D. R. Jennison’s research while affiliated with Sandia National Laboratories and other places

First principles density functional calculations are used to study the structure and properties of ultrathin films of alumina on Al(111) and Ru(0001) substrates. We focus on ∼5 Å two O-layer films, naturally produced by oxidizing NiAl, Ni3Al, and Al deposited on Ru(0001). The interface consists of chemisorbed l×1 oxygen on the underlying metal [1], with a nearly coplanar layer of Al2O3 above. The lowest energy structure of the Al-sublattice is found to consist of zig-zag rows of octahedral and tetrahedral Al ions, resembling the bulk κ-phase. Eleven different adsorbed metals, spanning the periodic table, have been studied [2]. At low coverage the bond is purely ionic; at high coverage most metals bind principally by polarization. Contrary to speculations, isolated vacancies are found on MgO(100) not to directly promote metal nucleation [3], and we suggest this behavior is general. However, ad-OH is found not only to promote island nucleation, but can also, in high concentrations, produce wetting of alumina by Cu [4].

First principles density functional methods were used to investigate the atomistic behavior of hydrogen, helium, and oxygen in β -phase ErH ₂ . The ground state for hydrogen was indeed determined to be the tetrahedral position as commonly assumed, but if the surrounding tetrahedral sites are filled, any additional hydrogen will occupy the octahedral site. Only a small amount of thermally generated tetrahedral-vacancy octahedral-occupancy pairs are predicted at equilibrium since the formation energy is 1.21 eV. Other possible scenarios that result in octahedral hydrogen occupation include a H/Er ratio ≫2.0 and the presence of oxygen in the lattice. Our calculations indicate that oxygen impurities will reside in tetrahedral sites, even if that site is already occupied and hydrogen must be displaced into a neighboring octahedral site. Oxygen will migrate at moderate temperatures by jumping between tetrahedral and octahedral sites. The extent of hydrogen self-diffusion will depend on the concentration of tetrahedral vacancies and/or octahedral hydrogen and therefore can be modified by changing the H/Er ratio or by impurities (such as oxygen) that create octahedral hydrogen occupation. In samples where some of the hydrogen is replaced with tritium, helium generated by tritium decay will favor a tetrahedral site left vacant by a transmuted tritium. The barrier to helium migration between two unoccupied neighboring tetrahedral sites is 0.49 eV, where the path maximum corresponds to the octahedral site. If an extended network of neighboring vacancies exists, the relatively small barrier provides that helium may move throughout that network at room temperature. Given enough energy to escape the tetrahedral site(s), 1.31 eV, helium may continue to migrate by a 0.88 eV concerted-motion mechanism—temporarily displacing hydrogen as it moves between empty octahedral sites and fi- lled tetrahedral sites.

The work functions for W, Cu, and Al are calculated using density functional theory (DFT). We go beyond the perfect lattice at zero Kelvin by employing molecular dynamics techniques. Effects of surface alloying and -oxidation are also investigated. The effect of alloying and oxidation is, as expected, found to be significant, while the temperature dependence, although clearly seen, is in comparison weak. In addition, the exchange-correlation density functional AM05 is compared to the results of LDA and PBE. The calculated work functions compare well to available experimental results. This work was supported by the LDRD office and the simulations were performed at the High Performance Computing facilities at Sandia National Laboratories, NM. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

The response of ordered ultrathin Al2O3 films on NiAl(1 1 0) and Ni3Al(1 1 0) substrates to sequential exposures at varying pressures of H2O between 10−7 Torr and 10−3 Torr, ambient temperature, was characterized by LEED, AES and density functional theory (DFT) calculations. In all cases, an increase in average oxide thickness, as determined by AES, was observed, consistent with a field-induced oxide growth mechanism. Ordered oxide films of initial average thicknesses of 7 Å and 12 Å grown on NiAl(1 1 0) achieved a limiting thickness of 17(1) Å, while films of initial thickness of 7 Å and 11 Å grown on Ni3Al(1 1 0) achieved a limiting thickness of 12(1) Å. The LEED patterns for the thinner (7 Å) films were not observed after exposure to 10−5 Torr (NiAl(1 1 0)), or 10−4 Torr (Ni3Al(1 1 0)). In contrast, LEED patterns for the films of greater initial thickness persisted after exposures to 10−3 Torr UHV. DFT calculations indicate an Al vacancy formation energy that is significantly greater (by ∼0.5 eV) on the surface that has the thicker oxide film, directly opposite to what may be naively expected. A simple coordination argument supports these numerical results. Therefore, the greater limiting oxide thickness observed on NiAl(1 1 0) demonstrates that the rate determining step in the oxide growth process is not Al removal from the metal substrate and transport across the oxide/metal interface. Instead, the results indicate that the determining factor in the oxide growth mechanism is the kinetic barrier to Al diffusion from the substrate bulk to the oxide/metal interface. The persistence of the LEED patterns observed for the films of greater initial oxide thickness indicates that the surface disorder generally observed for alumina films grown on aluminide substrates and exposed to intermediate pressures of H2O is due to the growth of a disordered alumina layer over an ordered substrate, rather than to direct H2O interaction with terrace sites.

