2009. "Raman and XPS characterization of fuel-cladding interactions using miniature specimens." Journal of Nuclear Materials 383(3):237-243. Abstract Laser Raman spectroscopy was evaluated as a tool for studying fuel-cladding chemical interactions at elevated temperatures. Materials and conditions were selected to simulate the interface of oxide fuels and fission products with high-temperature cladding materials for TRU-MOX fueled reactors. Both ex-situ and in-situ spectroscopy measurements were performed using polished HT-9 disks, uncoated and coated with yttria-stabilized zirconia, that were exposed to air oxidation at temperatures between 873-973K. Ex-situ measurements (under ambient conditions) were conducted to identify oxide phases, determine oxidation mechanisms and approximate film growth rates with an optimal signal-to-noise for the equipment used. Subsequently performed in-situ measurements were used to evaluate the sensitivity of the technique for measurements at elevated temperature in a hot-stage. Raman spectra were supported with x-ray photoelectron spectroscopy depth profiling. The results, which are for non-fueled materials in this study, illustrated a method for fast screening of candidate alloys with actinide-based MOX fuel mixtures.
2009. "Morphology and Electronic Structure of the Oxide Shell on the Surface of Iron Nanoparticles." Journal of the American Chemical Society 131(25):8824–8832. doi:10.1021/ja900353f Abstract A iron nanoparticle exposed to air at room temperature will be instantly covered by an oxide shell of typical thickness of ~ 3 nm. This native oxide shell in combination with an underlying iron core determines the physical and chemical behavior of this type of core-shell nanoparticles. One of the great challenges for characterizing this type of nanoparticles is determination of the structure of the oxide shell, as it is FeO, Fe3O4, -Fe2O3, -Fe2O3, or anything else. Significant research effort, mostly based on x-ray diffraction and spectroscopy and electron diffraction and transmission electron microscopy imaging, has been made to determine the structure of this thin layer of iron oxide. Most of the experimental results have been framed with one of the known iron oxide structures, although it is not necessarily true that this thin layer of iron oxide consists of a standard iron oxide. In this paper, the structure of the oxide shell on iron nanoparticle is probed using electron energy loss spectroscopy (EELS) at O K-edge with a spatial resolution of several nanometers (individual particle). Two types of representative particles were studied: particles that are fully oxidized and core-shell particle which possesses a Fe core. We found that the O K-edge spectra collected on the oxide shell in the nanoparticles shows distinctive differences as compared with that of the known iron oxide. Based on finger printing and quantum mechanical calculations results, we conclude that the distances between the absorbing oxygen and the next-nearest neighbor oxygens are more widely distributed than that in bulk Fe3O4 for both of these two types of particles. For smaller and fully oxidized particles, there is also a broadened distribution between the absorbing oxygen and the nearest neighbor oxygens. These results clearly demonstrate that the coordination configuration in the oxide shell on Fe nanoparticle is defective as compared with that of their bulk counterpart. Of the two types particles examined in this work, the degree of disorder is larger for the smaller fully oxidized particles.
2009. "Fluorescent Dye Encapsulated ZnO Particles with Cell-specific Toxicity for Potential use in Biomedical Applications." Journal of Materials Science. Materials in Medicine 20(1):11-22. Abstract Fluorescein isothiocyanate (FITC)-encapsulated core-shell particles with a nanoscale ZnO finishing layer have been synthesized for the first time as multifunctional “smart” nanostructures for particle tracking and cell imaging using the visible fluorescence emission of the dye or UV fluorescence emission of ZnO, and anti-cancer/antibacterial treatments using the selective toxicity of the nanoscale ZnO outer surface. The chemical phase composition, morphology, size, and the layered core-shell architecture of the particles were characterized using detailed transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-vis-NIR spectrophotometry. Systematic XPS studies after removing nanometer thick layers confirmed the expected layered structure in the order ZnO-SiO2-APTMS-FITC proceeding from the surface to the core of the ~200 nm sized particles. Detailed investigation of the fluorescence properties of these hydrophilic particles in bio-compatible media using fluorescence spectroscopy, flow cytometry and fluorescence confocal microscopy demonstrated that the silica/ZnO outer layer offers considerable protection to the encapsulated dye molecules from photobleaching and quenching due to reactive species such as oxygen in the solvent. These particles showed promise toward cell imaging, for example when the bacterium Escherichia coli was used as a test system, the green fluorescence of the particles allowed confocal microscopy to image the cells. The FITC encapsulated ZnO (FITC-ZnO) particles demonstrated excellent selectivity in preferentially killing Jurkat cancer cells (18% cell viability) without any significant toxicity to normal primary immune cells (75% cell viability) at 60 g/mL concentrations and inhibited the growth of both gram-positive and gram negative bacteria at concentrations ≥ 250-500 g/mL (for Staphylococcus aureus and Escherichia coli, respectively). These results indicate that the novel FITC encapsulated multifunctional particles with nanoscale ZnO surface layer are smart nanostructures for particle tracking, cell imaging, antibacterial treatments and cancer therapy.
2009. "Changes in the quaternary structure of amelogenin when adsorbed onto surfaces." Biopolymers 91(2):103-107. doi:10.1002/bip.21095 Abstract The amelogenin protein is involved in the formation of highly controlled and anisotropic hydroxyapatite crystals in tooth enamel. Amelogenin is unique in that it self assembles to form supramolecular quaternary structures called “nanospheres,” spherical aggregates of amelogenin monomers typically 20-60 nm in diameter. Although nanospheres have been observed in solution, the quaternary structure of amelogenin adsorbed onto surfaces is not well known. A better understanding of the surface structure is of great importance, however, because the function of amelogenin depends on it. We report studies of the adsorption of amelogenin onto self-assembled monolayers (SAMs) containing COOH and CH3 end group functionality as well as single crystal fluoroapatite (FAP), a biologically relevant surface. The supramolecular structures of the protein in solution as determined by dynamic light scattering (DLS) were compared with the supramolecular structures of the protein physisorbed onto surfaces as studied by atomic force microscopy (AFM). We found that although our solutions contained only nanospheres of narrow size distribution, smaller structures such as monomers and dimers were observed onto both hydrophilic and hydrophobic surfaces. This suggests that amelogenin can adsorb onto surfaces as small structures that peel away or “shed” from the nanospheres that are present in solution.
2009. "Adsorption of Amelogenin onto Self-Assembled and Fluoroapatite Surfaces." Journal of Physical Chemistry B 113(7):1833-1842. Abstract Abstract. The interactions of proteins at surfaces are of great importance to biomineralizaton processes and to the development and function of biomaterials. Amelogenin is a unique biomineralization protein because it self-assembles to form supramolecular structures called “nanospheres,” spherical aggregates of monomers that are 20-60 nm in diameter. Although the nanosphere quaternary structure has been observed in solution, the quaternary structure of amelogenin adsorbed onto surfaces is also of great interest because the surface structure is critical to its function. We report studies of the adsorption of the amelogenin onto self-assembled monolayers (SAMs) with COOH and CH3 end group functionality and single crystal fluoroapatite (FAP). Dynamic light scattering (DLS) experiments showed that the solutions contained nanospheres and aggregates of nanospheres. Protein adsorption onto the various substrates was evidenced by null ellipsometry, x-ray photoelectron spectroscopy (XPS), and external reflectance Fourier transform infrared spectroscopy (ERFTIR). Although only nanospheres were observed in solution, ellipsometry and atomic force microscopy (AFM) indicated that the protein adsorbates were much smaller structures than the original nanospheres, from monomers to small oligomers in size. Monomer adsorption was promoted onto the CH3 surfaces and small oligomer adsorption was promoted onto the COOH and FAP substrates. In some cases, remnants of the original nanospheres adsorbed as multilayers on top of the underlying subnanosphere layers. This work suggests that amelogenin can adsorb by the “shedding” or disassembling of substructures from the nanospheres onto substrates and indicates that amelogenin may have a range of possible quaternary structures depending on whether it is in solution or interacting with surfaces.
2009. "A novel low-temperature dendritic cyclotrimerization of 2,6-diacetyl pyridine leading to mesoporous carbon containing pyridine rings." Microporous and Mesoporous Materials 123(1-3):345-348. doi:10.1016/j.micromeso.2009.03.033 Abstract A simple, direct synthesis of a mesoporous carbon containing pyridine rings is described. This synthesis utilizes the SiCl4 induced cyclotrimerization of 2,6-diacetylpyridine to make a dendritic polymer, built of alternating benzene and pyridine rings. The cyclotrimerization allows for a high degree of crosslinking to take place at low temperatures stabilizing the mesostructure and allowing the carbonization to be carried out at only 600°C, the lowest temperature reported to date for an N-doped mesoporous carbon. The functional mesoporous carbon so formed was found to have a surface area of 1275 m2/g, 35Å pores, and contain 6.8% N.
2009. "The Corrosion of PEM Fuel Cell Catalyst Supports and Its Implications for Developing Durable Catalysts." Electrochimica Acta 54:3109-3114. Abstract Studying the corrosion behavior of catalyst support materials is of great significance for understanding the degradation of PEM fuel cell performance and developing durable catalysts. The oxidation of Vulcan carbon black (the most widely-used catalyst support for PEM fuel cells) was investigated using various electrochemical stressing methods (fixed-potential holding vs. potential step cycling), among which the potential step cycling was considered to mimic more closely the real drive cycle operation of vehicle PEM fuel cells. The oxidation of carbon was accelerated under potential step conditions as compared with the fixed-potential holding condition. Increasing potential step frequency or decreasing the lower potential limit in the potential step can further accelerate the corrosion of carbon. The accelerated corrosion of carbon black was attributed to the cycle of consumption/regeneration of some easily oxidized species. These findings are being employed to develop a test protocol for fast screening durable catalyst support.
2009. "Microstructure and Secondary Phase Segregation Correlation in Epitaxial/Oriented ZnO Films with Unfavorable Cr Dopant." Journal of Materials Research 24(2):506-515. doi:DOI: 10.1557/JMR.2009.0054 Abstract We discuss the effect of microstructure on the secondary phase segregation region and mobility of carbon impurities in case of poorly soluble Cr as a dopant in ZnO thin films. Thin films of Cr:ZnO ~50 nm in thickness were grown by metal organic chemical vapor deposition (MOCVD) of Zn(TMHD) and Cr(TMHD) precursors in reactive oxygen partial pressure environment. For an accurate comparison among the differences among the grain-boundary density and degree of orientation on the secondary phase segregation and impurity mobility, simultaneous thin film growths were carried out on single crystals of Si (100), c-plane oriented Al2O3 (c-ALO) and r-plane oriented Al2O3 (r-ALO) substrates. High-resolution transmission electron microscopy (HRTEM) measurements across the film substrate interface indicate that growths on Si(100) and c-ALO resulted in highly oriented Cr:ZnO films whereas a good epitaxial growth was observed on r-ALO. The trace carbon impurity detection, secondary phase formation and their mobility properties were studied by sensitive x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). We have observed that secondary phase segregation regions occur near the surface for Cr:ZnO films grown on ALO whereas the region moves near the interface for the growth on Si. Considering the presence of grain boundaries in Cr:ZnO grown on c-ALO and Si, it appears to be a weak relationship between grain boundary density and unfavorable dopant mobility as well as preferred segregation region. A near uniform stress distribution observed at r-ALO/ZnO interface indicates good epitaxial growth by domain matching epitaxy process. We also observe that low carbon impurity distribution in the studied thickness regime remains more or less uniform inside Cr:ZnO. This gives strong evidence that trace amount of carbon is soluble in the Cr:ZnO system as a direct result of oxygen vacancy defects.
2009. "Defining Active Catalyst Structure and Reaction Pathways from ab Initio Molecular Dynamics and Operando XAFS: Dehydrogenation of Dimethylaminoborane by Rhodium Clusters ." Journal of the American Chemical Society 131(30):10516-10524. Abstract We present the results of a detailed operando XAFS and density functional theory (DFT) based ab initio molecular dynamics (AIMD) investigation of the proposed mechanism of dehydrogenation of dimethylaminoborane (DMAB) by a homogeneous Rh4 cluster catalyst. Our AIMD simulations reveal that the previously proposed Rh structures are highly fluxional exhibiting both metal cluster and ligand isomerizations and dissociaton which can only be accounted for by a examining finite temperature ensemble as generated by AIMD. It is found that a highly fluxional species Rh4((H2BNMe2)82+ is fully compatible with operando XAFS measurements which suggest that this species may be the catalyst resting state. Based on this assignment we propose a catalytic mechanism for DMAB dehydrogenation which exhibits a maximum energy barrier of 24 kcal/mol, which is half that observed for the uncatalyzed thermal reaction. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program, and was performed in part using the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a DOE national scientific user facility located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. Department of Energy.
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. "Interparticle Chiral Recognition of Enantiomers: A Nanoparticle-Based Regulation Strategy." Analytical Chemistry 81(2):689-698. doi:10.1021/ac802119p Abstract The ability to regulate how molecular chirality of enantiomeric amino acids operates in biological systems constitutes the basis of drug design for specific targeting of diseases. We report herein nanoparticle-administered chiral recognition of enantiomeric cysteines as a general molecular tuning strategy to regulate the interactions of chiral amino acids. This strategy exploits the pairwise zwitterion interaction of enantiomeric cysteines adsorbed on gold nanoparticles to create a chiral footprint for interparticle enantiomeric recognition. The chiral recognition is evidenced by the sharp kinetic contrast between interparticle homochiral and heterochiral reactivities. The experimental-theoretical correlation of the interparticle reactivity with the enantiomeric ratio reveals that the chiral recognition is tunable both molecularly and by nanoparticles, which has potential applications in drug design for targeting or delivery.
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. "Enhanced Activity and Stability of Pt catalysts on Functionalized Graphene Sheets for Electrocatalytic Oxygen Reduction ." Electrochemistry Communications 11(5):954-957. Abstract Electrocatalysis of oxygen reduction using Pt nanoparticles supported on functionalized graphene sheets (FGSs) was studied. FGSs were prepared by thermal expansion of graphite oxide. Pt nanoparticles with average diameter of 2 nm were uniformly loaded on FGSs by impregnation methods. Pt-FGS showed a higher electrochemical surface area and oxygen reduction activity with improved stability as compared with commercial catalyst. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical characterization suggest that the improved performance of Pt-FGS can be attributed to smaller particle size and less aggregation of Pt nanoparticles on the functionalized graphene sheets.
2009. "Effects of sulfation level on the desulfation behavior of pre-sulfated Pt BaO/Al2O3 lean NOx trap catalysts: a combined H2 Temperature-Programmed Reaction, in-situ sulfur K-edge X-ray Absorption Near-Edge Spectroscopy, X-ray Photoelectron Spectroscopy, and Time-Resolved X-ray Diffraction Study." Journal of Physical Chemistry C 113(17):7336-7341. doi:10.1021/jp900304h Abstract Desulfation by hydrogen of pre-sulfated Pt(2wt%) BaO(20wt%)/Al2O3 with various sulfur loading (S/Ba = 0.12, 0.31 and 0.62) were investigated by combining H2 temperature programmed reaction (TPRX), x-ray photoelectron spectroscopy (XPS), in-situ sulfur K-edge x-ray absorption near-edge spectroscopy (XANES), and synchrotron time-resolved x-ray diffraction (TR-XRD) techniques. We find that the amount of H2S desorbed during the desulfation in the H2 TPRX experiments is not proportional to the amount of initial sulfur loading. The results of both in-situ sulfur K-edge XANES and TR-XRD show that at low sulfur loadings, sulfates were transformed to a BaS phase and remained in the catalyst, rather than being removed as H2S. On the other hand, when the deposited sulfur level exceeded a certain threshold (at least S/Ba = 0.31) sulfates were reduced to form H2S, and the relative amount of the residual sulfide species in the catalyst was much less than at low sulfur loading. Unlike samples with high sulfur loading (e.g., S/Ba = 0.62), H2O did not promote the desulfation for the sample with S/Ba of 0.12, implying that the formed BaS species originating from the reduction of sulfates at low sulfur loading are more stable to hydrolysis. The results of this combined spectroscopy investigation provide clear evidence to show that sulfates at low sulfur loadings are less likely to be removed as H2S and have a greater tendency to be transformed to BaS on the material, leading to the conclusion that desulfation behavior of Pt BaO/Al2O3 lean NOx trap catalysts is markedly dependent on the sulfation levels.
2009. "Suppression of conductivity in Mn-Doped ZnO Thin Films." Journal of Applied Physics 105(1):013715. doi:10.1063/1.3063730 Abstract We studied the dopant concentration distribution and conductivity in ZnO:Mn films grown by metalorganic chemical vapor deposition (MOCVD). The ion beam, surface and microstructural properties of undoped ZnO films were compared with Mn-doped ZnO films. Suppression of ZnO conductivity was noticed up to ~ 4.5 atom% Mn doping. The presence of Mn2+, confirmed by X-ray photoelectron spectroscopy (XPS), is correlated with the reduction in conductivity. No major change in the activation energy (~40 meV) and a reduction in the Zn/O ratio as a function of Mn concentration in highly sensitive proton induced X-ray emission (PIXE) technique also support this hypothesis. We discuss our results from a view point of homogeneous Mn distribution, elemental XPS ratio offsets and secondary phase formations in ZnO films.
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. "On the relationship between non-stoichiometry and passivity breakdown in ultra-thin oxides: combined depth-dependent spectroscopy, Mott-Schottky analysis and molecular dynamics simulation studies." Journal of Physical Chemistry C 113(9):3502-3511. Abstract Understanding the relationship between non-stoichiometry and physical properties of ultra-thin oxides is of great importance from both scientific and technological aspects. A specific example includes the onset of passivity breakdown in an ultra-thin oxide film in aqueous medium leading to the onset of corrosion. In this work, using the model system of ultra-thin oxide of alumina on aluminum synthesized by natural oxidation and photon-assisted oxidation processes; we demonstrate a direct correlation between passivity and quality of the oxide film quantitatively. Depth-dependent high resolution X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and nuclear reaction analysis (NRA) have been performed to characterize the physical and chemical properties of the oxide films, while detailed impedance measurements and Mott-Schottky studies have been performed to understand electronic transport. Combined NRA and TEM analysis reveal an 18% increase in oxygen density (for oxide films with near identical thicknesses ~3.8nm) in case of photon-assisted oxidation. The denser oxide film results in a ~34% more blockage of chloride ions transport as indicated by XPS analysis. Mott Schottky measurements on these oxide films indicates a 43% reduction of defect levels for UV-synthesized alumina when compared to native one, suggestive of chloride ion transport via oxygen vacancies. Additionally, molecular dynamics simulations have been performed to provide insights into the structure of the oxides at the atomic level to correlate with the experimental measurements. These simulations employ dynamic charge transfer between atoms and are used to investigate nanoscale oxides grown on Al (100) surfaces due to atomic and molecular oxygen. Oxidation using molecular and atomic oxygen resulted in an amorphous oxide scale with self limiting thickness of ~ 16 and 22 Å, respectively at 300 K. Structural and dynamic correlations indicate significant charge transfer to exist in the oxide film in both the cases. The oxide growth in both the cases occurs due to the inward oxygen and outward cation diffusion. The calculated in-plane and out-of-plane atomic diffusivities are 40-70% higher in case of atomic oxidation. In the presence of atomic oxygen, the O/Al ratio is more uniform and varies from 1.37 at the oxide-gas interface to 1.30 at metal-oxide interface whereas that formed by natural oxidation was sub-stoichiometric and oxygen deficient with O/Al values varying from 1.27 (oxide-gas interface) to 1.05 (metal-oxide interface) at room temperature. The simulation results are consistent with the reported experimental investigations.
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. "Synthesis of Lutetium Phosphate/Apoferritin Core-Shell Nanoparticles for Potential Applications in Radioimmunoimaging and Radioimmunotherapy of Cancers." Journal of Materials Chemistry 18(15):1779-1783. Abstract We report a novel approach for synthesizing LuPO4/apoferritin core-shell nanoparticles based on an apoferritin template, conjugated to the protein biotin. To prepare the nanoparticle conjugates, we used non-radioactive lutetium as a model target or surrogate for radiolutetium (177Lu). The central cavity, multi-channel structure, and chemical properties of apoferritin are well-suited for sequentially diffusing lutetium and phosphate ions into the cavity--resulting in a stable core-shell composite. We characterized the synthesized LuPO4/apoferritin nanoparticle using transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). We tested the pre-targeting capability of biotin-modified lutetium/apoferritin nanoparticle using streptavidin-modified magnetic beads and streptavidin-modified fluorescein isothiocyanate (FITC) tracer. This paper presents a simple, fast, and efficient method for synthesizing LuPO4/apoferritin nanoparticle conjugates with biotin for potential applications in radioimmunotherapy and radioimmunoimaging of cancer.
