| Description |
Semiconductor nanocrystal quantum dots are a relatively recent area of study in materials science and engineering, but their unique, size-dependent properties have resulted in active growth over the past three decades. The motivation for this thesis has been exploiting the ability to tune the energy band gap and develop new families of geothermal reservoir tracers. While colloid transport in porous media has been studied extensively for groundwater systems, there is little existing research appropriate to high temperature geothermal systems. In this research, a multitiered approach is used to characterize quantum dot behavior at temperatures above 100 °C. First, a model system of cadmium selenide (CdSe) quantum dots is used to investigate fundamental aspects of nanocrystal growth and dissolution. Observing quantum dot dissolution and modeling the kinetic parameters yields critically important thermodynamic properties. These parameters are necessary for optimizing large-scale reactor conditions and design, and predicting fluid-phase quantum dot behavior. Insight into these thermodynamic properties provides the basis for experimentally studying transport in high temperature porous media that are surrogates for a geothermal reservoir. Core/shell quantum dots were pumped through Ottawa sand columns under a range of temperatures and salinities. Retardation and deposition were investigated as the principal transport parameters, while also considering the dynamics of quantum dot solubility and the interaction energy between quantum dots and the sand surfaces. Elevated temperatures increased the amount of quantum dot retention, following a multilayer deposition model. Finally, a novel method for detecting optically active species is introduced. Existing techniques for optical detection of quantum dots fail in turbid or high temperature environments. We demonstrate how the characteristic absorption - coupled with a long-wavelength overtone band - can be used to detect QDs in a variety of industrially relevant mixtures. |
