Sensors and Actuators B 220 (2015) 154–161 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo u r nal homep age: www.elsevier.com/locate/snb Novel scheme to improve SnO 2 /SAW sensor performance for NO 2 gas by detuning the sensor oscillator frequency V. Bhasker Raj a , A.T. Nimal b , Monika Tomar c , M.U. Sharma b , Vinay Gupta a,∗ a Department of Physics, Acharya Narendra Dev College (University of Delhi), Govindpuri, Delhi 10019, India b Solid State Physics Laboratory (DRDO), Lucknow Road, Timarpur, Delhi 110054, India c Physics department, Miranda House college, University of Delhi, Delhi 1007, India a r t i c l e i n f o Article history: Received 3 January 2015 Received in revised form 13 May 2015 Accepted 16 May 2015 Available online 29 May 2015 Keywords: SAW NO2 SnO2 Sensor Mechanism Oscillator a b s t r a c t A new method has been devised for obtaining fast NO 2 gas sensing characteristics by using detuned SnO 2 /SAW sensor oscillator. The deposited SnO 2 thin films were polycrystalline showing XRD reflections corresponding to (1 0 1) and (2 1 1) planes of rutile structure. The frequency of the sensor remained tuned for SnO 2 layer of small thickness (< 90 nm), and sensing response was moderate (< 140 kHz) with relatively slow response time and recovery time. However with increase in thickness (≥ 90 nm), the frequency of SAW sensor detuned resulting in enhanced sensing response (≥ 16 MHz) along with fast response time and recovery time. Independent experiments confirmed the change in elasticity of the sensing layer as the dominant sensing mechanism for SnO 2 /SAW sensor towards NO 2 gas. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Nitrogen dioxide (NO 2 ) is one of the hazardous gases with seri- ous concern as it can lead to severe health hazards [1]. It is an intermediate product in the industrial synthesis of nitric acid which is produced in millions of tonnes every year [2]. The exposure of NO 2 gas can cause harmful effects on the lungs, worsen cough, increase the possibility of asthma attacks and make susceptible to respiratory infection [3]. It can also lead to smog and acid rain [1]. IDLH (Immediately dangerous to life or health) concentration of NO 2 gas has been declared to be 20 ppm by Occupational Safety and Health Administration, USA [1]. Hence detection of NO 2 gas at low concentration is crucial. Different sensing techniques have been used for the detection of NO 2 gas like Chemiresistive, Electro- chemical, Optical, Acoustic, Chromatography, etc. [1,4–7]. However surface acoustic wave (SAW) is the most suitable technique for the detection of toxic gases because of high sensitivity, fast response speed, good reliability, low cost, real time measurement and wire- less sensing in inaccessible areas [8]. ∗ Corresponding author at: Electronic Materials and Device Laboratory, Depart- ment of Physics and Astrophysics, University of Delhi, Delhi-110007, India. E-mail addresses: drguptavinay@gmail.com, drvin gupta@rediffmail.com (V. Gupta). Few efforts have been made in literature by utilizing polymers or metal oxide thin films as sensing layer for the detection of NO 2 gas using SAW sensors [9–12]. The sensing layer of copper pthalo- cyanine (CuPc) film yielded good sensitivity allowing sub ppm level detection of NO 2 gas [11]. However, the major issue with the usage of CuPc film was the high response and recovery time (∼ few min- utes), besides high operating temperature [11]. Further, the effect of other interferants on the performance of SAW sensor perfor- mance was not studied using CuPc layer. Thin film of polypyrole was also used as sensing layer for detection of NO 2 gas [10,12]. However, high values of response time and recovery time were the major drawbacks of the sensor [10,12]. Dewan et al. deposited TeO 2 thin films on the surface of LiNbO 3 based SAW devices and demonstrated the detection of NO 2 gas at room temperature [9]. However the concentration of NO 2 gas was too high (>100 ppm) and selectivity issue was not even accounted [9]. Other reports uti- lized metal oxides or bilayer structure of polymers and metal oxides as sensing layers for NO 2 gas detection [13,6,14,15]. However, the obtained results were not promising due to slow response and recovery speed and lack of selectivity [13,6,14,15]. Love wave SAW devices with ZnO as guiding layer and single walled carbon nano- tubes as sensing layer were also utilized for the detection of NO 2 gas [16]. However the prepared sensor was also sensitive to other gases [16]. This emphasize the importance of further study towards the development of SAW gas sensor having suitable sensing layer to obtain efficient response characteristics towards NO 2 gas. Recently, http://dx.doi.org/10.1016/j.snb.2015.05.043 0925-4005/© 2015 Elsevier B.V. All rights reserved.