2008. "Low-Temperature Synthesis of Tunable Mesoporous Crystalline Transition Metal Oxides and Applications as Au Catalyst Supports." Journal of Physical Chemistry C 112(35):13499-13509. doi:10.1021/jp804250f Abstract Mesoporous transition metal oxides are of great potential as catalyst supports, shape-selective catalysts, photocatalysts, and sensor materials. Previously stable crystalline mesoporous oxides were mostly obtained by thermally induced crystallization or by segregating the nanocrystals with an amorphous phase. Here we report a novel direct approach to crystalline mesoporous frameworks via the spontaneous growth and assembly of transition metal oxide nanocrystals (i.e., rutile TiO2, fluorite CeO2, cassiterite SnO2, and anatase SnxTi1-xO2) by oxidative hydrolysis and condensation in the presence of anionic surfactants. The influences of synthesis time, surfactants with different chain lengths, concentrations of the oxidant (i.e., hydrogen peroxide), and synthesis temperatures on the composition and morphologies of the resulting materials were investigated by X-ray diffraction (XRD), N2-sorption, transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). A mechanism for the templated synthesis of crystalline mesoporous metal oxides was tentatively proposed. To demonstrate the catalytic applications of these materials, gold nanoparticles were loaded on mesoporous rutile TiO2 and fluorite CeO2 supports, and their catalytic performance in CO oxidation and water-gas shift was surveyed. Au nanoparticles supported on the mesoporous crystalline metal oxides exhibit higher reactivity and excellent on-stream stability towards CO oxidation and water-gas shift reaction compared with commercial TiO2 and CeO2.
2008. "Direct Measurement of Oxygen Incorporation into Thin Film Oxides at Room Temperature Upon Ultraviolet Phton Irradiation." Applied Physics Letters 93(26):263109. doi:10.1063/1.3058691 Abstract Minute changes in oxygen concentration in complex oxides even of the order of ~0.001% can significantly influence functional properties ranging from onset of superconductivity to colossal dielectric constant and ferroic response. In this letter, we report on direct experimental measurement of enhanced oxygen incorporation into ultra-thin oxide films at room temperature under gentle UV photon exposure. Oxygen concentration changes in nanoscale yttria-doped-zirconia (YDZ) films grown on Ge substrate were quantified using the 16O(d,p)17O nuclear reaction. The oxygen concentration was consistently ~ 3 % larger in UV irradiated YDZ films compared to as-grown YDZ films and can be kinetically controlled. Possible incorporation mechanisms are discussed. This suggests a novel approach to modulate oxygen concentration in complex oxides. There is tremendous interest in the science and applications of ultra-thin oxide films, such as electrolyte membranes for solid oxide fuel cells 1, high-dielectric constant (high-) oxides for metal-oxide-semiconductor (MOS) devices 2 and multi-ferroics 3. In addition, thin film oxides also serve as model systems to investigate space charge effects on carrier transport and strongly correlated phenomena such as phase transitions. An overarching problem of central importance is the controlled synthesis of oxide films and how they impact functional properties. Particularly, the role of oxygen vacancies or non-stoichiometry has been found to be crucial in this regard. Examples include large magneto resistance effect and metal-to-insulator transition introduced by reducing oxygen stoichiometry of poly crystalline La0.67Ba0.33MnOz 3, nonsuperconducting-to-superconducting transformation by minute amount of oxygen incorporation upon annealing YBa2Cu3O7-(YBCO)films 4, and blue light emission at room temperature in oxygen deficient SrTiO3 (STO) 5, 6. These studies revealed broad spectra of properties with subtle changes (e.g., of the order of ~0.01% to introduce semiconductor-to-metal transition in SrTiO3)7 in oxygen-related defects, thereby it is extremely important to develop an understanding of oxygen concentration in thin films.
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. "Controlling Size of Gold Clusters in Polyaniline from Top-Down and from Bottom-Up." Journal of Electroanalytical Chemistry 621(2):238-244. doi:10.1016/j.jelechem.2007.11.025 Abstract Polyaniline forms a strong complex with chloroaurate at the protonated imine sites. Here we report on electrochemical procedure that allows preparation of gold clusters by adding gold atoms one-by-one (“bottom up” approach). It is contrasted with the “top down” approach in which the growth of multi-atom Au clusters was also controlled electrochemically. Our results confirm that both the amount and the size of gold clusters affects the properties of the composite material.
2008. "Long-term Black Carbon Dynamics in Cultivated Soil." Biogeochemistry 89(3):295-308. doi:10.1007/s10533-008-9220-9 Abstract Black carbon (BC) is a quantitatively important C pool in the global carbon cycle due to its relative recalcitrance against decay compared with other C pools. However, how rapidly BC is oxidized and in what way the molecular structure changes during decomposition over decadal time scales, is largely unknown. In the present study, the long-term dynamics in quality and quantity of BC were investigated in cultivated soil using X-ray Photoelectron Spectroscopy (XPS), Fourier-Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR) techniques. BC particles, obtained from soil samples at 8 conversion ages stretching over 100 years and from a forest soil sample from Kenya, were manually picked under a light microscope for characterization and quantification. BC contents rapidly decreased from 12.7 to 3.8 mg C gˉ¹ soil during the first 30 years since conversion, after which they slowly decreased to a steady state at 3.51 mg C g ˉ¹soil. BC-derived C losses over 100 years were estimated at 6000 kg C haˉ¹ to a depth of 0.1 m. The initial rapid changes in BC stocks resulted in a mean residence time of only around 8.3 years, which was likely a function of both decomposition as well as transport processes. The molecular properties of BC changed more rapidly on surfaces than in the interior of BC particles and more rapidly during the first 30 years than during the following 70 years. The Oc/C ratios (Oc is O bound to C) and carbonyl groups (C=O) increased over time by 133 and 192 %, respectively, indicating oxidation was an important degradation process controlling BC quality. Al, Si, polysaccharides, and to a lesser extent Fe were rapidly adsorbed on BC particle surfaces within the first few years after BC deposition to soil. The protection by physical and chemical stabilization was apparently sufficient to not only minimize decomposition below detection between 30 and 100 years after deposition, but also physical export by erosion and vertical transport below 0.1 m.
2008. "Role of dopant incorporation on the magnetic properties of Ce1-xNixO2 nanoparticles: An electron paramagnetic resonance study." Journal of Applied Physics 103(7):Art. No. 07D122. Abstract Nickel doping has been found to produce weak room-temperature ferromagnetism in CeO2 [1]. The saturation magnetization of the chemically synthesized Ce1-xNixO2 samples showed a maximum for x = 0.04, above which the magnetization decreased gradually. For Ce1-xNixO2 samples with x ≥ 0.04, an activation process involving slow annealing of the sample to 500 oC increased the saturation magnetization by more than two orders of magnitude [1]. However, no such activation effect was observed in samples with x < 0.04. Electron paramagnetic resonance (EPR), a sensitive technique to investigate the ionic states and local environments and interactions, has been used here in this work to investigate (i) why the ferromagnetic behavior gradually weakened and disappeared for x > 0.04 and (ii)_what causes the saturation magnetization to dramatically increase in the activated Ce1-xNixO2 samples with x ≥ 0.04 and why this process is absent in samples with x < 0.04. Our X-band (~9.4 GHz) EPR experiments and detailed simulation analysis on several as-prepared Ce1-xNixO2 samples with 0.01 ≤ x ≤ 0.10 at 5 K and 300 K indicate the presence of two magnetically inequivalent Ni2+ ions with the ionic spin of 1, one Ce3+ ion with spin ½, and three O2-. Spectra of samples with x < 0.04 are dominated by a single Ni2+ EPR line ascribed to dopant ions in substitutional sites whereas in samples with x ≥ 0.04, an additional signal attributed to Ni2+ ions in interstitial sites is also present. In the activated sample, the EPR line due to the interstitial Ni2+ is completely absent and only the lines due to substituional Ni2+ ions are present suggesting that the enhanced ferromagnetism results from conversion of interstitial Ni2+ ions to substitutional sites.
2008. "Stability of Biomass-derived Black Carbon in Soils ." Geochimica et Cosmochimica Acta 72(24):6069-6078. doi:10.1016/j.gca.2008.09.028 Abstract Black carbon (BC) may play an important role in the global C budget, due to its potential to act as a significant removal (sink) of atmospheric CO2. In order to fully evaluate the influence of BC on the global C cycle, a sound understanding of the stability of BC is required. The biochemical stability of BC was assessed in a chronosequence of high-BC containing Anthrosols from the central Amazon, Brazil, using a range of spectroscopic and biological methods. Results revealed that the Anthrosols had 61-80% lower (P<0.05) CO2 evolution over 532 days compared to that in the corresponding adjacent soils with low BC contents. No significant (P>0.05) differences of CO2 respiration were observed between Anthrosols with contrasting ages of BC and soil textures. Molecular forms of core regions of micrometer-sized BC particles quantified by synchrotron-based near-edge x-ray fine structure (NEXAFS) spectroscopy coupled to scanning x-ray transmission microscopy (STXM) remained similar regardless of their ages (600 to 8,700 years) and closely resembled the spectral characteristics of fresh BC. Deconvolution of NEXAFS spectra revealed greater oxidation on the surfaces of BC particles with little penetration into the core of the particles. The similar C mineralization between different BC-rich soils regardless of soil texture underpins the importance of chemical recalcitrance for the stability of BC, in contrast to adjacent soils which showed the highest mineralization in the sandiest soil. However, C distribution between free, intra-aggregate and organo-mineral pools was significantly different between soils with high and low BC contents, suggesting some degree of physical stabilization, and BC-rich Anthrosols had higher proportions (72-90%) of C in the organo-mineral fraction than BC-poor adjacent soils (2-70%).
2008. "The Synthesis of Ag-Doped Mesoporous TiO2 ." Microporous and Mesoporous Materials 111(1-3):639-642. doi:10.1016/j.micromeso.2007.07.042 Abstract Ag-doped mesoporous titanium oxide was prepared using non-ionic surfactants and easily handled titanium precursors, under mild reaction conditions. In contrast to the stabilizing effect of Cd-doping on mesoporous TiO2, Ag-doping was found to significantly destabilize the mesoporous structure.
2008. "Effects of atmospheres on bonding characteristics of silver and alumina." International Journal of Hydrogen Energy 33(14):4001-4011. doi:10.1016/j.ijhydene.2007.12.042 Abstract Joints prepared using a silver-copper oxide based reactive air brazing (RAB) technique is known to experience a significant decrease in joint strength when exposed in a reducing atmosphere at high temperature. To investigate the effects of atmospheres on the bonding characteristics of ceramic joints brazed with Ag-CuO braze filler metals, alumina joints prepared using a series of Ag-CuO compositions were exposed to a reducing atmosphere in hydrogen and also reoxidized in air at 800C. All the brazed joints exposed to hydrogen revealed significant reduction in flexural strength and exhibited debonding of the interface between the braze filler and the alumina substrate. In the case of the joints brazed with a braze filler containing a high copper content of 8 mol%, the formation of interfacial porosity caused by the reduction of interfacial oxide phases led to an extremely weak interface, which was not recovered after subsequent reoxidation in air at 800C. However, no significant microstructural change or interfacial porosity formation was observed in the braze filler metals containing no or low copper contents, and the interface remained intact even though interfacial strength was weak. Subsequent reoxidation of the joints with these filler materials resulted in the recovery of interfacial strength and flexural strength. This result clearly indicates that the bonding characteristics of the silver/alumina interface are by and large influenced by atmospheres of high temperature exposure. XPS analysis conducted on the in-situ¬ fractured surfaces of as-brazed and hydrogen-treated samples prepared using a braze filler with 2 mol% Cu indicated that oxygen in the silver matrix plays a critical role in bond strength between silver and alumina. The sample exposed in inert atmosphere also revealed low flexural strength and debonding of the silver/alumina interface, confirming the role of oxygen on the bond strength between silver and alumina.
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. "Effect of Nitrogen Additives on Flame Retardant Action of Tributyl Phosphate: Phosphorus – Nitrogen Synergism." Polymer Degradation and Stability 93(1):99-108. doi:10.1016/j.polymdegradstab.2007.10.013 Abstract The effect of nitrogen additives like urea, guanidine carbonate and melamine formaldehyde on the flame retardant efficacy of tributyl phosphate (TBP) has been investigated. From the LOI tests on treated cotton it is clear that the nitrogen additives have synergistic action. Estimation of activation energy of decomposition of treated cotton indicated that nitrogen additives enhance the thermal stability during the burning process. SEM pictures of chars formed after LOI test showed the formation of protective polymeric coating on the surface. The surface of chars formed were evaluated using FTIR-ATR and XPS analysis which showed that the coating was composed of Phosphorus-Nitrogen-Oxygen containing species. Formation of this coating during the burning process could lead to the synergistic interaction of phosphorus and nitrogen. Based on the experimental data we have further proposed several reaction mechanisms which could contribute to synergistic action and formation of protective coating on the surface of char.
2008. "Characterization of CeO2-Supported Cu-Pd Bimetallic Catalyst for the Oxygen-Assisted Water-Gas Shift Reaction." Journal of Catalysis 260(2):358-370. doi:10.1016/j.jcat.2008.08.018 Abstract This study was focused to investigate the roles of Cu and Pd in CuPd/CeO2 bimetallic catalysts containing 20-30 wt% Cu and 0.5-1 wt% Pd used in the oxygen-assisted water-gas shift (OWGS) reaction employing a combined bulk and surface characterization techniques such as XRD, TPR, CO chemisorption, and in-situ XPS. The catalytic activity for CO conversion and the stability of catalyst during on-stream operation increased by the addition of Cu to Pd/CeO2 or Pd to Cu/CeO2 monometallic catalysts, especially when the OWGS reaction was performed under low temperatures, below 200oC. The bimetallic catalyst after leaching with nitric acid retained about 60% of its original activity. The TPR of monometallic Cu/CeO2 showed reduction of CuO supported on CeO2 in two distinct regions, around 150 and 250oC. The high temperature peak disappeared and reduction occurred in a single step around 150oC upon Pd addition. The Pd dispersion decreased from 38.5% for Pd/CeO2 to below 1% for CuPd/CeO2 bimetallic catalyst. In-situ XPS studies showed a shift in Cu 2p peaks toward lower binding energy (BE) with concommitant shift in the Pd 3d peaks toward higher BE. Addition of Pd decreased the surface Cu concentration while the concentration of Pd remained unaltered. All these observations indicated the formation of Cu-Pd surface alloy. The valence band XP spectra collected below 10 eV corroborated the core level XP spectra and indicated that Cu is mainly involved in the catalytic reaction. The improved catalytic activity and stability of CuPd/CeO2 bimetallic catalyst was attributed to the alloy formation.
2008. "Electrodeposition of Technetium on Platinum for Thermal Ionization Mass Spectrometry (TIMS)." Journal of Radioanalytical and Nuclear Chemistry 276(2):493-498. doi:10.1007/s10967-008-0532-y Abstract A novel device has been fabricated for the electrodeposition of technetium metal onto platinum filaments for thermal ionization mass spectrometric (TIMS) measurements. The ability of the device to focus the deposition to diameters of hundreds of micrometers on pre-mounted TIMS filaments coupled with the ease of use and simplicity of design permit for an extremely sensitive yet economical TIMS filament loading technique. Electrodeposition parameters were varied in order to maximize deposition efficiency. X-ray photoelectron spectroscopy (XPS) was used to confirm and characterize the technetium deposit. The technetium is deposited in the metallic state, although surface oxides in the 4+ and 7+ state form readily. Initial TIMS measurements of the electrodeposited technetium in the presence of a barium sulfate ionization enhancer show potential for excellent sensitivity.
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. "Natural Oxidation of Black Carbon in Soils: Changes in Molecular Form and Surface Charge along a Climosequence ." Geochimica et Cosmochimica Acta 72(6):1598-1610. Abstract The aim of this work was to investigate changes in molecular form and surface charge of black carbon (BC) due to longtermnatural oxidation and to examine how climatic and soil factors affect BC oxidation. Black C was collected from 11 historical charcoal blast furnace sites with a geographic distribution from Quebec, Canada, to Georgia, USA, and compared to BC that was newly produced (new BC) using rebuilt historical kilns. The results showed that the historical BC samples were substantially oxidized after 130 years in soils as compared to new BC or BC incubated for one year. The major alterations by natural oxidation of BC included: (1) changes in elemental composition with increases in oxygen (O) from 7.2% in new BC to 24.8% in historical BC and decreases in C from 90.8% to 70.5%; (2) formation of oxygen-containing functional groups, particularly carboxylic and phenolic functional groups, and (3) disappearance of surface positive charge and evolution of surface negative charge after 12 months of incubation. Although time of exposure significantly increased natural oxidation of BC, a significant positive relationship between mean annual temperature (MAT) and BC oxidation (O/C ratio with r = 0.83;P < 0.01) explained that BC oxidation was increased by 87 mmole kg Cˉ1 per unit Celsius increase in MAT. This long-term oxidation was more pronounced on BC surfaces than for entire particles, and responded 7-fold stronger to increases in MAT. Our results also indicated that oxidation of BC was more important than adsorption of non-BC. Thus, natural oxidation of BC may play an important role in the effects of BC on soil biogeochemistry.
2008. "Superior Nanoscale Passive Oxide Layers Synthesized Under Photon Irradiation for Environmental Protection ." Applied Physics Letters 92(26):263103. doi:10.1063/1.2952282 Abstract We report on synthesis and functional properties of ultra-thin oxide layers synthesized on metal surfaces by room temperature photon irradiation. We show that the impedance of a passive aluminum oxide film synthesized under ultraviolet photon irradiation is an order of magnitude larger than that of native oxide in a 0.5 M NaCl solution. Further, the structure and impedance of existing native oxide layers can be dramatically improved by minutes-long exposure to photon-irradiation. Depth profiling studies with X-ray photoelectron spectroscopy shows that chlorine uptake in UV-synthesized oxides, compared to that of native oxides, is reduced which can contribute to the improvement in corrosion resistance. The results are of significance to synthesis of ultra-thin passive layers on metal and alloy structures for environmental protection.
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.
2008. "Spectroscopic Characterization of Extracellular Polymeric Substances from Escherichia coli and Serratia marcescens: Suppression using Sub-Inhibitory Concentrations of Bismuth Thiols." Biomacromolecules 9(11):3079-3089. doi:10.1021/bm800600p Abstract Free and capsular EPS produced by Escherichia coli and Serratia marcescens were characterized in detail using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Total EPS production decreased upon treatment with sub-inhibitory concentrations of lipophilic bismuth thiols (bismuth dimercaptopropanol, BisBAL; bismuth ethanedithiol, BisEDT; and bismuth pyrithione, BisPYR), BisBAL being most effective. Bismuth thiols also influenced acetylation and carboxylation of polysaccharides in EPS from S. marcescens. Extensive homology between EPS samples in the presence and absence of bismuth was observed with proteins, polysaccharides, and nucleic acids varying predominantly only in the total amount expressed. Second derivative analysis of the amide I region of FTIR spectra revealed decreases in protein secondary structures in the presence of bismuth thiols. Hence, anti-fouling properties of bismuth thiols appear to originate in their ability to suppress O-acetylation and protein secondary structures in addition to total EPS secretion.
