2009. "Molecular Depth Profiling of Sucrose Films: A Comparative Study of C₆₀n⁺ Ions and Traditional Cs⁺ and O₂⁺ Ions." Analytical Chemistry 81(20):8272-8279. doi:10.1021/ac900553z Abstract Time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling of sucrose thin films were investigated using 10 keV C60+, 20 keV C602+, 30 keV C603+, 250 eV, 500 eV and 1000 eV Cs+ and O2+ as sputtering ions. With C60n+ ions, the molecular ion signal initially decreases, and reaches a steady-state that is about 38-51% of its original intensity, depending on the energy of the C60n+ ions. On the contrary, with Cs+ and O2+ sputtering, molecular ion signals decrease quickly to the noise level, even using low energy (250 eV) sputtering ions. In addition, the sucrose/Si interface by C60+ sputtering is much narrower than that of Cs+ and O2+ sputtering. To understand the mechanisms of sputtering-induced damage by these ions, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to characterize the bottoms of these sputter craters. XPS data show very little chemical change in the C60+ sputter crater, while considerable amorphous carbon was found in the O2+ and Cs+ sputter craters, indicating extensive decomposition of the sucrose molecules. AFM images show a very flat bottom in the C60+ sputter crater, while the Cs+ and O2+ sputter crater bottoms are significantly rougher than that of the C60+ sputter crater. Based on above data, we developed a simple model to explain different damage mechanisms during sputtering process.
2009. "Fe-Al interface intermixing and the role of Ti, V, and Zr as a stabilizing interlayer at the interface." Journal of Applied Physics 105(5):Art. No. 053504. Abstract Fe-Al bilayer interfaces with and without interface stabilizing layers (Ti, V or Zr) were fabricated using dc magnetron sputtering. Intermixing layer thickness and the effectiveness of the stabilizing layer (Ti, V, Zr) at the interface were studied using Rutherford backscattering spectrometry (RBS) and X-ray reflectometry (XRR). The result for the intermixing thickness of the AlFe layer is always larger when Fe is deposited on Al as compared to when Al is deposited on Fe. By comparing measurements with computer simulations, the thicknesses of the AlFe layers were determined to be 20.6 Å and 41.1 Å for Al/Fe and Fe/Al bilayer systems, respectively. The introduction of Ti and V stabilizing layers at the Fe-Al interface reduced the amount of intermixing between Al and Fe, consistent with the predictions of model calculations. The Zr interlayer, however, was ineffective in stabilizing the Fe-Al interface in spite of the chemical similarities between Ti and Zr. In addition, analysis suggests that the Ti interlayer is not effective in stabilizing the Fe-Al interface when the Ti interlayer is extremely thin (~3 Å) for these sputtered metallic films.
2009. "Growth and characterization of barium oxide nanoclusters on YSZ(111)." Journal of Physical Chemistry C 113(32):14324-14328. doi:10.1021/jp9020068 Abstract Barium oxide (BaO) was grown on YSZ(111) substrate by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). In-situ reflection high-energy electron diffraction, ex-situ x-ray diffraction, atomic force microscopy and x-ray photoelectron spectroscopy have confirmed that the BaO grows as clusters on YSZ(111). During and following the growth under UHV conditions, BaO remains in single phase. When exposed to ambient conditions, the clusters transformed to BaCO3 and/or Ba(OH)2 H2O. However, in a few attempts of BaO growth, XRD results show a fairly single phase cubic BaO with a lattice constant of 0.5418(1) nm. XPS results show that exposing BaO clusters to ambient conditions results in the formation BaCO3 on the surface and partly Ba(OH)2 throughout in the bulk. Based on the observations, it is concluded that the BaO nanoclusters grown on YSZ(111) are highly reactive in ambient conditions. The variation in the reactivity of BaO between different attempts of the growth is attributed to the cluster size.
2009. " Using Thin Films to Screen Possible Scintillator Materials ." IEEE Transactions on Nuclear Science 56(3, PT 3): 1650-1654. Abstract The discovery and optimization of new scintillators has traditionally been a rather slow process due to the difficulties of single crystal growth. This paper discusses the production of polycrystalline scintillator thin films (a few microns thick) which were tested in order to determine what characterizations could be made concerning a material’s ultimate potential as a scintillator prior to pursuing crystal growth. Thin films of CaF2(Eu), CeF3, and CeCl3, all known scintillators, were produced by vapor deposition. The hygroscopic CeCl3 was coated with multiple polymer-aluminum oxide bi-layers. Emission spectra peak wavelengths and decay times agreed with single crystal values. The films were too thin to measure gamma photopeaks, but using alpha energy deposition peaks, one could compare the relative photon yield/MeV between materials. The values obtained appear to give a relevant indication of a material’s light yield potential. The technique also appears useful for quickly determining the proper dopant amount for a given material.
