Probing Nanomagnetism through a Materials Approach: Paramagnetic Ions within Nanomaterials

Title: Probing Nanomagnetism through a Materials Approach: Paramagnetic Ions within Nanomaterials
Authors: Holmberg, Rebecca Jane
Date: 2016
Abstract: This thesis will describe the magnetic behavior found in a scaling array of magnetic nanomaterials that have been uniquely designed, synthesized and characterised in order to better understand their properties with regards to potential future applications. Within Chapter 1 will be a detailed, yet accessible, introduction to nanomagnetism and the fundamental principles and practical techniques essential to the study of this unique mélange of physics and chemistry. This chapter will be designed to give the reader the necessary tools to understand key literature concepts found in Chapter 2, as well as the work presented in the following chapters. Chapter 2 will provide an overview of relevant literature in the field of magnetic nanomaterials, including: nanoparticles, single-molecule magnets, single-chain magnets and metal-organic frameworks. Chapter 3 will describe work performed on nanoparticles doped with lanthanide ions in order to explore their resulting size, shape, crystallinity and magnetic properties. The relevance of the chosen particles (NaYF4) pertains to their proposed use in a variety of applications due to their known luminescent properties, which we sought to hybridize with interesting magnetic properties, thus creating multimodal imaging capabilities. Doping with a variety of desired ratios of lanthanide ions (GdIII, TbIII, DyIII, ErIII and YbIII) was successful, producing crystalline nanoparticles with tunable size and shape. Magnetic measurements displayed a clear absence of superparamagnetic behavior, indicating that these materials have the potential to be well-suited to applications in biomedicine as multimodal imaging probes and MRI contrast agents. Chapter 4 will build on the previously explored doped nanomaterials through creating a hybrid nanomaterial by tethering lanthanide-based magnetic molecules to the surface of nanoparticles. This is performed through the synthetic design of a SMM with two anisotropic DyIII ions, which was synthesized and designed to bear terminal S-groups in order to promote the binding of the magnetic molecule to capping agent free gold nanoparticles. Upon confirmation of the successful surface attachment of the molecules, magnetic measurements displayed that the magnetic molecules maintained their static properties, however, their dynamic properties were altered. This system was the first example of this type of novel approach to the study of magnetic molecules on surfaces for data storage, spintronics, and quantum computing applications. Chapter 5 will expand on the previous study of ordering arrays of magnetic molecules on the surface of nanoparticles by tethering them into 1D chain networks. We successfully synthesized chain networks with YIII, EuIII, GdIII, TbIII and DyIII lanthanide ions. Magnetic characterisation revealed slow relaxation of the magnetization with no significant interactions between magnetic ions, thus these are discrete magnetic molecules in 1D. Rather surprisingly, the isotropic GdIII analogue displayed field induced slow relaxation of the magnetisation, necessitating the use of ab initio calculations in order to shed light on the potential causes of this unexpected behavior. Overall, through the formation and study of these structures, we observed a new potential method of SMM assembly for the study of ordered arrays of molecular magnets. Chapter 6 will focus on ordering of discrete magnetic systems in 3D. With this in mind, we successfully isolated the first Co8 cuboctahedron MOF. Magnetic measurements displayed that each SBU was well-isolated, with significant antiferromagnetic coupling between CoII ions, leading to an S = 0 ground state. These interactions were then modelled using density functional theory. This type of study promotes the future development of novel high-nuclearity MOF structures with interesting and tuneable magnetic properties, as well as the potential for assembly of discrete molecular magnetic units in 3D using MOFs. Chapter 7 utilizes the principles of Chapter 3, wherein magnetic ions are doped into a diamagnetic material; in this case, MOF-5. We sought to isolate one CoII ion in each SBU, and build upon this by adding additional magnetic ions and probing their interactions. Through magnetic measurements we observed a scaling magnetic moment with CoII content, and with higher dopant percentages we began to observe magnetic interactions occurring within the SBUs. Interestingly, we also observed a change in coordination environment with higher dopant percentages, likely as a result of the previously suggested capability of one ZnII ion within the MOF-5 SBU to become hexacoordinate, allowing CoII doping up to a maximum of 25%. Consequently, this study points to the cause of the structural instability that plagues MOF-5 in the presence of air and moisture. We probed this system further in Chapter 8 using FeIII as a dopant ion, and were able to obtain the first crystallographic evidence of the coordination change of ZnII in MOF-5. Furthermore, the structure obtained with FeIII was the first example of metal ion addition within a MOF that bound two interpenetrated frameworks together. This new MOF was found to have the potential to be a more practical material for gas storage and separation, and/or for catalysis. Thus, this study was informative in regards to the inherent instability of the parent framework, as well as a new method of metal addition to a known MOF structure. Chapter 9 will conclude the work with a discussion of what was performed in, and learned from, each thesis section, as well as provide an outlook and perspective on the novel work that may be derived from these projects going forward.
CollectionThèses, 2011 - // Theses, 2011 -