Fabrication and integration of metal oxide nanowire sensors using dielectrophoretic assembly and improved post-assembly processing

Sensors and Actuators B 148 (2010) 404–412

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Fabrication and integration of metal oxide nanowire sensors using dielectrophoretic assembly and improved post-assembly processing Xiaopeng Li a , Erica Chin a , Hongwei Sun b , Pradeep Kurup c , Zhiyong Gu a,∗ a b c

Department of Chemical Engineering, University of Massachusetts Lowell, One University Ave., Lowell, MA 01854, USA Department of Mechanical Engineering, University of Massachusetts Lowell, One University Ave., Lowell, MA 01854, USA Department of Civil and Environmental Engineering, University of Massachusetts Lowell, One University Ave., Lowell, MA 01854, USA

a r t i c l e

i n f o

Article history: Received 30 December 2009 Received in revised form 17 May 2010 Accepted 25 May 2010 Available online 4 June 2010 Keywords: Metal oxide sensor Nanowire Template synthesis Dielectrophoretic assembly Thermal bonding

a b s t r a c t We report the fabrication and integration of metal oxide nanowire sensors using dielectrophoretic assembly and a novel bonding process. Metal oxide nanowires were successfully prepared by a two-step thermal oxidation process from their corresponding metal nanowires (indium, tin, and indium–tin) that have been synthesized by electroplating in nanoporous templates. Before oxidation, dielectrophoretic (DEP) assembly and a novel post-assembly bonding process were applied to integrate high-density nanowire arrays on interdigitated microelectrodes. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDS) elemental analysis provide morphological and compositional informations of the metal and metal oxide nanowires. Electrical measurements of the nanowire arrays show how the resistance changed after each process, which demonstrates that the post-assembly bonding is an important and effective step in reducing contact resistance between the nanowires and interdigitated microelectrodes. Metal oxide nanowire sensor chips were fabricated using microelectrodes embedded with a microheater for temperature control. The performance of these metal oxide nanowire sensors was investigated towards common volatile organic compounds, including methanol, ethanol, isopropanol, acetone, chloroform, and benzene, and the sensors showed high sensitivity, fast response, and good repeatability. The assembly and improved post-assembly bonding processes developed in this research provide a new platform for nanowire-based sensor integration. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Metal oxide nanowires, one of the most important onedimensional nanostructures, have gained great attention as a key component in catalysts [1,2], sensors [3–5], electronic [6,7] and optical devices [8–10]. Among these potential applications, nanowire-based chemical sensors have been extensively studied in recent years and enhanced sensor performance has been reported compared to conventional metal oxide sensors [11,12]. Synthesis methods of metal oxide nanowires include physical deposition (vapor–solid/vapor–liquid–solid/solution–liquid–solid growth), thermal oxidation growth [13,14], electrospinning [15], and nanolithography [16,17]. Using these methods, various transition metal oxides, such as indium oxide [18,19], tin oxide [20–22], tungsten oxide [23,24], copper oxide [25,26] and zinc oxide [8,27,28] have been successfully synthesized in the shape of nanowires.

∗ Corresponding author. Tel.: +1 978 934 3540; fax: +1 978 934 3047. E-mail address: Zhiyong [email protected] (Z. Gu). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.05.062

However, future sensor development relies on not only the high surface-to-volume ratio and diversity in chemical composition of nanoelements including metal oxide, but also on the ease and effectiveness of how sensor elements are assembled and integrated. Up to date, most of the nanosensor devices fabricated only utilize one or few nanoelements, and thus lack stability and repeatability. On the other hand, sensor arrays with a large number of nanoelements (one or more types) provide more stability, repeatability, error tolerance, and possibility for higher selectivity of various analysts. However, the challenging issues of nanoelement uniformity and dispersibility prevent efficient assembly of sensor array. Besides the issues above, effective contact formation, which is a requirement for interconnect formation and device integration, has emerged as a new challenge in nanoscience and nanotechnology applications [29], including sensor assembly and integration. As the focus of nanotechnology research shifts to practical implementations and nanomanufacturing, both top-down [21,30] and bottom-up [31,32] approaches are being developed to integrate nanowires for device applications. The top-down approach depends heavily on photolithography techniques involving many steps, which are serial, time-consuming and expensive. On the other hand, in the bottom-up approach, various interaction forces and an external electric or magnetic field have been applied for

