EuO and Eu on metal crystals and graphene: interface effects and epitaxial films

Growth of the ferromagnetic semiconductor EuO was studied on the metal crystals Ni(100) and Ir(111) and on graphene. Primarily, characterisation was done by means of in-situ scanning tunnelling microscopy (STM) and low energy electron diffraction. The epitaxy on the metal crystals is strongly influenced by interface effects which lead to a complicated growth behaviour in the sub-monolayer regime, especially on Ni(100). Therefore, also films of sub-monolayer thickness were analysed in detail for these substrates. Eu oxide on Ni(100) shows a variety of different surface phases in the sub-monolayer regime, depending on the growth temperature and the ratio of the Eu and O fluxes. Hence, a careful selection of the initial growth parameters is decisive to obtain a surface oxide suitable for subsequent epitaxy of single phase EuO(100). After creation of a 3 layer thick coalesced oxide film, for subsequent growth a distillation technique can be applied. Ex-situ X-ray adsorption spectroscopy and magneto-optical Kerr effect microscopy measurements of thicker films on Ni(100) are consistent with stoichiometric single phase EuO with bulk properties. On Ir(111) initially only islands of polar EuO(111) grow, but formation of EuO(100) sets in before the first oxide layer is completed. The ratio of EuO(100) to EuO(111) is thereby influenced by the ratio of the Eu and O fluxes. Thus, the EuO films on Ir(111) consist of a phase mixture of EuO(111) and three rotational domains of EuO(100). The thinnest structure of the EuO(111) is a bilayer. Field emission resonances revealed a work function increase of 6 eV for this structure compared to EuO(100). Despite the polarity, the bilayer shows no obvious reconstruction which could reduce the high electric field. Triangular reconstruction motifs were found for the third EuO(111) layer. On graphene EuO can be grown as thin film of distinct, {100}-faceted grains which are oriented to the substrate at a sufficiently high growth temperature. As the EuO on graphene is not affected by interface effects, the initial growth stage is not crucial. Thus, the growth of these grains is far less sensitive to the ratio of Eu and O fluxes than the EuO growth on Ni(100). Appropriate annealing of EuO(100) films generates sufficient conductivity for STM and electron spectroscopies, even for films of 100 nm thickness. Oxygen vacancies were directly imaged by STM. They are of decisive importance for the metal-to-insulator transition of EuO around the temperature of the ferromagnetic-to-paramagnetic transition. Tunnelling spectra of EuO were recorded for the first time. For EuO(100) with 1% O vacancies in the topmost layer they exhibit states about 500 meV above the Fermi level which are most probably related to O vacancies. On all substrates, monolayer high EuO(100) films have a contracted lattice which expands with increasing film thickness. Even if the substrate applies compressive biaxial stress, the EuO bulk lattice constant is almost reached for 5 nm film thickness. This leaves little hope for an increase of the Curie temperature through epitaxial compression. During the investigation of the EuO on graphene, intercalation of Eu between the graphene and its Ir(111) substrate was observed and analysed further. For Eu deposition at 720 K a variety of equilibrium intercalate structures occur, dependent on the deposited Eu amount, all of which have a height of one monolayer. The dimensions and orientations of these structures are determined by binding energy differences within the unit cell of the graphene moiré on Ir(111). The energetically preferred lattice of the intercalated Eu is a p(2x2) structure, but intercalation continues until a denser (1.73x1.73)R30° structure is saturated. Angular resolved photoemission spectroscopy finds a shift of the graphene's Dirac cone by -1.5 eV for both of these structures. For closed graphene films, intercalation is hindered by a penetration barrier for temperatures below 400 K. The adsorption and equilibrium surface phases of Eu on graphene were investigated in the temperature range from 35 K to 400 K and for coverages ranging from a small fraction of a saturated monolayer to the second layer. Using density functional theory, including the 4f-shell Coulomb interactions and modelling of the electronic interactions, excellent agreement with the experimental results for the equilibrium adsorbate phase, adsorbate diffusion, and work function was obtained. Most remarkable, at 300 K in an intermediate coverage range a phase of uniformly distributed Eu clusters coexists in two dimensional equilibrium with large Eu-islands in a (1.73x1.73)R30° structure. The formation of the cluster phase is driven by the interplay of three effects: First, the metallic Eu-Eu binding leads to the local stability of (1.73x1.73)R30° structures. Second, electrons lower their kinetic energy by leaving the Eu clusters, thereby doping graphene. Third, the Coulomb energy penalty associated with the charge transfer from Eu to graphene is strongly reduced for smaller clusters

Polarity in GaN and ZnO: Theory, measurement, growth, and devices

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919.The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade

Engineered Nanoparticles Generation, Characterization and Applications

It is predicted that novel nanometer-sized structures incorporating nanoparticles will have a considerable impact on our lives during the coming decades. Engineered nanoparticles are already found in a number of commercially available products. However, many applications of these nanoparticles have only reached the stage of promising ideas or research demonstrations. The number of nanoparticle-based products on the market is therefore expected to increase considerably during the coming decades. For engineered nanoparticles to be useful in different commercial applications, it is important that their generation can be controlled. This means a stable generation process resulting in reproducible, high-quality nanoparticles with properties tailored for specific applications. In order to develop such production methods, thorough characterization of the particles generated is essential. In addition, since the impact of nanoparticles on human health and the environment has not been fully explored, the entire lifecycle of engineered nanoparticles must be thoroughly investigated. Engineered nanoparticles should not cause any harm to human health or the environment, during manufacturing or use of the product, or during disposal of the product after use. This thesis describes the manufacture of engineered nanoparticles, mainly by inert gas evaporation using a spark discharge generator and an evaporation/condensation furnace. Considerable effort has been put into investigating how different generation parameters affect particle production, so that the particle properties can be controlled and tailored to meet specific applications. To achieve this, the as-generated nanoparticles have been systematically characterized by various methods; transmission electron microscopy being the key characterization tool. The nanoparticles generated were then used in three different areas of application: as seed particles for so-called nanowires which may be useful in future devices, as model catalyst systems to provide deeper knowledge about the atomic-scale mechanisms involved in catalysis, and finally for research in the area of nano safety to learn how nanoparticles should be handled in a safe and sustainable manner

Dissociation of molecules on silicon surfaces studied by scanning tunneling microscopy

xviii, 175 leaves : ill. (some col.) ; 29 cmDissociation of trichloroethylene (TCE) molecules on the Si(111)-7x7 and the Si(100)-2x1 surfaces was studied using STM. Though molecular adsorption may also be observed on the Si(111)-7x7 surface, dissociation is the dominant process. From the STM images acquired, products of dissociation were identified, namely chlorine atoms and dichlorovinyl groups. Dissociation of chlorine from the TCE molecule was confirmed by studying not just appearance in STM images but also from studies of tip-induced diffusion. Different binding configurations were proposed for the vinyl group on the Si (111)-7x7 and the Si(100)-2x1 surfaces. Site preference for each product of dissociation is reported on the Si(111)-7x7 surface. Dissociation of molecules such as ammonia, dimethylamine and methyl chloride on the Si(111)-7x7 and Si(100)-2x1 surfaces is reviewed. The field emission process is explained in detail. The usefulness of making field emission measurements is in evaluating the sharpness of STM tips

Toward atomic-based understanding of some reactive and non-reactive surfaces

The thesis is composed of two broad themed sections with the underlying aim of understanding on a precise atomic basis, the electronic and structural factors governing the reactive and non-reactive surfaces of two metal oxides belonging to the same group in the periodic table; boron (B) and aluminium (Al). Using accurate density functional theory (DFT) computations, we first elucidate the initial reaction steps of the surface oxidation of elemental boron into its respective oxide; boron trioxide (B₂O₃). The highly exoergic reaction obtained for the dissociative adsorption of molecular oxygen over the boron surface coincides with the widely used boron oxidation reaction as secondary energy source in rockets. The relatively large activation energy for the O-O dissociation step marks the non-spontaneity of elemental boron oxidation at room temperature. Having established routes for the formation of B₂O₃-like precursors, we then investigate the relative stability of four low-index surfaces of the low-pressure B₂O₃ phase; namely the B₂O₃-I configuration. We demonstrate that none of the investigated low-index surfaces have dangling bonds, which reasonably relates to the experimentally observed low reactivity of this compound. The most stable surface terminations of B₂O₃ orientations entail tetrahedral BO₄ units. Such termination incurs a lower surface energy than orientations that consist of only triangular BO₃ units. Electronic and structural factors provide atomic-base elucidation of the observed inertness of B₂O₃. Combined experimental techniques (i.e. diffuse reflectance infrared spectroscopy) and DFT simulation are used to answer some of the most intriguing questions pertinent to factors underpinning the well-documented catalytic inhibition by B₂O₃ and its hygroscopic behaviour. We investigate the adsorption and dissociation mechanisms of two hydrogen chalcogenides, namely water (H₂O) and hydrogen sulfide (H₂S) molecules over B₂O₃-I (101) surfaces. We show that the diboron trioxide surface exhibits high physiochemical reactivity towards water molecules. The Lewis acid properties of B₂O₃-I lead to the formation of a molecular adsorption state (rather than dissociative adsorption) of the H₂S molecule via the acceptance of an electron pair into the low-energy orbital of the boron valence shell. While acting as water scavenger to generate dissociated radicals, B₂O₃ exhibits an inhibitor characteristic towards the dissociation of H₂S molecules, representing an ideal reactor wall coating in such systems. Alumina have been widely utilised as independent catalysts or as support materials for other catalysts. From an environmental perspective, alumina nanoclusters dispersed on surfaces of particulate matter PM₁₂ generate from various combustion processes play a critical role in the synthesis of environmental persistent free radicals (EPFR). Of particular importance are phenoxy-type EPFR that often acts as building blocks for the formation of notorious pollutants. Herein, we provide a comprehensive thermo-mechanistic account of alumina-surface mediated formation of phenoxy-type EPFR on different structural alumina models encompassing the following surfaces: dehydrated alumina surface, fully hydrate alumina surface, surfaces with different hydration coverage, and silicon-alumina doped surface. We show that fission of the phenol’s hydroxyl bond over dehydrated alumina systematically incurs lower energy barriers in reference to the hydrate surfaces. The catalytic activity of the alumina surface in producing the phenoxy/phenolate species reversibly correlates with the degree of hydroxyl coverage. Furthermore, we clarify the effect doping on the catalytic activity of alumina. The activation energy barrier required to form phenoxy moiety on Si-substituted Al₂O₃(0001) surface is ~40% lower than that of analogous barriers encountered over undoped dehydrate alumina surface. Overall, all considered models of alumina configurations are shown to produce adsorbed phenolate; however, desorption of the latter into the gas phase requires a rather sizable energy. Thus, the fate of absorbed phenolate is most likely to be dictated by decomposition affording carboneous layer of self-decomposition into other stable molecules