2009. "Morphology, Orientation Relationship and Stability Analysis of Cu2O nanoclusters on SrTiO3 (100) ." Applied Physics Letters 95(5):Art. No. 053111. doi:10.1063/1.3193530 Abstract Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM) and theoretical studies based on classical nucleation theory have been used to understand the morphology, orientation relationship and stability of Cu2O nanoclusters on SrTiO3 (100) (STO). We propose that the competing interfacial and elastic energies facilitate an in-plane rotation of the Cu2O clusters by 45o with respect to the STO substrate and stabilize Cu2O clusters on STO(100) with an orientation relationship of (001) Cu 2o //(001) SrTiO3 and <100> Cu 2o //<110> SrTiO3. Preliminary theoretical analysis also suggests that this particular orientation results in smaller critical nucleus sizes and lower nucleation barriers. The study also indicates a chemical potential (growth rate) dependence of the orientation relationship.
2009. "Growth-Rate Induced Epitaxial Orientation of CeO2 on Al2O3(0001)." Applied Physics Letters 94(20):204101:1-3. doi:10.1063/1.3139073 Abstract High-quality ceria (CeO2) films were grown on sapphire (Al2O3) (0001) substrates using oxygen plasma-assisted molecular beam epitaxy. The epitaxial orientation of the ceria films has been found to be (100) and (111) at low (< 8 Å/min) and higher growth rates (up to ~30 Å/min), respectively. Evidence shows that CeO2 (100) film grows as three-dimensional islands, while CeO2 (111) proceeds with layered growth. Three in-plane domains at 30° to each other are observed in the CeO2 (100), which is attributed to the close match of the oxygen sub-lattices in the film and substrate that has a three-fold symmetry. Molecular dynamic simulations have further confirmed that the CeO2 film retains (100) orientation on the Al2O3 (0001) substrate.
2009. "Response of Nanocrystalline 3C Silicon Carbide to Heavy-Ion Irradiation." Physical Review. B, Condensed Matter 80(16):Art. No.161301(R). doi:10.1103/PhysRevB.80.161301 Abstract Nanostructured materials are generally believed to be more radiation resistant. This study reports on Au ion induced amorphization in nanocrystalline 3C-SiC, characterized using x-ray diffraction, transmission electron microscopy and Raman spectroscopy. Full amorphization at room temperature occurs at a comparable dose to that for bulk SiC single crystals. The behavior is attributed to a high ion flux and sluggish migration of point defects produced during irradiation. The results may have a significant implication of using nanophased SiC in extremely high radiation environments.
2009. "Influence of samaria doping on the resistance of ceria thin films and its implications to the planar oxygen sensing devices." Sensors and Actuators. B, Chemical 139(2):380-386. doi:10.1016/j.snb.2009.03.021 Abstract In order to evaluate and analyze the effect of samarium (Sm) doping on the resistance of cerium oxide, we have grown highly oriented samaria doped ceria (SDC) thin films on sapphire, Al2O3 (0001) substrates by using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE). The film growth was monitored using reflection high-energy electron diffraction (RHEED) which shows two-dimensional growth throughout the deposition. Following growth, the thin films were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution X-ray diffraction (HRXRD), and Rutherford backscattering spectrometry (RBS). XPS depth-profile shows Sm atoms are uniformly distributed in ceria lattice throughout the bulk of the film. The valence states of Ce and Sm in doped thin films are found to be Ce4+ and Sm3+, respectively. HRXRD shows the samaria doped ceria films on Al2O3(0001) exhibit (111) preferred orientation. Ion-channeling in RBS measurements confirms high quality of the thin films. The resistance of the samaria doped ceria films, obtained by two probe measurement capability under various oxygen pressure (1mTorr-100Torr) and temperatures (623K to 973K), is significantly lower than that of pure ceria under same conditions. The 6Sm% doped ceria film is the optimum composition for highest conductivity. This is attributed to the increased oxygen vacant sites in fluorite crystal structure of the epitaxial thin films which facilitate faster oxygen diffusion through hopping process.
