CDSE Quantum Dot Surface Chemistry Thermodynamics via Isothermal Titration Calorimetry: An Emphasis on the Fundamentals

For several decades, the study and development of colloidal semiconductor nanocrystals, or quantum dots (QD), has become a rich field heralding improved integration into applications ranging from photovoltaics and photocatalysis to biomedical imaging and drug delivery. CdxSey is the most extensively studied QD system, however numerous compositional details still confound the nanocrystal field. Although CdSe QDs with native ligand coatings can show high fluorescence quantum yield and may be suitable for some applications, often times these original ligand layers are comprised of long aliphatic chains that preclude incorporation into biological matrices or severely impede charge transfer – depending on the end goal functionality. While the innermost core can be highly crystalline, due to the QD size regime a large fraction of the constituent atoms is found at the surface; the nature of which strongly influences optoelectronic properties. Indeed, the necessary ligand surfactant layer is anything but innocuous; dictating synthetic morphology, determining solubility, quenching or enhancing photoluminescence, or even modulating the nanocrystal’s band gap. A detailed, consistent and unambiguous profile for QD surface composition and thermodynamics would be extremely advantageous toward controlling and improving photophysical properties. This dissertation highlights several caveats for appropriately compiling a thermodynamic profile in situ for the dynamic nature of QD surfaces, and to describe approaches to address them. I have focused on developing commonly employed metrics for investigating CdSe QD surface chemistries. I begin by thoroughly considering how various purification techniques alter the most significant aspects of QD investigations and performance. Among these, I illustrate the gel permeation chromatography (GPC) approach that I helped to establish as a highly effective technique for nanoparticle purification. Finally, I delineate in several fundamental CdSe-based QD systems the capacity of isothermal titration calorimetry as a sensitive and precisely quantitative technique to directly probe reaction thermodynamics in organic phase. Even in cases where common spectroscopic techniques have been of limited use, ITC is employed to elucidate complex binding phenomena. Beginning with the highly reproducible GPC purification technique for a consistent QD starting material, this dissertation depicts my efforts to provide consistent equilibrium thermodynamic data for relevant QD surface chemistry interactions