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From the fabrication strategy to the device integration of gas nanosensors based on individual nanowires

R. Jiménez-Diaz, J.D. Prades, O. Casals, T. Andreu, J.R. Morante, S. Mathur, L. Fonseca, C. Cane, J. Santander, S. Barth, F. Hernandez-Ramirez, A. Romano-Rodriguez
Universitat de Barcelona (UB), ES

Keywords: nanowires, gas nanosensors, metal oxides, contact fabrication, gas response


In the last years, nanowires have emerged as potential building blocks of new devices and circuit architectures due to their performance derived from their reduced size and well-controlled chemical and physical properties [1]. Large efforts have been devoted to the optimization of the synthesis conditions of these nanomaterials, their electrical characterization, and their manipulation and positioning. However, the development and testing of nanodevices, as well as their integration with a control and read-out electronics that enables their incorporation into real life applications is quite scarce. Among the different possible materials, we have focused our activity on metal oxide (tin dioxide, tungsten trioxide, …) because these materials, in micro- and nanoparticle form, are used in the fabrication of commercial solid state gas sensors, operating as chemiresistors [2]. In this work we report the activity carried out on the fabrication and test of advanced gas nanosensors based on individual metal oxide nanowires. Defect-free monocrystalline nanowires with radii in the range from few ten to few hundred nm were synthesized by chemical vapor deposition (CVD), using molecular precursors [3]. Controlled dispersion and manipulation of the nanowires on top of photolithographically prepatterned microelectrodes, prepared either on bulk or on suspended substrates, and the contact fabrication were also addressed. Different techniques have been employed for the fabrication of nanocontacts between the nanowires and the microelectrodes, allowing from high precision [4] (and being very useful for prototyping, Fig. 1) to mass-production oriented contact fabrication. Characterization of these chemiresistors towards different oxidizing and reducing gases at normal operation temperatures of these materials (between 100 and 300ºC) has shown their sensitivity and fast and reliable operation (Fig. 2). The selectivity of the produced devices was still an issue which has to be addressed. As chemiresistors, nanowires suffer from Joule effect due to their small dimensions, strongly heating themselves when driving current required for their measurement. This generally degrading effect due to uncontrolled probing currents can be positively used for local heating of the nanowires when using controlled and stabilized currents (Fig. 3), giving rise to ultralow power consumption devices that only require few W for their heating and readout [5]. Furthermore, pulsed exposure of the sensors to UV light in combination with the presence of gases allows for the operating of these sensors at room temperature, enabling, for example, their use in explosive ambient thanks to the absence of heating. Finally, the read-out and control electronics for these devices is also addressed in this work. Low power consumption, portable devices using these nanosensors as active elements, have been designed and tested. This contribution attempts to critically discuss the fabrication strategy, its impact on the electrical behaviour of the fabricated nanostructures, the fabrication of operative nanodevices, the limitations of the methodology and the guidelines for future work.
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