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J. Sama, S. Barth, R. Jimenez-Diaz, J. Prades, O. Casals, I. Gracia, C. Calaza, C. Cane, A. Romano-Rodriquez:
"Grown In situ Metal Oxide Nanowires onto MEMS Microhotplates for Advanced Gas Sensors";
Poster: MRS Spring Meeting 2013, San Francisco CA; 2013-04-01 - 2013-04-05; in: "MRS 2013 Spring Meeting", (2013).

Nanowires have been the subject of intense research the last years due to their optimal characteristics acting as a a sensing element due to their reduced size and high volume / surface ratio, and their well-controlled physical and chemical properties [1].
Reliability and homogeneity of nanodevice properties are relevant requirements for large scale production devices. Integration of nanowires or nanostructures in general into electronic and functional devices is a challenging issue because of the handicaps presented in their handling and assembly steps.
Manipulation and preparation of nanomaterials for sensor purposes often is divided in several phases, where the nanostructure synthesis stage is separated from the contacting one. This assembly approach implies following a high time-consuming process and adds uncertainty factors to the fabrication process. A strategy to achieve a more reliable and reproducible nanomanipulation method would be to perform the synthesizing and contacting processes in one step, reducing fabrication stages.
With this objective, here we present the localized growth of tin oxide nanowires on top of CMOS compatible micromembranes with incorporated heater in order to provide the thermal energy necessary to synthesize the nanowires.
An Au film is deposited on top of the microhotplate surface in order to use Au as nucleation seed of the metal oxide nanowires. The growth is performed by chemical vapor deposition (CVD), using the molecular precursor [Sn(OtBu)4] [3] and the high temperature required for growing metal oxide nanowires (above 925K) is provided by the heater. HRTEM analysis showed monocrystalline tin dioxide nanowires, with predominant [101] direction of the tetragonal SnO2 phase, showing the same cristalline quality as nanowires of large area deposition.
The device has been proven to be effective as gas sensor, as a direct application of the one-step nanodevice fabricated. Nanowires have been tested towards NO2 and CO pulses in a synthetic air atmosphere on a constant gas flow, showing a high response to low concentrations of toxic gas species. Moreover, the grown nanowires have been operating for more than two months, keeping its gas response at high values and demonstrating high robustness and durability.
The here presented strategy can be extended to other chemistries and paves the way to a new fabrication route to nanodevices.
[1] S. Barth, et al. Prog. Mater. Sci. (2010) 55 563
[2] S. Barth et al. Chem. Comm., (2012) 48 4734
[3] S. Mathur et al., Small 1 (2005) 713.

http://www.mrs.org/s13-program-s/