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Innovative prototype of a zinc-oxide based optical gas sensor
Sensors and Actuators B 173 (2012) 391–395
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Innovative prototype of a zinc-oxide based optical gas sensor Sami Youssef a,b,c,∗ , Jean Podlecki a , Roland Habchi b , Marwan Brouche c , Alain Foucaran a , David Bouvier d , Nicolas Brillouet d , Paul Coudray d a
Institut d’Electronique du Sud, IES, Université Montpellier II, UMR CNRS 5214, Place Eugène Bataillon, 34095 Montpellier, France Laboratoire de Physique Appliquée (LPA), Département de Physique, Université libanaise, Faculté des sciences II, 90656 Jdeidet, Lebanon Ecole supérieure d’ingénieurs de Beyrouth, Campus des Sciences et Technologies, Mar Roukoz, Lebanon d Société KLOE, Hôtel d’Entreprise du Millénaire, 1068 Rue de la Vieille Poste, 34000 Montpellier, France b c
a r t i c l e
i n f o
Article history: Received 25 September 2011 Received in revised form 26 June 2012 Accepted 3 July 2012 Available online 17 July 2012 Keywords: Zinc oxide Bragg reflector Waveguide Gas sensor
a b s t r a c t A zinc-oxide based optical sensor is realized. This prototype is based on the properties of Bragg reflectors, first the measurement principles are described then the experimental results are presented. This sensor has proven to be sensitive and reliable in the case of detecting low concentrations of butane. The sensor is stabilized and desorbed by a flow of nitrogen. In the case of methane detection, the use of nitrogen was found inappropriate, ambient air has proven to be more suitable. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide ZnO has been known to have changeable electric properties in the presence of gases [1,2] so, many devices have been reported [3–7] to have a ZnO part as a sensing material to detect gas concentrations. Developing highly sensitive devices is of major importance for the scientific community [8–10] especially when it concerns environmental and security issues, such as monitoring gas leakages in oil companies and industrial facilities or even observing the evolution of pollutant gases that could reach the environment. In this paper we report the realization of a gas sensor used to detect small concentrations of methane and butane. But unlike the majority of ZnO based gas sensors, the reported device is based on a new approach that uses the optical properties of a Bragg grating. When a certain area of the grating is covered with active ZnO, a shift in the Bragg wavelength is observed when gas molecules are adsorbed at the surface which provides a detection mechanism to monitor gas leakages. 2. Measurement principle In order to study the response of the optical sensor, a special waveguide was conceived using a photosensitive hybrid
∗ Corresponding author at: Institut d’Electronique du Sud, IES, Université Montpellier II, UMR CNRS 5214, Place Eugène Bataillon, 34095 Montpellier, France. E-mail address: [email protected] (S. Youssef). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.07.025
organomineral material based on acrylate. This material is sensitive to UV light. The synthesis procedure is realized by a sol–gel technique. The sol is obtained by the use of two organic–inorganic precursors while the mineral lattice is attained from a hydrolysis and a polycondensation of alkoxides. A UV light is used to polymerize C C bonds. Polymerization of C C bonds could be done be either light or high temperature. In this case we have chosen to use UV light due to the nature of our material which is a photosensitive hybrid organomineral material. Any damage resulting from high temperature is therefore prevented. Once the solution is synthesized, low cost deposition methods such as spin-coating and dip-coating are used to put the layers over a silicon substrate. The gel obtained is thermally annealed to get a final dry glass product. The technique used in this work is the dip-coating which is a fast, low cost and easily industrialized process. UV light is then used to create a periodic difference of refractive index. No masks are used with this technique where the material is exposed to the UV beam which induces successive changes of the refractive index resulting into a high quality Bragg grating. The optimized structure is a buried waveguide as shown in Fig. 1a, a photo of the actual sensor is given in Fig. 1b, where a buffer layer having a refractive index n1 is deposited over the silicon substrate followed by another layer having a refractive index n2 . The layer of index n1 covers the device where a window is engraved to grow a layer of ZnO by reactive magnetron sputtering at room temperature [11]. Two successive distributed Bragg reflectors are obtained; the first is the reference reflector and the second is covered by an active layer of ZnO. This particular disposition is used to realize a differential
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Fig. 1. (a) Representation of the sensor with its two Bragg reflectors. (b) A photo of the optical gas sensor installed in its final package.
