2009. "Damage Profile and Ion Distribution of Slow Heavy Ions in Compounds." Journal of Applied Physics 105(10):104901:1-12. doi:10.1063/1.3118582 Abstract Slow heavy ions inevitably produce a significant concentration of defects and lattice disorder in solids during their slowing-down process via ion-solid interactions. For irradiation effects research and many industrial applications, atomic defect production, ion range and doping concentration are commonly estimated by the Stopping and Range of Ions in Matter (SRIM) code. In this study, ion-induced damage and projectile ranges of low energy Au ions in SiC are determined using complementary ion beam and microscopy techniques. Considerable errors in both disorder profile and ion range predicted by the SRIM code indicate an overestimation of the electronic stopping power, by a factor of 2 in most cases, in the energy region up to 25 keV/nucleon. Such large discrepancies are also observed for slow heavy ions, including Pt, Au and Pb ions, in other functional materials, such as GaN, AlN and SrTiO3. Due to the importance of these materials for advanced device and nuclear applications, better electronic stopping cross section predictions, based on a reciprocity principle developed by Sigmund, is suggested with fitting parameters for possible improvement.
2009. "Glucose Biosensor Based on Immobilization of Glucose Oxidase in Platinum Nanoparticles/Graphene/Chitosan Nanocomposite Film." Talanta 80(1):403-406. doi:10.1016/j.talanta.2009.06.054 Abstract The bionanocomposite film consisting of glucose oxidase/Pt/functional graphene sheets/chitosan (GOD/Pt/FGS/chitosan) for glucose sensing was described. With the electrocatalytic synergy of FGS and Pt nanoparticles to hydrogen peroxide, a sensitive biosensor with detection limit of 0.6 µM glucose was achieved. The biosensor also had good reproducibility, long term stability and negligible interfering signals from ascorbic acid and uric acid comparing to the response to glucose. The large surface area and good conductivity of graphene suggests that graphene is a potential candidate for sensor material. The hybrid nanocomposite glucose sensor provides new opportunity for clinical diagnosis and point-of-care applications.
2009. "Self-assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-ion Insertion ." ACS Nano 3(4):907-914. Abstract We used anionic sulfate surfactants to assist the stabilization of graphene in aqueous solutions and facilitate the self-assembly of in-situ grown nanocrystalline TiO2, rutile and anatase, with graphene. These nanostructured TiO2-graphene hybrid materials were used for investigation of Li-ion insertion properties. The hybrid materials showed significantly enhanced Li-ion insertion/extraction in TiO2. The specific capacity was more than doubled at high charge rates, as compared with the pure TiO2 phase. The improved capacity at high charge-discharge rate may be attributed to increased electrode conductivity in presence of a percolated graphene network embedded into the metal oxide electrodes.
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. "Microstructure and ionic-conductivity of alternating-multilayer structured Gd-doped ceria and zirconia thin films." Journal of Materials Science 44(8):2021-2026. Abstract Multilayer thin-film of consisting of alternating Gd-doped ceria and zirconia have been grown by sputter-deposition on -Al2O3 (0001) substrates. The films were characterized using x-ray diffraction (XRD), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The Gd-doped ceria and zirconia layers had the fluorite structure and are highly textured such that the (111) plane of the films parallel to the (0001) plane of the -Al2O3. The epitaxial relationship can be written as (111)ZrO2/CeO2//(0001)Al2O3 and [11-2]ZrO2/CeO2//[-2110]Al2O3.. The absence of Ce3+ features in the XPS spectra indicates that the Gd-doped ceria films are completely oxidized. The ionic conductivity of this structure shows great improvement as compared with that of the bulk crystalline material. This research provides insight on designing of material for low-temperature electrolyte applications.
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. "Crystal and Electronic Structure of Lithiated Nanosized RutileTiO2 by Electron Diffraction and Electron Energy-loss Spectroscopy." Applied Physics Letters 94(23):Art. No.: 233116. doi:10.1063/1.3152783 Abstract The electronic structure of the nanosized rutile TiO2 before and after mechanical lithiation were studied using TEM and EELS. EELS reveals the Li K-edge at the energy-loss position of ~ 61 eV. After lithiation, the separation of the t2g-eg crystal-field splitting on both Ti L2,3-edge and O K-edge decreases, the O K-edge shifts towards a higher energy-loss position and the separation between the pre-edge peak and main peak on the O K-edge decreases. These results suggest that the lithiation of rutile TiO2 was accompanied by the reduction of Ti ion and a charge transfer from Li to Ti.
2009. "Controlled deposition of covalently bonded tantalum oxide on carbon supports by solvent evaporation sol-gel process." Surface Science 603(15):2290-2293. doi:10.1016/j.susc.2009.05.006 Abstract A simple strategy for covalently attaching Ta2O5 particles onto functionalized graphitic carbon supports has been developed to fabricate hybrid nanocomposites. In this process, tantalum ethoxide was directly reacted with functional groups on the carbon surface to form covalent bonding, which caused the carbonyl stretches of the carbon supports to be blue-shifted to 50-70 cm-1 after Ta2O5 particle deposition. Homogeneously distributed Ta2O5 particles with smaller than 100 nm have been homogeneously deposited on the carbon supports. X-ray diffraction (XRD), FT-IR spectroscopy, and scanning electron microscopy (SEM) have been used to study the formation of Ta2O5/C hybrid structure.
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. "SO2-induced stability of Ag-alumina catalysts in the SCR of NO with methane." Applied Catalysis. B, Environmental 88(1-2):98-105. Abstract We report on a stabilization effect on the structure and activity of Ag/Al2O3 for the selective catalytic reduction (SCR) of NOx with CH4 imparted by the presence of SO2 in the exhaust gasmixture. The reaction is carried out at temperature above 600 8C to keep the surface partially free of sulfates. In SO2-free gases, catalyst deactivation is fast and measurable at these temperatures. Time-resolved TEM analyses of used samples have determined that deactivation is due to sintering of silver from well-dispersed clusters to nanoparticles to micrometer-size particles with time-on-stream at 625 8C. However, sintering of silver was dramatically suppressed by the presence of SO2 in the reaction gas mixture. The structural stabilization by SO2 was accompanied by stable catalyst activity for the NO reduction to N2. The direct oxidation of methane was suppressed, thus the methane selectivity was improved in SO2-laden gas mixtures. In tests with high-content silver alumina with some of the silver present in metallic form, an increase in the SCR activity was found in SO2-containing gas mixtures. This is attributed to redispersion of the silver particles by SO2, an unexpected finding. The catalyst performance was reversible over many cycles of operation at 625 8C with the SO2 switched on and off in the gas mixture.
2009. "The durability dependence of Pt/CNT electrocatalysts on the nanostructures of carbon nanotubes: hollow- and bamboo-CNTs." Journal of Nanoscience and Nanotechnology 9(10):5811-5815. Abstract The electrochemical durability of Pt/CNT with hollow- and bamboo-structured carbon nanotubes as the support for PEM fuel cells was investigated using cyclic voltammetry (CV, 0.6-1.1V) accelerated degradation test method. Pt/CNT catalysts were characterized with cyclic voltammograms, rotating disk electrodes, and TEM images. The changes in the electrochemical surface area of Pt and the activity toward oxygen reduction reaction (ORR) before and after the degradation indicate that bamboo-structured carbon nanotubes supported Pt (Pt/B-CNT) catalyst exhibited much higher durability. TEM images indicate that the sintering of Pt nanoparticles was much less for Pt/B-CNT. These are attributed to the specific bamboo-like nanostructures which provide more “bamboo-knot” defects and edge plane-like defects. Pt-support interaction was therefore enhanced and the durability was improved.
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. "Effect of Chemical Lithium Intercalation into Rutile TiO2 Nanorods." Journal of Physical Chemistry C 113(32):14567-14574. doi:10.1021/jp904148z Abstract Rutile TiO2 nanorods were synthesized by hydrolysis of TiCl4 followed by a hydrothermal method. Lithium insertion into the rutile nanorods was achieved by a chemical lithium intercalation process. The structural evolution of nano-structured rutile upon lithium intercalation was characterized by several experimental techniques, namely, XRD, TEM and 6Li MAS NMR. The XRD and TEM studies indicate the formation of a new lithium titanate phase (LixTiO2) during lithium intercalation. Additionally, SAED patterns show that the lithium titanate phase has cubic symmetry. Finally, ultra-high magnetic field (21.1T) 6Li MAS NMR reveals that the lithium titanate phase adopts two different structures depending on lithium content. Taken together, the three techniques consistently show that the intercalation of lithium into rutile TiO2 nanorods causes two consecutive structural phase transformations to lithium titanate phases with spinel (Fd m) and rocksalt (Fm m) structures at x=0.46 and 0.88, respectively. In addition, the broad line widths in the 6Li MAS NMR spectrum of the rocksalt phase are indicative of a disordered structure. Density functional theory calculations of the rutile, spinel and rocksalt bulk phases as a function of lithium content corroborate the observed phase transformations. These phase transitions could account for the large irreversible capacity loss of nano-structured rutile anodes observed in electrochemical cycling experiments.
