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Distributed chemical sensing
Sensors and Actuators B 45 (1997) 71 – 77
Distributed chemical sensing T. Eklo¨v *, H. Sundgren, I. Lundstro¨m Laboratory of Applied Physics and S-SENCE, Linko¨ping Uni6ersity, S-58251 Linko¨ping, Sweden Received 3 December 1996; received in revised form 28 July 1997; accepted 18 August 1997
Abstract When a gas mixture is passed over a catalytic surface its composition will change due to the catalytic activity of the surface. Different molecules have different reaction rates and this fact can be used in combination with distributed chemical sensors to increase the discriminating capability of the sensors. In a measurement cell (palladium covered or non-covered) a row of identical Pd-MOSFET sensors was mounted along the flow direction. The catalytically active measurement cell changed the concentration of detected molecules (hydrogen or ethanol) considerably compared to the non-catalytic cell. It was observed that even catalytic combustion in the lid 1 mm above the sensor row influenced the sensor responses significantly. When comparing hydrogen and ethanol it was found that the two molecules were influenced to different degrees when passing the catalytic surfaces. Hydrogen was consumed at a much faster rate, which could be seen when comparing the sensor responses along the row. A distributed sensor system thus shows an increased discriminating power. It was also found that chemical sensing was much more temperature dependent for ethanol than for hydrogen. © 1997 Elsevier Science S.A. Keywords: Pd-MOSFET gas sensor; Distributed sensor system; Catalytic surface; Selectivity
1. Introduction It is of considerable interest to find ways of changing the selectivity patterns of chemical sensors. A number of different possibilities have been described in connection with the different physical principles used for the sensing devices [1–4]. Catalytic metals are used, both as gates of gas sensitive field effect devices, [5,6] and as additives in gas sensors based on semiconducting metal oxides [1,7]. The choice of catalytic metal and the operation temperature of the device determine the selectivity pattern of such sensors. Catalysts have also been used to combust certain components in a gas mixture before it reaches the sensor, which changes the response pattern of the catalyst-sensor combination and thus increases the selectivity of the system [8,9]. We have previously shown that this possibility can be further developed by using a large catalytic sensing surface, combining the combustion and chemical sensing in a distributed but locally resolved way [10,11]. Such sensing surfaces can be used * Corresponding author. tel.: + 46 13 282632; fax: +46 13 288969; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 7 2 - 4
to produce response patterns that are different for different molecules or odours [10,11]. This was achieved by using a scanning light pulse technique to monitor chemically induced work function changes in the thin catalytic metal used as a gate of a metal-oxide-semiconductor structure. The work function changes were monitored as changes in the photo capacitive current generated by the scanning light pulse. A schematic of the experimental set-up used to produce the response patterns is shown in Fig. 1. The patterns occurred due to a varying gas composition along the catalytic metal surface and a varying chemical sensitivity due to a temperature gradient along the sensing surface. The details of the obtained response patterns are, however, not only determined by the catalytic activity of the metal surface but also by the turbulence in the gas flow and edge effects, etc. Furthermore, the scanning light pulse technique, interesting from a physical and fundamental point of view, is rather impractical for a device utilising the ‘distributed chemical sensing’ concept. A question of fundamental interest is whether it is possible to separate the combustion from the detection as schematically illustrated in Fig. 2. The present contribution deals with the performance of a system con-
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sisting of a (continuous) catalytic surface and distributed discrete chemical sensors. In our case the chemical sensors were gas sensitive field effect devices [5], but the sensors in Fig. 2 can, of course, be of any kind suitable for mounting or fabricating in an array. To obtain some basic knowledge of a system like that in Fig. 2 we chose to study one catalytic metal, palladium (Pd), and two molecules, hydrogen and ethanol, which behave differently on palladium. The gates of the distributed field effect transistors used were also made of palladium. Furthermore, we had a constant temperature along the sample to more specifically study the influence of the catalytic combustion on the sensor signals.
2. Experimental set-up In this study palladium gate metal-oxide-semiconductor field effect transistors (MOSFET) were used as sensors [5]. The sensors, fabricated at our laboratory, had the gate oxide covered with a thin (6 nm) discontinuous palladium film. The area covered with palladium was 0.1× 0.1 mm2. Seven sensors were mounted in a row, evenly spaced at the bottom of a measurement cell (Fig. 2). The measurement cell had the dimensions 10× 20× 1 mm3 and the separation between neighbouring sensors was 2.8 mm. The bottom and/or the top of the cell could be provided with a thick (100 nm) catalytically active, thermally evaporated layer of palladium. The sensors had a height of 0.2 mm, giving a distance of 0.8 mm to the top. The cell was made of stainless steel and was provided with tubes (ø 0.7 mm) to provide a gas inlet and outlet. The gas entered the cell at one end and flowed over the sensors to the other end (Fig. 2). A temperature diode and two heating resistors, one at the top and one below the sensors, covering the whole length of the measurement cell, were also placed in contact with the measurement cell (Fig. 2). The total gas flow rate was kept constant at 10 ml min − 1 during all measurements. One measurement cycle consisted of first flowing technical air (20% O2 in nitrogen) over the sensors for 4 min. Then the sensors were exposed to the test gas (H2 or ethanol) mixed with technical air for 2 min. The oxygen level was constant at 20% of the total flow. During the measurements the gate voltage of each MOSFET sensor was measured at a constant drain current, using a sampling frequency of 1 Hz. The response from a sensor was defined as the difference between gate voltages just before the exposure to the gas (hydrogen or ethanol) and at the end of the exposure. The gas mixing system was controlled by a PC, which also collected data from the sensors.
Fig. 1. Schematic experimental set-up for the scanning light pulse technique used to produce two-dimensional response maps of gas mixtures. The induced photo capacitive current iph is measured as the chopped light beam scans over the surface. With a temperature gradient (T1 " T2) and different catalytic metals, e.g. Pt, Pd and Ir, a response map of the gas mixture is produced. The gas flow is parallel to the sensor surface.
