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Suspended CuO nanowires for ppb level H2S sensing in dry and humid atmosphere
Sensors and Actuators B 186 (2013) 550–556
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Suspended CuO nanowires for ppb level H2 S sensing in dry and humid atmosphere S. Steinhauer ∗ , E. Brunet, T. Maier, G.C. Mutinati, A. Köck AIT Austrian Institute of Technology, Health & Environment Department, Molecular Diagnostics, 1220 Vienna, Austria
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Article history: Received 20 December 2012 Received in revised form 31 May 2013 Accepted 15 June 2013 Available online 24 June 2013 Keywords: CuO nanowire Gas sensor H2 S sensing Humidity interference
a b s t r a c t The ppb level detection of the toxic gas H2 S is of great importance for industrial and safety applications. We report on CuO nanowire gas sensors, which are capable to detect H2 S concentrations as low as 10 ppb in dry as well as humid atmosphere. In particular, measurements with different humidity levels up to 65% have been performed, which is of high practical relevance regarding H2 S detection in ambient atmosphere. Three different types of conductometric gas sensors have been investigated: a single CuO nanowire configuration and two different multiple CuO nanowire configurations that have been developed in order to optimize sensor performance and to enable CMOS integration of CuO nanowire gas sensors in the future. All sensor devices employ suspended CuO nanowires as gas sensitive components. This is a highly favorable configuration because the nanowires are entirely surrounded by the gas atmosphere. The devices based on multiple CuO nanowires show enhanced H2 S response in humid air compared to dry synthetic air. Furthermore, we have found that the sensor design, which employs CuO nanowires with the smallest average diameters around 20 nm, has the highest gas response. The estimated detection limit approaches the ppt range, which shows the excellent sensor performance even in humid atmosphere. These results and the CMOS backend compatibility of the optimized CuO nanowire sensor design are of high importance for the realization of low power silicon integrated gas sensors for daily life applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen sulfide (H2 S) is known as a flammable, highly toxic gas which occurs in industrial processes as well as natural sources. Depending on exposure time and H2 S concentration, humans can suffer from headache, nausea, unconsciousness or even death [1]. Long term ppb level H2 S exposure was reported to cause increased odds ratio for central nervous and respiratory symptoms [2]. The human nose is able to discern very small concentrations of H2 S as rotten-egg smell, but nevertheless highly sensitive and reliable gas sensor devices are needed for safety applications [3]. Moreover, H2 S concentrations as low as 10 ppb are known to deteriorate hydrogen fuel cell performance [4]. The p-type metal oxide semiconductor cupric oxide (CuO) is widely used in conductometric gas sensors for selective H2 S detection, either as additive in polycrystalline SnO2 layers [5] or employing CuO thin films [6], nanofibers [7], nanosheets [8] and nanowires [9,10] as sensing elements. Regarding practical applications, the influence of humidity on the gas sensor performance is of crucial importance as water vapor
∗ Corresponding author. Tel.: +43 505504302. E-mail address: stephan.steinhauer.fl@ait.ac.at (S. Steinhauer). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.06.044
is present in ambient atmosphere. As is known from SnO2 thick film sensors, humidity has a very complex influence on the sensing mechanism [11] and rather sophisticated models are needed for a theoretical understanding of the gas response [12]. In the case of CuO gas sensors, few experimental data on humidity interference is reported in literature. In [13] the authors used CuO thick film devices in order to investigate the humidity influence on CO sensing and report a decrease in sensor signals. In the case of H2 S sensing with CuO nanosheets, decreasing sensor response for increasing relative humidity was found [8]. In our previous publication we reported on CuO nanowire array devices with increased H2 S response at 50% relative humidity compared to dry synthetic air [10]. In this paper we show results on three different CuO nanowire sensors, which are all capable to detect H2 S concentrations as low as 10 ppb in dry as well as humid atmosphere. We perform measurements with three different relative humidity levels from 35% to 65%, which is of high practical relevance regarding gas measurements in ambient atmosphere. In order to optimize sensor performance we compare three different device designs: a single CuO nanowire device and two different multiple CuO nanowire devices. All the three devices employ suspended CuO nanowires, which is a highly favorable configuration for gas sensing. For the multiple CuO nanowire devices, a fabrication technology is chosen,
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Fig. 1. Single suspended CuO nanowire bridging two oxidized Cu lines being contacted by a thermal oxidation process (Image 60◦ tilted).
which was explicitly developed in order to enable CMOS integration of CuO nanowire gas sensors in the future. The multiple CuO nanowire device, which employs nanowires with the smallest average diameters around 20 nm, shows the highest H2 S response with an estimated detection limit approaching the ppt range. 2. Materials and methods For the realization of the first sensor design, the single CuO nanowire device, high-aspect-ratio CuO nanowires are synthesized by resistive heating of highly pure Cu wires. An optimized synthesis process, which is described in detail in [14], is performed at 500 ◦ C for 200 min in ambient atmosphere and results in CuO nanowires with diameters ranging from 50 nm to 200 nm and lengths up to 100 m. The CuO nanowires fabricated by this
method exhibit a typical bi-crystalline structure with twin characteristics. The oxidized Cu wire is ultrasonicated in isopropanol resulting in a CuO nanowire suspension, which is subsequently drop coated on a Si substrate with specifically patterned Cu structures (thickness 2.5 m) in order to bridge two adjacent Cu lines with a single CuO nanowire. Finally, a two step thermal oxidation process (30 min at 200 ◦ C, 60 min at 400 ◦ C) is performed, which leads to copper oxide growth around the CuO nanowire and the formation of Ohmic contacts (see [15] for details). Here, a single CuO nanowire with a diameter of 170 nm and a length around 5 m is employed as gas sensing component in a suspended configuration (Fig. 1). The electrical resistance is dominated by the single CuO nanowire with a resistance value in the order of 100 M and linear IV characteristics at the sensor operation temperature of 325 ◦ C.
Fig. 2. Suspended CuO nanowire arrays fabricated by thermal oxidation of (a) electroplated and (b) thermally evaporated Cu lines. The gas sensor resistance is dominated by (a) a parallel circuit and (b) a series circuit of several CuO nanowire arrays and is measured between opposing Ti/Au contact pads.
