Scientific Publications 2004
2004. "High Temperature Oxidation Resistance and Surface Electrical Conductivity of Stainless Steels with Filtered Arc Cr-Al-N Multilayer and/or Superlattice Coatings." Surface & Coatings Technology 188-189:55-61. Abstract The requirements for low cost and high-tempurater corrosion resistance for bipolar interconnect plates in solid oxide fuel cell (SOFC) stacks has directed attention to the use of metal plates with oxidation resistant coatings. Candidate coatings must exhibit chemical and thermal-mechanical stability and high electrical conductivity during long-term (>400,000 hrs) exposure to SOFC operatong conditions. The high temperature oxidation resistance and surface electrical donductivity of 304, 440A,a dn Crofer-22 APU steel coupons, with and without multilayer and/or superlattice coatings from a Cr-Al-N system were investigated as a function of exposure in an oxidization atmosphere at high temperatures. The coatins were deposited using large area filtered arc depsition (LAFAD) technology , and subsequently annealed in air at 800 degrees C for varying times. Area specific resistance and activation energy for electrical conductivity of oxidized coupons were measured using a 4-point technique with Pt paste for electrical contact between facing oxidized coupon surfaces. The surface compositon, structure and morphology of the coupons were characterized using RBS, nuclear reaction analysis, XPS, SEM, and AFM techniques. The structure of the CRN/CrAlN multilayered superlattice coatings was characterized by TEM. By altering the architecture of the coating layers, both surface electrical conductivity and oxidation resistance  improved signigicantly for some of the coated samples tested up to ~100hrs.
2004. "Atomic Computer Simulations of Defect Migration in 3C and 4H-SiC." Materials Science Forum 457-460(2004):457-460. Abstract Knowledge of the migration of intrinsic point defects is crucial to understand defect recovery, various annealing stages and microstructural evolution after irradiation or ion implantation. Molecular dynamics (MD) and the nudged-elastic band method have been applied to investigate long-range migration of point defects in SiC over the temperature range from 0.36 to 0.95 Tm , and the defect diffusion coefficient, activation energy and defect correlation factor have been determined. The results show that the activation energies for C and Si interstitials in 3C-SiC are about 0.74 and 1.53 eV, respectively, while it is about 0.77 eV for a C interstitial in 4H-SiC. The minima energy paths reveal that the activation energies for C and Si vacancies are about 4.1 and 2.35 eV, respectively. Finally, the results are discussed and compared with experimental observations and available ab initio data.
2004. "Atomistic Study of Intrinsic Defect Migration in 3C-SiC." Physical Review. B, Condensed Matter and Materials Physics 69(24):245205, 1-5. Abstract Atomic-scale computer simulations, both molecular dynamics (MD) and the nudged-elastic band method, have been applied to investigate long-range migration of point defects in 3C-SiC over the temperature range from 0.36 to 0.95 Tm (melting temperature). A wide set of diffusion characteristics has been obtained, including the self-diffusion coefficient, activation energy and defect correlation factor. Stable C split interstitials can migrate via the first or second neighbor sites, but the relative probability for the later mechanism is very low. Si interstitials migrate directly from one tetrahedral position to another neighboring equivalent position by a kick-in/kick-out process via a split interstitial configuration. Both C and Si vacancies jump to one of their equivalent sites through a direct migration mechanism. The migration energies obtained for C and Si interstitials are consistent with those obtained experimentally for the recovery processes in irradiated SiC. Also, energy barriers for C interstitial and vacancy diffusion are in reasonable agreement with ab initio data.
