Scientific Publications 2005
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2005. "Single Particle Laser Ablation Time-of-Flight Mass Spectrometer: An Introduction to SPLAT." Aerosol Science and Technology 39(6):554-568. Abstract We present our single particle mass spectrometer we call SPLAT. SPLAT was designed to make possible the detection and characterization of particles down to 100nm, generate reproducible single particle mass spectra, and collect data at a high rate. The final instrument presented here is capable of characterizing individual particles down to 50nm at the rate of 20 particles per second. In SPLAT, mass spectra are generated by a two-step process, that uses a pulsed infrared laser to heat the particle and a time delayed pulsed UV laser to create ions. We describe a mode of operation that makes it possible to take advantage of the gas phase ionization of the semivolatile components of the particle, while also generating mass spectral signatures of the nonvolatile fraction, thereby providing complete particle mass spectra. We present some sample results from two field deployments of SPLAT.
2005. "High Precision Density Measurements of Single Particles: The Density of Metastable Phases." Aerosol Science and Technology 39(10):972-986. Abstract We describe a system designed to measure the size, composition and density of individual particles in real-time. It uses a DMA to select a monodisperse particle population and the single particle mass spectrometer to measure individual particle mass spectrometer to measure individual particle aerodynamic diameter and composition. Mobility and aerodynamic diameters are used to extract particle density. The addition of individual particle density to the mass spectrum is intended to improve the data classification process. In the present paper we demonstrate that the system has the requisite accuracy and resolution to make this approach practicable. We also present a high precision variant that uses an internal calibrant to remove any of the systematic errors and significantly improves the measurement quality. The high precision scheme is most suitable for laboratory studies making it possible to follow slight changes in particle density. An application of the system to measure the density of hygroscopic particles of atmospheric importance in metastable phases near zero relative humidity is presented. The density data are consistent with conclusions reached in a number of other studies that some particle systems of atmospheric significance once deliquesced persist as droplets down to near zero relative humidity.
2005. "Chemisorption Sites of CO on Small Gold Clusters and Transitions from Chemisorption to Physisorption." Journal of Chemical Physics 122(5):051101-1-4. Abstract Gold clusters adsorbed with CO, Aum(CO)n- (m=2-5; n=0-7), were studied by photoelectron spectroscopy (PES). The first few CO adsorption was observed to induce significant red-shifts to the PES spectra relative to pure gold clusters. For each Au cluster, a critical CO number (nc) was observed, beyond which the PES spectra of Aum(CO)n- change very little with increasing n. nc was shown to correspond exactly to the available low coordination sites in each Au cluster. CO first chemisorbs to these sites and additional CO then only physisorbs to the chemisorption-sautrated Aum(CO)n-complexes.
2005. "Unique CO Chemisorption Properties of Gold Hexamer: Au₆(CO)n⁻(n = 0-3)." Journal of the American Chemical Society 127(34):12098-12106. Abstract Elucidating the chemisorption properties of CO on gold clusters is essential to understanding the catalytic mechanisms of gold nanoparticles. The gold hexamer Au₆ is a highly stable cluster, which is known to possess a D₃h triangular ground state structure with an extremely large HOMO-LUMO gap. Here we report a photoelectron spectroscopy (PES) and quasi-relativistic density functional theory (DFT) study of Au₆-CO complexes, Au₆(CO)n⁻ and Au₆(CO)n (n = 0-3). CO chemisorption on Au₆ was observed to be highly unusual. While the electron donor capability of CO is known to decrease the electron binding energies of Aum(CO)n⁻ complexes, CO chemisorption on Au₆ was observed to have very little effect on the electron binding energies of the first PES band of Au₆(CO)n⁻ (n = 1-3). However, the second PES band is significantly red-shifted upon successive CO chemisorption, resulting in a rapid closing of the HOMO-LUMO energy gap from 2.30 eV in Au₆ to 1.72, 1.45, and 1.17 eV for Au₆(CO)n (n = 1-3), respectively. Extensive DFT calculations showed that the first three CO successively chemisorb to the three apex atoms of the D₃h Au₆. It is shown that the LUMO (6a₁’) and LUMO+1 (8e’) of Au₆ are energetically near-degenerate, which are well separated from the HOMO (7e’), giving rise to the unusually large HOMO-LUMO gap in Au₆. Upon CO chemisorption, the degeneracy of the HOMO and LUMO orbitals are both lifted, leading to a₁+b₂ components under C₂v symmetry. In the Au₆(CO)n complexes, one of the a₁+ b₂ components of the LUMO+1 orbital, which mainly involves the inner triangle of the Au₆ motif, becomes the LUMO. Thus CO chemisorption on the apex Au sites (outer triangle) has little effect on this orbital, resulting in the roughly constant electron binding energies for the first PES band in Au₆(CO)n⁻ (n = 0-3). On the other hand, the a₁+b₂ components of the HOMO of Au₆ are significantly destabilized through HOMO-LUMO mixing in Au₆ and electron donation from the 5σ orbital of CO, resulting in the smaller HOMO-LUMO gaps observed in the Au₆(CO)n complexes.
