Applications for chemical sensors are diverse and include areas such as combustion products analysis, detection of chemical warfare agents, air quality monitoring, and medical diagnostics. Semiconducting metal-oxides such as SnO2, TiO2, and WO3 are widely used as gas sensing materials because the electrical conductivities of these materials increase by orders of magnitude when they are exposed to reducing gases such as carbon monoxide or hydrogen. However, commercial oxide sensors are not selective (i.e. they cannot differentiate between different analytes), and their relatively large masses result in high power requirements and long thermal stabilization times. Recent developments in microfabrication have provided opportunities to reduce the power requirement of sensor devices by utilizing micromachining techniques to thermally isolate sensor platforms from their underlying substrates. In addition to lower power requirements, such microsensor devices fabricated on silicon wafers have dramatically shorter thermal equilibration times than their traditional counterparts. Thus, it is possible to cycle microsensor temperatures on millisecond timescales, and we can take advantage of chemical kinetic effects to enhance sensor selectivity.
Miniaturization of chemical sensor technologies through micromachining techniques promises many advantages such as portability, low power operation, rapid thermal cycling, and array-based implementation for improved selectivity. Miniaturization is also accompanied by many challenges that include engineering issues involved in the design of micromachined devices as well as development of platform-compatible sensing materials to achieve chemical sensitivity and selectivity on the microscale. The University of Maine has an established research program dedicated to chemical sensor development in the Laboratory for Surface Science and Technology (LASST), and members of my group collaborate with LASST to develop and characterize sensor systems based on chemically selective sensor materials.
One goal of our research is to take advantage of chemical kinetics mechanisms to enhance the performance of gas sensors. Therefore, we study how temperature and catalytic activity of sensor materials affect the sensitivity of a sensor to particular gas analytes. The differences in chemical activity make it possible to design sensor systems that are selective to particular gas analytes. In this work we are interested in developing chemical sensor systems, as well as using surface science techniques such as temperature programmed desorption and infrared absorption spectroscopy to elucidate the underlying chemical reaction mechanisms.
Another area of current interest is the study of coupling or “cross-talk” between various sensor elements as we work to miniaturize sensor platforms and include multiple sensors in array-based platforms. For instance, experiments using arrays of SnO2-based microsensors have demonstrated that chemical reactions occurring on one sensor can affect the performance of adjacent sensors (see publications below). In addition to chemical crosstalk, we are also interested in addressing issues of thermal and electrical interference in multi-sensor arrays.
M. C. Wheeler, J. E. Tiffany, R. M. Walton, R. E. Cavicchi, S. Semancik, “Chemical Crosstalk between Heated Gas Microsensor Elements Operating in Close Proximity,” Sensors and Actuators B 77:167-176 (2001).
S. Semacik, R. Cavicchi, M. C. Wheeler, J. E. Tiffany, G. E. Poirier, R.M. Walton, J. S. Suehle, B. Panchapakesan, D. L. DeVoe, “Microhotplate Platforms for Chemical Sensor Research,” Sensors and Actuators B 77:579-591 (2001).
M. C. Wheeler, C. T. Reeves, D. C. Seets, C. B. Mullins, “Experimental Study of CO Oxidation by an Atomic Oxygen Beam on Pt(111), Ir(111), and Ru(001),” Journal of Chemical Physics 108:3057-3063 (1998).
P. D. Nolan, M. C. Wheeler, J. E. Davis, C. B. Mullins, “Mechanisms of Initial Dissociative Chemisorption of Oxygen on Transition-Metal Surfaces,” Accounts of Chemical Research 31:798-804 (1998).
M. C. Wheeler, D. C. Seets, C. B. Mullins, “Angular Dependence of the Dynamic Displacement of O2 from Pt(111) by Atomic Oxygen,” Journal of Chemical Physics 107:1672-1675 (1997).
D. C. Seets, M. C. Wheeler, C. B. Mullins, “Trapping-Mediated and Direct Dissociative Chemisorption of Methane on Ir(110): A Comparison of Molecular Beam and Bulb Experiments,” Journal of Chemical Physics 107:3986-3998 (1997)
M. C. Wheeler, D. C. Seets, C. B. Mullins, “Kinetics and Dynamics of the Initial Dissociative Chemisorption of Oxygen on Ru(001),” Journal of Chemical Physics 105:1572-1583 (1996).
D. C. Seets, M. C. Wheeler, C. B. Mullins, “Kinetics and Dynamics of Nitrogen Adsorption on Ru(001): Evidence for Direct Molecular Chemisorption,” Chemical Physics Letters 257:280-284 (1996).
M. C. Wheeler, D. C. Seets, C. B. Mullins, “Kinetics and Dynamics of the Trapping-Mediated Dissociative Chemisorption of Oxygen on Ru(001),” Journal of Vacuum Science & Technology A 14:1572-1577 (1996).
Image Description: M. Clayton Wheeler