Surface acoustic wave (SAW) sensors are a class of piezoelectric MEMS sensors which can achieve high sensitivity and excellent robustness. A surface acoustic wave ethanol sensor using ZnO nanorods has been developed and tested. Vertically oriented ZnO nanorods were produced on a ZnO/128° rotated Y-cut LiNbO3 layered SAW device using a solution growth method with zinc nitrate, hexamethylenetriamine, and polyethyleneimine. The nanorods have average diameter of 45？nm and height of 1？μm. The SAW device has a wavelength of 60？um and a center frequency of 66？MHz at room temperature. In testing at an operating temperature of 270 with an ethanol concentration of 2300？ppm, the sensor exhibited a 24？KHz frequency shift. This represents a significant improvement in comparison to an otherwise identical sensor using a ZnO thin film without nanorods, which had a frequency shift of 9？KHz. 1. Introduction Sensing of ethanol vapour has important applications in industry and society. At high temperatures (200°C to 300°C) zinc oxide absorbs ethanol vapour, causing a significant change in conductivity [1, 2] and also leading to a change in mass. This change in properties can be used to create an ethanol sensor. Surface acoustic wave (SAW) sensors are a class of piezoelectric MEMS sensor which can achieve high sensitivity and excellent robustness. Since their discovery by Rayleigh in 1885 [3] surface acoustic waves have been extensively researched. The energy of a surface acoustic wave is concentrated within several wavelengths of the surface [4, 5]. For this reason, the propagation characteristics of surface acoustic waves are highly sensitive to any change in the properties of the surface on which they travel. Due to the sensitivity of SAW devices to small changes in mass loading and surface conductivity, SAW devices have been extensively studied as gas sensors [6–8]. A typical SAW gas sensor uses an interdigital transducer (IDT) to generate a surface acoustic wave in a piezoelectric substrate. The surface acoustic wave propagates along a delay line coated in some material which absorbs the target gas. A second IDT at the end of the delay line is then used to transduce the SAW to an electrical signal. Absorption of gas onto the sensing layer causes a change in the propagation velocity of the surface acoustic wave, resulting in a shift in the resonant frequency of the device. The majority of existing SAW gas sensors have used thin film sensing layers, with limited surface area. The application of nanostructured sensing layers, such as ZnO nanorods can potentially lead to
References
[1]
Q. Wan, Q. H. Li, Y. J. Chen et al., “Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors,” Applied Physics Letters, vol. 84, no. 18, pp. 3654–3656, 2004.
[2]
G. Heiland, “Homogeneous semiconducting gas sensors,” Sensors and Actuators, vol. 2, pp. 343–361, 1981.
[3]
Rayleigh, “On waves propagated along the plane surface of an elastic solid,” Proceedings of the London Mathematical Society, vol. 1, supplement 1–17, pp. 4–11, 1885.
[4]
R. White, “Surface elastic waves,” Proceedings of the IEEE, vol. 58, no. 8, pp. 1238–1281, 1970.
[5]
A. Mamishev, K. Sundara-Rajan, F. Yang, Y. Du, and M. Zahn, “Interdigital sensors and transducers,” Proceedings of the IEEE, vol. 92, no. 5, pp. 808–845, 2004.
[6]
A. D'Amico, A. Palma, and E. Verona, “Surface acoustic wave hydrogen sensor,” Sensors and Actuators, vol. 3, pp. 31–39, 1982.
[7]
Y. J. Lee, H. B. Kim, Y. R. Roh, H. M. Cho, and S. Baik, “Development of a SAW gas sensor for monitoring SO2 gas,” Sensors and Actuators A: Physical, vol. 64, no. 2, pp. 173–178, 1998.
[8]
R. A. McGill, V. K. Nguyen, R. Chung et al., “The “NRL-SAWRHINO”: a nose for toxic gases,” Sensors and Actuators B: Chemical, vol. 65, no. 1, pp. 10–13, 2000.
[9]
F. C. Huang, Y. Y. Chen, and T. T. Wu, “A room temperature surface acoustic wave hydrogen sensor with Pt coated ZnO nanorods,” Nanotechnology, vol. 20, no. 6, Article ID 065501, 2009.
[10]
A. Z. Sadek, W. Wlodarski, Y. X. Li et al., “A ZnO nanorod based layered ZnO/64° YX LiNbO3 SAW hydrogen gas sensor,” Thin Solid Films, vol. 515, no. 24, pp. 8705–8708, 2007.
[11]
W. S. Wang, T. T. Wu, T. H. Chou, and Y. Y. Chen, “A ZnO nanorod-based SAW oscillator system for ultraviolet detection,” Nanotechnology, vol. 20, no. 13, Article ID 135503, 2009.
[12]
J. Hornsteiner, E. Born, G. Fischerauer, and E. Riha, “Surface acoustic wave sensors for high-temperature applications,” in Proceedings of the 1998 IEEE International Frequency Control Symposium, pp. 615–620, May 1998.
[13]
W. Soluch, “Design of SAW delay lines for sensors,” Sensors and Actuators A: Physical, vol. 67, no. 1, pp. 60–64, 1998.
[14]
P. Hari, M. Baumer, W. D. Tennyson, and L. A. Bumm, “ZnO nanorod growth by chemical bath method,” Journal of Non-Crystalline Solids, vol. 354, no. 19–25, pp. 2843–2848, 2008.
[15]
B. B. Rao, “Zinc oxide ceramic semi-conductor gas sensor for ethanol vapour,” Materials Chemistry and Physics, vol. 64, no. 1, pp. 62–65, 2000.
[16]
H. Wohltjen, “Mechanism of operation and design considerations for surface acoustic wave device vapour sensors,” Sensors and Actuators, vol. 5, no. 4, pp. 307–325, 1984.
