Date of Award

8-2007

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Computer Engineering

Advisor

M. Clayton Wheeler

Second Committee Member

Brian G. Frederick

Third Committee Member

Robert J. Lad

Abstract

The population of adsorbates on a semiconductor surface directly influences the physical and chemical properties of the semiconductor. In the case of a metal-oxide semiconductor, the adsorbing species can change its electrical conductivity, a phenomenon which forms the operating principle of gas sensors. The interaction of adsorbed oxygen species on a metaloxide surface with reducing or oxidizing gases leads to an increase or decrease in electrical conductivity respectively. Miniature gas sensors called microhotplates (developed at the National Institute of Standards and Technology) are excellent surface science tools to explore surface reactions on semiconducting metal-oxide films. This thesis outlines how the desorption kinetics may be modeled in situations where the effects of finite heating rate, and system pumping rate are intertwined with the desorption rate, and how it is possible to estimate these time constants from isothermal desorption of sub-femtomolar coverages. Benzoic acid on reduced SnO2 was used as a model system to demonstrate the technique. It was observed to adsorb at coverages below 0.005 monolayers with an activation energy for desorption of 97 kJ/mol. The uptake, reaction pathways, and desorption kinetics of 2-propanol on TiO2 and SnO2 films were studied to demonstrate new microhotplate-based techniques to probe the fundamental surface processes that lead to electrical conductivity changes in chemiresistive gas sensors. Uptake and pulsed desorption measurements showed that reproducible coverages of 2-propanol could be prepared during low temperature adsorption, while interlaced, mass-resolved desorption pulses quantified indications of conversion to propene on oxidized TiO2 and SnO2 that correlate with conductivity changes. Fractional isothermal desorption data for 2-propanol on the oxidized TiO2 film suggest that the surface is energetically heterogeneous. A Monte Carlo model gives an average binding energy of 102 kJ/mol with a standard deviation of 15.7 kJ/mol, assuming diffusion is negligible on the timescale of the microhotplate’s heating pulse. The technique can thus show how a microsensor platform can provide a better understanding of the principles of sensor operation by determining, from sub-femtomolar quantities of adsorbates on a single microsensor, coverage, pumping speed, desorption rate, and reactivity of surface interactions and their effect on the sensing film conductivity.

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