2009. "Oxidative Dissolution Potential of Biogenic and Abiogenic TcO2 in Subsurface Sediments." Geochimica et Cosmochimica Acta 73(8):962-976. doi:10.1016/j.gca.2009.01.027 Abstract Technetium-99 (Tc) is an important fission product contaminant associated with sites of nuclear fuels reprocessing and geologic nuclear waste disposal. Exhibiting an intermediate redox potential, Tc is highly mobile in its anionic, oxidized state [Tc(VII)O4-]; and less mobile as a poorly soluble oxyhydroxide precipitate [Tc(IV)O2•nH2O] in its reduced state. Here we investigate the potential for oxidation of Tc(IV) that was heterogeneously reduced by reaction with biogenic Fe(II) in two sediments differing in mineralogy and aggregation state (FRC, RG). Both sediments contained Fe(III) and Mn(III/IV) as redox active phases, but FRC also contained mass-dominant Fe-phyllosilicates of different types. Biogenic Tc(IV)O2•nH2O was oxidized in anoxic, but unreduced RG and FRC sediments through redox interaction with Mn(III/IV) oxides. Bioreduction by Shewanella putrefaciens CN32 dissolved Mn(III/IV) oxides and generated biogenic Fe(II) that was reactive with Tc(VII) in heat-killed, bioreduced sediment. Biogenic Fe(II) in the FRC exceeded that in RG by a factor of two. More rapid reduction rates were observed in the RG that had lower biogenic Fe(II), and less particle aggregation. EXAFS measurements indicated that the primary reduction product was a TcO2-like phase in both sediments. Redox product Tc(IV) oxidized rapidly and completely in RG when contacted with air. Oxidation, in contrast, was slow and incomplete in the FRC, in spite of similar molecular speciation to RG. X-ray microprobe, electron microprobe, x-ray absorption spectroscopy, and micro x-ray diffraction were applied to the whole sediment and isolated Tc-contained particles. These analyses revealed that non-oxidizable Tc(IV) in the FRC existed as complexes with octahedral Fe(III) within intra-grain domains of 50-100 µm-sized, Fe-containing micas presumptively identified as celadonite. The markedly slower oxidation rates in FRC as compared to RG were attributed to mass-transfer-limited migration of O2 into intra-aggregate and intraparticle domains where Tc(IV) existed; and the formation of unique, oxidation-resistant, intragrain Tc(IV)-Fe(III) molecular species.
2009. "New Approaches for Characterizing Sensor and Other Modern Complex Materials." ECS Transactions 19(6):137-148. doi:10.1149/1.3118546 Abstract Advances in understanding of sensor and other modern complex materials are often enabled by new research tools. This paper highlights three capability development themes used to identify new research tools to be provided to users of the U. S. Department of Energy’s Environmental Molecular Sciences Laboratory. These capability development directions address the importance of dynamic measurements in realistic environments, the need for increased resolution in three dimensional analyses as well as the importance of linking theory and experiment. Capability development involves expanding the range of operation for a number of important techniques, developing and applying new capabilities, and advancing methods of data processing. Examples of current developments are provided including those related to magnetic resonance, x-ray diffraction, application of a focused beam capability to fuel cell aging, and near real time analysis of XPS spectra.
2008. "Growth and structure of epitaxial Ce0.8Sm0.2O1.9 by oxygen-plasma-assisted molecular beam epitaxy." Journal of Crystal Growth 310(2008):2450-2456. Abstract The epitaxial growth of Ce0.8Sm0.2O1.9 films on sapphire (0001) substrate by oxygen-plasma-assisted MBE has been characterized using RHEED, XPS, XRD, AFM, HRTEM and RBS in order to determine their structure and compositions. The composition of the films was determined to be Ce: Sm: O of 0.8:0.2:1.9 by RBS. The film/substrate epitaxial relationship can be written as CeO2 (111)// -Al2O3 (0001) and CeO2 [110]// -Al2O3 . The Ce has only 4+ oxidation state in the films and Sm is fully oxidized in the films with formal oxidation of 3+. CeO2 (111) face is preferred orientation and the thin films are cubic phases.
2008. "Conductivity of Oriented Samaria-Doped Ceria Thin Films Grown by Oxygen-plasma-assisted Molecular Beam Epitaxy." Electrochemical and Solid-State Letters 11(5):B76-B78. doi:10.1149/1.2890122 Abstract We have used oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) to grow highly oriented Ce1-xSmxO2-δ films on single crystal c-Al2O3. The samarium concentration, x, was varied in the range 1-33 atom%. It was observed that dominant (111) orientation in Ce1-xSmxO2-δ films can be maintained up to about 10 samarium atom% concentration. Films higher than 10 atom% Sm concentration started to show polycrystalline features. The highest conductivity of 0.04 S.cm-1, at 600 0C, was observed for films with ~ 5 atom% Sm concentration. A loss of orientation, triggering an enhanced grain boundary scattering, appears to be responsible for the decrease in conductivity at higher dopant concentrations.
2008. "Photoluminescence of SnO2 nanoparticles embedded in Al2O3." Journal of Physics D. Applied Physics 41(22):225102. doi:10.1088/0022-3727/41/22/225102 Abstract Tetragonal Sn nanoparticles of ∼15 nm diameter are produced in Al2O3 by direct Sn implantation at room temperature. After thermal annealing at 1000 ◦C in oxygen, the implantation-induced amorphous region recrystallized and the Sn nanoparticles turned into SnO2 nanoparticles with an average diameter of ∼30 nm as revealed by transmission electron microscopy. While no absorption and photoluminescence (PL) are observed from the metallic Sn nanoparticles, SnO2 nanoparticles exhibit an absorption edge at ∼280 nm and three emission bands at 410 nm, 520 nm and 700 nm, respectively. In addition to the previously reported blue and green emission from SnO2 nanostructures, a red PL band was observed due to the unique surface state of SnO2 nanoparticles embedded in Al2O3 substrate fabricated by ion implantation.
