angle of incidence (u :/198) for the PSM used for vapor and liquid sensing. The peak width depends on optical losses that have not been considered in the theoretical model. In our case the FWHM is about 10 nm. The simulation of the PSM reflectivity is performed using a transfer matrix method and includes the effect of porosity of the layer on the refractive index [8]. When the PSM is exposed to vapors or dipped in liquid organic solvent a repeatable and completely reversible change in the reflectivity spectrum is observed. The interaction between PSM and external agent de-tunes the optical microcavity so that an On/Off detector can be realised. We assume that the partial substitution of air by the organic liquid phases in the pores of each layer determines an increase of the average refractive index of the microcavity, resulting in a marked red-shift of its characteristic peak. Regarding measurement resolution, a microcavity based sensor offers better performances than those achieved by a single PSi layer sensor, even with DBR structure. In fact, in these cases, the references peaks FWHM are larger (about 30 [9] and 75 nm [1], respectively) than the PSM structure FWHM (about 10 nm). A quantitative analysis of this net red-shift of the peak cavity has been realized by applying the Bruggeman effective medium approximation theory [10]. So that, we can describe the change of the average refractive index
Fig. 1. Experimental and simulated spectrum of the PSM. Solid line is the experimental registered spectrum; dot line is the numerical simulated one.
and calculate the expected peak shift as a function of the pore filling. In this approach, it has been assumed that at equilibrium, the liquid volume fraction of the filled layer is homogeneous across the whole stack and cannot obviously exceed layer porosity. This point of view can be justified by the capillary condensation theory [11]. In Fig. 2, the calculated peak shift as a function of the layer liquid fraction for exposure to different solvents is reported. The slope change at a layer liquid fraction of 0.68 corresponds to the low porosity layer saturated with liquid. In the simulation, we took into account the dispersion of silicon refractive index with respect to the wavelength, except for in the infrared region this variation is about 3%.
3. Experimental Changing the current during the electrochemical etching process on p-type silicon wafers produces the PSMs. The thickness of each layer and its porosity depends on etch time and etch current density. The PSMs used in this study were produced by electrochemical etching on p-type (r $/10 mV cm) standard silicon wafers. Both the distributed Bragg reflectors, above and under the optical cavity, were constituted by thirteen periods of alternating high and low refractive index l/4-layers. An etching current density of 80 mA cm 2 for 1.989 s, resulting in a porosity of 68%, is used for high refractive index layers while an etching current density of 250 mA cm 2 for 0.940 s, resulting in a porosity of 72%, for low ones. The optical cavity is a low index layer. This structure is
Fig. 2. Calculated wavelength shift of the PSM peak as a function of the organic solvent liquid fraction. Squares dotted line: acetone. Circles dotted line: iso-propanol. Up triangles dotted line: methanol. Down triangles dotted line: chlorobenzene. Rhombus dotted line: ethanol.
L.D. Stefano et al. / Materials Science and Engineering B100 (2003) 271 /274
273
Table 1 Flammable chemical organics used in sensing experiments and some relevant physical /chemical properties: n , liquid refractive index; r , density (at 25 8C); STC, surface tension coefficient (at 25 8C); VP, vapour pressure; BP, boiling point; Dl , the peak shift and LLF is the liquid layer fraction Solvent
n
r (g cm 3)
STC (mN m 1)
VP (kPa)
BP (8C)
Dl (nm)
LLF
Acetone Iso-propanol Methanol Chlorobenzene Ethanol
1.359 1.377 1.329 1.524 1.360
0.791 0.785 0.791 1.106 0.785
23.5 20.9 22.1 33.5 22.8
30.8 6.8 16.9 1.58 5.8
56 82.4 64.7 132 78
104 111 95 150 93
0.30 0.33 0.30 0.30 0.27
characterised by a single resonance peak at 1313 nm in a stop band between 1200 and 1500 nm. Each measurement starts after adding a small amount of volatile liquid to the glass vial so that its vapor quickly saturates the atmosphere surrounding the sensor. A white source impinges, through an optical fiber and a collimator, on the microcavity and the output is collected by an objective and coupled into a multimode fiber. The signal is directed in an optical spectrum analyser (Ando, Mod. AQ-6315B) and measured with a 0.2 nm resolution.
