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Single Metal Oxide Nanowire devices for Ammonia and Other Gases Detection in Humid Atmosphere
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 1052 – 1055
30th Eurosensors Conference, EUROSENSORS 2016
Single metal oxide nanowire devices for ammonia and other gases detection in humid atmosphere M. Donarellia,b,*, M. Ferronia,b, A. Ponzonib, F. Rigonia,b, D. Zappab, C. Barattob, G. Fagliaa,b, E. Cominia,b, G. Sberveglieria,b a
Sensor Lab., Dept. of Information Engineering, University of Brescia, Via Branze 38, 25123 Brescia, Italy b CNR-INO Brescia, Via Branze 38, 25123 Brescia, Italy
Abstract Metal oxide nanowires have been deposited on 100 nm SiO 2/Si substrates and SnO2 and ZnO single nanowire devices have been fabricated by electron beam lithography technique. Gas sensing tests have been carried out, in order to detect ammonia, CO and NO2 under standard humidity conditions. The devices have been also tested in either dark condition or exposed to ultraviolet light at room temperature. SnO2 nanowires are able to detect ammonia at concentration of the order of few ppm also in dark conditions, they are sensitive to NO2 and cannot detect CO. ZnO devices show good sensitivity to 5 ppm of ammonia in 30% relative humidity atmosphere. The samples have been mounted on a TO-39 case in a field effect transistor configuration in order to study the influence of the back-gate voltage on the single nanowire devices gas sensing performances. 2016The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © 2016 Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: single nanowire; NH3 sensing; FET; EBL.
1. Introduction The discovery and synthesis of low dimensional materials have paved the way to the development of novel gas sensors, exploiting their intrinsically high surface to volume ratio. Semiconducting nanowires (NWs) and nanobelts have been reported to be effective nanostructures for gas sensing applications [1,2]. The gas sensors based on metal oxide NWs and nanobelts usually work at high temperatures (higher than 200 °C). This represents an obstacle to the
* Corresponding author. Tel.: +39 030 3715875. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.338
M. Donarelli et al. / Procedia Engineering 168 (2016) 1052 – 1055
1053
fabrication of low power consumption devices. Fabrication of single NW devices has been reported to be a viable method to lower the power consumption, taking advantage from the NW self-heating due to the Joule effect [3]. In this kind of devices, the current flowing in the NW has two roles: it constitutes the gas sensing signal and it is used for the self-heating. Focused ion beam lithography has been intensively used to connect metal electrodes to NW [4,5]. Some authors have reported the ability of single NW devices to detect ammonia and NO2 at concentrations of the order of ppm [6]. Other works report the improved NO2 sensing performances of SnO2 nanoribbons under ultraviolet (UV) illumination [7]. In this paper, we report the gas sensing performances of single ZnO and SnO 2 nanowire devices, in dark and under UV illumination. We focus on the ability of the devices to detect ammonia also in dark conditions at concentration down to 5 ppm, under standard relative humidity (RH) conditions (RH = 30%) and at room temperature. We finally discuss the selectivity of the devices towards different target gases. 2. Experimental Metal oxide NWs have been fabricated by vapor-liquid-solid (VLS) technique, extensively reported in previous works [8,9]. This method allows to fabricate NWs with diameters ranging from ten to few hundreds nm’s and lengths from few to tens μm’s, whose morphology and crystallinity have been deeply analyzed and reported in literature [2,8,9]. The ZnO and SnO2 NWs bundles have been scratched from the alumina substrates and dispersed in
Fig. 1. (a): optical image of the single ZnO NW device. (b): sketch of the single NW FET device.