2008. "Tilted domain growth of metalorganic chemical vapor (MOCVD)-grown ZnO(0001)on a-Al2O3(0001)." Journal of Materials Research 23(1):13-17. doi:10.1557/JMR.2008.0026 Abstract ZnO grown on -Al2O3 (0001) generally possesses an orientation such that -Al2O3 (0001)//ZnO(0001) and two in-plane domains nucleate such that: -Al2O3 [11-20]//ZnO[11-20] and/or -Al2O3 [11-20]//ZnO[10-10]. In this paper, we report a new growth mode for ZnO grown on -Al2O3 (0001) using MOCVD. We find that -Al2O3 [11-20]//ZnO[10-10] but the (0001) plane of ZnO is tilted relative to the (0001) plane of -Al2O3 such that ZnO(0001) is almost parallel to the -Al2O3 (-1104) plane. This orientation reduces the extent of lattice mismatch. The interface between ZnO and -Al2O3 is abrupt and possesses periodic dislocations.
2008. "Surface & Interface Properties of 10-12 Unit Cells Thick Sputter Deposited Epitaxial CeO2 Films." Research Letters in Materials Science 2008:Article ID 206019. Abstract Ultra-thin and continuous epitaxial films with relaxed lattice strain can potentially maintain more of its bulk physical and chemical properties and are useful as buffer layers. We study surface, interface and micro-structural properties of ultra-thin (~10-12 unit cells thick) epitaxial ceria films grown on single crystal YSZ substrates. The out-of -plane and in-plane lattice parameters indicate relaxation in the continuous film due to misfit dislocations seen by high-resolution transmission electron microscopy (HRTEM) and substrate roughness of ~1-2 unit cells, confirmed by atomic force microscopy and HRTEM. A combination of secondary sputtering, substrate roughness and surface reduction creating secondary phase were likely causes of surface roughness which should be reduced to a minimum level for effective use of it as buffer layers.
2008. "Interface mixing of Al/Fe and Fe/Al bilayer systems and the role of Ti as a stabilizing interlayer using Rutherford backscattering spectrometry and x-ray reflectometry." Journal of Applied Physics 103(1):Art. No. 014508. doi:10.1063/1.2829803 Abstract Al/Fe and Fe/Al bilayer films with and without a Ti stabilizing interlayer at the interface have been grown on Si wafers using dc magnetron sputtering. X-ray reflectometry and Rutherford backscattering spectrometry were used to probe individual layer thicknesses and intermixing lengths. It is observed that the intermixing length is always higher when the Fe layer is on top of the Al layer. The samples with the Ti stabilizing layer, particularly when the Al layer is on top of the Fe, show that the Ti layer promotes the formation of abrupt interfaces.
2008. "Electronic properties of H and D doped ZnO epitaxial films." Applied Physics Letters 92(15):Art. No. 152105. Abstract ZnO epitaxial films grown by pulsed laser deposition in an ambient of H2 or D2 exhibit qualitatively different electronic properties compared to films grown in vacuum or O2, or bulk single crystals annealed in H2. These include temperature-independent resistivities of ~0.1 -cm, carrier (electron) concentrations in the 1018 cm-3 range, mobilities of 20-40 cm2/V-sec, and negligible (a few meV) activation energies for conduction. These transport properties are consistent with H (D) forming an ultra-shallow donor or conduction band states not achievable by post-growth annealing in H2.
2008. "A Study of H and D doped ZnO epitaxial films grown by pulsed laser deposition." Journal of Applied Physics 104(5):Article no. 053711. doi:10.1063/1.2975219 Abstract We examine the crystal structure, electrical and optical properties of ZnO epitaxial films grown by pulsed laser deposition in a H2 or D2 ambient. We compared with pure ZnO films grown in O2 and vacuum. N-type electrical conductivity is enhanced by two to three orders of magnitude as a result of growing in H2 or D2. Temperature dependent Hall effect measurements reveal small (a few meV) carrier activation energies, along with carrier concentrations of 2-7 x 1018 cm-3, and mobilities of 20-40 cm2/Vs in ZnO films doped with H or D in the 1018 cm-3 range. We have modeled the low-temperature electrical properties of H- and D-doped ZnO films using variable range hopping and surface layer conductivity models, but our data do not fit well with these models. Rather, it appears that growth in H2 or D2 promotes the formation of an exceedingly shallow or conduction-band degenerate donor state, possibly associated with H or D substitution at O sites in the lattice.