measurement where two signals are obtained out of the waveguide, a first one unchanged and a second one modified by the presence of a gas on the sensitive area. Here each reflector has a specific Bragg wavelength but they both undergo the same temperature and pressure variations. Fig. 2 highlights the wavelength shift experienced by the two Bragg gratings as the temperature varies. The blue curve is the sensors’ response in initial conditions, while the curve in red represents the new response after a temperature increase. Therefore we can notice that the two gratings, one as reference and one
for measuring, experienced the same wavelength shift. However, the adsorption of a gas on the sensitive area induces a change in the refractive index and therefore a change of the propagation properties in the waveguide, as a result a shift in the transmitted Bragg wavelength is observed. Indeed, pressure stresses the entire chip evenly, same as when temperature contracts or dilates the device also evenly. But the temperature effect on the refractive index is neglected with respect to the variation induced by pressure; this is an experimental fact that was controlled during our work. The experimental protocol is as follows: a nitrogen cylinder and a methane (or butane) cylinder are connected to an hermetically sealed chamber in such a way to be able to alternatively inject the methane or butane and then flush it with nitrogen which is considered to be a neutralizing gas used to ensure the desorption of the reflector. Fig. 3 shows a schematic of the testing setup. The length of each pipe and the position of the valves have been chosen in such a way to prevent any dead zones that are likely to retain small amounts of one gas while another gas is injected into the chamber. That would be a potential source of contamination and misinterpretation of the detected signal. The chamber is also equipped with a thermometer to monitor any temperature changes that would justify a shift in wavelength of the two Bragg reflectors. A pressure gauge is installed on the injection pipe to ensure that there will be no pressure variations that may influence the measurement. The injection and the extraction of gases are done through valves to guarantee a good tightness of the chamber, which is essential in saturation and stabilization phases. The sensor is inserted into the chamber, and optical fibers are coupled through cable glands. The monitoring system is composed of a tunable laser source, a photodetector, and special software controlling all the equipment. The laser is a class 1M single mode tunable laser, centered in the C-band window, and which is thermally regulated and is part of complete interrogating system for photonics sensors. Emission wavelength is permanently measured thanks to a wavelength meter associated to the experimental setup. The tunable laser provides a linear sweep of wavelength with a given start/stop wavelength and a given rate, and the photodetector is used to make an accurate optical power measurement. The control software includes several features such as the synchronization of the laser source with the detector, processing and finally displaying data. A measurement sequence begins when the laser and the photodetector are synchronized, then the laser scans the chosen wavelength range with a specified increment and goes into a continuous scanning mode. During the scan, the photodetector retrieves the optical power that corresponds to each wavelength and finally a spectrum is displayed. The following measurement sequence begins immediately after the end of the first. 3. Results A series of sensitivity tests are performed first with butane. At the beginning nitrogen is introduced into the chamber for 15 min to evacuate all possible pollutants and to ensure desorption of the sensor. When the inner atmosphere is stabilized for several minutes and no gas is detected, butane is injected. Considering that the volume of the chamber is 0.15 m3 and the flow of gas is 0.48 m3 /h at 28 mbar, the necessary injection time for a certain concentration of butane could be calculated using the following equation: tinj =
Fig. 2. The transmission spectrum of the two Bragg gratings of our waveguide, the peaks indicate the Bragg wavelength. The two Bragg reflectors experienced the same wavelength shift as the temperature changes.
%gas × Vchamber debit
(1)
Therefore an injection time of 11.2 s is needed to get a butane concentration of 1%. Fig. 4 represents data for successive injections of nitrogen and butane taken at real time. The measured parameter is the relative variation of the specific Bragg wavelength as a function of time. The
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Fig. 3. A schematic view of the experimental set up, showing the gas tubes, the valves, and the position of the detector.
figure is divided into successive injections of nitrogen, stabilization time and butane injection. The first injection of nitrogen is stopped after 20 min (10:10), here the curve is not perfectly leveled to consider a full desorption. A stabilization period is started followed by an 11 s injection of butane (1% of the volume) at “10:17”, 30 s later the butane starts to be detected and eventually a 0.055 nm shift of the Bragg wavelength is observed after 10 min. The curve is then leveled, which means that all active sites on the surface are occupied by butane molecules and no more adsorption is possible. The decrease of the curve between 10:31 and 10:40 is due to temperature variation in the chamber as noticed from the readings of our reference sensor, because when temperature changes, the entire material is subjected to some dilation or contraction, which leads to a shift in both Bragg peaks by the same amplitude. At 10:40 nitrogen is introduced again for 15 min and a rapid change in the Bragg wavelength is observed after less than 5 min scoring a shift of −0.08 nm. When the nitrogen flow is stopped at 10:55, the system is left to stabilize until 11:46 where the curve is perfectly leveled and the wavelength is stable at a value less than the first introduction of nitrogen. In fact, the cleaning process is done with the introduction of a nitrogen flow in the chamber and when the gas molecules are desorbed, the wavelength shift is negative
Fig. 4. Optical response of the sensor to butane. The curve is divided into consecutive injections of nitrogen, stabilization time and butane injection. A 0.03 and 0.055 nm shift of the Bragg wavelength is recorded for 0.5 and 1% butane respectively. The sensor is desorbed by a flow of nitrogen.
