Multiscale Study of BaTiO3 Nanostructures and Nanocomposites

Advancements in integrated nanoelectronics will continue to require the use of unique materials or systems of materials with diverse functionalities in increasingly confined spaces. Hence, research on finite-dimensional systems strive to unearth and expand the knowledge of fundamental physical properties in certain key materials which exhibit numerous concurrent and exploitable functions. Correspondingly, ferroelectric nanostructures, which particularly display a plethora of complex phenomena, prevalent in countless fields of research, are noteworthy candidates. Presently, however, the assimilation of zero-(0D) and one-dimensional (1D) ferroelectric into micro- or nano-electronics has been lagging, in part due to a lack of applied and fundamental studies but also due to the paucity of synthetic strategies yielding high quality monocrystalline structures. In this work, the problematics of size reduction, which affects many aspects of electronic devices, was addressed. Furthermore, the depolarizing effects associated with finite thickness in ferroelectric nanostructures was investigated in connection with other crucial boundary conditions. The work reported in this dissertation concerned isolated 0D and 1D BaTiO3 nanocrystals and nanocomposites composed of periodic arrays of BaTiO3 nanowires embedded in a matrix formed by another ferroelectric material. A systematic investigation was conducted for those three types of nanostructures from a quantum mechanical and atomistic perspective using both direct-first-principles and first-principles-derived methods. Using first-principles-based calculations, the structural phase sequences in 0D (cubic-to-tetragonal-to-monoclinic-to-rhombohedral) and 1D (cubic-to-tetragonal-to-orthorhombic-to-monoclinic) BaTiO3 nanoparticles revealed differences from that of the bulk and thin film systems. The monoclinic symmetry found in the 0D compounds, and as for the ground-state of 1D systems, were also affected by size effects and tuned by varying parameters related to the depolarizing effect. Strong electromechanical responses characteristic to the monoclinic symmetry, were also found. In addition, by partially screening the uncompensated charges at the surface of the nanodots, a small range existed (∼87% to ∼95% screening) where both the polarization and toroidal moment coexisted within the nanoparticles. Ferroelectric $nanocomposites$ are novel systems that were also examined and were found to exhibit completely original properties not yet observed in either constituents alone. The temperature-dependent properties such as the structural phases and behavior of the polarization within these nanocomposites were obtained. Interesting new features related to flux-closure configurations were discovered. Transitions associated with the cores of electric dipole vortices were correlated to the direction of in-plane polarization. In addition, vortex-antivortex pairs in a peculiar phase-locked configuration were ascertained in these structures. Complementary density-functional theory calculations were also performed for BaTiO3 nanowires with dissociated-water adsorbates as a function of the out-of-plane lattice constant. Topological defects with winding numbers ranging from 1 to -3 were found in the water-covered nanowires. The ground-state was found to be of triclinic symmetry. Ab-initio calculations were also performed for nanocomposites to investigate the electronic properties of the phase-locked configuration. Similarly to the Monte-Carlo simulations, a configuration containing both vortices (not localized in the nanowires though) and antivortices was found to be the ground state. Mastery of nanomaterials requires merging theoretical research with experimental observation, hence a synthesis project was developed to obtain BaTiO3 nano-tubes and wires using direct pore filling of nanoporous templates. The preliminary results suggested the synthesis of polycrystalline nanostructures depend on the template pore surface polarity and size. The results presented in this dissertation suggested that ferroelectric nanostructures continue to be of great fundamental value and may substantially impact advancement in certain technologies. Furthermore, the work on nanocomposites offered a glimpse to the novel functionalities in ferroelectrics

