Brett Leedahl

Insight into photon conversion of Nd3+ doped low temperature grown p and n type tin oxide thin films

The synthesis of multifunctional high-quality oxide thin films is a major current research challenge given their potential applications. Herein, we report on p and n type tin oxides thin films as functional TCOs with photon management properties through doping with Nd3+ rare earth ions. We show that the structure, composition, carrier transport and optical properties of the sputtered Nd:SnOx films can be easily tuned by simply varying the Ar/O2 gas flow ratio (R) during the deposition step. The increase of the oxygen content leads to drastic changes of the material properties from p-type SnO to n-type SnO2. Furthermore, all Nd:SnOx films are found to be highly conductive with resistivities as low as 1 × 10−3 Ω cm−1 and carrier mobilities up to 129 cm2 V−1 s−1. Thanks to deep XPS and NEXAFS spectroscopies, we gained insight into the coordination and oxidation degrees of the elements within the matrices. The insertion and optical activation of the incorporated Nd3+ ions have been successfully achieved in both matrices. As a consequence, strong NIR luminescence lines, typical of Nd3+ ions, were recorded under UV laser excitation. We experimentally show that the efficient Nd3+ photoluminescence in the near infrared region originates from efficient sensitization from the host matrix, through energy transfer. We found that the SnO2 host matrix provides more efficient sensitization of Nd3+ as compared to the SnO matrix. An energy transfer mechanism is proposed to explain the observed behaviour.

Selective Area Band Engineering of Graphene using Cobalt-Mediated Oxidation.

This study reports a scalable and economical method to open a band gap in single layer graphene by deposition of cobalt metal on its surface using physical vapor deposition in high vacuum. At low cobalt thickness, clusters form at impurity sites on the graphene without etching or damaging the graphene. When exposed to oxygen at room temperature, oxygen functional groups form in proportion to the cobalt thickness that modify the graphene band structure. Cobalt/Graphene resulting from this treatment can support a band gap of 0.30 eV, while remaining largely undamaged to preserve its structural and electrical properties. A mechanism of cobalt-mediated band opening is proposed as a two-step process starting with charge transfer from metal to graphene, followed by formation of oxides where cobalt has been deposited. Contributions from the formation of both CoO and oxygen functional groups on graphene affect the electronic structure to open a band gap. This study demonstrates that cobalt-mediated oxidation is a viable method to introduce a band gap into graphene at room temperature that could be applicable in electronics applications.

Structural defects induced by Fe-ion implantation in TiO2

X-ray photoelectron spectroscopy and resonant x-ray emission spectroscopy measurements of pellet and thin film forms of TiO2 with implanted Fe ions are presented and discussed. The findings indicate that Fe-implantation in a TiO2 pellet sample induces heterovalent cation substitution (Fe2+ → Ti4+) beneath the surface region. But in thin film samples, the clustering of Fe atoms is primarily detected. In addition to this, significant amounts of secondary phases of Fe3+ are detected on the surface of all doped samples due to oxygen exposure. These experimental findings are compared with density functional theory calculations of formation energies for different configurations of structural defects in the implanted TiO2:Fe system. According to our calculations, the clustering of Fe-atoms in TiO2:Fe thin films can be attributed to the formation of combined substitutional and interstitial defects. Further, the differences due to Fe doping in pellet and thin film samples can ultimately be attributed to different ...

Searching for pure iron in nature: the Chelyabinsk meteorite

Herein we aimed to use thermomagnetic analysis (TMA) to determine the nature of iron and nickel in the Chelyabinsk meteorite, and their effect on the meteorites magnetism. Our magnetic measurements show that 3% of the meteorite is metallic and consists of two ferromagnetic phases with Curie temperatures of TC1 = 1049 K and TC2 = 800 K. Using an Fe–Ni phase diagram, we show that the lower of the two temperatures is due to an Fe–Ni alloy with 51% Ni, while the higher Curie temperature phase is due to a pure or nearly pure (Ni-free) iron phase, for which we can be certain the Ni content is less than 1%. X-ray absorption (XAS) measurements show there are two clearly distinct iron oxidation environments: metallic and 2+, with the 2+ regions differing significantly from the standard FeO phase. We also demonstrate that beneath the immediate surface, iron exists virtually entirely in a metallic state. We are then able to estimate the surface composition using XPS, for which we found that 10% of iron on the surface is still surprisingly unoxidized. Finally, our theoretical calculations show how the density of states for both Fe and Ni atoms is affected for different nickel concentrations.

Bulk vs. Surface Structure of 3 d Metal Impurities in Topological Insulator Bi 2 Te 3

Topological insulators have become one of the most prominent research topics in materials science in recent years. Specifically, Bi2Te3 is one of the most promising for technological applications due to its conductive surface states and insulating bulk properties. Herein, we contrast the bulk and surface structural environments of dopant ions Cr, Mn, Fe, Co, Ni, and Cu in Bi2Te3 thin films in order to further elucidate this compound. Our measurements show the preferred oxidation state and surrounding crystal environment of each 3d-metal atomic species, and how they are incorporated into Bi2Te3. We show that in each case there is a unique interplay between structural environments, and that it is highly dependant on the dopant atom. Mn impurities in Bi2Te3 purely substitute into Bi sites in a 2+ oxidation state. Cr atoms seem only to reside on the surface and are effectively not able to be absorbed into the bulk. Whereas for Co and Ni, an array of substitutional, interstitial, and metallic configurations occur. Considering the relatively heavy Cu atoms, metallic clusters are highly favourable. The situation with Fe is even more complex, displaying a mix of oxidation states that differ greatly between the surface and bulk environments.

Tunability of room-temperature ferromagnetism in spintronic semiconductors through nonmagnetic atoms

The implementation and control of room temperature ferromagnetism (RTFM) by adding magnetic atoms to a semiconductors lattice has been one of the most important problems in solid state state physics in the last decade. Herein we report for the first time, to our knowledge, on the mechanism that allows RTFM to be tuned by the inclusion of \emph{non-magnetic} aluminum in nickel ferrite. This material, NiFe

The electronic structure of ε′-V2O5: an expanded band gap in a double-layered polymorph with increased interlayer separation

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Structure-induced switching of the band gap, charge order and correlation strength in ternary vanadium oxide bronzes

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Study of the Structural Characteristics of 3d Metals Cr, Mn, Fe, Co, Ni, and Cu Implanted in ZnO and TiO2—Experiment and Theory

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Contrasting 1D tunnel-structured and 2D layered polymorphs of V2O5: relating crystal structure and bonding to band gaps and electronic structure

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