Date of Award

Fall 2004

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Environmental Science - (Ph.D.)

Department

Chemistry and Environmental Science

First Advisor

Barbara B. Kebbekus

Second Advisor

Joseph W. Bozzelli

Third Advisor

Durgamadhab Misra

Fourth Advisor

S. Mitra

Fifth Advisor

Vladimir Zaitsev

Abstract

Two thin-film sensors, composed of different thicknesses (approximately 0.5 microns and 4.5 microns) of a cadmium sulfide layer coated with Rhodamine B fluorescent dye on a glass substrate, were tested for change in photoconductivity due to exposure to different concentrations of gaseous organic molecules in the parts-per-million (ppm) range.

It was theorized that the gaseous organic molecules would adsorb to the dyed semiconductor surface and that some energy would be transferred to the adsorbed analyte rather than to the cadmium sulfide semiconductor layer through the dye, thereby decreasing the photoconductivity of the surface in an amount proportional to the analyte concentration to which the sensor was exposed. Toluene was chosen as a typical organic vapor to test for the purpose of this study. [1]

The effect of oxygen, nitrogen, helium, humidity and temperature on these sensors was also studied and the results obtained are outlined in this report. Both sensors were able to detect toluene concentrations in the parts-per-million range in the absence of oxygen. The thicker sensor had a photoelectric response that was about ten times as large as the thinner sensor, presumably due to a greater porosity and surface area.

The sensor with the thicker cadmium sulfide layer was able to detect toluene in the 0 to 30 ppm concentration range in air at room temperature. At a concentration higher than 30 ppm of toluene, it is believed that the concentration of toluene was enough to provide an alternate pathway for the surface current, which led to a sudden increase in the surface photoconductivity.

The thicker sensor was also tested at a higher temperature and it was shown that a higher temperature led to lower resistances, presumably due to the desorbing of gases from the sensor surface plus an increased ability of electrons to partially overcome some of the intergrain resistance at the polycrystalline grain boundaries in the cadmium sulfide layer. [2]

Results from experimental studies showed that this sensor could detect both the presence and concentration of oxygen in the sensor chamber with the 0.5 and 4.5 micron cadmium sulfide thin-film sensor, whether the surface had a Rhodamine B dye layer or not. The dye was shown, however, to increase the photosensitivity of the sensor.

Exposure of the sensor to a large enough quantity of a vaporous polar substance like water, caused a decrease of the resistance due to the provision of an alternate pathway for the surface current, and in certain instances, concentrations could be predicted.

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