Defect Induced Magnetism in Titanium Dioxide

Epitaxial anatase TiO2 thin films were grown by pulsed laser deposition and ion beam sputter deposition, on STO and LAO substrates. Their phases and the crystallographic orientations were confirmed using X-ray diffraction measurements; the impurities concentration of the samples were examined using particle induced X-ray emission. The impurity concentration is too low to be the origin of the measured ferromagnetic signal after irradiation with low energetic ions. The as-grown samples show a small ferromagnetic signal without magnetic anisotropy and with Curie temperatures of TC ≈ 450 K. The origin of this magnetic signal may be related to a lattice mismatch between substrate and film and the resulting induced defects, defects generated during the growth process or annealing, or impurities. Irradiation with low energy Ar+ ions was shown to be a simple way to induce magnetism in anatase thin films. After the first irradiation, the magnetic moment at saturation increases by one order of magnitude with a high Curie temperature of TC ≈ 792 K. Further, a considerable out-of-plane magnetic anisotropy in the magnetization has been found. When increasing the irradiation fluence, the magnetic moment increases further until saturation is reached, whereas TC was reduced and the anisotropy vanished. XAS and XMCD experiments of the O K and Ti L3,2 absorption edges showed that the magnetic moment arises at the Ti 3d shell and not at the oxygen. The obtained magnetic moment per Ti di-Frenkel pair (FP) of m ≈ 2 µB agrees with literature reports. XAS and XMCD calculations of Ti di-FPs within an anatase lattice are in agreement with the results and the assumption that di-Frenkel pairs are responsible for the observed magnetism and anisotropy. Magnetic force microscopy proved the existence of oppositely aligned magnetic domains with out-of-plane magnetization directions. This explains the low remanence of these samples. The production method is efficient and non-destructive, and can be easily combined with other techniques, such as electron beam lithography. This allows the production of arbitrary magnetic patterns with perpendicular magnetic anisotropy at the anatase surface. There are some questions that could not be answered in depth, e.g. the connection between irradiation fluence and ion energy, and the saturation magnetic moment as well as the strength of the perpendicular magnetic anisotropy. More systematic experiments are necessary, preferably using a more sophisticated setup. The electric transport properties of single TiO2 nanotubes were measured. The temperature dependence of the resistance of the polycrystalline anatase nanotubes show a Mott variable range hopping behaviour. The results obtained with two contacts indicate the existence of a potential barrier between the Cr/Au contacts and samples surfaces. Impedance spectroscopy at room temperature indicates that the electronic transport of these polycrystalline tubes is dominated by the grain cores. Similar experiments were conducted on ZnO nanowires. The measurements were done on the as-prepared and after low-energy ion irradiation. The temperature dependence of the resistance of the wire before irradiation, can be described by two processes in parallel; the fluctuation induced tunneling conductance and an usual thermally activated process. Electron backscatter diffraction confirms the existence of different crystalline regions. After irradiation an additional thermally activated process appears that can be explained by taking into account the impurity band splitting. The previously mentioned experimental findings and methods where then applied to several different TiO2 nanotubes. Amorphous nanotubes were anodically grown on titanium foil and partially annealed to obtain anatase samples. Non-linear current–voltage characteristics were explained using the fluctuation induced tunneling conduction model. A clear enhancement of the conductance was induced in an insulating anatase nanotube through low-energy Ar/H ion irradiation. Confocal Raman spectroscopy shows that the annealed samples were in anatase phase and a blueshift due to phonon confinement was observed. Magnetic force microscopy is well known and established method to investigate magnetic samples of nanometer size. Focused electron beam induced deposition of cobalt was used to functionalize atomic force microscopy Akiyama tips for application in magnetic force microscopy. The grown tips have a content of ≈ 90 % Co after exposure to ambient air. In order to investigate the magnetic properties of the tips, current loops were prepared. Magnetic Akiyama tips open new possibilities for wide-range temperature magnetic force microscopy measurements. To continue the work on magnetic nanotubes, further experiments with single nanotubes would be interesting. These samples could be characterized with the help of MFM measurements or NV magnetometry. Also, experiments on nanotube bundles can be of interest, since the fabrication, irradiation and measurements of such more robust samples is easier to implement.:Declaration of Authorship iii List of Publications v Abstract vii Acknowledgements ix 1 Introduction 1 1.1 Defect-Induced Magnetism in Oxides . . . . . . . . . . . . . . . . . . . 1 1.1.1 Open Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Exchange Interactions and Magnetic Anisotropy . . . . . . . . . . . . . 3 1.3 Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.4 Magnetic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.5 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 Introduction to Magnetism 7 2.1 Orbital Magnetic Moment . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Spin Magnetic Moment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3 Localized Electron Model . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.4 Spin-Orbit Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.5 Multiplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.6 Classes of Magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . . 11 2.6.1 Diamagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.6.2 Paramagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.6.3 Antiferromagnetism . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.6.4 Ferromagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.6.5 Ferrimagnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.7 Exchange Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.7.1 Coulomb Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.7.2 Direct Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.7.3 Superexchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.7.4 Ferromagnetic Superexchange . . . . . . . . . . . . . . . . . . . 22 2.7.5 Double Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.7.6 Orbital Ordering . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.8 Magnetism in Transition Metal Oxides . . . . . . . . . . . . . . . . . . . 27 2.8.1 Oxygen Coordination . . . . . . . . . . . . . . . . . . . . . . . . 28 2.8.2 Crystal Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.8.3 Weak-Field Solution for Single Electron . . . . . . . . . . . . . . 29 2.8.4 Interionic Exchange Interaction . . . . . . . . . . . . . . . . . . . 34 2.9 Magnetic Anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.9.1 Cubic Symmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 2.9.2 Tetragonal Symmetry . . . . . . . . . . . . . . . . . . . . . . . . 39 xii 2.10 Defect-Induced Magnetism in TiO2 . . . . . . . . . . . . . . . . . . . . . 40 3 Defect Induced Magnetism in TiO2 Anatase Thin Films 43 3.1 Strong out-of-plane magnetic anisotropy in ion irradiated anatase TiO2 thin films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.2 Titanium 3d ferromagnetism with perpendicular anisotropy in defec- tive anatase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 4 Defect Induced Magnetism and Electrical Properties of TiO2 and ZnO Nan- otubes 69 4.1 Electrical properties of ZnO single nanowires . . . . . . . . . . . . . . . 71 4.2 Electrical transport properties of polycrystalline and amorphous TiO2 single nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4.3 Functionalized Akiyama tips for magnetic force microscopy measure- ments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5 Summary and Outlook 93 Bibliography 9

