The Role of Precursor Conversion in the Nucleation and Growth of Zinc Pnictide Based Semiconductor Quantum Dots

Abstract

Interest in the synthesis of colloidal semiconductor nanocrystals has boomed over the last thirty years. This is, in part, due to the tunable optoelectronic properties of these colloids, which stems from the quantum confinement effect where the electronic structure evolves as a function of crystal volume. The other promising attribute of these colloidal semiconductors is their ability to be prepared and processed in solution, allowing device fabrication under ambient conditions and enabling deposition onto a wide variety of substrates. Most research to date has focused on synthesizing materials in the II-VI, IV-VI, and III-V families. The II-V class of semiconductors, including Zn3P2, Cd3P2, Zn3As2, and Cd3As2, have not been given much attention from the colloidal nanocrystal community despite their enormous promise for light harvesting applications. These four semiconductors have bulk band gaps ranging between 1.5 eV (Zn3P2) and 0 eV (Cd3As2). They are also completely soluble within each other, meaning an alloy of any composition is achievable and thus the bulk band gap is completely tunable over this 1.5 eV range. As a result, these semiconductors may offer advantages for a wide variety of light absorbing applications including the high earth abundance of zinc and phosphorus for cost effective light absorbing layers in solar cells, or less toxic alternatives to cadmium mercury telluride alloys for IR light detection applications. This thesis dissertation describes our efforts to elucidate and exploit novel precursor chemistry for the synthesis of Zn3P2, larger Zn3P2, and alloys of composition (CdyZn1-y)3As2. Chapter 1 serves as an introduction into the synthesis of II-V colloidal nanocrystals as well as describing typical synthetic challenges and structural differences among the products that result from the various methodologies. Chapter 2 describes a novel synthesis of 3 nm Zn3P2 nanocrystals that utilizes an activated intermediate, [Zn5(O2CR)6(Et)4], to promote reactivity with P(SiMe3)3. A novel P-Zn containing species then forms in a rate-limiting step prior to the nucleation and growth of the 3 nm Zn3P2 quantum dots. Chapter 3 will describe efforts to grow larger Zn3P2 particles from 3 nm seeds, which led to the identification of a different P-Zn containing intermediate with lower relative reactivity that afforded access to either bulk zinc phosphide or large 5 nm particles depending on the supporting ligands present. This chapter will also discuss the structural evolution that occurs as the particles grow from 3 to 5 nm in diameter. Chapter 4 discusses an extension of these synthetic methods for the synthesis of (CdyZn1-y)3As2 alloys. Lastly, Appendix B serves as a reference towards the synthesis of a family of arylsilylphosphines with varying electron withdrawing or electron donating groups in the para position of the aryl rings to serve as a library of potential phosphide precursors where the electronics of the P-Si bonds could be toggled.