Understanding Magnetic and Optical Properties of Lanthanide-Doped Oxide Nanospinels and Heterometallic Formate Metal-Organic Frameworks

This dissertation is composed into two primary portions defined by the material types investigated. The first part is concerned with the magneto- and thermo-optical properties of trivalent lanthanide-doped metal-oxides using europium as a probe of the photophysical mechanisms in play. These works offer important insight to observed properties that have not previously been understood and will allow for more directed studies for the use of such materials in commercial applications. The second part is concerned with the synthesis and magnetic properties of novel hetermetallic perovskite-like metal-organic frameworks, providing a route to tune the multiferroic behavior through variations in the composition of the two metals involved. In the first part of this dissertation the magneto- and thermo-optical properties of known nanospinels (AB2X4), zinc aluminate and zinc gallate, and analogous β-gallium oxide nanoparticles doped with trivalent europium are studied using optical spectroscopy methodologies under applied high magnetic field and extreme temperature conditions. Lanthanide photophysics have been studied in depth for many decades due to their narrow line-like absorption and emission lines, despite being parity forbidden transitions. Lanthanides differ considerably from transition metal emitters in that the characteristics which lift the degeneracy in energy states are very different, resulting in many differences in the photophysical properties of materials containing those atoms. Lanthanides, however, differ from other common emitters in other ways, particularly in their relative quantum efficiencies and quantum yields. Due to the aforementioned parity forbidden nature of trivalent lanthanide excitation the quantum yields are very low due to poor absorption, despite very high quantum efficiencies. Research into materials containing lanthanides has continued to increase recently as methods of circumventing the poor absorption have increased quantum yields significantly and paved the way for commercial applications in solid-state lighting. In the last twelve years there has been additional interest in understanding the effects of temperature and applied magnetic field on lanthanide containing materials. Theoretical predictions have been made to explain observations made at moderately low temperatures (∼77 K) but experimental confirmation through a wide range of host materials and methods is limited. Studies of various metal-oxide hosts ranging from the nano- to bulk size regimes have been undertaken under applied high magnetic field using pulsed magnet systems. These studies have made a range of observations, including significant emission quenching, hysteresis-like behavior in emission recovery, and small shifts and changes in Stark splitting patterns as a function of applied magnetic field. There has been very little proposed to explain the mechanism that leads to emission quenching and hysteresis-like behavior (bistability), but the minute changes in Stark patterns and peak positions has been proposed to be the result of Zeeman splitting effects and changes in the local symmetry of the lanthanide ion. Lanthanides have also made an increased appearance in magnetic applica- tions as single-molecule magnets, most notably europium oxide, due to the unique behaviors of lanthanides under an applied magnetic field. This has widened the field of potential applications to include quantum computing and other spinelectronic applications. In this work a series of analogous metal-oxide nanoparticles are doped with trivalent europium and probed using optical spectroscopy at extreme temperatures as low as 4.2 K and, for the first time, in a persistent field magnet at high applied field. Using established theory the Zeeman splitting effects will be calculated to predict any relative shifts in Stark peak positions. Additionally, accepted theory will be used to verify that changes in Stark splitting patterns are likely due to Zee- man splitting alone, and not changes in local symmetry, as this splitting can lead to the separation of energy levels such that J-level mixing is reduced in at least one level and a secondary emission line exists. A simple mechanism is also proposed that functionally explains the emission quenching and bistabilities observed in recovery that is supported by thermo-optical data and a previously proposed model for temperature behavior in lanthanides based on Mott-Seitz theory. By using analogous hosts as controls it has been determined that the observed properties are due, not just to the field effects on lanthanide ions, but a combination of lanthanide properties and those of the host material. In fact, while the extremely limited tuning potential of lanthanide energy states are extremely important in facilitating the observed behaviors, their nature prohibits them from being useful in tuning them. Thus it has been concluded that the ability to manipulate these properties is primarily dependent on changes to the composition and structure of the host. In the second part of this dissertation a range of metal-metal ratios in a hetermetallic perovskite-like metal-organic framework is synthesized and probed to study the relationships between metal composition and magnetic properties. Since multiferroic behavior was first reported in metal-organic framework materials the field has grown considerably, with the past ten years seeing an explosion of studies to better understand the nature of these materials and the various ways these properties can be manipulated. This field has predominantly focused on homometallic ABX3 perovskite-like metal-organic frameworks and the tuning of these materials through structural manipulation via substitutions of the three ions (A, B and X). More recent studies have focused on tuning through changes in particle size and limited studies on heterometallic structures, typically with 50/50 metal ratios, on a handful of transition metal combinations. The investigation of the effects of particle size have also led to new synthetic routes, including microwave-assisted synthesis, which provide size control, substantially reduce the time of synthesis, and increase energy efficiency while lowering the environmental impact. This work relies on traditional solvothermal synthesis methods to investigate a range of metal-metal ratios in dimethylammonium nickelx manganese1-x formate metal-organic framework materials and their relative magnetic and electrical properties. Several relationships between these properties and the metal composition have been identified in addition to crystal growth size. It is also reported that the combination of metals, in which one is known to be multiferroic and the other is not, can and, in this case, will result in a hybrid material that maintains the desired multiferroic behavior. This dissertation seeks to broadly increase the understanding of various magnetic and electronic properties and relationships to structure in materials with significant promise in important fields and allow for more efficiently directed research in technical applications. Chapter 1 provides a thorough background and introduction to the theories and ideas used throughout this dissertation. In the first half an introduction to conventional lanthanide pho- tophysics provides an understanding of the unique nature of lanthanides relative to organic and transition metal emitters that in part leads to the difficulties of understanding the magnetic and thermal properties. A brief introduction is also provided to provide context for the analogous hosts used in this work which have allowed preliminary examination of the effects of manipulating various sites in the lattice. The second half of chapter 1 describes metal-organic frameworks and introduces the concept of multiferroics and provides an understanding of the significance of such materials. Chapter 2 provides introductions to the various methodologies used throughout this dissertation to probe the properties of various methods. Chapters 3 and 4 describe the synthetic methods and primary experimental results and discus- sion obtained through the study of trivalent europium doped at 5% in zinc aluminate, zinc gallate, and β-gallium oxide nanoparticles under applied high magnetic field and extreme temperature conditions respectively. Chapters 5 and 6 are concerned with the synthetic methods and structural, magnetic, and dielectric properties of dimethylammonium nickelx manganese1-x formate metal-organic framework single crystals and the relationships that relate many of these properties. Chapter 7 provides a summary of the most significant results of the prior chapters and a direction for future work to better understand and further validate the ideas proposed by this dissertation.