2005. "Electronic and Structural Evolution and Chemical Bonding in Ditungsten Oxide Clusters: W₂On⁻ and W₂On (n = 1-6)." Journal of Physical Chemistry A 109(27):6019-6030. Abstract We report a systematic and comprehensive investigation of the electronic structures and chemical bonding in a series of ditungsten oxide clusters, WOn⁻ and WOn (n = 1-6), using anion photoelectron spectroscopy and density functional theory (DFT) calculations. Well-resolved photoelectron spectra were obtained at several photon energies (2.331, 3.496, 4.661, 6.424, and 7.866 eV) and W 5d-based spectral features were clearly observed and distinguished from O 2p-based features. More complicated spectral features were observed for the oxygen-deficient clusters due to the W 5d electrons. With increasing oxygen content in WOn⁻, the photoelectron spectra were observed to shift gradually to higher binding energies, accompanied by a decreasing number of W 5d-derived features. A behavior of sequential oxidation as a result of charge transfers from W to O was clearly observed. A large energy gap (2.8 eV) was observed in the spectrum of WO₆⁻, indicating the high electronic stability of the stoichiometric WO₆ molecule. Extensive DFT calculations were carried out to search for the most stable structures of both the anion and neutral clusters. Time-dependent DFT method was used to compute the vertical detachment energies and compare to the experimental data. Molecular orbitals were used to analyze the chemical bonding in the ditungsten oxide clusters and elucidate their electronic and structural evolution.
2005. "Photoelectron Spectroscopy of Doubly and Singly Charged Group VIB Dimetalate Anions: M2O72-, MM'072-, and M207- (M, M'=Cr, Mo, W)." Journal of Physical Chemistry A 109(46):10512-10520. doi:10.1021/jp055122y Abstract We produced both doubly and singly charged Group VIB dimetalate species-M2O7 2-, MM'O72-, and M2O7 - (M, M'=) Cr, Mo, W)susing two different experimental techniques (electrospray ionization for the doubly charged anions and laser vaporization for the singly charged anions) and investigated their electronic and geometric structures using photoelectron spectroscopy and density functional calculations. Distinct changes in the electronic and geometric structures were observed as a function of the metal and charge state. The electron binding energies of the heteronuclear dianions MM'O7 2- were observed to be roughly the average of those of their homonuclear counterparts (M2O7 2- and M'2O7 2-). Density functional calculations indicated that W2O7 2-, W2O7-, and W2O7 possess different ground-state structures: the dianion is highly symmetric (D3d,1A1g) with a single bridging oxo ligand, the monoanion is a doublet (C1, 2A) with two bridging oxo ligands and a radical terminal oxo ligand, whereas the neutral is a singlet (C1, 1A) with two bridging oxo ligands and a terminal peroxo ligand. The combined experimental and theoretical study provides insights into the evolution of geometric and electronic structures as a function of charge state. The clusters identified might provide insights into the possible structures of reactive species present in early transition-metal oxide catalysts that are relevant to their reactivity and catalytic function.
2005. "Potassium Chloride Nanowire Formation Inside a Microchannel Glass Array." Applied Physics Letters 86:Article 263110. Abstract The synthesis of KCl nanowires has been achieved by atomic layer deposition inside high aspect ratio channels of microchannel glass. The average diameter of the KCl nanowires is 250 nm, with a minimum observed diameter of 50 nm, and lengths up to 5 µm. The Cl precursor was TaCl5, while the source of K was determined to be impurities in the microchannel glass substrate. The process for KC1 nanowire formation is a three-step chemical process that simultaneously etches K from the substrate concomitant with the formation of chlorine gas. It is postulated that the curvature of the channels may influence the diameters of the KCl nanowires.