isopropanol. The solutions have been sonicated for 1 hour, to avoid the formation of “piles”. One drop of each solution has been deposited on 3x3 mm2 100 nm SiO2/Si substrates and, in case, the sample with the deposited drop has been spinned, to obtain a more uniform distribution of the NWs on the substrate. For the single NW device fabrication, ZnO (SnO2) NWs with diameters of 50-150 nm (150-300 nm) have been selected, checking their dimension by scanning electron microscope (FE-SEM, Zeiss LEO 1525). 500 nm thick layer of PMMA has been deposited on the sample by spin coating and subsequent annealing at 180 °C. The FE-SEM electron beam directly patterned the electrodes and the pads. After that, the sample has been developed in a MIBK:IPA, 1:3 solution for 90 s and rinsed for 2 min in IPA to remove exposed areas. About 20 nm of Ti-W (adhesion layer) and 300 nm of Pt have been deposited on the sample by sputtering. The sample has been rinsed in acetone to remove the unexposed areas. The electrodes are about 2 µm wide and the pads areas are 100x100 µ m2 for the ZnO samples (Fig. 1a), while for the SnO2 ones the pads are patterned in order to directly contact the NW. The samples have been mounted on a TO-39 case in a back-gate FET configuration (a sketch of the device is reported in Fig. 1b). DC volt-amperometric measurements have been performed to monitor samples conductance, applying a constant bias (5 V) between electrodes and measuring the current through a pico ammeter (Keithley 6485). Gas sensing measurements have been carried out in an ad-hoc designed test chambers assembled by Kenosystec (temperature-stabilized at 20 °C, volume 1 L) by flow through technique, with a constant flux of about 220 sccm. Target gases are supplied by certified bottles. Mass flow controllers control the atmosphere composition. RH inside the tests chamber has been maintained equal to 30% in all the sensing tests. UV illumination has been provided by 8 W UV lamp which can be switched between dark and 254 and 365 nm wavelengths manually. The sensing responses are calculated as (R g-Ra)/Ra for NO2 and (Gg-Ga)/Ga for NH3, where R indicates the resistance, G the conductance, subscript “a” in air and “g” during gas exposure. Response
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time (τresp) is the time required to reach the 90% of the gas sensor response, while the recovery time (τrec) is the time the sensor takes to cover the 90% of the difference Rg-Ra (or Gg-Ga), once the injection has been switched off. 3. SnO2 and ZnO single NW devices sensing properties The SnO2 single NW device has been exposed to UV (λ = 254 nm) for three hours, then three hours in dark conditions and finally exposed again to UV light, at different wavelength (λ = 365 nm). During these three steps, NH 3 has been inserted in the test chamber, at three concentrations (namely 10, 20 and 30 ppm). The device has been exposed to NH3 for 5 minutes (Fig. 2a). The SnO2 single NW device does not show a clear response to NH 3 when it is exposed to UV light, while it can detect NH3 in dark conditions. Further analyses show that the device can detect ammonia in dark conditions, down to 5 ppm (Fig. 2b). Increasing the gate voltage (from 0 to 5 V) does not sensibly affect or improve the gas sensing performances of this device. The SnO2 has been exposed to NO2 (at concentrations of 2 and 4 ppm) under UV illumination (λ = 254 nm) and in dark conditions (Fig. 2c).
Fig. 2. SnO2 single NW electrical response to 10, 20 and 30 ppm of NH3 (a), to 5 and 10 ppm of NH3 (b), to 2 and 4 ppm of NO2 (c).
The device shows a remarkable decrease of the current flowing between electrodes when exposed to the target gas in dark conditions (the current decreases of one order of magnitude). However, it does not reach a saturation regime after 30 min of exposure, nor a complete recovery of the baseline. Under UV illumination, for 4 ppm of NO2, the gas sensing response is 0.21, the current reaches a steady state value during the gas exposure (τresp 75 s), and the baseline is completely recovered after few minutes the gas is switched off (τrec 460 s). Finally, the device has been exposed
Fig. 3. Left: ZnO single NW device electrical response to 10, 20 and 30 ppm of NH3. Right: ZnO electrical response to 5 and 10 ppm of NH3. Gate voltage 0 V, RH = 30%.
also to 200 and 400 ppm of CO under UV light and in dark conditions. No clear signal has been detected in both cases. In dark conditions, a very slight increase of the current is recorded during exposure to 400 ppm of CO, consistent to what previously reported by our group, for a different SnO2 single NW device, in dark and dry air conditions [10]. The ZnO single NW device has been exposed to 10, 20 and 30 ppm of ammonia, following the same measurement protocol adopted for SnO2 device (Fig. 3, left). The current value decreases by two orders of magnitude when the UV lamp is switched off. After one hour from the UV is switched off, residual photocurrent is still present, as deduced from the current exponential decrease in dark conditions. The current value under UV illumination at the two
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Fig. 4. Left: ZnO single NW electrical response to 5 and 10 ppm of NH3 (gate voltage 5 V). Right: ZnO single NW electrical response to 2 and 4 ppm of NO2 (gate voltage 0 V). In both cases, RH = 30%.
