Scientific Publications 2004
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E
2004. "Measurement of the Band Offsets between Amorphous LaAlO₃ and Silicon." Applied Physics Letters 84(5):726-728. Abstract The conduction and valence band offsets between amorphous LaAlO₃ and silicon have been determined from X-ray photoelectron spectroscopy (XPS) measurements. These films, which are free of interfacial SiO₂, were made by molecular beam deposition (MBD). The band line-up is type I with measured band offsets of 1.86 ± 0.2 eV for electrons and 3.23 ± 0.1 eV for holes. The band offsets are independent of the doping concentration in the silicon substrate as well as the amorphous LaAlO3 film thickness. These amorphous LaAlO3 films have a bandgap of 6.2 ± 0.1 eV.
2004. "Measurements of the Band Offsets Between Amorphous LaAIO₃ and Silicon." Applied Physics Letters 84(5):726-728. Abstract The conduction and valence band offsets between amorphous LaAlO₃ and silicon have been determined from X-ray photoelectron spectroscopy (XPS) measurements. These films, which are free of interfacial SiO₂, were made by molecular beam deposition (MBD). The band line-up is type I with measured band offsets of 1.86 ± 0.1 eV for electrons and 3.23 ± 0.1 eV for holes. The band offsets are independent of the doping concentration in the silicon substrate as well as the LaAlO₃ film thickness. These amorphous LaAlO₃ films have a bandgap of 6.2 ± 0.1 eV.
2004. "Suppression of Subcutaneous Oxidation during the Deposition of Amorphous Lanthanum Aluminate on Silicon." Applied Physics Letters 84(23):4629-4631. Abstract Amorphous LaAlO₃ thin films have been deposited by molecular beam deposition directly on silicon without detectable oxidation of the underlying substrate. We have studied these abrupt interfaces by Auger electron spectroscopy, high-resolution transmission electron microscopy, medium-energy ion scattering transmission infrared absorption spectroscopy, and x-ray photoelectron spectroscopy. Together these techniques indicate that the films are fully oxidated and have less than 0.1 A of SiO₂ at the interface between the amorphous LaAlO₃ and silicon. These heterostructures are being investigated for alternative gate dielectric applications and provide a unique opportunity to control the interface between the silicon and the gate dielectric.
2004. "Microwave-Assisted Sample Treatment in a Fully Automated Flow-Based Instrument: Oxidation of Reduced Technetium Species in the Analysis of Total Technetium-99 in Caustic Aged Nuclear Waste Samples." Analytical Chemistry 76(14):3869-3877. Abstract An automated flow-based instrument for microwave-assisted treatment of liquid samples has been developed and characterized. The instrument utilizes a flow-through reaction vessel design that facilitates the addition of multiple reagents during sample treatment, removal of the gaseous reaction products, and enables quantitative removal of liquids from the reaction vessel for carryover-free operations. Matrix modification and speciation control chemistries that are required for the radiochemical determination of total 99Tc in caustic aged nuclear waste samples have been investigated. A rapid and quantitative oxidation procedure using peroxydisulfate in acidic solution was developed to convert reduced technetium species to pertechnetate in samples with high content of reducing organics. The effectiveness of the automated sample treatment procedures has been validated in the radiochemical analysis of total 99Tc in caustic aged nuclear waste matrixes from the Hanford site.
2004. "Chemical Processing in High-Pressure Aqueous Environments. 7. Process Development for Catalytic Gasification of Wet Biomass Feedstocks." Industrial and Engineering Chemistry Research 43(9):1999-2004. Abstract Through the use of a metal catalyst, gasification of wet biomass can be accomplished with high levels of carbon conversion to gas at relatively low temperature (350 C). In the pressurized-water environment (20 MPa) near-total conversion of the organic structure of biomass to gases has been accomplished in the presence of a ruthenium metal catalyst. The process is essentially steam reforming as there is no added oxidizer or reagent other than water. In addition, the gas produced is a medium-heating value gas due to the synthesis of high-levels of methane, as dictated by thermodynamic equilibrium. Biomass trace components cause processing difficulties using the fixed catalyst bed tubular reactor system. Results are described for both bench-scale and scaled-up reactor systems.
2004. "The Effects of Trace Contaminants on Catalytic Processing of Biomass-Derived Feedstocks ." Applied Biochemistry and Biotechnology 115(1-3):807-825. Abstract Trace components in biomass feedstocks are potential catalyst poisons when catalytically processing these materials to value-added chemical products. Trace components include inorganic elements such as alkali metals and alkaline earths, phosphorus or sulfur, aluminum or silicon, chloride, or transition metals. Protein components in biomass feedstocks can lead to formation of peptide fractions (from hydrolysis) or ammonium ions (from more severe breakdown) both of which might interfere with catalysis. The effects of these components on catalytic hydrogenation processing has been studied in batch reactor processing tests
2004. "Synthesis, Structure and Reactions of Stable Oxametallacycles from Styrene Oxide on Ag(111)." Journal of Physical Chemistry B 109(6):2227-2233. doi:10.1021/jp048939k S1089-5647(04)08939-4 Abstract the abstract for this product is not available at this time.