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. "Electrochemical Effects of S Accumulation on Ion-implanted Alloy 22 in 1 M NaCl Solutions." Corrosion Science 49(6):2497-2511. doi:10.1016/j.corsci.2006.12.003 Abstract The objective of this study was to examine the effects of high levels of S in the near-surface region on the passivity of Alloy 22, a corrosion resistant Ni-Cr-Mo alloy, in deaerated 1 M NaCl solution. Near-surface concentrations of S up to 2 at.% were achieved in Alloy 22 test specimens by implanting them with S. The S-implanted samples were then evaluated in short-term electrochemical tests in the salt solution and subsequently analyzed with X-ray Photoelectron Spectroscopy (XPS) for film thickness and composition. Specimens tested included non-implanted and annealed Alloy 22 samples, samples implanted with S, and “blanks” implanted with Ar as an ion that would simulate the “damage” of S implantation without the chemical effect. A sample of S-implanted Alloy 22 was also exposed to solution for 29 days and analyzed for evidence of S accumulation at the surface over longer times
2007. "Morphology and Oxide Shell Structure of Iron Nanoparticles Grown by Sputter-Gas-Aggregation." Nanotechnology 18(25):Art. No. 255603. doi:10.1088/0957-4484/18/25/255603 Abstract Much recent research effort has been made on the synthesis, characterization, and property evaluation of core-shell structured Fe nanoparticles. Fundamental properties of these particles depend on both their external crystal faceting planes and the nature of a protective oxide layer. In this paper, the crystal faceting planes and oxide coating structures of core-shell structured iron/iron oxide nanoparticles synthesized by a sputter-gas-aggregation process were studied using transmission electron microscopy (TEM), electron diffraction and Wulff shape construction. The particles grown by this process and deposited on a support at room temperature process have been compared with particles grown and deposited at high temperature as reported in literature. Most synthesis processes produce round particles for particles less than 20 nm in diameter. For larger particles crystallographic facets are observed. It has been found that the Fe nanoparticles formed at RT are invariantly faceted on the {100} lattice planes and truncated by the {110} planes at different degrees. Substantial fraction of particles are confined only by the 6 {100} planes (not truncated by the {110} planes), this contrasts with the Fe particles formed at high temperature (HT) for which a predominance of {110} planes has been reported. Furthermore, at RT no particle was identified to be only confined by the 12 {110} planes which is relatively common for the particles formed at HT. The Fe cubes defined by the 6 {100} planes show a characteristic inward relaxation along the <100> and <110> directions and the reason for this behavior is not fully understood. The oxide shell on the Fe {100} plane maintains an orientation relationship: Fe(001)//Fe3O4(001) and Fe[100]//Fe3O4[110], which is same as the oxide formed on a bulk Fe(001) through thermal oxidation. Orientation of the oxide that forms on the Fe{110} facets differs from that on Fe{001}, therefore, properties of core-shell structured Fe nanoparticle faceted primarily with one type of lattice plane may be fully different from that faceted with another type of lattice planes.
2007. "Electron Beam Induced Thickening of the Protective Oxide Layer around Fe Nanoparticles ." Ultramicroscopy 108(1):43-51. doi:10.1016/j.ultramic.2007.03.002 Abstract There are many circumstances in science where the process of measuring the properties of a system alters the system. An imaging process can exert an inadvertent effect on the object being observed. Consequently, what we observe does not necessarily represent what had been present before the observation. Normally this effect can be ignored if the consequence of such a change is believed not to be significant. The expansion of nanostructured materials has made high resolution transmission electron microscopy one of the indispensable tools for probing the characteristics of nano-materials. Modification of nanoparticles by the electron beam during their imaging has been widely noticed and this is generally believed to be due to electron beam induced heating effect, defect formation in the particles, charging of the particle, or excitation of surrounding gases. However, an explicit experimental identification of which process dominates is often very hard to establish. We report the thickening of native oxide layer on iron nanoparticle under electron beam irradiation. Based on atomic level imaging, electron diffraction, and computer simulation, we have direct evidence that the protecting oxide layer formed on Fe nanoparticle at room temperature in air continues to grow during an electron beam bombardment in the vacuum system typical of most TEM systems. Partial illumination of a nanoparticle and observation of the shell thickening conclusively demonstrates that many of the mechanisms postulated to explain such processes are not occurring to a significant extent. The observed growth is not related to the electron beam induced heating of the nanoparticle, or residual oxygen ionization, or establishment of an electrical field, rather it is related to electron beam facilitated mass transport across the oxide layer (a defect related process).
2007. "Ferromagnetism in chemically synthesized CeO2 nanoparticles by Ni doping." Physical Review. B, Condensed Matter 76(16):165206-1 - 165206-8. doi:10.1103/PhysRevB.76.165206 Abstract This work reports the discovery of room-temperature ferromagnetism in 5 - 9 nm sized Ce1-xNixO2 nanoparticles (0.01 ≤ x ≤ 0.10) prepared using a sol-gel based chemical method at room temperature and under ambient conditions. Particle induced x-ray emission studies were used to determine the dopant concentrations. Magnetic measurements of the chemically synthesized Ce1-xNixO2 samples at room temperature showed coercivity in the 40 - 120 Oe range, and the saturation magnetization showed a maximum of 1.21 memu/g (8.59×10-4 μB/Ni ion) for x = 0.04. Average crystallite sizes and lattice parameters estimated from x-ray diffraction and transmission electron microscopy studies showed a gradual decrease with x in the entire doping range while the lattice strain showed a minimum for x = 0.04. Optical studies revealed direct band gap energies ranging from 3.23 to 3.99 eV with a minimum for x = 0.04. A high Curie temperature of TC = 550 K was obtained for x = 0.04.
2007. "Synthesis and Characterization of Phosphate-coated Mesoporous Titania and Cd-doping of Same via Ion-Exchange." Inorganic Chemistry Communications 10(6):642-645. doi:10.1016/j.inoche.2007.02.016 Abstract Phosphate-based mesoporous TiO2 materials were prepared by surfactant-directed method using an alkylphosphate surfactant, which produced a mesoporous titania with high surface area (~200 m2/g) and a phosphate monolayer interface. Calcination of the as-synthesized greenbody in an ozone atmosphere generated materials with higher surface area, and higher purity, than did calcination in air. These interfacial phosphate groups are convenient functionality for chemically modifying the surface via ion-exchange processes. High doping ratios of P/Ti (0.47-0.69) and Cd/P (0.37-0.40) were observed. Materials were characterized by XRD, FE-SEM, TEM, and XPS.
2007. "Functional Mesoporous Carbon Built from the 1,10-Phenathroline Building Block: A New Class of Catalyst Support ." Inorganic Chemistry Communications 10(12):1541-1544. doi:10.1016/J.INOCHE.2007.09.021 Abstract A simple synthesis of a high surface area catalyst support composed of a nanoporous carbon architecture built from a 1,10-phenanthroline precursor is described.
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. "Metalorganic chemical vapor deposition of carbon-free ZnO using the bis(2,2,6,6 tetramethyl-3,5-heptanedionato)zinc precursor." Journal of Materials Research 22(5):1230-1234. doi:10.1557/JMR.2007.0146 Abstract We report the growth of c-axis oriented ZnO films on silicon (100) single crystal substrates by MOCVD. A relatively uninvestigated precursor, Zn(TMHD)2, was used in a cold-wall MOCVD reactor. XPS and RBS analysis yielded a zinc-to-oxygen atom percent ratio of 0.98 and 1.00 at the surface and in the overall film, respectively indicative of stoichometric ZnO. Due to the presence of carbon at the surface, the excess oxygen at the surface was in the form of C-O bonding. The c-axis orientation was confirmed by HRTEM and XRD. We look at these results from a viewpoint of an ongoing effort to ensure cleaner decompositions using Zn(TMHD)2.
2007. "Fabrication of SiO2 Microdisk Arrays for Optics and Light Trapping Experiments." Microelectronic Engineering 84(12):2799-2803. doi:10.1016/j.mee.2007.02.001 Abstract We present a simple silicon based microfabrication process that produces an array of SiO2 microdisks using UV lithography. High-resolution SEM images of these structures indicate a smooth outer microdisk cavity surface. Photoemission measurements were performed at different spots on the microdisk and compared with measurements inside the cavity. A silicon to oxygen atomic concentration ratio of 1:2 obtained during depth profiling confirms that the entire microdisk is made up of stoichometric SiO2. In contrast, the inner cavity is mostly silicon with native oxide on top. We discuss the usefulness of SiO2 microdisks in optics for light trapping experiments.
2007. "On the room-temperature ferromagnetism of Zn1-xCrxO thin films deposited by reactive co-sputtering." Solar Energy Materials and Solar Cells 91(15-16):1496-1502. doi:10.1016/j.solmat.2007.03.012 Abstract We report on the preparation and detailed characterization of ferromagnetic Zn1-xCrxO thin films deposited on Si substrates using reactive co-sputtering of Cr and Zn in controlled oxygen atmospheres. X-ray diffraction data showed wurtzite ZnO peaks in the ferromagnetic films prepared with lower Cr powers, whose position and intensities are influenced by Cr doping. However, samples prepared with higher Cr powers did not show ferromagnetism but displayed evidence of Cr2O3 and ZnCr2O4 phases with no ZnO phase. The magnetization is higher (saturation magnetization Ms = 18 emu/cm3) for lower Cr concentrations and decreases for higher Cr doping. The samples were investigated extensively to understand the film composition, dopant distribution, homogeneity and potential origin of the observed ferromagnetism. Particle induced x-ray emission studies were employed to determine the chemical composition as well as the Cr/Zn ratio in the films. Film uniformity and homogeneity, investigated using Rutherford backscattering spectrometry, showed a relatively uniform ZnO layer in the as-prepared samples but, in a sample annealed at 800 oC, showed some diffusion of Si from the substrate. X-ray photoelectron spectroscopy studies indicated that Cr ions are in the oxidized state, but showed changes in the binding energy and Cr concentration when measured after removing 10 nm from the surface using Ar ion sputtering. Possible origins of the observed ferromagnetic behavior are discussed based on the comprehensive characterization results.
2007. "Effect of Mn doping on the structural, morphological, optical and magnetic properties of indium tin oxide films." Journal of Materials Science. Materials in Electronics 18(12):1197-1201. doi:10.1007/s10854-007-9277-6 Abstract We report high temperature ferromagnetism in Mn (3 - 4 at.%) doped optically transparent indium tin oxide (ITO) films prepared by a sol-gel based technique. The films showed 16 - 18 nm sized uniformly distributed particles with high phase purity. Magnetic measurements yield a coercivity Hc ~ 80 Oe and a saturation magnetization Ms ~ 0.39 B/Mn2+ ion at 300 K with a Tc > 600 K for the 3.2% Mn doped ITO film. Magnetic force microscopy showed convincing evidence of a uniformly distributed ferromagnetic phase with well defined magnetic domains spread over hundreds of independent nanoparticles.
2007. "Novel Magnetic Hydrogen Sensing: A Case Study Using Antiferromagnetic Hematite Nanoparticles." Nanotechnology 18(16):Art. No. 165502. doi:10.1088/0957-4484/18/16/165502 Abstract Hydrogen sensing is a critical component of safety to address wide spread public perceptions of the hazards of production, storage, transportation and use of hydrogen in proposed future automobiles and in various other applications. A nanoscale magnetic hydrogen sensor is proposed based on the experimental observation of systematically varying the saturation magnetization and remanence of nanoscale antiferromagnetic hematite with hydrogen flow. The saturation magnetization and remanence of the nanoscale hematite sample showed an increase of one to two orders of magnitude in the presence of flowing hydrogen gas at concentrations in the 1 to 10% range and at 575 K, suggesting that a practical magnetic hydrogen sensor could be developed using this material and the novel magnetic sensing method. Thermogravimetric analysis of the hematite sample shows significant mass loss when hydrogen gas is introduced. Xray diffraction and x-ray photoelectron spectroscopy studies ruled out any impurity phase formation as a result of gas-sample interaction. This work thus facilitates the use of the magnetic properties of an antiferromagnetic material as gas sensing parameters, thus exploring the concept of ‘magnetic gas sensing’.
2007. "Effect of Coal Gas Contaminants on Solid Oxide Fuel Cell Operation." ECS Transactions 11(33):63-70. Abstract The operation of solid oxide fuel cells (SOFC) was evaluated on simulated coal gas in the presence of several coal gas impurities that are expected to remain in low concentration after warm gas cleanup. Phosphorus, arsenic and sulfur were considered in this study. The presence of phosphorus and arsenic in low, 1-2 ppm, concentrations led to the slow and irreversible SOFC degradation due to the formation of the secondary phases with nickel in the upper part of the nickel-based anode close to the gas inlet. Sulfur interactions with the nickel were limited to the surface only. Cell performance losses due to sulfur exposure were reversible and independent of the presence of other impurities.
2007. "The Synthesis of Cadmium Doped Mesoporous TiO2." Inorganic Chemistry Communications 10(6):639-641. doi:10.1016/j.inoche.2007.02.018 Abstract Cd doped mesoporous titanium oxide was prepared using non-ionic surfactants and easily handled titanium precursors. The Cd doping was found to be able to significantly inhibit the growth of anatase crystal size, stabilize the mesoporous structure, and retard the densification of nanoporous TiO2 at elevated temperatures.
2007. "Behavior of Si and C atoms in ion amorphized SiC." Journal of Applied Physics 101(2):Art. No. 023524. doi:10.1063/1.2431941 Abstract Single crystal 6H-SiC wafers were fully amorphized at room temperature or 200 K using 1.0 or 2.0 MeV Au+ ion irradiation. The thickness of the amorphized layers has been determined using Rutherford backscattering spectrometry under ion channeling conditions. Microstructures of the irradiated SiC have been examined using cross-sectional transmission electron microscopy. The depth profiles of both the Si and C atoms have been studied using both x-ray photoelectron spectroscopy (XPS) and time-of-flight energy elastic recoil detection analysis. Neither Si nor C in the amorphized SiC exhibits a significant mass transport by diffusion during the irradiation and subsequent storage at room temperature. There is no observable phase segregation of either Si or C in the amorphized SiC. Ar+ ion sputtering leads to modifications of the composition, structure and chemical bonding at the 6H-SiC surface. The Si-Si bonds at the sputtered surface (amorphized) do not appear, as suggested by the XPS; however, Raman backscattering data reveals the existence of the Si-Si bonds in the bulk amorphized SiC, in addition to the C-C and Si-C bonds that the XPS also identified.
2007. "Effect of Co Doping on the Structural, Optical and Magnetic Properties of ZnO Nanoparticles." Journal of Physics. Condensed matter 19(26):Art. No. 266203. doi:10.1088/0953-8984/19/26/266203 Abstract We report the results of a detailed investigation of sol-gel synthesized nanoscale Zn1-xCoxO powders processed at 350 °C with 0 @ x @ 0.12 to understand how the structural, morphological, optical and magnetic properties of ZnO are modified by Co doping, in addition to searching for the theoretically predicated ferromagnetism. With x increasing to 0.03, both lattice parameters a and c of the hexagonal ZnO decreased suggesting substitutional doping of Co at the tetrahedral Zn2+ sites. For x > 0.03, these trends reversed and the lattice showed a gradual expansion as x approached 0.12, probably due to additional interstitial incorporation of Co. Raman spectroscopy measurements showed a rapid change in the ZnO peak positions for x > 0.03 suggesting significant disorder and changes in the ZnO structure, in support of additional interstitial Co doping possibility. Combined x-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectroscopy, photoluminescence spectroscopy and diffuse reflectance spectroscopy showed clear evidence for tetrahedrally coordinated high spin Co2+ ions occupying the lattice sites of ZnO host system, which became saturated for x > 0.03. Magnetic measurements showed a paramagnetic behavior in Zn1-xCoxO with increasing antiferromagnetic interactions as x increased to 0.10. Surprisingly, a weak ferromagnetic behavior was observed for the sample with x = 0.12 with a characteristic hysteresis loop showing a coercivity Hc ~ 350 Oe, 25% remanence Mr, a low saturation magnetization Ms ~ 0.04 emu/g and with a Curie temperature Tc ~ 540 K. The XPS data collected from Zn1-xCoxO samples showed a gradual increase in the oxygen concentration, changing the oxygen deficient undoped ZnO to an excess oxygen state for x = 0.12. This indicates that such high Co concentrations and appropriate oxygen stoichiometry may be needed to achieve adequate ferromagnetic exchange coupling between the incorporated Co2+ ions.
2007. "N incorporation and electronic structure in N-doped TiO2(110) rutile." Surface Science 601(7):1754-1762. doi:10.1016/j.susc.2007.01.051 Abstract Epitaxial TiO2-xNx film growth under anion-rich conditions is characterized by nearly balanced incorporation rates for substitutional N (NO) and interstitial Ti (Tii). Tii donors fully compensate and stabilize N3-, but preclude the formation of p-type material. Hybridization occurs between Tii(IV) and NO3-, but the value of x is limited to ~0.02 under these conditions. Tii(IV)-NO3- states occur above the valence band maximum of pure TiO2, riving rise to enhanced optical absorption in the visible up to ~2.5 eV. Much higher NO and Tii concentrations result from using cation-rich conditions.
2007. "Ferromagnetic Semiconductor Nanoclusters: Co-doped Cu2O." Applied Physics Letters 90(1):art. no. 013106 (3 pages). doi:10.1063/1.2429018 Abstract 5% Co-doped cuprous oxide dilute magnetic semiconductor (DMS) cluster film composed of crystalline nanoclusters in the range of ~ 4.2 nm average crystallite size, prepared using sputtering-aggregation technique is found to be ferromagnetic at 400 K with a coercivity of 29 Oe. With the increase in average crystallite size from 4.2 nm to 8 nm, the coercivity increased, whereas the saturation magnetization was achieved at 3 KOe field in both cases. Cu2O phase is observed from the cluster film deposited on the Si wafer when analyzed using XRD. Co in Cu2O host reveals a +2 oxidation state via XPS measurements. Positive magneto resistance (MR) data from the sample exhibits a temperature dependent decrease.
2006. "Irradiation behavior of SrTiO3 at temperatures close to the critical temperature for amorphization." Journal of Applied Physics 100(11):113533 (8 pages). doi:10.1063/1.2399932 Abstract Damage accumulation on both the Sr and Ti sublattices in strontium titanate (SrTiO3) has been investigated under 1.0 MeV Au+ irradiation at 360 and 400 K, close to the critical temperature for amorphization (~ 370 K). Under irradiation at 360 K, the relative disorder on both sublattices follows a nonlinear dependence on ion dose. Amorphization starts from the damage peak region (at a depth of 60 nm) and grows toward the surface and into the bulk. At 400 K, evolution of point defects to extended defects occurs as ion fluence increases. The disorder initially peaks at a depth of 60 nm, saturates at disorder level of ~0.75, and then decreases with further irradiation. At an ion fluence of 6.0×1015 cm-2, an amorphous layer of ~ 10 nm thickness is formed at the sample surface. After annealing at 375 K for one hour, the microstructural features indicate that the buried amorphous layer formed during irradiation at 360 K is re-crystallized with planar defects and dislocation loops. The surface amorphous layer formed at 400 K irradiation remains amorphous and less defects are observed at the irradiated region. The irradiation-enhanced recrystallization due high flux electron energy deposition is observed.
2006. "Sensitive Immunoassay of a Biomarker Tumor Necrosis Factor-[alpha] Based on Poly(guanine)-Functionalized Silica Nanoparticle Label." Analytical Chemistry 78(19):6974-6979. doi:10.1021/ac060809f Abstract A novel electrochemical immunosensor for the detection of tumor necrosis factor-alpha (TNF-a) based on poly(guanine)-functionalized silica nanoparticles (NPs) label is presented. The detection of mouse TNF-a via immunological reaction is based on a dual amplification: 1) a large amount of guanine residues is introduced on the electrode surface through the silica nanoparticle and immunoreaction, 2) mediator-induced catalytic oxidation of guanine, which results in great enhancement of anodic current. The synthesized silica NP conjugates were characterized with atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemistry. These experiments confirmed that poly[G] and avidin were immobilized on the surface of silica NPs. The performance of the electrochemical immunosensor was evaluated and some experiment parameters (e.g., concentration of Ru(bpy)32+, incubation time of TNF-a, etc.) were optimized. The detection of limit for TNF-a is found to be 5.0x10-11 g mL-1 (2.0 pM), which corresponds to 60 attomoles TNF-a in 30 uL. This immunosensor based on the poly[G] functionalized silica NP label offers great promise for rapid, simple, cost-effective analysis of biological samples.
