Thermoelectric transport properties of thin metallic films, nanowires and novel Bi-based core/shell nanowires

Thermoelektrische Phänomene können in Nanomaterialien im Vergleich zum Volumenmaterial stark modifiziert werden. Die Bestimmung der elektrischen Leitfähigkeit, des absoluten Seebeck-Koeffizienten (S) und der Wärmeleitfähigkeit ist eine wesentliche Herausforderung für die Messtechnik in Hinblick auf Mikro- und Nanostrukturen aufgrund dessen, dass die Transporteigenschaften vom Volumenmaterial sich durch Oberflächen- und Einschränkungseffekte verändern können. Im Rahmen dieser Abschlussarbeit wird der Einfluss von Größeneffekten auf die thermoelektrischen Eigenschaften von dünnen Platinschichten untersucht und mit dem Volumenmaterial verglichen. Dafür wurde eine Messplattform als standardisierte Methode entwickelt, um S einer dünnen Schicht zu bestimmen. Strukturelle Eigenschaften wie Schichtdicke und Korngröße werden variiert. Grenz- und Oberflächenstreuung reduzieren S der dünnen Schichten im Vergleich zum Volumenmaterial. Außerdem wird eine Methode demonstriert um S von einzelnen metallischen Nanodrähten zu bestimmen. Für hochreine und einkristalline Silber-Nanodrähte wird der Einfluss von Nanostrukturierung auf die Temperaturabhängigkeit von S gezeigt. Ein Modell ermöglicht die eindeutige Zerlegung des temperaturabhängigen S von Platin und Silber in einen Thermodiffusions- und Phononen-Drag-Anteil. Des Weiteren werden die thermoelektrischen Transporteigenschaften von einzelnen auf Bismut-basierenden Kern/Hülle-Nanodrähten untersucht. Der Einfluss des Hüllenmaterials (Tellur oder Titandioxid) und der räumlichen Dimension des Nanodrahts auf die Transporteigenschaften wird diskutiert. Streuung an Oberflächen, Einkerbungen und Grenzflächen zwischen dem Kern und der Hülle reduzieren die elektrische und thermische Leitfähigkeit. Eine Druckverformung induziert durch die Hülle kann zu einer Bandöffnung bei Bismut führen, sodass S gesteigert werden kann. Das Kern/Hülle-System zeigt in eine Richtung, um die thermoelektrischen Eigenschaften von Bismut erfolgreich anzupassen.Thermoelectric phenomena can be strongly modified in nanomaterials compared to the bulk. The determination of the electrical conductivity, the absolute Seebeck coefficient (S) and the thermal conductivity is a major challenge for metrology with respect to micro- and nanostructures because the transport properties of the bulk may change due to surface and confinement effects. Within the scope of this thesis, the influence of size effects on the thermoelectric properties of thin platinum films is investigated and compared to the bulk. For this reason, a measurement platform was developed as a standardized method to determine S of a thin film. Structural properties, like film thickness and grain size, are varied. Boundary and surface scattering reduce S of the thin films compared to the bulk. In addition, a method is demonstrated to determine S of individual metallic nanowires. For highly pure and single crystalline silver nanowires, the influence of nanopatterning on the temperature dependence of S is shown. A model allows the distinct decomposition of the temperature-dependent S of platinum and silver into a thermodiffusion and phonon drag contribution. Furthermore, the thermoelectric transport properties of individual bismuth-based core/shell nanowires are investigated. The influence of the shell material (tellurium or titanium dioxide) and spatial dimension of the nanowire on the transport properties are discussed. Scattering at surfaces, indentations and interfaces between the core and the shell reduces the electrical and the thermal conductivity. A compressive strain induced by the shell can lead to a band opening of bismuth increasing S. The core/shell system points towards a route to successfully tailor the thermoelectric properties of bismuth

Rich Chemistry in Inorganic Halide Perovskite Nanostructures.