3. Results and discussion
3.1. Hydrogen: basic measurements During the measurements on hydrogen the measurement cell was kept at a temperature of 140°C. A typical result from two measurements, with the gas flowing through the measurement cell in different directions, is shown in Fig. 3. The sensors were exposed to two pulses of hydrogen (20 and 10 ppm) in technical air. The slow transient response is partly due to the gas
Fig. 2. Top and side view of the measurement cell indicating sensor configuration, catalytic metal placement and gas flow.
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Fig. 3. Raw data from measurements on hydrogen using different flow directions. The seven sensors are exposed to two pulses of hydrogen, 20 and 10 ppm, respectively, in a mixture with technical air and between pulses only technical air. The dashed arrow defines the response of a sensor.
mixing time in the sample cell and partly due to the response characteristics of the sensors. The effect from the catalytic surface can be seen as the decrease of the height of the response peaks along the flow direction. By calculating the responses for each peak the result is easier to interpret. The results for the 20 ppm measurements using a catalytic lid and bottom are summarised in Fig. 4. One observation is that the sensors are not exactly similar but have slightly different response characteristics, causing the responses not to change continuously along the row of sensors. Obviously, the sensors could not be fabricated to exactly the same characteristics, resulting in slightly different characteristics for each sensor. Because of this, the absolute difference between flow directions for each sensor, as defined in Eq. (1), is used in the following analysis. absolute difference= responseflow 1 7 −responseflow 7 1 (1) The absolute difference is less sensitive than the responses to differences in sensor response characteris-
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Fig. 5. Absolute difference of the responses between flow directions for different hydrogen concentrations. Measurements were made using either catalytic lid and bottom or no catalytic surface at all.
tics. If the gas is catalytically changed the absolute difference is a monotonically decreasing function along the catalytic area. In the further analysis the absolute difference will be used to characterise the properties of the catalytic or non-catalytic cell.
3.2. Hydrogen: catalytic effect In Fig. 5 the absolute difference in response between the two flow directions is calculated for all seven sensors, exposing them to pulses of 10, 20, 50 and 100 ppm H2 and using both a palladium covered lid and bottom or a non-catalytic measurement cell. It is, thus, possible to detect catalytic consumption that occurs physically separated from the sensors, since the absolute difference (or change in response), between the two flow directions is considerably larger for the catalytic sample cell. Furthermore, for the measurements using the catalytic cell, the absolute difference increases as the hydrogen concentration increases. This is reasonable since, to a first approximation, the amount of hydrogen consumed is proportional to the total amount of hydrogen. For increasing hydrogen concentration this results in an increasing change in concentration and, concomitantly, an increasing change in sensor response due to the catalytic surface. Also, in the non-catalytic case there is an observable catalytic consumption since the difference between the flow directions increases for a higher H2-concentration. Each sensor has a small area covered with palladium that can account for this effect. Still, this difference is small compared to the catalytic case and is therefore neglected in the further analysis.
3.3. Hydrogen: concentration effect Fig. 4. Comparison between the two flow directions when exposing the sensors to 20 ppm H2 in technical air using the palladium coated measurement cell.
The relative difference between flow directions for each sensor is defined as:
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relati6e difference =absolute difference/a6erage ×(responseflow 1 7, responseflow 7 1) (2) The relative difference should be independent of the concentration, assuming that the sensors have a linear concentration dependence. However, this is not the case as the following analysis will show. The response of the Pd-MOSFET sensors to hydrogen is a saturated function of the hydrogen concentration and the response (DV) follows closely [12]: response= DVmaxa PH 2/(1 +a PH 2)
(3)
where DVmax and a are sensor parameters and PH 2 is the hydrogen concentration in ppm. Such a so called dissociative Langmuir isotherm has been found also in previous studies [13]. Even if it can be justified by a simple model for the detection of hydrogen by an oxygen covered Pd-surface, we regard it as an empirical relationship. For the devices used in the present investigation typical values of the constants in Eq. (3) were DVmax : 0.6 V and a : 0.03 (ppm) − 1/2, at a temperature of 140°C. These values were obtained from pulse responses similar to those in Fig. 3. In comparison with the results in [13], DVmax is somewhat smaller (0.6 V compared to 0.83 V) and the parameter a is approximately half of that in [13]. These differences can be due to many factors, e.g. different palladium structure, or different surface contaminations depending on different operating conditions. The influence of the saturated response is displayed in Fig. 6, where the relative catalytic effect of the measurement cell compared to an ideal non-catalytic cell is calculated as half of the relative response change between the first and last sensor in the row. The calculations were made for varying hydrogen concentrations and both the catalytic and non-catalytic measurement cell. For the catalytic measurement cell the curve is clearly decreasing with
Fig. 7. Measured relative response change between flow directions when exposing the sensors to pulses of 100 ppm hydrogen in technical air. Measurements were made having a catalytic metal both at the bottom and top or only at the bottom of the measurement cell.
increasing H2-concentration, confirming that the relative effect is affected by the saturation in the response characteristics. For the non-catalytic case the relative difference is constant and close to zero, again showing that if the measurement cell is non-catalytic the difference between flow directions can be neglected. From Eq. (3) it is possible to estimate the change in hydrogen concentration from the first to the last sensor in the case of a catalytic measurement cell. The response to 20 ppm hydrogen is approximately 80 mV and the change is about 20 mV. A sensor with DVmax = 0.6 V and a= 0.034 (ppm) − 1/2 gives DV =80 mV for 20 ppm H2 and DV =60 mV for 10 ppm H2, i.e. a decrease in DV of 20 mV corresponds to a relative change in hydrogen concentration of 50%. Even if this is a crude estimate we can conclude that an appreciable amount of the hydrogen is catalytically combusted, but the form of the response isotherm (Eq. (3)) makes the change in sensor response relatively smaller (around 25%). The decrease in the hydrogen concentration is also visible from the decrease in the flow direction of the initial slope of the responses (Fig. 3).
3.4. Hydrogen: effect of the size of the catalytic area
Fig. 6. The relative catalytic effect of the measurement cell as compared to an ideal non-catalytic cell. The calculations were made for varying hydrogen concentrations and both the catalytic and non-catalytic measurement cell.