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the self-assembly or directed assembly of nanowires. For example, dielectrophoresis [33] provides an easy way to manipulate nanoparticles in a non-uniform electric field as long as the geometry of the particle allows for generating a dipole moment. Magnetic assembly is another powerful way to accurately align nanowires or place nanowires into desired locations [34,35]; however, this method normally needs the presence of magnetic materials on the nanowires. The methods mentioned above enable the integration of nanowire sensor devices where a single nanowire [21], nanowire array [36] or randomly dispersed nanowire network [37] is integrated as a transistor [38] or chemoresistor [15] to detect chemicals or biomolecules. In this article we present a novel method of fabricating metal oxide nanowires, which takes advantage of the template-assisted electrodeposition method and a two-step thermal oxidation process. Through this method, indium oxide, tin oxide, and indium–tin oxide nanowires were successfully fabricated. Compared to nanowires synthesized through other methods, these nanowires are more uniform and easy to process and assemble with good control of chemical compositions. Dielectrophoretic (DEP) assembly was used to align a large number of nanowires onto interdigitated microelectrodes. More importantly, a novel post-assembly bonding process was demonstrated to effectively join nanowires with interdigitated microelectrodes, which helped to lower the contact resistance between the aligned nanowires and microelectrodes and increased the stability of the nanowire assembled. After effective bonding, a two-step oxidation process was used to convert the metal nanowires into metal oxide nanowires to form the nanowire sensor device. Finally, sensor performance was investigated towards different volatile organic compounds (VOCs), as well as the temperature and composition effects on the sensor performance. 2. Experimental 2.1. Materials and chemicals Silver (Ag) plating solution (Techni Silver E-2) was purchased from Technic, Inc. Tin (Sn) plating solution was prepared using the Techni Tin solution together with corresponding makeup solution and antioxidant (also obtained from Technic, Inc). Indium (In) plating solution (indium sulfamate) was purchased from Indium Corp. Indium/tin (In/Sn) plating solution was prepared in accordance with a recipe in reference [39]. Sodium hydroxide (NaOH), dichloromethane, methanol, ethanol, isopropanol, acetone, chloroform, and benzene were purchased from Fisher Scientific. 1H-Benzotriazole (99+ %) was purchased from Acros Organics. All the chemicals were used as received. Nitrogen gas (industrial grade, 99.8%) was purchased from Airgas East. Deionized water was prepared by a Barnstead E-pure system (model no. D4541) at 18.2 M cm. Alumina and polycarbonate membranes were purchased from Whatman (now part of GE Healthcare). Silver (99.99% purity) was purchased from Alfa Aesar, and silver thin film deposition was conducted in a NTE-3000 thermal evaporator (Nano-Master, Inc.). Nanowire fabrication was performed on a Princeton Applied Research (PAR) model 362 potentiostat. The bonding process and thermal oxidation of nanowires were carried out in a Thermo Scientific Lindberg Blue M tube furnace with programmable temperature control. 2.2. Nanowire fabrication Nanowire fabrication using the template-assisted electroplating method has been discussed in detail in our previous work [40]. Briefly, a 200 nm thick silver layer was evaporated on one side of an anodized aluminum oxide (AAO) template or track-etched polycarbonate membrane as a sacrificial layer. Later this side of