wavelengths (254 and 365 nm) is almost the same, as expected, being the ZnO band gap about 3.3 eV, which is lower than the incident UV photons energies. As for the single SnO2 NW device, the single ZnO NW can detect NH3 in dark conditions, after the UV lamp is switched off. The single ZnO NW shows a higher response to 5 and 10 ppm of ammonia than the single SnO2 NW (Fig. 3, right). Furthermore, increasing the gate voltage of the single ZnO NW device up to 5 V results in a slight decrease of the response time (Fig. 4, left): for 10 ppm of NH3, the response time passes from about 370 s at 0 V to 330 s at 5 V. The single ZnO NW has been exposed also to 2 and 4 ppm of NO2, under UV illumination (λ = 254 nm) and in dark conditions. As reported in Fig. 4, right, the device shows good response in dark conditions, even if the current is very low and the baseline is not recovered after the gas is switched off. The ZnO single NW device does not detect CO. 4. Conclusions We have shown that electron beam lithography is a viable way to fabricate single NW devices for gas sensing tests. Single SnO2 and ZnO NW can be used to detect NH3 and NO2 at concentration of the order of few ppm, under UV illumination and after the UV lamp is switched off, in humid atmosphere. In the case of NO 2, the UV illumination of the SnO2 NW helps it to recover the baseline after the target gas is switched off. For ZnO single NW FET, increasing the back-gate voltage results in a slight decrease of the response time. Finally, the fabricated devices do not show a clear response to CO in humid atmosphere, which is important in order to obtain selective gas sensors. References [1] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z. L. Whang, Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts, Appl. Phys. Lett. 81 (2002) 1869-1871. [2] G. Sberveglieri, C. Baratto, E. Comini, G. Faglia, M. Ferroni, A. Ponzoni, A. Vomiero, Synthesis and characterization of semiconducting nanowires for gas sensing, Sensor. Actuat. B-Chem. 121 (2007) 208-213. [3] J. D. Prades, F. Hernández-Ramirez, T. Fischer, M. Hoffman, R. Müller, N. López, S. Mathur, J. R. Morante, Quantitative analysis of COhumidity gas mixtures with self-heated nanowires operated in pulse mode, Appl. Phys. Lett. 97 (2010) 243105-243107. [4] F. Hernández-Ramirez, A. Tarancon, O. Casals, E. Pellicer, J. Rodriguez, A. Romano-Rodriguez, J. R. Morante, S. Barth, S. Mathur, Electrical properties of individual tin oxide nanowires contacted to platinum electrodes, Phys. Rev. B 76 (2007) 085429. [5] Q. Kuang, C. Lao, Z. L. Wang, Z. Xie, L. Zheng, High-sensitivity humidity sensor based on a single SnO2 nanowire, J. Am. Chem. Soc. 129 (2007) 6070-6071. [6] Z. Fan, J. G. Lu, Chemical sensing with ZnO nanowire field-effect transistor, IEEE Trans. Nanotechnol. 5 (2006) 393-396. [7] M. Law, H. Kind, B. Messer, F. Kim, P. Yang, Photochemical sensing of NO2 with SnO2 nanoribbon nanosensors at room temperature, Angew. Chem. Int. Ed. 41 (2002) 2405-2408. [8] E. Comini, G. Faglia, M. Ferroni, A. Ponzoni, A. Vomiero, G. Sberveglieri, Metal oxide nanowires: preparation and application in gas sensing, J. Mol. Catal. A: Chem. 305 (2009) 170-177. [9] S. Kaciulis, L. Pandolfi, E. Comini, G. Faglia, M. Ferroni, G. Sberveglieri, S. Kandasamy, M. Shafiei, W. Wlodarski, Nanowires of metal oxides for gas sensing application, Surf. Interface Anal. 40 (2007) 575-578. [10] M. Donarelli, R. Milan, M. Ferroni, G. Faglia, E. Comini, G. Sberveglieri, A. Ponzoni, C. Baratto, Fabrication of single-nanowire sensing devices by electron beam lithography, Proceedings of the 2015 18th AISEM Annual Conference (2015) 1-4.