2008. "Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films." New Journal of Physics 10:Art. No. 055010. doi:10.1088/1367-2630/10/5/055010 Abstract Epitaxial thin films of cobalt-doped ZnO (Co:ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10-4 – 105 -cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and/or addition of Al during deposition. Despite the high structural quality of the Co:ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co:ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.
2008. "Hidden Ferromagnetic Secondary Phases in Cobalt-doped ZnO Epitaxial Thin Films." Physical Review. B, Condensed Matter 77(20):201303. doi:10.1103/PhysRevB.77.201303 Abstract The quest to discover a dilute magnetic semiconductor which is ferromagnetic at room temperature has led to extensive research on doped semiconducting oxides. However, the wide range of reported properties has raised doubts regarding the presence of intrinsic ferromagnetism in these materials. Here we explore the origin of ferromagnetism in epitaxial Co:ZnO thin films, which are paramagnetic but become weakly ferromagnetic (~0.05 μB/Co) after annealing in Zn vapor to introduce interstitial Zn. Conventional bulk materials characterization techniques indicate no phase segregation or Co reduction has occurred. However, x-ray photoelectron spectroscopy sputter depth profiling clearly indicates the presence of Co(0) in the Zn-treated films; x-ray absorption spectroscopy is utilized to identify the secondary phase as ferromagnetic CoZn (1.5 μB/Co, TC ~ 400 – 450 K). This work demonstrates that the potential for ferromagnetic secondary phases in doped oxides must be thoroughly discounted, through painstaking materials characterization, before claims of intrinsic ferromagnetism can be made.
2008. "Experimentally determined dissolution kinetics of Na-rich borosilicate glass at far from equilibrium conditions: Implications for Transition State Theory." Geochimica et Cosmochimica Acta 72(12):2767-2788. doi:10.1016/j.gca.2008.02.026 Abstract Abstract—The dissolution kinetics of five chemically complex and two chemically simple borosilicate glass compositions (Na-B-Si±Al) were determined over a range of solution saturation values by varying the flow-through rates (1 to 100 mL d-1) in a dynamic single-pass flow-through (SPFT) apparatus. The chemically complex borosilicate glasses are representative of prospective hosts for radioactive waste disposal and are characterized by relatively high molar Si/(Si+Al) and Na/(Al+B) ratios (>0.7 and >1.0, respectively). Analysis by x-ray absorption spectroscopy (XAS) indicates that the fraction of ivB to iiiB (N4) varies from 0.66 to 0.70. Despite large differences in bulk chemistry, values of 29Si peak shift determined by MAS-NMR varies only by about 7 ppm (29Si = -94 to -87 ppm), indicating small differences in polymerization state for the glasses. Forward rates of reaction measured in dynamic experiments converge (average log10 rate [40°C, pH 9] = -1.87±0.79 [g/(m2•d)]) at high values of flow-rate (q) to sample surface area (S). Dissolution rates are independent of total Free Energy of Hydration (FEH) and this model appears to overestimate the impact of excess Na on chemical durability. For borosilicate glass compositions in which molar Na > Al + B, further addition of Na appears to stabilize the glass structure with respect to hydrolysis and dissolution. Compared to other borosilicate and aluminosilicate glasses, the glass specimens from this study dissolve at nearly the same rate (0 to ~55×) as the more polymerized glasses, such as vitreous reedmergnerite (NaBSi3O8), albite, and silica. Dissolution of glass follows the order: boroaluminosilicate glass > vitreous reedmergnerite > vitreous albite > silica glass, which is the same order of increasingly negative 29Si chemical shifts. The chemical shift of 29Si is a measure of the extent of bond overlap between Si and O and correlates with the forward rate of reaction. Thus, dissolution appears to be rate-limited by rupture of the Si—O bond, which is consistent with the tenants of Transition State Theory (TST). Therefore, dissolution at far from equilibrium conditions is dependent upon the speed of the rate-controlling elementary reaction and not on the sum of the free energies of hydration of the constituents of boroaluminosilicate glass.
