atmospheric pressure fabrication of sno2 nanowires for

Accepted Manuscript Title: Atmospheric pressure fabrication of SnO2 -nanowires for highly sensitive CO and CH4 detection Authors: Anton Kă ck, Alexandra Tischner, Thomas Maier, o Michael Kast, Christian Edtmaier, Christian Gspan, Gerald Kothleitner PII: DOI: Reference: S0925-4005(09)00172-5 doi:10.1016/j.snb.2009.02.055 SNB 11361 To appear in: Sensors and Actuators B Received date: Revised date: Accepted date: 22-10-2008 17-12-2008 20-2-2009 Please cite this article as: A Kă ck, A Tischner, T Maier, M Kast, C Edtmaier, o C Gspan, G Kothleitner, Atmospheric pressure fabrication of SnO2 -nanowires for highly sensitive CO and CH4 detection, Sensors and Actuators B: Chemical (2008), doi:10.1016/j.snb.2009.02.055 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain us cr i Fig M an carrier gas N2 Si wafer ce pt ed wafer carrier shutter Ac hot plate air atomizing nozzle siphon height solution Page of 40 Ac ce pt ed M an us cr i Figure Page of 40 Ac ce p te d M an us cr ip t Figure Page of 40 Ac ce p te d M an us cr ip t Fig Page of 40 Ac ce p te d M an us cr ip t Figure Page of 40 Ac ce p te d M an us cr ip t Figure Page of 40 Ac ce pt ed M an us cr i Fig new as pdf Page of 40 Ac c ep te d M an us cr ip t Fig new as ppt Page of 40 90 2.7E+06 M 70 2.8E+06 an Sensor R [Ω] 80 2.6E+06 d 60 2.5E+06 pt e 50 40 2.4E+06 ce 30 20 2.3E+06 Ac Humidity [%] us rH meas (%) 2.9E+06 10 300 600 900 Sensor Resistance [Ω] 100 cr ip t Fig color 1200 1500 1800 2100 2.2E+06 2400 Time [s] Page of 40 control All results presented in Fig – 11 have been achieved with the specific single nanowire sensor element shown in Fig The current-voltage curves, which have been measured well below a potential breakdown voltage, show a linear ohmic behaviour and indicate ohmic contacts of the Ti-Au contact pads to the SnO2- ip t nanowires The operating mode of the sensors is based on a change in the electrical conductance along the SnO2-nanowires due to interacting gas molecules on the surface Sensor response to humidity, CO, cr and CH4 was detected by applying a constant DC-current of typically 50 µA to a single sensor element and by measuring the voltage drop, which gives the sensor resistance used in all diagrams as a measure us of the responsivity an Sensing performance is investigated in an automated measurement setup, which has been described in detail in Ref [42] Synthetic air (20% O2, 80% N2, Linde Gas) is used in all sensing experiments as M background gas with a flow rate of 1000 sccm monitored by a mass flow controller The test gases are ready-made mixtures of 300 ppm CO, and 2.5 Vol% CH4 (Linde Gas) in synthetic air, respectively, d which are mixed to the background gas in a gas mixing vessel The total flow of background gas and test te gas was held constant at 1000 sccm in order to avoid any cooling during measurement due to flow rate thermocouple ce p variations In addition the temperature of the sensor chip is continuously monitored by the Fig shows the resistance of the single SnO2-nanowire sensor at an operating temperature of 250°C Ac when exposed to CO-gas pulses with concentrations of ppm in dry air Response time is found to be ~ 25 sec The resistance of the SnO2-nanowire is increased in presence of CO, which is basically opposite to theory This behaviour might be due to different forms of oxygen ionosorbed on the SnO2surface, which depend on temperature Since O2- has a lower activation energy it dominates up to about 200°C, at higher temperature the O- form dominates [51, 52] The observed increase of sensor resistance indicates an electron transfer from the surface to the CO molecules and thus a O2- dominated surface For temperatures above 300oC we observe a decrease of sensor resistance, which indicates an Odominated surface in the higher temperature range We have observed a similar transition from 10 Page 26 of 40 increasing to decreasing the sensor resistance around a temperature of 250oC in case of our nanocrystalline SnO2-sensors This characteristic might be of high value to achieve a certain selectivity Fig shows the response of the SnO2-nanowire sensor at an operating temperature of 200°C response when exposed to CH4-gas pulses with concentrations of in dry air 66 ppm in dry air Response time is ip t found to be ~ 25 sec The sensor resistance is decreased in presence of CH4, which is according to the theory Fig 10 represents the sensor response at an operating temperature of 200°C when exposed to cr CH4-gas pulses with a concentration of only ppm Fig 11 shows the humidity response of the SnO2-nanowire sensor at 200°C operating temperature, us when the humidity is switched in steps from 4% to 96% rH (time period 300 sec) The full line an represents the response of a reference humidity sensor; the other line represents the response of the SnO2-nanowire sensor As expected from theory the resistance of the