2005. "Synthesis and Luminescence of ZnMgS:Mn2+ Nanoparticles." Journal of Nanoscience and Nanotechnology 5(9):1465-1471. Abstract Efficient green emission from ZnMgS:Mn2+ nanoparticles prepared by co-doping Mg2+ and Mn2+ ions into ZnS lattices has been observed. The synthesis is carried out in aqueous solution, followed by a post-annealing process, thus showing the features of less complexity, low cost, and easy incorporation of dopants. In comparison with the emission of ZnS:Mn2+ nanoparticles, which is located generally around 590 nm, the photoluminescence of ZnMgS:Mn2+ nanoparticles is blue-shifted by 14 nm in wavelength, leading to the enhanced green emission. The X-ray diffraction, electron spin resonance, and pressure dependent photoluminescence measurements suggest that the change of the crystal field caused by Mg2+ ionic doping and the lower symmetry in the nanoparticles may account for the blue-shift of the photoluminescence. The ZnMgS:Mn2+ nanoparticles with 1% Mn2+ doping exhibit the strongest luminescence, which could potentially meet the requirements for the construction of green light emitting diodes.
2005. "Mixed-Metal Pt Monolayer Electrocatalysts for Enhanced Oxygen Reduction Kinetics ." Journal of the American Chemical Society 127:12480-12481. doi:10.1021/ja053695i Abstract An abstract is not available at this time for this article.
2005. "Studies of Damage Accumulation in 4H Silicon Carbide by Ion-Channeling Techniques." Materials Science Forum 475-479(1-5):1341-1344. Abstract Single crystal 4H-SiC was irradiated with 2 MeV Au ions at 165 K. Ion-induced defect configurations and damage accumulation were studied by ion-channeling techniques along the <0001>, <4403> and <2201> directions. A nonlinear dependence of damage accumulation is observed for both the Si and C sublattices along all three directions, and the relative disorder observed along the <4403> and <2201> directions is much higher than that along the <0001> direction. The damage accumulation can be described by a disorder accumulation model, which indicates that defect-stimulated amorphization is the primary amorphization mechanism in SiC, and the high disorder level for the large off-axis angles is partially attributed to a geometrical effect. Molecular dynamics (MD) simulations demonstrate that most single interstitial configurations are shielded by Si and C atoms on the lattice sites along the <0001> direction, which significantly reduces their contribution to the yield along the <0001> direction.
2005. "Ion-Induced Damage Accumulation and Electron-Beam-Enhanced Recrystallization in SrTiO3." Physical Review. B, Condensed Matter and Materials Physics 72(9):094112, 1-8. Abstract Damage accumulation in strontium titanate (SrTiO3) from 1.0 MeV Au irradiation has been investigated at temperatures from 150 to 400 K. The relative disorder on the Sr and Ti sublattices at the damage peak has been determined as a function of local dose and temperature. A disorder accumulation model has been fit to data from this study and from the literature, indicating that defect-stimulated amorphization is the primary amorphization mechanism up to ~360 K. High-dose irradiation at 400 K leads to formation of an amorphous surface layer. Analyses of the temperature dependence for amorphization indicate that the amorphization kinetics are consistent with irradiation-enhanced and thermal recovery processes with activation energies of 0.1 eV and 0.7 eV, respectively. Under 200 keV electron-beam irradiation, the epitaxial recrystallization rates are orders of magnitude higher than thermal rates, and an activation energy of 0.1 eV is determined for the e-beam enhanced recrystallization processes.
2005. "Experimental Determination of Electronic Stopping for Ions in Silicon Dioxide." Applied Physics Letters 87(10):104103. Abstract The electronic energy loss for 4He, 7Li, 9Be, 12C, 16O, 19F and 28Si ions in self-supported SiO2 films has been measured in transmission geometry using a time of flight detection system over a continuous range of energies. SRIM (The Stopping and Range of Ions in Matter) predicts stopping power well for He and C ions within the stated uncertainties. Deviations around the Bragg peak for Li, Be and C ions and different energy dependence for Be, O, F and Si ions, as compared with the SRIM predictions, are observed. The results indicate that the modified Bohr formula is suitable for scaling the stopping number of heavy ions (except He, Li and Be) in the classical interaction regime. The measured electronic stopping powers are parameterized for easily implementation in other applications.