To find out how much the lid and the bottom contributed to the total catalytic consumption, measurements were made comparing only a catalytic bottom with both a catalytic top and bottom. The sensors were exposed to pulses of 100 ppm hydrogen with the gas flowing in both directions. The relative difference between the flow directions using different catalytic areas is plotted in Fig. 7. When considering sensor numbers 1, 2, 6 and 7 the relative change is approximately halved if the catalytic area is halved. For the other sensors the relative difference is small compared to the noise and therefore gives no information. Thus,
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the lid and the bottom contribute about equally to the catalytic consumption detected by the sensors. However, the distance from a sensor to the lid is 0.8 mm and to the bottom 0.2 mm. If the flow was perfectly laminar the distance between the catalytic metal and the sensor would have a great influence on the spatial hydrogen concentration profiles, due to mass transport limitations. Hence, a catalyst close to the sensor would affect the measured response more than a catalyst further away [14]. In this case we have similar impact from the lid and bottom. The interpretation of this is that the flow is not perfectly laminar, thus giving a spatial mixing and a more homogeneous spatial gas concentration than in the case with laminar flow. This is an advantage in the present case since the gas concentration at the sensor surface then is more directly influenced by the catalytic area than for a laminar flow. Also, a turbulent flow increases the overall reaction rate as more molecules hit the catalytic surface per unit time.
3.5. Hydrogen and ethanol: temperature effect The hydrogen response for a Pd-MOSFET sensor is relatively independent of the operating temperature [5]. In Fig. 8 the results from measurements on 20 ppm hydrogen in technical air are shown. The operating temperature is 175°C and the response is approximately 50 mV for the first sensor in the row, using both the catalytic and the non-catalytic measurement cell. However, if the responses are compared along the row of sensors, the non-catalytic cell gives almost no decrease in response, while the catalytic surface causes the response to decrease substantially. The decrease is 40 mV when comparing different flow directions, giving a relative decrease of approximately 80% for the first sensor in the row. The catalytic consumption of hydrogen is
Fig. 8. Absolute change in response between flow directions when exposing the sensors to 20 ppm hydrogen. The figure includes measurements when using both the catalytic and the non-catalytic measurement cell.
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Fig. 9. Comparison between ethanol and hydrogen in a palladium covered measurement cell. The temperature was set to 175°C and the gas entered the measurement cell from the left.
therefore substantial at this temperature. Thus, the catalytic consumption clearly depends on the temperature. There is no direct correlation between the chemical sensing and catalytic combustion of hydrogen on palladium. Ethanol can also be monitored with Pd-MOSFET sensors. At temperatures below about 140°C, ethanol is very difficult to detect [15]. It has been previously observed that the response of palladium gate devices to ethanol is strongly temperature dependent [15]. A comparison between hydrogen and ethanol at 175°C is shown in Fig. 9, where succeeding measurements on 20 ppm hydrogen and 400 ppm ethanol were made, with the gas passing through a catalytic measurement cell. In this figure, a large difference in response pattern (catalytic consumption) between hydrogen and ethanol can be seen. The difference in response is small for the first sensor in the row, while hydrogen shows a much smaller response than ethanol for the last sensor. By using information from both the first and last sensors, it is thus possible to discriminate between the two molecules. This shows that using a catalytic metal spatially separated from the sensors can further increase the discriminatory ability of the sensors. The difference between ethanol and hydrogen is also clear from Table 1, where the response and change in response between the first and last sensors in the row to hydrogen and ethanol is compared at different operating temperatures. The change in response value indicates the rate of catalytic consumption, as well as the influence of temperature on the catalytic consumption and the chemical sensing for hydrogen and ethanol. Some observations can be made from Table 1. The predictions in the literature concerning the temperature dependence of the response is validated. Chemical sensing of hydrogen is relatively constant at the temperatures investigated, the detected decrease in response being mainly due to ageing of the device. Also, ethanol sensing shows strong temperature dependence, which is
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Table 1 Relative change in response between the first and the last sensors in the row in a catalytic measurement cell Test molecule
Concentration (ppm)
Temperature (°C)
Response (mV)
Change in response (%)
Hydrogen Hydrogen Ethanol Ethanol
20 20 400 400
140 175 140 175
70 50 10 50
25 82 Not detected 12
consistent with previous studies [15]. Catalytic consumption shows a strong dependence on temperature, especially for hydrogen. At a temperature of 175°C, most of the hydrogen entering the cell is consumed. The behaviour of hydrogen and ethanol is also consistent with the previously observed response patterns using the scanning light pulse technique [11].
3.6. Future outlook The experiments presented in this contribution were made to illustrate the features of distributed chemical sensing using two molecules, which behave differently when reacting with an oxygen covered palladium surface. We have used only one catalytic metal and one row of sensors. Fig. 10 shows schematically some other possibilities available by combining sensor arrays with catalytic surfaces. It is obvious that there is a large number of degrees of freedom. It should therefore be possible by a proper choice of catalysts and sensors to optimise the response for a given situation, e.g. for the identification or classification of molecules or mixtures of molecules (odours). It should be pointed out again that the sensors do not need to be of the MOSFET type. Any chemical sensor suitable to mount or produce in an array can be used. Furthermore, any catalytic surface, metallic or non-metallic can be employed. It is even possible to use a lid and/or bottom in the sample cell, which (selectively) let constituents in the gas diffuse out through from the sample cell. This will also create a decrease of analytes in the flow direction. By having a temperature gradi-
Fig. 10. Schematics of catalyst patterns (in the lid and at the bottom) and sensor arrays (mounted in the bottom of the measurement cell) giving different response patterns to given gas mixtures. T1 and T2 indicates the possibility of influencing these response patterns using a temperature gradient. Different layout of the sensors and catalytic areas can also be used to influence the response pattern.
ent in the sample cell it is possible to combine a decrease in the amount of detectable molecules with a change, normally an increase, in the chemical sensitivity to obtain further discriminating power of the distributed sensor system [11]. It is observed in Fig. 9 that the decrease of the sensor response for hydrogen is approximately exponential in the distance along the sensor surface. From such curves the catalytic reaction probability for a given molecule on the catalytic surface can be estimated, using appropriate models for the consumption of molecules in the sample cell. Such models are under development [14] and will be used in a separate publication to explain e.g. the data in Fig. 9.