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. "Effect of the Ligand Shell Composition on the Dispersibility and Transport of Gold Nanocrystals in Near Critical Solvents." Langmuir 25(9):4900-4906. doi:10.1021/la804058x Abstract The development of more efficient and greener methods for the synthesis and manipulation of nanomaterials has been a major focus of research among the scientific community. Supercritical (ScFs) and near-critical fluids (NcFs) offer numerous advantages over conventional solvents for these purposes. Among them, ScFs and NcFs offer dramatic reductions in the volume of organic waste typically generated during advanced material processes with the feasibility of changing a number of physicochemical properties by discrete variations in their pressure or temperature. In this work we study the dispersibility of gold nanocrystals of 3.8 nm core size stabilized with different ligand shells in near-critical (NcF) ethane and propane over a wide range of densities by fine-tuning the pressure of these fluids. Dispersibility-vs-density plots are obtained by following the variation in the Surface Plasmon Resonance (SPR) absorption spectra of the nanoparticles. To better understand the results obtained in this study three models, the total interaction theory, the sedimentation coefficient equation, and the Chrastil method, are briefly discussed. The dispersibility of the nanocrystals and its behavior with the variation of the fluid density is strongly dependent on the composition of the ligand shell and solvent employed. A correlation between the measured dispersibility values and the calculated sedimentation coefficients was observed in both compressed solvents. In addition, we successfully applied the Chrastil equation to predict and describe the dispersibility of gold nanocrystals with different shells as a function of density determining that the reason for the high stabilities of some of the nanocrystal dispersions is the strong solvent-nanocrystal interactions. While NcF propane showed larger nanocrystal dispersibilities, NcF ethane led to a greater tunability of nanoparticle dispersion in the pressure range of the study. Therefore, with a judicious selection of the fluid, NcFs seem to offer a remarkable advantage over conventional solvents for manipulation of nanomaterials, including transport, purification, and separation of nanocrystals.
2009. "Advancements Toward the Greener Processing of Engineered Nanomaterials -- Effect of Core Size on the Dispersibility and Transport of Gold Nanocrystals in Near-Critical Solvents." Small 5(8):961-969. doi:10.1002/smll.200801207 Abstract In this work, we explore the dispersibility of octanethiol-stabilized gold nanocrystals of different core sizes in compressed ethane and propane over a wide range of fluid conditions. The dispersibility of the nanocrystals was obtained through the Surface Plasmon Resonance (SPR) absorption spectra of solutions. Three models, the total interaction theory, the sedimentation coefficient equation, and the Chrastil method, are briefly discussed as tools to interpret the experimental results. Nanoparticle dispersibility-versus-density plots are strongly dependent on nanoparticle size and solvent conditions, with the dispersion of larger nanocrystals more dependant on changes of pressure or density at a given temperature. These results showed a notable correlation with the calculated sedimentation coefficients of the nanocrystals in both solvents. The Chrastil equation was successfully applied to predict and describe the dispersibility of the gold nanocrystals as a function of density, showing that the high stabilities of the nanocrystals dispersions are a result of the very strong solvent-nanocrystal interactions. For the range of nanoparticle sizes studied, compressed ethane at 25 ºC led to a greater tunability of nanoparticle dispersion when compared with compressed propane at 65 ºC. On the other hand, for equivalent pressures compressed propane was found to provide better solubility than ethane due to its higher density. The results of this study quantitatively demonstrate that compressed fluids can offer pressure tunable, size selective control of nanoparticle solvation and transport. This ability has clear advantages over conventional solvents and direct application to various nanomaterials processes, such as separation, transport and purification of nanocrystals.
2009. "Band Offsets at the Epitaxial Anatase TiO2/n-SrTiO3(001) Interface." Surface Science 603(5):771-780. Abstract We have used high-energy-resolution x-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO2(002)/n-SrTiO3(001) heterojunction prepared by molecular beam epitaxy, Within experimental error, the valance band offset is zero for anatase thicknesses between 1 and 7 monolayers. The conduction band offset is also zero by virtue of the fact that both anatase and SrTiO3 exhibit the same bandgap value (~3.2 eV). In one set of experiments, the interface included a partial monolayer of fluorine remaining from the HF etch that was used to prepare the substrate. The F could not be removed without Ar ion sputtering and annealing, which in turn resulted in ~0.15 eV of band bending, indicating the presence of interfacial defects. The band offsets were measured to be approximately 0 eV as well when the F was removed. Density functional theory predicts the valence band offset for the clean interface to be 0.5 eV. Inclusion of interfacial F reduces the theoretical band offset to 0.2 eV, much closer to experiment, and suggesting that the interface dipoles created by F and sputter defects have a major effect on the band offset.
2009. "Thermal evolution of microstructure in ion-irradiated GaN." Journal of Applied Physics 105(8):083514, 1-7. doi:10.1063/1.3106606 Abstract The thermal evolution of the microstructure created by irradiation of a GaN single crystal with 2 MeV Au2+ ions at 150 K is characterized following annealing at 973 K using transmission electron microscopy. In the as-irradiated sample characterized at 300 K, Ga nanocrystals with the diamond structure, which is an unstable configuration for Ga, are directly observed together with nitrogen bubbles in the irradiation-induced amorphous layer. Upon thermal annealing, the thickness of the amorphous layer decreases by ~13.1 %, and nano-beam electron diffraction analysis indicates no evidence for residual Ga nanocrystals, but instead reveals a mixture of hexagonal and cubic GaN phases in the annealed sample. Nitrogen molecules, captured in the as-irradiated bubbles, appear to debond and react with the Ga nanocrystals during the thermal annealing to form crystalline GaN. In addition, electron energy loss spectroscopy measurements reveal an atomic volume change of 18.9 % for the as-irradiated amorphous layer relative to the virgin single crystal GaN. This relative swelling of the damaged layer reduces to 7.7 % after thermal annealing. Partial recrystallization and structural relaxation of the GaN amorphous state are believed responsible for the volume change.
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. "Investigation of Iron-Chromium-Niobium-Titanium Ferritic Stainless Steel for Solid Oxide Fuel Cell Interconnect Applications." Journal of Power Sources 183(2):660-667. doi:10.1016/j.jpowsour.2008.05.037 Abstract As part of an effort to develop cost-effective ferritic stainless steel-based interconnects for solid oxide fuel cell (SOFC) stacks, AL 441 HPTM was studied in terms of its metallurgical characteristics, oxidation behavior, and electrical performance. Minor alloying elements (Nb and Ti) captured interstitials such as C by forming carbides, stabilizing the ferritic structure and mitigating the risks of sensitization and inter-granular corrosion. Laves phases rich in Nb and Si precipitated along grain boundaries during high temperature exposure, improving the steel’s high temperature mechanical strength. The capture of Si in the Laves phase minimized the Si activity in the steel substrate and prevented formation of an insulating silica layer at the scale/metal interface. However, the relatively high oxidation rate, and thus increasing ASR over time, necessitates the application of a conductive protection layer on the steel. In particular, Mn1.5Co1.5O4 spinel protection layers drastically improved the electrical performance of the ferritic stainless steel 441, acting as barriers to chromium outward and oxygen inward diffusion.
2008. "Tilted domain growth of metalorganic chemical vapor (MOCVD)-grown ZnO(0001)on a-Al2O3(0001)." Journal of Materials Research 23(1):13-17. doi:10.1557/JMR.2008.0026 Abstract ZnO grown on -Al2O3 (0001) generally possesses an orientation such that -Al2O3 (0001)//ZnO(0001) and two in-plane domains nucleate such that: -Al2O3 [11-20]//ZnO[11-20] and/or -Al2O3 [11-20]//ZnO[10-10]. In this paper, we report a new growth mode for ZnO grown on -Al2O3 (0001) using MOCVD. We find that -Al2O3 [11-20]//ZnO[10-10] but the (0001) plane of ZnO is tilted relative to the (0001) plane of -Al2O3 such that ZnO(0001) is almost parallel to the -Al2O3 (-1104) plane. This orientation reduces the extent of lattice mismatch. The interface between ZnO and -Al2O3 is abrupt and possesses periodic dislocations.
