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Sensors and Actuators B 227 (2016) 198–203

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

Ultrasensitive NO2 gas sensors using tungsten oxide nanowires with multiple junctions self-assembled on discrete catalyst islands via on-chip fabrication Phung Thi Hong Van, Do Duc Dai, Nguyen Van Duy, Nguyen Duc Hoa, Nguyen Van Hieu ∗ International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), Dai Co Viet Road, Hanoi, Viet Nam

a r t i c l e

i n f o

Article history: Received 7 September 2015 Received in revised form 6 November 2015 Accepted 14 December 2015 Available online 23 December 2015 Keywords: Tungsten oxide On-chip fabrication NO2 gas sensor

a b s t r a c t An effective fabrication of networked nanowire sensors with multiple junctions, which were obtained by selectively growing tungsten oxide nanowires directly on the arrays of discrete catalyst islands, was demonstrated for highly sensitive NO2 gas sensors. Junction density was controlled by altering growth time for superior NO2 gas response. The optimum sensor exhibited remarkably high response (Rg /Ra ) to 10 ppm NO2 (Rg /Ra ∼ 144.3), negligible cross-responses to 100 ppm CO, H2 , and NH3 (Rg /Ra ∼ 1.3–1.7), and ultralow detection limit (∼0.33 ppb) at 250 ◦ C. In addition, the sensor exhibited good reversibility for up to 15 cycles of air-to-gas switching under sensing at relatively high temperatures of up to 350 ◦ C. The methodology illustrated in the present work aims to overcome the disadvantages of nanowire sensors grown on an ordinary continuous catalyst layer and to make these sensors suitable for large-scale fabrication via microelectronic technology. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Semiconducting metal oxide nanowires (SMO-NWs), such as SnO2 , In2 O3 , ZnO, and tungsten oxide NWs (TONWs), are promising sensitive materials for next-generation semiconductor gas sensors [1]. However, a suitable integration method for the large-scale fabrication of SMO-NWs remains unidentiﬁed. Numerous indirect methods for effective SMO-NW integration have been developed to produce nanoelectronic devices and nanosensors [2,3], such as the Langmuir–Blodgett technique [4], ﬂuidic ﬂow [5], contact printing [6], and blow bubble [7], among others [2,3]. However, these methods still involve problems, such as contamination and weak adhesion (instability to mechanical vibration). Furthermore, the aforementioned methods are apparently suitable for assembling SMO-NWs synthesized via wet-chemical methods but not for integrating SMO-NWs grown on solid substrates because of the inherent series of tedious processes, such as the dispersion of NWs in a solution, the deposition of NWs on a substrate, and the lithography for deﬁning electrical contacts. Therefore, the indirect integration of SMO-NWs is apparently an impractical general approach for the mass production of SMO-NW-based gas sensors.

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (N. Van Hieu). http://dx.doi.org/10.1016/j.snb.2015.12.054 0925-4005/© 2015 Elsevier B.V. All rights reserved.

By contrast, the direct growth of SMO-NWs on the desired areas of solid substrates does not only offer a potential for large-scale fabrication but also an enhanced gas-sensing performance because of the forming Ohmic contacts between NWs and metal electrodes as well as the stable junctions between NWs [8–19]. According to reports in literature, most studies have successfully grown SnO2 [8–12] and ZnO [13–17] NWs directly onto Si/SiO2 substrate for on-chip fabrication of those NW gas sensors. In particular, TONWs are one of the important materials for highly sensitive gas sensors [20,21]. Surprisedly, a very limited work has reported on the on-chip fabrication of TONW gas sensors. This method enables the fabrication of reliable and reproducible TONW sensors as well as efﬁcient large-scale fabrication [19]. In our earlier work, we have successfully fabricated TONWs by growing them directly on unpolished Al2 O3 substrate with a seed layer of tungsten (W) for NO2 gas sensors [19], in which the roughness of the Al2 O3 substrate is a key issue for successful on-chip fabrication. However, this fabrication technique still exhibits the following disadvantages that must be overcome for practical applications. (1) This fabrication method is not generally compatible with Si processing because using an unpolished substrate is unsuitable for the lithography technique. Thus, it cannot be used to downscale sensor sizes for high-density integration, microarray sensor, and minimum power consumption. (2) The number of NW–NW junctions is difﬁcult to control. Whereas, this issue plays an important role in gas-sensing performance in terms of gas response,