4. Conclusions Our studies were made in two steps. Firstly, the possibility of detecting the influence of a catalytic surface spatially separated from a row of chemical sensors was investigated using hydrogen as the test molecule. A comparison between the response to hydrogen in a palladium coated cell and a non-coated cell showed that the effect was clearly visible, with the difference increasing for higher hydrogen concentrations. If the relative effect of a catalytic surface was examined it was smaller at higher hydrogen concentrations, mainly due a saturation in the response to hydrogen. Secondly, two chemically different molecules, hydrogen and ethanol, were compared using the catalytic measurement cell. The comparison showed that the two molecules had visibly different concentration profiles along the row of sensors, where hydrogen apparently was consumed at a much higher rate than ethanol. Furthermore, the sensitivity was considerably more temperature dependent for ethanol than for hydrogen. Thus, it has been shown that by using a catalytically induced concentration gradient in combination with chemical sensors it was possible to increase the discriminatory capability of the sensors. Finally, it was suggested that sensor arrays together with patterned catalytic surfaces with or without a temperature gradient provide many possibilities for making tailor-made sensor systems for odour identification and analysis.
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Acknowledgements Stimulating discussions on the present topic with Drs Per Ma˚rtensson, Anita Lloyd Spetz and Fredrik Winquist are gratefully acknowledged. Our research on chemical sensors is supported by grants from the Swedish Research Council for Engineering Sciences and the National Swedish Board for Industrial and Technical Development (NUTEK). S-SENCE (Swedish Sensor Centre) is financed by Swedish Industry, NUTEK and Linko¨ping University.
References [1] S.J. Gentry, T.A. Jones, The role of catalysis in solid-state gas sensors, Sensors and Actuators 10 (1986) 141–163. [2] M. Nakamura, I. Sugimoto, H. Kuwano, R. Lemos, Chemical sensing by analysing dynamics of plasma polymer film-coated sensors, Sensors and Actuators B 20 (1994) 231–237. [3] R.E. Cavicchi, J.S. Suehle, K.G. Kreider, M. Gaitan, P. Chaparala, Fast temperature programmed sensing for micro-hotplate gas sensors, IEEE Electron Device Lett. 16 (1995) 286 – 288. [4] M.E.H. Amrani, K.C. Persaud, P.A. Payne, High-frequency measurements of conducting polymers: development of a new technique for sensing volatile compounds, Meas. Sci. Technol. 6 (1995) 1500 – 1507. [5] I. Lundstro¨m, A. Spetz, F. Winquist, U. Ackelid, H. Sundgren, Catalytic metals and field-effect devices-a useful combination, Sensors and Actuators B 1 (1990) 15–20. [6] I. Lundstro¨m, C. Svensson, A. Spetz, H. Sundgren, F. Winquist, From hydrogen sensors to olfactory images-twenty years, Sensors and Actuators B 13 (1993) 16–23. [7] K.D. Shierbaum, J. Geiger, U. Weimar, W. Go¨pel, Specific palladium and platinum doping for SnO2-based thin film sensor arrays, Sensors and Actuators B 13–14 (1993) 143–147. [8] J.R. Stetter, S. Zaromb, M.W. Findlay, Monitoring of electrochemical inactive compounds by amperometric gas sensors, Sensors and Actuators 6 (1984) 269–288. [9] J.R. Stetter, M.W. Findlay, G.J. Maclay, J. Zhang, S. Vaihinger, W. Go¨pel, Sensor array and catalytic filament for chemical analysis of vapors and mixtures, Sensors and Actuators B 1 (1990) 43 – 47. [10] I. Lundstro¨m, R. Erlandsson, U. Frykman, E. Hedborg, A. Spetz, H. Sundgren, S. Welin, F. Winquist, Artificial ‘olfactory’ images from a chemical sensor using a light-pulse technique, Nature 352 (1991) 47–50. [11] I. Lundstro¨m, H. Sundgren, F. Winquist, Generation of response maps of gas mixtures, J. Appl. Phys. 11 (1993) 6953–6961.
.
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[12] I. Lundstro¨m, M. Armgarth, L.-G. Pettersson, Physics with catalytic metal gate chemical sensors, CRC Crit. Rev. Solid State Mater. Sci. 15 (1989) 201 – 278. [13] Y. Morita, K. Nakamura, C. Kim, Langmuir analysis on hydrogen gas response of palladium gate FET, Sensros and Actuators B 33 (1996) 96 – 99. [14] M. Johansson, D. Loyd and I. Lundstro¨m, Influence of mass transfer on the steady state response of catalytic metal gate gas sensors, to be published in Sensors and Actuators B 40 (1997) 125 – 133. [15] U. Ackelid, M. Armgarth, A. Spetz, I. Lundstro¨m, Ethanol sensitivity of palladium-gate metal-oxide-semiconductor structures, IEEE Electron Device Letters, EDL- 7 (1986) 353–355.
Biographies Tomas Eklo¨6 received a M.Sc. in physics and electrical engineering in 1994 from Linko¨ping University, Sweden. He is now working towards a Ph.D. at the Laboratory of Applied Physics and the Swedish Sensor Centre (S-SENCE), Linko¨ping University. He is focusing his research on methods to increase the selectivity of chemical sensors and sensor arrays. Hans Sundgren has been a senior research engineer at the Laboratory of Applied Physics, Linko¨ping University since 1979, where he became a Licenciate of Engineering in 1992. His main research area is in developing sensor systems with electronics and signal conditioning, like pattern recognition methods, mainly for semiconductor based gas sensors. Ingemar Lundstro¨m received his Ph.D. in 1970, in electrical engineering (solid-state electronics) from Chalmers University of Technology, Gothenburg, Sweden. He was an assistant professor at the Research Laboratory of Electronics, Chalmers, until 1978, when he was appointed as a professor at the technical faculty of Linko¨ping University, Linko¨ping, Sweden, where he now heads the Laboratory of Applied Physics. The Laboratory, which has an interdisciplinary research staff, conducts research on chemical sensors and biosensors, catalysis, thin films, conductive polymers, surface modifications, biomaterials, and interface biology. He is presently involved e.g. in the development of high temperature chemical sensors, electronic noses and surface oriented biospecific interaction analysis.