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the membrane was placed in contact with a copper plate, which served as the cathode, and restrained by a glass tube and o-ring seal. The growth of nanowires by electrodeposition was achieved by applying current through the plating bath, which was filled with the electrolytic solution of choice. The template was first soaked in water for 5 min and then filled with the appropriate electrolytic solution. Current density was well controlled when the plating solution for indium, tin, and indium–tin was utilized. The length of nanowires was controlled by the duration of the applied current. The sacrificial silver layer was etched away using a silver-etching solution, which consisted of methanol, ammonium hydroxide (30%) and hydrogen peroxide (30%) with the volume ratio of 4:1:1 [41]. After being rinsed several times in water, the membrane was dissolved in NaOH (for the AAO membrane) or dichloromethane (for the polycarbonate membrane) to release the nanowires. To protect tin and indium from corrosion, benzotriazole was added into NaOH solution when dissolving the template. Released nanowires were rinsed repeatedly with water and ethanol (using repeated sonication and centrifugation cycles) and finally stored in ethanol for further utilization. 2.3. Dielectrophoretic assembly of metal nanowires A schematic of the sensor assembly and integration process is shown in Fig. 1. Two types of microelectrodes were used in this work: the first one was fabricated in-house through a typical photolithography process (gold electrode), while the second one was a commercially available product (Heraeus, MSP332, platinum electrode) with an integrated micro-hotplate for temperature control up to 440 ◦ C. Nanowires stored in ethanol were dispersed in an ultrasonic water bath for 3 min before the DEP assembly process. A few droplets of nanowire suspensions were transferred using a pipette onto the microelectrode substrate and the DEP assembly was applied to align nanowires onto the electrodes (step 1 in Fig. 1). The electrical field was generated by connecting a function generator (Tektronix FG502 11 MHz Function Generator) to the interdigitated electrodes, while a sinusoidal wave with a frequency of 5–6 MHz and peak-to-peak voltage of 6 V was applied during the process (step 2). At room temperature, ethanol volatized within minutes, leaving nanowires aligned between the electrode pairs (step 3). 2.4. Thermal bonding in nitrogen and two-step oxidation in air Two distinct thermal treatment processes were carried out after the DEP assembly of nanowires. First, aligned metal nanowires were heated to about 10◦ below their respective melting points; the temperature varied according to different material compositions for the indium–tin nanowires. Here the melting points were assumed to be the same as their bulk materials since the size-effect on melting behavior is not obvious for nanowires with a diameter larger than 20 nm [40]. To avoid unnecessary oxidation, this bonding process was carried out in an inert nitrogen environment (step 4). Next, nanowires were heated at the temperature just below the melting point of the respective material in an atmospheric environment for about 2 h of oxidation. The temperature was later raised to 400–500 ◦ C for another 3 h to achieve full oxidation (step 5). When the assembly and thermal treatment processes were finished, the nanowire sensor chip was ready for electrical measurement and chemical response testing (step 6). 2.5. Structural and compositional characterization Images of nanowires before and after assembly were taken using a JEOL JSM-7401F field emission scanning electron microscope (FESEM). An EDAX Genesis V4.61 X-ray detector on the FE-SEM was

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Fig. 1. Schematic of nanowire sensor integration: (1) nanowire suspension drop casting on interdigitated microelectrodes; (2) DEP assembly of metal nanowires; (3) final aligned nanowires on microelectrodes; (4) bonding of metal nanowires onto interdigitated electrodes in nitrogen atmosphere; (5) two-step thermal oxidation of metal nanowires into oxide nanowires in air and (6) final integrated sensor chip.

used to analyze elemental information of nanowires before and after thermal oxidation. Optical images were obtained from an Olympus CX-41 microscope equipped with a DP-71 CCD camera. 2.6. Measurement of electrical properties and gas-sensing property The resistance measurements were conducted using a sourcemeter (Keithley 2400) at a wide temperature range 25–440 ◦ C. Current–voltage (I–V) characteristics of these semiconducting metal oxide materials were determined using a sweeping voltage from −3 to 3 V to determine the specific resistance at a corresponding voltage. The gas-sensing set-up consists of an airtight chamber of a fixed volume with an inlet and outlet for air-flow as well as a septum port for analyte injection. The size of the chamber was either 50 mL or 4.4 L based on the choice of chemical and related concentration range. The sensor chip (DEP aligned nanowires after thermal oxidation with the electrode and integrated micro-hotplate) was hung inside the chamber with all connection wires protruding out through a small port on the cap of the chamber, which was later airtight sealed. Before any test, a low flow rate of dry compressed air was used to purge the chamber for over 30 min, then both the inlet and outlet were closed and a stable baseline was obtained for resistance monitoring. Sensor response was measured by injecting appropriate amount of analyte into the chamber, which caused a step change in concentration. A syringe or syringe pump was utilized to accurately control the injection amount. At the same time the corresponding resistance change was captured using the sourcemeter. The injection amount is determined by the vapor pressure of the target chemical and desired concentration. To study the recovery process, after the resistance became stable and was maintained for 30 s, the inlet and outlet were opened to flush the gas inside the chamber that lowered the concentration of analyte chemical back to the original status. Temperature control up to 440 ◦ C was enabled by the micro-hotplate. 3. Results and discussion 3.1. Metal and metal oxide nanowires Various metal and mixed metal nanowires, including Sn, In, and In/Sn have been successfully fabricated. Fig. 2A shows a SEM image