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Fig. 1. Schematic diagram of procedure for on-chip fabrication of multi-junction joints WO3 nanowires gas sensors: (a) oxidized Si substrate; (b) patterned Pt electrodes and heater; (c) deposited Cr/W islands and (d) directly grown WO3 nanowires onto the islands.

response-recovery speed, and reliability because of the basic principle of NW-based gas sensors, that is, the increased number of junctions in the networked NW sensors leads to effective sensing capabilities [12]. (3) The use of a continuous catalyst layer cannot prevent the shortcoming of leakage current through the buffer layer under NW ﬁlms, which typically exits under NW ﬁlms grown via thermal evaporation. This shortcoming may decrease the gas response of TONW sensors fabricated via on-chip growth. In the present work, we introduce an efﬁcient design for the onchip fabrication of TONW gas sensors to solve the aforementioned problems of TONW ﬁlms. Networked TONW sensors with multiplejunctions are fabricated by selectively growing TONWs directly on the arrays of discrete W islands laid out between a pair of Pt electrodes. Conventional lithography and metal deposition techniques are employed to pattern the electrodes and islands on the Si/SiO2 substrate. The controllable multiple junctions of TONWs are a key to enhancing their sensing performance to NO2 gas signiﬁcantly. 2. Experimental TONWs were grown via thermal evaporation on a W catalyst layer as reported in another work [19]. The schematic for the on-chip fabrication of WO3 NW sensors with multiple junctions is presented in Fig. 1. In the ﬁrst step, a pair of electrodes and micro-heater was fabricated on Si/SiO2 substrate through conventional lithography technique. After depositing and patterning the photoresist layer, the electrodes and heater, which consisted of two layers of Cr (20 nm) and Pt (200 nm), were sequentially deposited using the sputtering method. Then, the lift-off technique was employed to pattern the heater and electrodes. In the second step, the discrete W islands were fabricated. A similar lithography step was employed to deﬁne the discrete islands on the area between a pair of electrodes. The discrete islands consisted of two layers of Cr (20 nm) and W (20 nm) that were sequentially deposited using the sputtering method. A Cr layer was used to enhance adhesion between the W and SiO2 layers. In particular, the gap between the electrodes was approximately 40 ␮m, and the distance between the islands and their radius were both approximately 5 ␮m. The ﬁnal step involved growing TONWs directly on the chip via the thermal evaporation of WO3 powder

source at 1000 ◦ C, as previously described [19]. The growth time in this experiment was varied from 1 h to 3 h with a step of 0.5 h while keeping the other growth conditions identical to control the density of the multiple-junction network between electrodes. The morphologies of the TONWs were characterized by ﬁeld emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Hitachi, Ltd., Japan) and high-resolution transmission electron microscopy (HR-TEM, JEM-3010 HR, JEOL, Ltd., Japan). The gassensing characteristics were determined via a ﬂow-through technique with a standard ﬂow rate of 200 standard cubic centimeters per minute (sccm) for both reference (dry air) and target gases at different temperatures (200, 250, 300, and 350 ◦ C) and NO2 gas (1–5 ppm) and other gases, such as NH3 (100 ppm), H2 (100 ppm), and CO (100 ppm). The gas sensor was mounted on a hot plate and equipped with two W needles as two DC probes, and the gas line was directly exposed to the surface of TONW sensors. Although the micro-heater is available for gas-sensing test, the hotplate is used for generating stable and reproducible operating temperature and the simplicity. The resistance of the sensors was monitored with a two-point probe using a voltage source meter (Keithley 2602, Keithley Instruments, Inc., USA) interfaced with a computer.

3. Results and discussion Fig. 2 presents the microstructure of the TONW sensors fabricated at different growth times. Fig. 2(a) represents the overall plane-view FE-SEM image of the TONW sensors with a growth time of 1 h. Fig. 2(b)–(f) are the FE-SEM images focused on the small areas between the two electrodes of the TONW sensors that correspond with the growth times of 1, 1.5, 2, 2.5, and 3 h, respectively. These images clearly showed that the TONWs were selectively grown on the catalyst islands. The array of the catalyst islands was designed between a pair of Pt electrodes. Thus, the grown TONWs on the islands formed a network with multiple TONW junctions. NW density also increased with growth time. Fig. S1 under “Supplementary Information” illustrates that the TONWs grown on adjacent islands were not entangled, and the network of TONWs was not accordingly formed between a pair of Pt electrodes for the short growth time of 0.5 h. The insets in Fig. 2(b) and (c) show FE-SEM images with high magniﬁcation, which indicate that the diameters