2008. "Synthesis and Li-ion Insertion Properties of Highly Crystalline Mesoporous Rutile TiO2." Chemistry of Materials 20(10):3435-3442. Abstract Mesoporous TiO2 has attracted great attention as a promising Li insertion electrode material with improved cycling life, rate capability and high power density. Up to date, mesoporous anatase TiO2 has been investigated for Li insertion. Recent studies have shown that nanosized rutile could be an excellent candidate for anode materials for higher Li insertion capacity and improved stability. However, synthesis of highly crystalline mesoporous rutile has met with limited success so far. There has been no report on Li insertion of mesoporous rutile TiO2. In this paper, we report a new low-temperature solution growth of TiO2 nanocrystals within an anionic surfactant matrix to produce highly crystalline mesoporous rutile and investigate Li insertion properties of the mesoporous crystalline rutile. X-ray diffraction (XRD) patterns and N2 sorption isotherms reveal mesoporous structure in the highly crystalline mesoporous TiO2 directly results from the anionic surfactant templating effects with high surface area (245~300 m2/g) and tunable mesopore diameter ranging from 2.2 to 3.8 nm after calcination. Transmission electron microscopy (TEM) measurements show that framework of the highly crystalline mesoporous TiO2 are composed of aligned rutile nanorod building blocks grown along [001] direction. The new mesoporous crystalline rutile can accommodate more than 0.7 Li (Li0.7TiO2, 235 mAh/g) during the first discharge at C/5 rate between 1–3 V versus Li+/Li, with a reversible capacity of 0.55 Li (Li0.55TiO2, 185 mAh/g). The mesoporous crystalline rutile shows excellent capacity retention with less than 10% capacity loss after over 100 cycles. XRD and TEM characterization on electrochemically lithiated sample show that the rutile nanorods were transformed into cubic rocksalt LiTiO2 nanorods, but the mesostructures remained stable after the phase transformation and cycling. Furthermore, the crystalline mesoporous rutile may also have good potentials for other applications such as stable catalyst supports.
2008. "Microstructures of ZnO films deposited on (0001) and r-cut α-Al2O3 using metal organic chemical vapor deposition." Thin Solid Films 516(23):8337-8342. doi:10.1016/j.tsf.2008.04.001 Abstract Zinc oxide films were deposited on (0001) and r-cut α-Al2O3 under identical conditions using metal organic chemical vapor deposition. Microstructures of the ZnO films were studied in detail using conventional and high-resolution transmission electron microscopy (HRTEM), electron diffraction, and HRTEM image simulations. The films deposited on these two substrates show distinctive structural differences. The film grown on r-cut α-Al2O3 shows a high quality single crystal with an orientation relationship of α-Al2O3[-101-1]//ZnO[0001] and α-Al2O3(10-1-2)//ZnO(2-1-10). The interface between the film and the substrate was abrupt and decorated with high density of misfit dislocations. Film grown on α-Al2O3 (0001) shows several orientation domains. Typically, one domain correspond to the classic growth model such that α-Al2O3 (0001)//ZnO(0001) and α-Al2O3 [11-20]//ZnO[10-10]. Another domain corresponds to the growth mode such that α-Al2O3 [11-20]//ZnO[10-10] but the (0001) plane of ZnO is tilted relative to the (0001) plane of α-Al2O3 such that ZnO(0001) is almost parallel to the α-Al2O3 (-1104) plane. This orientation reduces the extent of lattice mismatch as compared with the classic growth mode. The interface between ZnO and α-Al2O3 is abrupt and possesses periodic dislocations.
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. "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. "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. "Effect of Nickel Microstructure on Methane Steam-Reforming Activity of Ni-YSZ Cermet Anode Catalyst ." Journal of Catalysis 258(2):356-365. doi:10.1016/j.jcat.2008.06.031 Abstract The activity of nickel-yttria stabilized zirconia (Ni-YSZ) solid oxide fuel cell (SOFC) cermet anodes for the steam reforming of methane has been investigated in the absence of electrochemical effects. The cermet was prepared by co-milling and sintering NiO and 5YSZ powders at 1375oC in air. During the high temperature sintering step, NiO dissolved into the YSZ particles to form a solid NiO-YSZ solution. During the subsequent catalyst reduction step, Ni exolved from the YSZ. As a result, many small Ni particles on the order of 10-20 nm formed at the surface of the YSZ. These small particles contribute significantly to the overall reforming activity, along with the large bulk Ni particles within the Ni-YSZ cermet. We have observed high initial activity that decreases by as much as an order of magnitude with time on stream, until the anode catalyst reaches a stable steady state activity. The time to reach this stable activity is a function of the reaction conditions and feed gas composition. Higher temperature, hydrogen partial pressure, and space velocity all accelerated the deactivation rate at a constant steam-to-carbon ratio of 3. Initial and lined out activities and average turnover frequencies were obtained for both Ni-YSZ and bulk Ni, based on a rate expression that is first order in methane and zero order in steam. Comparative tests at 750oC show high initial activity on a per-Ni site basis with both materials, but these turnover rates decline over a period of a few hours. Following lineout, there appears to be a negligible effect of Ni particle size on turnover rate. These results indicate the presence of structure sensitivity for methane reforming, but only with freshly calcined and reduced catalysts that may contain highly coordinatively unsaturated sites. There is an apparent structure insensitivity with aged catalysts where Ni particle sizes are generally 50 nm and greater. Under reaction conditions that employ high space velocities and low methane conversions, the water-gas-shift reaction does not establish thermodynamic equilibrium.
2008. "Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films." New Journal of Physics 10:Art. No. 055010. doi:10.1088/1367-2630/10/5/055010 Abstract Epitaxial thin films of cobalt-doped ZnO (Co:ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10-4 – 105 -cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and/or addition of Al during deposition. Despite the high structural quality of the Co:ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co:ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.
2008. "Rapid Synthesis and Size Control of CuInS2 Semi-Conductor Nanoparticles Using Microwave Irradiation." Journal of Nanoparticle Research 10(4):633-641. doi:10.1007/s11051-007-9294-7 Abstract The properties of CuInS2 semi-conductor nanoparticles make them attractive materials for use in next-generation photovoltaics. We have prepared CuInS2 nanoparticles from single source precursors via microwave irradiation. Microwave irradiation methods have allowed us to increase the efficiency of preparation of these materials by providing uniform heating and rapid reaction times. We have also controlled nanoparticle growth in the 3 to 5 nm size range by varying thiolated capping ligand concentrations as well as reaction temperatures and times. Investigation of the photophysical properties of the colloidal nanoparticles were performed using electronic absorption and luminescence emission spectroscopy. Qualitative nanoparticles sizes were determined from the photoluminescence (PLE) data and compared to HRTEM images.
2008. "Rapid synthesis and size control of CuInS2 semi-conductor nanoparticles using microwave irradiation." Journal of Nanoparticle Research 10(4):633-641. Abstract The properties of CuInS2 semi-conductor nanoparticles make them attractive materials for use in next-generation photovoltaics. We have prepared CuInS2 nanoparticles from single source precursors via microwave irradiation. Microwave irradiation methods have allowed us to increase the efficiency of preparation of these materials by providing uniform heating and rapid reaction times. The synergistic effect of varying thiol capping ligand concentrations as well as reaction temperatures and times resulted in fine control of nanoparticle growth in the 3–5 nm size range. Investigation of the photophysical properties of the colloidal nanoparticles were performed using electronic absorption and luminescence emission spectroscopy. Qualitative nanoparticles sizes were determined from the photoluminescence (PLE) data and compared to HRTEM images.
2008. "Tuning and Quantifying the Dispersibility of Gold Nanocrystals in Liquid and Supercritical Solvents ." Journal of Physical Chemistry C 112(36):13947-13957. doi:10.1021/jp8038237 Abstract The application of nanomaterials relies on the ability to synthesize, purify, transport, and deposit them in a controllable fashion. The capacity to adjust the density, and thus the solvent strength, of a supercritical or near-critical fluid can be used to tune reaction and separation processes as well as to assemble nanomaterials in a controllable fashion. Herein we demonstrate and quantify density-tunable and reversible size-dependent dispersibility of octanethiol-stabilized gold nanocrystals with a size of 3.7 ± 2.2 nm in near-critical and supercritical solvents as a way to show the significant potential of these fluids for nanomaterials processing. This study introduced discrete variations on the pressure of nanocrystals dispersions in compressed ethane and propane at temperatures of 25, 45, and 65 °C until they reached a saturation region, at which point actual measurements of nanocrystal dispersibility were obtained using UV-Vis absorption spectroscopy. Transmission electron microscopy (TEM) was employed to correlate the dispersibility results with the actual size of the nanoparticles fractions at different densities. The results showed that stable dispersions of nanocrystals could be obtained at pressures as low as 50 atm in both solvents. Compressed ethane in its liquid or supercritical state was found to provide better dynamic tunability while propane provided higher dispersibility of these nanocrystals under the studied pressure-temperature conditions. Two theoretical models, the total interaction theory and Chrastil equation, are briefly presented as a means of interpreting the experimental observations. It was determined that dispersibility depends strongly on the nanocrystal size, solvent density and carbon chain length of the solvent. These results clearly show that selected supercritical fluids can be remarkably effective for the manipulation of nanoparticles.