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The second device design, a multiple CuO nanowire configuration, is fabricated by a different process based on thermal oxidation of patterned Cu structures on a Si substrate, which has been developed to enable the efficient integration of CuO nanowire devices with CMOS technology. This device design (Fig. 2a), which will be referred to as interdigitated CuO nanowire sensor in the following, relies on structured Cu lines (width 25 m, thickness 2.5 m) deposited by electroplating on a Si wafer covered by thermal oxide. After an optimized thermal oxidation process (T = 400 ◦ C, 3 h, O2 atmosphere), the gap between the oxidized Cu structures (initial distance 5 m) is bridged by CuO nanowires. A detailed description of the fabrication process and characterization results can be found elsewhere [10]. CuO nanowires employed in this device exhibit a polycrystalline structure with an average diameter around 40 nm (diameters ranging from 15 nm to 70 nm) and lengths of few m. In this case, the electrical resistance can be attributed to a parallel circuit of several suspended CuO nanowire arrays with a resistance value in the order of 100 k and linear IV characteristics at the sensor operation temperature of 325 ◦ C. The third device design is shown in Fig. 2b and will be referred to as bridge-to-bridge CuO nanowire sensor in this paper. It is realized by the fabrication of thermally evaporated Cu lines (width 5 m, thickness 500 nm) on a Si/SiO2 substrate and a subsequent thermal oxidation process (T = 350 ◦ C, 3 h, ambient atmosphere). Details of the fabrication process are described in [16]. Similar to the interdigitated CuO nanowire devices, suspended CuO nanowires bridge the gap between the oxidized Cu structures (initial distance 2 m). In this case, CuO nanowires show an average diameter around 20 nm (diameters ranging from 10 nm to 30 nm) and lengths around 200 nm. The electrical resistance is dominated by a series circuit of multiple CuO nanowire arrays with a resistance value in the order of few M and linear IV characteristics at the sensor operation temperature of 325 ◦ C. All CuO nanowire gas sensing devices are operated at a constant temperature by means of commercially available microheaters (10 × 2 Pt6,8-0,4, Delta-R GmbH) and a Pt100 temperature sensor (4 × 1 Pt100A, Delta-R GmbH). Electrical connections from the sensor chip to a ceramic mounting are established by wedge bonding of a 25 m thick Au wire to Ti/Au contact pads (thickness 5 nm and 200 nm, respectively), which are used in all three types of devices. The electrical resistance of the sensor devices is measured by a Keithley 2400 SourceMeter in constant current operation. Gas measurements are performed in an automated gas measurement setup under a constant total gas flow of 1000 sccm. Mass flow controllers are used to mix the background gas (synthetic air, Linde Gas, 80% N2 with 20% O2 ) with small concentrations of H2 S (Linde Gas, 10.1 ppm in N2 ), whereas humidity is added by a separate gas flow of synthetic air through bubble humidifiers. All three types of gas sensor devices, i.e. the single CuO nanowire device, the interdigitated CuO nanowire sensor and the bridge-to-bridge CuO nanowire sensor are exposed to different concentrations of H2 S (10 ppb, 100 ppb, 200 ppb, 300 ppb, 400 ppb, 500 ppb) in dry as well as humid (35% r.H., 50% r.H. and 65% r.H.) synthetic air and are operated at a constant temperature of 325 ◦ C.
3. Results and discussion The H2 S response of the first sensor design is shown in Fig. 3 (concentration range 10 ppb–500 ppb). As can be seen, the single suspended CuO nanowire sensor is able to detect H2 S concentrations in the ppb range in dry as well as humid atmosphere. The results show that the conductivity of a single CuO nanowire is strongly modulated by H2 S exposure even for low concentrations down to 10 ppb. Slight drifts of the sensor resistance occur in the
Fig. 3. Sensor resistance of the single suspended CuO nanowire sensor (operation temperature 325 ◦ C) during H2 S exposure in humid and dry synthetic air.
Fig. 4. Sensor resistance of the interdigitated CuO nanowire sensor (operation temperature 325 ◦ C) during H2 S exposure in humid and dry synthetic air.
presence of humidity but are decreased in dry synthetic air. No correlations with small relative humidity changes could be found and a similar sensor behavior is again observed in repeated measurements. At the moment, however, a detailed understanding of the observed resistance drifts in humid atmosphere is lacking. The H2 S sensing results for the second device design, the interdigitated CuO nanowire sensor, are shown in Fig. 4 (concentration range 10 ppb–500 ppb). Again, H2 S concentrations down to 10 ppb are clearly detected in dry as well as humid synthetic air as background gas, but with an increased gas response compared to the single suspended CuO nanowire. The H2 S response of the third device design, the bridge-to-bridge CuO nanowire sensor, is presented in Fig. 5. This sensor device is also able to detect H2 S concentrations between 10 ppb and 500 ppb in dry as well as humid atmosphere and shows the highest gas response of all three CuO nanowire sensors. A detailed comparison and interpretation of the measurement results will be given later on. In order to clearly demonstrate the excellent sensing performance of the bridge-to-bridge CuO nanowire sensor, an enlarged view of the response to 10 ppb H2 S at 65% relative humidity is depicted in Fig. 6. The signal to noise ratio of this particular measurement, which is estimated by calculating the ratio between sensor signal (difference between Rgas and Rair ) and the standard deviation of the baseline resistance (300 data points before the H2 S pulse), lies around 120. Consequently, we assume that the detection limit is considerably lower than 10 ppb and should approach the ppt range. However, it cannot be evaluated with the current gas measurement setup, because the mass flow controllers do not allow further dilution of the H2 S gas.
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species, which in turn leads to an increase of sensor resistance. In our experimental data all three CuO nanowire devices show increasing baseline resistance for increasing relative humidity levels, which is in qualitative agreement with this model for the interaction between water vapor and a CuO surface. In order to quantify and compare the sensing results of the three CuO nanowire sensors we evaluate the gas response S by calculating the ratio S=
Fig. 5. Sensor resistance of the bridge-to-bridge CuO nanowire sensor (operation temperature 325 ◦ C) during H2 S exposure in humid and dry synthetic air.
Fig. 6. Sensor resistance of the bridge-to-bridge CuO nanowire sensor (operation temperature 325 ◦ C) during exposure to 10 ppb H2 S at 65% relative humidity.
In contrast to the single suspended CuO nanowire, both multiple CuO nanowire devices exhibit more stable sensor signals in dry as well as humid atmosphere. Therefore, the observed resistance drifts of the single CuO nanowire are considered as a property of this device rather than instabilities in the measurement setup. The enhanced stability of the sensor signal is a clear advantage of the multiple CuO nanowire devices, where the device resistance depends on a large number of CuO nanowires. The gas sensing behavior of the three CuO nanowire devices can be explained as follows: On the surface of a p-type metal oxide semiconductor, an accumulation layer is formed due to ionosorbed oxygen O− species [17]. For CuO gas sensors, the sensing mechanism for low concentrations of H2 S is usually explained by the reaction with ionosorbed oxygen species O− [9]: H2 S + 3O− → H2 O + SO2 + 3e− (ad)
(1)
Due to a reduction of the hole concentration in the accumulation layer, the H2 S reaction leads to an increase of sensor resistance. This qualitative behavior is observed for the single suspended CuO nanowire, the interdigitated CuO nanowire sensor as well as the bridge-to-bridge CuO nanowire sensor. For the case of water vapor molecules, the following reaction was proposed [13]: H2 O + O − + 2CuCu + h+ → 2(Cu+ − OH− ) + SO Cu (ad)
(2)
where CuCu indicates a Cu site on the surface and SO a surface site for chemisorption of oxygen. Considering this equation, the reaction with water vapor leads to the formation of terminal hydroxyl groups and a decrease of the concentration of ionosorbed oxygen
Rgas Rair
(3)
with the sensor resistance Rgas during H2 S exposure, whereas Rair refers to the resistance in dry or humid synthetic air. In Fig. 7a, the H2 S response of the single suspended CuO nanowire is presented. In this case, the resistance drifts in humid atmosphere are taken into account by evaluating the mean value of the sensor resistance of 300 data points (corresponds to approximately 5 min) before the H2 S pulses (Rair ) and at the end of the H2 S pulses (Rgas ). As can be seen, no distinctive increase or decrease in H2 S response can be found when comparing the results for dry and humid atmosphere. However, the interdigitated CuO nanowire sensor (Fig. 7b) and the bridge-to-bridge CuO nanowire sensor (Fig. 7c) both show significantly enhanced gas response in humid atmosphere, rather independent of relative humidity level. Regarding a practical application, this sensing behavior is beneficial, as it facilitates sensor calibration in ambient atmosphere considerably. The sensor performances of the three CuO nanowire sensors at 50% relative humidity are compared in Fig. 7d. The bridge-to-bridge CuO nanowire sensor, which employs CuO nanowires with the smallest diameters, clearly exhibits the highest gas response over the whole concentration range. A comparison with recent literature results [18] confirms the excellent H2 S sensing performance of the presented devices. The device designs of the interdigitated CuO nanowire sensor and the bridge-to-bridge CuO nanowire sensor obviously differ from the single suspended CuO nanowire. Thus the experimental data of the three different CuO nanowire sensors have to be correlated with their specific device designs and circuit models for proper interpretation of the measurement results. The IV characteristics of all three CuO nanowire devices have been measured at the sensor operation temperature of 325 ◦ C (see Fig. 8). All three IV curves can be very well approximated by a linear fit with a coefficient of determination of at least 99.99%. Therefore we assume that the sensor resistance is dominated by the CuO nanowire channels rather than potential barriers between CuO nanowires contacting each other or the adjacent oxidized Cu structure. Consequently it is reasonable to compare the results of the single suspended CuO nanowire and the multiple CuO nanowire devices as the same transduction mechanism of the H2 S reaction should be dominant. Our results on multiple CuO nanowire configurations differ from literature reports on CuO nanowire arrays that are mechanically contacted after synthesis [9]. In that case, the H2 S response is attributed to modulation of the contact potential barrier between touching CuO nanowires. As the device designs of the interdigitated CuO nanowire sensor and the bridge-to-bridge CuO nanowire sensor differ in terms of circuit model (parallel circuit of CuO nanowire arrays in contrast to series circuit of CuO nanowire arrays), the specific sensor structure might influence the gas response. Despite the low ppb level concentrations it can be assumed that the H2 S molecule impingement rates are high enough so that the conductivities of the multiple CuO nanowire channels are modulated homogeneously across the whole sensor area. Consequently the described differences in circuit model should have no influence on the gas sensing mechanism of the two multiple CuO nanowire devices.