4. Results and discussion The results obtained for all the substances are reported in Table 1 and shown in Fig. 3. It can be noted that even for substances with very close values of refractive index n , i.e. acetone and ethanol, the peak shift Dl is well resolved. Same results, within the experimental error, have been obtained by directly wetting the sensor with a small amount of liquid. In Table 1 are also listed the corresponding calculated layer liquid fraction (LLF) due to capillary condensation of the organic liquid into the silicon pores. The LLF
Fig. 3. Reflectivity spectra of the PSM after the exposure to different flammable organic substances. Curve 1, unperturbed PSM; 2, to ethanol; 3, to methanol; 4, to acetone; 5, to iso-propanol; 6, to chlorobenzene.
values found with our devices indicate, for these solvents, a filling of the pores between 0.27 and 0.33, according to the results reported in Ref. [1]. We performed time-resolved measurements in order to characterise the sensor dynamic: using a monochromator, we have measured, as a function of time, the signal at the wavelength corresponding to the shifted peak characteristic of each substance. In the same way, after the exposure, we have verified the recovery time of the sensor by monitoring the unperturbed cavity peak wavelength. As an example, the results of time-resolved measurements, in the case of iso-propanol, are shown in Fig. 4: identification of the solvent occurs in less then 10 s (tid /8.7 s), while the signal returns to its original value in even shorter time (trec /1.7 s).
5. Conclusions We have presented a simple and well performing optical sensor of flammable chemicals based on PSMs. A well-defined shift of the reflectivity spectrum can be repeatably observed when the sensor is exposed to the organic vapors. The shift is characteristic of each species and depends on their physical and chemical properties. The sensing process is completely reversible. We attribute the effect to the capillary condensation of the liquid
Fig. 4. Time-resolved measurement in the case of iso-propanol.
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L.D. Stefano et al. / Materials Science and Engineering B100 (2003) 271 /274
into the pores. Numerical simulation of the PSM based on the Bruggeman effective medium approximation gives values of the liquid volume fraction adsorbed in the layers very similar to that reported in literature. Time-resolved measurements indicate that stationary conditions between the vapor and the sensor are reached in a short time scale, easily resolved by optical techniques. For its intrinsic simplicity and good features, we believe that PSMs have great potential for liquid and vapor sensing of chemical substances.
References [1] P.A. Snow, E.K. Squire, P.J. Russel, St., L.T. Canaham, J. Appl. Phys. 86 (1999) 1781.
[2] V.S.Y. Lin, K. Motesharei, K.P.S. Dancil, M.J. Sailor, M.R. Ghadiri, Science 278 (1997) 840. [3] S. Zangooie, R. Jansoon, H. Arwin, J. Vac. Sci. Technol. A 16 (1998) 2901. [4] V. Mulloni, L. Pavesi, Appl. Phys. Lett. 76 (2000) 2523. [5] M. Ben-Chorin, A. Kux, I. Schecter, Appl. Phys. Lett. 64 (1994) 481. [6] H.F. Arrand, A. Loni, R. Arens-Fischer, M.G. Kruger, M. Thoenissen, H. Luth, S. Kershaw, N.N. Vorazov, J. Lumin. 80 (1999) 119. [7] T.M. Benson, H.F. Arrand, P. Sewell, D. Niemeyer, A. Loni, R.J. Bozeat, M. Kruger, R. Arens-Fischer, M. Thoenissen, H. Luth, Mater. Sci. Eng. B69 /B70 (1999) 92 /99. [8] M.A. Muriel, A. Carballar, IEEEP. Technol. Lett. 9 (1997) 955. [9] V.S.Y. Lin, K. Motesharei, K.P.S. Dancil, M.J. Sailor, M.R. Ghadiri, Science 278 (1997) 840. [10] J.E. Spanier, I.P. Herman, Phys. Rev. 61 (2000) 10 437. [11] A.V. Neimark, P.I. Ravikovitch, Micro. Meso. Mat. 44 /45 (2001) 697 /707.