of an indium–tin mixed metal nanowire fabricated using an AAO template, and Fig. 2B shows a layout of EDS elemental analysis of indium, tin and indium–tin nanowires. The upper two spectra demonstrate that the nanowires are pure indium or tin, while the bottom spectrum proves the coexistence of indium and tin elements on the In/Sn nanowires. In principle, other compositions of the metal alloy nanowires can also be obtained using the same method. The diameter of the nanowires was fixed by the actual pore size of either the AAO template or polycarbonate membrane, while the length of the nanowires was varied based on the time duration of electrical current. Normally, the AAO template was used for nanowires with a nominal diameter of 200 nm, while the polycarbonate membranes were used for smaller diameter nanowires, such as 30 nm and 50 nm. In order to effectively convert metal nanowires into metal oxide nanowires, a two-step thermal oxidation process was employed [42]. The result of successful conversion of metal into metal oxide is supported by the strong oxygen peak on the EDS spectrum (lower left, Fig. 3) and quantitative information (tin/oxygen atomic ratio) obtained; in contrast, only a small oxygen peak was observed in this position before thermal oxidation (as shown in Fig. 2B), indicating the presence of only a natural surface oxide layer (normally a few nanometers) on the metallic nanowires. Supporting evidence of successful metal oxide conversion also includes the element mappings to the right of the original SEM image in Fig. 3C. 3.2. DEP assembly results Pohl [33] reported the behavior of neutral matter in nonuniform electric fields, and the following studies on DEP assembly [43,44] revealed the correlation between DEP forces with dipole moments and electric fields. Metal nanowires are non-charged, polarizable particles; when dispersed in a liquid dielectric medium like ethanol, they could be manipulated by dielectrophoretic force, which is influenced by two major factors – electric field gradient and frequency [45]. Electric field gradient is determined by the applied potential and electrode geometry. In this case, the interdigitated electrodes feature long parallel edges, resulting in the electric field uniformity only existing vertically to the substrate surface, so most of the nanowires align perpendicularly to the vertical axis direction of the electrodes. In this work, a series of experiments were conducted to finetune the applied potential and frequency of the DEP assembly

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Fig. 2. (A) SEM image of indium/tin mixed nanowires (inset is an enlarged image of a single nanowire at 10,000×) and (B) a layout of EDS analysis on nanowires of different compositions (from top to bottom: pure indium, pure tin, indium/tin).

process. First, at a fixed frequency, nanowires were observed to be randomly placed after the ethanol dried out until the amplitude of the applied sine wave reached 1 V. The best assembly result appeared when a sine wave with peak-to-peak amplitude of 6 V was applied. At a low frequency (below 50 kHz), only a small portion of nanowires were trapped in the electrode gaps, while a moderate frequency value for assembly would be approximately 6 MHz. SEM images of nanowires (fabricated using both the PC membrane and AAO template) after DEP assembly are shown in Fig. 4. Due to its limited length, normally 5–6 ␮m, a single nanowire fabricated using a PC membrane is not able to bridge two parallel electrodes, which has a separation gap of 10 ␮m. Thus, the electrical connections are mainly established by overlapping nanowire junctions as shown in Fig. 4A. Unlike Fig. 4A, a single AAO-fabricated nanowire has a length of more than 10 ␮m; therefore, it is capable of being positioned between two electrodes with both ends in contact with the metal pads as evident in Fig. 4B. The nanowire density was easily controlled by either the amount of nanowire suspension used or the cutoff of the electric field, since the number of aligned nanowires is proportional to the elapsed time. The overview of the final integrated sensor chip on the micro-hotplate is shown in Fig. 4C. 3.3. Post-assembly thermal treatment The main purpose of this two-step thermal treatment is, first, to reduce the contact resistance between the metal nanowires and

