1,721,217 research outputs found
Normal-State Transport Properties of Infinite-Layer Sr(1−x)La(x)CuO(2) Electron-Doped Cuprates in Optimal- and Over-Doped Regimes
No full textTransport properties of electron-doped cuprate Sr [Formula: see text] La [Formula: see text] CuO [Formula: see text] thin films have been investigated as a function of doping. In particular, optimal- and over-doped samples were obtained by tuning the Sr:La stoichiometric ratio. Optimal-doped samples show a non-Fermi liquid behavior characterized by linear dependence of the resistivity from room temperature down to intermediate temperature (about 150–170 K). However, by approaching temperatures in the superconducting transition, a Fermi-liquid behavior-characterized by a [Formula: see text]-scaling law-was observed. Once established, the transition from a linear-T to a quadratic- [Formula: see text] behavior was successfully traced back in over-doped samples, even occurring at lower temperatures. In addition, the over-doped samples show a crossover to a linear-T to a logarithmic dependence at high temperatures compatible with anti-ferromagnetic spin fluctuations dominating the normal state properties of electron-doped cuprates
Doping evolution and polar surface reconstruction of the infinite-layer cuprate Sr1-xLaxCuO2
Get PDFWe use angle-resolved photoemission spectroscopy to study the doping evolution of infinite-layer Sr1-xLaxCuO2 thin films grown by molecular-beam epitaxy. At low doping, the material exhibits a dispersive lower Hubbard band typical of the superconducting cuprate parent compounds. As carriers are added to the system, a continuous evolution from charge-transfer insulator to superconductor is observed, with the initial lower Hubbard band pinned well below the Fermi level and the development of a coherent low-energy band with electron doping. This two-component spectral function emphasizes the important role that strong local correlations play even at relatively high doping levels. Electron diffraction probes reveal a p(2×2) surface reconstruction of the material at low doping levels. Using a number of simple assumptions, we develop a model of this reconstruction based on the polar nature of the infinite-layer structure. Finally, we provide evidence for a thickness-controlled transition in ultrathin films of SrCuO2 grown on nonpolar SrTiO3, highlighting the diverse structural changes that can occur in polar complex oxide thin films
Simulation, Design, and Fabrication of Deep UV Aluminum Gallium Nitride Distributed Bragg Reflectors
Get PDFThe AlGaN materials system has been widely explored for its applications in wide-bandgap optoelectronics active in the UV-C spectrum between 300 - 200 nm. However, serious challenges to its use exist, including a lack of high reflectivity mirror components. Distributed Bragg Reflectors (DBRs) offer some of the most efficient mirrors available in this range. These one dimensional photonic crystals can be grown epitaxially on existing deep UV optical devices within the same growth chamber, making them easy to integrate with existing designs to potentially improve light extraction efficiency. Serious challenges still exist with the use of AlGaN DBRs, including the low contrast of the material system and difficulty of consistent crystal growth. The design of AlGaN DBRs is approached from initial principles based on the optical simulation of these DBRs using the 1D transfer matrix method combined with existing models of the optical parameters of AlGaN films. Exotic approaches such as porous films are also included in the models to expand the range of design parameters. Design rules of thumb based on analysis of these simulations are used to simplify design choices into a systematic approach. Finally, the growth of AlGaN DBRs using molecular beam epitaxy (MBE) and optical and structural characterization methods are introduced
UTILIZING COMPLEX OXIDE SUBSTRATES TO CONTROL CARRIER CONCENTRATION IN LARGE-AREA MONOLAYER MOS2 FILMS
No full text56 pagesBand-gap engineering is central to the design of heterojunction devices. It is a powerful technique to overcome the constrain of natural material properties and realize novel functionalities of materials. Monolayer transitional metal dichalcogenides (TMDs) provides unprecedented opportunities for band-gap engineering with their atomically-thin thickness, direct bandgap in the near-infrared to the visible region, large excitonic effect, and strong spin-orbit coupling.For heterojunctions involving monolayer TMDs, the electronic properties of the atomically thin films are widely tunable by external perturbations. For instance, the carrier concentration of monolayer MoS2 can vary significantly depending on the amount of charge transfer between the MoS2 and the substrate. This makes substrates with a range of charge neutrality levels—as is the case for complex oxide substrates—a powerful addition to electrostatic gating or chemical doping to control the doping of overlying MoS2 layers. In this thesis, I demonstrate this approach (charge transfer doping) by growing monolayer MoS2 on perovskite (SrTiO3 and LaAlO3), spinel (MgAl2O4), and SiO2 substrates with multi-inch uniformity. The as-grown MoS2 films on these substrates exhibit a controlled, reproducible, and uniform carrier concentration ranging from (1-4) ×1013 cm-2, depending on the oxide substrate employed. The observed carrier concentrations are further confirmed by our density-functional theory calculations based on ab initio mismatched interface theory (MINT). This approach is relevant to large-scale heterostructures involving monolayer-thick materials in which it is desired to precisely control carrier concentrations for applications
The Impact of Sample Tilt on Scanning Transmission Electron Microscopy in Strontium Titanate
