Investigation of Cantilever Resonance Applied to Potentiometric Sensing
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0951-E05-03
Investigation of Cantilever Resonance Applied to Potentiometric Sensing Goutam Koley1 and Lakshminarayanan Lakshmanan2 1 Electrical Engineering, University of South Carolina, 3A12 Swearingen Center, Columbia, SC, 29208 2 Electrical Engineering, University of South Carolina, 1B27 Swearingen Center, Columbia, SC, 29208
ABSTRACT We demonstrate a highly sensitive potentiometric gas sensor based on a resonating Si microcantilever. Using a scanning probe microscope based set up in non-contact mode, the microcantilever was made to oscillate at its resonance frequency with periodically changing amplitude, using simultaneous mechanical and electrical excitation sources. The variation of the oscillation amplitude was found to be extremely sensitive to changes in surface potential, and served as a linear indicator for surface work function changes caused by molecular adsorption. The microcantilever sensor was found to be able to detect changes in surface potential down to 50 µV, which is basically limited by the system noise. When applied to sensing hydrogen using platinum coated cantilevers, it was observed that the microcantilever sensor can detect 1000 ppm hydrogen with an estimated lower limit of the detection time of 70 ms, at a cantilever-ground electrode distance of ~10 micron. Several parameters, such as ac signal amplitude, cantilever – reference electrode distance, quality factor, area, and spring constant of the cantilever, can be adjusted to significantly enhance the sensitivity, possibly by orders of magnitude. Excitation of the cantilever at subharmonic resonance frequencies was also performed to study possible parametric resonance effects. In this system it was possible to observe sub-harmonic resonance of order more than 50 (i.e. lower than one-fiftieth the resonance frequency).
INTRODUCTION Microelectromechanical systems (MEMS) have been extensively used as sensors for a variety of mechanical parameters such as pressure, flow, mass, and stress.1-4 In addition, they have been used for several electrical applications, such as, filters and switches in Integrated Circuits, high quality factor (high-Q) oscillators, etc.5, 6 The microcantilever used in a scanning probe microscope (SPM) in non-contact or tapping modes is essentially a MEMS device. The advantage of using a microcantilever in measurements such as in a SPM, include, high quality factors of such oscillatory systems, easy batch fabrication using standard lithographic techniques, and a base material (commonly Si) with tunable electrical properties. In recent years, there has been a large focus on using the micro(nano)cantilevers as sensor elements for specific chemical and biological specie.7-9 These applications exploit the above mentioned advantages associated with micro(nano)cantilevers, in addition to the requirements of low power, and very small sample quantity for detection.
One of the most common techniques to detect specific type of molecules (usually larger biomolecules) is to detect the change in resonance frequency of very high-
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