Albert Romano4Rodriguez1, Francisco Hernandez4Ramirez1,2, Joan Daniel .... S. Mathur, S. Barth, H. Shen, J. C. Pyun, U. Werner, Small 2005, 1, 713. 5.
Bottom-up Fabrication of Individual SnO2 Nanowires-based Gas Sensors on Suspended Micromembranes Albert Romano-Rodriguez1, Francisco Hernandez-Ramirez1,2, Joan Daniel Prades1, Albert Tarancon1, Olga Casals1, Roman Jimenez-Diaz1, Miguel Angel Juli1,2, Joan Ramon Morante1, Sven Barth3,4, Sanjay Mathur3,4, Andreas Helwig5, Jan Spannhake5, and Gerhard Mueller5 1 Electronics, University of Barcelona, Martí i Franquès 1, Barcelona, E-08028, Spain 2 NTEC106, S.L., Mare de Deu dels Desemparats 12 baixos, L'Hospitalet de Llobregat, E-08903, Spain 3 Chemistry, University Würzburg, Würzburg, D-97070, Germany 4 Nanocrystaline Materials and Thin Film Systems, Leibniz Institut of New Materials, Saarbruecken, D-66123, Germany 5 IW-SI Sensors, Electronics & System Integration, EADS Deutschland GmbH, Muenchen, D81663, Germany ABSTRACT Bottom – up techniques were used to obtain gas sensors based on individual SnO2 nanowires placed over microhotplates with integrated heaters. These nanowires were electrically contacted to pre-patterned microelectrodes by means of Focused Ion Beam (FIB) nanofabrication methodologies. The performance of these sensors, which exhibit reproducible and stable responses, was evaluated as function of different gas atmospheres and dissipated power by the heater, demonstrating that this technological approach could be used to develop functional devices based on nanomaterials. INTRODUCTION The new properties of nanomaterials with respect to bulk materials have attracted great research interest because of their potential applications in functional devices [1]. Although great advances have been produced in the synthesis and the characterization of their fundamental properties, the fabrication of reliable and reproducible devices based on these nanomaterials is still scarce due to the difficulties to study them [2]. In this work, bottom – up techniques were used to fabricate sensors based on individual SnO2 nanowires placed over microhotplates with heaters, which allows a fast modulation of their working temperature and their sensing characteristics. This approach is based on previous work in which similar nanowires have been contacted on standard photolighographically defined microelectodees on oxidized silicon wafers [3]. The stability of some of these devices was evaluated as function of the operating time and the applied current, showing good performance for weeks and currents below I = 100 nA. At higher currents, self – heating related problems, such as degradation of the electrical contacts and irreversible damage of the nanowires were observed.
EXPERIMENT Monocrystalline SnO2 nanowires were synthesized by chemical vapor deposition following a process explained elsewhere [4]. These nanowires were grown single-crystalline with dislocation-free bodies. Their main growth direction was [100] with interplanar spacing according to the rutile structure of SnO2. The length and radii of these nanowires varied between 2 and 7 microns, and between 30 and 70 nm respectively. The grown nanowires were dispersed over the microhotplate with an integrated heater and lithographically pre-patterned platinum interdigitated electrodes (figure 1.a.). Similar structures were reported elsewhere [5].
Figure 1. (a) Image of one microhotplate with integrated heater and (b) detail of a single nanowire contacted with Pt strips to the interdigitated electrodes. Contacts between the electrodes and nanowires were fabricated with a FEI Dual-Beam Strata 235 FIB instrument with a trimethyl - methylcyclopentadienyl – platinum ((CH3)3CH3C5H4Pt) injector to deposit platinum [6]. To guarantee that the nanofabrication process did not alter the properties of nanowires, deposition of platinum strips over these structures were performed by means of an electron beam induced process while the rest of the contacts, until arrive at pre – patterned microelectrodes, was continued with ion – beam induced depositions (figure 1.b). This step reduces the required fabrication time, which takes approximately two hours for each device, limiting this technique in large scale processes [3, 5]. Electrical measurements were performed with the help of a circuit designed to guarantee low currents and avoid any undesired current fluctuation [7, 8]. All the experiments were performed in a home-made chamber, where the gas flow was maintained between 50 and 200 ml min-1. Accurate gas mixtures were prepared by combining different gases passing through mass flow controllers.