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Fig. 2. FESEM images of WO3 nanowires sensors fabricated at difrent growth times: (a, b) 1 h, (c) 1.5 h, (d) 2 h, 2.5 h and (e) 3 h (f).

of the NWs are within a wide range of 50–500 nm. The morphology of the NWs was highly similar to those of TONWs grown on W foil [22] and TONWs grown on a W seed layer deposited on unpolished Al2 O3 substrate [19], as reported previously. The adhesion between W seed layer and Si/SiO2 substrate played an important role in the successful growth of TONWs. We deposited a Cr layer before depositing the W layer to enhance adhesion. We performed numerous TONW time growths on a single W seed layer, which was directly deposited on Si/SiO2 substrate. However, we failed to grow TONWs directly on the substrate. This ﬁnding indicated that the enhanced adhesion of the W catalyst layer by the Cr interlayer is a key issue in the successful on-chip growth of TONWs on Si substrate. The microstructure characterization of TONWs was performed via HR-TEM. Fig. 3(b) shows a typical TEM image of a single TONW; no metal nanoparticle was detected on the NW ends, and the diameter of the NWs was uniform along their length. The HR-TEM lattice image shown in Fig. 3(a) indicated the single-crystalline nature of

as-synthesized TONWs. This observation also agrees with the 2D Fourier transform pattern of the TEM image shown in Fig. 3(c), which clearly indicates that the lattice fringes are perpendicular to the long axis of NW growth. Fig. 3(d) illustrates the scanning of a length of 10 nm along the same TONW, which shows approximately 26 planes. This ﬁnding indicated that the lattice fringes of the TONWs were approximately 0.38 nm, which was indexed to be the (010) plane of monoclinic W18 O49 . The single crystalline of TONWs was determined using the growth mechanism that was explained in detail in another study [19]. To determine their gas-sensing performance, the TONW sensors fabricated at different growth times were ﬁrst annealed at 600 ◦ C in air for 5 h and then systematically investigated with regard to NO2 at different sensing temperatures. The typical gas-sensing characteristics of the TONW sensors fabricated at a growth time of 2 h are summarized in Fig. 4, whereas those of the other TONW sensors are presented in Figs. S3–S5 (“Supplementary Information”). Fig. 4(a)–(d) shows the dynamic response transient to 1–10 ppm

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Fig. 3. (a) HRTEM and (b) TEM images of a typical TONW; (c) the two-dimensional Fourier transform pattern of the TEM image and (d) element mapping on a distance of 10 nm corresponding to 26 planes.

Fig. 4. Typical gas sensing performance of the WO3 nanowires sensor grown for 2 h: (a) transient response to NO2 gas (1–10 ppm) measured at 200 (a), 250 (b), 300 (c) and 350 ◦ C (d); the response (Rg /Ra ) as a function of operating temperature (e) and gas concentration (f).

NO2 at 200, 250, 300, and 350 ◦ C, respectively, which exhibits stable sensing and recovery characteristics regardless of sensing temperature. The TONW sensors showed that the resistance increased upon exposure to NO2 gas and then decreased and regained their original values upon exposure to dry air. This observation indicated that the sensors demonstrated a typical n-type semiconducting sensing behavior with respect to NO2 gas. The increased resistance upon exposure to NO2 gas is attributed to the extension of the depletion layer along NWs as well as to the increase in the potential barrier height of the NW junction [14]. The relative contribution of these two mechanisms to the gas response of TONW sensors depends on NW diameter [23]. The TONWs prepared in the present work have a relatively large diameter compared with their Debye length. Thus, the potential barrier height of an NW junction can be considered the main contribution for the gas-sensing performance of the networked TONW sensors [24]. The gas response (Rg /Ra ), as a function of sensing temperature for NO2 gas concentration ranging from 1 ppm to 10 ppm, is plotted in Fig. 3(e). At a given NO2 gas concentration, the response slightly increased as the temperature