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Fig. 7. H2 S response in humid and dry synthetic air of the (a) single suspended CuO nanowire sensor, the (b) interdigitated CuO nanowire sensor and the (c) bridge-to-bridge CuO nanowire sensor. The H2 S response of the three different devices is compared in (d) at 50% relative humidity.
Therefore we conclude that the main difference between all the three investigated devices is the diameter of the employed CuO nanowire sensing elements. For the single suspended CuO nanowire, it can be assumed that the nanowire diameter of 170 nm considerably exceeds the Debye length, which was estimated to lie between 3 nm and 23 nm for CuO [13]. As a consequence, the resistance of the single CuO nanowire consists of a bulk contribution and a surface accumulation layer contribution, which is influenced by the surrounding gas atmosphere. As no potential barriers are present at the Ohmic contacts to the single suspended CuO nanowire sensor, the H2 S response can be exclusively attributed to the modulation of the carrier concentration in the accumulation
layer at the CuO nanowire surface. The single suspended CuO nanowire exhibits a rather large diameter of 170 nm; thus a lower gas response is observed compared to the CuO nanowire array devices, where nanowires with much smaller diameters are employed as sensing elements. It is well known from literature that metal oxide nanowires with smaller diameters show a larger gas response. The authors of [19] compared single SnO2 nanowires with diameters between 41 nm and 117 nm and found differences in gas response up to a factor of 2, which is in qualitative agreement with our results. As mentioned earlier we clearly observe increased H2 S response in the presence of humidity for both multiple CuO nanowire
Fig. 8. IV characteristics of the (a) single suspended CuO nanowire sensor, the (b) interdigitated CuO nanowire sensor and the (c) bridge-to-bridge CuO nanowire sensor at the sensor operation temperature of 325 ◦ C.
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sensors. In our previous publication [10] we proposed the following model for explaining the increased H2 S response in the presence of humidity: Due to the large number of water vapor molecules, the initial density of bulk free holes Nh is modified to a reduced density of free holes Nh rH in humidity. The Debye length is inversely proportional to the square root of the free hole density and is therefore increased in humid air. As the Debye length is a measure how far the surface space charge layer extends into the material, this leads to a more efficient modulation of the CuO nanowire gas sensor resistance by H2 S when assuming that the sensor resistance consists of a surface contribution and a bulk contribution. When comparing the H2 S response of the single suspended CuO nanowire in dry and humid atmosphere, no distinct increase or decrease can be observed as was already mentioned earlier. We assume that humidity induced Debye length modulation is not observed in this case as it is expected to be small compared to the large nanowire diameter (170 nm). However, for both the interdigitated CuO nanowire sensor (average nanowire diameter around 40 nm) and the bridge-to-bridge CuO nanowire sensor (average nanowire diameter around 20 nm) we observe increased H2 S response in humid atmosphere. In these two devices we expect that the proposed humidity induced change of the Debye length is comparable to the magnitude of the nanowire diameters and can be therefore observed in our measurements. Moreover, the ratio Shumid /Sair is larger for the bridge-to-bridge CuO nanowire sensor, which is another indication that the enhancement of H2 S response in humid air is dependent on the diameter of the nanowires supporting the discussed H2 S sensing model. Of course, more data with nanostructures of different sizes are needed for verification and therefore additional experiments are planned in the future. For different relative humidity levels only a minor influence on the free hole density Nh rH is expected as the CuO nanowire sensor surface is almost saturated for the investigated humidity concentrations. Thus we also observe only small differences in the H2 S gas response for the different relative humidity levels. However, this sensing behavior is highly beneficial regarding a practical application in ambient conditions. Although the single suspended CuO nanowire sensor is well suited as a model system for the characterization of the gas sensitivity of a single CuO nanowire channel, the fabrication technology, in particular the nanowire transfer process, is problematic with regard to device reproducibility and process realization issues. In the CuO nanowire array devices, the nanowires are synthesized directly on the Si substrate using a specific technology which does not require an additional nanowire transfer process. In addition, the maximum process temperature for CuO nanowire synthesis is below 400 ◦ C. Thus the whole fabrication process is CMOS backend compatible and is suitable for industrial fabrication on wafer scale. In a next step we will integrate the CuO nanowire array devices on specific CMOS fabricated microhotplates which is a prerequisite for the realization of low power, silicon integrated gas sensors for daily life applications [20].