and it will keep shifting as long as the nitrogen is kept flowing. Indeed the first introduction of butane induces a first shift of the Bragg wavelength; the second introduction will also induce another shift that will be added to the first one. No desorption was realized between the two introduction of butane, so the final cleaning is more complete and induces a bigger negative shift. This is due to the fact that several tests were performed successively without a systematic complete cleaning after each test. As the nitrogen is kept flowing, the wavelength will keep shifting until we get a perfectly clean surface. A new injection of butane is then started for 5 s (0.5% of butane); the same reactivity is observed again with a wavelength shift of 0.03 nm this time as the half of the previous shift for half the concentration. A new injection of 10 s at 12:02 increases the shift by 0.04 nm before a final cleanout by nitrogen. In order to verify the repetitiveness of the results, several injections of butane were performed, lasting 15 s each (1% concentration). The tests repeated on several days reproduced the same wavelength shift each time (∼0.05 nm for 1%). Fig. 5 shows the case of two successive injections, verifying the sensibility and the speed of detection when the sensor is exposed to the same concentration and hence the reproducibility of the detection sequence. The reversibility is also confirmed with the nitrogen flow. Consequently a ZnO sensor is proven very suitable for butane detection as sensitivity of 0.05 nm was detected
Fig. 5. Optical response of the sensor to butane. This figure shows the reproducibility and the reversibility of the sensor when it was exposed to 1% butane.
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initial state (zero shift with respect to the reference). The actual set up of the experiment is not considered suitable for detecting low concentration of methane due the relatively large response time which is about 3 min. 4. Conclusion
Fig. 6. Optical response of the sensor to methane in a nitrogen atmosphere. Continuous shifting of the wavelength is observed with no reversibility of the sensor with nitrogen.
for only 1% gas concentration with the possibility of desorption by a nitrogen flow. Better sensitivity is thought to be possible with a better desorption of the material’s surface, sensibilities of 0.1% seem to be achievable with the current monitoring resolution. The same experiment is repeated using methane instead of butane. Methane is proven very difficult to detect by this technique due to the fact that its refractive index (1.000444) is very close to the refractive index of nitrogen (1.000297) so a sensibility of at least 10−4 is needed to observe a significant shift of the Bragg wavelength. Successive injections of methane and nitrogen are shown in Fig. 6, continuous shifting of the wavelength is observed with no reversibility of the sensor with nitrogen. In our device the adsorption of gas molecules at the surface will change the refractive index around the waveguide. This variation will change the effective refractive index of the guided mode at the level of the Bragg grating, so the wavelength shifts. When the refractive index of the tested gas is similar to the index of nitrogen, the variation is very small to be detected. In the case of methane the introduction of nitrogen in the chamber does not prove to be useful to reverse the response of the sensor. Instead, ambient air was introduced for 20 min as shown in Fig. 7. After a stabilization time, the sensor regains its initial Bragg wavelength at 12:43 when a new introduction of methane begins for 3 min. Between 12:46 and 13:13, a significant shift of the Bragg wavelength is noticed then ambient air is introduced again until 13:19. A stabilization period is started so the sensor stabilizes at an intermediate level due to simultaneous presence of both air and methane in the chamber. Air introduction is continued starting at 13:23 until 13:34, the sensor regains its