2004. "Intrinsic Defect Properties in GaN Calculated By Ab Initio and Empirical Potential Methods." Physical Review. B, Condensed Matter and Materials Physics 70(24):245208-1-245208-8. doi:10.1103/PhysRevB.70.245208 Abstract Density functional theory (DFT) has been used to investigate the formation, properties, and atomic configurations of vacancies, antisite defects and interstitials in GaN, and the DFT results are compared with those calculated by molecular dynamics (MD) simulations using two representative potentials. The DFT calculations reveal that the relaxation of vacancies is generally small, but the relaxation around antisite defects is large, especially for the Ga antisite that is not stable and converts to a N+-N<0001> split interstitial plus a Ga vacancy at the original site. The N interstitials, starting from all possible sites, eventually relax into a N+-N<11-20> split interstitial. In the case of Ga interstitials, the most stable configuration is a Ga octahedral interstitial, but the energy difference among all the interstitials is small. The Ga+-Ga<11-20> split interstitial can bridge the gap between non-bonded Ga atoms, thereby leading to a chain of four Ga atoms along the <11-20> direction in GaN. The formation energies of vacancies and antisite defects obtained using the Stillinger-Weber potential (SW) are in reasonable agreement with those obtained by DFT calculations, whereas the Tersoff-Brenner (TB) potential better describes the behavior of N interstitials. In the case of Ga interstitials, the most stable configuration predicted by the TB-model is a Ga+-N<11-20> split interstitial; while for the SW-model the Ga tetrahedral configuration is more stable, which is in contrast to DFT results.
2004. "Mechanical Properties and Elastic Constants Due to Damage Accumulation and Amorphization in SiC." Physical Review. B, Condensed Matter and Materials Physics 69(22):224108, 1-10. Abstract Damage accumulation due to cascade overlap, which was simulated previously, has been used to study the changes of elastic constants, bulk and elastic moduli as a function of dose. These mechanical properties generally decrease with increasing dose, and the rapid decrease at low-dose level indicates that point defects and small clusters play an important role in the changes of elastic constants rather than topological disorder. The internal strain relaxation has no effect on the elastic constants, C11 and C12, in perfect SiC, but it has a significant influence on all elastic constants calculated in damaged SiC. The elastic constants in the cascade-amorphized (CA) SiC decrease about 19%, 29% and 46% for C11, C12 and C44, respectively. The bulk modulus decrease 23% and the elastic modulus decreases 29%, which is consistent with experimental measurements. The stability of both the perfect SiC and CA-SiC under hydrostatic tension has been also investigated. All mechanical properties in the CA-SiC exhibit behavior similar to that in perfect SiC, but the critical stress at which the CA-SiC becomes structurally unstable is one order of magnitude smaller than that for perfect SiC.
2004. "Structures and Energetics of Defects: A Comparative Study of 3C- and 4H-SiC." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 218:74-79. Abstract The structures, formation energies and stable configurations of elementary defects (vacancies, antisite defects and self interstitials) in 3C- and 4H-SiC are studied using classical molecular dynamics simulation with a recently developed interatomic potential. The defect structures in 3C-SiC are relatively simple, but those in 4H-SiC are more complicated. The interstitials between hexagonal and trigonal rings are characteristic for 4H-SiC and other hexagonal polytypes, but not for 3C-SiC. The number of non-equivalent defects in 4H-SiC is much higher than that in 3C-SiC, and a considerable difference is found for some complex and anisotropic defects, in particular for the dumbbells D1Si-Si, D1Si-C and D2Si-C. The lattice deformation beyond the first nearest neighbor shell, which depends strongly on the polytype structure, plays an important role on these effects. However, the polytypism does not have a significant influence on the structure and energetics of the more compact and isotropic defects, such as vacancies and antisite defects. Despite the complexity of defect configurations, the tetrahedral interstitials have very similar properties in 3C- and 4H-SiC because their first, second and third nearest neighbor shells are identical.
2004. "Wannier Orbitals and Bonding Properties of Interstitial and Antisite Defects in GaN." Applied Physics Letters 85(23):5565-5567. Abstract Intrinsic interstitial and antisite defects in GaN have been studied using density functional theory (DFT), and their configurations, electronic structures and bonding properties have been characterized using the Wannier function. All N interstitial configurations eventually transform into N-N split interstitials, between which two π orbitals exist. The relaxation of a Ga antisite defect also leads to the formation of a N-N split configuration; however, its local Wannier orbitals are remarkably different from the N-N split interstitial. The different local Wannier orbitals around Ga interstitial configurations demonstrate that Ga interstitials are critical defects in GaN. The electronic orbitals of the Ga octahedral interstitial is, for example, greatly delocalized, and there are no covalent bonds formed between the interstitial and the surrounded atoms. The most striking feature is that Ga-Ga split interstitials can bridge the gap between non-bonded Ga atoms, thereby leading to a chain of four metallic-like-bonded Ga atoms in GaN, which may exhibit novel quantum properties.