SnO2-nanowire decreases in M presence of water The sensor reacts very sensitively to the first increase of humidity and tends to saturate at high values The single steps, however, can still be resolved As soon as the humidity is d switched off the sensor response recovers within ~140 sec to the initial value te The experiments have been conducted over several days and as is obvious from the results, the resistance of the nanowire sensor has increased over that time period from the 104 Ohm range (Fig 8) up ce p to the 106 Ohm range (Fig - Fig.11), where the resistance is reaching its final value All measurements have been performed without intermediate heating of the sensor up to temperatures exceeding 250oC, which is most probably responsible for the observed drift In case of the nanocrystalline SnO2-sensor Ac [Ref 42] intermediate heating up of the sensor to 450oC has been found to be a useful strategy for recovering the device response The influence of intermediate heating on the nanowire resistance will be studied in detail in future experiments The measurements were performed in a wide range of gas concentrations Many applications, however, demand high sensitivity, i.e which are the lowest gas concentrations that can be still detected with a sensor We have found that the nanowire sensors react sensitively in the low gas concentration regime up to 10 ppm and start to saturate for higher concentrations This is due to the high surface to 11 Page 27 of 40 volume ratio of the SnO2-nanowire, where the sensor response is dominated by the surface and its interaction with surrounding gases The ability for detection of CO and CH4 in the low ppm concentration regime is the main feature of our nanowire sensor device As in our case the resistance is used as the measure of sensor response, we have measured the resistance change, when the sensor is ip t exposed to the target gas, in dependence of gas concentration By stepwise reducing the gas concentration we can approach the sensitivity limit of the nanowire sensor As long as the target gas cr exposure results in a distinct resistance change, we define the device to be sensitive to the specific target gas concentration The response of the sensor to ppm CO-concentration (see Fig 9) can be clearly us resolved and indicates a sensitivity limit well below ppm, which exceeds conventional metal oxide an based sensors on the market The response to ppm CH4-concentration (see Fig 10) is close to the limit of resolution and reflects the absolute sensitivity towards CH4 M Due to different measurement setups, such as different background gases [53] or measurement in vacuum conditions prior to target gas exposure [37, 38], it is not easy to compare the achieved results d with other reported results Our results can be most likely compared to the results reported by Comini te et.al [26], who have achieved a clear sensor response to 250 and 500 ppm CO with tin oxide nanobelts Kolmakov et.al [53] have reported tin oxide nanowire sensor response to a CO concentration of ppm, ce p which has been also achieved by Hernandez-Ramirez et.al [41] The detection of NO2, for example, seems to be easier than for the reported gases CO and CH4, where sensitivity limits down to ppb levels Ac have been reported by several groups [26, 27, 32] We have achieved a response time around 25 sec for both CO and CH4, which compares with the value of 30 sec reported by Kolmakov et.al [53] However, we think that in our case the measured response time is considerably limited by the measurement setup Further measurements on repeatability and long-term signal drift of the nanowire sensors are presently under progress Although the presented SnO2-nanowire device demonstrates its high potential for practical supersensitive gas detecting applications, it suffers from the required SnO2-nanowire transfer process from the growth substrate to the sensor substrate This is basically feasible; the final fabrication process 12 Page 28 of 40 requires only one photolithographic step for the evaporation of metal contacts, which makes the process very simple Depending on the statistic arrangement of the nanowires on the wafer this procedure results in devices each of them incorporating either a single or a few SnO2-nanowires in parallel as sensing elements In addition the diameters of the SnO2-nanowires are different from device to device, which is ip t a problem with respect to sensor calibration Outlook and summary cr Further development of the present SnO2-nanowire sensor to a real-world device is focused on improvement of sensor selectivity and on specific control of nanowire growth Both issues will be us tackled by means of our fabrication technology Selectivity towards specific gas species is a central issue an with all metal-oxide-based sensors and is also a problem for the presented single crystalline SnO2nanowire sensor device In order to achieve well defined