In summary, ZnO based sensor is proven to be appropriate as a detector for low concentrations of gases. The dynamics of detection are important, achieving a quasi-instantaneous response time depending on the concentration of the tested gas. Two explosive gases were examined, butane and methane, where the detection of butane was significant and satisfying; Bragg wavelength shifts of 0.05 nm are achievable for a 1% concentration of gas. The possibility of desorption with a nitrogen flow is demonstrated, the sensor returns systematically to a lower wavelength. Tests with methane were not as successful but not impossible, using a substitute for nitrogen could eventually improve the sensibility. However, zinc oxide is a very easy material in terms of fabrication, the quality of its surface is proved to be very efficient in collecting gas molecules by adsorption. The total cost of fabrication of our sensor is relatively low due to the use of very simple techniques such as the dip coating to obtain the waveguide. Our method also benefits from the advantages of using UV light to directly induce a refractive index difference with a much defined structure and better resolution than the classic photolithography. The ultimate aim of this work is to define a detection protocol to rapidly localize and quantify small leakages of explosive gases for an eventual installation of a security system. References [1] T. Seiyama, A. Kato, K. Fulishi, M. Nagatani, Analytical Chemistry 34 (1962) 1502. [2] M. Ahn, K. Park, J. Heo, D. Kim, K. Choi, J. Park, Sensors and Actuators B 138 (2009) 168. [3] H. Xu, X. Liu, D. Cui, M. Li, M. Jiang, Sensors and Actuators B 114 (2006) 301. [4] Z. Yanga, Y. Huang, G. Chena, Z. Guoc, S. Cheng, S. Huang, Sensors and Actuators B 140 (2009) 549. [5] H. Gong, J. Hu, J. Wang, C. Ong, F. Zhu, Sensors and Actuators B 115 (2006) 247. [6] B. Geng, J. Liu, C. Wang, Sensors and Actuators B 150 (2010) 742. [7] N. Al Hardan, M. Abdullah, A. Abdul Aziz, Physica B 405 (2010) 4509. [8] C. Chang, S. Hung, C. Lin, C. Chen, E. Kuo, Thin Solid Films 519 (2010) 1693. [9] D. Patil, L. Patil, Sensors and Actuators B 123 (2007) 546. [10] P. Bhattacharyya, P. Basu, H. Saha, S. Basu, Sensors and Actuators B 124 (2007) 62. [11] S. Youssef, P. Combette, J. Podlecki, R. Al Asmar, A. Foucaran, Crystal Growth and Design 9 (2009) 1088.
Biographies Sami Youssef received the Ph.D. degree in electronics, optoelectronics, and systems from Montpellier II University, France, in 2009. He is a specialist in the fabrication and characterization of pyroelectric and piezoelectric materials for the realization of microelectromechanical systems and gas sensors. His research interests include the modeling of these MEMS devices. He is currently involved in the study of magnetic hybrid materials for biomedical applications and metal oxide materials for nanoscaled optical sensors. Jean Podlecki received the Ph.D. degree in electronics, optoelectronics, and systems from Montpellier II University, in 1995. Since then, he has worked on MEMS and has been involved in the setup of two startups. He is currently an assistant professor with the Southern Electronics Institute, Montpellier II University. His research interests include the study of thermoelectric materials and their applications to fast micro and nano-thermal sensors. Roland Habchi is actually an associate professor of physics and microelectronics at the Lebanese University. He obtained his Ph.D. in 2007 from the University of Perpignan, France. He has written a number of research papers in the field of microelectronics and materials science. His current research activities include magnetic materials, nanoscaled sensors and mechanical properties of thin films using AFM.
Fig. 7. Optical response of the sensor to methane, as tested in ambient air.
Marwan Brouche holds a Ph.D. degree in energetic from the Grenoble Institute of Technology, in 1994. He is actually an associate professor of physics at Saint Joseph University. His research interests include the study of metal oxide materials for gas sensor applications.
S. Youssef et al. / Sensors and Actuators B 173 (2012) 391–395 Alain Foucaran was born in Le Vigan, France. Since 1986, he has been a professor of electronics at Montpellier II University, France. He is currently director of the Southern Electronics Institute, specializing in preparation and properties of porous silicon. He is involved in thermal sensors for humidity measurements and biochemicals for pesticides detection. David Bouvier, with a Ph.D. in optronics and an engineering degree in microelectronics and automation, joined KLOE in 2006 to establish an Electronic Department within the company. Now responsible for the equipment line, he contributes to the development of new micro-technology equipment.