Distributed chemical sensing T. Eklo¨v *, H. Sundgren, I. Lundstro¨m Laboratory of Applied Physics and S-SENCE, Linko¨ping Uni6ersity, S-58251 Linko¨ping, Sweden Received 3 December 1996; received in revised form 28 July 1997; accepted 18 August 1997
Abstract When a gas mixture is passed over a catalytic surface its composition will change due to the catalytic activity of the surface. Different molecules have different reaction rates and this fact can be used in combination with distributed chemical sensors to increase the discriminating capability of the sensors. In a measurement cell (palladium covered or non-covered) a row of identical Pd-MOSFET sensors was mounted along the flow direction. The catalytically active measurement cell changed the concentration of detected molecules (hydrogen or ethanol) considerably compared to the non-catalytic cell. It was observed that even catalytic combustion in the lid 1 mm above the sensor row influenced the sensor responses significantly. When comparing hydrogen and ethanol it was found that the two molecules were influenced to different degrees when passing the catalytic surfaces. Hydrogen was consumed at a much faster rate, which could be seen when comparing the sensor responses along the row. A distributed sensor system thus shows an increased discriminating power. It was also found that chemical sensing was much more temperature dependent for ethanol than for hydrogen. © 1997 Elsevier Science S.A. Keywords: Pd-MOSFET gas sensor; Distributed sensor system; Catalytic surface; Selectivity
1. Introduction It is of considerable interest to find ways of changing the selectivity patterns of chemical sensors. A number of different possibilities have been described in connection with the different physical principles used for the sensing devices [1–4]. Catalytic metals are used, both as gates of gas sensitive field effect devices, [5,6] and as additives in gas sensors based on semiconducting metal oxides [1,7]. The choice of catalytic metal and the operation temperature of the device determine the selectivity pattern of such sensors. Catalysts have also been used to combust certain components in a gas mixture before it reaches the sensor, which changes the response pattern of the catalyst-sensor combination and thus increases the selectivity of the system [8,9]. We have previously shown that this possibility can be further developed by using a large catalytic sensing surface, combining the combustion and chemical sensing in a distributed but locally resolved way [10,11]. Such sensing surfaces can be used * Corresponding author. tel.: + 46 13 282632; fax: +46 13 288969; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 2 7 2 - 4
to produce response patterns that are different for different molecules or odours [10,11]. This was achieved by using a scanning light pulse technique to monitor chemically induced work function changes in the thin catalytic metal used as a gate of a metal-oxide-semiconductor structure. The work function changes were monitored as changes in the photo capacitive current generated by the scanning light pulse. A schematic of the experimental set-up used to produce the response patterns is shown in Fig. 1. The patterns occurred due to a varying gas composition along the catalytic metal surface and a varying chemical sensitivity due to a temperature gradient along the sensing surface. The details of the obtained response patterns are, however, not only determined by the catalytic activity of the metal surface but also by the turbulence in the gas flow and edge effects, etc. Furthermore, the scanning light pulse technique, interesting from a physical and fundamental point of view, is rather impractical for a device utilising the ‘distributed chemical sensing’ concept. A question of fundamental interest is whether it is possible to separate the combustion from the detection as schematically illustrated in Fig. 2. The present contribution deals with the performance of a system con-
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sisting of a (continuous) catalytic surface and distributed discrete chemical sensors. In our case the chemical sensors were gas sensitive field effect devices [5], but the sensors in Fig. 2 can, of course, be of any kind suitable for mounting or fabricating in an array. To obtain some basic knowledge of a system like that in Fig. 2 we chose to study one catalytic metal, palladium (Pd), and two molecules, hydrogen and ethanol, which behave differently on palladium. The gates of the distributed field effect transistors used were also made of palladium. Furthermore, we had a constant temperature along the sample to more specifically study the influence of the catalytic combustion on the sensor signals.
2. Experimental set-up In this study palladium gate metal-oxide-semiconductor field effect transistors (MOSFET) were used as sensors [5]. The sensors, fabricated at our laboratory, had the gate oxide covered with a thin (6 nm) discontinuous palladium film. The area covered with palladium was 0.1× 0.1 mm2. Seven sensors were mounted in a row, evenly spaced at the bottom of a measurement cell (Fig. 2). The measurement cell had the dimensions 10× 20× 1 mm3 and the separation between neighbouring sensors was 2.8 mm. The bottom and/or the top of the cell could be provided with a thick (100 nm) catalytically active, thermally evaporated layer of palladium. The sensors had a height of 0.2 mm, giving a distance of 0.8 mm to the top. The cell was made of stainless steel and was provided with tubes (ø 0.7 mm) to provide a gas inlet and outlet. The gas entered the cell at one end and flowed over the sensors to the other end (Fig. 2). A temperature diode and two heating resistors, one at the top and one below the sensors, covering the whole length of the measurement cell, were also placed in contact with the measurement cell (Fig. 2). The total gas flow rate was kept constant at 10 ml min − 1 during all measurements. One measurement cycle consisted of first flowing technical air (20% O2 in nitrogen) over the sensors for 4 min. Then the sensors were exposed to the test gas (H2 or ethanol) mixed with technical air for 2 min. The oxygen level was constant at 20% of the total flow. During the measurements the gate voltage of each MOSFET sensor was measured at a constant drain current, using a sampling frequency of 1 Hz. The response from a sensor was defined as the difference between gate voltages just before the exposure to the gas (hydrogen or ethanol) and at the end of the exposure. The gas mixing system was controlled by a PC, which also collected data from the sensors.
Fig. 1. Schematic experimental set-up for the scanning light pulse technique used to produce two-dimensional response maps of gas mixtures. The induced photo capacitive current iph is measured as the chopped light beam scans over the surface. With a temperature gradient (T1 " T2) and different catalytic metals, e.g. Pt, Pd and Ir, a response map of the gas mixture is produced. The gas flow is parallel to the sensor surface.
3. Results and discussion
3.1. Hydrogen: basic measurements During the measurements on hydrogen the measurement cell was kept at a temperature of 140°C. A typical result from two measurements, with the gas flowing through the measurement cell in different directions, is shown in Fig. 3. The sensors were exposed to two pulses of hydrogen (20 and 10 ppm) in technical air. The slow transient response is partly due to the gas
Fig. 2. Top and side view of the measurement cell indicating sensor configuration, catalytic metal placement and gas flow.