4. Conclusion We have reported on ppb level H2 S sensing in dry and humid atmosphere with three different types of CuO nanowire sensors: a single CuO nanowire device and two different multiple CuO nanowire devices. Our results show that the multiple CuO nanowire devices show superior sensor performance, especially in humid atmosphere. The best results are achieved for a sensor configuration employing multiple CuO nanowires with very small diameters between 10 nm and 30 nm, which shows the importance of the CuO nanowire dimensions. Furthermore, it was found that the H2 S response is almost independent of relative humidity level, which is
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crucial regarding a practical application in ambient conditions. The fabrication process of the optimized CuO nanowire sensor design is CMOS backend compatible, which is of high importance for the realization of highly sensitive, silicon integrated H2 S sensors. The integration of the CuO nanowire sensors with CMOS fabricated microhotplates in order to achieve miniaturized, low power consumption devices is currently under progress. Acknowledgements The authors are grateful to H. Homolka from the Institute of Sensor and Actuator Systems, Vienna University of Technology, for preparation of the ceramic sensor mountings. This work was partly funded by the FFG – Austrian Research Promotion Agency within the MNT-Eranet project “NanoSmart” (No. 828691) and the WWTF Vienna Science and Technology Fund within the project “Mathematics and Nanosensors” (No. MA09-028). References [1] P. Patnaik, A Comprehensive Guide to the Hazardous Properties of Chemical Substances, 3rd ed., John Wiley & Sons, Hoboken, 2007. [2] M.S. Legator, C.R. Singleton, D.L. Morris, D.L. Philips, Health effects from chronic low level exposure to hydrogen sulfide, Archives of Environmental Health 56 (2001) 123–131. [3] N. Yamazoe, K. Shimanoe, Overview of gas sensor technology, in: D.K. Aswal, S.K. Gupta (Eds.), Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, New York, 2007, pp. 1–31. [4] F. Garzon, F.A. Uribe, T. Rockward, I.G. Urdampilleta, E.L. Brosha, The impact of hydrogen fuel contaminates on long-term PMFC performance, ECS Transactions 3 (2006) 695–703. [5] J. Tamaki, K. Shimanoe, Y. Yamada, Y. Yamamoto, N. Miura, N. Yamazoe, Dilute hydrogen sulfide sensing properties of CuO–SnO2 thin film prepared by lowpressure evaporation method, Sensors and Actuators B 49 (1998) 121–125. [6] N.S. Ramgir, S. Kailasa Ganapathi, M. Kaur, N. Datta, K.P. Muthe, D.K. Aswal, S.K. Gupta, J.V. Yakhmi, Sub-ppm H2 S sensing at room temperature using CuO thin films, Sensors and Actuators B 151 (2010) 90–96. [7] J. Hennemann, T. Sauerwald, C.D. Kohl, T. Wagner, M. Bognitzki, A. Greiner, Electrospun copper oxide nanofibers for H2 S dosimetry, Physica Status Solidi A 209 (2012) 911–916. [8] F. Zhang, A. Zhu, Y. Luo, Y. Tian, J. Yang, Y. Qin, CuO nanosheets for sensitive and selective determination of H2 S with high recovery ability, Journal of Physical Chemistry C 114 (2010) 19214–19219. [9] J. Chen, K. Wang, L. Hartman, W. Zhou, H2 S detection by vertically aligned CuO nanowire array sensors, Journal of Physical Chemistry C 112 (2008) 16017–16021. [10] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, O. Freudenberg, C. Gspan, W. Grogger, A. Neuhold, R. Resel, Gas sensing properties of novel CuO nanowire devices, Sensors and Actuators B (2012) http://dx.doi.org/10.1016/j.snb.2012.09.034 [11] N. Barsan, U. Weimar, Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO2 sensors in the presence of humidity, Journal of Physics: Condensed Matter 15 (2003) R813–R839. [12] A. Fort, M. Mugnaini, I. Pasquini, S. Rocchi, V. Vignoli, Modeling of the influence of H2 O on metal oxide sensor responses to CO, Sensors and Actuators B 159 (2011) 82–91. [13] M. Hübner, C.E. Simion, A. Tomescu-St˘anoiu, S. Pokhrel, N. Barsan, U. Weimar, Influence of humidity on CO sensing with p-type CuO thick film gas sensors, Sensors and Actuators B 153 (2011) 347–353. [14] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, W.-D. Schubert, C. Edtmaier, C. Gspan, W. Grogger, Synthesis of high-aspect-ratio CuO nanowires for conductometric gas sensing, Procedia Engineering 25 (2011) 1477–1480. [15] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, O. Freudenberg, Single suspended CuO nanowire for conductometric gas sensing, Procedia Engineering 47 (2012) 17–20. [16] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, On-Chip Synthesis of CuO Nanowires for Direct Gas Sensor Integration, Nanotechnology (IEEE-NANO) 2012 12th IEEE Conference on, 2012, http://dx.doi.org/10.1109/NANO.2012.6321957. [17] N. Barsan, C. Simion, T. Heine, S. Pokhrel, U. Weimar, Modeling of sensing and transduction for p-type semiconducting metal oxide based gas sensors, Journal of Electroceramics 25 (2010) 11–19. [18] X. Li, Y. Wang, Y. Lei, Z. Gu, Highly sensitive H2 S sensor based on templatesynthesized CuO nanowires, RSC Advances 2 (2012) 2302–2307. [19] M. Tonezzer, N.V. Hieu, Size-dependent response of single-nanowire gas sensors, Sensors and Actuators B 163 (2012) 146–152. [20] C. Griessler, E. Brunet, T. Maier, S. Steinhauer, A. Köck, T. Jordi, F. Schrank, M. Schrems, Tin oxide nanosensors for highly sensitive toxic gas detection and their 3D system integration, Microelectronic Engineering 88 (2011) 1779–1781.
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Biographies Stephan Steinhauer received a BSc in mechatronics/microsystems engineering at the University of Applied Sciences in Wiener Neustadt and an MSc in microelectronics at the Vienna University of Technology. In addition, he holds a BSc degree in technical physics. In 2010 he joined the business unit Molecular Diagnostics, H&E Department, AIT Austrian Institute of Technology GmbH as PhD student in the gas sensor research group lead by Anton Köck. The main focus of his work is the fabrication of copper oxide nanowires for gas sensing applications and the integration with CMOS devices. Elise Brunet studied physical chemistry at the University Paris Diderot–Paris 7, where she obtained her master’s degree in 2009. In 2010 she joined the business unit Molecular Diagnostics, H&E Department, AIT Austrian Institute of Technology GmbH as PhD student in the gas sensor research group lead by Anton Köck. The main focus of her work is the fabrication of tin dioxide nanowires for gas sensing applications. Thomas Maier studied technical physics at the Technical University of Graz, Austria, and at the Paul Scherrer Institute of ETH Zurich, Université de Neuchâtel, Switzerland. He received his graduate degree (DI) in 1995 and finished his PhD
in 2000 on surface-emitting laser diodes at the Institute of Solid State Electronics, VUT. As senior researcher in the business unit Molecular Diagnostics, H&E Department, AIT Austrian Institute of Technology GmbH he is developing bolometers and nanosensors for gas detection. Giorgio C. Mutinati studied physics at the Univerisity of Pisa where he graduated in 2002. In 2003 he attended a post-degree master in material science for micro- and nanotechnology at IUSS in Pavia. Since 2003 he worked in semiconductor technology companies: nanotechnology group of Pirelli Labs (then PGT Photonics) and R&D department of Numonyx. The main activities have been R&D process development and process integration. In 2010 he joined the business unit Molecular Diagnostics, H&E Department, AIT Austrian Institute of Technology GmbH as scientist in the gas sensor research group lead by Anton Köck. The main focus of his work is the heterogeneous and CMOS integration of gas sensors. Anton Köck received his master’s degree and PhD in experimental physics at the University of Innsbruck, Austria. After a Post Doc position at the Walter Schottky Institute, Technical University Munich, he was head of the Optoelectronics research group at the Institute for Solid State Electronics, VUT, where he habilitated in 1998. He was head of the MEMS research group at the Wiener Neustadt University for Applied Sciences. Since 2004 he is in the H&E Department, AIT Austrian Institute of Technology GmbH and is developing nanosensors for gas detection.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Suspended CuO nanowires for ppb level H2 S sensing in dry and humid atmosphere S. Steinhauer ∗ , E. Brunet, T. Maier, G.C. Mutinati, A. Köck AIT Austrian Institute of Technology, Health & Environment Department, Molecular Diagnostics, 1220 Vienna, Austria