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. "Growth and characterization of highly oriented gadolinia-doped ceria (111) thin films on zirconia (111)/sapphire (0001) substrates." Thin Solid Films 516(18):6088-6094. doi:10.1016/j.tsf.2007.11.007 Abstract Highly-oriented pure and gadolinia-doped ceria thin films have been grown on pure and ZrO2 (111)-buffered Al2O3 (0001) substrates using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) to understand the oxygen ionic transport processes in ceria based oxide thin films. Gadolinia-doped ceria films grown on pure Al2O3(0001) substrate show polycrystalline features due to structural deformations resulting from the large lattice mismatch between the Al2O3(0001) substrate and the films. However, the films, grown on a thin layer of ZrO2(111) buffered Al2O3 (0001) substrate, appears to be highly oriented. These films were characterized using high resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS) depth profiling. Oxygen ionic conductivity in gadolinia-doped ceria films was measured as a function of Gd concentration and these results were compared with the ion conductance data of the polycrystalline and single crystalline yttria-stabilized zirconia (YSZ).
2008. "Characterization Challenges for Nanomaterials." Surface and Interface Analysis 40(3-4):529-537. doi:10.1002/sia.2726 Abstract Nanostructured materials are increasingly subject to nearly every type of chemical and physical analysis possible. Because of their small feature size there is a significant focus on tools with high spatial resolution. Because of their high surface area, it is also natural to characterize nanomaterials using tools designed to analyze surfaces. Regardless of the approach, nanostructured materials present a variety of obstacles to adequate, useful and needed analysis. This paper provides short overviews to some of the issues and complications including: particle stability, environmental effects, specimen handling, surface coating, contamination and time. Some specific examples are provided from a our work focused on ceria nanoparticles and iron metal-core/oxide-shell nanoparticles in which we use a combination of tools for routine analysis including XPS, TEM, and XRD and apply other methods as needed to obtain essential information.
2007. "Synthesis and Characterization of Compositionally Graded Si1-xGex Layers on Si substrate." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 261(1-2):723-726. Abstract Thin film of silicon germanium (Si1-xGex) with tailored composition was grown on Si (100) substrate at 650oC in an ultrahigh vacuum molecular beam epitaxy system. The nominal x-value is ranged from 0 to 0.14. The quality of the film was investigated by Rutherford backscattering spectrometry (RBS) in random and channeling geometries, glancing angle x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDX), and atomic force microscopy (AFM). RBS/Channeling measurements indicate that the strain associated with lattice mismatch is compressive in the film. Both RBS and EDX analyses indicate the compositional graded incorporation of Ge in the film with x ranging from 0 to 0.14. The film shows island growth with each island centering around an interface dislocation.
2007. "Epitaxial Growth and Microstructure of Cu2O Nanoparticle/thin Films on SrTiO3(100)." Nanotechnology 18:Art. No. 115601. doi:10.1088/0957-4484/18/11/115601 Abstract Cuprous oxide (Cu2O) was grown on SrTiO3 (STO)(100) by oxygen plasma assisted molecular beam epitaxy. Microstructure of the grown layer and Cu valence state were analyzed using x-ray diffraction (XRD), x-ray photo-electron spectroscopy (XPS), atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM) as well as electron diffractions. The grown layer was dominated by Cu2O phase, possessing an epitaxial orientation with the substrate such that: Cu2O[001]//STO[001] and Cu2O(100)//STO(100). Cu2O film morphologically shows dependence on the growth rate. Typically, a fast growth will lead to the formation of a thin film with a relatively smooth surface. A slow growth will lead to the development of nanoparticles, featuring the formation of Cu2O pyramid. The pyramids are invariantly defined by the Cu2O {111} planes. Given the fact that the {111} planes correspond to the lowest surface energy of Cu2O, a slow growth will lend the system enough time to allow it to adopt the pyramid configuration by which the overall energy of the system was minimized.
2007. "Understanding Practical Catalysts Using a Surface Science Approach: The Importance of Strong Interaction between BaO and Al2O3 in NOx Storage Materials." Journal of Physical Chemistry C 111(41):14942-14944. doi:10.1021/jp0763376 Abstract Modern surface science techniques have been commonly applied to understand issues arising from practical catalytic systems.[1-4] However, the applicability of most of the results obtained from model systems has been limited, due, primarily, to the vastly different conditions studies on model and practical systems are carried out (catalyst composition, reaction conditions etc.).[5, 6] Therefore, the need to conduct experiments on compositionally similar systems (model and practical) is necessary to obtain valuable information on the workings of real catalysts. In this communication we demonstrate the utility of surface science studies on model catalysts in understanding the properties of high surface area, BaO-based NOx storage-reduction (NSR) catalysts.[7] We present evidence for the facile formation of surface barium aluminate-like species even at very low coverages of BaO. This Ba-aluminate layer, however, can react with NO2 resulting in the formation of a bulk-like Ba(NO3)2 phase. In order to construct model catalysts that are representative of the practical NOx storage systems, we first needed to estimate the BaO covareges on the high surface area catalysts. Since the publication of the work by Fanson et al.[8], BaO loadings of 8 – 10 wt.% on a γ-alumina support (200 m2/g) have been regarded as corresponding to one monolayer (ML) coverage, based on the unit cell size of bulk BaO. The coverage equivalent of one ML, however, was significantly underestimated. Assuming complete spreading of the BaO layer and using a Ba–O distance of ~ 2.77 Å (one unit of BaO occupies 1.53 × 10-19 m2), 10 wt.% loading of BaO would cover only about 1/3 of the alumina surface. Table 1 shows our calculated estimates of two-dimensional BaO coverages as a function of loading on a -Al2O3 surface (200 m2/g) based on the lattice parameters of bulk BaO[9] (5.54 Å). Based on these values, for our model system studies we prepared BaO/Al2O3/NiAl(110) materials in which the BaO coverages were very close to those of 4, 8, and 20 wt.% BaO/γ-Al2O3 high surface area catalysts used in prior studies.
2007. "Hydrothermal Dehydration of Aqueous Fructose Solutions in a Closed System ." Journal of Physical Chemistry C 111(42):15141-15145. doi:10.1021/jp074188l Abstract The synthesis of materials with targeted size and shape has attracted much attention. Specifically, colloidal spheres with targeted and uniform sizes have opened the door for a variety of applications associated with drug delivery, and manipulation of light (photonic band-gap crystals). Surface modification is a key to realizing many of these applications owing to the inherent inert surface.The remarkable transformation of carbohydrate molecules including sugars to homogeneous carbon spheres is found to readily occur by a dehydration mechanism and subsequent sequestering in aqueous solutions that are heated at 160-180oC in a pressurized vessel. Under such conditions, these molecules actually dehydrate even though they are dissolved in water. Size-tunable metal and metal oxides with uniform shells have also been prepared by using carbon spheres as templates.
2007. "HIGH TEMPERATURE OXIDATION/CORROSION BEHAVIOR OF METALS AND ALLOYS UNDER A HYDROGEN GRADIENT." International Journal of Hydrogen Energy 32(16):3770-3777. doi:10.1016/j.ijhydene.2006.08.056 Abstract Metallic interconnects in SOFC stacks, perform in challenging environment, as they are simultaneously exposed to a reducing environment (e.g. hydrogen, reformate) on one side and an oxidizing environment (e.g. air) on the other side at elevated temperatures. To understand the oxidation/corrosion behavior of metals and alloys under the dual exposures and assess their suitability, selected metals and alloys, including nickel, Fe-Cr and Ni-Cr base chromia forming alloys, alumina forming Fecralloy®, were investigated. It was found that the oxidation/corrosion behavior of metals and alloys in the presence of dual environment can be significantly different in terms of scale structure and/or chemistry from their exposure in a single oxidizing or reducing atmosphere. The anomalous oxidization/corrosion is attributed to the presence of hydrogen diffusion flux from the fuel side to the air side under the influence of a hydrogen gradient across the metallic substrates.