Optical sensing of flammable substances using porous silicon microcavities Luca De Stefano a,*, Ivo Rendina a, Luigi Moretti b, Andrea Mario Rossi c a
IMM-CNR-Sezione di Napoli, Via P. Castellino 111, 80131 Napoli, Italy b DEIS-University of Calabria, 87036 Rende (CS), Italy c Istituto Elettrotecnico Nazionale ‘G. Ferraris’, Strada delle Cacce 91, Torino, Italy Received 22 October 2002; received in revised form 10 March 2003; accepted 31 March 2003
Abstract A porous silicon multilayer, constituted by a Fabry /Pe`rot cavity between two distributed Bragg reflectors, is exposed to vapor of several organic species. Different resonant peak shifts in the reflectivity spectra, ascribed to capillary condensation of the vapor in the silicon pores, have been observed. Starting from experimental data, the layer liquid volume fractions condensed in the sensing stack have been numerically estimated. Values ranging between 0.27 (for ethanol) and 0.33 (for iso-propanol) have been found. Time-resolved measurements show that the solvent identification occurs in less then 10 s. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Optical sensors; Microcavities; Low-dimensional silicon structure
1. Introduction In presence of harsh environment, optical sensing is by far the best technique for in situ monitoring of hazardous compounds: in fact, it does not require electric contacts that may cause explosions or fire, nor any sample preparation. Moreover, response times can be very fast. Many optical sensors of organic chemicals and solvents, based on Porous Silicon (PSi), have recently been presented, due to its very peculiar structural properties, in particular its large internal surface. In the last few years, PSi has generated deep interest in photonics and sensing, but if the great reactivity of this material could affect the photonic device characteristics, on the other hand, its a great advantage in sensing applications. The specific surface area, on the order of 500 m2 cm 3, assures a rapid and effective interaction between the organic substances and the PSi. This way, evident changes in several physical quantities such as reflectivity [1 /3], photoluminescence [4], electrical con-
* Corresponding author. Tel.: /39-081-613-2375; fax: 39-081-6132598. E-mail address: [email protected] (L.D. Stefano).
ductivity [5], and optical waveguiding [6,7], can be used for sensing purposes. Thus, liquid, gas, vapor and biological sensors based on PSi technology have been proposed. Furthermore, PSi offers many other advantages: its layers can be used to produce compact and low cost sensor systems on a chip, where both the sensing element and the read-out electronics can be effectively integrated on the same wafer. In this work, we present some very interesting results about the detection of several flammable chemical compounds by using porous silicon microcavities (PSMs). We have systematically studied the sensor response on exposure to each substance both from experimental and theoretical point of view.
2. Theory The microcavity is made of two Distributed Bragg Reflectors (DBRs) with a Fabry/Pe`rot cavity of l /2thickness in the middle. Several alternating pairs of PSilayers, having different refractive indexes, obtained modulating the porosity, constitute the DBRs. The optical thickness (nd ) of each single-layer is l/4, where
0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(03)00114-4
272
L.D. Stefano et al. / Materials Science and Engineering B100 (2003) 271 /274
d is the physical thickness of the layer, n its refractive index and l is the Bragg wavelength, thus the whole stack is a resonating structure at the Bragg wavelength l. The optical characteristic is a high reflectivity stop band having a narrow peak of transmittance approximately in the centre. Its width is controlled by a properly design of the layer stack. Fig. 1 shows the experimental measured and simulated reflectivity at a fixed angle of incidence (u :/198) for the PSM used for vapor and liquid sensing. The peak width depends on optical losses that have not been considered in the theoretical model. In our case the FWHM is about 10 nm. The simulation of the PSM reflectivity is performed using a transfer matrix method and includes the effect of porosity of the layer on the refractive index [8]. When the PSM is exposed to vapors or dipped in liquid organic solvent a repeatable and completely reversible change in the reflectivity spectrum is observed. The interaction between PSM and external agent de-tunes the optical microcavity so that an On/Off detector can be realised. We assume that the partial substitution of air by the organic liquid phases in the pores of each layer determines an increase of the average refractive index of the microcavity, resulting in a marked red-shift of its characteristic peak. Regarding measurement resolution, a microcavity based sensor offers better performances than those achieved by a single PSi layer sensor, even with DBR structure. In fact, in these cases, the references peaks FWHM are larger (about 30 [9] and 75 nm [1], respectively) than the PSM structure FWHM (about 10 nm). A quantitative analysis of this net red-shift of the peak cavity has been realized by applying the Bruggeman effective medium approximation theory [10]. So that, we can describe the change of the average refractive index
Fig. 1. Experimental and simulated spectrum of the PSM. Solid line is the experimental registered spectrum; dot line is the numerical simulated one.