a r t i c l e
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Article history: Received 20 December 2012 Received in revised form 31 May 2013 Accepted 15 June 2013 Available online 24 June 2013 Keywords: CuO nanowire Gas sensor H2 S sensing Humidity interference
a b s t r a c t The ppb level detection of the toxic gas H2 S is of great importance for industrial and safety applications. We report on CuO nanowire gas sensors, which are capable to detect H2 S concentrations as low as 10 ppb in dry as well as humid atmosphere. In particular, measurements with different humidity levels up to 65% have been performed, which is of high practical relevance regarding H2 S detection in ambient atmosphere. Three different types of conductometric gas sensors have been investigated: a single CuO nanowire configuration and two different multiple CuO nanowire configurations that have been developed in order to optimize sensor performance and to enable CMOS integration of CuO nanowire gas sensors in the future. All sensor devices employ suspended CuO nanowires as gas sensitive components. This is a highly favorable configuration because the nanowires are entirely surrounded by the gas atmosphere. The devices based on multiple CuO nanowires show enhanced H2 S response in humid air compared to dry synthetic air. Furthermore, we have found that the sensor design, which employs CuO nanowires with the smallest average diameters around 20 nm, has the highest gas response. The estimated detection limit approaches the ppt range, which shows the excellent sensor performance even in humid atmosphere. These results and the CMOS backend compatibility of the optimized CuO nanowire sensor design are of high importance for the realization of low power silicon integrated gas sensors for daily life applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen sulfide (H2 S) is known as a flammable, highly toxic gas which occurs in industrial processes as well as natural sources. Depending on exposure time and H2 S concentration, humans can suffer from headache, nausea, unconsciousness or even death [1]. Long term ppb level H2 S exposure was reported to cause increased odds ratio for central nervous and respiratory symptoms [2]. The human nose is able to discern very small concentrations of H2 S as rotten-egg smell, but nevertheless highly sensitive and reliable gas sensor devices are needed for safety applications [3]. Moreover, H2 S concentrations as low as 10 ppb are known to deteriorate hydrogen fuel cell performance [4]. The p-type metal oxide semiconductor cupric oxide (CuO) is widely used in conductometric gas sensors for selective H2 S detection, either as additive in polycrystalline SnO2 layers [5] or employing CuO thin films [6], nanofibers [7], nanosheets [8] and nanowires [9,10] as sensing elements. Regarding practical applications, the influence of humidity on the gas sensor performance is of crucial importance as water vapor
∗ Corresponding author. Tel.: +43 505504302. E-mail address: stephan.steinhauer.fl@ait.ac.at (S. Steinhauer). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.06.044
is present in ambient atmosphere. As is known from SnO2 thick film sensors, humidity has a very complex influence on the sensing mechanism [11] and rather sophisticated models are needed for a theoretical understanding of the gas response [12]. In the case of CuO gas sensors, few experimental data on humidity interference is reported in literature. In [13] the authors used CuO thick film devices in order to investigate the humidity influence on CO sensing and report a decrease in sensor signals. In the case of H2 S sensing with CuO nanosheets, decreasing sensor response for increasing relative humidity was found [8]. In our previous publication we reported on CuO nanowire array devices with increased H2 S response at 50% relative humidity compared to dry synthetic air [10]. In this paper we show results on three different CuO nanowire sensors, which are all capable to detect H2 S concentrations as low as 10 ppb in dry as well as humid atmosphere. We perform measurements with three different relative humidity levels from 35% to 65%, which is of high practical relevance regarding gas measurements in ambient atmosphere. In order to optimize sensor performance we compare three different device designs: a single CuO nanowire device and two different multiple CuO nanowire devices. All the three devices employ suspended CuO nanowires, which is a highly favorable configuration for gas sensing. For the multiple CuO nanowire devices, a fabrication technology is chosen,
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Fig. 1. Single suspended CuO nanowire bridging two oxidized Cu lines being contacted by a thermal oxidation process (Image 60◦ tilted).
which was explicitly developed in order to enable CMOS integration of CuO nanowire gas sensors in the future. The multiple CuO nanowire device, which employs nanowires with the smallest average diameters around 20 nm, shows the highest H2 S response with an estimated detection limit approaching the ppt range. 2. Materials and methods For the realization of the first sensor design, the single CuO nanowire device, high-aspect-ratio CuO nanowires are synthesized by resistive heating of highly pure Cu wires. An optimized synthesis process, which is described in detail in [14], is performed at 500 ◦ C for 200 min in ambient atmosphere and results in CuO nanowires with diameters ranging from 50 nm to 200 nm and lengths up to 100 m. The CuO nanowires fabricated by this
method exhibit a typical bi-crystalline structure with twin characteristics. The oxidized Cu wire is ultrasonicated in isopropanol resulting in a CuO nanowire suspension, which is subsequently drop coated on a Si substrate with specifically patterned Cu structures (thickness 2.5 m) in order to bridge two adjacent Cu lines with a single CuO nanowire. Finally, a two step thermal oxidation process (30 min at 200 ◦ C, 60 min at 400 ◦ C) is performed, which leads to copper oxide growth around the CuO nanowire and the formation of Ohmic contacts (see [15] for details). Here, a single CuO nanowire with a diameter of 170 nm and a length around 5 m is employed as gas sensing component in a suspended configuration (Fig. 1). The electrical resistance is dominated by the single CuO nanowire with a resistance value in the order of 100 M and linear IV characteristics at the sensor operation temperature of 325 ◦ C.
Fig. 2. Suspended CuO nanowire arrays fabricated by thermal oxidation of (a) electroplated and (b) thermally evaporated Cu lines. The gas sensor resistance is dominated by (a) a parallel circuit and (b) a series circuit of several CuO nanowire arrays and is measured between opposing Ti/Au contact pads.
