Biological and colloidal synthesis of fluorescent quantum dots

Student thesis: Doctoral ThesisDoctor of Philosophy


Part I of this thesis shall focus on the colloidal synthesis of InZnP-based quantum dots (QDs). Tris(dimethylamino)phosphine (P(NMe2)3) was used for the P-precursor because it was safer than the commonly-used tris(trimethylsilyl)phosphine. Recent studies have indicated that zinc halides do not contribute to the formation of alloyed InZnP with P(NMe2)3; instead, Zn is a spectator to the reaction. Zinc carboxylates are well characterised in the reaction with silylphosphine precursors, but their role remains unclear with aminophosphines. Zinc stearate was therefore chosen as the zinc precursor. Surprisingly, the product nanoparticles were fluorescent, when most bare InP is not. The emission wavelength was dependent on the concentration of zinc stearate, which implied band-gap behaviour. High-resolution STEM images could not resolve a core/shell structure, suggesting an alloyed structure; this was confirmed by XPS, which additionally indicated the formation of a Zn-rich surface. However, oxides were prevalent within the QDs. A surface phase of ZnS or CdS was subsequently deposited on InZnP, forming alloys in both cases. InZnPCdS was of particular interest because it formed a quasi-type II band alignment, because it had a bright quantum yield (36%), and because it had a long average fluorescence lifetime of 133 ns. Surprisingly, XPS revealed that it was homogeneously alloyed with respect to Cd, unlike the more common InP/CdS core/shell structure.

Part II of this thesis considers the biological synthesis of cadmium chalcogenide QDs in the roots of Allium fistulosum (spring onion). Cadmium acetate and sodium selenite were added to the hydroponic growth solution; within 6 hours, visible orange fluorescence that was associated with QDs had formed in the root tips. The QD photoluminescence was observed to spread fast through the roots radially, but slower axially. Spectral-scanning confocal microscopy indicated a photoluminescence wavelength of ca. 560–620 nm that was chiefly located within the cortical symplast. Likewise, these symplastic QDs had long lifetimes, ca. 140 ns, that allowed them to be distinguished from the fast lifetimes of the autofluorescent root tissue. Correlative imaging allowed the exact same subcellular features to be analysed with with multimodal techniques. First, elemental analysis with NanoSIMS located CdSe-containing material, chiefly along the plasma membranes. This was imaged at high-resolution with STEM, which identified CdSeS QDs with <10 nm diameters; these were bound to the plasma membrane. Cl appeared to be associated with the surface of the QDs, which may account for their bright fluorescence. Bright cathodoluminescence was observed in a QD-rich region, with an emission wavelength of ca. 590 nm. Thus, fluorescent CdSeS QDs were synthesised in situ by the roots of A. fistulosum, the first report of its kind.
Date of Award1 Sept 2019
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorMark Green (Supervisor) & Roland Fleck (Supervisor)

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