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Nicolas Brillouet, an optoelectronics and photonics engineer by training, began his career in Canada as a product and optical test engineer. With over 10 years of experience in photonics, he is the technical lead for major industrial projects in which KLOE participates. Paul Coudray holds a Ph.D. in optoelectronics and photonics and over 20 years of experience in optics and photonics. He helped create two photonic start-up companies in Canada as well as a photonics university research team prior to founding his own company in France. He is the author of more than 60 scientific papers in refereed journals and international conferences, and over a dozen patents.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Innovative prototype of a zinc-oxide based optical gas sensor Sami Youssef a,b,c,∗ , Jean Podlecki a , Roland Habchi b , Marwan Brouche c , Alain Foucaran a , David Bouvier d , Nicolas Brillouet d , Paul Coudray d a
Institut d’Electronique du Sud, IES, Université Montpellier II, UMR CNRS 5214, Place Eugène Bataillon, 34095 Montpellier, France Laboratoire de Physique Appliquée (LPA), Département de Physique, Université libanaise, Faculté des sciences II, 90656 Jdeidet, Lebanon Ecole supérieure d’ingénieurs de Beyrouth, Campus des Sciences et Technologies, Mar Roukoz, Lebanon d Société KLOE, Hôtel d’Entreprise du Millénaire, 1068 Rue de la Vieille Poste, 34000 Montpellier, France b c
a r t i c l e
i n f o
Article history: Received 25 September 2011 Received in revised form 26 June 2012 Accepted 3 July 2012 Available online 17 July 2012 Keywords: Zinc oxide Bragg reflector Waveguide Gas sensor
a b s t r a c t A zinc-oxide based optical sensor is realized. This prototype is based on the properties of Bragg reflectors, first the measurement principles are described then the experimental results are presented. This sensor has proven to be sensitive and reliable in the case of detecting low concentrations of butane. The sensor is stabilized and desorbed by a flow of nitrogen. In the case of methane detection, the use of nitrogen was found inappropriate, ambient air has proven to be more suitable. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide ZnO has been known to have changeable electric properties in the presence of gases [1,2] so, many devices have been reported [3–7] to have a ZnO part as a sensing material to detect gas concentrations. Developing highly sensitive devices is of major importance for the scientific community [8–10] especially when it concerns environmental and security issues, such as monitoring gas leakages in oil companies and industrial facilities or even observing the evolution of pollutant gases that could reach the environment. In this paper we report the realization of a gas sensor used to detect small concentrations of methane and butane. But unlike the majority of ZnO based gas sensors, the reported device is based on a new approach that uses the optical properties of a Bragg grating. When a certain area of the grating is covered with active ZnO, a shift in the Bragg wavelength is observed when gas molecules are adsorbed at the surface which provides a detection mechanism to monitor gas leakages. 2. Measurement principle In order to study the response of the optical sensor, a special waveguide was conceived using a photosensitive hybrid
∗ Corresponding author at: Institut d’Electronique du Sud, IES, Université Montpellier II, UMR CNRS 5214, Place Eugène Bataillon, 34095 Montpellier, France. E-mail address: [email protected] (S. Youssef). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.07.025
organomineral material based on acrylate. This material is sensitive to UV light. The synthesis procedure is realized by a sol–gel technique. The sol is obtained by the use of two organic–inorganic precursors while the mineral lattice is attained from a hydrolysis and a polycondensation of alkoxides. A UV light is used to polymerize C C bonds. Polymerization of C C bonds could be done be either light or high temperature. In this case we have chosen to use UV light due to the nature of our material which is a photosensitive hybrid organomineral material. Any damage resulting from high temperature is therefore prevented. Once the solution is synthesized, low cost deposition methods such as spin-coating and dip-coating are used to put the layers over a silicon substrate. The gel obtained is thermally annealed to get a final dry glass product. The technique used in this work is the dip-coating which is a fast, low cost and easily industrialized process. UV light is then used to create a periodic difference of refractive index. No masks are used with this technique where the material is exposed to the UV beam which induces successive changes of the refractive index resulting into a high quality Bragg grating. The optimized structure is a buried waveguide as shown in Fig. 1a, a photo of the actual sensor is given in Fig. 1b, where a buffer layer having a refractive index n1 is deposited over the silicon substrate followed by another layer having a refractive index n2 . The layer of index n1 covers the device where a window is engraved to grow a layer of ZnO by reactive magnetron sputtering at room temperature [11]. Two successive distributed Bragg reflectors are obtained; the first is the reference reflector and the second is covered by an active layer of ZnO. This particular disposition is used to realize a differential
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Fig. 1. (a) Representation of the sensor with its two Bragg reflectors. (b) A photo of the optical gas sensor installed in its final package.