2007. "Proton Dynamics in ZnO Nanorods Quantified by In Situ Solid-State 1H Nuclear Magnetic Resonance." Applied Physics Letters 91(17):paper number 173107. doi:10.1063/1.2799734 Abstract Zinc oxide (ZnO) adopts wurtzite structure and possesses a direct wide band gap (Eg ~ 3.3 eV at 300 K), similar to that of GaN (Eg ~ 3.4 eV at 300 K), which enables ZnO as an alternative candidate to replace GaN for use in optoelectronic devices. The present controversy is centered at the microscopic origin of the “native donors”, particularly after ab initio calculations by Van de Walle, which indicate that hydrogen is soluble in ZnO at the interstitial sites, effectively forming a donor level just below the conduction band in ZnO. Hence, the origin of n type conductivity in ZnO is proposed due to the presence of hydrogen. Electron paramagnetic resonance and spectroscopic observations of muons provide experimental evidence of hydrogen presence in ZnO. Whereas, Look et al. suggests that the complex of zinc interstitial and nitrogen defect is a stronger candidate for donor than hydrogen interstitials under N ambient. Hydrogen-oxygen complex is claimed to be stable even at T > 1000°C in the hydrothermally synthesized ZnO. Therefore, the thermodynamic nature of hydrogen characteristics remains controversial, particularly its role on resident defects. In this letter, in situ temperature dependent solid state 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is employed to probe the local chemical environments of hydrogen in ZnO nanorods. To best knowledge of ours, this is the first time that the presence of hydrogen, its concentration, and local transport dynamics are directly chemically determined. Moreover, in situ NMR allows a new approach to investigate the absorption and desorption of protons from different sites on the ZnO nanorods, thus study of site-specific proton dynamics in ZnO becomes feasible.
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. "HRTEM Study of Diesel Soot Collected from Diesel Particulate Filters ." Carbon 45(1):70-77. doi:10.1016/j.carbon.2006.08.005 Abstract HRTEM study of several soot samples collected on Diesel Particulate Filters (DPF) under conditions relevant to practical applications of DPF technology, revealed nano-structure, to our knowledge, not reported previously for diesel soot. In particular, some of the primary particles were found to have hollow interior, and the outer shell exhibiting evidence of graphitization, with a higher crystallinity compared to the non-hollowed particles. The percentage of such particles varied between different soot samples and tentatively appeared to be related to the oxidation history of the sample. Remarkably, similar effect was not reproduced for a carbon black sample, Printex-U, suggesting that propensity to such oxidation-induced graphitization is related to the original nano-structure of the particle. These initial observations were independently confirmed for the same set of soot samples by two different HRTEM facilities, at NASA-Glenn and PNNL.
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 of SiC nanorods from bleached wood pulp." Materials Letters 61(13):2814-2817. doi:10.1016/j.matlet.2006.10.035 Abstract Unbleached and bleached soft wood pulps have been used as templates and carbon precursors to produce SiC nanorods. Hydrolyzed tetraethylorthosilicate (TEOS), Silicic acid was infiltrated into the pulps followed by a carbothermal reduction to form SiC nanorods at 1400oC in Ar. Residual carbon formed along with SiC was removed by gasification at 700oC in air. The SiC materials prepared from unbleached pulp were non-uniform SiC with a thick SiO2 coating, while the SiC nanorods prepared from the bleached pulp were uniform and straight with dimensions of 250 nm in diameter and 5.0 mm long. The formation of uniform camelback structure of SiC in the reaction between silica and bleached pulp is attributed to more silica deposited in the amorphous region of cellulose.
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. "Growth and structure of MBE grown TiO2 anatase films with rutile nano-crystallites ." Surface Science 601(6):1582-1589. doi:10.1016/j.susc.2007.01.039 Abstract We have grown TiO2 anatase films with rutile nanocrystalline inclusions using molecular beam epitaxy under different growth conditions. This model system is important for investigating the role of rutile/anatase interfaces in heterogeneous photocatalysis. To control the film structure, we grew a pure anatase (001) layer at a slow rate and then increased the growth rate to drive the nucleation of rutile particles. Structure analysis indicates that the rutile phase has four preferred orientations in the anatase film.
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. "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. "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. "Templating Mesoporous Hierarchies in Silica Thin Films Using the Thermal Degradation of Cellulose Nitrate." Microporous and Mesoporous Materials 99(3):308-318. Abstract Materials containing a hierarchical pore structure (i.e. large pores leading to small pores) are highly desirable because they combine the advantages of high surface area with the rapid mass transport. Mesoporous SiO2 with hierarchical pore structure was prepared by a novel dual templating approach using a combination of cellulose nitrate and surfactants as the templates. Both ionic and nonionic surfactants, or mixtures of surfactants, in conjunction with cellulose nitrate were used as the pore templates. Low angle XRD patterns show well-defined pore structures and BET shows surface areas from 500 to over 800m2/g, with tunable bimodal or trimodal pore-size distributions from 18Å to 0.3µm. The hierarchical pore structure can be controlled by manipulating the template composition.
2007. "Synthesis and Photoexcited Charge Carrier Dynamics of beta-FeOOH Nanorods." Applied Physics Letters 90(10):Art. No. 103504. doi:10.1063/1.2711395 Abstract Akaganeite(B-FeOOH) nanorods of dimensions 15 nm diameter and 200 nm length were prepared by aqueous synthesis. Charge carrier dynamics following femtosecond excitation displays three timescales. The first is a sub-picosecond decay of initially excited carriers to the band edge followed by trapping or nonradiative decay within 2 ps. The trapped electrons and holes persist for significantly longer times (at least tens-of-ps), similar to previous results from a-Fe2O3 materials. The short carrier lifetimes in these materials are attributed to fast trapping to Fe d-d and midgap states.
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. "Structure, Magnetism and Conductivity in Epitaxial Ti-doped -Fe2O3 Hematite: Experiment and density functional theory calculations." Physical Review. B, Condensed Matter and Materials Physics 75(10):, doi:10.1103/PhysRevB.75.104412 Abstract We explore the feasibility of growing epitaxial Ti-doped -Fe2O3 in which Ti(IV) substitutes for Fe(III) preferentially in one magnetic sublattice, but not the other. Such a structure has been predicted by first-principles theory to be energetically likely, and is expected to yield interesting and useful magnetic and electronic properties. However, we find that a majority of Ti dopants disperse and occupy random cation sites in both magnetic sublattices. Density functional theory predicts that the magnetically ordered and magnetically random structures are nearly isoenergetic.
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. "Formation of Cobalt Silicide Films by Ion Beam Deposition." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):602-604. Abstract Thin films of cobalt silicide are widely used as metallization in very large-scale integrated electronic circuits. In this study, Co ions were deposited on Si (111) wafers by a high beam current filter metal vacuum arc deposition (FMEVAD) system. Surface silicide films were formed after annealing from 500 to 700 C for 30 minutes. Cobalt depth profiles and contaminations were determined using Rutherford backscattering spectrometry (RBS) and time-of-flight energy elastic recoil detection analysis (ToF-E ERDA). The polycrystalline cobalt silicide phases formed were characterized by grazing-incidence x-ray diffraction (GIXRD). The surface topography development and interfaces have been investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that a thin CoSi2 surface layer with both a smooth surface topography and sharp interface can be achieved by annealing at 700 C. The CoSi phase and O contamination were observed in the samples that were annealed at lower temperatures.
2006. "Reproducible Tip Fabrication and Cleaning for UHV STM ." Ultramicroscopy 108(9):873-877. doi:10.1016/j.ultramic.2008.02.010 Abstract Several technical modifications related to the fabrication and ultra-high vacuum (UHV) treatment of the tips have been implemented to improve a reliability of the tip preparation for high-resolution scanning tunneling microscopy. The widely used drop-off technique for the tip electrochemical etching has been further refined to enable a reproducible fabrication of the tungsten tips with a radius 3 nm. Simple and flexible setup for the tip UHV annealing has been developed and employed. The Ar ion sputtering with subsequent annealing has been adopted for a final tip treatment. The proper tip preparation has been demonstrated by imaging an atomic structure of the rutile TiO2(110) surface.
2006. "Room Temperature Solvent-free Synthesis of Monodisperse Magnetite Nanocrystals." Journal of Nanoscience and Nanotechnology 6(3):852-856. doi:10.1166/jnn.2006.135 Abstract We have successfully demonstrated a facile, solvent-free synthesis of highly crystalline and monodisperse Fe3O4 nanocrystallites at ambient temperature avoiding any heating. Solid state reaction of inorganic Fe(II) and Fe(III) salts with NaOH was found to produce highly crystalline Fe3O4 nanoparticles. The reaction, if carried out in the presence of surfactant such as oleic acid-oleylamine adduct, generated monodisperse Fe3O4 nanocrystals extractable directly from the reaction mixture. The extracted nanoparticles were capable of forming self-assembled, two-dimensional and uniform periodic array. The new process utilizes inexpensive and nontoxic starting materials, and does not require a use of high boiling point and toxic solvents, thus is amenable to an environmentally desirable, large-scale synthesis of nanocrystals.