ScienceDirect Procedia Engineering 168 (2016) 1052 – 1055
30th Eurosensors Conference, EUROSENSORS 2016
Single metal oxide nanowire devices for ammonia and other gases detection in humid atmosphere M. Donarellia,b,*, M. Ferronia,b, A. Ponzonib, F. Rigonia,b, D. Zappab, C. Barattob, G. Fagliaa,b, E. Cominia,b, G. Sberveglieria,b a
Sensor Lab., Dept. of Information Engineering, University of Brescia, Via Branze 38, 25123 Brescia, Italy b CNR-INO Brescia, Via Branze 38, 25123 Brescia, Italy
Abstract Metal oxide nanowires have been deposited on 100 nm SiO 2/Si substrates and SnO2 and ZnO single nanowire devices have been fabricated by electron beam lithography technique. Gas sensing tests have been carried out, in order to detect ammonia, CO and NO2 under standard humidity conditions. The devices have been also tested in either dark condition or exposed to ultraviolet light at room temperature. SnO2 nanowires are able to detect ammonia at concentration of the order of few ppm also in dark conditions, they are sensitive to NO2 and cannot detect CO. ZnO devices show good sensitivity to 5 ppm of ammonia in 30% relative humidity atmosphere. The samples have been mounted on a TO-39 case in a field effect transistor configuration in order to study the influence of the back-gate voltage on the single nanowire devices gas sensing performances. 2016The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © 2016 Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: single nanowire; NH3 sensing; FET; EBL.
1. Introduction The discovery and synthesis of low dimensional materials have paved the way to the development of novel gas sensors, exploiting their intrinsically high surface to volume ratio. Semiconducting nanowires (NWs) and nanobelts have been reported to be effective nanostructures for gas sensing applications [1,2]. The gas sensors based on metal oxide NWs and nanobelts usually work at high temperatures (higher than 200 °C). This represents an obstacle to the
* Corresponding author. Tel.: +39 030 3715875. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.338
M. Donarelli et al. / Procedia Engineering 168 (2016) 1052 – 1055
1053
fabrication of low power consumption devices. Fabrication of single NW devices has been reported to be a viable method to lower the power consumption, taking advantage from the NW self-heating due to the Joule effect [3]. In this kind of devices, the current flowing in the NW has two roles: it constitutes the gas sensing signal and it is used for the self-heating. Focused ion beam lithography has been intensively used to connect metal electrodes to NW [4,5]. Some authors have reported the ability of single NW devices to detect ammonia and NO2 at concentrations of the order of ppm [6]. Other works report the improved NO2 sensing performances of SnO2 nanoribbons under ultraviolet (UV) illumination [7]. In this paper, we report the gas sensing performances of single ZnO and SnO 2 nanowire devices, in dark and under UV illumination. We focus on the ability of the devices to detect ammonia also in dark conditions at concentration down to 5 ppm, under standard relative humidity (RH) conditions (RH = 30%) and at room temperature. We finally discuss the selectivity of the devices towards different target gases. 2. Experimental Metal oxide NWs have been fabricated by vapor-liquid-solid (VLS) technique, extensively reported in previous works [8,9]. This method allows to fabricate NWs with diameters ranging from ten to few hundreds nm’s and lengths from few to tens μm’s, whose morphology and crystallinity have been deeply analyzed and reported in literature [2,8,9]. The ZnO and SnO2 NWs bundles have been scratched from the alumina substrates and dispersed in
Fig. 1. (a): optical image of the single ZnO NW device. (b): sketch of the single NW FET device.