electrodes and second, to convert metallic nanowires into metal oxide nanowires, which are capable of gas and chemical detection. One of the most critical problems regarding DEP-assembled nanowires is high contact resistance. The most recent solutions to this issue include high temperature thermal annealing [46] and a hot-pressing method [47]. In our case, a unique bonding process was performed to enhance the contact property. This may relate to the fact that both indium, tin, and their alloys are widely used as solder materials due to their low melting point, and previous research has proved that both indium and tin nanowires are capable of forming solder joints [40,48]. Originally, the actual area of contact points is much less than the apparent contact area due to the surface roughness of both electrodes and nanowires. This asperity leads to limited electron paths and a small current flow from two parallel electrodes through the bridging nanowires, resulting in non-ohmic high contact resistance. This non-ohmic I–V characteristic may also be caused by the thin oxide layer on the nanowire surface due to inevitable very short exposure of metal nanowires to air. The thermal bonding process that we adopted here, which is similar to performing the solder reflow process to join metallic nanowires as discussed by Gu et al. [49], was aimed to address this critical issue. However, in this case the treatment temperature was not over the metal melting temperature. Thus, we believe this treatment only helped soften the metal at around the peak temperature (not completely melting), which increased the contact area as well as the diffusion at the metal–metal (nanowire–electrode) interface, resulting in low contact resistance and increased join strength.

Fig. 3. (A) SEM image and (B) EDS spectrum of randomly dispersed indium/tin mixed oxide nanowires on silicon substrate and (C) EDS mapping of four major elements.

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Fig. 4. DEP assembly results of pure tin nanowires. (A) Concentrated tin nanowires fabricated using PC membrane. (B) Less concentrated and well dispersed indium/tin nanowires fabricated using AAO template. (C) Overview of the integrated sensor chip on micro-hotplate.

To prove that this bonding process actually improves the contact, we measured the I–V characteristics immediately following each step, and plotted the I–V curves as shown in Fig. 5. Justified by the I–V curves before and after bonding in nitrogen atmosphere, the resistance of aligned nanowires drops from the mega ohm range (Fig. 5A) to several hundred ohms (Fig. 5B). Furthermore, the thermal oxidation process which converted the metal nanowires into metal oxide nanowires caused another increase in electrical resistance (Fig. 5C). A typical thermal oxidation process consists of the formation of an oxidation shell and the conversion of the inside material [42]. The existence of oxygen after the oxidation process was determined by EDS analysis and was shown in Fig. 3.

Fig. 5. I–V characteristics of indium oxide nanowires: (A) just after the DEP assembly; (B) after the bonding in nitrogen atmosphere and (C) after the two-step thermal oxidation.

3.4. Electrical characteristics and sensor performance I–V curves of indium oxide nanowires on microelectrodes were obtained by utilizing a sweeping voltage range from −3 to 3 V (repeated 5 times) at eight selected temperature points from 25 to 440 ◦ C as shown in Fig. 6A. Their non-linearity indicates the semiconducting nature of the metal oxide nanowires – the resistance is not constant and its value must be presented with the corre-

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Fig. 7. (A) Sensor response of tin oxide nanowire sensor towards ethanol of different concentrations at 400 ◦ C and (B) a zoomed in view of the second peak showing the response time and recovery time.

Fig. 6. (A) I–V curves of indium oxide nanowires at eight different temperatures with a voltage scanning range from −3 to 3 V and (B) The resistance–temperature relationships of indium oxide nanowires, measured at 0.1 V.

