Sensors and Actuators B 106 (2005) 708–712 Gas sensing by thermoelectric voltage fluctuations in SnO 2 nanoparticle films Janusz M. Smulko a,1 , Jesper Ederth a,2 , Yingfeng Li a , Laszlo B. Kish a,∗ , Marcus K. Kennedy b , Frank E. Kruis b a Department of Electrical Engineering, Texas A&M University, College Station, TX 77843-3128, USA b Faculty of Engineering Sciences, Process- and Aerosol Measurement Technology, University Duisburg-Essen, Bismarckstrasse 81, D-47057 Duisburg, Germany Received 16 June 2004; received in revised form 18 September 2004; accepted 21 September 2004 Available online 11 November 2004 Abstract Experimental results of gas sensing by thermoelectric voltage fluctuation measurements in gas sensors are presented. The applied sensors consist of monodispersed SnO 2 nanoparticles films, with a mean diameter of 20 nm. The voltage fluctuations in the sensor films were observed with and without external voltage bias and its power density spectrum exceeded at least a thousand times its thermal noise. The power density spectrum of the observed stochastic signal changed in different gases by a factor of three while the change of the sensor resistance was only 2–3%. The stochastic signal observed in the sensor without external voltage bias is caused by the temperature gradient in the film. The results show that thermoelectric fluctuations, without external voltage bias, can be applied for use in gas sensing. © 2004 Elsevier B.V. All rights reserved. Keywords: Fluctuation-enhanced sensing; Nanoparticles; Stochastic processes; Bias-free Taguchi sensor 1. Introduction It has been known [1] for a long time that the processes of gas adsorption on porous semiconductor surfaces can change the electrical properties of the surface. Taguchi gas sensors are based on grainy metal-oxide semiconductor films (e.g. SnO 2 ) with transport properties that are very sensitive to vari- ations of the composition of the ambient gas. Usually, the DC resistance is measured and used for gas sensing in many prac- tical applications [2]. Significantly greater sensitivity (DC re- sistance change) has been observed with smaller grain sizes, even at lower substrate temperatures [3]. Presently, most arti- ficial noses need an array of different sensors in order to ana- ∗ Corresponding author. Tel.: +1 979 847 9071; fax: +1 979 845 1729. E-mail address: laszlo.kish@ee.tamu.edu (L.B. Kish). 1 Gdansk University of Technology, WETiI, ul. G. Narutowicza 11/12, 80-952 Gdansk, Poland. 2 The ˚ Angstr¨ om Laboratory, Department of Materials Science, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden. lyze gas mixtures [4] because each sensor provides only one component (point) of a pattern which is needed for gas recog- nition. This fact implies a variety of practical problems that limit their practical applications. For example, the power con- sumption, and the strong need and high cost of sensor main- tenance are among the most significant problems. Promising results have been observed when the fluctuations of the gas- sensitive film are applied for sensing [5,6]. Then, the statisti- cal measures of the fluctuations provide the required pattern, thus a single sensor can be used as a complete electronic nose detecting complex mixtures [6]. 2. Description of the sensors The gas sensors were prepared from monodispersed SnO 2 nanoparticles with a mean particle diameter of 20 nm [7]. The gas-sensitive film consists of a particle size of about 20 nm. The structure of the sensors is presented in Fig. 1 (top view) 0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.09.021