2008. "Ultraviolet stimulation of hydrogen peroxide production using aminoindazole, diaminopyridine, and phenylenediamine solid polymer complexes of Zn(II)." Journal of Photochemistry and Photobiology. A, Chemistry 197(2-3):245-252. doi:10.1016/j.jphotochem.2007.12.031 Abstract Hydrogen peroxide is a valuable chemical commodity whose production relies on expensive methods. If an efficient, sustainable, and inexpensive solar-mediated production method could be developed from the reaction between dioxygen and water then its use as a fuel may be possible and gain acceptance. Hydrogen peroxide at greater than 10 M possesses a high specific energy, is environmentally clean, and is easily stored. However, the current method of manufacturing H2O2 via the anthraquinone process is environmentally unfriendly making the unexplored nature of its photochemical production from solar irradiation of interest. Here the concentration and quantum yield of hydrogen peroxide produced in an ultraviolet (UV-B) irradiated environment using aromatic and nitrogen-heterocyclic ring complexes of zinc(II) as solid substrates was studied. The amino-substituted isomers of the substrates indazole, pyridine, and phenylenediamine solid polymer complexes are examined. Samples exposed to the ambient atmosphere (e.g., aerated) were irradiated with a low power lamp with emission from 280-360 nm. Irradiation of various zinc complexes revealed Zn-5-aminoindazole to have the greatest first-day production of 63 mM/day with a 37% quantum yield. Para-phenylenediamine (PPAM) showed the greatest long-term stability and thus suggests H2O2 is produced photocatalytically. Isomeric forms of the catalyst’s organic components (e.g., amino groups) did have an effect on the production. Irradiation of diaminopyridine isomers indicated 2,3-diamino and 3,4-diamino structures were the most productive, each generating 32 mM/day hydrogen peroxide. However, the 2,5-diamino isomer showed no peroxide production. A significant decrease in hydrogen peroxide production in all but PPAM was noticed in the samples, suggesting the possibility of a catalyst poisoning mechanism. The samples ability to produce H2O2 is rationalized by proposing a reaction mechanism and examining the stability of the resonance structures of the different isomers.
2008. "A Simple Method for the Prevention of Non-Specific Adsorption by Nanocrystals onto Surfaces." Journal of Nanoscience and Nanotechnology 8(11):5781-5786. doi:10.1166/jnn.2008.320 Abstract In this work we introduce an efficient method for averting non-specific adsorption of various nanoparticles to typical oxide-coated surfaces, such as glass, quartz, and sapphire through the attachment of a fluorinated self-assembled monolayer (SAM) that will minimize the interactions between stabilized nanoparticles and these surfaces. This surface treatment will be shown to be effective for a variety of nanoparticles in a range of solvent systems. As a result, loss of usually expensive nanomaterials to different surfaces can be prevented and the monitoring of nanomaterial processes can be readily performed.
2008. "N incorporation, composition and electronic structure in N-doped TiO2(001) anatase epitaxial films grown on LaAlO3(001)." Surface Science 602(1):133-141. doi:10.1016/j.susc.2007.09.061 Abstract We have investigated the properties of N-doped TiO2 anatase grown by plasma-assisted molecular beam epitaxy on LaAlO3(001) substrates. Phase-pure epitaxial films in which N substitutes for O with no secondary phases formation occur only over a narrow range of fluxes. The N solubility is limited to ~0.2 at. % of the anions and is an order of magnitude lower than that found in N-doped rutile. N substitution for O results in N 2p derived states off the top of the anatase valence band and the associated red shift in the optical bandgap.
2008. "Thermal stability and oxidation resistance of TiCrAlYO coatings on SS430 for solid oxide fuel cell interconnect applications." Surface & Coatings Technology 202(19):4820-4824. doi:10.1016/j.surfcoat.2008.04.059 Abstract Chromia-forming ferritic stainless steels are being considered for interconnects applications in planar solid oxide fuel cell (SOFC) stacks because of their low cost and physical properties. At high temperatures, ferritic steels lack environmental stability in the SOFC operating environment, and gradually degrade the cell performance. In this study, an effective, dense and well adherent TiCrAlYO coating was deposited on an SS430 alloy using the filtered arc deposition technique. High-energy ion backscattering was used to characterize the composition and the thermal stability of the coatings. The chromium volatility of the coated steel plates at 800 oC was also measured using ion beam analysis. Significant reductions in oxidation rates as well as reduced Cr volatility were observed for the coated alloys.
2008. "Growth and characterization of highly oriented gadolinia-doped ceria (111) thin films on zirconia (111)/sapphire (0001) substrates." Thin Solid Films 516(18):6088-6094. doi:10.1016/j.tsf.2007.11.007 Abstract Highly-oriented pure and gadolinia-doped ceria thin films have been grown on pure and ZrO2 (111)-buffered Al2O3 (0001) substrates using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) to understand the oxygen ionic transport processes in ceria based oxide thin films. Gadolinia-doped ceria films grown on pure Al2O3(0001) substrate show polycrystalline features due to structural deformations resulting from the large lattice mismatch between the Al2O3(0001) substrate and the films. However, the films, grown on a thin layer of ZrO2(111) buffered Al2O3 (0001) substrate, appears to be highly oriented. These films were characterized using high resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS) depth profiling. Oxygen ionic conductivity in gadolinia-doped ceria films was measured as a function of Gd concentration and these results were compared with the ion conductance data of the polycrystalline and single crystalline yttria-stabilized zirconia (YSZ).