and calculate the expected peak shift as a function of the pore filling. In this approach, it has been assumed that at equilibrium, the liquid volume fraction of the filled layer is homogeneous across the whole stack and cannot obviously exceed layer porosity. This point of view can be justified by the capillary condensation theory [11]. In Fig. 2, the calculated peak shift as a function of the layer liquid fraction for exposure to different solvents is reported. The slope change at a layer liquid fraction of 0.68 corresponds to the low porosity layer saturated with liquid. In the simulation, we took into account the dispersion of silicon refractive index with respect to the wavelength, except for in the infrared region this variation is about 3%.
3. Experimental Changing the current during the electrochemical etching process on p-type silicon wafers produces the PSMs. The thickness of each layer and its porosity depends on etch time and etch current density. The PSMs used in this study were produced by electrochemical etching on p-type (r $/10 mV cm) standard silicon wafers. Both the distributed Bragg reflectors, above and under the optical cavity, were constituted by thirteen periods of alternating high and low refractive index l/4-layers. An etching current density of 80 mA cm 2 for 1.989 s, resulting in a porosity of 68%, is used for high refractive index layers while an etching current density of 250 mA cm 2 for 0.940 s, resulting in a porosity of 72%, for low ones. The optical cavity is a low index layer. This structure is
Fig. 2. Calculated wavelength shift of the PSM peak as a function of the organic solvent liquid fraction. Squares dotted line: acetone. Circles dotted line: iso-propanol. Up triangles dotted line: methanol. Down triangles dotted line: chlorobenzene. Rhombus dotted line: ethanol.
L.D. Stefano et al. / Materials Science and Engineering B100 (2003) 271 /274
273
Table 1 Flammable chemical organics used in sensing experiments and some relevant physical /chemical properties: n , liquid refractive index; r , density (at 25 8C); STC, surface tension coefficient (at 25 8C); VP, vapour pressure; BP, boiling point; Dl , the peak shift and LLF is the liquid layer fraction Solvent
n
r (g cm 3)
STC (mN m 1)
VP (kPa)
BP (8C)
Dl (nm)
LLF
Acetone Iso-propanol Methanol Chlorobenzene Ethanol
1.359 1.377 1.329 1.524 1.360
0.791 0.785 0.791 1.106 0.785
23.5 20.9 22.1 33.5 22.8
30.8 6.8 16.9 1.58 5.8
56 82.4 64.7 132 78
104 111 95 150 93
0.30 0.33 0.30 0.30 0.27
characterised by a single resonance peak at 1313 nm in a stop band between 1200 and 1500 nm. Each measurement starts after adding a small amount of volatile liquid to the glass vial so that its vapor quickly saturates the atmosphere surrounding the sensor. A white source impinges, through an optical fiber and a collimator, on the microcavity and the output is collected by an objective and coupled into a multimode fiber. The signal is directed in an optical spectrum analyser (Ando, Mod. AQ-6315B) and measured with a 0.2 nm resolution.