2006. "Distribution of Oxygen Vacancies and Gadolinium Dopants in ZrO2-CeO2 Multi-Layer Films Grown on α-Al2O3." Solid State Ionics 177(15-16):1299-1306. doi:10.1016/j.ssi.2006.05.036 Abstract Gd-doped ZrO2 and CeO2 multi-layer films were deposited on α-Al2O3 (0001) using oxygen plasma assisted molecular beam epitaxy. Oxygen vacancies and Gd dopant distributions were investigated in these multi-layer films using x-ray diffraction (XRD), conventional and high-resolution transmission electron microscopy (HRTEM), annular dark-filed imaging in scanning transmission electron microscopy (STEM), energy dispersive x-ray spectroscopy (EDS) elemental mapping and x-ray photoelectron spectroscopy (XPS) depth profiling. EDS and XPS reveal that Gd concentration in the ZrO2 layer is lower than that in the CeO2 layer. As a result, higher oxygen vacancy concentration exists in CeO2 layers compared to that in ZrO2 layers. In addition, Gd is found to segregate only at the interfaces formed during the deposition of CeO2 layers on ZrO2 layers. On the other hand, the interfaces formed during the deposition of ZrO2 layers on CeO2 layers did not show any Gd segregation. The Gd segregation behavior at the every other interface is believed to be associated with the low solubility of Gd in ZrO2.
2006. "Degradation Mechanisms of La-Sr-Co-Fe-O3 SOFC Cathodes." Electrochemical and Solid-State Letters 9(10):A478-A481. Abstract The long-term stability of anode-supported YSZ electrolyte SOFCs utilizing (La0.6Sr0.4)0.98Co0.2Fe0.8O3- (LSCF-6428) cathodes was assessed. Samples tested for 500 hours at 750ºC and 0.7V indicated ≈50% degradation. While scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) analysis indicated no obvious microstructural or chemical phenomena that could explain the high degradation, x-ray photon spectroscopy (XPS) revealed that enrichment of Sr at the cathode-electrolyte and cathode-current collector interfaces was at least partially responsible for the observed degradation.
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. "Correlation among Channeling, Morphological and Micro-structural Properties in Epitaxial CeO2 Films." Electrochemical and Solid-State Letters 9(5):J17-J20. doi:10.1149/1.2186029 Abstract We report an evidence of a critical thickness at ~ 64 nm in epitaxial CeO2 films grown at 750 0C on YSZ substrates by dc magnetron sputtering where optimum ion channeling can be correlated with overall strain relaxation and film surface roughness. The occurrence of saturation in ion channeling yield, enhancement in the average surface roughness and relaxation in c-axis strain is clearly evident in thicker films beyond the critical thickness. Despite excellent surface smoothness and overall epitaxial growth, CeO2 films grown at 650 0C did not show optimum ion channeling properties due to high misfit dislocation and defect density. These results are discussed from a viewpoint of the need for such an optimum thickness to develop multilayers with smooth interfaces with relative overall lattice relaxation.
2006. "Pressure-temperature dependence of nanowire formation in the arsenic-sulfur system ." Physics and Chemistry of Glasses 47(6):675-680. Abstract Nanowire Formation in Arsenic Trisulfide Brian J. Riley, S.K. Sundaram*, Bradley R. Johnson, Mark Engelhard Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352 * Corresponding author: Phone: 509-373-6665; Fax: 509-376-3108, E-mail: sk.Sundaram@pnl.gov Abstract: Arsenic trisulfide (As2S3) nanowires, nano-droplets, and micro-islands were synthesized on fused silica substrates, using a sublimation-condensation process at reduced pressures (70 mtorr – 70 torr) in a sealed ampoule. Microstructural control of the deposited thin film was achieved by controlling initial pressure, substrate temperature and substrate surface treatment. Microstructures were characterized using scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). Surface topography and chemistry of the substrates were characterized using x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Semi-quantitative image analysis and basic curve-fitting were used to develop empirical models to mathematically describe the variation of microstructure as a function of initial pressure and substrate temperature and map out the regions of different microstructures in P-T space. Thermodyamic properties (available from literature) of this system are also incorporated in this map. Nanowires of an amorphous, transparent in visible-LWIR region, semi-conducting material, like As2S3, provide new opportunities for the development of novel nano-photonic and electronic devices. Additionally, this system provides an excellent opportunity to model (and control) microstructure development from nanometer to micron scales in a physical vapor deposition process, which is of great value to nanoscience and nanotechnology in general.
2006. "Magnetic Gas Sensing Using a Dilute Magnetic Semiconductor." Applied Physics Letters 89(Art. No. 112509):, doi:10.1063/1.2349284 Abstract The authors report on a magnetic gas sensing methodology to detect hydrogen using the ferromagnetic properties of a nanoscale dilute magnetic semiconductor Sn0.95Fe0.05O2. This work demonstrates the systematic variation of saturation magnetization, coercivity, and remanence of Sn0.95Fe0.05O2 with the hydrogen gas flow rate, thus providing clear experimental evidence of the concept of magnetic gas sensing (using the magnetic property of a material as a gas sensing parameter). Based on the results of using hydrogen as an example for reducing gases, it is believed that any reducing gas capable of changing the oxygen stoichiometry of Sn0.95Fe0.05O2 can be detected using this method. Furthermore, this method presents an alternative gas sensing technology without the use of the electrical contacts.
2006. "Dopant Distribution, Oxygen Stoichiometry and Magnetism of Nanoscale Sn0.99Co0.01O₂." Solid State Communications 139:434-438. doi:10.1016/j.ssc.2005.09.040 Abstract In a recent work, we have shown that chemically synthesized Sn₁-xCoxO₂ nanoscale powders with x ≤ 0.01 are ferromagnetic at room-temperature when prepared by annealing the reaction precipitate in the narrow temperature window of 350 to 600 °C. Combined high resolution x-ray photoelectron spectroscopy and magnetometry measurements on as-prepared and Ar⁺ ion sputtered samples showed that a uniform distribution of Co observed in the 350-600 °C annealed Sn0.99Co0.01O₂ samples is essential to produce stable ferromagnetism, while surface segregation of the dopant atoms in samples annealed at >600 °C destroys it.
2006. "Chemical bonding and electronic structures of the Al2SiO5 polymorphs, andalusite, sillimanite, and kyanite: X-ray photoelectron- and electron energy loss spectroscopy studies." American Mineralogist 91(5-6):740-746. doi:10.2138/am.2006.1887 Abstract We have undertaken a detailed analysis of the X-ray photoelectron spectra obtained from the three polymorphs of Al2SiO5; andalusite, sillimanite, and kyanite. Comparison of the spectra was made based on the chemical bonding and structural differences in the Al- and Si-coordination within each polymorph. The spectra for Si(2p) for all three polymorphs are nearly identical, consistent with the fact that all the Si atoms are in 4-fold (tetrahedral) coordination, whereas the binding energies, peak shapes, and peak widths for Al(2p) vary depending on the type of polymorph. The upper-valence band for all three polymorphs is characterized by four main features derived from O(2p), Al(3s), Al(2p), Si(3s), and Si(3p), and the differences in their contributions are observed. The density of state of the Al2SiO5 polymorphs is relatively featureless compared to those observed from α-SiO2 and α-Al2O3, suggesting that the orbital overlaps span a greater range in energy. The observed band gap energy for Al2SiO5 (sillimanite) was ~9.1eV, a value in between those for α-SiO2 (~8.6eV) and α-Al2O3 (~9.6eV). The conduction band feature of Al2SiO5 was experimentally compared to those of α-SiO2 and α-Al2O3, and shown that it is indeed intermediate between the α-SiO2 and α-Al2O3 phases.
2006. "Surface and Interface Control on Photochemically Initiated Immobilization ." Journal of the American Chemical Society 128(43):14067 (6 pages). Abstract Surface and interface properties are important in controlling the yield and efficiency of the photochemically initiated immobilization. Using a silane-functionalized perfluorophenylazide (PFPA-silane) as the photoactive crosslinker, the immobilization of polymers was studied by adjusting the density of the surface azido groups. Dilution of the photolinker resulted in a gradual decrease in the density of surface azido groups as well as the thickness of the immobilized film. When a non-photoactive silane was added to PFPA-silane, the film thickness decreased more rapidly, indicating that the additive competed with PFPA-silane and effectively reduced the density of the surface azido groups. The effect of surface topography was studied by adding a non-photoactive silane with either a shorter (n-propyltrimethoxysilane (PTMS)) or a longer spacer (n-octadecyltrimethoxysilane (ODTMS)). In most cases the long chain ODTMS shielded the surface azido groups resulting in more rapid decrease in film thickness as compared to PTMS treated under the same conditions. As the density of the surface azido groups decreased, the immobilized polymer changed from smooth films to patched structures, and eventually single polymer molecules.
2006. "Effects of nanocrystalline CeO2 supports on the properties and performance of Ni–Rh bimetallic catalyst for oxidative steam reforming of ethanol." Journal of Catalysis 238(2):430-440. doi:10.1016/j.jcat.2006.01.001 Abstract This study focuses on the effects of the CeO2 support properties on the catalyst properties and performance of bimetallic Ni–Rh/CeO2 catalysts containing 5 wt% Ni and 1 wt% Rh for the oxidative steam reforming (OSR) of ethanol for hydrogen production and fuel cell applications. Three CeO2 supports with different crystal sizes and surface areas were examined. The surface areas of these supports increases in the order of CeO2-I (74 m2/g) < CeO2-II (92 m2/g) < CeO2-III (154 m2/g), but their crystallite sizes were about 10.2, 29.3, and 6.5 nm, respectively. The properties of Ni–Rh/CeO2 catalysts were investigated by XRD, TPR, H2 chemisorption, and in situ XPS techniques. The Rh metal dispersion increased while the Ni metal dispersion decreased with decreasing crystallite sizes of CeO2. TPR studies revealed the existence of a Rh–CeO2 metal–support interaction as well as Ni–Rh interaction in the Ni–Rh bimetallic catalyst supported on CeO2-III with a crystallite size of about 6.5 nm. The in situ XPS studies corroborated the TPR results. The reduced Ni and Rh species were reversibly oxidized, suggesting the existence of Ni–Rh redox species rather than NiRh surface alloy in the present catalyst system. The Rh species became highly dispersed when the crystallite size of CeO2 support was smaller. Comparing the catalytic performance in the OSR of ethanol was compared with the properties of the catalysts demonstrated that both ethanol conversion and H2 selectivity increased and the selectivity for undesirable byproducts decreased with increasing Rh metal dispersion. Best catalytic performance for OSR was achieved by supporting Ni–Rh bimetallic catalysts on the nanocrystalline CeO2-III. The Ni–Rh/CeO2-III catalyst exhibited stable activity and selectivity during on-stream operations at 450 ◦C and as well as at 600 ◦C.
2006. "Development and Processing Temperature Dependence of Ferromagnetism in Zn0.98Co0.02O." Journal of Applied Physics 99(8):Art. no. 08M123. doi:10.1063/1.2173956 Abstract We report the development of room-temperature ferromagnetism (FM), with coercivity Hc = 2850 Oe and saturation magnetization Ms ~ 0.01 emu/g, in chemically synthesized powders of Zn0.98Co0.02O processed at 150 °C, and paramagnetism with antiferromagnetic interactions between the Co2+ spins (S = 3/2) in samples processed at higher temperatures 200 ≤ TP ≤ 900 °C. X-ray diffraction data show a decrease in the lattice parameters a and c with TP indicating progressive incorporation of 0.58 Å sized tetrahedral Co2+ at the substitutional sites of 0.60 Å sized Zn2+. Diffuse reflectance spectra show three well defined absorption edges at 660, 615 and 568 nm due to the d – d crystal field transitions 4A2(F) 2E(G), 4A2(F) 4T1(P) and 4A2(F) 2A1(G) of high spin (S = 3/2) Co2+ in tetrahedral crystal field, whose intensities increase with processing temperature. X-ray photoelectron spectroscopy shows that the doped Co2+ ions in the 150 °C processed samples are located mostly on the surface of the particles and they disperse into the entire volume of the particles when processed at higher temperatures. The observations suggest that the FM results from Co2+ attached to the surface sites and it is lost in well dispersed samples formed at TP > 150 °C.
2006. "Preparation, Characterization and Anion Exchange Properties of Polypyrrole/Carbon Nanotube Nanocomposite." Journal of Nanoscience and Nanotechnology 6(2):547-553. Abstract In this study, polypyrrole (PPy) thin film was electrodeposited on carbon nanotube (CNT) backbones by applying a constant deposition potential in solution with 0.1 M pyrrole with different electrolytes such as NaCl, NaNO3, or NaClO4. The hybrid films were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. SEM images revealed the nanostructrure of PPy film generated on CNTs surface. The electrochemical and anion exchange properties of PPy-CNT composite film have been investigated. Nanostructured composite thin films of polypyrrole/CNTs were studied by cyclic votammetry between 0.4 and -0.8 V in aqueous solution to evaluate their cycling stability and capacity for electrically switched anion exchange. It is found that the PPy/CNTs nanocomposites can improve the anion exchange capacity and stability of the PPy-CNTs composite film, which may be attributed to the nanostructure of the polyprrole film, which offer the high aspect ratio of the film and ease of diffusion of anions in the nanostructured film, and the interaction between CNTs and PPy.
2006. "Oxidation of Black Carbon by Biotic and Abiotic Processes." Organic Geochemistry 37(11):1477-1488. doi:10.1016/j.orggeochem.2006.06.022 Abstract The objectives of this study were to quantify the relative importance of either biotic or abiotic oxidation of biomass-derived black carbon (BC) and to characterize the surface properties and charge characteristics of oxidized particulate BC. We incubated BC and BC-soil mixtures at two different temperatures (30ºC and 70ºC) with and without microbial inoculation, nutrient additions, or manure amendments for four months. Abiotic processes were more important for oxidation of BC than biotic processes during this short-term incubation, as inoculation with microorganisms did not change any of the measured parameters. Black C incubated at both 30ºC and 70ºC without microbial activity showed dramatic decreases in pH (in water) from 5.4 to 5.2 and 3.4, as well as increases in cation exchange capacity (CEC at pH 7) by 53% and 538% and in oxygen (O) contents by 4% and 38%, respectively. Boehm titration and Fourier transform infrared (FTIR) spectroscopy suggested that the formation of carboxylic functional groups was the reason for the enhanced CEC during oxidation. The analyses of BC surface properties by X-ray photoelectron spectroscopy (XPS) indicated that the oxidation of BC particles initiated on the surface. Incubation at 30ºC only enhanced oxidation on particle surfaces, while oxidation during incubation at 70ºC penetrated into the interior of particles. Such short-term oxidation of BC has great significance for the stability of BC in soils as well as for its effects on soil fertility and biogeochemistry.
2006. "Ferromagnetism in Ti-Doped ZnO Nanoclusters above Room Temperature." IEEE Transactions on Magnetics 42(10):2697-2699. doi:10.1109/TMAG.2006.879723 Abstract Ferromagnetism is observed at room temperature, when a small percentage (5%) of non-magnetic titanium is added to zinc atoms to form Ti-doped ZnO nanoclusters in the oxygen atmosphere. The nanocluster films are prepared at room temperature by a technique that is a combination of high pressure sputtering along with aggregation. A Super-conducting Quantum Interference Device (SQUID) measures the magnetic properties of this cluster films at various temperatures. Ti dopant exhibits +4 oxidation state through out the cluster film. Coercivity of the samples decreased exponentially with the increase of temperature.
2006. "Surface Degradation of As2S3 Thin Films." Physics and Chemistry of Glasses 47(6):681-687. Abstract Thin films of as-deposited As2S3 glass developed a white haze under ambient conditions. To study this surface degradation, we evaluated the effects of visible light, oxygen, humidity, and temperature. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to characterize the surface of these films. Results showed that As2S3 films simultaneously exposed to light and humidity formed arsenolite (As2O3) crystals on the film surface, correlating with the observed haze. A dry and dark environment is concluded to provide the best protection for storing As2S3 thin films. Additionally, a water vapor barrier layer was shown to inhibit degradation under typically corrosive conditions.
2005. "Supercritical Fluid Immersion Deposition: A New Process for Selective Deposition of Metal Films on Silicon Substrates." Surface & Coatings Technology 190(1):25-31. Abstract Supercritical CO2 is used as a new solvent for immersion deposition, a galvanic displacement process traditionally carried out in aqueous HF solutions containing metal ions, to selectively develop metal films on featured or non-featured silicon substrates. Components of supercritical fluid immersion deposition (SFID) solutions for fabricating Cu and Pd films on silicon substrates are described along with the corresponding experimental setup and procedure. Only silicon substrates exposed and reactive to SFID solutions can be coated. The highly pressurized and gas-like supercritical CO2, combined with the galvanic displacement property of immersion deposition, enables the SFID technique to selectively deposit metal films in small features. SFID may also provide a new method to fabricate palladium silicide in small features or to metallize porous silicon.
2005. "Monodispersed core-shell Fe3O4@Au nanoparticles ." Journal of Physical Chemistry B 109(46):21593-21601. Abstract The ability to synthesize and assemble monodispersed core-shell nanoparticles is important for exploring the unique properties of nanoscale core, shell, or their combinations in technological applications. This paper describes findings of an investigation of the synthesis and assembly of core (Fe3O4)-shell (An) nanoparticles with high monodispersity. Fe3O4 nanoparticles of selected sizes were used as seeding materials for the reduction of gold precursors to produce gold-coated Fe3O4 nanoparticles (Fe3O4@Au). Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, core-shell composition, surface reactivity, and magnetic properties have confirmed the formation of the core-shell nanostructure. The interfacial reactivity of a combination of ligand-exchanging and interparticle cross-linking was exploited for molecularly mediated thin film assembly of the core-shell nanoparticles. The SQUID data reveal a decrease in magnetization and blocking temperature and an increase in coercivity for Fe3O4@Au, reflecting the decreased coupling of the magnetic moments as a result of the increased interparticle spacing by both gold and capping shells. Implications of the findings to the design of interfacial reactivities via core-shell nanocomposites for magnetic, catalytic, and biological applications are also briefly discussed.
2005. "Iron Oxide-Gold Core-Shell Nanoparticles and Thin-Film Assembly." Journal of Materials Chemistry 15(18):1821-1832. Abstract This paper reports findings of an investigation of the synthesis of monolayer-capped iron oxide and core (iron oxide)–shell (gold) nanocomposite and their assembly towards thin film materials. Pre-synthesized and size-defined iron oxide nanoparticles were used as seeding materials for the reduction of gold precursors, which was shown to be effective for coating the iron oxide cores with gold shells (Fe oxide@Au). The unique aspect of our synthesis is the formation of Fe oxide@Au core–shell nanoparticles with controllable surface properties. The novelty of our assembly strategy is the exploitation of the ligand-exchange reactivity at the gold shells for the thin film assembly of the core–shell nanoparticles. The core–shell nanocomposites and assemblies have been characterized using TEM, XRD, XPS, FTIR, TGA, and DCP-AES techniques. In addition to evidence from TEM detection of the change in particle size, UV-Vis observation of the change in the surface plasmon resonance band, and XRD detection of disappearance of the magnetite diffraction peaks after coating the gold shell, the formation of the core–shell morphology was further confirmed by DCP-AES composition analysis of Au and Fe in the molecularly-mediated thin film assembly of Fe oxide@Au particles. The interparticle ligand exchange–precipitation chemistry at the gold shell is to our knowledge the first example demonstrating the inter-shell reactivity for constructing thin films of Fe oxide@Au particles. The results have provided important insights into the design of interfacial reactivities via core–shell nanocomposites for magnetic, catalytic and biosensing applications.
2005. "Adsorption and Reaction of Methanol on Stoichiometric and Defective SrTiO3(100) Surfaces ." Journal of Physical Chemistry B 109(10):4507-4513. doi:10.1021/jp048338t Abstract The adsorption and reaction of methanol (CH3OH) on stoichiometric (TiO2-terminated) and reduced SrTiO3(100) surfaces have been investigated using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and first-principles density-functional calculations. Methanol adsorbs mostly nondissociatively on the stoichiometric SrTiO3(100) surface that contains predominately Ti4+ cations. Molecular desorption of methanol from the stoichiometric surface is observed at ~250 K whereas the multilayer is found to desorb at ~160 K. Theoretical calculations predict weak adsorption of methanol on TiO2–terminated SrTiO3(100) surfaces, in agreement with the experimental results. However, the reduced SrTiO3(100) surface containing substantial amounts of Ti3+ cations exhibits higher reactivity towards adsorbed methanol and H2, C2H4, and CO are the major decomposition products observed on the reduced surface. The surface defects on the reduced SrTiO3(100) surface are partially reoxidized upon saturation exposure of CH3OH onto this surface at 300 K.
