Thomas M. McCleskey

Effect of spin-orbit coupling on the actinide dioxides AnO2 (An=Th, Pa, U, Np, Pu, and Am): A screened hybrid density functional study

We present a systematic comparison of the lattice structures, electronic density of states, and band gaps of actinide dioxides, AnO(2) (An=Th, Pa, U, Np, Pu, and Am) predicted by the Heyd-Scuseria-Ernzerhof screened hybrid density functional (HSE) with the self-consistent inclusion of spin-orbit coupling (SOC). The computed HSE lattice constants and band gaps of AnO(2) are in consistently good agreement with the available experimental data across the series, and differ little from earlier HSE results without SOC. ThO(2) is a simple band insulator (f(0)), while PaO(2), UO(2), and NpO(2) are predicted to be Mott insulators. The remainders (PuO(2) and AmO(2)) show considerable O2p/An5f mixing and are classified as charge-transfer insulators. We also compare our results for UO(2), NpO(2), and PuO(2) with the PBE+U, self interaction correction (SIC), and dynamic mean-field theory (DMFT) many-body approximations.

Epitaxial growth of Eu2O3 thin films on LaAlO3 substrates by polymer-assisted deposition

Eu2O3 thin films were epitaxially grown on (001)LaAlO3(LAO) substrates using a polymer-assisted deposition technique. Microstructural studies by x-ray diffraction and transmission electron microscopy show that the films are epitaxial with an orientational relationship of (001)Eu2O3∥(001)LAO and [11¯0]Eu2O3∥[100]LAO. We have found a systematic change in the out-of-plane lattice parameter of Eu2O3(a⊥) with the annealing temperature, which is believed to be from different strain effects.

Epitaxial Superconducting δ-MoN Films Grown by a Chemical Solution Method

The synthesis of pure δ-MoN with desired superconducting properties usually requires extreme conditions, such as high temperature and high pressure, which hinders its fundamental studies and applications. Herein, by using a chemical solution method, epitaxial δ-MoN thin films have been grown on c-cut Al(2)O(3) substrates at a temperature lower than 900 °C and an ambient pressure. The films are phase pure and show a T(c) of 13.0 K with a sharp transition. In addition, the films show a high critical field and excellent current carrying capabilities, which further prove the superior quality of these chemically prepared epitaxial thin films.

Magnetoresistance in polymer-assisted deposited Sr- and Ca-doped lanthanum manganite films

We have grown epitaxial films of La0.67Sr0.33MnO3 (LSMO) and La0.67Ca0.33MnO3 (LCMO) on single crystalline LaAlO3 substrates by a cost effective polymer-assisted deposition technique. Film crystallinity, microstructure, resistivity, magnetization, and magnetoresistance (MR) were highly dependent on the annealing temperature in the film processing. High negative MR values of −50% (at 305K) and −88% (at 250K) were observed at magnetic field of 5T, for high temperature annealed LSMO and LCMO films, respectively. These results are comparable to those for films grown by pulsed laser deposition and rf sputtering techniques.

Highly aligned carbon nanotube forests coated by superconducting NbC

The formation of carbon nanotube and superconductor composites makes it possible to produce new and/or improved functionalities that the individual material does not possess. Here we show that coating carbon nanotube forests with superconducting niobium carbide (NbC) does not destroy the microstructure of the nanotubes. NbC also shows much improved superconducting properties such as a higher irreversibility and upper critical field. An upper critical field value of ~5 T at 4.2 K is much greater than the 1.7 T reported in the literature for pure bulk NbC. Furthermore, the aligned carbon nanotubes induce anisotropy in the upper critical field, with a higher upper critical field occurring when the magnetic field is parallel to the carbon nanotube growth direction. These results suggest that highly oriented carbon nanotubes embedded in superconducting NbC matrix can function as defects and effectively enhance the superconducting properties of the NbC.

