Date of Award

12-14-2015

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Andrew B. Greytak

Abstract

Progress in the colloidal synthesis of semiconductor quantum dots (QDs) has triggered a burst of research toward applications in QD based light emitting diodes, biological imaging as fluorescent tags, and advanced solar cell designs. In order to adjust and optimize the photo physical properties of QDs, QD heterostructures have been introduced and widely explored. This dissertation is focused on studies of growth of semiconductor colloidal CdSe/CdS core/shell quantum dots and the influences of neutral surface ligands to their photo-physical properties. Chapter 1 provides an introduction of the concept of semiconductor colloidal nanoparticles, core/shell heterostructures and the synthesis methods as well as the importance and applications. In chapter 2, we monitor effective bandgap energy shifts and free reagent concentration during the formation of CdS shells on CdSe nanocrystals to test the hypothesis that alternating addition of stoichiometric doses of precursors can effectively saturate surface sites and thereby enforce conformal shell growth. The selective ionic layer addition and reaction (SILAR) mechanism has been proposed to describe growth under such conditions and the method is attractive because of the opportunity to (1) avoid cross-reaction of precursors in growing binary films in solution and (2) enforce conformal growth, rather than regioselective growth, by saturating all available surface sites in a self-limiting manner in each half-cycle. The strong redshift that takes place when CdS shells are grown on CdSe cores provides a convenient process monitoring tool that complements Scanning Transmission Electron Microscopy (STEM) imaging and analytical measurements of free reagent concentration. We find that under commonly-used conditions, a cadmium oleate precursor reacts incompletely at chalcogenide-saturated nanocrystal surfaces. Although approximately spherical particles are obtained, the growth does not proceed via saturating cycles as described in the SILAR mechanism. This has implications for the rational control of conformal and regioselective growth of epilayers on nanocrystal quantum dots and higher-dimensional chalcogenide semiconductor nanostructures via solution processes. In chapter 3, we describe an experiment designed to identify the role of specific molecular ligands in maintaining the high photoluminescence (PL) quantum yield (QY) observed in as-synthesized CdSe/CdZnS and CdSe/CdS quantum dots (QDs). Although it has been possible for many years to prepare core/shell quantum dots with near-unity quantum yield through high-temperature colloidal synthesis, purification of such colloidal particles is frequently accompanied by a reduction in quantum yield. Here, a recently established gel permeation chromatography (GPC) technique is used to remove weakly associated ligands without a change in solvent: a decrease in ensemble QY and average PL lifetime is observed. Minor components of the initial mixture that were removed by GPC are then added separately to purified QD samples to determine whether reintroduction of these components can restore the photophysical properties of the initial sample. We show that among these putative ligands trioctylphosphine and cadmium oleate can regenerate the initial high QY of all samples, but only the “L-type” ligands (trioctyphosphine and oleylamine) can restore the QY without changing the shapes of the optical spectra. On the basis of the PL decay analysis, we confirm that quenching in viii GPC-purified samples and regeneration in ligand-introduced samples are associated chiefly with changes in the relative population fraction of QDs with different decay rates. The reversibility of the QY regeneration process has also been studied; the introduction and removal of trioctylphosphine and oleylamine tend to be reversible, while cadmium oleate is not. Finally, isothermal titration calorimetry (ITC) has been used to study the relationship between the binding strength of the neutral ligands to the surface and photophysical property changes in QD samples to which they are added. In chapter 4, the influence of different mixtures of solvents (such as amines), are studied as to increase the synthetic yield of the shell for core/shell nanoparticles when using SILAR based techniques for shell growth. Conversion of shell precursors to surface-adsorbed equivalents should be maximized for effective control of shell growth. Here, UV-vis absorption and photoluminescence spectroscopy are applied to monitor shell growth. Additionally, during the shell growth, the free precursor concentration is measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and fitted with Langmuir isotherm model which reveals the influence of different solvents on the fractional occupation of shell precursor equivalents on the QD surface. The binding affinities of the solvent molecules to the QD surface are also studied to understand the influence of such interactions on shell growth. This study is important for understanding the mechanism of growing the core-shell nanoparticles via SILAR technique and provides conditions under which precursor binding and synthetic yield can be increased, which could be applicable to synthesis of isotropic and anisotropic core/shell nanoparticles in an advanced and controllable manner.

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© 2015, Rui Tan

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