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The second device design, a multiple CuO nanowire configuration, is fabricated by a different process based on thermal oxidation of patterned Cu structures on a Si substrate, which has been developed to enable the efficient integration of CuO nanowire devices with CMOS technology. This device design (Fig. 2a), which will be referred to as interdigitated CuO nanowire sensor in the following, relies on structured Cu lines (width 25 m, thickness 2.5 m) deposited by electroplating on a Si wafer covered by thermal oxide. After an optimized thermal oxidation process (T = 400 ◦ C, 3 h, O2 atmosphere), the gap between the oxidized Cu structures (initial distance 5 m) is bridged by CuO nanowires. A detailed description of the fabrication process and characterization results can be found elsewhere [10]. CuO nanowires employed in this device exhibit a polycrystalline structure with an average diameter around 40 nm (diameters ranging from 15 nm to 70 nm) and lengths of few m. In this case, the electrical resistance can be attributed to a parallel circuit of several suspended CuO nanowire arrays with a resistance value in the order of 100 k and linear IV characteristics at the sensor operation temperature of 325 ◦ C. The third device design is shown in Fig. 2b and will be referred to as bridge-to-bridge CuO nanowire sensor in this paper. It is realized by the fabrication of thermally evaporated Cu lines (width 5 m, thickness 500 nm) on a Si/SiO2 substrate and a subsequent thermal oxidation process (T = 350 ◦ C, 3 h, ambient atmosphere). Details of the fabrication process are described in [16]. Similar to the interdigitated CuO nanowire devices, suspended CuO nanowires bridge the gap between the oxidized Cu structures (initial distance 2 m). In this case, CuO nanowires show an average diameter around 20 nm (diameters ranging from 10 nm to 30 nm) and lengths around 200 nm. The electrical resistance is dominated by a series circuit of multiple CuO nanowire arrays with a resistance value in the order of few M and linear IV characteristics at the sensor operation temperature of 325 ◦ C. All CuO nanowire gas sensing devices are operated at a constant temperature by means of commercially available microheaters (10 × 2 Pt6,8-0,4, Delta-R GmbH) and a Pt100 temperature sensor (4 × 1 Pt100A, Delta-R GmbH). Electrical connections from the sensor chip to a ceramic mounting are established by wedge bonding of a 25 m thick Au wire to Ti/Au contact pads (thickness 5 nm and 200 nm, respectively), which are used in all three types of devices. The electrical resistance of the sensor devices is measured by a Keithley 2400 SourceMeter in constant current operation. Gas measurements are performed in an automated gas measurement setup under a constant total gas flow of 1000 sccm. Mass flow controllers are used to mix the background gas (synthetic air, Linde Gas, 80% N2 with 20% O2 ) with small concentrations of H2 S (Linde Gas, 10.1 ppm in N2 ), whereas humidity is added by a separate gas flow of synthetic air through bubble humidifiers. All three types of gas sensor devices, i.e. the single CuO nanowire device, the interdigitated CuO nanowire sensor and the bridge-to-bridge CuO nanowire sensor are exposed to different concentrations of H2 S (10 ppb, 100 ppb, 200 ppb, 300 ppb, 400 ppb, 500 ppb) in dry as well as humid (35% r.H., 50% r.H. and 65% r.H.) synthetic air and are operated at a constant temperature of 325 ◦ C.
3. Results and discussion The H2 S response of the first sensor design is shown in Fig. 3 (concentration range 10 ppb–500 ppb). As can be seen, the single suspended CuO nanowire sensor is able to detect H2 S concentrations in the ppb range in dry as well as humid atmosphere. The results show that the conductivity of a single CuO nanowire is strongly modulated by H2 S exposure even for low concentrations down to 10 ppb. Slight drifts of the sensor resistance occur in the
Fig. 3. Sensor resistance of the single suspended CuO nanowire sensor (operation temperature 325 ◦ C) during H2 S exposure in humid and dry synthetic air.
Fig. 4. Sensor resistance of the interdigitated CuO nanowire sensor (operation temperature 325 ◦ C) during H2 S exposure in humid and dry synthetic air.
presence of humidity but are decreased in dry synthetic air. No correlations with small relative humidity changes could be found and a similar sensor behavior is again observed in repeated measurements. At the moment, however, a detailed understanding of the observed resistance drifts in humid atmosphere is lacking. The H2 S sensing results for the second device design, the interdigitated CuO nanowire sensor, are shown in Fig. 4 (concentration range 10 ppb–500 ppb). Again, H2 S concentrations down to 10 ppb are clearly detected in dry as well as humid synthetic air as background gas, but with an increased gas response compared to the single suspended CuO nanowire. The H2 S response of the third device design, the bridge-to-bridge CuO nanowire sensor, is presented in Fig. 5. This sensor device is also able to detect H2 S concentrations between 10 ppb and 500 ppb in dry as well as humid atmosphere and shows the highest gas response of all three CuO nanowire sensors. A detailed comparison and interpretation of the measurement results will be given later on. In order to clearly demonstrate the excellent sensing performance of the bridge-to-bridge CuO nanowire sensor, an enlarged view of the response to 10 ppb H2 S at 65% relative humidity is depicted in Fig. 6. The signal to noise ratio of this particular measurement, which is estimated by calculating the ratio between sensor signal (difference between Rgas and Rair ) and the standard deviation of the baseline resistance (300 data points before the H2 S pulse), lies around 120. Consequently, we assume that the detection limit is considerably lower than 10 ppb and should approach the ppt range. However, it cannot be evaluated with the current gas measurement setup, because the mass flow controllers do not allow further dilution of the H2 S gas.
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species, which in turn leads to an increase of sensor resistance. In our experimental data all three CuO nanowire devices show increasing baseline resistance for increasing relative humidity levels, which is in qualitative agreement with this model for the interaction between water vapor and a CuO surface. In order to quantify and compare the sensing results of the three CuO nanowire sensors we evaluate the gas response S by calculating the ratio S=
Fig. 5. Sensor resistance of the bridge-to-bridge CuO nanowire sensor (operation temperature 325 ◦ C) during H2 S exposure in humid and dry synthetic air.
Fig. 6. Sensor resistance of the bridge-to-bridge CuO nanowire sensor (operation temperature 325 ◦ C) during exposure to 10 ppb H2 S at 65% relative humidity.
In contrast to the single suspended CuO nanowire, both multiple CuO nanowire devices exhibit more stable sensor signals in dry as well as humid atmosphere. Therefore, the observed resistance drifts of the single CuO nanowire are considered as a property of this device rather than instabilities in the measurement setup. The enhanced stability of the sensor signal is a clear advantage of the multiple CuO nanowire devices, where the device resistance depends on a large number of CuO nanowires. The gas sensing behavior of the three CuO nanowire devices can be explained as follows: On the surface of a p-type metal oxide semiconductor, an accumulation layer is formed due to ionosorbed oxygen O− species [17]. For CuO gas sensors, the sensing mechanism for low concentrations of H2 S is usually explained by the reaction with ionosorbed oxygen species O− [9]: H2 S + 3O− → H2 O + SO2 + 3e− (ad)
(1)
Due to a reduction of the hole concentration in the accumulation layer, the H2 S reaction leads to an increase of sensor resistance. This qualitative behavior is observed for the single suspended CuO nanowire, the interdigitated CuO nanowire sensor as well as the bridge-to-bridge CuO nanowire sensor. For the case of water vapor molecules, the following reaction was proposed [13]: H2 O + O − + 2CuCu + h+ → 2(Cu+ − OH− ) + SO Cu (ad)
(2)
where CuCu indicates a Cu site on the surface and SO a surface site for chemisorption of oxygen. Considering this equation, the reaction with water vapor leads to the formation of terminal hydroxyl groups and a decrease of the concentration of ionosorbed oxygen
Rgas Rair
(3)
with the sensor resistance Rgas during H2 S exposure, whereas Rair refers to the resistance in dry or humid synthetic air. In Fig. 7a, the H2 S response of the single suspended CuO nanowire is presented. In this case, the resistance drifts in humid atmosphere are taken into account by evaluating the mean value of the sensor resistance of 300 data points (corresponds to approximately 5 min) before the H2 S pulses (Rair ) and at the end of the H2 S pulses (Rgas ). As can be seen, no distinctive increase or decrease in H2 S response can be found when comparing the results for dry and humid atmosphere. However, the interdigitated CuO nanowire sensor (Fig. 7b) and the bridge-to-bridge CuO nanowire sensor (Fig. 7c) both show significantly enhanced gas response in humid atmosphere, rather independent of relative humidity level. Regarding a practical application, this sensing behavior is beneficial, as it facilitates sensor calibration in ambient atmosphere considerably. The sensor performances of the three CuO nanowire sensors at 50% relative humidity are compared in Fig. 7d. The bridge-to-bridge CuO nanowire sensor, which employs CuO nanowires with the smallest diameters, clearly exhibits the highest gas response over the whole concentration range. A comparison with recent literature results [18] confirms the excellent H2 S sensing performance of the presented devices. The device designs of the interdigitated CuO nanowire sensor and the bridge-to-bridge CuO nanowire sensor obviously differ from the single suspended CuO nanowire. Thus the experimental data of the three different CuO nanowire sensors have to be correlated with their specific device designs and circuit models for proper interpretation of the measurement results. The IV characteristics of all three CuO nanowire devices have been measured at the sensor operation temperature of 325 ◦ C (see Fig. 8). All three IV curves can be very well approximated by a linear fit with a coefficient of determination of at least 99.99%. Therefore we assume that the sensor resistance is dominated by the CuO nanowire channels rather than potential barriers between CuO nanowires contacting each other or the adjacent oxidized Cu structure. Consequently it is reasonable to compare the results of the single suspended CuO nanowire and the multiple CuO nanowire devices as the same transduction mechanism of the H2 S reaction should be dominant. Our results on multiple CuO nanowire configurations differ from literature reports on CuO nanowire arrays that are mechanically contacted after synthesis [9]. In that case, the H2 S response is attributed to modulation of the contact potential barrier between touching CuO nanowires. As the device designs of the interdigitated CuO nanowire sensor and the bridge-to-bridge CuO nanowire sensor differ in terms of circuit model (parallel circuit of CuO nanowire arrays in contrast to series circuit of CuO nanowire arrays), the specific sensor structure might influence the gas response. Despite the low ppb level concentrations it can be assumed that the H2 S molecule impingement rates are high enough so that the conductivities of the multiple CuO nanowire channels are modulated homogeneously across the whole sensor area. Consequently the described differences in circuit model should have no influence on the gas sensing mechanism of the two multiple CuO nanowire devices.