isopropanol. The solutions have been sonicated for 1 hour, to avoid the formation of “piles”. One drop of each solution has been deposited on 3x3 mm2 100 nm SiO2/Si substrates and, in case, the sample with the deposited drop has been spinned, to obtain a more uniform distribution of the NWs on the substrate. For the single NW device fabrication, ZnO (SnO2) NWs with diameters of 50-150 nm (150-300 nm) have been selected, checking their dimension by scanning electron microscope (FE-SEM, Zeiss LEO 1525). 500 nm thick layer of PMMA has been deposited on the sample by spin coating and subsequent annealing at 180 °C. The FE-SEM electron beam directly patterned the electrodes and the pads. After that, the sample has been developed in a MIBK:IPA, 1:3 solution for 90 s and rinsed for 2 min in IPA to remove exposed areas. About 20 nm of Ti-W (adhesion layer) and 300 nm of Pt have been deposited on the sample by sputtering. The sample has been rinsed in acetone to remove the unexposed areas. The electrodes are about 2 µm wide and the pads areas are 100x100 µ m2 for the ZnO samples (Fig. 1a), while for the SnO2 ones the pads are patterned in order to directly contact the NW. The samples have been mounted on a TO-39 case in a back-gate FET configuration (a sketch of the device is reported in Fig. 1b). DC volt-amperometric measurements have been performed to monitor samples conductance, applying a constant bias (5 V) between electrodes and measuring the current through a pico ammeter (Keithley 6485). Gas sensing measurements have been carried out in an ad-hoc designed test chambers assembled by Kenosystec (temperature-stabilized at 20 °C, volume 1 L) by flow through technique, with a constant flux of about 220 sccm. Target gases are supplied by certified bottles. Mass flow controllers control the atmosphere composition. RH inside the tests chamber has been maintained equal to 30% in all the sensing tests. UV illumination has been provided by 8 W UV lamp which can be switched between dark and 254 and 365 nm wavelengths manually. The sensing responses are calculated as (R g-Ra)/Ra for NO2 and (Gg-Ga)/Ga for NH3, where R indicates the resistance, G the conductance, subscript “a” in air and “g” during gas exposure. Response
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M. Donarelli et al. / Procedia Engineering 168 (2016) 1052 – 1055
time (τresp) is the time required to reach the 90% of the gas sensor response, while the recovery time (τrec) is the time the sensor takes to cover the 90% of the difference Rg-Ra (or Gg-Ga), once the injection has been switched off. 3. SnO2 and ZnO single NW devices sensing properties The SnO2 single NW device has been exposed to UV (λ = 254 nm) for three hours, then three hours in dark conditions and finally exposed again to UV light, at different wavelength (λ = 365 nm). During these three steps, NH 3 has been inserted in the test chamber, at three concentrations (namely 10, 20 and 30 ppm). The device has been exposed to NH3 for 5 minutes (Fig. 2a). The SnO2 single NW device does not show a clear response to NH 3 when it is exposed to UV light, while it can detect NH3 in dark conditions. Further analyses show that the device can detect ammonia in dark conditions, down to 5 ppm (Fig. 2b). Increasing the gate voltage (from 0 to 5 V) does not sensibly affect or improve the gas sensing performances of this device. The SnO2 has been exposed to NO2 (at concentrations of 2 and 4 ppm) under UV illumination (λ = 254 nm) and in dark conditions (Fig. 2c).
Fig. 2. SnO2 single NW electrical response to 10, 20 and 30 ppm of NH3 (a), to 5 and 10 ppm of NH3 (b), to 2 and 4 ppm of NO2 (c).
The device shows a remarkable decrease of the current flowing between electrodes when exposed to the target gas in dark conditions (the current decreases of one order of magnitude). However, it does not reach a saturation regime after 30 min of exposure, nor a complete recovery of the baseline. Under UV illumination, for 4 ppm of NO2, the gas sensing response is 0.21, the current reaches a steady state value during the gas exposure (τresp 75 s), and the baseline is completely recovered after few minutes the gas is switched off (τrec 460 s). Finally, the device has been exposed
Fig. 3. Left: ZnO single NW device electrical response to 10, 20 and 30 ppm of NH3. Right: ZnO electrical response to 5 and 10 ppm of NH3. Gate voltage 0 V, RH = 30%.
also to 200 and 400 ppm of CO under UV light and in dark conditions. No clear signal has been detected in both cases. In dark conditions, a very slight increase of the current is recorded during exposure to 400 ppm of CO, consistent to what previously reported by our group, for a different SnO2 single NW device, in dark and dry air conditions [10]. The ZnO single NW device has been exposed to 10, 20 and 30 ppm of ammonia, following the same measurement protocol adopted for SnO2 device (Fig. 3, left). The current value decreases by two orders of magnitude when the UV lamp is switched off. After one hour from the UV is switched off, residual photocurrent is still present, as deduced from the current exponential decrease in dark conditions. The current value under UV illumination at the two
M. Donarelli et al. / Procedia Engineering 168 (2016) 1052 – 1055
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Fig. 4. Left: ZnO single NW electrical response to 5 and 10 ppm of NH3 (gate voltage 5 V). Right: ZnO single NW electrical response to 2 and 4 ppm of NO2 (gate voltage 0 V). In both cases, RH = 30%.