2005. "Performance of the DFTB method in comparison to DFT and semiempirical methods for geometries and energies of C20-C86fullerene isomers." Chemical Physics Letters 412:210-216. doi:10.1016/j.cplett.2005.06.105 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The performance of both non-iterative (NCC) and self-consistent charge (SCC) versions of the density functional tight binding (DFTB) method, as well as AM1 and PM3 methods, has been compared with the B3LYP method, a hybrid density functional theory (DFT) method, for equilibrium geometries and relative energies of various isomers of C20–C86 fullerenes. Both NCC- and SCCDFTB methods compare very favorably with B3LYP both in geometries and isomer relative energies, while AM1 and PM3 do noticeably worse.
2005. "Performance of the DFTB method in comparisonto DFT and semiempirical methods for geometriesand energies of C20–C86 fullerene isomers." Chemical Physics Letters 412(1-3):210-216. doi:10.1016/j.cplett.2005.06.105 Abstract An abstract for this journal article is not available at this time.
2005. "Electronic Structure Differences in ZrO2 vs. HfO2." Journal of Physical Chemistry A 109(50):11521-11525. Abstract While ZrO2 and HfO2 are, for the most part, quite similar chemically, subtle differences in their electronic structures appear to be responsible for differing MO2/Si (M = Zr, Hf) interface stabilities. In order to shed light on the electronic structure differences between ZrO2 and HfO2, we have conducted joint experimental/theoretical studies. Since electron affinities are a sensitive probe of electronic structure, we have measured them by conducting photoelectron spectroscopic experiments on ZrO2- and HfO2-. The electron affinity of HfO2 was determined to be 2.14 0.03 eV, while that of ZrO2 was determined to be 1.64 0.03 eV. Concurrently, electronic structure calculations were conducted to determine electron affinities, vibrational frequencies, and geometries of these systems. The calculated electron affinities of HfO2 and ZrO2 were found to be 2.05 and 1.62 eV, respectively. The molecular results confirm earlier predictions from solid phases that HfO2 is more ionic than ZrO2. The excess electron in MO2- occupies an sd-type hybrid orbital localized on the M atom (M=Zr, Hf). The structural parameters of ZrO2 and HfO2 were found to be very similar. The difference in geometries between the neutral and the anion is along the symmetrical stretching and bending modes. Together, these studies unveil significant differences in the electronic structures of ZrO2 and HfO2.
2005. "Oxidative Remobilization of Biogenic Uranium (IV) Precipitates: Effects of Iron (II) and pH." Journal of Environmental Quality 34(5):1763-1771. Abstract The oxidative dissolution of biogenic U(IV) precipitates was investigated in bioreduced sediment suspensions in contact with atmospheric O2 with an emphasis on the influence of Fe(II) and pH on the rate and extent of U release from the solid to the aqueous phase. The sediment was collected from the US Department of Energy (DOE) Field Research Center (FRC) site at Oak Ridge, Tennessee. Biogenic U(IV) precipitates and bioreduced sediment were generated through anaerobic incubation with a dissimilatory metal reducing bacterium Shewanella putrefaciens strain CN32. The oxidative dissolution of freshly prepared and aged biogenic U(IV) was conducted in 0.1 mol/L NaNO3 electrolyte with variable pH and Fe(II) concentrations. Biogenic U(IV)O2(s) was oxidized with the highest rate and extent at pH 4 and 9. U release to the aqueous phase was the lowest at circumneutral pH. Increasing Fe(II) significantly decreased the release of U(VI) to the aqueous phase. From 70 to 100% of the U in the sediments was extractable at the experiment termination (40-80 days) with a bicarbonate solution (0.2 mol/L), indicating that biogenic U(IV) was oxidized regardless of Fe(II) concentration and pH. Sorption experiments and modeling calculations indicated that the inhibitive effect of Fe(II) on U(IV) oxidative remobilization was consistent with the Fe(III) oxide precipitation and U(VI) sorption to this secondary phase.
2005. "Hydrophilic Properties of Aged Soot." Geophysical Research Letters 32:LO1807. Abstract The global presence of soot has significant effects on regional and global climate, as well as human health. Influence of soot on radiation budget, rain formation and heterogeneous chemistry, and its residence time in the atmosphere are largely dependent on its ability to interact with water. While freshly emitted soot is extremely hydrophobic oxidation during aging causes soot to become more hydrophilic. Laboratory studies demonstrate that aged soot attracts and retains water, and can be efficiently removed from the troposphere by entrapment in existing liquid clouds droplets or by activation as cloud condensation nuclei.