sponding voltage. To illustrate how the conductance responds to temperature, the values of resistance at 0.1 V were plotted as a function of temperature as shown in Fig. 6B, which showed a semiconducting behavior that resistance dropped with an increase of the temperature. Error bars of the data at each temperature were added in the figure, which were smaller than the size of the symbols, indicating good repeatability. A polynomial fitted curve based on these measurements showed a slight positive slope above 260 ◦ C, and this could be explained by the physisorption and chemisorption on the metal oxide surface [50]. At temperatures lower than 260 ◦ C, the resistance change is mostly related to physisorbed oxygen. Chemisorption becomes an important factor at higher temperatures, bringing more oxygen to the surface, hence increasing the resistance. Both the temperature effect and the sensing properties of metal oxide materials rely mainly on their surface reactions with gas molecules. In ambient conditions, this reaction is dominated by the absorption of oxygen in various species (O2 , O2 − and O− ) [51] and capture of electrons [52]; however when oxidizing/reducing gases appear in the atmosphere, resulting in charge exchange between the metal oxide material and absorbed molecules, the conductance of the system will change correspondingly. Tin oxide is among the most studied and widely used sensor materials for gas detection. Fig. 7A gives the sensor response of a tin oxide nanowire sensor towards the detection of ethanol of different concentrations. The response is expressed by the percentage of resistance change to the original resistance (R/R0 ) and plotted as a function of elapsed time. For an n-type semiconductor, the presence of ethanol, a reducing agent, may cause an increase in its conductance due to its dissociation. Ethanol molecules can be either dehydrogenated or dehydrated to acetaldehyde or ethylene, since these two competing reactions take place at the same time, both resulting in desorption of oxygen ions on a metal oxide

surface [53,54]. Without oxygen ions removing electrons from the conduction band of semiconductor materials, the resistance drops. Distinguishable resistance drops (1.2%) can be detected even at the level of about 1 ppm (1.6 ppm), implying a high sensitivity of the nanowire sensor at the current condition. With more optimized conditions including composition variation and smaller nanowire diameter, lower concentration at the ppb level may be detected. In Fig. 7B, a zoomed in view of the second peak illustrates the response time (tS ) and the recovery time (tR ). Response times, which were measured as 90% full scale of the peak value of each cycle, are typically within 10 s. The response time might be slightly different if the injection amount changed, which is mainly attributed to the vapor diffusion rate inside the chamber. The sensor device also showed good repeatability; for the same concentration of the analyst injected into the sensing chamber, the output response almost remained the same. The nanowire sensor requires a longer time to recover when it is exposed to more concentrated ethanol vapor, as shown in Fig. 7A. Besides the intrinsic property of the nanowire sensor itself, the recovery time may also depend on the geometry of the testing chamber and the purging flow rate. It takes about 8–15 min to completely flush away the analyte residue and the response returns back to the original condition. We believe that the actual recovery time for our nanowire sensor will be shorter if operated in a more ideal system.

Fig. 8. Response of tin oxide nanowire sensor to methanol, ethanol, isopropanol, acetone, chloroform, and benzene as a function of temperature at a constant concentration of 1500 ppm.

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Fig. 8 summarizes the response of a tin oxide nanowire sensor chip towards six different analytes at a relatively high concentration, 1500 ppm, as a function of temperature. No obvious response was observed below 200 ◦ C, and sensor performance improved as the working temperature became higher for all analytes. We found that for this type of sensor, oxygenated compounds were detected with higher sensitivity than aromatic and chlorinated compounds. The sensor performance was significantly enhanced when operated at an elevated temperature of 440 ◦ C. A similar work has been conducted by Gong et al. [55], which showed a maximum sensitivity at 450 ◦ C. However, we only tested sensor performance up to 440 ◦ C which was the maximum operating temperature of the micro-hotplates in this work. Tin oxide, as a Lewis acid, can accept a lone pair of electrons to its incomplete orbital of 5s2 5p2 , and both the hydroxyl group from acetone and carbonyl group from alcohols would donate lone-paired electrons from the oxygen atom, resulting in a strong adsorption [55]. In general, among those four reducing agents tested here, the sensitivity of three alcohols follows an order (highest to lowest) as methanol, ethanol, isopropanol, and this order coincides with the number of methyl groups in their chemical structure. Two competitive dissociative reactions were proposed [56] to explain the alcohol molecules adsorbed on the tin oxide surface, simply described as O–H and C–O bond-breaking