Program and the Book of Abstracts / Fourteenth Young Researchers' Conference Materials Sciences and Engineering, December 9-11, 2015, Belgrade, Serbia

Young Researchers' meetings are held annually late in December since 2002 and they are organized by the Materials Research Society of Serbia. Originally conceived as seminars, since 2007 these meetings were transformed into conferences. The previous eleven meetings featured presentations based on the research of various young scientists from Serbia, Bosnia and Herzegovina, Montenegro, Slovenia, Brazil, Germany, United States of America, China, Poland, Belgium, Spain, Romania, United Kingdom, Austria, Italy, Hungary, Russia, Canada, etc. At the Conference, young researchers, students of doctoral, master and undergraduate studies, are given the opportunity to make an overview of their research into materials science and engineering through oral and poster presentations. As for the scientific content of the conference, we have given full priority to research topics that are currently considered as being on the frontier of the field. Nanotechnology and Advanced Materials, Synthesis and Engineering of Biomaterials, Application of Biomaterials, Theoretical Modeling of Materials and Advanced Methods for Synthesis and Processing present only some of those exciting topics that will be given the central stage and most attention during this meeting. The 14th Young Researchers' Conference Materials Science and Engineering was held in Belgrade, Serbia on December 9-11, 2015, Belgrade, Serbia. It was organized by the Materials Research Society of Serbia and Institute of Technical Sciences of the Serbian Academy of Sciences and Arts

21st Century Nanostructured Materials

Nanostructured materials (NMs) are attracting interest as low-dimensional materials in the high-tech era of the 21st century. Recently, nanomaterials have experienced breakthroughs in synthesis and industrial and biomedical applications. This book presents recent achievements related to NMs such as graphene, carbon nanotubes, plasmonic materials, metal nanowires, metal oxides, nanoparticles, metamaterials, nanofibers, and nanocomposites, along with their physical and chemical aspects. Additionally, the book discusses the potential uses of these nanomaterials in photodetectors, transistors, quantum technology, chemical sensors, energy storage, silk fibroin, composites, drug delivery, tissue engineering, and sustainable agriculture and environmental applications

Synthesis, Transport, and Thermoelectric Studies of Topological Dirac Semimetal Cd3AS2 for Room Temperature Waste Heat Recovery and Energy Conversion