Electronic excitationsin adsorbate layers stimulate desoprtion and dissociation of adsorbed melecules as well as chemical reactions between adsorbates. The highest-probability stimulated processes produce neutral desorbates and determine how surface composition is altered by elctron or photon radiation. A basic understanding has emerged, due largely to laser resonance-enhanced multiphoton ionization (REMPI) experiments, which provide quantum-state resolution of the gas-phase products. Auger phenomena enter this understanding in several ways. For example, CVV Auger spectroscopy determines the screened hole-hole interaction, U, in adsorbates, which in turn provides insight into the degree of charge-transfer screening from the substrate. In those systems where screening charge is used in excitation Auger decay, screening direcly determines the lifetime, which in turn can exponentially affect the yield. Reductions in screening, e.g. induced by coadsorption of electro-negative species, thus can result in giant yield enhancements. As separate issues, a finite U may prevent the fast resonant decay and this increase the yield from two-hole excitations, as has been suggested for NO2 dissociation on Pt(111), or may assist in the localization (self-trapping) of two-hole excitations in dense adsorbate layers, as apparently is the case for NO desorption from the same surface. The latter causes the yields from one- and two-hole excitations to differ in their coverage dependence. Finally, CVV Auger spectroscopy, of course, measures the energies of two-hole excitations, which can be correlated with observed stimulated thresholds.

The electronically-stimulated dissociation of NO2 on Pt(111) has been studied through state-selective, time-of-flight detection of the NO product above the surface. The NO leaves as a direct dissociation product resulting from 5-800 eV electron impact, whereas the O atom remains on the surface. The quantum-resolved analysis of the NO reveals fundamental aspects of the stimulated surface process such as dominant excitation channels and dynamics. Because of rapid decay via resonant tunneling from substrate levels, the 3 eV shallow valence excitations prominent in gas-phase photodissociation of NO2 have lifetimes that are too short to produce observable dissociation on the metal surface. Instead, we find a 10-15 eV threshold which corresponds to ionization of 3b2 and double ionization of 1a2 levels which cannot be resonantly filled by substrate electrons and thus have the longest lifetimes relative to Auger decay. Screening of the hole(s) by the metal through the 6a1 orbital of the molecule not only determines the lifetime, but is found to influence the extent of internal excitation in the NO product.

Recently, X-ray diffraction (XRD) and scanning tunneling microscopy (STM) experiments confirmed a theoretical prediction that the surface of the two O-layer, or 5Å, alumina film on NiAl(110) has 50% tetrahedral and 50% octahedral Al-site occupancy, a distorted A-plane of κ-alumina. Now density functional theory (DFT) calculations for a 7Å (i.e., three oxygen layer) alumina film are compared with experiments on Ni3Al(110). To minimize the surface energy, we assume that the first two planes of the thicker film are the A- and B- planes of the κ-phase. As with the thinner film, significant distortions occur from the crystalline form. However, a 2×1 unit cell occurs with rows separated by ∼1nm. STM of this film confirms this structure. Finally, an earlier theoretical conjecture is confirmed using low energy electron diffraction (LEED) on the same film: the bottom oxygen layer is chemisorbed on a plane of Al(111), and these are rotated against the substrate to produce the observed domain structures. Thus this system is tri-layered as κ-alumina (A–B planes)/O(111) on Al(111)/Ni3Al.

Thermionic energy conversion in a miniature format shows potential as a viable, high efficiency, micro to macro-scale power source. A microminiature thermionic converter (MTC) with inter-electrode spacings on the order of microns has been prototyped and evaluated at Sandia. The remaining enabling technology is the development of low work function materials and processes that can be integrated into these converters to increase power production at modest temperatures (800 - 1300 K). The electrode materials are not well understood and the electrode thermionic properties are highly sensitive to manufacturing processes. Advanced theoretical, modeling, and fabrication capabilities are required to achieve optimum performance for MTC diodes. This report describes the modeling and fabrication efforts performed to develop micro dispenser cathodes for use in the MTC.

Density functional theory is used to predict workfunctions, φ. For relaxed clean W(100), the local density approximation (LDA) agrees with experiment better than the newer generalized gradient approximation, probably due to the surface electron self-energy. The large Ba metallic radius indicates it covers W(100) at about 0.5 monolayer (ML). However, Ba2+, O2−, and metallic W all have similar radii. Thus 1 ML of BaO (one BaO unit for each two W atoms) produces minimum strain, indicating commensurate interfaces. BaO (1 ML) and Ba (1/2 ML) have the sameφ to within 0.02 V, so at these coverages reduction or oxidation is not important. Due to greater chemical activity of ScO vs. highly ionic BaO, when mixing the latter with this suboxide of scandia, the overlayer always has BaO as the top layer and ScO as the second layer. The BaO/ScO bilayer has a rocksalt structure, suggesting high stability. In the series BaO/ScO/, BaO/YO/, and BaO/LaO/W(100), the latter has a remarkably low φ of 1.3 V (LDA), but 2 ML of rocksalt BaO also has φ at 1.3 V. We suggest BaO (1 ML) does not exist and that it is worthwhile to attempt the direct synthesis and study of BaO (2 ML) and BaO/LaO.

Chambers et al. [Science 297 (2002) 827] recently reported room temperature laminar growth of Co deposited in vacuum from an evaporation source on fully hydroxylated but otherwise clean α-Al2O3(0 0 0 1). We extend this work to a number of metals using first-principles calculations. The exothermicity of the suggested core reaction, 2OH− + M(a) → H2 + 2O2− + M2+, where M represents any metal, is investigated for Cr, Fe, Ni, Cu, Mo, Ru, Rh, Pd, and Al. We find that the reactions are strongly exothermic for most metals. Rh is slightly endothermic, but has sufficient heat of adsorption to react immediately upon contact. The behavior of Cu cannot be determined within the current accuracy of density functional theory, and Pd is strongly endothermic. This suggests noble metals will not react. By first-principles molecular dynamics simulations of Rh, we show that the likelihood of a direct “hot” reaction, driven by the heat of adsorption, is significant.