2005. "Adsorptive Removal of Organic Sulfur Compounds from Jet Fuel over K-exchanged NiY Zeolites Prepared by Impregnation and Ion Exchange." Industrial and Engineering Chemistry Research 44(15):5740-5749. Abstract NiY zeolites with different Ni loadings were synthesized by incipient wetness impregnation and liquid phase ion-exchange methods using NH4Y and KY zeolites. These Ni-containing Y zeolites were tested as adsorbents for removing organic sulfur compounds from a model jet fuel containing 510 ppmw sulfur and a real JP-8 jet fuel containing 380 ppmw sulfur under ambient conditions either without reduction or after reduction around 600oC. At the adsorption temperature of 80oC, the NiY zeolite containing 30 wt % Ni synthesized by incipient wetness impregnation of NH4Y zeolite was able to clean only about 10 ml of a model jet fuel per g of the adsorbent to produce a desulfurized fuel containing below 1 ppmw sulfur. Under the same experimental conditions, the NiY zeolite prepared using KY zeolite cleaned about 30 ml of the fuel per g of the adsorbent. Better sulfur adsorption performance was observed when the NiY zeolites were synthesized by ion-exchange, and reduced before sulfur adsorption. The reducibility and surface properties of some of the selected NiY zeolites were investigated by temperature-programmed reduction (TPR) and in-situ X-ray photoelectron spectroscopy (XPS). TPR studies indicated that the reducibility of NiY-zeolite was improved when K was present as a co-cation. The in-situ XPS studies of unreduced and reduced samples revealed that presence of K as co-cation in the zeolite matrix helps Ni dispersion at the surface. The promotional effect of K on the sulfur adsorption performance of NiY zeolites was therefore attributed to improved reducibility and surface dispersion of Ni when K was present as a co-cation.
2005. "Gold-polyaniline composites: Part II. Effects of nanometer sized particles." Physical Chemistry Chemical Physics. PCCP 7(20):3619-3625. Abstract The amount of electronic charge transferred between gold particles and polyaniline depends not only on the electron affinity of the two materials but also on the size of the gold particles. As measured by X-ray photoelectron spectroscopy, for particles < 5 nm the binding energy of the electrons is size dependent. This nano-effect has its origin in the electrostatics of particles. It is demonstrated as a measurable shift of the binding energy of the Au4f7/2 photoelectrons emitted from Au particles embedded in a polyaniline matrix. Gold nanoparticle size was evaluated by high resolution transmission electron microscopy.
2005. "Thiophene Hydrodesulfurization over Nickel Phosphide Catalysts: Effect of the Precursor Composition and Support ." Journal of Catalysis 231(2):300–313. Abstract Silica- and alumina-supported nickel phosphide (NixPy) catalysts have been prepared, characterized by bulk and surface sensitive techniques, and evaluated for the hydrodesulfurization (HDS) of thiophene. Series of 30 wt% NixPy/SiO2 and 20 wt% NixPy/Al2O3 catalysts were prepared from oxidic precursors having a range of P/Ni molar ratios by temperature programmed reduction (TPR) in flowing H2. Oxidic precursors with molar ratios of P/Ni = 0.8 and 2.0 yielded catalysts containing phase-pure Ni2P on the silica and alumina supports, respectively. At lower P/Ni ratios, significant Ni12P5 impurities were present in the NixPy/SiO2 and NixPy/Al2O3 catalysts as indicated by X-ray diffraction. The HDS activities of the NixPy/SiO2 and NixPy/Al2O3 catalysts depended strongly on the P/Ni molar ratio of the oxidic precursors with optimal activities obtained for catalysts containing phase pure Ni2P and minimal excess P. After 48 h on-stream, a Ni2P/SiO2 catalyst was 20 and 3.3 times more active than sulfided Ni/SiO2 and Ni-Mo/SiO2 catalysts, respectively. A Ni2P/Al2O3 catalyst was 2.7 times more active than a sulfided Ni/Al2O3 catalyst but only about half as active as a Ni-Mo/Al2O3 catalyst.
2005. "Hydrogen Bubbles and Formation of Nanoporous Silicon during Electrochemical Etching." Surface and Interface Analysis 37(6):555-561. Abstract Many nanoporous Si structures, including those formed by common electrochemical etching procedures, produce a uniformly etch nanoporous surface. If the electrochemical etch rate is slowed down, details of the etch process can be explored and process parameters may be varied to test hypotheses and obtain controlled nanoporous and defect structures. For example, after electrochemical etching of a heavily n-doped (R = 0.05-0.5 Ω -cm) <100> silicon at a current density of 10 mA/cm² in buffer oxide etch (BOE) electrolyte solution defect craters, containing textured nanopores, were observed to occur in ring shaped patterns of rings. The defect craters apparently originate at the hydrogen-BOE bubble interface, which forms during hydrogen evolution in the reaction. The slower hydrogen evolution due to low current density allows sufficient bubble residence time so that a high defect density appears at the bubble edges where local reaction rates are highest. Current carrying Si-OH species are most likely responsible for the widening in the craters. Reducing the defect/doping density in silicon lowers the defect concentration and thereby the density of nanopores. Measurements of photoluminescence lifetime and intensity show a distinct feature when the low density of nanopores formed at ring edges are isolated from each other. Overall features observed in photoluminescence (PL), X-ray photoelectron spectroscopy (XPS) intensity strongly emphasize the role of surface oxide that influences these properties.
2005. "Development of High-Temperature Ferromagnetism in SnO₂ and Paramagnetism in SnO by Fe Doping." Physical Review. B, Condensed Matter and Materials Physics 72(5):054402 (14 p.). Abstract We report the development of room-temperature ferromagnetism in chemically synthesized powder samples of Sn₁-xFexO₂ (0.005 ≤ x ≤ 0.10) and paramagnetic behavior in an identical set of Sn₁-xFexO. The ferromagnetic Sn₀․₉₉Fe₀․₀₁O₂ showed a Curie temperature Tc = 850 K, which is among the highest reported for dilute magnetic semiconductors. No evidence of dopant segregation was detected in Sn₁-xFexO₂ or Sn1-xFexO, suggesting that the emerging magnetic interactions in these systems are strongly related to the properties of the host systems SnO and SnO₂.
2005. "Development of high-temperature ferromagnetism in SnO2 and paramagnetism in SnO by Fe doping." Virtual Journal of Nanoscale Science & Technology 12(7):, Abstract We report the development of room-temperature ferromagnetism in chemically synthesized powder samples of Sn1−xFexO2 *0.005*x*0.05* and paramagnetic behavior in an identically synthesized set of Sn1−xFexO. The ferromagnetic Sn0.99Fe0.01O2 showed a Curie temperature TC=850 K, which is among the highest reported for transition-metal-doped semiconductor oxides. With increasing Fe doping, the lattice parameters of SnO2 decreased and the saturation magnetization increased, suggesting a strong structure-magnetic property relationship. When the Sn0.95Fe0.05O2 was prepared at different temperatures between 200 and 900 °C, systematic changes in the magnetic properties were observed. Combined Mössbauer spectroscopy and magnetometry measurements showed a ferromagnetic behavior in Sn0.95Fe0.05O2 samples prepared at and above 350 °C, but the ferromagnetic component decreased gradually as preparation temperature approached 600 °C. All Sn0.95Fe0.05O2 samples prepared above 600 °C were paramagnetic. X-ray photoelectron spectroscopy, magnetometry, and particle induced x-ray emission studies showed that the Fe dopants diffuse towards the surface of the particles in samples prepared at higher temperatures, gradually destroying the ferromagnetism. Mössbauer studies showed that the magnetically ordered Fe3+ spins observed in the Sn0.95Fe0.05O2 sample prepared at 350 °C is only *24% of the uniformly incorporated Fe3+. No evidence of any iron oxide impurity phases were detected in Sn1−xFexO2 or Sn1−xFexO, suggesting that the emerging magnetic interactions in these systems are most likely related to the properties of the host systems SnO2 and SnO, and their oxygen stoichiometry.
2005. "Functionalized TiO2 nanoparticles for use for in-situ anion immobilization." Environmental Science and Technology 39(18):7306-7310. Abstract 40-60 nm anatase nanoparticles were coated with an organosilane monolayer terminated with an ethylenediamine (EDA) ligand. This functionalized nanoparticle (FNP) was then treated with an aqueous solution of Cu(II) to create a cationic Cu-EDA complex bound to the nanoparticle surface. The Cu-EDA FNP was then studied for its binding affinity for pertechnetate anion from a Hanford groundwater matrix. The Cu-EDA FNP was also evaluated for its injectability into a porous medium for possible application as a subsurface semi-permeable reactive barrier. Injection was readily accomplished, and resulted in a highly uniform distribution of the FNP sorbent in the test column.
2005. "Surface Electronic Properties and Site-Specific Laser Desorption Processes of Highly Structured Nanoporous MgO Thin Films." Surface Science 593(1-3):242-247. Abstract The surface electronic properties of metal oxides critically depends on low-coordinated sites, such as kinks, corners and steps [1]. In order to characterize experimentally those surface states as well as their role for laser desorption processes, we prepare defect enriched surfaces by growing thin MgO films using reactive ballistic deposition [2] on crystalline dielectric substrates. With samples held at room temperature, the resulting MgO films are highly textured and consist of porous columns with column lengths ranging from tens of nanometers up to six micrometers. Measurements by x-ray photoelectron spectroscopy (XPS) are carried out in-situ for MgO films, vacuum-cleaved MgO crystals, and water vapor exposed samples. In the case of thin films, we observe O 1s spectra with a significant shoulder feature at 2.3 eV higher binding energy (HBE) than the corresponding peak at 530.0 eV representing regular lattice oxygen. We evaluate this feature in terms of non-stoichiometric oxygen and formation of an oxygen-rich layer at the topmost surface of the MgO columns. In contrast, no HBE-features are detectable from clean single crystal MgO surfaces, while the hydroxyl O 1s band peaks at 531.6 eV. Under excitation with 266-nm-laser-pulses, known to be resonant with low-coordinated surface anions [3], we observe preferential depletion of defective oxygen-states (HBE signal) and temporary restoration of ideal surface stoichiometry. Furthermore, auxiliary signals are observed on several micrometer thick films, acting like satellites to major photoelectron-peaks (O 1s, Mg 2s, and Mg 2p) but shifted by approximately 4 eV towards lower kinetic energy. These features are depleted by UV-light exposure, pointing to the occurrence of surface-charge imbalance, accompanied by photon stimulated charge-transfer reactions. These results are in line with desorption experiments of neutrals, stimulated by laser excitation at 266 nm. According to the low-coordination nature of nanoporous MgO films, we find that the laser fluence required for oxygen desorption is much lower in comparison to the single-crystal MgO (100) surface. The detected neutral oxygen desorption occurs with a single photon power dependence from nanoporous MgO thin films. This contrasts to the two-photon power dependence observed from MgO single crystals. In fact, the single photon energy of the laser lies within the charge-transfer transition in surface ion pairs with three-coordinated anions (~ 4.7 eV), while a two-photon absorption regime easily lies within the four-coordinated anion charge transfer transition near 5.4 eV [3]. In summary, our XPS studies and laser desorption experiments indicate new surface-site-specific excitation regimes, which might eventually allow for site-specific manipulation of surface morphology.
2005. "Relationship between the structural and magnetic properties of Co-doped SnO₂ nanoparticles." Physical Review. B, Condensed Matter and Materials Physics 72(7):518-524. Abstract In this paper, we present the results of a detailed investigation of the structural, optical and magnetic properties of chemically synthesized pure and Co doped SnO₂ powders using x-ray diffraction (XRD), diffuse reflectance spectroscopy, Raman spectroscopy, x-ray photoelectron spectroscopy and magnetometry. For low doping concentrations of Co (≤ 1%), the SnO₂ lattice contracts and a ferromagnetic behavior is developed. Increasing the Co doping concentration to > 1% leads to a rapid expansion of the lattice and significant structural disorder evidenced by changes in the XRD and Raman spectra presumably due to additional interstitial incorporation of Co. This higher Co doping completely destroys the ferromagnetism. The striking similarity between the changes in the lattice parameters and the magnetic properties of Sn₁-xCoxO₂ indicates a structure-magnetic property relationship.
2005. "Aluminum Nitride-Silicon Carbide Alloy Crystals Grown on SiC Substrates by Sublimation." MRS Internet Journal of Nitride Semiconductor Research 10(5):1-8. Abstract AlN-SiC alloy crystals, with a thickness greater than 500 µm, were grown on 4H- and 6H-SiC substrates from a mixture of AlN and SiC powders by the sublimation-recondensation method at 1860-1990 °C. On-axis SiC substrates produced a rough surface covered with hexagonal grains, while 6H- and 4H- off-axis SiC substrates with different miscut angles (8° or 3.68°) formed a relatively smooth surface with terraces and steps. The substrate misorientation ensured that the AlN-SiC alloy crystals grew two dimensionally as identified by scanning electron microscopy (SEM). X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed that the AlN-SiC alloys had the wurtzite structure. Electron probe microanalysis (EPMA) and x-ray photoelectron spectroscopy (XPS) demonstrated that the resultant alloy crystals had non-stoichiometric ratios of Al:N and Si:C and a uniform composition throughout the alloy crystal from the interface to the surface. The composition ratio of Al:Si of the alloy crystals changed with the growth temperature, and differed from the original source composition, which was consistent with the results predicted by thermodynamic calculation of the solid-vapor distribution of each element. XPS detected the bonding between Si-C, Si-N, Si-O for the Si 2p spectra. The dislocation density decreased with the growth, which was lower than 106 cm-2 at the alloy surface, more than two orders of magnitude lower compared to regions close to the crystal/substrate interface, as determined by TEM.
2005. "Erosion Rate Variations during XPS Sputter Depth Profiling of Nanoporous Films." Surface and Interface Analysis 37(4):417-423. Abstract Sputter depth profiling is commonly used to obtain valuable information regarding the three dimensional distribution of elements within a sample, and is one of the best ways to measure the composition of a buried interface or the uniformity of a thin film. X-ray photoelectron spectroscopy (XPS) is one of the analysis tools often used in conjunction with ion beam erosion to obtain sputter depth profiles. However, to obtain accurate depth information it is often necessary to better understand the sputtering process for a specific materials system. Artifacts such as differential sputtering, varying sputter rates and ion beam-induced chemistry are well known. Here, however, we present evidence from experiments on a porous thin film deposited on a Si wafer that relatively small chemical and/or structural changes in a nanoporous film can affect the rate of erosion measured during sputter depth profiling. Reproducible variations in sputter rate are found with chemical modification leading to compositional changes of the nanoporous thin film. The origin of the sputter rate changes is discussed with the aid of results obtained using Fourier transform infrared spectroscopy, profilometry, nuclear reaction analysis, electron microscopy and XPS-based depth profiling.
2005. "Simple Method for Estimating and Comparing of X-Ray Damage Rates." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 23(6):1740-1744. doi:10.1116/1.2073387 Abstract This note describes an approach for estimating and comparing rates or thresholds for x-ray induced specimen damage during surface analysis. The method uses a common reference material to compare x-ray damage rates reported in the literature and in publications to the rates of damage that may occur on a specific instrument. Although the method makes several assumptions that are only partially valid, results from a few damage data-sets appear acceptably consistent when comparing estimates to a time for 10% damage or signal change.
2005. "Challenges in Applying Surface Analysis Methods to Nanoparticles and Nanostructured Materials." Journal of Surface Analysis 12(2):101-108. Abstract Nanostructured materials of various types and forms are formulated in a variety of novel ways and increasingly subject to many types of chemical and physical analysis. Since nanomaterial systems contain a relatively large amount of surface or interface area, it should be natural to characterize them using tools designed to analyze surfaces and interfaces. We have found that nanoparticles and other nanostructured materials present a variety of challenges. This paper reviews environmental effects on measurements of Ce-oxide nanoparticles and nanoporous silica films and focuses on efforts to quantify the ion damage and sputter rates for the Fe-oxide nanoparticles. We have found that nanoparticles appear more readily damaged and to have sputter rates that exceed “bulk” materials. To verify such effects, we need to know many details about size, size distribution, density, and shape that are not always easily obtained.
2005. "Nanoscale Effects on Ion Conductance of Layer-by-Layer Structures of Gadolinia-doped Ceria and Zirconia." Applied Physics Letters 86(13):131906-131909. Abstract Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.
2005. "Adsorption and Reaction of CO and CO₂ on Oxidized and Reduced SrTiO₃ (100) Surfaces." Journal of Physical Chemistry B 109(20):10327-10331. Abstract The adsorption and reaction of CO and CO2 on oxidized and reduced SrTiO3(100) surfaces have been studied using temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). XPS results indicate that the fully oxidized and vacuum annealed SrTiO3(100) surfaces are nearly defect-free with predominantly Ti4+ ions whereas the sputter-reduced surfaces contain substantial amounts of defects (~ 40% reduced Ti sites). Both CO and CO2 are found to adsorb weakly on the oxidized SrTiO3(100) surfaces. The desorption activation energies of CO and CO2 from the oxidized SrTiO3(100) surfaces are 38 and 32 kJmol-1, respectively, following adsorption at 110 K. On sputter-reduced surfaces, enhanced reactivity of CO and CO2 is observed due to the presence of oxygen vacancy sites, causing dissociative adsorption of these molecules. Our studies indicate that CO and CO2 molecules exhibit relatively weaker binding to SrTiO3(100) compared with TiO2(110) and TiO2(100) surfaces. These differences in reactivity can be attributed primarily to the influence of the Sr cations on the electronic structure of the mixed oxide of SrTiO3.
2005. "Electrochemical Corrosion Behavior of Low Carbon I-Beam Steels In Simulated Yucca Mountain Repository Environment." Corrosion 61(4):381-391. Abstract The electrochemical corrosion behavior of low carbon steel was examined in a simulated Yucca Mountain (YM) ground water by varying the electrolyte concentration and temperature under aerated and deaerated conditions. The results show that in deaerated conditions, the corrosion rate is low in the order of 0.6 to 4.5mpy, between 25 to 85oC, respectively. However, in aerated conditions the measured rates were expectedly very high, in the order of 3-55mpy in the above mentioned temperature levels. The rates initially increased up to 45oC, and a decreasing trend was observed with further increase in temperature from 65 to 85oC. The maximum corrosion rate was occurred at 45oC (54.5mpy). The low corrosion rates observed in all deaerated conditions, and in aerated solutions at higher temperatures were due to the preferential adsorption of Mg-species on the steel surface, as identified by XPS analyses. The results also indicate possible localized corrosion behavior of carbon steel in aerated conditions up to 45oC.
2005. "ZnO nanoclusters: Synthesis and photoluminescence." Applied Physics Letters 87:241917. doi:10.1063/1.2147715 Abstract ZnO nanoclusters were prepared and deposited at room temperature using a newly developed cluster source. The nanoclusters act as a building block for the cluster films deposited on various substrates. The cluster films were characterized by transmission electron microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and photoluminescence. We prepared monodispersed crystalline ZnO nanoclusters of ~7 nm diameter. These clusters have a significant blueshift of ~125 meV (compared to the results published so far) within the ultraviolet region at room temperature. No PL in our samples was observed in the visible region, which implies negligible defect formation in ZnO nanocluster films.
2005. "Effects of Reduction Temperature and Metal-support Interactions on the Catalytic Activity of Pt/γ-Al2O3 and Pt/TiO2 for the Oxidation of CO in the Presence and Absence of H2." Journal of Physical Chemistry B 109(49):23430-23443. doi:10.1021/jp054888v Abstract TiO2- and -Al2O3-supported Pt catalysts were characterized by HRTEM, XPS, EXAFS, and in-situ FTIR after activation at various conditions and their catalytic properties were examined for the oxidation of CO in the absence and presence of H2 (PROX). When -Al2O3 was used as the support, the catalytic, electronic, and structural properties of the Pt particles formed were not affected substantially by the pretreatment conditions. In contrast, the surface properties and catalytic activity of Pt/TiO2 were strongly influenced by the pretreatment conditions. In this case, an increase in the reduction temperature led to higher electron density on Pt, altering its chemisorptive properties, weakening the Pt-CO bonds, and increasing its activity for the oxidation of CO. The in-situ FTIR data suggest that both the terminal and bridging CO species adsorbed on fully reduced Pt are active for this reaction. The high activity of Pt/TiO2 for the oxidation of CO can also be attributed to the ability of TiO2 to provide or stabilize highly reactive oxygen species at the metal-support interface. However, such species appear to be more reactive towards H2 than CO. Consequently, Pt/TiO2 shows substantially lower selectivities towards CO oxidation under PROX conditions than Pt/-Al2O3.