T. Eklo¨6 et al. / Sensors and Actuators B 45 (1997) 71–77
Fig. 3. Raw data from measurements on hydrogen using different flow directions. The seven sensors are exposed to two pulses of hydrogen, 20 and 10 ppm, respectively, in a mixture with technical air and between pulses only technical air. The dashed arrow defines the response of a sensor.
mixing time in the sample cell and partly due to the response characteristics of the sensors. The effect from the catalytic surface can be seen as the decrease of the height of the response peaks along the flow direction. By calculating the responses for each peak the result is easier to interpret. The results for the 20 ppm measurements using a catalytic lid and bottom are summarised in Fig. 4. One observation is that the sensors are not exactly similar but have slightly different response characteristics, causing the responses not to change continuously along the row of sensors. Obviously, the sensors could not be fabricated to exactly the same characteristics, resulting in slightly different characteristics for each sensor. Because of this, the absolute difference between flow directions for each sensor, as defined in Eq. (1), is used in the following analysis. absolute difference= responseflow 1 7 −responseflow 7 1 (1) The absolute difference is less sensitive than the responses to differences in sensor response characteris-
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Fig. 5. Absolute difference of the responses between flow directions for different hydrogen concentrations. Measurements were made using either catalytic lid and bottom or no catalytic surface at all.
tics. If the gas is catalytically changed the absolute difference is a monotonically decreasing function along the catalytic area. In the further analysis the absolute difference will be used to characterise the properties of the catalytic or non-catalytic cell.
3.2. Hydrogen: catalytic effect In Fig. 5 the absolute difference in response between the two flow directions is calculated for all seven sensors, exposing them to pulses of 10, 20, 50 and 100 ppm H2 and using both a palladium covered lid and bottom or a non-catalytic measurement cell. It is, thus, possible to detect catalytic consumption that occurs physically separated from the sensors, since the absolute difference (or change in response), between the two flow directions is considerably larger for the catalytic sample cell. Furthermore, for the measurements using the catalytic cell, the absolute difference increases as the hydrogen concentration increases. This is reasonable since, to a first approximation, the amount of hydrogen consumed is proportional to the total amount of hydrogen. For increasing hydrogen concentration this results in an increasing change in concentration and, concomitantly, an increasing change in sensor response due to the catalytic surface. Also, in the non-catalytic case there is an observable catalytic consumption since the difference between the flow directions increases for a higher H2-concentration. Each sensor has a small area covered with palladium that can account for this effect. Still, this difference is small compared to the catalytic case and is therefore neglected in the further analysis.
3.3. Hydrogen: concentration effect Fig. 4. Comparison between the two flow directions when exposing the sensors to 20 ppm H2 in technical air using the palladium coated measurement cell.
The relative difference between flow directions for each sensor is defined as:
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relati6e difference =absolute difference/a6erage ×(responseflow 1 7, responseflow 7 1) (2) The relative difference should be independent of the concentration, assuming that the sensors have a linear concentration dependence. However, this is not the case as the following analysis will show. The response of the Pd-MOSFET sensors to hydrogen is a saturated function of the hydrogen concentration and the response (DV) follows closely [12]: response= DVmaxa PH 2/(1 +a PH 2)
(3)
where DVmax and a are sensor parameters and PH 2 is the hydrogen concentration in ppm. Such a so called dissociative Langmuir isotherm has been found also in previous studies [13]. Even if it can be justified by a simple model for the detection of hydrogen by an oxygen covered Pd-surface, we regard it as an empirical relationship. For the devices used in the present investigation typical values of the constants in Eq. (3) were DVmax : 0.6 V and a : 0.03 (ppm) − 1/2, at a temperature of 140°C. These values were obtained from pulse responses similar to those in Fig. 3. In comparison with the results in [13], DVmax is somewhat smaller (0.6 V compared to 0.83 V) and the parameter a is approximately half of that in [13]. These differences can be due to many factors, e.g. different palladium structure, or different surface contaminations depending on different operating conditions. The influence of the saturated response is displayed in Fig. 6, where the relative catalytic effect of the measurement cell compared to an ideal non-catalytic cell is calculated as half of the relative response change between the first and last sensor in the row. The calculations were made for varying hydrogen concentrations and both the catalytic and non-catalytic measurement cell. For the catalytic measurement cell the curve is clearly decreasing with
Fig. 7. Measured relative response change between flow directions when exposing the sensors to pulses of 100 ppm hydrogen in technical air. Measurements were made having a catalytic metal both at the bottom and top or only at the bottom of the measurement cell.
increasing H2-concentration, confirming that the relative effect is affected by the saturation in the response characteristics. For the non-catalytic case the relative difference is constant and close to zero, again showing that if the measurement cell is non-catalytic the difference between flow directions can be neglected. From Eq. (3) it is possible to estimate the change in hydrogen concentration from the first to the last sensor in the case of a catalytic measurement cell. The response to 20 ppm hydrogen is approximately 80 mV and the change is about 20 mV. A sensor with DVmax = 0.6 V and a= 0.034 (ppm) − 1/2 gives DV =80 mV for 20 ppm H2 and DV =60 mV for 10 ppm H2, i.e. a decrease in DV of 20 mV corresponds to a relative change in hydrogen concentration of 50%. Even if this is a crude estimate we can conclude that an appreciable amount of the hydrogen is catalytically combusted, but the form of the response isotherm (Eq. (3)) makes the change in sensor response relatively smaller (around 25%). The decrease in the hydrogen concentration is also visible from the decrease in the flow direction of the initial slope of the responses (Fig. 3).
3.4. Hydrogen: effect of the size of the catalytic area
Fig. 6. The relative catalytic effect of the measurement cell as compared to an ideal non-catalytic cell. The calculations were made for varying hydrogen concentrations and both the catalytic and non-catalytic measurement cell.