2008. "Characterization Challenges for Nanomaterials." Surface and Interface Analysis 40(3-4):529-537. doi:10.1002/sia.2726 Abstract Nanostructured materials are increasingly subject to nearly every type of chemical and physical analysis possible. Because of their small feature size there is a significant focus on tools with high spatial resolution. Because of their high surface area, it is also natural to characterize nanomaterials using tools designed to analyze surfaces. Regardless of the approach, nanostructured materials present a variety of obstacles to adequate, useful and needed analysis. This paper provides short overviews to some of the issues and complications including: particle stability, environmental effects, specimen handling, surface coating, contamination and time. Some specific examples are provided from a our work focused on ceria nanoparticles and iron metal-core/oxide-shell nanoparticles in which we use a combination of tools for routine analysis including XPS, TEM, and XRD and apply other methods as needed to obtain essential information.
2007. "Synthesis and Characterization of Compositionally Graded Si1-xGex Layers on Si substrate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 261(1-2):723-726. Abstract Thin film of silicon germanium (Si1-xGex) with tailored composition was grown on Si (100) substrate at 650oC in an ultrahigh vacuum molecular beam epitaxy system. The nominal x-value is ranged from 0 to 0.14. The quality of the film was investigated by Rutherford backscattering spectrometry (RBS) in random and channeling geometries, glancing angle x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDX), and atomic force microscopy (AFM). RBS/Channeling measurements indicate that the strain associated with lattice mismatch is compressive in the film. Both RBS and EDX analyses indicate the compositional graded incorporation of Ge in the film with x ranging from 0 to 0.14. The film shows island growth with each island centering around an interface dislocation.
2007. "Epitaxial Growth and Microstructure of Cu2O Nanoparticle/thin Films on SrTiO3(100)." Nanotechnology 18:Art. No. 115601. doi:10.1088/0957-4484/18/11/115601 Abstract Cuprous oxide (Cu2O) was grown on SrTiO3 (STO)(100) by oxygen plasma assisted molecular beam epitaxy. Microstructure of the grown layer and Cu valence state were analyzed using x-ray diffraction (XRD), x-ray photo-electron spectroscopy (XPS), atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM) as well as electron diffractions. The grown layer was dominated by Cu2O phase, possessing an epitaxial orientation with the substrate such that: Cu2O[001]//STO[001] and Cu2O(100)//STO(100). Cu2O film morphologically shows dependence on the growth rate. Typically, a fast growth will lead to the formation of a thin film with a relatively smooth surface. A slow growth will lead to the development of nanoparticles, featuring the formation of Cu2O pyramid. The pyramids are invariantly defined by the Cu2O {111} planes. Given the fact that the {111} planes correspond to the lowest surface energy of Cu2O, a slow growth will lend the system enough time to allow it to adopt the pyramid configuration by which the overall energy of the system was minimized.
2007. "Nucleation and Growth of MOCVD Grown (Cr, Zn)O Films – Uniform Doping vs. Secondary Phase Formation." Journal of the Electrochemical Society 154(3):D134-D138. doi:10.1149/1.2424422 Abstract We report a detailed study of chromium solubility and secondary phase formation in MOCVD grown (Cr, Zn)O-based films on silicon (100). Simultaneous deposition of 0.15M Cr(TMHD) and 0.025M Zn(TMHD) based precursors in an oxidizing environment with a flow ratio of 1:10 resulted in secondary phase formation rather than uniform Cr doping. Based on several surface and micro-structural techniques, we have identified nano-crystalline ZnCr2O4 and disordered Cr2O3 as the secondary Cr-containing phases that nucleate. Analysis suggests that ZnCr2O4 crystallites are dispersed throughout the film and that disordered Cr2O3 layer may form at the interface. These results reveal a strong tendency for Cr to exist in octahedral, rather than tetrahedral coordination.
2007. "Variation in lattice parameters of 6H-SiC irradiated to extremely low doses." Applied Physics Letters 91(9):091918, 1-3. doi:10.1063/1.2778630 Abstract Irradiation of 6H-SiC single crystals was performed using 4 MeV H+ ions at 340 and 210 K. The changes in lattice parameters in the basal plane and along the c-axes were measured as a function of dose using high-resolution x-ray diffraction. The c-axis lattice parameter increases monotonically with the increasing dose, while a-axis lattice parameter decreases at extremely low doses. An initial volumetric contraction of the unit cell is observed. The decrease in the a parameter may originate from the irradiation-induced vacancies and the possible formation of antisite defects that cause the lattice structure on the basal plane to shrink.
2006. "Synthesis of Room-Temperature Ferromagnetic Cr-doped TiO₂(110) Rutile Single Crystals using Ion Implantation." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):198-200. doi:10.1016/j.nimb.2005.08.149 Abstract Ferromagnetic Cr-doped rutile TiO₂ single crystals were synthesized by high-temperature ion implantation. The associated structural, compositional and magnetic properties were studied by x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, proton induced x-ray emission, x-ray diffraction, Cr K- and L-shell near-edge x-ray absorption spectroscopy, and vibrating sample magnetometry. Cr was distributed uniformly to the depth of about 300 nm with an average concentration of ~1 at. %. The samples are semiconducting and ferromagnetic as implanted, with a saturation magnetization of 0.29B/Cr atom at room temperature. Cr is in a formal oxidation state of +3 throughout the implanted region, and no CrO₂ is detected.