2004. "Supercritical Fluid Synthesis and Characterization of Catalytic Metal Nanoparticles on Carbon Nanotubes." Journal of Materials Chemistry 14(5):908-913. Abstract A rapid, convenient and environmentally benign method has been developed for the fabrication of metal nanoparticlemultiwall carbon nanotube (MWCNT) composites. Nanoparticles of palladium, rhodium and ruthenium are deposited onto functionalized MWCNTs through a simple hydrogen reduction of metal--diketone precursors in supercritical carbon dioxide, and are characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses. These highly dispersed nanoparticles, with a narrow range of size distribution and good adhesion on MWCNT surfaces, are expected to exhibit promising catalytic properties for a variety of chemical reactions. Preliminary experiments demonstrate that Pd nanoparticles supported on MWCNTs are effective catalysts for hydrogenation of olefins in carbon dioxide. The Pd nanoparticleMWCNT composite also shows a high electrocatalytic activity in oxygen reduction for potential fuel cell application.
2004. "Electrophilic Aromatic Substitutions of Amine and Sulfonate onto Fine-Grained Activated Carbon for Aqueous-Phase Metal Ion Removal ." Separation Science and Technology 39(14):3263-3279. Abstract This paper describes the synthesis and characterization of functionalized activated carbon. Evaluation of the activated carbon materials for metal removal from aqueous solution is also discussed.
2004. "Adsorption and Reaction of Acetaldehyde on Stoichiometric and Defective SrTiO₃(100) Surfaces." Journal of Physical Chemistry B 108(5):1646-1652. Abstract The adsorption and reaction of acetaldehyde (CH₃CHO), on stoichiometric (TiO₂-terminated) and reduced SrTiO₃(100) surfaces, have been investigated using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Acetaldehyde adsorbs molecularly on the stoichiometric SrTiO₃(100) surface that contains predominately Ti₄⁺ cations. The Ti₄⁺ sites on the stoichiometric SrTiO₃(100) surface are not sufficiently active for surface reactions such as aldol condensation, as opposed to the Ti₄⁺ ions on the TiO₂ (001) surface. However, decomposition and redox reactions of acetaldehyde occur in the presence of surface defects created by Ar⁺ sputtering. The decomposition products following reactions of acetaldehyde on the defective surface include H₂, C2H₄, CO, C₄H6 and C₄H₈. Reductive coupling, to produce C₂H₄ and C₄H₈, is the main reaction pathway for decomposition of acetaldehyde on the sputter reduced SrTiO₃ (100) surface.
2004. "Adsorption and Reaction of Acetaldehyde on Stoichiometric and Defective SrTiO₃ (100) Surfaces." Journal of Physical Chemistry B 108(5):1646 - 1652. Abstract The adsorption and reaction of acetaldehyde (CH₃CHO), on stoichiometric (TiO₂-terminated) and reduced SrTiO₃(100) surfaces, have been investigated using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Acetaldehyde adsorbs molecularly on the stoichiometric SrTiO₃(100) surface that contains predominately Ti₄⁺ cations. The Ti₄⁺ sites on the stoichiometric SrTiO₃(100) surface are not sufficiently active for surface reactions such as aldol condensation, as opposed to the Ti₄⁺ ions on the TiO₂(001) surface. However, decomposition and redox reactions of acetaldehyde occur in the presence of surface defects created by Ar⁺ sputtering. The decomposition products following reactions of acetaldehyde on the defective surface include H₂, C₂H₄, CO, C₄H₆ and C₄H₈. Reductive coupling, to produce C₂H₄ and C₄H₈, is the main reaction pathway for decomposition of acetaldehyde on the sputter reduced SrTiO₃(100) surface.
2004. "Epitaxial Growth and Properties of Cobalt-doped ZnO on α-Al₂O₃ Single-Crystal Substrates." Physical Review. B, Condensed Matter 70(5):054424. Abstract Co-doped ZnO (CoxZn₁-xO) is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial CoxZn₁-xO films on Al₂O₃(012) substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of Co (x < 0.35) can be uniformly incorporated into the film without phase segregation. Co is found to be in the ⁺² oxidation state, independent of x. This material can be grown n type by the deliberate incorporation of oxygen vacancies, but not by inclusion of ~1 at. % Al. Semiconducting films remain ferromagnetic up to 350 K. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per Co basis was very small (< 0.1 B/Co), even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the Co(II) ions behave as magnetically isolated S = 3/2 spins.
2004. "Reaction of Hydroquinone with Hematite I. Study of Adsorption by Electrochemical-Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy." Journal of Colloid and Interface Science 274 (2004)(2):433-441. Abstract The reaction of hematite with quinones and the quinone moieties of larger molecules may be an important factor in limiting the rate ofreductive dissolution, especially by iron-reducing bacteria. Here, the electrochemical and physical properties of hydroquinone adsorbed on hematite surfaces at pH 2.5–3 were investigated with cyclic voltammetry (CV), electrochemical-scanning tunneling microscopy (EC-STM), and X-ray photoelectron spectroscopy (XPS). An oxidation peak for hydroquinone was observed in the CV experiments, as well as (photo)reduction of iron and decomposition of the solvent. The EC-STM results indicate that hydroquinone sometimes forms an ordered monolayer with ∼1.1 H₂/nm², but can be fairly disordered (especially when viewed at larger scales). XPS results indicate that hydroquinone and benzoquinone are retained at the interface in increasing amounts as the reaction proceeds, but reduced iron is not observed. These results suggest that quinones do not adsorb by an inner-sphere complex where adsorbate–surface interactions determine the adsorbate surface structure, but rather in an outer-sphere complex where interactions among the adsorbate molecules dominate.
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.
2004. "Probing Cation Antisite Disorder in Gd₂Ti₂O₇ Pyrochlore by Site-Specific Near-Edge X-ray-Absorption Fine Structure and X-ray Photoelectron Spectroscopy." Physical Review. B, Condensed Matter and Materials Physics 70(10):100101(R), 1-4. Abstract Disorder in Gd₂Ti₂O₇ is investigated by near-edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy (XPS). NEXAFS shows Ti⁺⁴ ions occupy octahedral sites with a tetragonal distortion induced by vacant oxygen sites. O 1s XPS spectra obtained with a charge neutralization system from Gd₂Ti₂O₇ (100) and the Gd₂Ti₂O₇ pyrochlore used in Phys. Rev. Lett. 88, 105901 (2002), both yielded a single peak, unlike the previous result on the latter that found two peaks. The current results give no evidence for an anisotropic distribution of Ti and O. The extra features reported in the aforementioned communication resulted from charging effects and incomplete surface cleaning. Thus, a result confirming the direct observation of simultaneous cation–anion antisite disordering and lending credence to the split vacancy model has been clarified.
2004. "Spectroscopic Characterizations of Molecularly-Linked Gold Nanoparticle Assemblies Upon Thermal Treatment." Langmuir 20(10):4254-4260. Abstract Nanostructured gold catalysts have attracted increasing interest since the pioneer work of Haruta which demonstrated unusually high catalytic activities for CO oxidation when the nanoparticles were made less than ~10 nm diameter in size and supported on oxides. It is possible to use core-shell gold nanoparticles to construct the catalyst and activate the catalytic activity by applying a thermal-treatment process. The understanding of how the core-shell surface components reconstitute in such a catalytic activation process will have important implications in expanding this approach to the design and fabrication of nanostructured catalysts. We report herein the results of a study of decanethiolate-capped gold nanoparticles of 2-nm and 5-nm core sizes assembled on planar substrates using 1,9-nonanedithiol (NDT) and 11-mercaptoundecanoic acid (MUA) as molecular linkers were studied as a model system. XPS and IR are employed to detect the identity of surface species and to analyze the elemental composition or oxidation states of the nanomaterials, from which we derive structural information about the surface reconstitution of the core-shell nanostructured catalysts. Electrochemical methods are used to characterize the electrocatalytic activities.
2004. "Nanoparticle-Structured Ligand Framework as Electrode Interfaces." Electroanalysis 16(1-2):120-126. Abstract Nanostructured electrode based on gold nanoparticle network was developed for selective detection of metal ions.
2004. "Composition-Controlled Synthesis of Bimetallic Gold-Silver Nanoparticles." Langmuir 20(25):11240-11246. Abstract This paper reports finding of an investigation of the synthesis of monolayer-capped binary gold-silver bimetallic nanoparticles that is aimed at understanding the control factors governing the formation of the bimetallic compositions. The findings have important implications to the exploration of gold-based bimetallic nanoparticles for biosensing and fuell cell catalytic nanomaterials.
2004. "Direct Determination of Volume Changes in Ion-Beam-Irradiated SiC." Journal of Applied Physics 95(9):4687-4690. Abstract A single crystal 6H-SiC wafer was sequentially implanted in two areas at 873 and 295 K using 2.0 MeV Au₂⁺ ions under off-axis conditions. Identical Au profiles, as a function of atomic areal density, were produced at 873 and 295 K. The linear expansion in the amorphous state produced at 295 K was measured relative to the slightly damaged state produced at 873 K, using the Au profiles as references. The red-shift of the plasmon-loss peak was also used to directly measure the local density changes. Based on these measurements, the volume expansion of the amorphous state in 6H-SiC at 295 K is 11.51.9%, while that in the slightly damaged state at 873 is 0.9%.
2004. "Noncovalent Functionalization of Carbon Nanotubes with Molecular Anchors using Supercritical Fluids ." Journal of Physical Chemistry B 108(25):8737-8741. Abstract In this article we describe a facile and effective method for the modification of multi wall carbon nanotubes with molecular anchor molecules using supercritical fluids. Through choice of deposition conditions the degree of loading in these nanotube-anchor structures can be controlled to achieve sub-monolayer, monolayer, or greater-than-monolayer coverage. This level of control represents a potential advantage of supercritical fluids over liquid solvents for anchor deposition. Employment of the described technique is expected to enable the direct addition of desired chemical functionality to many carbon nanotube structures for a variety of applications.
2004. "X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced CeO₂(111) Surfaces." Surface Science Spectra 11(1-4):73-81. Abstract We have studied the electronic structure of oxidized and reduced CeO₂ (111) surfaces using X-ray photoelectron spectroscopy (XPS). The 50 nm thick Co₂(111) film was grown on a YSZ(111) substrate using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in-situ RHEED (reflection high energy electron diffraction) and ex-situ XRD (X-ray diffraction), HRTEM (high resolution transmission electron microscopy) and RBS (Rutherford backscattering spectroscopy). The lattice mismatch between CeO₂(111) and YSZ(111) is less than 5% and yields a flat surface that is comprised of an equivalent number of Ce⁴⁺ and O₂₋ ions. Oxidation with O₂ at 773 K under UHV conditions was sufficient to fully oxidize the CeO₂(111). Surface reduction was carried out by annealing in UHV at 973 K.
2004. "Chemical Processing in High-Pressure Aqueous Environments. 7. Process Development for Catalytic Gasification of Wet Biomass Feedstocks." Industrial and Engineering Chemistry Research 43(9):1999-2004. Abstract Through the use of a metal catalyst, gasification of wet biomass can be accomplished with high levels of carbon conversion to gas at relatively low temperature (350 C). In the pressurized-water environment (20 MPa) near-total conversion of the organic structure of biomass to gases has been accomplished in the presence of a ruthenium metal catalyst. The process is essentially steam reforming as there is no added oxidizer or reagent other than water. In addition, the gas produced is a medium-heating value gas due to the synthesis of high-levels of methane, as dictated by thermodynamic equilibrium. Biomass trace components cause processing difficulties using the fixed catalyst bed tubular reactor system. Results are described for both bench-scale and scaled-up reactor systems.
2004. "X-ray Photoelectron Spectroscopic Analyses of Corrosion Products Formed on Rock Bolt Carbon Steel in Chloride Media with Bicarbonate and Silicate Ions." Corrosion Science 46(11):2629-2649. Abstract The passivation behavior of Yucca Mountain Repository rock bolt carbon steel in deaerated 3.5% NaCl solution containing SiO₂_₃ and HCO_₃ ions was investigated by potentiodynamic polarization, electrochemical impedance spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopic methods. Polarization results indicate that combinations of silicate and bicarbonate anions decrease the passive current density and raise the pitting potential. XPS results indicate the enrichment of silica at passive potentials and the formation of mixed FeCO₃ and silica film at lower potentials. This change in film composition was responsible for the changes in corrosion rate at lower and higher potentials. XPS results also support the thermodynamic data with regard to the occurrence of second oxidation peak observed in the polarization curves to be due to the oxidation of FeCO₃ to Fe2O₃
2004. "X-ray Photoelectron Spectroscopy Studies of Oxidized and Reduced Ce₀․₈Zr₀․₂O₂(111)." Surface Science Spectra 11(1-4):82-90. doi:10.1116/11.20050202 Abstract We have studied the electronic structure of oxidized and reduced Ce₀․₈Zr₀․₂O₂(111) using x-ray photoelectron spectroscopy (XPS). The 50 nm thick Ce₀․₈Zr₀․₂O₂(111) film was grown on a YSZ(111) substrate using oxygen assisted molecular beam epitaxy (OPA-MBE). This film has been characterized using in-situ RHEED (reflection high energy electron diffraction) and ex-situ XRD (x-ray diffraction), HRTEM (high energy resolution transmission electron spectroscopy) and RBS (Rutherford backscattering spectroscopy). Surfaces of the Ce₀․₈Zr₀․₂O₂(111) film used in this study is found to be unreconstructed and exhibits the structure of bulk CeO₂(111) where Zr atoms occupy the lattice sites of Ce in the fluorite structure of ceria. The extent of surface reduction as a result of vacuum annealing has been reported here in addition to the electronic structure of defect-free Ce₀․₈Zr₀․₂O₂(111) surface.
2003. "Beam Effects During AES and XPS Analysis." Chapter 9 in Surface Analysis by Auger and X-ray Photoelectron Spectroscopy. IM Publications, Chichester, United Kingdom. Abstract It is important to realize that any surface analysis method may alter the specimen in some way. Alterations that complicate the ability to collect the desired information are usually considered damage. Damage (like beauty) is in the eye of the beholder. In some cases, analysis-induced changes to a sample will have little or no impact on the information sought. In other cases, similar changes will be totally unacceptable and considered information-destroying damage. The analyst must therefore be able to recognize damage in all its various forms, understand its origins, and be able to compensate for, or limit, its effects on the analysis.
2003. "Appendix F: Comparing Beam Damage Rates Using Susceptibility Tables." In Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, ed. D. Briggs and J.T. Grant, pp. 845-856. SurfaceSpectra Ltd. & IM Publications, Manchester, United Kingdom. Abstract Many research groups have observed electron and x-ray damage on a variety of materials. It is, of course, highly desirable to take advantage of the considerable experience gained by others in observing the presence of damage. Ideally, damage processes and rates would be determined for the samples of interest on the instrument in which measurements are to be made. However, in many cases such measurements are not possible and many important speciments are one of a kind. Several compilations of damage rates are available in the literature 1,2, or from Companies 3. In most cases the data from these potentially useful data sets are not likely to directly relate to the damage rates that might be observed for other instruments. There are two related reasons for this. First there is over an order of magnitude difference between x-ray damage rates observed on instruments in current use, as reported by Yoshihara and Tanaka.4 In addition, many new instruments have higher x-ray fluxes than many older instruments. The damage rates observed in the Pacific Northwest National Laboratory (PNNL) PHI Quantum 2000, are in the lower 1/3 of the instruments reported in Reference 4 but are approximately 5 times faster than those reported by Beamson and Briggs1 in 1992 for the Scienta 300 system.
2003. "Immersion Deposition of Metal Films on Silicon and Germanium Substrates in Supercritical Carbon Dioxide." Chemistry of Materials 15(1):83-91. Abstract A low temperature carbon dioxide based on immersion deposition technology (SFID) has been developed for producing palladium, copper, silver, and other metal films on silicon-based substrates in supercritical CO2. The reaction is initiated by oxidation of elemental silicon to SiF4 or H2SiF6 by HF with the release of electrons that cause the reduction of metal ions in an organometallic precursor to the metallic form on silicon surface in CO2. Only the substrate surfaces are coated with metals using this method. Based on surface analysis of the films and spectroscopic analysis of the reaction products, the mechanism of metal film deposition is discussed. The metal films (Pd, Cu, and Ag) formed on silicon surfaces by the SFID method exhibit good coverage, smooth and dense texture, high purity and a metallic behavior. Similarly, metal films can also be deposited onto geranium substrates using SFID. The gas-like properties and the high pressure of the supercritical fluids, combined with the low reaction temperature, make this SFID method potentially useful for depositing thin metal films in small features, which are difficult to accomplish by conventional CVD methods.
2003. "Temperature-Induced Phase Separation in Chromium Films." Applied Physics Letters 82(14):2230-2232. Abstract Study of phase seperton at Cr/Si interface. Vacuum evaporation of chromium on Si(100) results in an interfacial nanophase layer followed by a mixed grain region. In this work, the mixed grain region in chromium is nearly transformed into a single-phase region, which resulted in a maximum phase separation between nano -A15 phase and bulk -bcc chromium. It is achieved by creating a temperature gradient across chromium-silicon interface by using rapid evaporation at a growth rate of ~3 m/min at a source-tosubstrate (SS) distance of 2 cm. Separately-grown nanophase chromium films at SS distance of 32 cm show predominant -A15 phase. Short-SS-distance rapid evaporation has a potential to produce selective phase separation by combining temperature gradient with the interfacial stress.
2003. "Investigation of Copper(I) Oxide Quantum Dots by Near-Edge X-ray Absorption Fine Structure Spectroscopy." Chemistry of Materials 20(15):3939-3946. Abstract Copper oxide quantum dots (CuOQD) were grown in various thicknesses on different SrTiO₃(001) surfaces and were investigated by near edge x-ray absorption fine structure (NEXAFS) spectroscopy. The experimental growth conditions for the CuOQD were optimized to obtain Cu₂O as the major phase. The CuOQD grown on clean SrTiO₃(001) surfaces at 825 K or higher with p(O₂) of 9.0x10-7 Torr or above contain mostly CuO contrasting to CuOQD grown at 800 K with p(O₂) of ~7.0x10-7 Torr that contain primarily Cu₂O. Furthermore, it is established that there is a strong interaction between the SrTiO₃(001) surface and the first few monolayers of the CuOQD, which induces the formation of Cu(II). However, this interaction is mitigated with increasing thickness of CuOQD resulting in the exclusive formation of Cu₂O in the topmost layers. The influence of the SrTiO₃(001) substrate on the formation of CuOQD can be reduced by modifying the substrate surface using chemical treatment and/or energetic Au₂⁺ ion-beam irradiation, since the substrate effect results from the reaction between the substrate oxygen and the copper atoms from the CuOQD. Examination of the photochemical properties of these CuOQD shows that prolonged soft x-ray irradiation under vacuum reduces Cu(II), which is present as a minor impurity in the CuOQD.
2003. "X-Ray Photoelectron Spectroscopic Study of the Activation of Molecularly-Linked Gold Nanoparticle Catalysts." Langmuir 19(1):125-131. Abstract This paper reports the results of a study of the activation of core-shell assembled gold nanoparticle catalysts using X-ray photoelectron spectroscopy (XPS). The goal is to determine the surface reconstitution of the nanostructured catalysts upon electrochemical activation for the electrocatalytic oxidation of methanol. The decanethiolate-capped gold nanoparticles of 2~5 nm core sizes were assembled as catalyst thin films on electrode surfaces using 1,9-nonanedithiol and 11-mercaptoundecanoic acid as molecular linkers. The XPS results have provided two important insights into the surface reconstitution of the activated nanostructure. First, the capping/linking thiolates or dithiolates are partially removed to produce the catalytic access, with the degree of removal being dependent on the nature of the molecular linker. Second, oxygenated species are detected on the activated gold nanocrystals, demonstrating the formation of surface gold oxide and its participation in the electrocatalytic oxidation of methanol. The findings are also correlated with results from studies of surface microscopic morphology and interfacial mass flux, and provide further insights into issues related to the design and preparation of highly active nanostructured gold catalysts.