To find out how much the lid and the bottom contributed to the total catalytic consumption, measurements were made comparing only a catalytic bottom with both a catalytic top and bottom. The sensors were exposed to pulses of 100 ppm hydrogen with the gas flowing in both directions. The relative difference between the flow directions using different catalytic areas is plotted in Fig. 7. When considering sensor numbers 1, 2, 6 and 7 the relative change is approximately halved if the catalytic area is halved. For the other sensors the relative difference is small compared to the noise and therefore gives no information. Thus,
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the lid and the bottom contribute about equally to the catalytic consumption detected by the sensors. However, the distance from a sensor to the lid is 0.8 mm and to the bottom 0.2 mm. If the flow was perfectly laminar the distance between the catalytic metal and the sensor would have a great influence on the spatial hydrogen concentration profiles, due to mass transport limitations. Hence, a catalyst close to the sensor would affect the measured response more than a catalyst further away [14]. In this case we have similar impact from the lid and bottom. The interpretation of this is that the flow is not perfectly laminar, thus giving a spatial mixing and a more homogeneous spatial gas concentration than in the case with laminar flow. This is an advantage in the present case since the gas concentration at the sensor surface then is more directly influenced by the catalytic area than for a laminar flow. Also, a turbulent flow increases the overall reaction rate as more molecules hit the catalytic surface per unit time.
3.5. Hydrogen and ethanol: temperature effect The hydrogen response for a Pd-MOSFET sensor is relatively independent of the operating temperature [5]. In Fig. 8 the results from measurements on 20 ppm hydrogen in technical air are shown. The operating temperature is 175°C and the response is approximately 50 mV for the first sensor in the row, using both the catalytic and the non-catalytic measurement cell. However, if the responses are compared along the row of sensors, the non-catalytic cell gives almost no decrease in response, while the catalytic surface causes the response to decrease substantially. The decrease is 40 mV when comparing different flow directions, giving a relative decrease of approximately 80% for the first sensor in the row. The catalytic consumption of hydrogen is
Fig. 8. Absolute change in response between flow directions when exposing the sensors to 20 ppm hydrogen. The figure includes measurements when using both the catalytic and the non-catalytic measurement cell.
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Fig. 9. Comparison between ethanol and hydrogen in a palladium covered measurement cell. The temperature was set to 175°C and the gas entered the measurement cell from the left.
therefore substantial at this temperature. Thus, the catalytic consumption clearly depends on the temperature. There is no direct correlation between the chemical sensing and catalytic combustion of hydrogen on palladium. Ethanol can also be monitored with Pd-MOSFET sensors. At temperatures below about 140°C, ethanol is very difficult to detect [15]. It has been previously observed that the response of palladium gate devices to ethanol is strongly temperature dependent [15]. A comparison between hydrogen and ethanol at 175°C is shown in Fig. 9, where succeeding measurements on 20 ppm hydrogen and 400 ppm ethanol were made, with the gas passing through a catalytic measurement cell. In this figure, a large difference in response pattern (catalytic consumption) between hydrogen and ethanol can be seen. The difference in response is small for the first sensor in the row, while hydrogen shows a much smaller response than ethanol for the last sensor. By using information from both the first and last sensors, it is thus possible to discriminate between the two molecules. This shows that using a catalytic metal spatially separated from the sensors can further increase the discriminatory ability of the sensors. The difference between ethanol and hydrogen is also clear from Table 1, where the response and change in response between the first and last sensors in the row to hydrogen and ethanol is compared at different operating temperatures. The change in response value indicates the rate of catalytic consumption, as well as the influence of temperature on the catalytic consumption and the chemical sensing for hydrogen and ethanol. Some observations can be made from Table 1. The predictions in the literature concerning the temperature dependence of the response is validated. Chemical sensing of hydrogen is relatively constant at the temperatures investigated, the detected decrease in response being mainly due to ageing of the device. Also, ethanol sensing shows strong temperature dependence, which is
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Table 1 Relative change in response between the first and the last sensors in the row in a catalytic measurement cell Test molecule
Concentration (ppm)
Temperature (°C)
Response (mV)
Change in response (%)
Hydrogen Hydrogen Ethanol Ethanol
20 20 400 400
140 175 140 175
70 50 10 50
25 82 Not detected 12
consistent with previous studies [15]. Catalytic consumption shows a strong dependence on temperature, especially for hydrogen. At a temperature of 175°C, most of the hydrogen entering the cell is consumed. The behaviour of hydrogen and ethanol is also consistent with the previously observed response patterns using the scanning light pulse technique [11].
3.6. Future outlook The experiments presented in this contribution were made to illustrate the features of distributed chemical sensing using two molecules, which behave differently when reacting with an oxygen covered palladium surface. We have used only one catalytic metal and one row of sensors. Fig. 10 shows schematically some other possibilities available by combining sensor arrays with catalytic surfaces. It is obvious that there is a large number of degrees of freedom. It should therefore be possible by a proper choice of catalysts and sensors to optimise the response for a given situation, e.g. for the identification or classification of molecules or mixtures of molecules (odours). It should be pointed out again that the sensors do not need to be of the MOSFET type. Any chemical sensor suitable to mount or produce in an array can be used. Furthermore, any catalytic surface, metallic or non-metallic can be employed. It is even possible to use a lid and/or bottom in the sample cell, which (selectively) let constituents in the gas diffuse out through from the sample cell. This will also create a decrease of analytes in the flow direction. By having a temperature gradi-
Fig. 10. Schematics of catalyst patterns (in the lid and at the bottom) and sensor arrays (mounted in the bottom of the measurement cell) giving different response patterns to given gas mixtures. T1 and T2 indicates the possibility of influencing these response patterns using a temperature gradient. Different layout of the sensors and catalytic areas can also be used to influence the response pattern.
ent in the sample cell it is possible to combine a decrease in the amount of detectable molecules with a change, normally an increase, in the chemical sensitivity to obtain further discriminating power of the distributed sensor system [11]. It is observed in Fig. 9 that the decrease of the sensor response for hydrogen is approximately exponential in the distance along the sensor surface. From such curves the catalytic reaction probability for a given molecule on the catalytic surface can be estimated, using appropriate models for the consumption of molecules in the sample cell. Such models are under development [14] and will be used in a separate publication to explain e.g. the data in Fig. 9.