2006. "Excited Carrier Dynamics of α-Cr2O3/α-Fe2O3 Core-Shell Nanostructures." Journal of Physical Chemistry B 110(34):16937-16940. doi:10.1021/jp062507n S1520-6106(06)02507-7 Abstract In this work α-Cr2O3/α-Fe2O3 core-shell polycrystalline nanostructures were synthesized using α-Cr2O3 nanoparticles as seed crystals during aqueous nucleation. The formation of α-Fe2O3 polycrystallites on α-Cr2O3 surfaces were confirmed by x-ray diffraction, transmission electron microscopy, and energy dispersive x-ray analysis. The excited state relaxation dynamics of as-grown core-shell structures and \pure" α-Fe2O3 particles of the same size were measured using femtosecond transient absorption spectroscopy. The results show the carrier lifetimes decay within a few picoseconds regardless of sample. This is likely due to fast recombination/trapping of carriers to defects and iron d-states.
2006. "Formation of cobalt silicide from filter metal vacuum arc deposited films." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 247(2):271-278. doi:10.1016/j.nimb.2006.02.016 Abstract The thermal reaction of Co film deposited on Si (111) surfaces by a high current filter metal vacuum arc (FMEVAD) system has been studied. After deposition the films were annealed over the 400-900 ◦C temperature range for 30 min. Rutherford Backscattering Spectrometry (RBS) was used to charactrise the elemental depth distributions in the films subjected to different annealing temperatures. Ordered chemical phases were determined by glancing-incidence X-ray diffraction (GIXRD) and the morphology was determined by cross section transmission electron microscopy (TEM). The results show that the phases formed are Co2Si at 400 ◦C, CoSi + CoO at 500 ◦C, CoSi + CoSi2 at 600 ◦C, and CoSi2 at (700-800 ◦C). At 900 ◦C, CoSi2 was formed with a mixture of cubic cobalt and probably an amorphous cobalt oxide surface layer. The interface morphology was a rough cusp-like crenellation at 600 ◦C which became less pronounced after annealing at 800 ◦C.
2006. "Synthesis of organically templated nanoporous tin (II/IV) phosphate for radionuclide and metal sequestration." Inorganic Chemistry 45(6):2382-2384. doi:10.1021/ic051949v Abstract Nanoporous tin (II/IV) phosphate materials, with spherical morphology, have been synthesized using cetyltrimethylammonium chloride (CH3(CH2)15N(CH3)3Cl) as the surfactant. The structure of the material is stable at 500°C; however, partial oxidation of the material occurs with redox conversion of Sn2+ to Sn4+, resulting in a mixed Sn(II)/ Sn(IV) material. Preliminary batch contact studies were conducted to assess the effectiveness of nanoporous tin phosphate, NP-SnPO, in sequestering redox sensitive metals and radionuclides, technetium(VII), neptunium(V), thorium(IV), and a toxic metal, chromium(VI), from aqueous matrices. Results indicate tin (II) phosphate removed > 95% of all contaminants investigated from solution.
2006. "Morphological Evolution of Ba(NO3)2 Supported on -Al2O3(0001): An In-Situ TEM Study." Journal of Physical Chemistry B 110(24):11878-11883. doi:10.1021/jp060235i Abstract One of the key questions for the BaO-based NOx catalyst system is the morphological evolution of Ba(NO3)2 to BaO upon heating for releasing of NOx or vice versa from BaO to Ba(NO3)2 upon uptaking of NOx. However, associated with the small crystallite size of high-surface area Al2O3, it can be difficult to extract structural and morphological features of Ba(NO3)2 supported on -Al2O3 by any direct imaging method including transmission electron microscopy. In this work, by choosing a model system of Ba(NO3)2 particles supported on single crystal -Al2O3, we have investigated the structural and morphological features of Ba(NO3)2 as well as the formation of BaO from Ba(NO3)2 during the release of NOx using ex-situ and in-situ TEM imaging, electron diffraction, energy dispersive spectroscopy (EDS), and Wulff shape construction. We find that Ba(NO3)2 supported on -Al2O3 possesses a platelet morphology, with the interface and facets being invariably the 8 {111} planes. Formation of the platelet structure leads to an enlarged interface area between Ba(NO3)2 and -Al2O3, indicating that the interfacial energy is lower than the Ba(NO3)2 surface free energy. In fact, Wulff shape constructions indicate that the interfacial energy is ~1/4 of the {111} surface free energy of Ba(NO3)2. The orientation relationship between Ba(NO3)2 and the -Al2O3 is: -Al2O3[0001]//Ba(NO3)2[111] and -Al2O3(1-2 10)//Ba(NO3)2(110).
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. "Atomic level imaging of Au nanocluster dispersed in TiO2 and SrTiO3 ." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):380-382. doi:10.1016/j.nimb.2005.08.144 Abstract Au nanoclusters dispersed in single crystal TiO2 and SrTiO3 have been prepared by ion implantation at 300 and 975 K and subsequent annealing at 1275 K for 10 hours. High resolution transmission electron microscopy and high-angle-annular-dark-field (HAADF) imaging in an aberration corrected scanning transmission electron microscope (STEM) have been used to characterize the microstructure of the dispersed gold nanoclusters. The results indicate that Au atoms substitute for cations in these systems. Cavities of up to several tens of nanometers are observed in TiO2 and SrTiO3. The nanometer-sized cavities and Au clusters are faceted along the same lattice plane of the matrix, indicating that the interfacial energy (defined by the Au cluster and the matrix) and the surface energy of the matrix (defined by cavity and the matrix ) follow a similar trend of change with respect to different lattice planes of the matrix.
2006. "Effects of Ba loading and calcination temperature on BaAl2O4 formation for BaO/Al2O3 NOx Storage and Reduction Catalysts." Catalysis Today 114(1):86-93. doi:10.1016/j.cattod.2006.02.016 Abstract The effect of thermal treatment on the structure and chemical properties of Ba-oxide-based NOx storage/reduction catalysts with different Ba loadings was investigated using BET, TEM, EDS, TPD and FTIR techniques. On the basis of the present and previously reported results, we propose that moderate (< ~873 K) temperature calcinations result in a single monolayer (ML) ‘coating’ of BaO on the alumina surface. At high Ba loading in excess of that required for a full monolayer ‘coating’ (> 8 wt.% BaO), small (~5 nm) particles of ‘bulk’ BaO are present on top of the 1 ML BaO/Al2O3 surface. We did not observe any detectable morphological changes upon higher temperature thermal treatment of 2 and 8 wt% BaO/Al2O3 samples, while dramatic changes occurred for the 20 wt% sample. In this latter case, the transformations included BaAl2O4 formation at the expense of the bulk BaO phase. In particular, we conclude that the surface (ML) BaO phase is quite stable against thermal treatment, while the bulk phase provides the source of Ba for BaAl2O4 formation.
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. "Preparation of Ultrafine Chalcopyrite Nanoparticles via the Photochemical Decomposition of Molecular Single-Source Precursors." Nano Letters 6(6):1218-1223. doi:10.1021/nl060661f Abstract The synthesis and characterization of ultrafine CuInS2 nanoparticles are described. Ultraviolet irradiation was used to decompose a molecular single source precursor, yielding organic soluble ~2 nm sized nanoparticles with a narrow size distribution. UV-vis absorption, 1H and 31P{1H} NMR, and fluorescence spectroscopies and mass spectrometry were used to characterize decomposition of the precursors and nanoparticle formation. The nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy energy dispersive X-ray spectroscopy, powder X-ray diffraction (XRD), electron diffraction, inductively coupled plasma analysis, UV-vis absorption spectroscopy, and fluorescence spectroscopy. They have a wurzite-type crystal structure with a copper-rich composition. The hypsochromic shift in their emission band due to quantum confinement effects is consistent with the size of the nanocrystals indicated in the HRTEM and XRD analyses.
2006. "Nitrocellulose Templated Hierarchical Pore Structure in Mesoporous Thin Films." Inorganic Chemistry Communications 9(1):7-9. Abstract Over the last decade, a great deal of effort has been expended on the templated synthesis of nanoporous materials. Many different templates have been used to create this nanostructure (surfactants, polymers, latex spheres, etc.), but by far the most widely used has been micelles composed of surfactants. This is a versatile, and highly useful, synthetic method, capable of producing a wide variety of materials and structures. More recently, the synthesis of hierarchical pore structures (i.e. small pores leading to large pores) has been of great interest as a means of enhancing mass transport within these materials.[1] Such hierarchical pore structures have been made by combining surfactant templating methods with latex beads [2], by assembling as-synthesized MCM-41 particles around block co-polymer micelles, followed by crosslinking and calcination.[3], by spray drying MCM-41 and MCM-48 agglomerates [4], and by using “evaporation induced self-assembly” [5-9].
