Study on electrical spin injection devices and synthesis of manganese fluoride for energy storage devices

Abstract

This dissertation mainly concerns two areas of intense pursuit, electrical spin injection into semiconductors and synthesis of nanomaterials for energy storage. The first part (Chapter1 and 2) deals with the electrical spin injection from ferromagnetic materials into semiconductors. Spintronics is considered as one of the most important emerging research areas whereby the spin degree of freedom in electronic devices is exercised. Especially, the spintronic devices based on semiconductor has attracted considerable critical attention due to the expectations for the future spintronic devices such as spin light-emitting diode, spin transistor, and spin resonant tunneling diode. To realize these advanced devices with high performance and energy efficiency, the control of the injection, transport, and dynamics of spins inside of a semiconductor material is a great goal of semiconductor spintronics. In particular, the electrical injection of spin-polarized electrons from ferromagnetic materials into semiconductors is currently one of the foremost challenges in semiconductor spintronics. Chapter 1 of this thesis begins with a brief overview of spintronics and the underlying physical phenomena of electrical spin injection devices. The non-local spin-valve devices and three-terminal Hanle signal devices which are considered typical devices for electrical spin injection were introduced. Especially, three-terminal Hanle signal devices have been popular due to their simplicity of setup compared to other methods that requiring complex device miniaturization. However, some scientists threw a question about the origin of the three-terminal Hanle signal devices, whether it really comes from spin injection in semiconductor or other unclear reasons. Because the three-terminal Hanle signal is quite larger than expected values based on conventional spin injection theory and has shown unexplainable dependencies. For example, three-terminal Hanle signals depend much on the tunneling resistances through the insulating tunneling barriers. In Chapter 2, in order to take a step towards and think deeply about these questions on the origin of the three-terminal Hanle signal, the comparative study of three-terminal Hanle signals in ferromagnet/insulator/semiconductor and non-ferromagnet/insulator/semiconductors devices were conducted. For the comparative study of Hanle signals coming from two types of devices depending on the existence of the source of spin-polarized current, many devices with various tunneling resistance were fabricated and measured at a wide range of temperatures. The characteristics of three-terminal Hanle signals obtained from our devices were discussed. The second part (Chapter 3–5) contains the synthesis and characterization of manganese fluorides for electrochemical energy storage applications. Controlled synthesis of nanomaterials is important in terms of both academic studies and practical application because nanomaterials exhibit many remarkable chemical and physical properties that are different from both individual metal atoms and bulk counterparts. Especially, morphology control of nanomaterials is one of the key strategies in optimizing the electrochemical performance of energy storage devices. In particular, hierarchical structures have drawn attention because they can increase the specific surface area, improve the contact area between electrode and electrolyte, shorten the diffusion path of electrolyte ion, and enhance structural stability. Recently, manganese fluoride (MnF2) nanomaterials have attracted interest owing to their potentials in electrochemical energy storage applications including as supercapacitors, the electrode for fluoride ion batteries, and anode materials for lithium-ion batteries. However, since harmful and toxic materials such as hydrogen fluoide are conventionally used as a fluorine source in the wet chemical synthesis process, the practical application of metal fluoride materials has been limited. Here, we successfully demonstrated the controllable synthesis of MnF2 different crystalline phases and morphologies using ionic liquids as alternative fluorine sources that are environmentally friendly and operationally safe compared to other erosive sources. In addition, the MnF2 were facile and rapidly synthesized by using the microwave irradiation method. The dominant factors affecting the crystalline phase and morphologies of MnF2 were investigated. Finally, the hierarchical structured MnF2 with the high surface area were optimized and evaluated as the anode of Li-ion batteries. Through the investigation of the electrochemical performance, the greatly enhanced reversible specific capacity is obtained. The second part provides a comprehensive understanding of the morphological control of the MnF2 using the microwave-ionic liquids combination method for energy storage devices. Chapter 3 starts with a brief introduction of microwave assisted synthesis of inorganic nanomaterials. The principles and advantages of microwave-assisted heating for synthesizing the nanoparticles were invetigated. Also, the ionic liquids which are compounds completely composed of ions with a melting point below 100 °C, and their applications were introduced. In Chapter 4, the MnF2 were synthesized using the solvent mixtures of ethylene glycol and ionic liquids under microwave irradiations, where the two ionic liquids with different alkyl lengths were used. The uniformly synthesized MnF2 with different crystalline phases and shapes were obtained. Depending on the volume ratio of solvent mixtures of ethylene glycol and ionic liquids and the type of ionic liquids, the crystalline phase and morphological shape of MnF2 were controlled. Especially, the photoluminescence characteristics of MnF2 show clear differences depending on the crystalline phase of MnF2. This research also shows that the solvent mixtures of ethylene glycol and ionic liquids are the proper way to synthesize the MnF2 by comparing MnF2 synthesized using the mixtures of deionized water and ionic liquids. In Chapter 5, in this research, we focused on the construction and optimization of hierarchical structured MnF2 for applications of anodes for Li-ion batteries. Here, the MnF2 were synthesized only using ionic liquids as the solvent, where the ionic liquid plays roles of solvent, soft-template, and fluorine sources. As considering that the supramolecular structure of ionic liquid could be affected by impurities, the hierarchical structure of MnF2 was controlled by purities of ionic liquid without any adding materials. Also, the kinetics of MnF2 formation which could be divided by nucleation and growth were controlled by changing the microwave heating rate. Finally, the findings from each research were summarized in chapter 6.