increased from 200 ◦ C to 250 ◦ C, and then it tended to decrease with further temperature increases of up to 350 ◦ C. Such typical bell-shaped response, as a function of sensing temperature, is common in TONW-based gas sensors [25,26]. However, this tendency is consistent with NO2 gas concentrations higher than 2.5 ppm. The response value to 2.5 ppm NO2 slightly increased from 42.3 to 43.2 with a temperature increase from 200 ◦ C to 250 ◦ C. This response value decreased to approximately 5.6 with further temperature increases of up to 350 ◦ C. Nevertheless, the best response to NO2 gas concentration ranging from 1 ppm to 10 ppm was approximately 250 ◦ C. The response to NO2 gas at 250 ◦ C was plotted as a function of gas concentration in Fig. 4(f). The relationship between response and NO2 gas concentration was good in terms of being linear within the range of 1 ppm to 10 ppm. The response values to 1 ppm to 10 ppm NO2 ranged from 19.9 to 144.3, which were among the highest values reported in the literature for TONW-based NO2 gas sensors regardless of NW diameter [19,26,22,27,25,28]. Such high response of the present sensor can be attributed to the unique conﬁguration of the on-chip growth of TONWs on discrete islands. The use of discrete catalyst islands instead of an ordinary continuous catalyst thin ﬁlm [19] can lead to (i) a considerable number of NW–NW junction of the sensors [24] and (ii) the prevention of the leakage of current through seed layers. Recently, Kim et al. developed various geometries of NW sensors to enhance the density of NW junctions, such as trench [11] and V-groove structures [12]. The approach proposed in the present work is less complicated than those in the aforementioned works. The NO2 gas detection limit (DL) of the present sensors could not be measured because of the limits of our experimental setup. However, we could extrapolate the DL value of the present sensor from the experimental data. Sensor noise could be calculated using the variation in gas response at the baseline through root–mean–square (rms) deviation. We took 10 experimental data points at the baseline of the transient response from Fig. 4(b) for the ﬁfth polynomial ﬁtting, as shown in the inset of Fig. 4(f). The [Si − S]2/N, where rmsnoise value was calculated as rmsnoise = Si is an experimental data point and S is the corresponding value calculated from the ﬁfth polynomial curve ﬁtting. The rmsnoise value was calculated to be approximately 0.0015 ± 0.0003. According to IUPAC deﬁnition, DL is calculated as DL (ppm) = 3 × rmsnoise /slope, where slope is the slope value of the linear curve ﬁtting of gas response (S) versus gas concentration (ppm). The DL value was estimated to be approximately 0.33 × 10−3 ppm (0.33 ppb) from the

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Fig. 5. Gas-sensing characteristics of WO3 nanowires grown at different times: (a) the response to 10 ppm NO2 as a function of growth time; (b) the response measured at 250 ◦ C as a function gas concentration; the response of WO3 nanowires grown at 2 h to 100 ppm CO, H2 , NH3 and 10 ppm NO2 ; (d) the response measured to 1 ppm NO2 at 350 ◦ C for 15 cycles.

slope value of 13.8 ± 0.1, which suggested that the present sensors could be used to detect NO2 gas at ultra-low concentrations down to sub-ppb level at a sensing temperature of 250 ◦ C. A general principle suggests that the density of networked NW sensors is strongly affected in their gas-sensing performance [10,13,29,30]. Thus, we represented the response of TONW sensors to 10 ppm NO2 gas at different sensing temperatures as a function of growth time in Fig. 5(a) for comparison. All of the as-prepared TONW sensors exhibited a unique optimal sensing temperature (approximately 250 ◦ C), which revealed that the growth time was independent from this parameter. Fig. 5(a) also clearly shows that the response increases as growth time increases from 1 hto 2 h, and decreases as growth time increases further. Thus, the growth time of 2 h can be considered optimal to surpass gas response. The response of TONW sensors grown at different times and plotted as a function of gas concentration in Fig. 5(b) indicated that growth time signiﬁcantly affected on the response, as measured at high NO2 gas concentrations. The response values (Rg /Ra ) of the TONW sensors grown at 1 h and 3 h to 10 ppm NO2 were approximately 35.9 and 29.9, respectively. These values are considerably lower than that of the TONW sensor grown at 2 h (Rg /Ra ∼ 144.3). This observation can be explained in the framework of NW junction density. When growth time increases from 1 h to 2 h, the lengths of NWs increase, which results in the increase of the number of NW junctions. However, increasing growth time further does not increase the number of NW junctions because the growth time of 2 h is sufﬁciently long for entangling all the TOWNs grown on adjacent catalyst islands. This ﬁnding indicated that the number of NW junctions did not increase with the increase in growth time from 2 h to 3 h. A long growth time can result in the entanglement of NWs, with not only the NWs grown on the adjacent islands but also the NWs grown on other faraway islands. This phenomenon can result in a decrease in