2003. "Electron Beam Damage in Poly(Vinyl Chloride) and Poly(Acrylonitrile) as Observed by Auger Electron Spectroscopy." Surface Science Spectra 10:67-79. Abstract AES spectra of spun-cast films of poly(vinyl chloride) (PVC) and poly(acrylonitrile) (PAN) were collected over a period of time to determine specimen damage during exposure to a 10kV electron beam. For the PVC, loss of chlorine was observed over a period of 203 minutes to the extent that the final chlorine concentration was only 20% of its original value. PAN exhibited a loss in nitrogen content over a period of 120 minutes, but the rate of damage to the polymer was significantly less than PVC. Figure 1 shows the atomic concentration in the PVC film as a function of dose (time). It takes a dose of approximately 7.0x10-5 Ccm-5 for the chlorine concentration to fall from its original value by 10% (one definition of critical dose). Figure 2 shows a similar drop in nitrogen concentration in the PAN film as a function of dose. For this polymer, it takes a dose of 1.3x10-3 Ccm-2 for the nitrogen concentration to fall by 10%.
2003. "Redox Properties of Water on the Oxidized and Reduced Surfaces of CeO₂ (111)." Surface Science 526(1-2):1-18. Abstract We present x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) results probing the surface chemistry of water on the oxidized and reduced surfaces of a 500 Å epitaxial CeO₂ (111) film grown on yttria-stabilized ZrO₂ (111). Oxidation with O₂ at 773 K under UHV conditions was sufficient to generate XPS spectra reflective of fully oxidized CeO₂ (111). Surface reduction was carried out by annealing in UHV between 773 and 973 K, and the level of reduction was quantified using changes in the Ce₃d₃/₂ 4f0 photoemission peak at 917 eV which results primarily from Ce₄⁺ sites. As expected, the level of surface reduction (generation of Ce₃⁺ sites) increased with increasing temperature. These Ce₃⁺ sites were primarily in the first layer based on the fact that exposure of the film to O₂ at RT resulted in nearly complete conversion of Ce₃⁺ to Ce₄⁺. Annealing at 773 K led to a surface in which approximately 40% of the surface Ce₄⁺sites were reduced to Ce₃⁺, whereas annealing at higher temperatures led to more substantial reduction of the first layer along with some subsurface reduction that was not reoxidized by RT exposure to O₂.
2003. "Beam Damage of Poly(Vinyl Chloride) [PVC] Film as Observed by X-ray Photoelectron Spectroscopy." Surface Science Spectra 10(101):57-66. Abstract XPS spectra of a spin-coated film poly(vinyl chloride) (PVC) were collected over a period of 243 minutes at 303 K to determine specimen damage during long exposures to monochromatic Al Ka x-rays. For this PVC film we measured the loss of chlorine as a function of time by rastering a focused 104.6 w 100 um diameter x-ray beam over a 1.4 mm x 0.2 mm area on the sample.
2003. "Beam damage of poly(vinyl chloride) [PVC] as observed by x-ray photoelectron spectroscopy at 143 K, 303 K and 373 K." Surface Science Spectra 10:101-116. Abstract X-ray beam damage is often observed during surface analysis of beam sensitive materials as indicated in the introduction to this issue and in a wide variety of references. While damage occurs in a wide range of materials, those that are most susceptible to damage are materials that contain low energy covalent bonds such as polymers or other organic materials. Even amongst the relatively easily damaged polymers, there is a wide range of damage rates. The data reported in this submission was collected in the interest of comparing the rates of damage produced by x-ray and electron irradiation of different materials. The focus of this submission is x-ray damage of bulk poly(vinyl chloride) PVC since it is a readily available material. The temperature of the sample holder was controlled during irradiation of the PVC in order to determine the extent to which localized sample heating affects the rate of damage. PVC degrades by photoionization, resulting in the production of HCl through H and Cl bond cleavage. Bulk PVC has previously been the focus of an x-ray damage study involving many laboratories throughout the world. For comparison to the bulk PVC data, damage rates for thin films of poly(2-chloroethyl methacrylate) (PCEMA) are also reported for the same three temperatures. Measurements of several relatively common materials on one instrument can provide a data base that allows damage rates on one instrument to be linked or compared to other damage data in the literature.3 , , , In addition to the PCEMA films and bulk PVC, other data collected at the same x-ray parameters include thin films of PVC (for which damage rates are essentially identical to the bulk material) and of poly(acrylonitrile) PAN which is more stable that either PCEMA or PVC. An additional set of PVC data for a different instrument is also included in the volume. X-ray beams used for XPS produce less damage in materials than electrons used for AES analysis. This is due to both the weaker interaction between x-rays and materials (relative to electrons) and the deeper penetration of x-rays into the material producing a lower damage density. Although the rates of damage for x-rays and electrons differ, many of the processes are similar. Both electron and x-ray damage has been collected for PVC and PAN as reported in this volume of SSS8,9,12 and summarized in the introduction. The overall damage rates for PVC and PAN differ by an order of magnitude, but the ratios of the electron and x-ray damage rates for these two materials are nearly the same.1,7
2003. "Beam damage of poly(2-chloroethyl methylacrylate) [PCEMA] films as observed by x-ray photoelectron spectroscopy at 143 K, 303 K, and 373 K." Surface Science Spectra 10:80-100. Abstract X-ray beam damage is often observed during surface analysis of beam sensitive materials as indicated in the introduction to this issue and in a wide variety of references. While damage occurs in a wide range of materials, those that are most susceptible to damage are materials that contain low energy covalent bonds such as polymers or other organic materials. Even amongst the relatively easily damaged polymers, there is a wide range of damage rates. The focus of this submission is on poly(2-chloroethyl methylacrylate) [PCEMA] films. In order to determine the extent to which localized sample heating could influence damage rates the temperature of the substrate holding the PECMA was controlled during irradiation. PCEMA presumably degrades primarily by photo-ionization, resulting in the production of HCl through H and Cl bond cleavage. PCEMA has been recommended as a polymer for use as a reference for evaluating x-ray damage. PCEMA has been shown to be more sensitive to degradation than PVC which has also been used as a damage sensitive material useful for comparison of damage rates. Measurements of several relatively common materials on one instrument can provide a data base that allows damage rates on one instrument to be linked or compared to other damage data in the literature.3,4,5, Therefore for purposes of comparison, damage rates for bulk PVC at the same three different temperatures used for the PCEMA data have been collected and are also presented in this volume. Other data collected at the same x-ray parameters include thin films of PVC (for which damage rates are essentially identical to the bulk material) and of poly(acrylonitrile) PAN which is more stable that either PCEMA or PVC.
2003. "Oxidation of H2S by Iron Oxides in Unsaturated Conditions." Environmental Science and Technology 37(10):2192-2199. Abstract Previous studies have demonstrated that gas phase H2S can immobilize certain redox-sensitive contaminants (Cr, U, Tc, etc.) in vadose zone environments. A key issue for effective and efficient delivery of H2S in these environments is its reactivity with indigenous iron oxides. To elucidate the factors that control the transport of H2S in the vadose zone, laboratory experiments were conducted to determine reaction mechanisms and rates of H2S oxidation by iron oxide coated sands using several carrier gas compositions and variable flow rates. Most experiments were conducted using ferrihydrite-coated sand. Additional studies were conducted with goethite and hematite coated sand and a natural sediment. Column experiments were conducted with 200 ppmv H2S in a carrier gas at three flow rates. Three carrier gases were used; N2, Air and O2. Selective extractions were conducted at the end of each column experiment to determine the mass balance of the reaction products for support of the postulated reaction pathways. XPS was used to confirm the presence of the reaction products. Results from the column experiments containing ferrihydrite indicate that when N2 is used as the carrier-gas, the major H2S oxidation products are FeS and elemental sulfur (S0). The ratios of FeS/S0 at the end of these experiments were consistent with the stoichiometry of the postulated reactions. When air or O2 were used as the carrier gas, S0 become the dominant reaction product along with FeS2 and smaller amounts of FeS, sulfate and thiosulfate.
2003. "Introduction to Surface Science Spectra Data on Electron and X-ray Damage: Sample Degradation during XPS and AES Measurements." Surface Science Spectra 10(1-4):47-56. Abstract The types of damage to sample surfaces that can occur during X-ray or electron irradiation used during Auger electron spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) are summarized. The material and instrumental dependence of the rate and nature of damage formation or sample degradation are highlighted. Particular note is made of an enhanced susceptibility of thin films to damage. A simple method to enable comparison of published damage rates to what may be observed in a specific system is described and discussed in relation to data included in this journal volume. Strategies for detecting and minimizing damage are presented.
2002. "XAS and XPS Characterization of Monolayers Derived from a Dithiol and Structurally Related Disulfide-Containing Polyamides." Langmuir 18(21):8123-8128. Abstract X-ray absorption spectroscopy and X-ray photoemission spectroscopy have been used to examine sulfur-gold bond formation in self-assembled monolayers derived from a dithiol monomer and disulfide-containing polyamide. These compounds were designed to allow the molecules to adsorb to gold through two terminal sulfurs, forming surface-attached loops. Element and site-specific density of unoccupied electronic states was probed by x-ray absorption spectroscopy at the C₁s, N₁s, O₁s (K-edge) and S 2p (L₂,3-edge) absorption edges. Photoemission measurements of the C₁s, N₁s, O₁s and S 2p core lines were also used to estimate relative coverage, confirm layer formation and evaluate chemical bonding of the monomer and polymer to the Au coated substrates. In case of the dithiol monomer SAM, the spectroscopic evidence clearly shows that most of the molecules adsorb through only one sulfur. The disulfide-containing polymer, in contrast to the monomer, attaches to the surface through both sulfurs to form the anticipated surface-attached loop.
2002. "High Energy Ion Beam Studies of Ion Exchange in a Na₂O-Al₂O₃-SiO₂ Glass." Journal of Applied Physics 91(4):1910-1920. Abstract As part of understanding the processes leading to sodium release and ion exchange, the surface and near surface reaction regions on several specimens of a Na₂O-Al₂O₃-SiO₂ glass have been examined after exposures to isotopicaly labeled aqueous solutions. The majority of the analyses describe here have been carried out using energetic ion beam analysis. Rutherford backscattering spectrometry (RBS) has been used to measure the overall glass composition and to determine the profiles and amounts of Na released from the surface. An important part of the ion exchange process is the uptake and incorporation of hydrogen and oxygen in the glass from the solution. To facilitate this analysis, the glasses were exposed to a solution containing 18O and deuterium and analyzed by accelerator based nuclear reaction analysis (NRA). To confirm some of the RBS depth profile data very near the surface, XPS depth profiles were collected on some samples. Although the Na concentration is decreased in the near surface region, it is not totally removed from the outer surface. In this same region, there is also a significant amount of 18O incorporated demonsrating considerable interaction between the water and the glass. Deeper into the material the amounts of deuterium and 18O are more consistent with water or H3O+ diffusion. These results suggest that there exist an outer reaction layer and an inner diffusion controlled layer in the surface region of the reacted glass.
2002. "Interfacial Chemistry and the Performance of Bromine-etched CdZnTe Radiation Detector Devices." IEEE Transactions on Nuclear Science 49(4):2005-2009. Abstract The interfacial chemistry and composition of Pt electrodes sputter deposited on bromine-etched CdZnTe surfaces was studied by XPS, SIMS, AES, NRA and RBS. The interfacial composition of a functioning and a non-functioning CdZnTe detector shows significant differences. The degree of cation out-diffusion into the Pt overlayer and the in-diffusion of Pt into the CdZnTe correlate with the degree of oxidation found at the metal-semiconductor interface. Practically all the oxide present at the interface was found to be TeO₂. The results suggest that the inter-diffusion of the atoms and associated charges contribute to stoichiometric variations at the metal-semiconductor interface and influence the electrical performance of the devices.
2002. "In2O3/Al2O3 Catalysts for NOx Reduction in Lean Condition." Journal of Catalysis 210(1):97-105. Abstract The lean NOx performance and catalytic properties of In2O3/Al2O3 catalysts were investigated. High lean NOx activity was observed when propene was used as a reductant in the presence of 9% O2 and 7% H2O at a space velocity of 30,000h-1. The optimum lean NOx activity of In2O3/Al2O3 catalysts was observed at a loading of 2.5 wt.% indium on -Al2O3 which was prepared by a sol-gel technique (230 m2/g). When propane was used as a reductant, the In2O3/Al2O3 catalyst did not promote NOx reduction compared to the alumina substrate. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and temperature programmed reduction (TPR) have been used to characterize a series of In2O3/Al2O3 catalysts to better understand the surface structure of indium oxide species on the alumina support. The XRD data indicated that crystalline In2O3 was present at In2O3 loadings > 5wt.% and the quantity of the crystalline phase increased as a function of indium loading. XPS results suggested that indium oxide existed as a well-dispersed phase up to 10wt.% indium. The well dispersed or reducible indium oxide species below 400 oC in TPR experiments were assigned as the sites which activate propene to oxygenated hydrocarbons such as acetaldehyde and acrolein. Alumina sites readily utilize the oxygenated hydrocarbons to reduce NOx. Dual-function mechanism was proposed to explain NOx reduction over In2O3/Al2O3 catalysts.
2002. "Electronic Structure of Ytterbium-Doped Strontium Fluoroapatite: Photoemission and Photoabsorption Investigation ." Journal of Applied Physics 91(8):5135-5140. Abstract X-ray photoemission and x-ray photoabsorption were used to study the composition and the electronic structure of ytterbium-doped strontium fluoroapatite (Yb:S-FAP). High resolution photoemission measurements on the valence band electronic structure and Sr 3d, P 2p and 2s, Yb 4d and 4p, F 1s and O 1s core lines were used to evaluate the surface and near surface chemistry of this fluoroapatite. Element specific density of unoccupied electronic states in Yb:S-FAP were probed by x-ray absorption spectroscopy (XAS) at the Yb 4d (N4,5-edge), Sr 3d (M4,5-edge), P 2p (L2,3-edge), F 1s and O 1s (K-edges) absorption edges. These results provide the first measurements of the electronic structure and surface chemistry of this material.
2002. "Surface Decontamination of Simulated Chemical Warfare Agents Using a Nonequilibrium Plasma with Off-Gas Monitoring." IEEE Transactions on Plasma Science 30(4):1454-1459. Abstract InnovaTek is developing a surface decontamination technology that utilizes active species generated in a nonequilibrium corona plasma. The plasma technology was tested against DMMP, a simulant for the chemical agent Sarin. GC-MS analysis showed that a greater than four log10 destruction of the DMMP on an aluminum surface was achieved in a 10 minute treatment. An ion-trap mass spectrometer was utilized to collect time-resolved data on the treatment off-gases. These data indicate that only non-toxic fragments of the broken down DMMP molecule were present in the gas phase. The technology is being further refined to develop a product that will not only decontaminate surfaces but will also sense when decontamination is complete
2002. "Evidence for Localization of Reaction Upon Reduction of Carbon Tetrachloride by Granular Iron." Langmuir 18(20):7688-7693. Abstract The distribution of reaction sites on iron particles exposed to water containing carbon tetrachloride has been examined by measuring the locations of reaction products. The uniformity or localization of reaction sites has implications for understanding and modeling the reduction of environmental contaminants by iron in ground water systems. Granular iron surfaces similar to those being used for environmental remediation applications were studied using surfaces analysis techniques to develop an understanding of the physical and chemical structure of the surface and oxide films. Scanning Auger microscopy and imaging time-of-flight secondary ion mass spectrometry revealed that granular iron exposed to carbon tetrachloride-saturated water exhibits chloride-enriched regions occurred at pits rather than on th passive oxide film on the metal. Understanding the nature of the local solute reduction sites will play an important role in modeling the kinetics of reaction at passive iron oxide films in environmental systems.
2002. "Hydration of Passive Oxide Films on Aluminum." Journal of Physical Chemistry B 106(18):4705-4713. Abstract Models for the corrosion and pitting of passive metals such as aluminum usually involve the migration of point defects through the native oxide film as the rate limiting step. Hydration of the surface oxide could also influence the protective nature of the film. Secondary ion mass spectrometry (SIMS) has been used in conjunction with isotopic labeling to determine the extent and rate of passive film hydration on aluminum. The rates at which oxygen- and hydrogen-contianing species migrate through the film has been determined as a function of temperature and applied potential (cathodic and anodic polarization). The results suggest that defects such as hyroxide ions are prevalent and mobile in the oxide film, influencing the kinetics and mechanisms of corrosion processes.
2002. "Use and Limitations of Electron Flood Gun Control of Surface Potential During XPS: Two Non-homogeneous Sample Types." Surface and Interface Analysis 33:781-790. Abstract The ability of charge compensation methods to control the surface potentials for two types of non-homogenous samples is examined. Results demonstrate that two newer types of charge compensation systems have improved performance in relation to some previous flood gun methods and reaffirm the concept that a primary objective of charge compensation is to find conditions for which the surface potential of the specimen is as uniform as possible. However, experiments involving both flood gun use and specimen grounding, demonstrate that peak broadening and shifting can occur when two (or more) potentials are present in the region of analysis. Finally, the ability of interface charge to shift specimen potentials and measured binding energies demonstrates fundamental limitations to the absolute accuracy of binding energy measurements, but also remind us that charging phenomena can be used to obtain important information about the sample.
2002. "Practical Aspects of Charging Phenomena in XPS as demonstrated in Oxidized-Al Films on Al and Al Alloys." Journal of Surface Analysis 9(3):396-403. Abstract Understanding the surface and interface structure, composition and chemistry of insulating materials has long been of importance in surface analysis. The relevance of insulating surfaces to the environment, and the increasing use of newer and more complex oxide films in electronic and optical applications significantly increases the information needed about these materials. Through a series of examples using Al-oxide films, this paper summarizes some conceptual and practical issues related to analysis of insulators, including vacuum-level and Fermi-level referencing, charge buildup at interfaces, the use of charge neutralization, the impact of electron and ion-beam damage, and the influence of impurities on oxide properties and measurements. Many of the measurements are understood through consideration of potential variation through a specimen. Current results, along with many in the literature, demonstrate that surface charging is not a problem that can be readily solved and ignored, but is a tool providing important information about materials and films.
2001. "Infrared Transparent Spinel Films with p -Type Conductivity." Thin Solid Films 398-399:45-52. Abstract Spinel oxide films containing at least two transition metal cations were found to exhibit p-type conductivity with high optical transparency from the visible to wavelengths near 15 micrometers. Resistivities as low as 0.003 ohm-cm were measured on 100 nm thick rf sputter deposited films that contained nickel and cobalt. Optical spectra, Raman scattering and XPS measurements indicated the valency of nickel localized on octahedral sites within the spinel lattice determines these properties. Electronic band structure calculations corroborated the experimental results. A resistivity minimum was found at the composition NiCo2O4 deposited from aqueous or alcoholic solutions followed by subsequent annealing at 400 degrees C in air. Solution deposited films richer in nickel than this stoichiometry always were found to phase separate into nickel oxide and a spinel phase with concomitant loss in conductivity. However, the phase stability region could be extended to higher nickel contents when rf-sputter deposition techniques were used. Sputter deposited spinel films having a nickel to cobalt ratio less than 2 were found to exhibit the highest conductivity. Results suggest that the phase stability region for these materials can be extended through appropriate choice of deposition conditions. A possible mechanism that promotes high conductivity in this system is thought to be charge transfer between the resident di- and trivalent cations that may be assisted by the magnetic nature of the oxide film.
2001. "Analyzing Localized Corrosion in Ion-Implanted Metals via XPS/AES." JOM. The Journal of the Minerals, Metals and Materials Society 53(7):37-41. Abstract Intergranular Stress Corrosion Cracking (IGSCC) of metals is often controlled by anodic (oxidative) dissolution of the metal at the crack tip. The anodic processes, in turn, depend on both crack-tip solution and grain boundary compositions. The latter are strongly influenced by alloying constituents and impurities that frequently segregate to (or away from) the grain boundaries during heat treatment and/or aging. For systems that exhibit anodic control, understanding the kinetics of the corrosion processes at crack tips and, in particular, determining how the kinetics are influenced by the presence and amount of grain boundary segregants, is critical to identifying both promising andproblematic alloys and in developing predictive crack growth models.