processes [57], where the former one has a greater contribution to the resistance drop. The sensitivity order may relate to the preference of each chemical over these two competitive reactions. The response measurements of the six analytes for the tin oxide nanowire sensor at a lower concentration (100 ppm) (Fig. 9A) showed similar trends to those at 1500 ppm, even though the sensitivity of the sensor at 100 ppm was smaller than that at 1500 ppm. The variation of the metal oxide composition also had an impact on sensitivity and possibly selectivity of the nanowire sensors. Indium–tin nanowires with a composition ratio of 25% indium, 75% tin (atomic ratio) were fabricated and integrated into a sensor device using the same method. The sensor response towards four chemicals (acetone, methanol, ethanol, and isopropanol) of 1500 ppm was tested and compared with that of the tin oxide nanowire sensor (shown in Fig. 9B). In general, the sensitivity of the indium–tin oxide sensor is lower than that of the tin oxide nanowire sensor. In the case of indium–tin oxide, the response mechanism to chemical vapor is similar to that of tin oxide – the adsorption/desorption of O2− and O− species has a dominant influence on gas-sensing characteristics. The incorporation of In3+ in Sn4+ or Sn2+ (surface) sites may improve the crystallinity and also the electron mobility which helps lower the resistance. On the other hand, the indium percentage used here is slightly larger than that

Fig. 9. (A) Response of tin oxide nanowire sensor to the six different chemicals at two different concentrations and (B) response of two different sensors (SnO2 and ITO) to different chemicals at 1500 ppm concentration.

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in the gamma-tin (In0.2Sn0.8) in the indium–tin phase diagram, which means that a small excess amount of indium may not be effectively doped in the tin lattice. After oxidation it may cause composition stoichiometric disorder [58]. Both of these factors may play a role in the final sensor response. Further research on optimization of the indium–tin oxide composition is needed to better control the sensitivity and selectivity of the sensors. 4. Conclusions Indium oxide, tin oxide and indium–tin oxide nanowires were successfully fabricated by thermal oxidation of their corresponding metal nanowires and this method can be further extended to fabricate nanowires of large diversity. DEP assembly was performed to place nanowires in contact with microelectrodes in an optimized condition. The unique bonding process in a nitrogen atmosphere was able to achieve good contact between the nanowires and the electrodes. Sensor chips were fabricated by integrating nanowires on a commercially available microelectrode with integrated microhotplate. Electrical characteristics and sensor performance tests showed that this type of sensor is highly sensitive and gives fast and repeatable responses at elevated temperatures. Chemical response studies to different volatile organic compounds revealed that oxygenated organic compounds caused higher response than aromatic or chlorinated compounds. Acknowledgements Financial support from the National Science Foundation (award number ECCS-0731125) is greatly acknowledged. Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the writers and do not necessarily reflect the views of the funding agency. References [1] Y. Zhang, A. Kolmakov, S. Chretien, H. Metiu, M. Moskovits, Control of catalytic reactions at the surface of a metal oxide nanowire by manipulating electron density inside it, Nano Lett. 4 (2004) 403–407. [2] Y. Zhang, A. Kolmakov, Y. Lilach, M. Moskovits, Electronic control of chemistry and catalysis at the surface of an individual tin oxide nanowire, J. Phys. Chem. B 109 (2005) 1923–1929. [3] A. Kolmakov, M. Moskovits, Chemical sensing and catalysis by one-dimensional metal-oxide nanostructures, Annu. Rev. Mater. Res. 34 (2004) 151–180. [4] R. Vander Wal, G. Hunter, J. Xu, M. Kulis, G. Berger, T. Ticich, Metal-oxide nanostructure and gas-sensing performance, Sens. Actuators B 138 (2009) 113–119. [5] G. Sberveglieri, C. Baratto, E. Comini, G. Faglia, M. Ferroni, A. Ponzoni, A. Vomiero, Synthesis and characterization of semiconducting nanowires for gas sensing, Sens. Actuators B 121 (2007) 208–213. [6] Y. Heo, L. Tien, Y. Kwon, D. Norton, S. Pearton, B. Kang, F. Ren, Depletion-mode ZnO nanowire field-effect transistor, Appl. Phys. Lett. 85 (2004) 2274–2276. [7] J. Goldberger, D. Sirbuly, M. Law, P. Yang, ZnO nanowire transistors, J. Phys. Chem. B 109 (2005) 9–14. [8] M. Zheng, L. Zhang, G. Li, W. Shen, Fabrication and optical properties of large-scale uniform zinc oxide nanowire arrays by one-step electrochemical deposition technique, Chem. Phys. Lett. 363 (2002) 123–128. [9] Y. Li, G. Meng, L. Zhang, F. Phillipp, Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties, Appl. Phys. Lett. 76 (2000) 2011–2014. [10] D. Zhang, C. Li, S. Han, X. Liu, T. Tang, W. Jin, C. Zhou, Ultraviolet photodetection properties of indium oxide nanowires, Appl. Phys. A 77 (2003) 163–166. [11] K. Ryu, D. Zhang, C. Zhou, High-performance metal oxide nanowire chemical sensors with integrated micromachined hotplates, Appl. Phys. Lett. 92 (2008) 093111–093111-3. [12] Q. Kuang, C. Lao, Z. Wang, Z. Xie, L. Zheng, High-sensitivity humidity sensor based on a single SnO2 nanowire, J. Am. Chem. Soc. 129 (2007) 6070–6071. [13] J. Chen, F. Zhang, J. Wang, G. Zhang, B. Miao, X. Fan, D. Yan, P. Yan, CuO nanowires synthesized by thermal oxidation route, J. Alloys Compd. 454 (2008) 268–273. [14] M. Kaur, K. Muthe, S. Despande, S. Choudhury, J. Singh, N. Verma, S. Gupta, J. Yakhmi, Growth and branching of CuO nanowires by thermal oxidation of copper, J. Cryst. Growth 289 (2006) 670–675. [15] K. Sawicka, A. Prasad, P. Gouma, Metal oxide nanowires for use in chemical sensing applications, Sens. Lett. 3 (2005) 31–35. [16] B. Basnar, I. Willner, Dip-pen-nanolithographic patterning of metallic, semiconductor, and metal oxide nanostructures on surfaces, Small 5 (2009) 28–44.

