Probing the Interplay Between Chemical Mechanisms and Crystal Growth in Synthesis using Sodium Yttrium Fluoride

Abstract

To fully understand crystal growth, we have to consider chemistry. To fully understand chemistry, we have to consider crystal growth. This dissertation aims to understand the relationship between these two processes that are too often thought of as independent. We also take a look at some of the techniques that can be used to study these processes andwhen and why one might choose one technique over another. The primary system in which we study these mechanisms is in the aqueous synthesis of sodium yttrium fluoride (NaYF). Here I present research in which we studied the nucleation and growth mechanisms during all stages of that synthesis, and in doing found multiple instances of non-classical crystallization that fundamentally alter not just how the crystals themselves form but also the chemical composition and phase of those crystals. Following the NaYF chapters, we take a look at the atom probe tomography (APT) of bulk diamond and the challenges that we face in making those measurements, allowing us to delve into a technique that is not heavily used but which could potentially allow us to study crystal growth and other processes in an entirely different way. This dissertation begins with a deep dive into the theory that underlies the scientific questions that we’re asking. This can be broken up into two parts: the synthesis and the measurement. In the discussion of the synthesis, I start with an overview of crystallization theory, beginning with classical crystallization and the mathematical formulae and concepts developed by Gibbs and others that describe how crystals are “supposed” to grow, and then look toward all of the non-classical ways that crystals have also been observed to grow. I then give a background on NaYF—why it is important in the greater scientific community and to our group’s research specifically, and why we should care about its crystallizationprocess. In the discussion of measurement, I provide a brief overview of the history, physical basis, and current use of the two microscopic techniques that I used most in this dissertation: transmission electron microscopy (TEM) and APT. The goal of this discussion is to convey the importance of these processes and techniques to our research and provides the background necessary to understand why we did the experiments that we did. The following four chapters describe the research that I have been conducting in my time here. Chapters 2-4 discuss the NaYF project, and Chapter 5 discusses the APT of diamond. We start our study of NaYF by using a combination of analytical TEM, powder X-ray diffraction (XRD), in situ liquid cell TEM, APT, and extended x-ray absorption fine structure (EXAFS) measurements to show that hexagonal NaYF nanowires form hydrothermally through a non-classical crystal growth mechanism involving the formation and subsequent oriented attachment of mesocrystals consisting of cubic (α) phase units. EXAFS spectroscopy also suggests that substitutional Yb3+ point defects within NaYF are distributed evenly throughout the crystal lattice without clustering, and also that they may exhibit selective substitution into one of the two possible trivalent yttrium sites in the unit cell for hydrothermally synthesized β-NaYF. We then transition to a study of the initial nucleation of NaYF in water. Two-step crystallization mechanisms based on spinodal decomposition followed by nucleation are commonly observed both in the laboratory and in nature. While this pathway may require chemical reactions, subsequent nucleation and growth are often considered as separate, discrete events from the reaction itself. In this chapter, using both experimental measurements and atomistic computational modeling, we report another step in the aqueous synthesis of sodium yttrium fluoride of solid state chemical diffusion, thus showing a mechanism of at least four discrete steps, including 1) the segregation of aqueous ions into a dense liquid phase, 2) the formation of an amorphous aggregate, 3) solid-state diffusion of sodium and fluoride ions into the amorphous aggregate toward a NaYF4 stoichiometry, and 4) the crystallization of a stable nonstoichiometric cubic sodium yttrium fluoride phase. The penultimate step involves a continuous, gradual change of the solid phase’s chemical stoichiometry from YF3 toward NaYF4. Unlike previously studied nucleation and growth mechanisms, the stoichiometry of the final solid phase evolves throughout the crystallization process rather than being determined at the time of the initial separation from solution. This novel four-step mechanism provides a new perspective into the nucleation and growth of many other crystalline materials given the ubiquity of nonstoichiometric compounds in nature. Following this, we conclude our study of the NaYF system by examining its relationship with YF3. In the previous chapter, we found that in the initial step of the aqueous NaYF synthesis, an amorphous YF3 phase separates from solution but is unable to crystallize until enough NaF diffuses into the matrix that it can form cubic NaYF. This prompts the question: what happens if there isn’t enough NaF left for that? We find that in that situation, the gel still initially forms, but then appears to dissolve away and then renucleate as orthorhombic YF3, rather than nucleate within the solid gel phase as the NaYF does. This creates a “switch” mechanism of sorts in which the relative concentrations of the reactants can be used to select between two different products with a different composition and crystal phase from each other, and the mechanism by which that occurs is directly via the crystallization process, showing that the chemistry and the crystallization are fundamentally intertwined. We then delve deeper into the YF3 nucleation process in which we observe with nuclear magnetic resonance spectroscopy (NMR) and predict with computational modeling that as the gel redissolves into the solution, it forms various solvated YFn species that can interact to form the final solid YF3 product. Finally, we show with dynamic light scattering (DLS) that the growth rate of the YF3 particles is not constant and likely involves some nonclassical processes that merit further study. The final research chapter represents a departure from the NaYF system, and rather we consider the APT of bulk diamond. APT is a technique that allows us to map out atoms in a sample in three dimensions by field evaporating ions off of a sharp tip, assisted by a pulsed laser, and detecting them with a two-dimensional mass spectrometer with the third dimension being detected in time as a function of the laser pulse rate. This technique can be challenging for samples like diamond both in terms of sample preparation and the measurement itself. APT samples are typically prepared with a focused ion beam (FIB), but due to diamond’s inherent hardness, it is difficult and time-consuming to mill bulk diamond, and the high beam dose needed to mill diamond introduces a considerable amount of damage. Once a sample is made, the measurement is also difficult due to the extremely high evaporation field of diamond, necessitating high voltages that introduce mechanical strain in the sample, often causing it to fracture. We address the fabrication problem by producing an array of tips using reactive ion etching to minimize the required interaction of the sample with the ion beam, and we consider strategies to improve data collection on diamond in APT. We find that diamond requires much higher pulse energies than most other materials in order to reduce the required voltage for field evaporation, but we show through computational modeling as well as experimental data that diamond is able to handle those increased pulse energies with negligible storage of heat, which is typically the greatest problem with high pulse energies. We were able to successfully collect APT data on diamond, showing the distribution of nitrogen and hydrogen, and we were also able to use our APT results to show the damage incurred by the FIB. These results show that APT can be a useful method for studying diamond, and that an optimization of the technique can reduce many of the problems classically associated with these measurements. Furthermore, this demonstrates a unique potential application of diamond in APT as a sample mount. APT sample mounts should have low background and high thermal conductivity, and diamond would be an ideal material for this. As such, this potential application warrants further study. This dissertation concludes by summarizing our results and, perhaps more importantly, by formally posing many of the questions that this research has helped us to uncover. A recurring theme among these projects is that these are complex systems that we are studying, and oftentimes every new thing we learn creates even more questions. This concept applies not just to these experiments but to all of science—many of the chemical and physical processes that occur in nature involve many steps and a high degree of interplay, and as such resist being fully characterized by simple models. From this perspective, as we consider NaYF or the APT of diamond, we can apply these lessons broadly to other systems and use them to learn more about how things happen in real life rather than how they happen in textbooks. It is my hope that the results in this dissertation nucleate many years of research into these fields so that we can make discoveries that I never could have imagined in my time here.