Sensors and Actuators B 166–167 (2012) 576–585 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical j o ur nal homep a ge: www.elsevier.com/locate/snb Investigations on the origin of mass and elastic loading in the time varying distinct response of ZnO SAW ammonia sensor V. Bhasker Raj a , A.T. Nimal b , Yashoda Parmar b , M.U. Sharma b , Vinay Gupta a,∗ a Electronic Materials and Devices Laboratory, Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India b Solid State Physics Laboratory, Lucknow Road, Timarpur, Delhi 110054, India a r t i c l e i n f o Article history: Received 5 November 2011 Received in revised form 26 January 2012 Accepted 5 March 2012 Available online 15 March 2012 Keywords: SAW Sensor Ammonia Sensing mechanism a b s t r a c t In our earlier report, we gave an account of an interesting observation of a distinct response of ZnO SAW sensor for liquor ammonia. While the sensor response consisted of a small initial decrease in differential frequency followed by large increase for moist ammonia, it showed decrease alone for dry ammonia, water vapors and other interferants. This paper attempts to investigate the fundamental cause of the observed distinct response, by exploring the contributions of various sensor mechanisms such as mass, elastic and acousto-electric loading. To carry out the investigation, four sensors having ZnO films on SAW resonators with varying structural properties were employed. ZnO thin films of same thickness but differing in crystallite size, surface roughness and stress were achieved by depositing the films using RF sputtering under different pressures (10–40 mTorr). Electrical measurements on ZnO thin film show a very small contribution of acoustoelectric effect toward sensing response for liquor ammonia. Initially the elastic loading and later mass loading are identified as the dominant mechanism. The response has been fitted theoretically to determine individual contributions toward the sensing. A mechanism has been proposed and a correlation between the properties of ZnO film and sensor response has been attempted. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Nowadays the use of acoustic devices as chemical sensors is well known for their high potential in terms of their high sensitivity, high speed, good reliability, high accuracy and low cost. The surface acoustic wave (SAW) sensors can be used for precise detection of hazardous gases, chemical warfare agents, explosive vapors, envi- ronmental pollutants, etc. [1–3]. The adsorption of chemical vapors by the sensitive layer on the SAW surface changes the SAW charac- teristics such as attenuation and velocity, which in turn affects the amplitude, frequency and delay of the SAW device. However there are a lot of issues and concerns related to the properties of the sensitive coating such as sensitivity, selectivity, robustness, reliability and long term stability. A good control of thickness, uniformity, viscosity and film adherence of the sensitive coating is necessary in order to obtain reliable and stable SAW sen- sor performance. In this respect, semiconductor metal oxide thin films are preferred over the polymer coatings as sensitive layer. The common techniques employed for the deposition of polymers, such as spin coating, dip coating or spray coating, do not enable ∗ Corresponding author. E-mail address: drvin gupta@rediffmail.com (V. Gupta). sufficient uniformity of the sensitive layers. The defects thus incor- porated are known to degrade the performance of the SAW sensors [4]. Further, the response signal of SAW sensors coated with poly- mers may also include a large contribution from swelling induced modulus changes in the sensitive film due to vapor sorption [5]. The rapid expansion of supercritical solutions (RESS) is a technique for depositing polymer films to increase sensitivity and reliability how- ever it requires specially designed equipment and is more complex and expensive [6]. Sensitivity and selectivity can also be improved using molecularly imprinted polymers (MIP). But the performance of MIPs in practical applications is limited because of the lack of simple and robust methods to synthesize them. Further, MIP pre- pared with one template molecule will not only be completely selective for that molecule but also for other target molecules with a similar three-dimensional arrangement of interacting functional groups [7]. Other coatings such as carbon nanotubes and graphene sheets have also been considered to increase the sensitivity and selectiv- ity of SAW sensor but they suffer from the lack of reproducibility and the difficulty in depositing uniform coatings onto the trans- ducer surfaces [8]. Alternatively metal oxides are attractive for enhanced and reproducible response due to ease in fabrication of nanostructures and increase in surface to volume ratio. However the selectivity issue remains a challenge. 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.03.013