selectivity, dopants will be added to the SnO2- M nanowires The spray pyrolysis process in particular offers a comparable simple way to modify the chemical composition of the nanocrystalline SnO2-layers by adding chemicals, such as InCl3 or SbCl3, d for example, to the spray solution This should result in a shift of nanowire material composition from te SnO2 to Sn1-xInxO2, and Sn1-xSbxO2, respectively, which should have considerable influence on sensor selectivity This approach is presently investigated in detail ce p Control of SnO2-nanowire growth is also based on the two-step fabrication technology In case of our fabrication procedure the nanowires grow only in the regions of the nanocrystalline SnO2-films Ac Patterning of the nanocrystalline SnO2-films prior to the annealing process step should thus allow for nanowire growth in well-defined areas of the sample, which is of high importance with respect to practical device fabrication In order to understand the influence of the intermediate metal layers on nanowire growth and geometry, Cu and Au dot arrays with different diameters are presently fabricated by electron beam lithography and lift-off prior to the spray pyrolysis deposition of the nanocrystalline SnO2-films In case that the intermediate metal layers trigger the nanowire growth, the metal dots should allow for better control of the nanowire diameter Extensive experiments for better understanding the growth mechanism, in particular the influence of the intermediate metal layers, are presently in progress 13 Page 29 of 40 A combination of both patterned SnO2-structures as well as metal dot arrays might solve the problem of controlling SnO2-nanowire growth directly on the sensor chip Presently we use an experimental setup with a single spray nozzle, which allows for coating a substrate size of about x cm2 homogeneously with the nanocrystalline SnO2-film Moving the spray nozzle or using additional ip t spraying nozzles would easily allow an upscale of the possible substrate to 6”-wafer size Thus our fabrication procedure might be the technology of choice for the controlled fabrication of SnO2- cr nanowires as gas sensing elements on a wafer scale In summary we have demonstrated a new approach for the fabrication of ultra-long single crystalline us SnO2-nanowires for gas sensing applications based on a combined spray pyrolysis and annealing an process The SnO2-nanowires have been grown on SiO2-coated Si-substrates with diameters 30 - 400 nm and lengths up to several 100 µm The SnO2-nanowire fabrication procedure is performed at M atmospheric pressure and requires no vacuum The experimental results suggest a competing evaporation and condensation process, which converts the nanocrystalline SnO2-films into single d crystalline SnO2-nanowires directly on the chip We believe that our fabrication procedure might be the te technology of choice for the controlled fabrication of SnO2-nanowires on a wafer scale TEM-analysis has revealed the single crystalline character of SnO2-nanowires with two preferred growth directions ce p For the realization of gas sensors the SnO2-nanowires have been transferred to another SiO2-coated Sisubstrate; final evaporation of Ti/Au contact pads on both ends of single SnO2-nanowires has enabled Ac their direct use as sensing elements The nanowire sensors are very sensitive and are able to detect concentrations of CO and CH4 in the low ppm-regime Experiments for better understanding of the growth mechanism, and for well-defined SnO2-nanowire growth directly on the sensor chip are presently under investigation Acknowledgements We thank Dr Theodore Dimopoulos1 for sputter coating the samples with Cu-layers, Thomas Narzt from IMS Nanofabrication AG, Vienna, Austria, for the SEM images, and Dr Angelika Reichmann2 for the supporting ESEM studies 14 Page 30 of 40 FIGURE CAPTIONS Figure Setup of spray pyrolysis process for parallel flow of atomized spray Figure SnO2-nanowires fabricated on Si-samples Nanowires have diameters of 30 - 400 nm and Figure Electron diffraction pattern from a nanowire in zone axis [001] ip t lengths up to several 100 µm cr Figure TEM bright field image from nanowires, which are found to grow either straight or alternating us along two preferred growth directions The frame indicates the region for the high resolution TEM image in Fig an Figure HRTEM image which shows the two preferred growth directions [100] and [110] M Figure HRTEM image from a needle with a diameter of about 40 nm The surface is pyramidal shaped d Figure SEM-photo of a single SnO2-nanowire sensor element with diameter of ~ 80 nm and a length te of ~ 55 µm The inset schematically shows the sensor geometry with the wire bonds, the arrows indicate ce p the current flow through the sample Figure Dependence of sensor resistance (blue) to stepwise increased humidity at 200°C operating Ac temperature and response of the reference sensor (black) Figure Sensor response to ppm CO exposure at 250°C operating temperature Figure 10 Sensor response to ppm CH4 exposure at 200°C operating temperature REFERENCES 15 Page 31 of 40 [1] H Meixner, J Gerblinger, U Lampe, M Fleischer, Thin-film gas sensors based on semiconducting metal oxides, Sens Act B 23 (1995) 119–125 [2] M Fleischer, H Meixner, Fast gas sensors based on metal oxides which are stable at high temperatures, Sens Act B 43 (1997) 1–10 ip t [3] G Korotcenkov, Metal oxides for solid-state gas sensors: What determines our choice?, Materials cr Science and Engineering B 139 (2007) 1–23 [4] V Malyshev, A Pislyakov, Investigation of gas-sensitivity of sensor structures to carbon us monoxide in a wide range of temperature, concentration and humidity of gas 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40 BIBLIOGRAPHIES Anton Köck ip t Anton Köck received his master’s degree (1986) and PhD (1989) in Experimental Physics at the University of Innsbruck, Austria After a Post Doc position at the Walter Schottky Institute, TUM, cr where he worked on quantum well IR detectors and laser diodes, he was head of the Optoelectronics research group at the Institute for Solid State Electronics, VUT, where he habilitated in 1998 He was us head of the MEMS research group at the Wiener Neustadt University for Applied Sciences Since 2004 an he is deputy head of the ARC-division Nano-System-Technologies and is developing nanosensors for gas detection and new photonic devices M Alexandra Tischner d Alexandra Tischner studied Technical Physics at the Vienna University of Technology (VUT), te Austria, and received her graduate degree (DI) in 2005 In addition she holds a baccalaureate degree in Medical Informatics Since 2006 she is PhD student at the VUT and works on her thesis "Development ce p of electronic noses based on nanosensors" at the division Nano-System-Technologies of Austrian Research Centers GmbH – ARC, Vienna, Austria Ac Thomas Maier 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 ARC-division Nano-System-Technologies he is developing bolometers and nanosensors for gas detection Michael Kast 22 Page 38 of 40 Michael Kast studied Technical Physics at the Johannes Kepler University Linz and at the Vienna University of Technology, Austria He received his graduate degree (DI) in 1999 and finished his PhD in 2003 on high-resolution hot-electron spectroscopy at the Institute of Solid State Electronics, VUT In 2004 he joined as research scientist and quality manager the division Nano-System-Technologies of the ip t Austrian Research Centers GmbH – ARC, where he is working on the synthesis of Zn and ZnO nanowires cr Christian Edtmaier us Christian Edtmaier received his master´s degree (1990) and PhD (1993) in mechanical and process engineering at the Vienna University of Technology, Austria Within a post doc position he worked for an Schott Glas-Mainz, Germany on dispersion reinforced precious metals Since 1996 he is working at the Institute of Chemical Technologies and Analytics within the Faculty of Chemistry, TU Vienna and is M head of the research group of Nano-Materials Main focus of work concerns nano-composites, tailoring d interfaces, growth and thermal transport phenomena of nano-structures te Christian Gspan ce p Christian Gspan received his master's degree (2003) in Technical Chemistry at the Graz University of Technology, Austria Since 2003 he is working in the field of Transmission Electron Microscopy at the Institute of Electron Microscopy of the Graz University of Technology Ac Gerald Kothleitner Gerald Kothleitner received his master's degree (1993) and PhD (1996) in Physical Chemistry at the Graz University of Technology, Austria After his PhD he took a position in industry with Gatan Inc, California, where he was responsible for analytical electron microscopy equipment As a product manager he significantly influenced the design and functionality of spectrometers and energy-filters for TEM analysis In 2000 he returned to the TU Graz, where he habilitated 2004 in Applied Physical 23 Page 39 of 40 Chemistry He is head of a research group, which is focused on improving electron microscopic imaging Ac ce p te d M an us cr ip t and analysis methods for the application to novel materials 24 Page 40 of 40 ... nanocrystalline SnO2- films into single d crystalline SnO2- nanowires directly on the chip We believe that our fabrication procedure might be the te technology of choice for the controlled fabrication of SnO2- nanowires. .. Centre for Electron Microscopy Graz, 8010 Graz, Austria Page 17 of 40 ABSTRACT ip t In this paper we report on a new approach for the fabrication of ultra long single crystalline SnO 2nanowires for. .. operating te temperatures of 200 – 250°C We believe that our fabrication procedure might be the technology of Ac wafer scale ce p choice for the controlled fabrication of SnO2- nanowires as highly sensitive