4. Conclusions Our studies were made in two steps. Firstly, the possibility of detecting the influence of a catalytic surface spatially separated from a row of chemical sensors was investigated using hydrogen as the test molecule. A comparison between the response to hydrogen in a palladium coated cell and a non-coated cell showed that the effect was clearly visible, with the difference increasing for higher hydrogen concentrations. If the relative effect of a catalytic surface was examined it was smaller at higher hydrogen concentrations, mainly due a saturation in the response to hydrogen. Secondly, two chemically different molecules, hydrogen and ethanol, were compared using the catalytic measurement cell. The comparison showed that the two molecules had visibly different concentration profiles along the row of sensors, where hydrogen apparently was consumed at a much higher rate than ethanol. Furthermore, the sensitivity was considerably more temperature dependent for ethanol than for hydrogen. Thus, it has been shown that by using a catalytically induced concentration gradient in combination with chemical sensors it was possible to increase the discriminatory capability of the sensors. Finally, it was suggested that sensor arrays together with patterned catalytic surfaces with or without a temperature gradient provide many possibilities for making tailor-made sensor systems for odour identification and analysis.
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Acknowledgements Stimulating discussions on the present topic with Drs Per Ma˚rtensson, Anita Lloyd Spetz and Fredrik Winquist are gratefully acknowledged. Our research on chemical sensors is supported by grants from the Swedish Research Council for Engineering Sciences and the National Swedish Board for Industrial and Technical Development (NUTEK). S-SENCE (Swedish Sensor Centre) is financed by Swedish Industry, NUTEK and Linko¨ping University.
References [1] S.J. Gentry, T.A. Jones, The role of catalysis in solid-state gas sensors, Sensors and Actuators 10 (1986) 141–163. [2] M. Nakamura, I. Sugimoto, H. Kuwano, R. Lemos, Chemical sensing by analysing dynamics of plasma polymer film-coated sensors, Sensors and Actuators B 20 (1994) 231–237. [3] R.E. Cavicchi, J.S. Suehle, K.G. Kreider, M. Gaitan, P. Chaparala, Fast temperature programmed sensing for micro-hotplate gas sensors, IEEE Electron Device Lett. 16 (1995) 286 – 288. [4] M.E.H. Amrani, K.C. Persaud, P.A. Payne, High-frequency measurements of conducting polymers: development of a new technique for sensing volatile compounds, Meas. Sci. Technol. 6 (1995) 1500 – 1507. [5] I. Lundstro¨m, A. Spetz, F. Winquist, U. Ackelid, H. Sundgren, Catalytic metals and field-effect devices-a useful combination, Sensors and Actuators B 1 (1990) 15–20. [6] I. Lundstro¨m, C. Svensson, A. Spetz, H. Sundgren, F. Winquist, From hydrogen sensors to olfactory images-twenty years, Sensors and Actuators B 13 (1993) 16–23. [7] K.D. Shierbaum, J. Geiger, U. Weimar, W. Go¨pel, Specific palladium and platinum doping for SnO2-based thin film sensor arrays, Sensors and Actuators B 13–14 (1993) 143–147. [8] J.R. Stetter, S. Zaromb, M.W. Findlay, Monitoring of electrochemical inactive compounds by amperometric gas sensors, Sensors and Actuators 6 (1984) 269–288. [9] J.R. Stetter, M.W. Findlay, G.J. Maclay, J. Zhang, S. Vaihinger, W. Go¨pel, Sensor array and catalytic filament for chemical analysis of vapors and mixtures, Sensors and Actuators B 1 (1990) 43 – 47. [10] I. Lundstro¨m, R. Erlandsson, U. Frykman, E. Hedborg, A. Spetz, H. Sundgren, S. Welin, F. Winquist, Artificial ‘olfactory’ images from a chemical sensor using a light-pulse technique, Nature 352 (1991) 47–50. [11] I. Lundstro¨m, H. Sundgren, F. Winquist, Generation of response maps of gas mixtures, J. Appl. Phys. 11 (1993) 6953–6961.
.
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[12] I. Lundstro¨m, M. Armgarth, L.-G. Pettersson, Physics with catalytic metal gate chemical sensors, CRC Crit. Rev. Solid State Mater. Sci. 15 (1989) 201 – 278. [13] Y. Morita, K. Nakamura, C. Kim, Langmuir analysis on hydrogen gas response of palladium gate FET, Sensros and Actuators B 33 (1996) 96 – 99. [14] M. Johansson, D. Loyd and I. Lundstro¨m, Influence of mass transfer on the steady state response of catalytic metal gate gas sensors, to be published in Sensors and Actuators B 40 (1997) 125 – 133. [15] U. Ackelid, M. Armgarth, A. Spetz, I. Lundstro¨m, Ethanol sensitivity of palladium-gate metal-oxide-semiconductor structures, IEEE Electron Device Letters, EDL- 7 (1986) 353–355.
Biographies Tomas Eklo¨6 received a M.Sc. in physics and electrical engineering in 1994 from Linko¨ping University, Sweden. He is now working towards a Ph.D. at the Laboratory of Applied Physics and the Swedish Sensor Centre (S-SENCE), Linko¨ping University. He is focusing his research on methods to increase the selectivity of chemical sensors and sensor arrays. Hans Sundgren has been a senior research engineer at the Laboratory of Applied Physics, Linko¨ping University since 1979, where he became a Licenciate of Engineering in 1992. His main research area is in developing sensor systems with electronics and signal conditioning, like pattern recognition methods, mainly for semiconductor based gas sensors. Ingemar Lundstro¨m received his Ph.D. in 1970, in electrical engineering (solid-state electronics) from Chalmers University of Technology, Gothenburg, Sweden. He was an assistant professor at the Research Laboratory of Electronics, Chalmers, until 1978, when he was appointed as a professor at the technical faculty of Linko¨ping University, Linko¨ping, Sweden, where he now heads the Laboratory of Applied Physics. The Laboratory, which has an interdisciplinary research staff, conducts research on chemical sensors and biosensors, catalysis, thin films, conductive polymers, surface modifications, biomaterials, and interface biology. He is presently involved e.g. in the development of high temperature chemical sensors, electronic noses and surface oriented biospecific interaction analysis.