The demand for power‐efficient micro‐and nanodevices is increasing rapidly. In this regard, electrothermal nanowire‐based heaters are promising solutions for the ultralow‐power devices required in IoT applications. Herein, a method is demonstrated for producing a 1D nanoheater by selectively coating a suspended pyrolyzed carbon nanowire backbone with a thin Au resistive heater layer and utilizing it in a portable gas sensor system. This sophisticated nanostructure is developed without complex nanofabrication and nanoscale alignment processes, owing to the suspended architecture and built‐in shadow mask. The suspended carbon nanowires, which are batch‐fabricated using carbon‐microelectromechanical systems technology, maintain their structural and functional integrity in subsequent nanopatterning processes because of their excellent mechanical robustness. The developed nanoheater is used in gas sensors via user‐designable localization of the metal oxide semiconductor nanomaterials onto the central region of the nanoheater at the desired temperature. This allows the sensing site to be uniformly heated, enabling reliable and sensitive gas detection. The 1D nanoheater embedded gas sensor can be heated immediately to 250 °C at a remarkably low power of 1.6 mW, surpassing the performance of state‐of‐the‐art microheater‐based gas sensors. The presented technology offers facile 1D nanoheater production and promising pathways for applications in various electrothermal devices.
Junction networks made of longitudinally connected metal oxide nanowires (MOx NWs) have been widely utilized in resistive-type gas sensors because the potential barrier at the NW junctions leads to improved gas sensing performances. However, conventional MOx–NW-based gas sensors exhibit limited gas access to the sensing sites and reduced utilization of the entire NW surfaces because the NW networks are grown on the substrate. This study presents a novel gas sensor platform facilitating the formation of ZnO NW junction networks in a suspended architecture by growing ZnO NWs radially on a suspended carbon mesh backbone consisting of sub-micrometer-sized wires. NW networks were densely formed in the lateral and longitudinal directions of the ZnO NWs, forming additional longitudinally connected junctions in the voids of the carbon mesh. Therefore, target gases could efficiently access the sensing sites, including the junctions and the entire surface of the ZnO NWs. Thus, the present sensor, based on a suspended network of longitudinally connected NW junctions, exhibited enhanced gas response, sensitivity, and lower limit of detection compared to sensors consisting of only laterally connected NWs. In addition, complete sensor structures consisting of a suspended carbon mesh backbone and ZnO NWs could be prepared using only batch fabrication processes such as carbon microelectromechanical systems and hydrothermal synthesis, allowing cost-effective sensor fabrication.
Introduction Over the last few decades, research has been conducted to build sensitive, reliable and high-performance gas monitoring systems that meet the demands of industrial sites. In particular, 1D metal oxide (MOx) nanomaterials have been actively studied as a gas sensing material because of its excellent sensing performance, low cost, and suitability for a micro/nano device [1]. Besides, the MOx 1D nanostructures are relatively easy to synthesize, easy to control morphology and have various options including surface functionalization to make even further enhancement of the sensing performance [2]. In particular, when the MOx 1D nanostructures linked together and form a junction structure, an additional current path is generated through the formed junction and this forms additional potential barriers. Thus, an additional gas sensing mechanism can be utilized to further improve sensing performance as shown in Figure 1 [3]. In general, however, the fabrication process of MOx nanostructure networks including dense junctions requires complex and precise techniques. In addition, the nanostructure networks are positioned on the substrates. Therefore, gas access to the junctions and mass transfer to the sensing sites are limited. In this study, we report a novel gas sensor architecture including a suspended carbon mesh integrated with ZnO nanowires networks. The nanowires are grown on single carbon wires of the mesh and meet at the void regions of mesh structures. Thus, dese nanowire junctions can be formed in a suspended form (Figure 2). Owing to the suspended architecture, gas access to the sensing sites is effective and the sensing signal is free from the effect of the substrate [4]. The suspended carbon nanomesh was fabricated using the carbon-MEMS process consisting of simple photolithography and pyrolysis processes. The ZnO nanowire networks were precisely patterned at the suspended mesh using the hydrothermal method. The length of the nanowires was adjusted for good junction formation and the advantage of the suspended junctions was evaluated. Method Suspended carbon nanomesh was fabricated by photolithography and pyrolysis (Vacuum, 700°C) processes. Via two successive exposure steps in photolithography, suspended polymer mesh was defined without complex lithography techniques. After the pyrolysis process, microscale polymer mesh was converted into nano-sized carbon mesh because of the large volume reduction (up to 90%) in pyrolysis. After a thin ZnO seed layer was selectively coated on the suspended carbon mesh, ZnO nanowires were grown on the patterend seed layer by a hydrothermal process (Zn(NO3)2 25mM, HMTA 25mM ). The pristine ZnO nanostructures were annealed using RTA (N2 atmosphere, 300°C) for better adhesion and connection. Results and Conclusions Figure 2 shows the SEM images of the fabricated suspended carbon nanomesh (width ~140μm, thickness ~400nm, line spacing ~6μm) and ZnO nanowire networks (diameter ~100 nm, length ~3.5 μm). ZnO nanowires were well grown to form uniform and dense junctions. To characterize the effect of the nanowire junctions, NO2 gas sensing responses were compared between a nanomesh-based gas sensor and a single suspended carbon wire-based sensor. As shown in Figure 3. the suspend mesh-based sensor exhibited higher response and wider linear range due to the effect of junctions. This novel sensor architecture can be widely utilized because all the fabrication processes of the presented sensor were carried out at a wafer-level making it cost-effective and various MOx nanowires can be simple integrated with a bunch of junctions. References [1] A. Dey, Semiconductor metal oxide gas sensors, Materials Science and Engineering: B, 229 (2018) 206 – 217. [2] Y. Lim, S. Kim, Y. M. Kwon, J. M. Baik, H. Shin, A highly sensitive gas-sensing platform based on a metal-oxide nanowire forest grown on a suspended carbon nanowire fabricated at a wafer level, Sensors and Actuators B : Chemical. 