2006. "Magnetic properties of epitaxial Co-doped anatase TiO2 thin films with excellent structural quality." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 24(4):2012-2017. doi:10.1116/1.2216723 Abstract The heteroepitaxy of Co-doped anatase TiO2 on LaAlO3(001) has been refined with the goal of determining the relationship between structural quality and magnetic ordering. By significantly reducing the deposition rate and substrate temperature, well-ordered Co:TiO2 films with unprecedented crystalline quality were obtained by oxygen-plasma-assisted molecular beam epitaxy, as characterized by x-ray diffraction. These films exhibit uniform Co doping, with no evidence of Co segregation or secondary phases throughout the film depth or on the surface. Despite the improvement in crystalline quality and Co distribution, the films exhibit negligible ferromagnetism, with saturation moments of only ~0.1 B/Co. This loss of ferromagnetism is in stark contrast to faster-grown Co:TiO2 films, where a higher growth rate and substrate temperature typically result in lower crystalline quality, a highly non-uniform Co distribution, and average saturation moments of ~1.2 B/Co. The presence of ferromagnetism in faster-grown Co:TiO2 does not appear to arise from intrinsic point defects present in the bulk material, such as charge-compensating oxygen vacancies, but is instead attributed to the presence of extended structural defects.
2005. "Oxygen Transport Studies in Nanocrystalline Ceria Films." Journal of Materials Research 20(5):1295-1299. Abstract Oxygen uptake and conductivity were measured by nuclear reaction analysis and ac impedance technique at the intermediate temperature range on sol-gel grown nanocrystalline ceria films with average grain-sizes 7 nm and 38 nm synthesized at 723 K and 1173 K, respectively. Higher oxygen uptake and lower ionic conductivity are observed in ceria films with ~7 nm grain-size. High permeation-assisted oxygen diffusion in nanocrystallites combined with oxygen trapping in the disordered region contributes in higher oxygen uptake. However the lower ionic conductivity in the film results from absence of long-range lattice ordering. Relationship between oxygen uptake and conductivity in ceria is discussed in details by considering grain-size dependent defect density, related surface area and enhanced oxygen mobility.
2005. "Near-edge X-ray Absorption Fine Structure Study of Disordering in Gd₂(Ti1-yZry)₂O₇ Pyrochlores." Journal of Physical Chemistry B 109(4):1337-1339. Abstract Disorder in Gd₂(Ti1-yZry)₂O₇ pyrochlores, for y=0.0-1.0, is investigated by Ti 2p and O 1s near-edge x-ray absorption fine structure spectroscopy. Ti⁴⁺ ions are found to occupy octahedral sites in Gd₂Ti₂O₇ with a tetragonal distortion induced by vacant oxygen sites. As Zr substitutes for Ti, the tetragonal distortion decreases, and Zr coordination increases from 6 to 8. The migration of oxygen ions from 48f or 8b sites to vacant 8a sites compensate for the increased Zr coordination, thereby reducing the number of vacant 8a sites, which further reduces the tetragonal distortion and introduces more disorder around Ti. This is evidence for simultaneous cation disorder with anion migration.
2004. "Room-Temperature Ferromagnetism in Ion-Implanted Co-Doped TiO₂(110) Rutile." Applied Physics Letters 84(22):4466-4468. Abstract Interest in diluted magnetic semiconductros (DMS) is growing rapidly within the emerging field of spintronics. For example, the ability to efficiently inject spin-polarized carriers into nonmagnetic semiconductor heterostructures creates new and exciting possibilities for utilizing DMS materials in spin-based devices. Several III-V and II-VI semiconductor materials doped with magnetic transition metal elements have been explored. Although these materials show promising behavior in some cases, most exhibit Curie temperatures of ~170 K or less. It has recently been shown that certain oxide semiconductors doped with magnetic transition elements show room-temperature ferromagnetism.
2004. "Room Temperature Ferromagnetism in Ion-implanted Co-doped TiO₂(110) Rutile." Applied Physics Letters 84(22):4466-4468. Abstract Ferromagnetic Co-doped rutile TiO₂ singel crystals were successfully synthesized by high temperature ion implantation and charaterized by a variety of techniques. Co is uniformly distrubuted to the depth of ~300nm with an average concentration of ~2 at. %, except in the near-surface region, where the concentration is ~3 at. %. Magnetic measurements reval ferromagnetic behavior at room temperature with an effective saturation magnetization of ~0.6 ub/Co atom. The Co formal oxidation state is found to be +2 throughouth the implanted region, and no Co(0) is detected.