2006. "Effect of Barium Loading on the Desulfation of Pt-BaO/Al2O3 Studied by H2 TPRX, TEM, Sulfur K-edge XANES, and in Situ TR-XRD." Journal of Physical Chemistry B 110(21):10441-10448. doi:10.1021/jp060119f Abstract Desulfation processes were investigated over sulfated Pt BaO/Al2O3 with different barium loading (8 wt% and 20 wt%) by using H2 temperature programmed reaction (TPRX), transmission electron microscope (TEM) with energy dispersive spectroscopy (EDS), sulfur K-edge X-ray absorption near-edge spectroscopy (XANES), and in situ time-resolved X-ray diffraction (TR-XRD) techniques. Both sulfated samples (8 wt% and 20 wt%) form sulfate species (primarily BaSO4) as evidenced by S K-edge XANES and in situ TR-XRD. However, the desulfation behavior is strongly dependant on the barium loading. Sulfated Pt BaO(8)/Al2O3, consisting predominantly of surface BaO/BaCO3 species, displays more facile desulfation by H2 at lower temperatures than sulfated Pt BaO(20)/Al2O3, a material containing primarily bulk BaO/BaCO3 species. Therefore, after desulfation with H2 up to 1073 K, the amount of the remaining sulfur species on the former, mostly as BaS, is much less than on the latter. This suggests that the initial morphology differences between the two samples play a crucial role in determining the extent of desulfation and the temperature at which it occurs. It is concluded that the removal of sulfur is significantly easier at lower barium loading. This finding can potentially be important in developing more sulfur resistant LNT catalyst systems.
2006. "Magnetic properties of epitaxial Co-doped anatase TiO2 thin films with excellent structural quality." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 24(4):2012-2017. doi:10.1116/1.2216723 Abstract The heteroepitaxy of Co-doped anatase TiO2 on LaAlO3(001) has been refined with the goal of determining the relationship between structural quality and magnetic ordering. By significantly reducing the deposition rate and substrate temperature, well-ordered Co:TiO2 films with unprecedented crystalline quality were obtained by oxygen-plasma-assisted molecular beam epitaxy, as characterized by x-ray diffraction. These films exhibit uniform Co doping, with no evidence of Co segregation or secondary phases throughout the film depth or on the surface. Despite the improvement in crystalline quality and Co distribution, the films exhibit negligible ferromagnetism, with saturation moments of only ~0.1 B/Co. This loss of ferromagnetism is in stark contrast to faster-grown Co:TiO2 films, where a higher growth rate and substrate temperature typically result in lower crystalline quality, a highly non-uniform Co distribution, and average saturation moments of ~1.2 B/Co. The presence of ferromagnetism in faster-grown Co:TiO2 does not appear to arise from intrinsic point defects present in the bulk material, such as charge-compensating oxygen vacancies, but is instead attributed to the presence of extended structural defects.
2006. "Ferromagnetism and structure of epitaxial Cr-doped anatase TiO2 thin films." Physical Review. B, Condensed Matter and Materials Physics 73(15):155327 (12 p.). doi:10.1103/PhysRevB.73.155327 Abstract The materials and magnetic properties of Cr-doped anatase TiO2 thin films deposited on LaAlO3(001) and SrTiO3(001) substrates by oxygen-plasma-assisted molecular beam epitaxy have been studied in detail to elucidate the origin of ferromagnetic ordering. Cr substitution for Ti in the anatase lattice, with no evidence of Cr interstitials, segregation, or secondary phases, was independently confirmed by transmission electron microscopy (TEM) with energy dispersive x-ray (EDX) spectroscopy, extended x-ray absorption fine structure (EXAFS), and Rutherford backscattering spectrometry (RBS) in the channeling geometry. Epitaxial films deposited at ~0.1 Å/s were found to have a highly defected crystalline structure, as quantified by high resolution x-ray diffraction (XRD). These films were also ferromagnetic at room temperature with a moment of ~0.5 B/Cr, Curie temperatures in the range of 400 – 700°C, and exhibited shape and in-plane magnetocrystalline anisotropy. However, no free carrier spin polarization was observed by Hall effect measurements, raising questions about the mechanism of magnetism. Films deposited slowly (~0.015 Å/s) possessed a nearly perfect crystalline structure as characterized by XRD. Contrary to expectations, these films exhibited negligible ferromagnetism at all Cr concentrations. Annealing in vacuum to generate additional oxygen defects and free carrier electrons did not significantly increase the ferromagnetic ordering in either fast- or slow-grown films. These results contradict both oxygen-vacancy-derived free-carrier-mediated exchange and F-center-mediated bound magnetic polaron exchange mechanisms, and instead indicate the primary role of extended structural defects in mediating the ferromagnetic ordering in doped anatase films.
2006. "Thermal Oxidation of Aluminum Nitride Powder ." Journal of the American Ceramic Society 89(7):2167–2171. doi:10.1111/j.1551-2916.2006.01065.x Abstract The kinetics of the thermal oxidation of AlN powder in flowing oxygen over temperatures from 800 to 1150 °C and the morphology and crystallinity of the resultant oxide were determined. The oxidation of two types of AlN powder was investigated and compared. Complex difference in the oxidation behavior was observed, probably due to their different morphology, particle size, particle size distribution, and residual impurities. Amorphous alumina formed at relatively low oxidation temperatures (800-1000 °C), with a linear oxidation rate governed by interfacial reaction. Crystalline alumina formed at higher temperatures (>1000 °C), and the oxidation rate was parabolic which suggested an oxidant diffusion controlled process.
2006. "Growth of aligned carbon nanotubes on carbon microfibers by dc plasma-enhanced chemical vapor deposition." Applied Physics Letters 88:033103. doi:10.1063/1.2166472 Abstract It is shown that unidirectionally aligned carbon nanotubes can be grown on electrically conductive network of carbon microfibers via control of buffer layer material and applied electric field during dc plasma chemical vapor deposition growth. Ni catalyst deposition on carbon microfiber produces relatively poorly aligned nanotubes with significantly varying diameters and lengths obtained. The insertion of Ti 5 nm thick underlayer between Ni catalyst layer and C microfiber substrate significantly alters the morphology of nanotubes, resulting in much better aligned, finer diameter, and longer array of nanotubes. This beneficial effect is attributed to the reduced reaction between Ni and carbon paper, as well as prevention of plasma etching of carbon paper by inserting a Ti buffer layer. Such a unidirectionally aligned nanotube structure on an open-pore conductive substrate structure may conveniently be utilized as a high-surface-area base electrodes for fuel cells, batteries, and other electrochemical and catalytic reactions.
2006. "Ferromagnetism in Oxide Semiconductors ." Materials Today 9(11):28-35. Abstract In order to become a practical technology, semiconductor spintronics requires the discovery and utilization of ferromagnetic semiconductors which exhibit spin polarization in the majority carrier band at and above room temperature. Intrinsic remanent magnetization would allow spin polarized currents to be propagated in such materials without the need for a continuous magnetic field. However, the discovery and understanding of such materials is proving to be a grand challenge in solid-state science. Indeed, one of the 125 critical unanswered scientific questions recently posed in Science magazine asks, “Is it possible to create magnetic semiconductors that work at room temperature?”
2006. "Site-Specific Laser Modification of MgO nanoclusters: Towards Atomic-Scale Surface Structuring." Physical Review. B, Condensed Matter 74:045404 (5 pages). Abstract Atomic emission from MgO nanostructures is induced using laser light tuned to excite specific surface sites at energies well below the excitation threshold of the bulk material. We find that near UV excitation of MgO nancrystalline films and nanocube samples desorbs neutral Mg-atoms with hyper-thermal kinetic energies in the range of 0.1-0.4 eV. Our ab initio calculations suggest that metal atom emission is induced predominantly by electron trapping at surface 3-coordinated Mg sites followed by electronic excitation at these sites. The proposed general mechanism can be used to control atomic scale modification of insulating surfaces.
2006. "Synthesis and Characterization of Stable Iron–Iron Oxide Core–Shell Nanoclusters for Environmental Applications." Journal of Nanoscience and Nanotechnology 6(2):568-572. doi:10.1166/jnn.2006.074 Abstract Iron–iron oxide core–shell nanoclusters are of great interest due to their potential applications as a remedy for environmental contamination. We report the room-temperature synthesis of core–shell iron–iron oxide nanoclusters using our novel cluster deposition system. Various types of measurements such as Transmission Electron Microscopy, X-ray Diffraction, X-ray Photon Spectroscopy, and Electron Energy Loss Spectroscopy are conducted in characterizing nanoclusters. Stable, monodispersive iron–iron oxide core–shell nanocrystals are identified.
2005. "Potassium Chloride Nanowire Formation Inside a Microchannel Glass Array." Applied Physics Letters 86:Article 263110. Abstract The synthesis of KCl nanowires has been achieved by atomic layer deposition inside high aspect ratio channels of microchannel glass. The average diameter of the KCl nanowires is 250 nm, with a minimum observed diameter of 50 nm, and lengths up to 5 µm. The Cl precursor was TaCl5, while the source of K was determined to be impurities in the microchannel glass substrate. The process for KC1 nanowire formation is a three-step chemical process that simultaneously etches K from the substrate concomitant with the formation of chlorine gas. It is postulated that the curvature of the channels may influence the diameters of the KCl nanowires.