wavelengths (254 and 365 nm) is almost the same, as expected, being the ZnO band gap about 3.3 eV, which is lower than the incident UV photons energies. As for the single SnO2 NW device, the single ZnO NW can detect NH3 in dark conditions, after the UV lamp is switched off. The single ZnO NW shows a higher response to 5 and 10 ppm of ammonia than the single SnO2 NW (Fig. 3, right). Furthermore, increasing the gate voltage of the single ZnO NW device up to 5 V results in a slight decrease of the response time (Fig. 4, left): for 10 ppm of NH3, the response time passes from about 370 s at 0 V to 330 s at 5 V. The single ZnO NW has been exposed also to 2 and 4 ppm of NO2, under UV illumination (λ = 254 nm) and in dark conditions. As reported in Fig. 4, right, the device shows good response in dark conditions, even if the current is very low and the baseline is not recovered after the gas is switched off. The ZnO single NW device does not detect CO. 4. Conclusions We have shown that electron beam lithography is a viable way to fabricate single NW devices for gas sensing tests. Single SnO2 and ZnO NW can be used to detect NH3 and NO2 at concentration of the order of few ppm, under UV illumination and after the UV lamp is switched off, in humid atmosphere. In the case of NO 2, the UV illumination of the SnO2 NW helps it to recover the baseline after the target gas is switched off. For ZnO single NW FET, increasing the back-gate voltage results in a slight decrease of the response time. Finally, the fabricated devices do not show a clear response to CO in humid atmosphere, which is important in order to obtain selective gas sensors. References [1] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z. L. Whang, Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts, Appl. Phys. Lett. 81 (2002) 1869-1871. [2] G. Sberveglieri, C. Baratto, E. Comini, G. Faglia, M. Ferroni, A. Ponzoni, A. Vomiero, Synthesis and characterization of semiconducting nanowires for gas sensing, Sensor. Actuat. B-Chem. 121 (2007) 208-213. [3] J. D. Prades, F. Hernández-Ramirez, T. Fischer, M. Hoffman, R. Müller, N. López, S. Mathur, J. R. Morante, Quantitative analysis of COhumidity gas mixtures with self-heated nanowires operated in pulse mode, Appl. Phys. Lett. 97 (2010) 243105-243107. [4] F. Hernández-Ramirez, A. Tarancon, O. Casals, E. Pellicer, J. Rodriguez, A. Romano-Rodriguez, J. R. Morante, S. Barth, S. Mathur, Electrical properties of individual tin oxide nanowires contacted to platinum electrodes, Phys. Rev. B 76 (2007) 085429. [5] Q. Kuang, C. Lao, Z. L. Wang, Z. Xie, L. Zheng, High-sensitivity humidity sensor based on a single SnO2 nanowire, J. Am. Chem. Soc. 129 (2007) 6070-6071. [6] Z. Fan, J. G. Lu, Chemical sensing with ZnO nanowire field-effect transistor, IEEE Trans. Nanotechnol. 5 (2006) 393-396. [7] M. Law, H. Kind, B. Messer, F. Kim, P. Yang, Photochemical sensing of NO2 with SnO2 nanoribbon nanosensors at room temperature, Angew. Chem. Int. Ed. 41 (2002) 2405-2408. [8] E. Comini, G. Faglia, M. Ferroni, A. Ponzoni, A. Vomiero, G. Sberveglieri, Metal oxide nanowires: preparation and application in gas sensing, J. Mol. Catal. A: Chem. 305 (2009) 170-177. [9] S. Kaciulis, L. Pandolfi, E. Comini, G. Faglia, M. Ferroni, G. Sberveglieri, S. Kandasamy, M. Shafiei, W. Wlodarski, Nanowires of metal oxides for gas sensing application, Surf. Interface Anal. 40 (2007) 575-578. [10] M. Donarelli, R. Milan, M. Ferroni, G. Faglia, E. Comini, G. Sberveglieri, A. Ponzoni, C. Baratto, Fabrication of single-nanowire sensing devices by electron beam lithography, Proceedings of the 2015 18th AISEM Annual Conference (2015) 1-4.