Scratching the Surface of Colloidal InP Nanoparticles: Tuning the Physical and Electronic Structure through Surface Chemistry

Abstract

The structural and compositional variety of materials afforded by nanocrystal synthetic chemistry has established a foundation for developing high quality and tailored materials for a broad spectrum of applications, including photovoltaics, display and lighting, biological imaging, and catalysis. In all of these applications, colloidal processing presents a low-cost, highly scalable strategy for manufacturing. Quantum-confined semiconductor nanocrystals, or quantum dots (QDs), have size-dependent optical and electronic properties that have been successfully exploited in commercial display technologies that require phosphors with high color purity and emission tunability. Cadmium-based materials have dominated the field with respect to efficient syntheses that can produce monodisperse and highly emissive QDs, but the toxic nature of cadmium has led to environmental regulations that limit the amount of cadmium permitted in consumer products in many countries. Indium phosphide nanomaterials, InP, have thus gained interest as lower toxicity phosphors that emit in the visible region. The synthetic development of emissive InP QDs has encountered obstacles related to the more covalent nature of the crystal lattice and a lack of diverse precursor compounds. Furthermore, as-synthesized InP QDs are characterized by prohibitively weak emission that requires a non-trivial effort involving synthetic re-design or post-synthetic processing to improve photoluminescence quantum yields (PL QYs). This thesis works seeks to develop methods for overcoming this challenge. Chapter 1 summarizes the current state of InP QDs as viable commercial phosphors, including the strategies chemists use to improve PL QYs, and sets the stage for the following four chapters that target the rational design of PL enhancement strategies. Chapter 2 describes the interaction between InP QD surfaces and exogenous M2+ Lewis acids, revealing that M2+ cations (cadmium and zinc) undergo Z-type ligand exchange with surface indium, and through this shallow degree of alloying and surface passivation, impart distinct electronic properties and PL enhancement. In Chapter 3, the state of InP QD surface oxidation is discussed wherein adventitious side-reactions occur in common InP syntheses, resulting in the presence of oxidized phosphorus species that contribute to the non-radiative trap landscape that limits the obtainable QYs of surface-modified InP QDs. X-ray emission spectroscopy was used as a powerful technique to systematically and rapidly assess the degree of oxidation imparted in a series of InP syntheses and industrially-relevant shelling reactions. In an alternative approach to direct and improve InP QD properties, InP magic-sized clusters (MSCs) were used as scaffolds for cation exchange where once again, surface chemistry plays a critical role in facilitating partial or complete cation exchange. Motivating these studies was an interest in developing a fundamental understanding of cation exchange mechanisms in the larger InP quantum dots, described in Chapter 2, and the appeal of employing alloyed clusters as single-source precursors to access emergent properties in InP QDs. Chapter 4 outlines the cation exchange reaction that takes place between InP MSCs and cadmium carboxylates, where incorporation of cadmium induces a complete structural rearrangement and full cation exchange to cadmium phosphide clusters. The findings from this study revealed the distinct reactivity differences between larger QDs and clusters, leading to the development of zinc alloyed clusters in Chapter 5 and their role as precursors for InP QDs.