measurement where two signals are obtained out of the waveguide, a first one unchanged and a second one modified by the presence of a gas on the sensitive area. Here each reflector has a specific Bragg wavelength but they both undergo the same temperature and pressure variations. Fig. 2 highlights the wavelength shift experienced by the two Bragg gratings as the temperature varies. The blue curve is the sensors’ response in initial conditions, while the curve in red represents the new response after a temperature increase. Therefore we can notice that the two gratings, one as reference and one
for measuring, experienced the same wavelength shift. However, the adsorption of a gas on the sensitive area induces a change in the refractive index and therefore a change of the propagation properties in the waveguide, as a result a shift in the transmitted Bragg wavelength is observed. Indeed, pressure stresses the entire chip evenly, same as when temperature contracts or dilates the device also evenly. But the temperature effect on the refractive index is neglected with respect to the variation induced by pressure; this is an experimental fact that was controlled during our work. The experimental protocol is as follows: a nitrogen cylinder and a methane (or butane) cylinder are connected to an hermetically sealed chamber in such a way to be able to alternatively inject the methane or butane and then flush it with nitrogen which is considered to be a neutralizing gas used to ensure the desorption of the reflector. Fig. 3 shows a schematic of the testing setup. The length of each pipe and the position of the valves have been chosen in such a way to prevent any dead zones that are likely to retain small amounts of one gas while another gas is injected into the chamber. That would be a potential source of contamination and misinterpretation of the detected signal. The chamber is also equipped with a thermometer to monitor any temperature changes that would justify a shift in wavelength of the two Bragg reflectors. A pressure gauge is installed on the injection pipe to ensure that there will be no pressure variations that may influence the measurement. The injection and the extraction of gases are done through valves to guarantee a good tightness of the chamber, which is essential in saturation and stabilization phases. The sensor is inserted into the chamber, and optical fibers are coupled through cable glands. The monitoring system is composed of a tunable laser source, a photodetector, and special software controlling all the equipment. The laser is a class 1M single mode tunable laser, centered in the C-band window, and which is thermally regulated and is part of complete interrogating system for photonics sensors. Emission wavelength is permanently measured thanks to a wavelength meter associated to the experimental setup. The tunable laser provides a linear sweep of wavelength with a given start/stop wavelength and a given rate, and the photodetector is used to make an accurate optical power measurement. The control software includes several features such as the synchronization of the laser source with the detector, processing and finally displaying data. A measurement sequence begins when the laser and the photodetector are synchronized, then the laser scans the chosen wavelength range with a specified increment and goes into a continuous scanning mode. During the scan, the photodetector retrieves the optical power that corresponds to each wavelength and finally a spectrum is displayed. The following measurement sequence begins immediately after the end of the first. 3. Results A series of sensitivity tests are performed first with butane. At the beginning nitrogen is introduced into the chamber for 15 min to evacuate all possible pollutants and to ensure desorption of the sensor. When the inner atmosphere is stabilized for several minutes and no gas is detected, butane is injected. Considering that the volume of the chamber is 0.15 m3 and the flow of gas is 0.48 m3 /h at 28 mbar, the necessary injection time for a certain concentration of butane could be calculated using the following equation: tinj =
Fig. 2. The transmission spectrum of the two Bragg gratings of our waveguide, the peaks indicate the Bragg wavelength. The two Bragg reflectors experienced the same wavelength shift as the temperature changes.
%gas × Vchamber debit
(1)
Therefore an injection time of 11.2 s is needed to get a butane concentration of 1%. Fig. 4 represents data for successive injections of nitrogen and butane taken at real time. The measured parameter is the relative variation of the specific Bragg wavelength as a function of time. The
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Fig. 3. A schematic view of the experimental set up, showing the gas tubes, the valves, and the position of the detector.
figure is divided into successive injections of nitrogen, stabilization time and butane injection. The first injection of nitrogen is stopped after 20 min (10:10), here the curve is not perfectly leveled to consider a full desorption. A stabilization period is started followed by an 11 s injection of butane (1% of the volume) at “10:17”, 30 s later the butane starts to be detected and eventually a 0.055 nm shift of the Bragg wavelength is observed after 10 min. The curve is then leveled, which means that all active sites on the surface are occupied by butane molecules and no more adsorption is possible. The decrease of the curve between 10:31 and 10:40 is due to temperature variation in the chamber as noticed from the readings of our reference sensor, because when temperature changes, the entire material is subjected to some dilation or contraction, which leads to a shift in both Bragg peaks by the same amplitude. At 10:40 nitrogen is introduced again for 15 min and a rapid change in the Bragg wavelength is observed after less than 5 min scoring a shift of −0.08 nm. When the nitrogen flow is stopped at 10:55, the system is left to stabilize until 11:46 where the curve is perfectly leveled and the wavelength is stable at a value less than the first introduction of nitrogen. In fact, the cleaning process is done with the introduction of a nitrogen flow in the chamber and when the gas molecules are desorbed, the wavelength shift is negative
Fig. 4. Optical response of the sensor to butane. The curve is divided into consecutive injections of nitrogen, stabilization time and butane injection. A 0.03 and 0.055 nm shift of the Bragg wavelength is recorded for 0.5 and 1% butane respectively. The sensor is desorbed by a flow of nitrogen.
