Title: The Synthesis and Surface Chemistry of Colloidal Quantum Dots
Description: Colloidal semiconductor nanocrystals, also known as quantum dots, are an extraordinary class of material, combining many of the most attractive properties of semiconductors with the practicality of solution chemistry. As such, they lie at a unique interface between inorganic chemistry, organic chemistry, solid-state physics, and colloidal chemistry. The rapid advance in knowledge of quantum dots over the past 30 years has largely been driven by interest in their fundamental physical properties and their broad applicability to challenges in nanoscience. However, much less attention has been paid to the chemistry underlying these features. In this dissertation, we discuss the state of nanocrystal chemistry and new insights we have unlocked by taking a bottom-up, chemistry-based approach to nanocrystal synthesis. We will cover these in a case-by-case fashion in the context of four chapters. Chapter 1 covers our CdTe nanocrystal synthesis surface chemistry studies with an eye toward CdTe photovoltaic technology, in which the role of CdTe surfaces is poorly understood.

CdTe nanocrystals are traditionally a difficult material to synthesize, particularly with well-defined surface chemistry. In order to enable quantitative surface studies, we looked upstream and re-evaluated CdTe synthesis from the ground up. We identified a CdTe precursor largely overlooked since 1990, cadmium bis(phenyltellurolate) (Cd(TePh)2), and harnessed its excellent reactivity toward a synthesis of CdTe nanocrystals solely bound by cadmium carboxylate (Cd(O2CR)2) ligands. We then use this well-defined material to show that Cd(O2CR)2 ligands bind less tightly to CdTe nanocrystals than CdSe nanocrystals. This finding holds promise for the development of photovoltaics from colloidal CdTe feedstocks. Chapter 2 covers a tunable library of substituted thiourea precursors to metal sulfide nanocrystals. Controlling the size of nanocrystals produced in a given reaction is paramount to their use in opto-electronic devices, but the most widely used technique to control size is prematurely arresting crystal growth.

We introduce a library of thiourea precursors whose organic substituents tune the rate of precursor conversion, which dictates the number of nanocrystals formed and the final nanocrystal size following complete precursor conversion. We use PbS as a model system to 1) demonstrate the concept of kinetically controlled nanocrystal size, 2) quantify substituent trends, and 3) optimize multigram scale syntheses. We then expand the thiourea methodology to a broad range of materials and nanocrystal morphologies. This work represents a paradigm shift that will greatly accelerate the pace of progress in nanocrystal science as it transitions from academia to a multibillion-dollar industry. Chapter 3 covers an analogously tunable library of substituted selenourea precursors, but focuses on the synthesis of PbSe nanocrystals. PbSe nanocrystal synthesis is notoriously low-yielding and poorly tunable, but the remarkable properties of PbSe nanocrystals in photovoltaics and electrical transport have driven interest in the material for decades.

We develop a library of N,N,N’-trisubstituted selenourea precursors and leverage their fine conversion rate tunability to synthesize PbSe nanocrystals of many sizes in quantitative yields. Interestingly, the nanocrystals produced in this reaction are demonstrably less polydisperse than literature samples, exhibiting absorption linewidths approaching the single-particle limit. We quantify this narrowness using a transient absorption spectroscopy technique called spectral hole burning. Chapter 4 covers our efforts to dig deeper into nanocrystal nucleation and growth and use that new knowledge to develop luminescent downconverters ready for on-chip integration into LED lighting. By studying early time points in PbS and PbSe nanocrystal synthesis, we estimate solute concentrations, nucleation thresholds, and nanocrystal growth rates. In particular, we find that metal selenides and sulfides have very different nucleation and growth behavior, as well as that PbS nucleation is a surprisingly slow process.

The lessons learned from these fundamental experiments have enabled us to rapidly develop red-emitting CdS/CdSe/CdS “spherical quantum well” emitters whose photoluminescence quantum yields are 90 – 95%.