2004. "Expression of Functional Human Coagulation Factor XIII A-domain in Plant Cell Suspensions and Whole Plants." Protein expression and purification 37(1):89-96. Abstract Coagulation factor XIII, a zymogen present in blood as a tetramer (A2B2) of A- and B-domains, is one of the components of many “wound sealants” which are proposed for use or currently in use as effective hemostatic agents, sealants and tissue adhesives in surgery. After activation by -thrombin cleavage, coagulation factor XIII A-domain, a transglutaminase, is formed and catalyzes the covalent crosslinking of the - and -chains of linear fibrin to form homopolymers, which can quickly stop bleeding. We have successfully expressed the A-domain of factor XIII in both plant cell cultures and whole plants. Transgenic plant cell culture allows a rapid method for testing production feasibility while expression in whole plants demonstrates an economic production system for recombinant human plasma-based proteins. The expressed factor XIII A-domain had a similar size as that of human plasma-derived factor XIII. Crude plant extract containing recombinant factor XIII A-domain showed transglutaminase activity with monodansylcadaverine and casein as substrates and crosslinking activity in the presence of linear fibrin. The expression of factor XIII A-domain was not affected by plant leaf position.
2004. "Growth and surface structure of vanadium oxide on anatase (001)." Surface Science 559(2-3):201-213. Abstract Oxygen plasma assisted molecular beam epitaxy (OPA-MBE) of vanadium oxide on (1×4)-reconstructed anatase (001) thin films was studied using reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), x-ray diffraction (XRD), and transmission electron microscopy (TEM). XPS and UPS results showed that the vanadium was predominantly in the 5+ oxidation state after deposition of a monolayer at 525 K. After 1 ML of vanadia was deposited, the anatase (1×4)/(4×1) LEED and RHEED patterns were replaced by (1×1) patterns indicating that the vanadia lifts the reconstruction and suggesting that the monolayer is pseudomorphic. At 525 K, the V⁵⁺ oxidation state predominated in thicker films, however, no discernible LEED or RHEED patterns were seen after a few monolayers were deposited indicating that V₂O₅ epitaxy cannot be continued beyond 1 ML. When the growth temperature was increased to 750 K, RHEED patterns indicated no change in the surface structure after more than 20 ML of vanadia were deposited. Under these conditions, XPS peak positions were consistent with VO₂. After growth at 775 K a c(2x2) LEED pattern attributed to half a monolayer of adsorbed oxygen on the VO₂ surface was observed. The surface characterization data all pointed towards pseudomorphic growth of VO₂ with a half monolayer of capping oxygen allowing the monolayer to achieve the V₂O₅ stoichiometry while maintaining the anatase structure. Bulk XRD data, however, were consistent with VO₂ (B), V₆O₁₃, and rutile VO₂ none of which expose surfaces with the periodicity observed with RHEED and LEED. The reasons for the differences between the surface and bulk characterization are discussed.
2004. "TOF-SIMS Analysis of Sea Salt Particles: Imaging and Depth Profiling in the Discovery of an Unrecogized Mechanism for pH Buffering." Applied Surface Science 231-232:520-523. doi:10.1016/j.apsusc.2004.03.046 Abstract As part of a broader effort at understanding the chemistry of sea salt particles, we have performed time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis of individual sea salt particles deposited on a transmission electron microscopy (TEM) grid. Environmental scanning electron microscopy (ESEM) and TOF-SIMS analysis have, in conjunction with OH exposure studies, led to the discovery of an unrecognised buffering mechanism in the uptake and oxidation of SO2 in sea salt particles in the marine boundary layer. This chemistry may resolve several discrepancies in the atmospheric chemistry literature. Several challenges during the acquisition and interpretation of both imaging and depth profiling data on specific particles on the TEM grid identified by the ESEM were overcome. A description of the analysis challenges and the solutions ultimately developed to them is presented here, along with an account of how the TOF-SIMS data were incorporated into the overall research effort. Several issues unique to the analysis of high aspect ratio particles are addressed.