Get PDFAnnular Bright Field Scanning Transmission Electron Microscopy (ABF-STEM) allows microscopers to image the location of atoms in films as thin as a single atomic layer. In high signal to noise images sub-picometer localization precision is achievable. Recent work has used ABF-STEM to measure oxygen displacements in complex oxides heterostructures, with intent of showing ferromagnetic and multiferroic properties. However, previous work on the accuracy of ABFSTGEM imaging has shown that when a sample is tilted by 6 mrad relative to the electron beam, it creates artificial displacements of 11.9 pm between oxygen and cation columns. Artifacts of this magnitude make picometer-scale measurements of oxygen displacements impossible. However, use of Convergent Beam Electron Diffraction (CBED) can aid sample alignment in the STEM and mistilts can typically be reduced to approximately 1 mrad or better. Thus, there remains an open question as to what kinds of tilt-induced artifacts exist at sample tilts expected in experimental ABF-STEM. In order to quantify the effects of a sample tilt of 1 mrad, I performed multislice image simulations on cubic SrTiO3 over a range of thicknesses from one atomic layer to just over 30 nm. I found that even with only 1 mrad of tilt, artificial displacements so large as 11:8pm between titanium/oxygen and oxygen columns and 4.2 pm between strontium and titanium/oxygen columns are present in ABF-STEM images. I further found that these displacements are not present in HAADF-STEM images, as the displacement between strontium and titanium/oxygen columns was below 0.2 pm at 1 mrad of sample tilt and less than 1.5 pm at sample tilts up to 10 mrad. Because tilts of this magnitude are difficult to control for experimentally, the apparent location of atomic positions in ABF-STEM images may not accurately reflect true atomic structure and measurements of picometer-scale oxygen distortions in complex oxides may not be possible unless sample tilt is carefully controlled
Thickness And Stacking Sequence Determination Of 1T-Tantalum Disulfide Using Scanning Transmission Electron Microscopy
No full textThe layered transition metal dichalcogenide (TMD), 1T-tantalum disulfide (1T-TaS2) exhibits a range of electronic properties when cooled down, including phase transitions associated with charge-density-waves (CDWs), metal-insulator transitions as well as superconductivity upon doping or high pressure. The CDW phase transitions can be modulated by the thickness of 1T-TaS2. To understand the material's potential for electronic applications it is therefore important to identify the dimension and atomic layer stacking of 1T-TaS2 samples with both accuracy and precision. In this work, our goal is to develop a reliable method to extract the thickness and stacking sequence of 1T-TaS2 directly from experimentally recorded images. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) is a powerful method to image materials with atomic resolution. However, considering the two-dimensional nature of projected HAADF-STEM images, we choose the technique of convergent-beam electron diffraction (CBED) performed with atomically small electron beams from STEM. We show that combining experimental and simulated CBED patterns with rich information in the third-direction (i.e. thickness and stacking sequence) is an effective approach to study the structures of layered dichalcogenides. I will discuss the development of a method to extract the thickness and stacking sequence of exfoliated 1T-TaS2 layered structures directly from experimental CBED patterns and STEM images by comparison with simulation results. Starting with multislice simulations of both HAADF-STEM images and CBED patterns for 1T-TaS2 thickness and stacking sequence, I will demonstrate the influence on the image contrast. By comparison of these results with experimental data I show that the thickness of 1TTaS2 with simple stacking (all-A stacking) can be determined with a ± 1 unit cells uncertainty. Variations in the symmetry and the intensity distribution in CBED patterns can be used to distinguish simple all-A (AA….A) stacking from other stacking. Finally, I will briefly discuss the limitations of this method and future work. i
Study of the Metal-Insulator Transition in LaCoO<sub>3-<i>x</i></sub> Epitaxial Films
No full textMaterials with temperature-dependent metal-to-insulator transitions (MIT) have gained attention for the abrupt collapse of the band gap during the transition. Various novel transistor structures which utilize MITs have been suggested and realized to produce more energy efficient transistors. To achieve practical operation temperatures for device applications, however, MIT materials with transition temperatures below 400 K are unsuitable. This work focuses on one MIT material with a high temperature transition, LaCoO3. Using oxide molecular-beam epitaxy (MBE) a series of LaCoO3-x thin films with varied La and Co compositions were grown. All films were grown on LaAlO3 (001)p substrates, which like LaCoO3, has a pseudocubic perovskite structure. The lattice mismatch is less than 0.9%. X-ray diffraction θ-2θ measurements were used to assess the structure of the films; the out-of-plane lattice constant of each film was calculated using a Nelson-Riley analysis. The temperature-dependent resistivity of each film was measured and each shows a change in electrical resistivity of more than two orders of magnitude in the MIT temperature range of 400 - 600 K, which is similar to that of bulk single crystal LaCoO3. The abruptness and magnitude of the MIT is found to be insensitive to the film composition for samples ranging from La1.1CoO3-x to LaCo1.1O3-x
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
In Situ Spectroscopy of Metal Oxides Reveal Electrocatalyst Structure-Property Relationships
No full text123 pagesThe structure-property relationship is a foundational concept of materials science and engineering. Tuning the catalytic properties of materials by varying their structural properties like stoichiometry and crystal facets have been integral to advances in electrocatalysis. Traditionally, these studies rely on ex situ, bulk structural characterizations to describe catalytic properties. Catalysis, however, occurs on a dynamic catalytic surface sensitive to its local environment. This dissertation uses in situ spectroscopy of metal oxide electrocatalysts to better capture the intricacies of such catalyst structure-property relationships. La2/3S1/3rMnO3 films are grown using molecular beam epitaxy for a controlled study on the effects of surface and sub-surface structure of metal oxides on the oxygen reduction reaction (ORR) catalysis. Ambient pressure X-ray photoelectron spectroscopy of the films show that the surface and sub-surface structures control the balance between electronic benefits vs. parasitic surface reactions for the ORR. The low atomic number group of an amorphous cobalt oxide catalyst in a phosphate network (CoPi) is studied using home-built stimulated Raman spectroscopy (SRS). In situ SRS reveals previously unreported phosphate motifs in CoPi under oxygen evolution reaction conditions. Isotopic studies show that the phosphate structure is intimately linked to the aqueous environment with implications for the catalyst microstructure and deposition. Extensions of the dissertation on the in situ spectroscopy of a quinone electrochemical systems is discussed in the end
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