ABSTRACT SYNTHESIS, TRANSPORT, AND THERMOELECTRIC STUDIES OF TOPOLOGICAL DIRAC SEMIMETAL CD3AS2 FOR ROOM TEMPERATURE WASTE HEAT RECOVERY AND ENERGY CONVERSION by The University of Wisconsin-Milwaukee, 2017 Under the Supervision of Professor Nikolai Kouklin Rising rates of the energy consumption and growing concerns over the climate change worldwide have made energy efficiency an urgent problem to address. Nowadays, almost two-thirds of the energy produced by burning fossil fuels to generate electrical power is lost in the form of the heat. On this front, increasing electrical power generation through a waste heat recovery remains one of the highly promising venues of the energy research. Thermo-electric generators (TEGs) directly convert thermal energy into electrical and are the prime candidates for application in low-grade thermal energy/ waste heat recovery. The key commercial TE materials, e.g. PbTe and Bi2Te3, have room temperature ZT of less than 1, whereas ZT exceeding 3 is required for a TEG to be economically viable. With the thermoelectric efficiency typically within a few percent range and a low efficiency-to-cost ratio of TEGs, there has been a resurgence in the search for new class of thermo-electric materials for developing high efficiency thermo-to-electric energy conversion systems, with phonon-glass electron-crystal materials holding the most promise. Herein, we focus on synthesis, characterization and investigation of electrical, thermo-electrical and thermal characteristics of crystalline Cd3As2, a high performance 3D topological Dirac semimetal with Dirac fermions dispersing linearly in k3-space and possessing one of the largest electron mobilities known for crystalline materials, i.e. ~104-105cm2V-1s-1. Suppression of carrier backscattering, ultra-high charge carrier mobility, and inherently low thermal conductivity make this semimetal a key candidate for demonstrating high, device-favorable S and in turn ZT. In this work, a low-temperature vapor-based crystallization pathway was developed and optimized to produce free standing 2D cm-size crystals in Cd3As2. Compared to the bulk crystals produced in previous studies, e.g. Piper-Polich, Bridgman, or flux method, Cd3As2 samples were synthesized over a considerably shorter time ( only a few hours), were single crystals and highly stochiometric. A high thermopower of up to 613 μV K−1 and the electrical conductivity of ~ 105 S/m were registered within the temperature range of 300–400 K. A 1ω-method based on the transfer function was applied to probe a thermal conductivity, k of Cd3As2 platelets. The results yield k of ~2.4 W/m.K in the confirmation that the thermal conductivity of Cd3As2 crystals is to approach the amorphous limit at the room temperature. With its peak thermopower attained at the low temperature range of ~300-400 K, high electrical conductivity and amorphous limit thermal conductivity, crystalline Cd3As2 grown via a low-T vapor based method demonstrates ZT \u3e3; the results confirm that as-produced Cd3As2 platelets hold a high promise and is another phonon-glass electron-crystal TE material for the development of next generation, high efficiency thermo-electric generators and refrigerators operating under normal conditions

Synthesis of nanostructured metal chalcogenides used for energy conversion and storage

Despite that now most of our energy is still originated from burning of fossil fuels which are non-renewable in short period and may cause serious pollution to the environment; it has become a common sense that human being is facing with more and more serious energy crisis as well as environmental problems including global warming and pollutions. To save our future, now the developing and utilization of green renewable energy, such as solar energy, waste heat recovery, fuel cells, and so on, has been one of the hottest topics all around the world\u27s scientists

Structural Properties of Ferroelectric Lead (zirconium0.5,Titanium0.5)Oxygen3 Nanotube Array and Electronic Structure of Lao delta-doped strontium titanate

In this Dissertation we begin with two introductions on: 1) ferroelectricity and related phenomena, and 2) novel properties of Oxide electronics and the generation of two dimensional electron gas. We then give theoretical background of density functional theory (including LDA+U) and pseudopotentials. The first part of research work is about structural, polarization, and dielectric properties of ferroelectric Lead Zirconate Titanate (PZT) solid solution in the form of a nanotube array, embedded in a matrix medium of different ferroelectric strengths. We use the effective Hamiltonian derived from first-principles and finite-temperature Monte Carlo methods to determine the various properties. We revealed different polarization phases of the system in the absence of an external electric field and explained these properties in microscopic detail. In the second part, we study the effects of compressive biaxial inplane strains on the electronic and structural properties of Lanthanum Oxide $\delta$-doped Strontium Titanate supercell. We use first-principles density functional calculations within the local density approximation including also on-site Coulomb interaction energy. We approached the problem by comparing the band structures, localization of electronic states, and cation-anion displacements of unstrained and strained systems. We found a critical strain above which there are abrupt changes in conduction band dispersions and cation-anion displacements, indicating that inplane biaxial strain can drastically tune the properties of this system, which may have potential technological applications