A Novel Sensor to Monitor Surface Charge Interactions: The Optically Stimulated Contact Potential Difference Probe

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Mess, Francis McCarthy
Danyluk, Steven
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This study addresses the development of a sensor to monitor chemical adsorption and charge transfer processes on a surface using a contact potential difference probe (CPD). The current investigation is an outgrowth of ongoing research on non-vibrating CPD probes (nvCPD) which led to the recent development of a novel measurement technique utilizing optical stimulation: optically stimulated CPD (osCPD). Primary outcomes of this thesis are the theoretical modeling, fabrication and demonstration of a functional osCPD sensor. The research also involved significant engineering and experimentation in the design, development, and application of this sensor to oil condition monitoring. This technique measures dielectric and chemical properties of a fluid at the interface between the fluid and a semiconductor substrate. Chopped visible light is used to stimulate the rear surface of a semiconductor substrate, and a CPD probe measures the work function response of the semiconductor on the front surface of the substrate. The work function response is influenced by the nature and quantity of adsorbed species on the top surface, allowing the probe to detect changes in chemical composition at the substrate/fluid interface. An analytical model is developed that relates the osCPD sensor output signal to the chemical and dielectric properties of the oil sample, as well as to the geometry, composition, and control inputs of the silicon substrate and test fixture. In this investigation, the osCPD sensor was used to evaluate dielectric and chemical properties of commercially available engine oil. Oil samples were intentionally degraded through thermal aging (oxidation) and through addition of known contaminants. The osCPD sensor shows good sensitivity to depletion of antioxidants in the oil, as well as to the presence of ferric chloride, an oil-soluble salt typically used to calibrate laboratory test equipment.
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