2004. "TOF-SIMS Analysis of Sea Salt Particles: Imaging and Depth Profiling in the Discovery of an Unrecognized Mechanism for pH Buffering." Applied Surface Science 231-2(Sp. Iss. SI):520-523. Abstract As part of a broader effort at understanding the chemistry of sea salt particles, we have performed time-of-flight secondary ion mass spectroscopy (TOF-SIMS) analysis of individual sea salt particles deposited on a transmission electron microscopy (TEM) grid. Environmental scanning electron microscopy (ESEM) and TOF-SIMS analysis have, in conjunction with OH exposure studies, led to the discovery of an unrecognised buffering mechanism in the uptake and oxidation of SO2 in sea salt particles in the marine boundary layer. This chemistry may resolve several discrepancies in the atmospheric chemistry literature. Several challenges during the acquisition and interpretation of both imaging and depth profiling data on specific particles on the TEM grid identified by the ESEM were overcome. A description of the analysis challenges and the solutions ultimately developed to them is presented here, along with an account of how the TOF-SIMS data were incorporated into the overall research effort. Several issues unique to the analysis of high aspect ratio particles are addressed.
2004. "Dioxygen Activation in Methane Monooxygenase: A Theoretical Study." Journal of the American Chemical Society 126(9):2978-2990. Abstract The abstract for this product is not available at this time.
2004. "A Connection Between Empirical Bond Strength and the Localization of the Electron Density at the Bond Critical Points of the SiO Bonds in Silicates ." Journal of Physical Chemistry A 108:7643-7645. Abstract The empirical bond strength of the SiO bond correlates with the value of the electron density at the bond critical point calculated for a large number of silicates and observed for the silica polymorphs stishovite and coesite. The greater the bond strength, the greater the localization of the electron density at the critical point, the shorter the bond, and the greater the covalent character of the bonded interaction. Bond strength and resonance bond number are considered to represent similar properties of the electronic structure of the bond.
2004. "Effect of Sn on the Reactivity of Cu Surfaces." Journal of Physical Chemistry B 108(37):14062 -14073. doi:10.1021/jp048013c Abstract Periodic, density functional theory (DFT-GGA) calculations, using PW91 (self-consistently) and RPBE functionals, have been employed to determine preferred binding sites, adsorbate structures, and binding energies for the adsorption of atomic (H, N, O, S, and C), molecular (NO and CO), and radical (OH) species on Cu(111) and CuSn(0001) alloy surfaces. Our results indicate the following order in the binding energies from the least to the most strongly bound: NO < CO < H < OH < N < O < S < C for Cu-terminated CuSn(0001). On Cu(111), the corresponding relative order of adsorbates from the least strongly bound to the most strongly bound is CO < NO < H < OH < N < O < S < C. On the Sn-terminated CuSn(0001) surface, CO does not adsorb and the relative order of adsorbates from the least strongly bound to the most strongly bound is NO < H < OH < N < S < O < C. For all adsorbates, the binding on Cu-terminated CuSn(0001) is stronger than on Cu(111), resulting from a combination of electronic and strain effects caused by the addition of Sn to Cu. CO dissociation is endothermic on Cu-terminated CuSn(0001) and Cu(111) surfaces, while CO oxidation is exothermic on these surfaces. OH dissociation is endothermic on all three surfaces. On all surfaces studied, thermodynamics of NO decomposition are much more favorable than those of CO and OH dissociation on the corresponding surfaces. Our microcalorimetric studies of the interaction of NO with Cu/SiO2 and Cu6Sn5/SiO2 samples give initial heats of 270 (2.80 eV) and 130 (1.35 eV) kJ/mol, respectively. These values correspond to the decomposition of NO to give adsorbed oxygen plus gaseous N2 on Cu/SiO2 and adsorbed oxygen plus gaseous N2O on the Sn-terminated phase of Cu6Sn5/SiO2.