2004. "X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced CeO₂(111) Surfaces." Surface Science Spectra 11(1-4):73-81. Abstract We have studied the electronic structure of oxidized and reduced CeO₂ (111) surfaces using X-ray photoelectron spectroscopy (XPS). The 50 nm thick Co₂(111) film was grown on a YSZ(111) substrate using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in-situ RHEED (reflection high energy electron diffraction) and ex-situ XRD (X-ray diffraction), HRTEM (high resolution transmission electron microscopy) and RBS (Rutherford backscattering spectroscopy). The lattice mismatch between CeO₂(111) and YSZ(111) is less than 5% and yields a flat surface that is comprised of an equivalent number of Ce⁴⁺ and O₂₋ ions. Oxidation with O₂ at 773 K under UHV conditions was sufficient to fully oxidize the CeO₂(111). Surface reduction was carried out by annealing in UHV at 973 K.
2004. "Laser-Induced Damage of Calcium Fluoride." Journal of Undergraduate Research Volume IV, 2004:60-65. Abstract Radiation damage of materials has long been of fundamental interest, especially since the growth of laser technology. One such source of damage comes from UV laser light. Laser systems continue to move into shorter wavelength ranges, but unfortunately are limited by the damage threshold of their optical components. For example, semiconductor lithography is making its way into the 157nm range and requires a material that can not only transmit this light (air cannot), but also withstand the highly energetic photons present at this shorter wavelength. CaF2, an alkaline earth halide, is the chosen material for vacuum UV 157 nm excimer radiation. It can transmit light down to 120 nm and is relatively inexpensive. Although it is readily available through natural and synthetic sources, it is often times difficult to find in high purity. Impurities in the crystal can result in occupied states in the band gap that induce photon absorption [2] and ultimately lead to the degradation of the material. In order to predict how well CaF2 will perform under irradiation of short wavelength laser light, one must understand the mechanisms for laser-induced damage. Laser damage is often a two-step process: initial photons create new defects in the lattice and subsequent photons excite these defects. When laser light is incident on a solid surface there is an initial production of electron-hole (e-h) pairs, a heating of free electrons and a generation of local heating around optically absorbing centers [3]. Once this initial excitation converts to the driving energy for nuclear motion, the result is an ejection of atoms, ions and molecules from the surface, known as desorption or ablation [3]. Secondary processes further driving desorption are photoabsorption, successive excitations of self-trapped excitons (STE’s) and defects, and ionization of neutrals by incident laser light [3]. The combination of laser-induced desorption and the alterations to the electronic and geometrical structure of the lattice result in defect formation. In the material CaF2 some of these defects take the form of F-centers, an electron trapped at a halogen vacancy [4], and H-centers, a F2- molecular ion at a single lattice site [5]. While the F-centers are stable, the H-centers are transient but can form into aggregates that are stable. There are many different configurations the defects can take based on the relative position of F and H centers in the lattice and this is extensively discussed in literature [1,4,5]. Once these defects have formed they cause further absorption of light, which ultimately induces particle emission and the production of even more defects. Various forms of laser-induced damage of CaF2 have been studied. For example, the mechanism for photon-stimulated desorption (PSD) of F+ from CaF2 (111) is discussed in ref. 6 and the energy threshold, distribution and kinetics governing electron-stimulated desorption (ESD) is investigated in ref. 7. The desorption of neutral Ca and F atoms has also been explored [1]. In this paper I focus on the emission of ions and neutrals from CaF2 under the irradiation of pulsed laser light at 266 nm, in addition to a brief study of its purity and transmittance.
2004. "Nuclear Waste Disposal—Pyrochlore (A₂B₂O(7)): Nuclear Waste Form for the Immobilization of Plutonium and "Minor" Actinides." Journal of Applied Physics 95(11 ):5949-5971 Part 1. Abstract During the past half-century, the nuclear fuel cycle has generated approximately 1,400 metric tons of plutonium and substantial quantities of the “minor” actinides, such as Np, Am and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A₂B₂O₇, for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1,000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compositions has revealed that zirconate pyrochlore does not become amorphous, but remains crystalline as a defect fluorite structure due to disordering of the A- and B-site cations. The zirconate pyrochlore will remain crystalline even to very high doses, greater than 100 displacements per atom. Systematic experimental studies of actinide-doped and ion beam-irradiated pyrochlore, studies of natural U-bearing pyrochlore, and simulations of the energetics of the disordering process now provide a rather detailed understanding of the structural and chemical controls of the pyrochlore structure on its response to radiation. These results provide a solid basis for predicting the behavior and durability of pyrochlore used to immobilize plutonium.