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Fig. 7. H2 S response in humid and dry synthetic air of the (a) single suspended CuO nanowire sensor, the (b) interdigitated CuO nanowire sensor and the (c) bridge-to-bridge CuO nanowire sensor. The H2 S response of the three different devices is compared in (d) at 50% relative humidity.
Therefore we conclude that the main difference between all the three investigated devices is the diameter of the employed CuO nanowire sensing elements. For the single suspended CuO nanowire, it can be assumed that the nanowire diameter of 170 nm considerably exceeds the Debye length, which was estimated to lie between 3 nm and 23 nm for CuO [13]. As a consequence, the resistance of the single CuO nanowire consists of a bulk contribution and a surface accumulation layer contribution, which is influenced by the surrounding gas atmosphere. As no potential barriers are present at the Ohmic contacts to the single suspended CuO nanowire sensor, the H2 S response can be exclusively attributed to the modulation of the carrier concentration in the accumulation
layer at the CuO nanowire surface. The single suspended CuO nanowire exhibits a rather large diameter of 170 nm; thus a lower gas response is observed compared to the CuO nanowire array devices, where nanowires with much smaller diameters are employed as sensing elements. It is well known from literature that metal oxide nanowires with smaller diameters show a larger gas response. The authors of [19] compared single SnO2 nanowires with diameters between 41 nm and 117 nm and found differences in gas response up to a factor of 2, which is in qualitative agreement with our results. As mentioned earlier we clearly observe increased H2 S response in the presence of humidity for both multiple CuO nanowire
Fig. 8. IV characteristics of the (a) single suspended CuO nanowire sensor, the (b) interdigitated CuO nanowire sensor and the (c) bridge-to-bridge CuO nanowire sensor at the sensor operation temperature of 325 ◦ C.
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sensors. In our previous publication [10] we proposed the following model for explaining the increased H2 S response in the presence of humidity: Due to the large number of water vapor molecules, the initial density of bulk free holes Nh is modified to a reduced density of free holes Nh rH in humidity. The Debye length is inversely proportional to the square root of the free hole density and is therefore increased in humid air. As the Debye length is a measure how far the surface space charge layer extends into the material, this leads to a more efficient modulation of the CuO nanowire gas sensor resistance by H2 S when assuming that the sensor resistance consists of a surface contribution and a bulk contribution. When comparing the H2 S response of the single suspended CuO nanowire in dry and humid atmosphere, no distinct increase or decrease can be observed as was already mentioned earlier. We assume that humidity induced Debye length modulation is not observed in this case as it is expected to be small compared to the large nanowire diameter (170 nm). However, for both the interdigitated CuO nanowire sensor (average nanowire diameter around 40 nm) and the bridge-to-bridge CuO nanowire sensor (average nanowire diameter around 20 nm) we observe increased H2 S response in humid atmosphere. In these two devices we expect that the proposed humidity induced change of the Debye length is comparable to the magnitude of the nanowire diameters and can be therefore observed in our measurements. Moreover, the ratio Shumid /Sair is larger for the bridge-to-bridge CuO nanowire sensor, which is another indication that the enhancement of H2 S response in humid air is dependent on the diameter of the nanowires supporting the discussed H2 S sensing model. Of course, more data with nanostructures of different sizes are needed for verification and therefore additional experiments are planned in the future. For different relative humidity levels only a minor influence on the free hole density Nh rH is expected as the CuO nanowire sensor surface is almost saturated for the investigated humidity concentrations. Thus we also observe only small differences in the H2 S gas response for the different relative humidity levels. However, this sensing behavior is highly beneficial regarding a practical application in ambient conditions. Although the single suspended CuO nanowire sensor is well suited as a model system for the characterization of the gas sensitivity of a single CuO nanowire channel, the fabrication technology, in particular the nanowire transfer process, is problematic with regard to device reproducibility and process realization issues. In the CuO nanowire array devices, the nanowires are synthesized directly on the Si substrate using a specific technology which does not require an additional nanowire transfer process. In addition, the maximum process temperature for CuO nanowire synthesis is below 400 ◦ C. Thus the whole fabrication process is CMOS backend compatible and is suitable for industrial fabrication on wafer scale. In a next step we will integrate the CuO nanowire array devices on specific CMOS fabricated microhotplates which is a prerequisite for the realization of low power, silicon integrated gas sensors for daily life applications [20].