Halide perovskites have emerged as a class of promising semiconductor materials owing to their remarkable optoelectronic properties exhibiting in solar cells, light-emitting diodes, semiconductor lasers, etc. Inorganic halide perovskites are attracting increasing attention because of the higher stability toward moisture, light, and heat as compared with their organic-inorganic hybrid counterparts. In particular, inorganic halide perovskite nanomaterials provide controllable morphology, tunable optoelectronic properties, and improved quantum efficiency. Here, the development controlled synthesis of desired inorganic halide perovskite nanostructures by various chemical approaches is described. Utilizing these nanostructures as platforms, anion exchange chemistry for wide compositional and optical tunabilities is described, and the rich structural phase transition phenomenon and mechanism investigated systematically. Furthermore, these nanostructures and extracted knowledge are applied to design photonic, photovoltaic, and thermoelectric devices. Finally, future directions and challenges toward improvement of the optical, electrical, and optoelectronic properties, exploration of the anion and cation exchange kinetics, and alleviation of the stability and toxicity issues in inorganic lead based halide perovskites are discussed to provide an outlook on this promising field

SYNTHESIS AND CHARACTERIZATION OF NANOSTRUCTURED MATERIALS FOR THERMOELECTRIC ENERGY CONVERSION

In 2012, more than 58% of the energy produced in the US was rejected in the form of heat. The rapid development of thermoelectric materials in the past decade has raised new hopes for the possibility of directly converting some of this waste thermal energy back to electricity. However, the large scale deployment of thermoelectric devices is still limited by the mediocre conversion efficiency. Nanostructured materials have been proved to be able to significantly improve conversion efficiency. My research is devoted to developing efficient solution phase reactions to synthesize nanostructured thermoelectric materials in an economical and scalable way. We also aim at exploring the unique applications of solution synthesized nanostructured materials, e.g. developing nanocrystal ink to coat on flexible substrates for applications in wearable thermoelectric devices

Design and Synthesis of Stable, Aligned and Welded Magnesium Silicide Nanowire Assemblies for Fabrication of Efficient and Reliable Thermoelectrics

The ever-increasing energy needs of humanity, coupled with geopolitics of fossil fuels, are imposing demand on the energy supply. A perennial supply of energy is possible by tapping into renewable sources as well as making the current process more efficient. This can be achieved via the use of thermoelectrics, i.e. they can be used to generate energy by converting sunlight into electricity via solar thermoelectrics and increase the efficiency of current processes by converting the wasted heat into electricity. However, the current state-of-the-art thermoelectrics are inefficient and expensive due to the use of rare earth materials. Nanomaterials, especially nanowires, are on the forefront of advancement in thermoelectrics. Recent theoretical and experimental reports indicate the superior energy conversion performances of nanowires compared to their bulk counter parts. However, this superior performance is only observed in single-nanowire devices. Despite more than two decades of research, macro-devices based on large-scale nanowire assemblies, have not been realized. The three main roadblocks for fabricating such large-scale assemblies are lack of 1) methods for mass producing nanowires of any desired chemical composition, 2) techniques for assembling these nanowires into energy conversion devices in an interface engineered manner, and 3) techniques to stabilize the nanowires at higher temperatures against air- water- and acid-assisted degradation. In this context, the objective of this work is to design a strategy to obtain stable and efficient nanowire-based thermoelectric devices. The material systems chosen for this effort are Zinc Phosphide Zn3P2, Magnesium Silicide, Mg2Si, and Manganese Silicide MnSi1.75. Methods for mass production of Zn3P2 and Mg2Si nanowires have been realized previously via a combination of chemical vapor deposition, electroless etching and solid-state diffusion. The assembly and aligning of nanowires was achieved using shear forces via Equal Channel Angular Extrusion (a severe plastic deformation technique). Assembly of the nanowires was also achieved via welding of nanowires utilizing the solid-state diffusion. The nanowires were chemically stabilized by nonconformally decorating them with boron nitride. The stabilization and welding of nanowires helped to achieve decreased thermal conductivity thereby improving their thermoelectric performance compared to the as-obtained nanowire pellets

USING CONVENTIONAL AND \u3cem\u3eIN SITU\u3c/em\u3e TRANSMISSION ELECTRON MICROSCOPY TECHNIQUES TO UNDERSTAND NANOSCALE CRYSTALLOGRAPHY

