Direct Printing of Suspended Metal Oxides Nanowires on MEMS Chip as Gas Sensor

Semiconducting metal oxides(MOs) are prevalently used in gas sensors thanks to its unique charge transport mechanism being sensitive to surrounding gas molecules. [1] To enhance the performance of the chemiresistor-type gas sensor, both the composition and structure of the sensing materials need to be optimized. [2] One-dimensional MOs nanowires(NWs), comparing to the conventional thick films, have exhibited outstanding gas sensing performance such as lower detection limit and shorter response time.[3] Currently, various synthesis methods of MOs NWs have been developed including hydrothermal, chemical vapor deposition, electrochemical deposition, and electrospinning, etc. However, the current existing methods fail to deliver precise point-to-point fabrication, which makes the integration of multiple gas sensing materials rather challenging. In addition, with the existing NWs synthesis technology, the fabrication of gas sensing NWs on a suspended heater membrane of the Micro-Electro-Mechanical System(MEMS) chip is complicated.[4]<br/>Here, inspired by the printing electronics, we present an alternative approach to fabricating freeform MOs NWs directly on the MEMS heater chip. The method exploits a nano-pipette filled with the ink containing polymer binder and metal precursors to print a preliminary composite wire with hundreds of nanometers in diameter and suspended arch architecture on micrometer-scale MEMS electrode pairs via a precise displacement and positioning system. After calcination, polycrystalline metal oxide NWs are successfully obtained. By optimizing the type or quantity of metal precursors in the ink, or adding precursors of noble metals such as gold, silver, platinum, and palladium, abounding MOs-based composite NWs are fabricated for diverse gas sensing. In addition, through such a point-to-point direct printing technology, various semiconducting metal oxides NWs can be assembled in a micron-scale region with high integration density, excellent precision, and nano-scale characteristics, which largely reduces power consumption and improves sensing performance. Furthermore, it also offers us a promising and straightforward process for multi-oxides material-based “E-nose” device fabrication.<br/>References:<br/>[1] Wang, Chengxiang, et al. "Metal oxide gas sensors: sensitivity and influencing factors." sensors 10.3 (2010): 2088-2106.<br/>[2] Dey, Ananya. "Semiconductor metal oxide gas sensors: A review." Materials Science and Engineering: B 229 (2018): 206-217.<br/>[3] Yang, Bingxin, Nosang V. Myung, and Thien Toan Tran. "1D metal oxide semiconductor materials for chemiresistive gas sensors: A review." Advanced Electronic Materials 7.9 (2021): 2100271.<br/>[4] Cho, Incheol, et al. "Localized liquid-phase synthesis of porous SnO2 nanotubes on MEMS platform for low-power, high performance gas sensors." ACS applied materials & interfaces 9.32 (2017): 27111-27119.