2001. "Interaction of HCOOH with stoichiometric and reduced SrTiO3(100) surfaces." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 18(4):1893-1899. Abstract Interaction of formic acid with stoichiometric (TiO2-terminated) and reduced SrTiO3(100) surfaces has been investigated using temperature programmed desorption (TPD), and x-ray photoelectron spectroscopy (XPS). Formic acid was dissociated to form formate and a surface proton below 250 K on both stoichiometric and reduced SrTiO3(100) surfaces. Formate was decomposed primarily through dehydration to produce CO and H2O, instead of through dehydrogenation to produce CO2 and H2, on both sufaces. Formaldehyde produced from decomposition of formate was also observed on both surfaces. On stoichiometric surfaces, formaldehyde was produced through bimolecular coupling of two formates on low-coordination Ti cation sites. However, on the reduced surface, formaldehyde formation involves the reduction of surface formates through the oxidation of reduced Ti cations. XPS results show that surface defects on reduced SrTiO3(100) surfaces were reoxidized significantly upon exposure to 30 L HCOOH at 300 K, in contrast to defects on reduced TiO2(110) surfaces where no reduction in defect intensity was observed under identical conditions.
2001. "Porous Silicon as a Versatile Platform for Laser Desorption/Ionization Mass Spectrometry." Analytical Chemistry 73 (3):612-619. Abstract Desorption/Ionization on porous silicon mass spectrometry (DIOS-MS) is a novel method for generating and analyzing gas phase ions that employs direct laser vaporization. The structure and phsicochemical properties of the porous silicon surfaces are crucial to DIOS-MS performance and are controlled by the selection of silicon and the electrochemical etching conditions. Porous silicon generation and DIOS signals were examined as function of silicon crystal orientation, resistivity, etching solution, etching current density, etching time, and irradiation. Pre- and post-etching conditions were also examined for their effect on DIOS signal as were chemical modifications to examine stability with respect to surface oxidation. Pore size and other physical characteristics were examined by scanning electron microscopy (SEM), atomic force microscopy (AFM), and Fourier transform infrared (FT-IR) spectroscopy, and correlated with DIOS-MS signal. Porous silicon surfaces optimized for DIOS response were examined for their applicability to quantitative analysis, organic reaction monitoring, post-source decay tandem mass spectrometry, and chromatogrophy.
2001. "The Structure of Na2O-Al2O3-SiO2 Glass: Impact on Sodium Ion Exchange in H2O and D2O." Journal of Non-crystalline Solids 296:10-26. Abstract The kinetics of matrix dissolution and alkali-exchange for a series of sodium aluminosilicate glass compositions was determined at constant temperature and solution pH(D) under conditions of silica-saturation. Steady state release rate for sodium was 10 to 50 times faster than the rate of matrix dissolution, demonstrating that alkali exchange is an important long-term reaction mechanism that must be considered when modeling systems near saturation with respect to dissolved glass components. Sodium release rates were 30% slower in D2O compared to rates in H2O; but matrix dissolution rates were unaffected. These results are consistent with rupture of the O-H bond as the rate-limiting reaction in Na+-H+ exchange whereas matrix dissolution is controlled by OH- or H2O catalyzed hydrolysis of Si-O-Si and Si-O-Al bonds. Changes in Na exchange rate with increasing Al2O3 content could not be reconciled with changes in the number of non-bridging oxygen (NBO) sites in the glass alone. A simple model was used to estimate a structural energy barrier for alkali ion exchange using Na-O bond length and coordination as measured by Na K-edge XANES spectroscopy, and binding energy shifts for Si-O-Na sites measured by XPS. The energy barrier was calculated to increase from 34 kJ/mol for Na2O?2SiO2 glass to 50 kJ/mol for a glass containing 15 mol% Al2O3, consistent with stronger bonding of Na on NBO sites and increasing mechanical stiffness of the glass network with increasing Al content. The calculated ion-exchange enthalpies were then used to calculate Na ion exchange rates as a function of glass composition. Agreement between the calculated and measured Na ion exchange rates was excellent.
2001. "Selective Sorption of Cesium Using Self-Assembled Monolayers on Mesoporous Supports ." Environmental Science and Technology 35(19):3962-3966. Abstract The synthesis of novel nano-composite sorbent material, copper ferrocyanide immobilized within a mesoporous ceramic matrix, and its use as a novel cesium sorbent material is reported in this paper. Complete removal of cesium was achieved in the presence of competing metal ions for solutions containing 2 ppm cesium under a variety of conditions.
2001. "First Step Towards the Growth of Single-Crystal Oxides on Si: Formation of a Two-Dimensional Crystalline Silicate on Si(001)." Applied Physics Letters 79(22):3591-3593. Abstract Sr-covered Si(001) surfaces hold promise for providing stable templates for growth of single-crystal oxide films. We have investigated the structural and chemical properties of reconstructed Sr/Si(001) surfaces at different Sr coverages using low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). Results show that upon low temperature oxidation and subsequent UHV annealing of the Sr/Si(001)-(1x2) surface, a crystalline Sr2SiO4 silicate layer was formed. Using this silicate as a template layer, single-crystal SrO thin films were grown on Si(001) substrates. Our results provided microscopic and spectroscopic evidence of the formation of a uniform, stable, two-dimensional crystalline silicate that can be used for growth of single-crystal oxides on Si(001) substrates.
2001. "A Study of Potassium Dihydrogen Phosphate (KDP) Crystal Surfaces by XPS." Surface Science Spectra 8(1):56-80. Abstract Potassium dihydrogen phosphate KH_2PO_4 (KDP) is a transparant dielectric material best known for its nonlinear optical and electro-optical properties. Because of its nonlinear optical properties, it has been incorporated into various laser systems for harmonic generation and optoelectrical switching. In addition, KDP is particularly suitable for use in large-aperature laser systems such as that located at the National Ignition Facility (NIF) because it can be grown as a single crystal to large size. Despite the importance of this material, surface composition and surface electronic structure data were found to be nonexistant. X-ray photoemission spectroscopy was thus used to characterize the composition and surface structure of (100) and (101) native crystals.
2001. "Chemical Sensors Based on Dielectric Response of Functionalized Mesoporous Silica Films." Journal of Materials Research 16(10):2810-2816. Abstract Dielectric response of mesoporous silica films was monitored as a function of several gas-phase chemical species. The effects of humidity, ammonia and methane on dielectric constant and dissipation factor of films subjected to different chemical treatments are described. Dielectric constant and dissipation factor of partially dehydroxylated films were found to be highly sensitive to both water vapor and ammonia in air. The capacitive devices based on mesoporous silica films show potential for use in chemical sensors.
2000. "Effectiveness of High Energy Ion Beam Techniques for the Characterization of Mesoporous Low Dielectric-Constant Materials." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 161-163:476-481. Abstract There is growing interest interest in integration of low dielectric materials in microelectric devices. Highly porous silica films can potentially reduce power dissipation, cross talk, and interconnection delay in the deep submicron device regime. Recently, low k dielectric "mesoporous" silica films have been synthesized [1] using micellar surfactants to template porosity in spin-on sol-gel silica. Since then we have made significant progress in developing low k films, using both cationic and non-ionic surfactants, that could meet device performance requirements. During this development we have used high-energy ion beam techniques along with optical profilometry to characterize the porosity of these films. Rutherford backscattering spectroscopy (RBS) and 16O(d,p0)17O nuclear reaction were used to determine the total number of Si and O atoms in the films. Interaction of these films with water was characterized by the 1H(19F, alpha-gamma)16O resonant nuclear reaction. Combination of these techniques provides fast accurate, and quantitative methods for characterizing these films. However, the high-energy ion beams appear to cause significant damage in the films. X-ray photoelectron spectroscopy (XPS) measurements from the ion beam interacted region interacted region show a tail in the low binding energy side of the Si 2p core level spectrum which is characteristic to metal Si. In addition, craters as deep as 100nm were left in the films where the ion beams interacted with the material. We will dicuss the effectiveness of these techniques for these porosity measurements in nanoporous low dielectric-constant silica films.
2000. "Temperature-Sensitive Surfaces Prepared by UV Photografting Reaction of Photosensitizer and N-Isopropylacrylamide ." Journal of Physical Chemistry B 104(49):11667-11673. Abstract A thermal-sensitive surface was prepared on the surface of a silicone wafer (substrate) by an ultraviolet photografting reaction between a photosensitizer, (N,N'-diethylamino)dithiocarbamoylpropyl(trimethoxy)silane, and N-isopropylacrylamide. Ellipsometery measurement revealed a thin grafting layer (~44 ?) consisting of a single terminal poly(N-isopropylacrylamide) (PNIPAAm) chain on the surface of the silicone wafers. X-ray photoelectron spectroscopy confirmed that the grafting layer was composed of the PNIPAAm structure. The properties of the grafting layer can be adjusted and manipulated by varying the photopolymerization time and the concentration of the monomer. Increasing the photopolymerization time and the concentration of the monomer increases both the thickness of the grafting layer and the wettability of the surface. The characteristics of the temperature-sensitive surface were investigated by dynamic contact angle as a function of temperature. The remarkable change on advancing contact angle can be observed around 32C. Compared with the substrate grafted by PNIPAAm gel, the substrate with a signal-terminal PNIPAAm chain exhibited a lower transition temperature and a narrower change range of transition temperature. This can be attributed to the mobile PNIPAAm chain with a single-terminal mode, which increases the characteristics of faster response to the temperature change.
2000. "Temperature-Sensitive Surfaces Prepared by UV Photografting Reaction of Photosensitizer and N-Isopropylacrylamide ." Journal of Physical Chemistry B 104:11667-11673. Abstract Temperature-sensitive surfaces were prepared by grafting poly(N-isopropylacrylamide) (PNIPAAm) hydrogel on the surface of silicone wafers. The silicone wafer substrates were modified by organosilane (vinyltriethoxylsilane). They were further reacted with N-isopropylacrylamide (NIPAAm), using N,N'-methylenebisacrylamide as the cross-linking agent, to generate the cross-linked PNIPAAm layer on the surface of the substrate. The surfaces modified by the cross-linked PNIPAAm layer were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and attenuated total reflectance-Fourier transform infrared (FTIR) spectroscopy. The reversible hydrophilic/hydrophobic properties of the surface were evaluated by the dynamic contact angle. The morphology of the surface modified by a cross-linked PNIPAAm layer can be turned from "sea island" to "mountain valley" by changing the molar ratio of NIPAAm to BisAAm and by varying the polymerization time. A completely hydrophilic surface (advancing contact angle = 0) was observed below 25 C, and the surface became extremely hydrophobic (advancing contact angle = 92) above 40 C. The sensitivity of the surfaces to temperature change can be improved by increasing the cross-linking density of the polymer layer and varying the polymerization time. The water meniscus height in a capillary tube, whose wall was coated by a cross-linked PNIPAAm layer, went up or down as the temperature changed to below or above the lower critical solution temperature of PNIPAAm. The differences in the water meniscus heights are 10 and 5 mm for a capillary tube with a diameter of 2 and 3 mm, respectively, corresponding to a change in temperature from 23 to 50 C. The temperature-sensitive characteristics, which produce remarkable and rapid changes of surface properties, make this technology applicable for use as actuators, modulators, sensors, and switches.
2000. "Surfaces with Reversible Hydrophilic/Hydrophobic Characteristics on Cross-linked Poly(N-Isopropylacrylamide) Hydrogels." Langmuir 16:8016-8023. Abstract Temperature-sensitive surfaces were prepared by grafting polyN-isopropylacrylamide (PNIPAAm) hydrogel on the surface of silicone wafers. The silicone wafers as the substrate were modified by organosilane (vinyltriethoxylsilane). They were further reacted with N-isopropylacrylamide (NIPAAm) using N, N'-methylenebisacrylamide as the crosslinking agent to generate the crosslinked PNIPAAm layer on the surface of the substrate. The surfaces modified by the crosslinked PNIPAAm layer were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and attenuated total reflectance-Fourier transform infrared. The reversible hydrophilic/hydrophobic properties of the surface were evaluated by dynamic contact angle. The morphology of the surface modified by a crosslinked PNIPAAm layer can be turned from "sea-island" to "mountain-valley" by changing the molar ratio of NIPAAm to BisAAm and by varying the polymerization time. The completely hydrophilic surface (advancing contact angle = 0?) was observed below 25?C, and the surface became extremely hydrophobic (advancing contact angle = 92?) above 40?C. The sensitivity of surfaces to temperature change can be improved by increasing the crosslinking density of the polymer layer and varying the polymerization time. The water meniscus height in a capillary tube, whose wall was coated by a crosslinked PNIPAAm layer, went up or down as the temperature changed to below or above the lower critical solution temperature of PNIPAAm. The differences of water meniscus height are 10 and 5 mm for the capillary tube with the diameter of 2 and 3 mm, respectively, corresponding to the change of temperature from 23?C to 50?C. The temperature-sensitive characteristics, which produce remarkable and rapid changes of surface properties, makes this technology applicable as actuators, modulators, sensors, and switches.
2000. "MOCVD Growth and Structure of Nb- and V-doped TiO2 Films on Sapphire." Journal of Crystal Growth 212 (1-2):178-190. Abstract Describes the results and process for growing Nb- and V-doped TiO2 thin films at a doping level up to 40 at% on sapphire.
2000. "Effects of precursors and substrate materials on microstructure dielectric properties, and step coverage of (Ba, Sr)TiO3 films grown by metalorganic chemical vapor deposition." Journal of Applied Physics 87(1):124-132. Abstract (Ba,Sr)TiO3 (BST) thin films have been grown on planar Ir/Si and Pt/Si substrates and on three-dimensional (3D) Ir electrodes by metalorganic chemical vapor deposition using two kinds of bata-diketonate-based BST precursors. Film growth was studied as a function of film thickness, composition, and substrate temperature. Growth rate was monitored by in situ spectroscopic ellipsometry. The BST films were characterized ex situ by a variety of techniques including x-ray photoelectron spectroscopy, Auger electron microscopy, atomic force microscopy, transmission and scanning electron microscopy, x-ray diffraction, and impedance analyzer. The results reveal that the two sets of BST precursors, albeit slightly different, show quite different reactivities that strongly affect the step coverage on the 3D structure. However, different reactivities have no apparent effect on the microstructure, surface morphology, and dielectric properties of the stoichiometric BST films. These properties strongly depend on the film composition, substrate material, and growth temperature. In general, the BST films grown on Pt exhibit better crystalline quality, surface smoothness, and dielectric compared to those grown on Ir under the optimal growth conditions.
2000. "Third Row Transition Metals by X-ray Photoelectron Spectroscopy." Surface Science Spectra 7(1):1-68. Abstract XPS Spectrum have been obtained from eleven metals in the third row of transition elements using a Quantum 2000 Scanning ESCA Microprobe. The metals analyzed include La, Hf, Ta, W, Re, Ir, Pt, Au, Tl, Pb, and Bi. Each sample was Ar> {+} ion etched before XPS analysis to remove any surface contamination and/or oxide. The spectrum include standard survey scans and high energy resolution scans of the photoelectron peaks, as well as selected x-ray induced Auger peaks. Each spectrum was collected using a 100 um monochromatic Al Ka x-ray beam scanned over a 1.5 mm x 0.2 mm area of each sample. Survey scans were collected using an 58.7 eV pass energy, while high energy resolution scans were collected using a 23.5 eV pass energy.
2000. "Influence of Mg on the Corrosion of Al ." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 18(1):131-136. Abstract This paper summarizes a series of experiments to determine the influence of Mg on the corrosion and electrochemical behavior of Al. Magnesium is commonly added to increase the strength of lightweight non-heat treatable Al alloys. However, these alloys are susceptible to grain boundary dissolution, stress corrosion cracking or hydrogen induced embrittlement due to changes in the alloy structure and the elemental distribution during processing,welding, or in-service exposure to elevated temperatures. Auger electron spectroscopy and transmission electron microscopy measurement show that alloys having a distributuion of Al3Mg2 (beta phase) precipitates and segregated Mg on grain boundaries are more susceptible to cracking. To understand the roles of Mg on the cracking process we compared the corrosion potential and film formation of pure AL, AL implanted with Mg, a 7 wt% Mg-Al alloy and pure Al3Mg2 phase. The surfaces of the specimens were cleaned and prepared in a surface analysis system and transferred in a vacuum transfer system to a corrosion cell. After solution exposure and electrochemical measurement the specimens were returned to the spectrometer and analyzed by x-ray photoelectron spectroscopy. The open circuit potentials for Al, Mg implanted Al, and the 7% alloy were nearly identical. However, the corrosion potential for the beta phase differs significantly. The thickness of the film formed on each of the samples is similar. Mg is observed to be depleted in the outer part of the oxide films, but somewhat enhanced near the oxide-metal interface. The results suggest that segregrated Mg plays little role in the cracking and that hydrogen production of the Beta phase particles may be the most significant factor.
2000. "Approach for Determining Area Selectivity in Small-Area XPS Analysis." Surface and Interface Analysis 29(11):766-772. Abstract This paper demonstrates the measurements on well defined "dots" of a material on a substrate provide a useful experimental approach for determining the area of a speciman that contributes to a small area XPS measurement. The method provides information that can verify instrument operation conditions and the adequacy of speciman alignment procedures. A data set collected primarily on one system demonstrates how some general properties of an instrument can be learned in order to understand and optimize data collection methods. The relationship of common methods of measuring spatial resolution and the dot test method are discussed.
1999. "Corrosion of Phosphorus-Implanted 304L Stainless Steel in 1 N H2SO4." Journal of the Electrochemical Society 146(3):984-987. Abstract Combined electrochemical and surface analysis measurements revealed that 304L stainless steel, implanted with P, oxidizes at near-transpassive anodic potential in 1 N H2SO4 to give corrosion products that affect the corrosion rate differently depending on the amount of P at the surface of the metal. At low concentrations, P assisted corrosion by facilitating dissolution of the oxidation products, while at high concentrations, the products were insoluble and precipitated to form a thick, electrically resistive film. The resulstant voltage drop contributed to lower currents as long as the thick film was stable. The dependence of corrosion rate on P concentration is believed to contribute to the "two-peak" current repsonse boserved for the material in potential-step measurements.
1999. "Interactions of Liquid and Vapor Water with Stoichiometric and Defective TiO2(100) Surfaces." Surface Science 440(1-2):60-68. Abstract Interactions of both liquids and vapor water with stoichiometric (nearly defect-free) and defective TiO2(100) surfaces have been studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). For an almost defect-free (100) 1X1 surface, water coverage was ~0.08ML (1 ML=7.36x10 4/cm2) at 10 4 L exposure to low-vapor-pressure water, ~0.32ML at 10 8 L exposure to higher-vapor-pressure water, and ~0.50 ML with liquid-water exposure, respectively. Defect intensities were greatly reduced after exposing defective surfaces to ~10 2 L low-vapor-pressure water. More significantly, electron-beam-induced defects were completely removed upon higher exposure (>10 4 L), while defects created by Ar+ bombardment were partially removed. The surface structural influence on defect reactivity has been examined by comparing the results obtained from both (100) and (110) surfaces. Defects on (100) surfaces were removed more readily than those on (110) surfaces.
1999. "The Ion Beam Materials Analysis Laboratory at the Environmental Molecular Sciences Laboratory." Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 420(1-2):81-89. Abstract Describes the equipment capabilities at EMSL with special emphasis on the accelerator, ion beam analysis and ion beam modification.
1999. "Vacuum Cleaved Calcium Carbonate by XPS." Surface Science Spectra 6:153-159. Abstract Calcite is a common mineral phase found in pure and mixed phase systems. The data reported here was collected for comparison with data collected on other systems and to determine the relative sensitivity for various Ca photopeaks that we were using for the analysis of other Ca-containing mineral phases. This data was collected on a PHI Quantum Scanning ESCA Microprobe. This instrument is unique in that the x-ray beam is focused and can be scanned over the sample. A single crystal of calcite was broken in vaccum (exposing the 1014 cleavage plane) and analyzed using monochromatic Al K alpha x-rays. The sample was neutralized using both low energy ions and electrons.
1998. "Electrochemistry of TiN in 6M KOH Solution." Journal of the Electrochemical Society 145(4):1211-1218. Abstract There is no abstract currently available for this item