Добірка наукової літератури з теми "Defect induced magnetism, Titanium dioxide"

AbstractIn this study, in situ electrical biasing was combined with transmission electron microscopy (TEM) in order to study the formation and evolution of Wadsley defects and Magnéli phases during electrical biasing and resistive switching in titanium dioxide (TiO2). Resistive switching devices were fabricated from single-crystal rutile TiO2 substrates through focused ion beam milling and lift-out techniques. Defect evolution and phase transformations in rutile TiO2 were monitored by diffraction contrast imaging inside the TEM during electrical biasing. Reversible bipolar resistive switching behavior was observed in these single-crystal TiO2 devices. Biased induced reduction reactions created increased oxygen vacancy concentrations to such an extent that shear faults (Wadsley defects) and oxygen-deficient phases (Magnéli phases) formed over large volumes within the TiO2 TEM specimen. Nevertheless, the observed reversible formation/dissociation of Wadsley defects does not appear to correlate to resistive switching phenomena at these length scales. These defect zones were found to reversibly reconfigure in a manner consistent with charged oxygen vacancy migration responding to the applied bias polarity.

Abstract Various surface bioactivation technology has been confirmed to improve the osteogenic ability of porous titanium (pTi) implants effectively. In this study, a three-layered composite coating, i.e. outer layer of hydroxyapatite (HA), middle layer of loose titanium dioxide (L-TiO2) and inner layer of dense TiO2 (D-TiO2), was fabricated on pTi by a combined processing procedure of pickling, alkali heat (AH), anodic oxidation (AO), electrochemical deposition (ED) and hydrothermal treatment (HT). After soaking in simulated body fluid for 48 h, the surface of the AHAOEDHT-treated pTi was completely covered by a homogeneous apatite layer. Using MC3T3-E1 pro-osteoblasts as cell model, the cell culture revealed that both the pTi without surface treatment and the AHAOEDHT sample could support the attachment, growth and proliferation of the cells. Compared to the pTi sample, the AHAOEDHT one induced higher expressions of osteogenesis-related genes in the cells, including alkaline phosphatase, Type I collagen, osteopontin, osteoclast inhibitor, osteocalcin and zinc finger structure transcription factor. As thus, besides the good corrosion resistance, the HA/L-TiO2/D-TiO2-coated pTi had good osteogenic activity, showing good potential in practical application for bone defect repair.

This article briefly reviews some of our recent work carried out both from an experimental point of view as well as from a theoretical perspective to gain further understanding of the events that take place in Heterogeneous Photocatalysis. Previously, the multitude of reports from our laboratory and from many others looked at the primary photocatalytic events as involving (a) absorption of light, (b) formation of the free (electrons and holes) and/or trapped charge carriers (Ti3+and •OH radicals), and (c) reaction of pre-adsorbed acceptor or donor molecules with the relevant trapped carrier. Our recent work notes that this view is reasonable if the only purpose of photocatalysis is elimination of undesirable environmental pollutants. But when we begin to query how to render a process more efficient, we need to address the primary events following photoexcitation of the photocatalyst, which in most instances has been titanium dioxide (in the anatase form). Owing to the nature of light absorption byTiO2we resorted to examining other metal oxides, most of which are dielectric insulators with very large bandgap energies, for example zirconia (ZrO2) and scandia (Sc2O3). These dielectrics have provided added information on the photophysical events, many of which are masked by the strong light absorption in titania. Despite some of our recent progress, much remains to be done for a fuller understanding of the events that occur at the surface, which we have often considered to be the greatest and most complex defect in metal oxide particulates.

Titanium dioxide (TiO2) films were prepared on glass substrates without external heating by DC magnetron reactive sputtering. Argon and oxygen were used as sputtering and reactive gases, respectively. Initially TiO2 films were produced under a constant discharge power of 300W, total operated pressure of 1.10 Pa, the Ar:O2 flow rate of 50:45 sccm and film thickness of 300 nm. After deposition, the films were annealed in air and in vacuum (8×10-1 Pa) at a temperature of 350 °C and an annealed time of 2 hours. Films structure, optical properties, photocatalytic activity (methylene blue degradation) and photo-induced hydrophilicity properties were mainly investigated to produce self-cleaning surface. Because of higher crystallinity of vacuum-annealed films lead to the highest MB degradation under UV irradiation. While, air-annealed films showed lower MB degradation than as-deposited films that could be affected of contamination on surface and defect from annealing. Vacuum-annealed films showed changes of water contact angle on the films surface higher than as-deposited films and air annealed- films.

Herein we report a simple fabrication of graphene and titanium dioxide nanotube composite using hydrothermal method. Photoluminescence emission of the composite were investigated to study defect states in bandgap of nanotubes when the content of graphene varied from 1 wt% to 8 wt%. With the content of graphene lower than 5 wt%, the photoluminescence spectra form of the composite showed similar to that of pristine titanium dioxide nanotube. When the content of graphene reached 8 wt%, the emission positions were unchanged. However the spectrum form was dramatically changed, the intensity of green emission at about 530 nm was dominated. Thanks to the formation of graphene and titanium dioxide nanotube, excited electron was easy to diffuse from nanotube to graphene. The recombination of excited electron and trap hole on nanotube surface induced by oxygen vacancy in titanium dioxide anatas phase was enhanced which was attributed to the 530 nm emission.

AbstractIn this work we investigated the structural, electrical, and optical properties of titanium dioxide (TiO2) nanotubes (NTs) formed by electrochemical anodization of Ti metal sheets in NH4F+glycerol electrolyte at different anodization voltages (Va) and acid concentrations. Our results revealed that TiO2 NTs can be grown in a wide range of anodization voltages from 10 V to 240 V. The maximum NH4F acid concentration, at which NTs can be formed, decreases with the anodization voltage, which is 0.7% for Va<60V, and decreases to 0.1% at Va =240 V. Glancing angle X-ray diffraction (GAXRD) experiments show that as-grown amorphous TiO2 transforms to anatase phase after annealing at 400 oC, and further transforms to rutile phase at annealing temperatures above 500 oC. Samples grown in 30-120 voltage range have higher crystal quality as seen from anatase (101) peak intensity and reduced linewidth. The electrical resistivity of the NTs varies with Va concentration and increases by eight orders of magnitude when Va increases from 10 V to 240 V. This is consistent with cathodoluminescense studies which showed improved optical properties for samples grown in this voltage range. Optical properties of samples were also studied by low temperature photoluminescence. Temperature dependent I-V and photo-induced current transient spectroscopy were employed to analyze electrical properties and defect structure on NT samples.

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 99