Summaries of FY 1997 Research in the Chemical Sciences

The objective of this program is to expand, through support of basic research, knowledge of various areas of chemistry, physics and chemical engineering with a goal of contributing to new or improved processes for developing and using domestic energy resources in an efficient and environmentally sound manner. Each team of the Division of Chemical Sciences, Fundamental Interactions and Molecular Processes, is divided into programs that cover the various disciplines. Disciplinary areas where research is supported include atomic, molecular, and optical physics; physical, inorganic, and organic chemistry; chemical energy, chemical physics; photochemistry; radiation chemistry; analytical chemistry; separations science; heavy element chemistry; chemical engineering sciences; and advanced battery research. However, traditional disciplinary boundaries should not be considered barriers, and multi-disciplinary efforts are encouraged. In addition, the program supports several major scientific user facilities. The following summaries describe the programs

Magnetoresistance of atomic structures studied with a Scanning Tunneling Microscope

In this thesis spin-dependent properties of adsorbates placed in different magnetic environments are investigated using a low-temperature scanning tunneling microscope. Spin-polarized currents and anisotropic magnetoresistance (AMR) are studied for single atoms and atomic structures adsorbed on a ferro- magnetic Fe bilayer on W(110). This layer, thanks to its out-of-plane and in-plane magnetized domains and domain walls, respectively, allows to study equivalent adsorbates in different magnetic environments without the need for an external magnetic field. Three different experiments on spin-properties of atomic structures are briefly introduced in the following. Using spin-polarized tips, the energy and distance dependence of the spin-polarized current of single Ir atoms is investigated. Non-magnetic tips are employed to investigate the effect of spin-orbit coupling (SOC) on the differential conductance of artificially built Pb dimers and on the conductance of single atoms that are contacted with the STM tip. The last two effects are linked to AMR, which describes the SOC mediated dependence of the electrical resistance on the magnetization direction

National synchrotron light source. Activity report, October 1, 1994--September 30, 1995

This report discusses research conducted at the National Synchrotron Light Source in the following areas: atomic and molecular science; energy dispersive diffraction; lithography, microscopy, and tomography; nuclear physics; scattering and crystallography studies of biological materials; time resolved spectroscopy; UV photoemission and surface science; x-ray absorption spectroscopy; x-ray scattering and crystallography; x-ray topography; the 1995 NSLS annual users` meeting; 17th international free electron laser conference; micro bunches workshop; VUV machine; VUV storage ring parameters; beamline technical improvements; x-ray beamlines; x-ray storage ring parameters; the NSLS source development laboratory; the accelerator test facility (ATF); NSLS facility improvements; NSLS advisory committees; NSLS staff; VUV beamline guide; and x-ray beamline guide

Defect Characterization of Cu2ZnSnSe4 Thin Film Solar Cells Using Advanced Microscopic Techniques

Thin film chalcogenide solar cells have been utilized in a broad range of application for their tunable direct bandgap and high efficiency. In this work, we performeda novel fabrication and multiple high-resolution characterizations of Cu2ZnSnSe4(CZTSe) solar cells, which is believed to be a better candidate compared to well-developed CuInxGa(1-x)Se2(CIGS)for its earth-abundant contents. The fabrication is based on nanoparticle precursor production by liquid-phase pulsed laser ablation, electrophoretic deposition of precursor thin film under ambient condition, and selenization. Such non-vacuum fabrication has the advantage of low cost and minimum impact on the environment. By studying the CZTSe and CIGS fabricated in the above methods using techniques including Raman integrated scanning probe microscope, electron holography, scanning transmission electron microscopy and in-situ transmission electron microscopy. We discoveredthe origin of the performance limit of the CZTSe compared to CIGS as well as the defect of our non-vacuum fabrication methods. The presented results, including the characterization methods, create a novel way to correlate the solar cell performance with the microstructure in a nanometer scale. It opens up the possibility for developing high performance solar cell devices from the prospective of nanostructure and defect engineering.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/140886/1/mjxu_1.pd