2004. "Effect of Sn on the Reactivity of Cu Surfaces." Journal of Physical Chemistry B 108(37):14062-14073. doi:10.1021/jp048013c Abstract Periodic, density functional theory (DFT-GGA) calculations, using PW91 (self-consistently) and RPBE functionals, have been employed to determine preferred binding sites, adsorbate structures, and binding energies for the adsorption of atomic (H, N, O, S, and C), molecular (NO and CO), and radical (OH) species on Cu(111) and CuSn(0001) alloy surfaces. Our results indicate the following order in the binding energies from the least to the most strongly bound: NO < CO < H < OH < N < O < S < C for Cu-terminated CuSn(0001). On Cu(111), the corresponding relative order of adsorbates from the least strongly bound to the most strongly bound is CO < NO < H < OH < N < O < S < C. On the Sn-terminated CuSn(0001) surface, CO does not adsorb and the relative order of adsorbates from the least strongly bound to the most strongly bound is NO < H < OH < N < S < O < C. For all adsorbates, the binding on Cu-terminated CuSn(0001) is stronger than on Cu(111), resulting from a combination of electronic and strain effects caused by the addition of Sn to Cu. CO dissociation is endothermic on Cu-terminated CuSn(0001) and Cu(111) surfaces, while CO oxidation is exothermic on these surfaces. OH dissociation is endothermic on all three surfaces. On all surfaces studied, thermodynamics of NO decomposition are much more favorable than those of CO and OH dissociation on the corresponding surfaces. Our microcalorimetric studies of the interaction of NO with Cu/SiO₂ and Cu₆Sn₅/SiO₂ samples give initial heats of 270 (2.80 eV) and 130 (1.35 eV) kJ/mol, respectively. These values correspond to the decomposition of NO to give adsorbed oxygen plus gaseous N₂ on Cu/SiO₂2 and adsorbed oxygen plus gaseous N₂O on the Sn-terminated phase of Cu₆Sn₅/SiO₂.
2004. "Rotationally Resolved Spectroscopy of the nu(8) Band of cis-Methyl Nitrite." Journal of Molecular Spectroscopy 225(2):182-188. Abstract The 770-880 cm-1 region of the methyl nitrite spectrum has been recorded at a resolution of 0.0015 cm-1 in a static cell. Consistent with published determinations of the barrier to internal rotation of the methyl group, bands belonging to the trans isomer are very congested while those belonging to the cis isomer are more tractable.
2004. "Alloy Catalysts Designed from First Principles." Nature Materials 3 (11):810-815. Abstract The rational design of pure and alloy metal catalysts from fundamental principles has the potential to yield catalysts of greatly improved activity and selectivity. A promising area of research concerns the role that near-surface alloys (NSAs) can play in endowing surfaces with novel catalytic properties. NSAs are defined as alloys wherein a solute metal is present near the surface of a host metal in concentrations different from the bulk; here we use density functional theory calculations to introduce a new class of these alloys that can yield superior catalytic behaviour for hydrogen-related reactions. Some of these NSAs bind atomic hydrogen (H) as weakly as the noble metals (Cu, Au) while, at the same time, dissociating H₂ much more easily. This unique set of properties may permit these alloys to serve as low-temperature, highly selective catalysts for pharmaceuticals production and as robust fuel-cell anodes.
2004. "Development and Evaluation of Gold-Centered Monolayer Protected Nanoparticle Stationary Phases for Gas Chromatography." Journal of Chromatography A 1060(1-2):225-236. Abstract The current status for the development of novel open-tubular gas chromatography (GC) stationary phases consists of thin films of gold-centered monolayer protected nanoparticles (MPNs) is reported. Dodecanethiol MPNs, in which the monolayer is dodecanethiol linked to the gold nanoparticle, have shown great promise as a GC stationary phase with efficient columns having been produced in a variety of capillary i.d.’s with stationary phase film depths ranging from 10-60 nm, +/- 2 nm at a given film depth. Stationary phase operational parameters are discussed including maximum operating temperature, sample capacity, and stationary phase lifetime and robustness.