4. Conclusion We have reported on ppb level H2 S sensing in dry and humid atmosphere with three different types of CuO nanowire sensors: a single CuO nanowire device and two different multiple CuO nanowire devices. Our results show that the multiple CuO nanowire devices show superior sensor performance, especially in humid atmosphere. The best results are achieved for a sensor configuration employing multiple CuO nanowires with very small diameters between 10 nm and 30 nm, which shows the importance of the CuO nanowire dimensions. Furthermore, it was found that the H2 S response is almost independent of relative humidity level, which is
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crucial regarding a practical application in ambient conditions. The fabrication process of the optimized CuO nanowire sensor design is CMOS backend compatible, which is of high importance for the realization of highly sensitive, silicon integrated H2 S sensors. The integration of the CuO nanowire sensors with CMOS fabricated microhotplates in order to achieve miniaturized, low power consumption devices is currently under progress. Acknowledgements The authors are grateful to H. Homolka from the Institute of Sensor and Actuator Systems, Vienna University of Technology, for preparation of the ceramic sensor mountings. This work was partly funded by the FFG – Austrian Research Promotion Agency within the MNT-Eranet project “NanoSmart” (No. 828691) and the WWTF Vienna Science and Technology Fund within the project “Mathematics and Nanosensors” (No. MA09-028). References [1] P. Patnaik, A Comprehensive Guide to the Hazardous Properties of Chemical Substances, 3rd ed., John Wiley & Sons, Hoboken, 2007. [2] M.S. Legator, C.R. Singleton, D.L. Morris, D.L. Philips, Health effects from chronic low level exposure to hydrogen sulfide, Archives of Environmental Health 56 (2001) 123–131. [3] N. Yamazoe, K. Shimanoe, Overview of gas sensor technology, in: D.K. Aswal, S.K. Gupta (Eds.), Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, New York, 2007, pp. 1–31. [4] F. Garzon, F.A. Uribe, T. Rockward, I.G. Urdampilleta, E.L. Brosha, The impact of hydrogen fuel contaminates on long-term PMFC performance, ECS Transactions 3 (2006) 695–703. [5] J. Tamaki, K. Shimanoe, Y. Yamada, Y. Yamamoto, N. Miura, N. Yamazoe, Dilute hydrogen sulfide sensing properties of CuO–SnO2 thin film prepared by lowpressure evaporation method, Sensors and Actuators B 49 (1998) 121–125. [6] N.S. Ramgir, S. Kailasa Ganapathi, M. Kaur, N. Datta, K.P. Muthe, D.K. Aswal, S.K. Gupta, J.V. Yakhmi, Sub-ppm H2 S sensing at room temperature using CuO thin films, Sensors and Actuators B 151 (2010) 90–96. [7] J. Hennemann, T. Sauerwald, C.D. Kohl, T. Wagner, M. Bognitzki, A. Greiner, Electrospun copper oxide nanofibers for H2 S dosimetry, Physica Status Solidi A 209 (2012) 911–916. [8] F. Zhang, A. Zhu, Y. Luo, Y. Tian, J. Yang, Y. Qin, CuO nanosheets for sensitive and selective determination of H2 S with high recovery ability, Journal of Physical Chemistry C 114 (2010) 19214–19219. [9] J. Chen, K. Wang, L. Hartman, W. Zhou, H2 S detection by vertically aligned CuO nanowire array sensors, Journal of Physical Chemistry C 112 (2008) 16017–16021. [10] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, O. Freudenberg, C. Gspan, W. Grogger, A. Neuhold, R. Resel, Gas sensing properties of novel CuO nanowire devices, Sensors and Actuators B (2012) http://dx.doi.org/10.1016/j.snb.2012.09.034 [11] N. Barsan, U. Weimar, Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO2 sensors in the presence of humidity, Journal of Physics: Condensed Matter 15 (2003) R813–R839. [12] A. Fort, M. Mugnaini, I. Pasquini, S. Rocchi, V. Vignoli, Modeling of the influence of H2 O on metal oxide sensor responses to CO, Sensors and Actuators B 159 (2011) 82–91. [13] M. Hübner, C.E. Simion, A. Tomescu-St˘anoiu, S. Pokhrel, N. Barsan, U. Weimar, Influence of humidity on CO sensing with p-type CuO thick film gas sensors, Sensors and Actuators B 153 (2011) 347–353. [14] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, W.-D. Schubert, C. Edtmaier, C. Gspan, W. Grogger, Synthesis of high-aspect-ratio CuO nanowires for conductometric gas sensing, Procedia Engineering 25 (2011) 1477–1480. [15] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, O. Freudenberg, Single suspended CuO nanowire for conductometric gas sensing, Procedia Engineering 47 (2012) 17–20. [16] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, On-Chip Synthesis of CuO Nanowires for Direct Gas Sensor Integration, Nanotechnology (IEEE-NANO) 2012 12th IEEE Conference on, 2012, http://dx.doi.org/10.1109/NANO.2012.6321957. [17] N. Barsan, C. Simion, T. Heine, S. Pokhrel, U. Weimar, Modeling of sensing and transduction for p-type semiconducting metal oxide based gas sensors, Journal of Electroceramics 25 (2010) 11–19. [18] X. Li, Y. Wang, Y. Lei, Z. Gu, Highly sensitive H2 S sensor based on templatesynthesized CuO nanowires, RSC Advances 2 (2012) 2302–2307. [19] M. Tonezzer, N.V. Hieu, Size-dependent response of single-nanowire gas sensors, Sensors and Actuators B 163 (2012) 146–152. [20] C. Griessler, E. Brunet, T. Maier, S. Steinhauer, A. Köck, T. Jordi, F. Schrank, M. Schrems, Tin oxide nanosensors for highly sensitive toxic gas detection and their 3D system integration, Microelectronic Engineering 88 (2011) 1779–1781.
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Biographies Stephan Steinhauer received a BSc in mechatronics/microsystems engineering at the University of Applied Sciences in Wiener Neustadt and an MSc in microelectronics at the Vienna University of Technology. In addition, he holds a BSc degree in technical physics. In 2010 he joined the business unit Molecular Diagnostics, H&E Department, AIT Austrian Institute of Technology GmbH as PhD student in the gas sensor research group lead by Anton Köck. The main focus of his work is the fabrication of copper oxide nanowires for gas sensing applications and the integration with CMOS devices. Elise Brunet studied physical chemistry at the University Paris Diderot–Paris 7, where she obtained her master’s degree in 2009. In 2010 she joined the business unit Molecular Diagnostics, H&E Department, AIT Austrian Institute of Technology GmbH as PhD student in the gas sensor research group lead by Anton Köck. The main focus of her work is the fabrication of tin dioxide nanowires for gas sensing applications. Thomas Maier studied technical physics at the Technical University of Graz, Austria, and at the Paul Scherrer Institute of ETH Zurich, Université de Neuchâtel, Switzerland. He received his graduate degree (DI) in 1995 and finished his PhD
in 2000 on surface-emitting laser diodes at the Institute of Solid State Electronics, VUT. As senior researcher in the business unit Molecular Diagnostics, H&E Department, AIT Austrian Institute of Technology GmbH he is developing bolometers and nanosensors for gas detection. Giorgio C. Mutinati studied physics at the Univerisity of Pisa where he graduated in 2002. In 2003 he attended a post-degree master in material science for micro- and nanotechnology at IUSS in Pavia. Since 2003 he worked in semiconductor technology companies: nanotechnology group of Pirelli Labs (then PGT Photonics) and R&D department of Numonyx. The main activities have been R&D process development and process integration. In 2010 he joined the business unit Molecular Diagnostics, H&E Department, AIT Austrian Institute of Technology GmbH as scientist in the gas sensor research group lead by Anton Köck. The main focus of his work is the heterogeneous and CMOS integration of gas sensors. Anton Köck received his master’s degree and PhD in experimental physics at the University of Innsbruck, Austria. After a Post Doc position at the Walter Schottky Institute, Technical University Munich, he was head of the Optoelectronics research group at the Institute for Solid State Electronics, VUT, where he habilitated in 1998. He was head of the MEMS research group at the Wiener Neustadt University for Applied Sciences. Since 2004 he is in the H&E Department, AIT Austrian Institute of Technology GmbH and is developing nanosensors for gas detection.