Title: Synthesis and Formation Mechanism of Metal Phosphide and Chalcogenide Nanocrystals
Description: Semiconductor nanocrystals, or quantum dots, have attracted significant interest for use in solid state lighting, biological imaging, photovoltaics, catalysis, and displays such as televisions or tablets. Quantum dots excel in these applications because of their narrow emission profiles, high absorptivity at high energies, and optoelectronic properties that can be easily tuned using colloidal chemistry. The last point in particular has driven the development of new synthetic methods for producing a range of semiconducting materials on the nanoscale. Academically, interest in the synthesis of quantum dots has also extended to the mechanism of their formation and its implications for the growth of nanoscale crystals more generally. This thesis addresses facets of both points above, first by developing several novel syntheses for indium and gallium phosphide nanocrystals, and second by leveraging the synthetic control it allows to study the mechanisms of homogeneous crystal growth. Chapter 1 provides a brief overview of the colloidal syntheses, optoelectronic properties, and formation mechanisms of quantum dots.

Emphasis is placed on the development of new chemical syntheses for nanoscale materials and how the size, size distribution, and morphology can be carefully controlled by thoughtful reaction design. The progression of quantum dot synthesis is presented and specific innovations to the precursor and surfactant design are highlighted. Next, a brief discussion about nanocrystal surface chemistry and its impact on the photophysical properties of the inorganic core is described along with its proposed influence on the kinetics of nanocrystal growth. Finally, classical theories of homogeneous crystal growth are presented and used to explain the origin of the exceptionally narrow size distributions accessible in a wide range of materials. Chapter 2 introduces two novel synthetic pathways to InP nanocrystals. The first describes a small library of substituted aminophosphines that can control the precursor conversion reactivity by over an order of magnitude. Leveraging the collection of aminophosphines, it is demonstrated that at sufficiently high temperatures, the rate of precursor conversion can be used to vary the final nanocrystal size—disputing previous findings for InP nanocrystals.

We show that the reactivity of the phosphine is governed by a pre-equilibrium between the precursor and an intermediate (P(NHR)3) that goes on to form InP. Variations to the initial aminophosphine substitution pattern change the position of the pre-equilibrium, thereby allowing the rate of [InP]i deposition to be controlled. The second synthetic method leverages metal phosphonate salts as a surfactant to synthesize large samples of InP. We find that the nanocrystals grow via a ripening mechanism and display excellent crystallinity as determined by powder X-ray diffraction and pair distribution function analysis. Finally, we demonstrate that the final nanocrystals are bound by both phosphonates and phosphines through the use of 31P nuclear magnetic resonance spectroscopy. Chapter 3 expands on the syntheses of InP in the previous chapter by developing methods to form GaP, InxGa1-xP, and InP-based core-shell structures. At the onset, two distinct syntheses of GaP are introduced, one similar to the metal phosphonate route used to form InP, and one that used a mixture of amines to stabilize GaP colloidally.

The phosphonate method results in small GaP with somewhat indistinct scattering patterns, while the amine method results in large GaP whose morphology can be varied depending on the solvent selected. Leveraging the newly developed InP and GaP syntheses we demonstrate that InxGa1-xP alloys could be directly synthesized from mixtures of In3+ and Ga3+ salts. We also show that InxGa1-xP can be accessed indirectly via cation exchange of Zn3P2 or Cd3P2, however attempts at synthesizing alloys via cation exchange with phosphonate bound GaP were found to be largely unsuccessful. Finally, the chapter contains initial attempts at synthesizing GaP/InP core-shells with the intention of producing GaP/InP/GaP spherical quantum well architectures. Preliminary data show that InP can be deposited using several different methods, though it remains unclear whether the optical properties will be suitable for integration in solid state lighting applications. Chapter 4 examines the crystal growth processes that precede the formation of monodisperse ensembles of InP, PbS, and PbSe nanocrystals.

Surprisingly, we find that nucleation persists for a substantial portion of the total reaction time—a stark departure from the canonical “burst” of nucleation proposed originally by Victor LaMer. We go on to measure the nucleation period for a variety of different reaction conditions and find that the fraction of reaction time nucleation extends over is sensitive to both the material and reaction temperature. This is consistent with a mechanism where faster kinetics of monomer attachment reduce the duration of crystal nucleation—a conclusion that can be surmised by nucleation mass balance models that show a clear material and temperature dependence on the rate of nanocrystal growth. We also interrogate the claim that solute molecules accumulate prior to the formation of mature nanostructures. In situ X-ray experiments clearly corroborate the appearance of solute-like species at early reaction times that build up prior to the appearance of crystals with extended structure. Finally, we propose a novel size-focusing mechanism predicated on a size dependent growth rate.

Using population mass balance modeling we show that the measurements of size and size distribution are qualitatively consistent with a growth rate inversely proportional to nanocrystal size.