NW junction density, and this case is similar to the continuous ﬁlm NW sensors directly grown between a pair of Pt electrodes [19]. NW density can also affect response and recovery times ( resp. and recov. ). In the present work, response and recovery times were deﬁned as the time to reach 90% of the ﬁnal equilibrium. Fig. 5(c) and (d) shows the response and recovery times of the TONW sensors grown at 1, 2, and 3 h as a function of operating temperature. Response and recovery times rapidly decreased with the increase in operating temperatures, but were only slightly affected by growth time. The response and recovery times of the present sensors were shorter than those of the continuous ﬁlm TONW sensors [19]. This ﬁnding can be also attributed to the unique device structure. The TONWs were selectively grown on discrete islands, which resulted in structural porosity. Thus, gas molecules can be easily adsorbed and desorbed on the surface of the NWs. In the last part of this report, we presented our investigation on the selectivity and stability of the present TONW sensors in detecting NO2 gas. The transient response to 100 ppm CO, H2 , and NH3 gases is recorded in Fig. S6 (“Supplementary Information”), and the obtained responses are represented together with the response to 10 ppm NO2 in Fig. 5(c). The response to 10 ppm NO2 was approximately 100 times higher than that to 100 ppm CO, H2 , and NH3 gases. This dedicated that the present TONW sensors have good selectivity to reductive interference gases. However, the selectivity of the sensor should further investigate with other oxidative gases. It have been reposted that thin ﬁlm TONW sensors exhibited excellent stability to NO2 gas [19]. Therefore, the stability of the present TONW sensors must be investigated. In general, the sensing response is quickly degraded at high operating temperature. Therefore, we have investigated the short term stability of TONWs sensors at 350 ◦ C. In addition, we can measure gas sensing response for a relatively large cycling without time consumption. Fig. 5(d) shows the transient response of the present TONW sensor with a growth time of 2 h at 350 ◦ C and 1 ppm NO2 gas. The response of the present sensor exhibited perfect reliability with a measurement time of up to 110 min at relatively high operating temperature, which indicated that the sensor had good stability at lower operating temperature. Good stability and repeatability could be achieved from the on-chip fabrication technique [19]. 4. Conclusion We presented an effective design for TONW gas sensors. The fabrication protocol of the sensors was utilized via an in situ growth process, which could be employed in the scalable fabrication of sensor chips via silicon microelectronic technology. The presented design can lead to advantages, such as increase in the number of NW–NW junctions and the elimination of leakage similar to that via a continuous catalyst layer. The sensors demonstrated a signiﬁcantly enhanced response to NO2 gas with extremely low DL compared with TONW thin-ﬁlm sensors, and consequently, improved selectivity. In addition, the sensors exhibited a relatively fast response and recovery times. The improved gas sensing performance could be attributed to the formation of multiple junctions on the arrays of discrete catalyst islands. Acknowledgments This research was funded by the Vietnam National Foundation for Science and Technology Development (Code 103.02-2014.18). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.12.054.

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Biographies Phung Thi Hong Van received MSc degree in materials science at the International Training Institute for Materials Science (ITIMS), HUST, in Viet Nam, in 2004. She is currently pursuing the PhD degree at ITIMS, where he is working on functionalized metal oxides nanowires for gas-sensing application. Do Duc Dai received MSc degree in materials science at the International Training Institute for Materials Science (ITIMS), HUST, in Viet Nam, in 2014. He is currently working for Rang Dong light source & vacuum ﬂask company. Nguyen Van Duy is currently working as a research lecture at International Training Institute for Material Science (ITIMS), Hanoi University of Science and Technology (HUST). He received PhD degree from the Department of Electrical and Electronics Engineering at Sungkyunkwan University, South Korea, in 2011. His current research interests include nanomaterials, nanofabrications, characterizations, and applications to electronic devices, gas sensors, and biosensors. Nguyen Duc Hoa obtained his PhD degree in materials science and engineering in 2009 at Chungnam National University in Korea. He awarded JSPS fellowship and conducted the research at National Institute for Materials Science (NIMS, Japan) from 2009 to 2011. His research activity has covered a wide range of nanostructured materials from synthesis, fundamental, and applications. Currently, he is a lecturer and scientist at Hanoi University of Science and Technology, Viet Nam. Nguyen Van Hieu joined the International Training Institute for Material Science (ITIMS) at Hanoi University of Science and Technology (HUST) in 2004, where he is currently Full Professor. He received his PhD degree from the Faculty of Electrical Engineering at University of Twente in The Netherlands in 2004. He worked as a post-doctoral fellow at the Korea University from 2006 to 2007. His current research interests include functional nanostructures, gas sensors, and biosensors.

Extending SOA/MDD to Sensors and Actuators for ...