and it will keep shifting as long as the nitrogen is kept flowing. Indeed the first introduction of butane induces a first shift of the Bragg wavelength; the second introduction will also induce another shift that will be added to the first one. No desorption was realized between the two introduction of butane, so the final cleaning is more complete and induces a bigger negative shift. This is due to the fact that several tests were performed successively without a systematic complete cleaning after each test. As the nitrogen is kept flowing, the wavelength will keep shifting until we get a perfectly clean surface. A new injection of butane is then started for 5 s (0.5% of butane); the same reactivity is observed again with a wavelength shift of 0.03 nm this time as the half of the previous shift for half the concentration. A new injection of 10 s at 12:02 increases the shift by 0.04 nm before a final cleanout by nitrogen. In order to verify the repetitiveness of the results, several injections of butane were performed, lasting 15 s each (1% concentration). The tests repeated on several days reproduced the same wavelength shift each time (∼0.05 nm for 1%). Fig. 5 shows the case of two successive injections, verifying the sensibility and the speed of detection when the sensor is exposed to the same concentration and hence the reproducibility of the detection sequence. The reversibility is also confirmed with the nitrogen flow. Consequently a ZnO sensor is proven very suitable for butane detection as sensitivity of 0.05 nm was detected
Fig. 5. Optical response of the sensor to butane. This figure shows the reproducibility and the reversibility of the sensor when it was exposed to 1% butane.
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initial state (zero shift with respect to the reference). The actual set up of the experiment is not considered suitable for detecting low concentration of methane due the relatively large response time which is about 3 min. 4. Conclusion
Fig. 6. Optical response of the sensor to methane in a nitrogen atmosphere. Continuous shifting of the wavelength is observed with no reversibility of the sensor with nitrogen.
for only 1% gas concentration with the possibility of desorption by a nitrogen flow. Better sensitivity is thought to be possible with a better desorption of the material’s surface, sensibilities of 0.1% seem to be achievable with the current monitoring resolution. The same experiment is repeated using methane instead of butane. Methane is proven very difficult to detect by this technique due to the fact that its refractive index (1.000444) is very close to the refractive index of nitrogen (1.000297) so a sensibility of at least 10−4 is needed to observe a significant shift of the Bragg wavelength. Successive injections of methane and nitrogen are shown in Fig. 6, continuous shifting of the wavelength is observed with no reversibility of the sensor with nitrogen. In our device the adsorption of gas molecules at the surface will change the refractive index around the waveguide. This variation will change the effective refractive index of the guided mode at the level of the Bragg grating, so the wavelength shifts. When the refractive index of the tested gas is similar to the index of nitrogen, the variation is very small to be detected. In the case of methane the introduction of nitrogen in the chamber does not prove to be useful to reverse the response of the sensor. Instead, ambient air was introduced for 20 min as shown in Fig. 7. After a stabilization time, the sensor regains its initial Bragg wavelength at 12:43 when a new introduction of methane begins for 3 min. Between 12:46 and 13:13, a significant shift of the Bragg wavelength is noticed then ambient air is introduced again until 13:19. A stabilization period is started so the sensor stabilizes at an intermediate level due to simultaneous presence of both air and methane in the chamber. Air introduction is continued starting at 13:23 until 13:34, the sensor regains its
In summary, ZnO based sensor is proven to be appropriate as a detector for low concentrations of gases. The dynamics of detection are important, achieving a quasi-instantaneous response time depending on the concentration of the tested gas. Two explosive gases were examined, butane and methane, where the detection of butane was significant and satisfying; Bragg wavelength shifts of 0.05 nm are achievable for a 1% concentration of gas. The possibility of desorption with a nitrogen flow is demonstrated, the sensor returns systematically to a lower wavelength. Tests with methane were not as successful but not impossible, using a substitute for nitrogen could eventually improve the sensibility. However, zinc oxide is a very easy material in