Transmission electron microscopy (TEM) is a powerful tool for studying solidstate crystalline systems. With the advances in aberration correction, monochromation, and in situ capabilities, these microscopes are now more useful for addressing fundamental materials chemistry problems than ever before. This dissertation will illustrate the ways in which I have been using high-resolution imaging and in situ heating in the TEM during my Ph.D. research to investigate unique solid state chemistry questions. This dissertation will focus on four unique crystal systems: thermoelectric skutterudite crystals, vapor-liquid-solid (VLS) grown nanowires, and hafnium dioxide nanorods. Although these systems are very different from one another, high resolution and/or in situ heating in TEM is an integral part of each study. Through these techniques, we gain insight and knowledge of these systems that may have gone unknown through different analysis techniques. The experiments I will describe in some cases provide surprising and unexpected results that arise from the nanoscale nature of the materials and would be difficult to observe through bulk analytical methods. The work presented here helps to demonstrate the strength and versatility of TEM to address solid state chemistry questions

Size-controllable synthesis and bandgap modulation of single-layered RF-sputtered bismuth nanoparticles

We here report a simple and efficient method to grow single-layer bismuth nanoparticles (BiNPs) with various sizes on glass substrates. Optimal conditions were found to be 200°C and 0.12 W/cm(2) at a growth rate of 6 Å/s, with the deposition time around 40 s. Scanning electron microscope (SEM) images were used to calculate the particle size distribution statistics, and high-resolution X-ray diffraction (XRD) patterns were used to examine the chemical interactions between BiNPs and the substrates. By measuring the transmission spectra within the range of 300 to 1,000 nm, we found that the optical bandgap can be modulated from 0.45 to 2.63 eV by controlling the size of these BiNPs. These interesting discoveries offer an insight to explore the dynamic nature of nanoparticles

Transition metal oxides - Thermoelectric properties

Transition metal oxides (TMOs) are a fascinating class of materials due to their wide ranging electronic, chemical and mechanical properties. Additionally, they are gaining increasing attention for their thermoelectric (TE) properties due to their high temperature stability, tunable electronic and phonon transport properties and well established synthesis techniques. In this article, we review TE TMOs at cryogenic, ambient and high temperatures. An overview of strategies used for morphological, compositing and stoichiometric tuning of their key TE parameters is presented. This article also provides an outlook on the current and future prospects of implementing TMOs for a wide range of TE applications

First-Principles Calculations of Optoelectronic and Transport Properties of Materials for Energy Applications.

Modern semiconductor technology and nanoengineering techniques enable rapid development of new materials for energy applications such as photovoltaics, solid- state lighting, and thermoelectric devices. Yet as materials engineering capabilities become increasingly refined, the space of controllable properties becomes increasingly large and complex. Selecting the most promising materials and parameters to focus on represents a significant challenge. We approach this challenge by applying state-of-the-art predictive first-principles calculation methods to guide research and development of materials for energy applications. This work describes our first-principles investigations of nanostructured group-III-nitrides for solid-state lighting applications and bulk titanium dioxides for thermoelectric applications. We demonstrate several remarkable properties of nanostructured group-III-nitrides. In InN nanowires with diameters on the order of 1 nm, we predict that quantum confinement shifts optical emission into the visible range at 2.3 to 2.5 eV (green to cyan) and results in a large exciton binding energy of 1.4 eV. These findings offer a new approach to addressing the ”green-gap” problem of low efficiency in solid-state lighting devices emitting in this part of the spectrum. In ultra-thin GaN-AlN quantum wells, we show how to adjust the well and barrier thicknesses for tuning the optical gap in the deep ultraviolet range between 3.85 and 5.23 eV. Furthermore, we predict that quantum confinement in ultra-thin GaN wells results in large exciton binding energies between 80 and 210 meV and enhances radiative recombination by reducing the exciton lifetime to as short as approximately 1 ns at room temperature. These findings highlight the capability of quantum-confined group-III-nitrides to improve the efficiency and utility of visible and ultraviolet solid-state light emitters. Additionally, we calculate the n-type thermoelectric transport properties of the naturally occurring rutile, anatase, and brookite polymorphs of TiO2 and predict optimal temperatures and free-carrier concentrations for thermoelectric energy conversion. We also predict a theoretical limit on the figure of merit ZT of 0.93 in the rutile polymorph, demonstrating that TiO2 can potentially achieve thermoelectric energy conversion efficiency comparable to that of commercialized thermoelectrics.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116701/1/bayerl_1.pd