4. Results and discussion The results obtained for all the substances are reported in Table 1 and shown in Fig. 3. It can be noted that even for substances with very close values of refractive index n , i.e. acetone and ethanol, the peak shift Dl is well resolved. Same results, within the experimental error, have been obtained by directly wetting the sensor with a small amount of liquid. In Table 1 are also listed the corresponding calculated layer liquid fraction (LLF) due to capillary condensation of the organic liquid into the silicon pores. The LLF
Fig. 3. Reflectivity spectra of the PSM after the exposure to different flammable organic substances. Curve 1, unperturbed PSM; 2, to ethanol; 3, to methanol; 4, to acetone; 5, to iso-propanol; 6, to chlorobenzene.
values found with our devices indicate, for these solvents, a filling of the pores between 0.27 and 0.33, according to the results reported in Ref. [1]. We performed time-resolved measurements in order to characterise the sensor dynamic: using a monochromator, we have measured, as a function of time, the signal at the wavelength corresponding to the shifted peak characteristic of each substance. In the same way, after the exposure, we have verified the recovery time of the sensor by monitoring the unperturbed cavity peak wavelength. As an example, the results of time-resolved measurements, in the case of iso-propanol, are shown in Fig. 4: identification of the solvent occurs in less then 10 s (tid /8.7 s), while the signal returns to its original value in even shorter time (trec /1.7 s).
5. Conclusions We have presented a simple and well performing optical sensor of flammable chemicals based on PSMs. A well-defined shift of the reflectivity spectrum can be repeatably observed when the sensor is exposed to the organic vapors. The shift is characteristic of each species and depends on their physical and chemical properties. The sensing process is completely reversible. We attribute the effect to the capillary condensation of the liquid
Fig. 4. Time-resolved measurement in the case of iso-propanol.
274
L.D. Stefano et al. / Materials Science and Engineering B100 (2003) 271 /274
into the pores. Numerical simulation of the PSM based on the Bruggeman effective medium approximation gives values of the liquid volume fraction adsorbed in the layers very similar to that reported in literature. Time-resolved measurements indicate that stationary conditions between the vapor and the sensor are reached in a short time scale, easily resolved by optical techniques. For its intrinsic simplicity and good features, we believe that PSMs have great potential for liquid and vapor sensing of chemical substances.
References [1] P.A. Snow, E.K. Squire, P.J. Russel, St., L.T. Canaham, J. Appl. Phys. 86 (1999) 1781.
[2] V.S.Y. Lin, K. Motesharei, K.P.S. Dancil, M.J. Sailor, M.R. Ghadiri, Science 278 (1997) 840. [3] S. Zangooie, R. Jansoon, H. Arwin, J. Vac. Sci. Technol. A 16 (1998) 2901. [4] V. Mulloni, L. Pavesi, Appl. Phys. Lett. 76 (2000) 2523. [5] M. Ben-Chorin, A. Kux, I. Schecter, Appl. Phys. Lett. 64 (1994) 481. [6] H.F. Arrand, A. Loni, R. Arens-Fischer, M.G. Kruger, M. Thoenissen, H. Luth, S. Kershaw, N.N. Vorazov, J. Lumin. 80 (1999) 119. [7] T.M. Benson, H.F. Arrand, P. Sewell, D. Niemeyer, A. Loni, R.J. Bozeat, M. Kruger, R. Arens-Fischer, M. Thoenissen, H. Luth, Mater. Sci. Eng. B69 /B70 (1999) 92 /99. [8] M.A. Muriel, A. Carballar, IEEEP. Technol. Lett. 9 (1997) 955. [9] V.S.Y. Lin, K. Motesharei, K.P.S. Dancil, M.J. Sailor, M.R. Ghadiri, Science 278 (1997) 840. [10] J.E. Spanier, I.P. Herman, Phys. Rev. 61 (2000) 10 437. [11] A.V. Neimark, P.I. Ravikovitch, Micro. Meso. Mat. 44 /45 (2001) 697 /707.