DISCUSSION The electrical stability as function of operating time and the applied current was measured for up to ten devices. No significant degradation was observed after several weeks operating with currents below I = 100 nA. On the contrary, they were quickly destroyed when currents overcame this value, because of uncontrolled self – heating effects produced during the measurement [9]. In this case, two degradation processes were clearly identified: breaking of the nanowires and melting of the contacts. This last effect is related to the evaporation of carbon present in high concentrations (around 70 %) in the FIB – assisted depositions. Similar modifications on these interconnections have been reported elsewhere [10]
Figure 2. Sensor destroyed after applying high current I = 100 µA. (a) The rupture of the nanowire, (b) and the degradation of the contacts due to the evaporation of carbon of the Pt depositions can be clearly observed. Depending on the diameter and length of the nanowires, electrical resistances were estimated ranging from a few kiloohms to a few megaohms. The results were in concordance with the previously reported resistivity values found on similar nanowires [3]. The effect of the temperature on the resistance of the device was also studied. Figure 3a shows the variation of the resistance along the time when different pulses of electrical power to the heater were applied. Increasing power, directly related to temperature (Figure 3b), leads to a decrease in the nanowire’s resistance, demonstrating the semiconductor characteristics of the nanowire. The modulation of the resistance produced by heater pulses disappeared after a few seconds of switching it off. If applied power above P = 45 mW (T ≈ 400 K), the recovery time increases a few minutes suggesting that modifications of the adsorbed species at the nanowire surface are produced, mainly oxygen and water molecules [11, 12].
Figure 3. (a) Evolution in time of the resistance of one single SnO2 nanowire (L = 6.4 µm, R = 50 ± 5 nm) to series of increasing pulses of electrical power to the heater. The experiment was performed in synthetic air. (b) Calibration of the dissipated power by the heater P and the effective temperature of the microhotplate T [13] as function of applied voltage V. Thus, the operating temperature of these devices can be varied applying controlled power to the heater. This process is fast enough to reach a complete thermal stabilization in few seconds (Figure 3a), demonstrating that the optimal working conditions were easily modulated in a fast and controlled way. Gas sensing experiments were performed with different atmospheres to evaluate the capabilities of these devices as functional elements. It is well known that the electrical resistance of SnO2 is modulated by oxygen adsorption and desorption, which are thermally activated processes [11, 12]. This point is demonstrated in figure 4 where one of these sensors responds to pulses of synthethic air (SA) (20 % of O2) alternated with nitrogen (N2) pulses for a dissipated power by the heater of P = 23 mW (T ≈ 340 K). On the contrary, no response is observed when the heater is switched off, as it was demonstrated elsewhere [8].
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Figure 4. Response of the device to nitrogen and synthetic air pulses with a dissipated power in the heater of P = 23 mW (T ≈ 340 K). The resistance drops when nitrogen is passed through the chamber demonstrating oxygen desorption. The dashed lines represent the synthetic air pulses.
In this work, we also report the possibility of using these devices to detect carbon monoxide (CO). CO is a reducing gas which leads to a reversible reduction of the SnO2 resistance. Applying a power to the heater of P =56 mW (T ≈ 393 K), the device is able to detect and discriminate CO concentrations of 50, 100 and 200 ppm (figure 5).
Figure 5. Response of the device to three CO pulses of 50, 100 and 200 ppm with a dissipated power in the heater of P = 56 mW (T ≈ 393 K). The dashed lines indicate CO pulses. CONCLUSIONS A bottom-up fabrication process of gas microsensors based on single nanowires is shown to be effective and reliable. Individual SnO2 nanowires were contacted to pre-patterned microelectrodes placed on microhotplates. These devices show good stability as function of operating time if currents below 100 nA are applied. Additionally, the integration of a heater in these devices leads to the possibility of enhancing the adsorption / desorption of gas molecules on the surface of these nanowires, improving their responses as gas sensors. The possibility of using this experimental methodology to obtain functional devices was demonstrated. ACKNOWLEDGMENTS This work has been partially supported by the EU through the project NANOS4 of the 6th FMP. The support of the Spanish Ministry of Education and Science (MEC) is also acknowledged through the projects MAGASENS and CROMINA, through the FPU grants of several authors (F-H.-R., O.C. and J.D.P) and through the Torres Quevedo PTQ05 –02 – 0301 program (F.H.-R.). Thanks are due to the German Science Foundation (DFG) for supporting this work in the frame of the priority program on nanomaterials – Sonderforschungsbereich 277 – at the Saarland University, Saarbruecken, Germany. The authors would like to acknowledge the valuable suggestions of Dr. O. Ruiz during the development of the electronic circuit. REFERENCES 1. Xing-Jiu Huang and Yang-Kyu Choi Sensors Actuators B 2007 122 659. 2. Y. Chen, C. Zhu, M. Cao, T. Wang, Nanotenology 2007, 18, 285502. 3. Francisco Hernández-Ramírez, Albert Tarancón, Olga Casals, Jordi Rodríguez, Albert Romano-Rodríguez, Joan R Morante, Sven Barth, Sanjay Mathur, Tae Y Choi, Dimos Poulikakos, Victor Callegari and Philipp M Nellen, Nanotechnology 2006 17 5577
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