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Biographies Xiaopeng Li is currently a Ph.D. student in the Department of Chemical Engineering at the University of Massachusetts Lowell, USA. He received his B.S. in Chemistry and B.Eng. in Chemical Engineering in a five-year double bachelor program from Shandong University, China, in 2007. His current project is focused on the synthesis and assembly of metal oxide nanowires and conducting polymer nanowires for sensor applications. Erica Chin received her B.S. and M.S. in Chemical Engineering from the University of Massachusetts Lowell in 2007 and 2009, respectively. During her master’s

study, she worked on the synthesis and characterization of metal nanowires and CdS nanowires. She received a student award from the New England Institute of Chemists (NEIC) in 2008. She is currently working at Pfizer (formerly Wyeth) in Andover, MA. Hongwei Sun is an Assistant Professor in the Department of Mechanical Engineering at the University of Massachusetts Lowell (UML). He graduated with a Ph.D. from Institute of Engineering Thermophysics at Chinese Academy of Science in 1998. Prior to joining UML in 2005, he was a postdoctoral researcher at the University of Rhode Island and later a research scientist at the Massachusetts Institute of Technology. His research interests are in the area of Power Microelectromechanical Systems (Power MEMS), MEMS acoustic sensors, microscale cooling systems. His other interests are in micro/nano fabrication technology, fundamental understanding of micro/nanoscale fluidics and their applications in biological analysis and energy areas. Pradeep U. Kurup is a Professor in the Department of Civil and Environmental Engineering at the University of Massachusetts Lowell. He received his B.Tech. in Civil Engineering in 1985 from the University of Kerala and obtained his M.Tech. from the Indian Institute of Technology Madras (1987). He holds a Ph.D. in Civil Engineering (1993) from Louisiana State University (LSU). Dr. Kurup has conducted extensive research in multi-sensor data fusion, artificial olfaction, and geotechnical & geoenvironmental site characterization. Dr. Kurup is a member of several professional societies, and is a registered Professional Engineer in the State of Louisiana. Zhiyong Gu is an Assistant Professor in the Department of Chemical Engineering at the University of Massachusetts Lowell. He is also affiliated with the CHN/NCOE Nanomanufacturing Center. He received his B.E. from Qingdao Institute of Chemical Technology, China, in 1996, his M.S. from the University of Notre Dame in 2001, and his Ph.D. from the State University of New York at Buffalo in 2004, respectively. He was a Postdoctoral Fellow at the Johns Hopkins University from 2004 to 2006. He has published 4 book chapters and over 30 refereed papers. His current research interests include synthesis of nanoparticles and nanowires, self-assembly, block copolymers, nanocomposites, and nanoscale-integration for electronics and sensors.