2004. "High-Speed Gas Chromatography using Synchronized Dual-Valve Injection." Analytical Chemistry 76(13):3517-3524. Abstract A novel injection technique for high-speed gas chromatography (HS-GC) is demonstrated. Synchronized dual-valve injection is demonstrated to provide peak widths as low as 1.5 ms (width-at-half-height) for the unretained analyte using a 0.5 m DB-5 column with an internal diameter of 100 m and a film thickness of 0.4 m using a head pressure of 70 psi at a temperature of 150 °C, for a resulting dead time of only to =26 ms (~1900 cm/s., 26 mL/min). Using the DB-5 column in a 1.0 m length under the same instrumental parameters, with a resulting linear flow velocity of 935 cm/sec (12.7 mL/min of carrier gas, to=117 ms), a minimum peak width of 3.3 ms was obtained. For these 0.5 m and 1.0 m column experiments, the resulting band broadening data is limited almost entirely by the band broadening terms in the Golay equation, and not by extra-column band broadening due to the injection process. During an isothermal separation, ten analytes were separated in a time window of 400 ms.
2004. "Monolayer-Protected Gold Nanoparticles as an Efficient Stationary Phase for Open Tubular Gas Chromatography using a Square Capillary Model for Chip-Based Gas Chromatography in Square Cornered Microfabricated Channels ." Journal of Chromatography A 1029(1-2):185-192. Abstract The application of a dodecanethiol monolayer protected gold nanoparticle (MPN) stationary phase within a microchannel environment was explored using a square capillary column as a model for a high-speed, microfabricated gas chromatography (GC). Successful deposition and evaluation of a dodecanethiol MPN phase within a 1.3 m long, 100 m by 100 m square capillary is reported. Depth of the MPN phase was evaluated using SEM analysis. An average thickness of 15 nm along the capillary walls was determined. While the film depth along the walls was very uniform, the corner depths were greater with the largest observed depth being 430 nm. Overall, an efficient chromatographic system was obtained with a minimum reduced plate height, hmin, of 1.2 for octane (k’ = 0.22). Characterization of the MPN column was completed using four compound classes (alkanes, alcohols, ketones, and aromatics) that were used to form a 7 component mixture with a 2 second separation. A mixture consisting of a nerve agent simulator in a sample containing analytes that may commonly interfere with detection was also separated in 2 seconds, much faster than a similar separation previously reported using a GC system in 50 seconds. Application of the square capillary MPN column for a high-speed separation as the second column of a comprehensive two-dimensional gas chromatography system (GC x GC) was also explored. Comparison of the MPN stationary phase was compared to phases employed in previously reported GC systems.
2004. "Solvation Free Energies and Transfer Free Energies for Amino Acids from Hydrophobic Solution to Water Solution from a Very Simple Residue Model ." Journal of Physical Chemistry B 108(18):5806-5814. Abstract Solvation free energies of neutral amino acids in water and in chloroform were computed from extensive molecular dynamics simulations using multiconfiguration thermodynamic integration. The values computed for the AMBER force field are in very good agreement with available experimental data (rms differences of 5.1 kJ mol⁻¹ for the solvation free energies and 6.4 kJ mol⁻¹ for the transfer free energies of amino acids between water and chloroform) and with existing calculations. We derived an additive residue-scale solvation model formulated as the sum of a nonpolar term that is proportional to the molecular surface area and an electrostatic term (Kirkwood-Onsager model) for the hydration free energy of a dipole in a solvated cavity. This model can surprisingly well describe the solvation free energies in water and chloroform as well as the transfer free energies of amino acids between the two solvents when suitably adapted cavity radii are used. Root-mean-square differences of the predicted values with respect to the values calculated from thermodynamic integration are 1.8, 5.9, and 7.7 kJ mol⁻¹, respectively.