Chemical Solution Deposition of Epitaxial Carbide Films

Carbide films exhibit many unique properties. The development of a versatile and simple technique for the deposition of carbide films will enable a wide range of technological applications. Here we report a cost-effective chemical solution deposition or polymer-assisted deposition method for growing epitaxial carbide (including TiC, VC, and TaC) films. These epitaxial carbide films exhibit structural and physical properties similar to the films grown by vapor deposition methods.

High Surface Area Molybdenum Nitride Support for Fuel Cell Electrodes

Alternative supports for polymer electrolyte membrane fuel cells were synthesized and catalytic activity was explored using electrochemical analysis. High surface area, molybdenum nitride supports were synthesized by rapidly heating a gel of polyethyleneimine bound molybdenum in a tube furnace under a forming gas atmosphere. Subsequent disposition of platinum through an incipient wetness approach lead to dispersed crystallites of platinum on the conductive support. All the ceramic materials were characterized with XRD, SEM, TEM and electrochemical analysis. The supports without platinum are highly stable to acidic aqueous conditions and show no signs of oxygen reduction reactivity (ORR). However, once the 20 wt % platinum is added to the material, ORR activity comparable to XC72 based materials is observed.

LaF3:Ce nanocomposite scintillator for gamma-ray detection

Nanophosphor LaF3:Ce has been synthesized and incorporated into a matrix to form a nanocomposite scintillator suitable for application to γ-ray detection. Owing to the small nanocrystallite size (sub-10 nm), optical emission from the γ / nanophosphor interaction is only weakly Rayleigh scattered (optical attenuation length exceeds 1 cm for 5-nm crystallites), thus yielding a transparent scintillator. The measured energy resolution is ca. 16% for 137Cs γ rays, which may be improved by utilizing brighter nanophosphors. Synthesis of the nanophosphor is achieved via a solution-precipitation method that is inexpensive, amenable to routine processing, and readily scalable to large volumes. These results demonstrate nanocomposite scintillator proof-of- principle and provide a framework for further research in this nascent field of scintillator research.

Upper critical magnetic field and vortex-free state in very thin epitaxial δ-MoN films grown by polymer-assisted deposition

We measured the thickness dependence of the superconducting properties in epitaxial δ-MoN thin films grown on α-Al2O3(001) substrates by polymer-assisted deposition. Our results indicate that the superconducting properties such as the upper critical field (μ0Hc2 ≈ 10 T) and the superconducting critical temperature (Tc = 12.5 K) are thickness independent for films thicker than ~36 nm. By measuring the critical current density (Jc) in the vortex-free state, which coincides with the depairing current density (J0), we estimate that films thicker than ~36 nm have a coherence length ξ(0) = 5.8 ± 0.2 nm and penetration depth λ(0) = 420 ± 50 nm. We found that it is possible to enhance the Hc2(0) values to close to 10 T without any appreciable reduction in Tc.

Polymer-assisted deposition of metal-oxide films

In the last several years, metal-oxides have become the basis for many revolutionary electronic devices because they exhibit a wide range of electronic properties that conventional metallic elements and covalent semiconductors do not possess. Metal-oxide films can be grown by physical vapor deposition, chemical vapor deposition, and chemical solution deposition techniques. One of the challenges in solution-based processes of complex functional metal-oxide films has been to produce high quality films and at the same time to control the stoichiometry. In this talk, we describe a new chemical solution method called polymer-assisted deposition (PAD) to grow epitaxial complex metal-oxides such as ferroelectric Ba1-xSrxTiO3, Pb0.40Sr0.60TiO3, and BaZr0.27Ti0.73O3 films. We use a new strategy to control the distribution of metals in solution at a molecular level and a mixture of metal precursor and soluble polymer to form a solution with desired viscosity. By actively binding the metal, the polymer serves to encapsulate the metal to prevent chemical reaction while maintaining an even distribution of the metal in solution.Ü This ensures a homogeneous metal distribution and prevents unwanted reactivity that can lead to the formation of undesired phases. The successful growth of epitaxial ferroelectric films with desired properties by PAD suggests that PAD is a feasible alternative approach to the growth of high quality ferroelectric materials.