260, (2018) 55-62. [3] R. Khan, H.W. Ra, J. Kim, W. Jang, D. Sharma, Y. Im, Nanojunction effects in multiple ZnO nanowire gas sensor, Sensors and Actuators B: Chemical, 150 (2010) 389 – 393. [4] Y. Lim, J. I. Heo, M. J. Madou, H. Shin, Monolithic carbon structures including suspended single nanowires and nanomeshes as a sensor platform, Nanoscale Research Letter. 8 (2013) 492. Figure 1
Introduction Over the past few decades, different types of gas sensors have been developed based on different sensing materials and methods. Among them, semiconducting metal oxide nanomaterials are suitable for gas sensing applications due to their high electron mobility, high surface-to-volume ratio, high crystallinity, and long-term stability. Considerable efforts have been made to improve the performance indicators of metal oxide gas sensors such as sensitivity, selectivity, reaction and recovery time [1]. However, in recent years, there has been growing demand for miniaturization, low power consumption, low cost and mass production, which are the characteristics required for continuous and systematically integrated monitoring and management in the sensor market [2]. Besides, widely used materials for metal oxide gas detection are wide-bandgap semiconducting oxides such as zinc oxide, tin oxide, gallium oxide or indium oxides, which are operate at elevated temperatures (250–600°C) since enough thermal energy of surface redox reaction is required to overcome the activation energy barrier and increase the reaction kinetics in order to realize sensing measurement. Therefore, additional heater systems are required to metal oxide gas sensor and it may cause high power consumption, high cost or occupying a lot of space. Hearin, we propose a nano-heater integrated metal oxide gas sensor that capable of sensing with low power consumption and fast response and of being fabricated at a wafer level through C-MEMS which facilitated sub-micrometer-level carbon patterning due to a dramatic volume reduction during the polymer pyrolysis. The temperature of the metal oxide, the sensing material, can be heated immediately to 600°C within a low power of 7mW. In addition, the maximum temperature of the metal oxide can be easily controlled according to the applied voltage. Therefore, only a voltage source is needed to heat the sensing material. Method Our sensor platforms as shown in Figure a, were fabricated in several steps. First, eave structures for the selective metal coating on a suspended carbon nanowire were fabricated by oxide etching and isotropic silicon etching. Then, the suspended carbon nanowire was fabricated through C-MEMS consisting of two successive photolithography and polymer pyrolysis [3]. Then, we deposited gold on a carbon wire using evaporation as a heater line. Owing to the eave structure and anisotropic evaporation, the gold layer could be connected only through the suspended wire as shown in Figure b. The next step was the deposition of HfO2 as an insulation layer by Atomic Layer Deposition. In what follows, the ZnO nanowire forest (Figure c) was integrated selectively on the middle of a suspended wire structure via seed layer patterning and a hydrothermal process in an autoclave [4]. Due to the high aspect ratio of the carbon nanowire, heat is hard to be released to both posts, resulting in a uniform temperature distribution over 80% of the total length of the wire. Therefore, we used ZnO nanowire forest as a sensing site by growing in the center of the carbon nanowire selectively where the temperature is kept uniform and within the range of the sensing target temperature according to the simulation results (Figure d). In the last, to obtain the electrical signal of sensing material, we deposited gold once more using an E-beam evaporator. Results and Conclusions We have identified the possibility of gas sensing with heater integrated metal oxide sensor platform. Observing that the resistance of the metal oxide decreased rapidly when a voltage was applied, we speculated that the temperature of the metal oxide would have risen immediately according to the Joule-heating principle. Furthermore, the resistance of the metal oxide decreased continuously as the applied-voltage gradually increased. When we applied 1.2V to a gold heater, the resistance of the ZnO dropped to about 1/20 of its room temperature resistance (Figure e) similar when it is heated by an external heater to about 250℃, which is occasionally used temperature condition in the MOS type gas sensor. Detailed gas sensing results will be presented at the conference. References [1] N. Barsan, D. Koziej, and U. Weimar. Metal oxide-based gas sensor research: How to?. Sensors and Actuators B: Chemical 121.1 18-35 (2007). [2] Y. S. Kim. Microheater-integrated single gas sensor array chip fabricated on flexible polyimide substrate. Sensors and Actuators B: Chemical 114.1 410-417 (2006). [3] Y. Lim, J. Heo, and H. Shin. Fabrication and application of a stacked carbon electrode set including a suspended mesh made of nanowires and a substrate-bound planar electrode toward for an electrochemical/biosensor platform, Sensors and Actuator B-Chemical 192 796-803 (2014). [4] Y. Lim, S. Kim, Y. M. Kwon, J. M. Baik, and H. Shin. highly sensitive gas-sensing platform based on a metal-oxide nanowire forest grown on a suspended carbon nanowire fabricated at a wafer level. Sensors and Actuators B: Chemical 260 55-62 (2018). Figure 1
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