Gas Sensing by Resistance Fluctuations in Pd x WO 3 Nanoparticle Films J. Ederth a , J. M. Smulko b , and L.B. Kish c , P. Heszler a , G. Niklasson a , C. Granqvist a a Dept. of Materials Science, The Angstrom Laboratory, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden; E-mail: jesper.edert@angstrom.uu.se b Gdansk Univ. of Technology, WETiI, ul. G. Narutowicza 11/12 80-952 Gdansk, Poland c Dept. of Electrical Engineering, Texas A&M Univ., College Station, TX 77843-3128, USA ABSTRACT Nanoparticle films of Pd x WO 3 , with x being 0.01 or 0.12, were made by dual-beam gas deposition. Resistance noise as well as dc resistance were measured during exposure to ethanol and hydrogen gas. For ethanol concentrations exceeding 50 ppm, changes in the resistance noise gave 300 times larger detection sensitivity than changes in the dc resistance. This sensitivity reached a maximum at 250 °C and was very reproducible for ethanol sensing. Keywords: gas sensors, gas detection, gas deposition, resistance fluctuations 1. INTRODUCTION Gas sensing technology is a rapidly growing research field, for which improved sensitivity and selectivity are among the strongest driving forces. With regard to applications, other crucial aspects include durability and reproducibility [1,2]. A large number of investigations have been performed on gas sensors, particularly on those based on thin semiconducting films for which changes in the dc transport have been used to probe the pertinent gases [3-13]. There are several types of sensors, based on a number of different sensing mechanisms. The most common sensors comprise thin semiconducting metal oxide layers that can be reduced or oxidized by the gas, thereby changing their dc resistance [14] by modifying either the mobility or the concentration of the charge carriers. The mobility is likely to be altered by a change in the grain boundary potential between adjacent particles, caused by the target gas. A change in the charge carrier concentration can be due to electron traps created on the surface by the target gas. The magnitude of the resistance change depends on the concentration of the target gas [1,2]. Thus the change in dc resistance, i.e., the sensitivity, is correlated to modifications of the dc transport parameters of the sensor caused by the target gas. Gas sensors with improved sensitivity can be accomplished by measuring fluctuations in the resistance rather than changes in the dc resistance; the former are correlated to low frequency components of the dynamic disorder [15]. This fluctuation (“noise”) spectroscopy provides a new detection principle, as elaborated recently [16]. The purpose of this paper is to present preliminary results showing that the gas detection ability of a sensor can be significantly improved by recording fluctuations in resistance. Our samples were porous films containing nanoparticles of WO 3 and Pd, prepared by the gas evaporation method [17-20]. Section 2 reports sample preparation in detail. Sample characterization is described in Sec. 3, and results related to gas sensing in air, ethanol, and hydrogen are presented in Sec. 4. Finally, the results are discussed and analyzed in Sec. 5. 2. SAMPLES 2.1 Gas Evaporation All samples were made by gas evaporation. A schematic drawing of the gas evaporation unit is given in Fig. 1, showing that dual sources were used. A more detailed description of the equipment can be found elsewhere [17,18]. The left-hand source was employed for Pd evaporation. The starting material was put in a carbon crucible and was evaporated by induction heating. A laminar flow of synthetic air was used to cool the metal vapor so that nucleation and growth of Pd Invited Paper Noise and Information in Nanoelectronics, Sensors, and Standards II, edited by Janusz M. Smulko, Yaroslav Blanter, Mark I. Dykman, Laszlo B. Kish, Proc. of SPIE Vol. 5472 (SPIE, Bellingham, WA, 2004) · 0277-786X/04/$15 · doi: 10.1117/12.547362 191 Downloaded from SPIE Digital Library on 22 Feb 2010 to 130.238.21.142. Terms of Use: http://spiedl.org/terms