terms of fabrication, the quality of its surface is proved to be very efficient in collecting gas molecules by adsorption. The total cost of fabrication of our sensor is relatively low due to the use of very simple techniques such as the dip coating to obtain the waveguide. Our method also benefits from the advantages of using UV light to directly induce a refractive index difference with a much defined structure and better resolution than the classic photolithography. The ultimate aim of this work is to define a detection protocol to rapidly localize and quantify small leakages of explosive gases for an eventual installation of a security system. References [1] T. Seiyama, A. Kato, K. Fulishi, M. Nagatani, Analytical Chemistry 34 (1962) 1502. [2] M. Ahn, K. Park, J. Heo, D. Kim, K. Choi, J. Park, Sensors and Actuators B 138 (2009) 168. [3] H. Xu, X. Liu, D. Cui, M. Li, M. Jiang, Sensors and Actuators B 114 (2006) 301. [4] Z. Yanga, Y. Huang, G. Chena, Z. Guoc, S. Cheng, S. Huang, Sensors and Actuators B 140 (2009) 549. [5] H. Gong, J. Hu, J. Wang, C. Ong, F. Zhu, Sensors and Actuators B 115 (2006) 247. [6] B. Geng, J. Liu, C. Wang, Sensors and Actuators B 150 (2010) 742. [7] N. Al Hardan, M. Abdullah, A. Abdul Aziz, Physica B 405 (2010) 4509. [8] C. Chang, S. Hung, C. Lin, C. Chen, E. Kuo, Thin Solid Films 519 (2010) 1693. [9] D. Patil, L. Patil, Sensors and Actuators B 123 (2007) 546. [10] P. Bhattacharyya, P. Basu, H. Saha, S. Basu, Sensors and Actuators B 124 (2007) 62. [11] S. Youssef, P. Combette, J. Podlecki, R. Al Asmar, A. Foucaran, Crystal Growth and Design 9 (2009) 1088.
Biographies Sami Youssef received the Ph.D. degree in electronics, optoelectronics, and systems from Montpellier II University, France, in 2009. He is a specialist in the fabrication and characterization of pyroelectric and piezoelectric materials for the realization of microelectromechanical systems and gas sensors. His research interests include the modeling of these MEMS devices. He is currently involved in the study of magnetic hybrid materials for biomedical applications and metal oxide materials for nanoscaled optical sensors. Jean Podlecki received the Ph.D. degree in electronics, optoelectronics, and systems from Montpellier II University, in 1995. Since then, he has worked on MEMS and has been involved in the setup of two startups. He is currently an assistant professor with the Southern Electronics Institute, Montpellier II University. His research interests include the study of thermoelectric materials and their applications to fast micro and nano-thermal sensors. Roland Habchi is actually an associate professor of physics and microelectronics at the Lebanese University. He obtained his Ph.D. in 2007 from the University of Perpignan, France. He has written a number of research papers in the field of microelectronics and materials science. His current research activities include magnetic materials, nanoscaled sensors and mechanical properties of thin films using AFM.
Fig. 7. Optical response of the sensor to methane, as tested in ambient air.
Marwan Brouche holds a Ph.D. degree in energetic from the Grenoble Institute of Technology, in 1994. He is actually an associate professor of physics at Saint Joseph University. His research interests include the study of metal oxide materials for gas sensor applications.
S. Youssef et al. / Sensors and Actuators B 173 (2012) 391–395 Alain Foucaran was born in Le Vigan, France. Since 1986, he has been a professor of electronics at Montpellier II University, France. He is currently director of the Southern Electronics Institute, specializing in preparation and properties of porous silicon. He is involved in thermal sensors for humidity measurements and biochemicals for pesticides detection. David Bouvier, with a Ph.D. in optronics and an engineering degree in microelectronics and automation, joined KLOE in 2006 to establish an Electronic Department within the company. Now responsible for the equipment line, he contributes to the development of new micro-technology equipment.
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Nicolas Brillouet, an optoelectronics and photonics engineer by training, began his career in Canada as a product and optical test engineer. With over 10 years of experience in photonics, he is the technical lead for major industrial projects in which KLOE participates. Paul Coudray holds a Ph.D. in optoelectronics and photonics and over 20 years of experience in optics and photonics. He helped create two photonic start-up companies in Canada as well as a photonics university research team prior to founding his own company in France. He is the author of more than 60 scientific papers in refereed journals and international conferences, and over a dozen patents.