2005. "Ion-Induced Damage Accumulation and Electron-Beam-Enhanced Recrystallization in SrTiO3." Physical Review. B, Condensed Matter and Materials Physics 72(9):094112, 1-8. Abstract Damage accumulation in strontium titanate (SrTiO3) from 1.0 MeV Au irradiation has been investigated at temperatures from 150 to 400 K. The relative disorder on the Sr and Ti sublattices at the damage peak has been determined as a function of local dose and temperature. A disorder accumulation model has been fit to data from this study and from the literature, indicating that defect-stimulated amorphization is the primary amorphization mechanism up to ~360 K. High-dose irradiation at 400 K leads to formation of an amorphous surface layer. Analyses of the temperature dependence for amorphization indicate that the amorphization kinetics are consistent with irradiation-enhanced and thermal recovery processes with activation energies of 0.1 eV and 0.7 eV, respectively. Under 200 keV electron-beam irradiation, the epitaxial recrystallization rates are orders of magnitude higher than thermal rates, and an activation energy of 0.1 eV is determined for the e-beam enhanced recrystallization processes.
2005. "Mechanical Stability of Templated Mesoporous Silica Thin Films." Microporous and Mesoporous Materials 85(3):260-266. Abstract Mesoporous silica thin films about 1 micron thick were prepared by spin casting using several organic templates that provide a range of pore structures from disordered (sponge-like) to more ordered (honeycomb-like) 2D hexagonal arrays. Nanoindentation measurements indicate that the elastic modulus (E), and thus the density, of the pore wall material are substantially lower than for fused silica. The corresponding lower dielectric constant for pore walls was used to calculate film porosities (P) with recent correlations from the literature. Curve fits to the standard modulus vs. porosity correlation, E/Eporewall=(1-P)n, for the films gave lower n=2.2 for the honeycomb-like film with higher E, but higher n=2.5 for the sponge-like film with lower E, in contrast to theoretical expectations (n=2 for sponge-like and n=3 for honeycomb-like). Although the dielectric constant depends primarily on first-order structural information (P), the elastic modulus of these structurally imperfect films required second-order (pore wall thickness/diameter ratio) and third-order (pore connectivity) parameters to resolve the data. The power law exponent n can vary continuously, depending on the details of the mesostructure, and should not be assumed a' priori unless justified by detailed structural information
2005. "Heteroepitaxial Growth and Structural Analysis of Epitaxial ⍺-Fe₂O₃(1010) on TiO₂(001)." Journal of Materials Research 20(5):1250-1256. Abstract We have grown epitaxial ⍺-Fe₂O₃(1010) on TiO₂(001) rutile by oxygen plasma assisted molecular beam epitaxy. High resolution transmission electron microscopy (HRTEM), reflection high energy electron diffraction (RHEED) and x-ray diffraction pole figures confirm that the film is composed of four different in-plane orientations rotated by 90º relative to one another. For a given Fe₂O₃ unit cell, the lattice mismatch along the parallel [0001]Fe₂O₃ and [100]TiO₂ directions is nominally +67%. However, due to a three-fold repetition of the slightly distorted square symmetry of anion positions within the Fe₂O₃ unit cell, there is a coincidental anion alignment along the [0001]Fe₂O₃ and [100]TiO₂ directions which results in an effective lattice mismatch of only -0.02% along this direction. The lattice mismatch is nearly 10% in the orthogonal [1120]Fe₂O₃ and [100]TiO₂ directions. The film is highly ordered and well registered to the substrate despite a large lattice mismatch in one direction. The film grows in registry with the substrate along the parallel [0001]Fe₂O₃ and [100]TiO₂ directions and nucleates dislocations along the orthogonal [1120]Fe₂O₃ [100]TiO₂ directions.
2005. "Void Formation during Early Stages of Passivation: Initial Oxidation of Iron Nanoparticles at Room Temperature." Journal of Applied Physics 98(9):094308(7). doi:10.1063/1.2130890 Abstract The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some uncertainties. Exposure of most clean metals to air or oxygen results in the nearly instant formation of an oxide layer. Because this initial layer normally forms in a relatively uncontrollable manner, the atomic level understanding of the initial oxidation is limited in comparison to the abundant experimental observation and theoretical derivation on thickening of oxidation layer on metal surface at high temperature. We report in this letter mMicrostructurale characterization of iron nanoparticles oxide passivated with iron oxide shell nanoparticleswere studied using high resolution transmission electron microscopy (HRTEM) and high angle annular dark-filed (HAADF) imaging in aberration corrected scanning transmission electron microcopy (STEM). Voids were readily observed on both small single crystal -Fe nanoparticles formed in a sputtering process and the more complex particles created by reduction of an oxide by hydrogen. Although the formation of hollow spheres of nanoparticle has been engineered for Co at higher temperatures1, they occur for iron at room temperature and provide insight into the initial oxidation processes of iron. The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some controversy. For example, void formation has been noticed in the single crystal -Fe nanoparticles as a consequence of iron outward diffusion during the initial oxidation at room temperature. There exists a critical size of ~ 8 nm for which the iron has been fully oxidized, leading to a hollow iron oxide nanoparticle. For particles larger than the critical size, an iron/iron oxide core-shell structure was formed and voids reside at the interface between the oxide shell and the iron core. The present observation provides new insight for tailoring of metal/metal-oxide core-shell structured nanoparticles for applications related to optics, magnetism, and nanoelectronics.
2005. "Synthesis, Characterization, and Optical Properties of Pristine and Doped Yttrium Aluminum Garnet Nanopowders." Journal of the American Ceramic Society 88(2):284–286. Abstract Pristine, Si-doped, and Si/Nd-doped yttrium aluminum garnet (YAG) nanoparticles were synthesized by pyrolysis of complex compounds of aluminum and yttrium with triethanolamine. It was found that the coexistence of Si⁺⁴ and Nd³⁺ increased the solubility of both ions and promoted the formation of YAG phase. Single-phase, nanocrystalline Si/Nd:YAG powders were obtained at calcinations temperatures as low as 920°C. The optical behaviour of the Si/Nd:YAG nanopowders was similar to that of single-crystal ND:YAG.
2005. "Self-assembling of nanocavities in TiO2 dispersed with Au nanoclusters." Physical Review. B, Condensed Matter 72(24):245421, 1-5. doi:10.1103/PhysRevB.72.245421 Abstract There has been considerable research effort on tailoring the non-linear optical properties of dielectric materials by dispersing nanometer-sized metallic clusters in them. It has been proposed that the optical response of this type of material is related to the quantum antidots (a vacancy cluster), which is spatially located at the interface between the metal cluster and the dielectric matrix. In order to clarify the vacancy clustering behavior as well as its correlation with Au clustering, single crystal TiO2 has been implanted with Au ions at 975 K and subsequently annealed at 1275 K for 10 hours. A characteristic self-assembling of nano-cavities along the boundary between the region of Au clusters and the free surface has been observed in the present system. These cavities are faceted along TiO2(110) and have a size of ~10 nm. High angle annular dark-field (HAADF) imaging in an aberration corrected scanning transmission electron microscope (STEM) revealed that vacancy clusters of ~ 2 nm in size also exist in the Au populated regions. Formation of cavities in Au-irradiated TiO2 strongly indicates that vacancy clustering processes prevail over Frenkel-pair recombination. Furthermore, the Au atoms substitution for Ti in TiO2 is also directly observed by STEM-HAADF imaging and by channeling Rutherford backscattering spectrometry (RBS).
2005. "Microstructure of ZrO2-CeO2 Hetero-Multi-Layer Films Grown on YSZ Substrate." Acta materialia 53(7):1921-1929. Abstract Multi-layer films of pure ZrO2 and CeO2 were grown using oxygen plasma assisted molecular beam epitaxy on yttria stabilized zirconia (YSZ) substrates. The microstructure of the film was analyzed using x-ray diffraction (XRD), conventional and high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), energy dispersive x-ray (EDX) elemental mapping, selected area electron diffraction, and dynamical electron diffraction calculations. The deposited pure CeO2 layers exist in the cubic fluorite structure, and the ZrO2 layers show a good epitaxial orientation with respect to the CeO2 layers. However, distinctive forbidden diffraction spots of (odd, odd, even) type were observed on the selected area electron diffraction patterns of the film. Dark-field imaging clearly reveals that these forbidden diffraction spots were contributed solely by the ZrO2 layers. Dynamical electron diffraction calculation based on the tetragonal phase of unity tetragonality (space group P42/nmc) with oxygen displaced along the c-axis does not match with the experimental pattern. Instead, a diffraction pattern calculated based on a cubic structure (space group P43m) for which the oxygen sub-lattice was displaced along the <111> matches with the experimental data. It is further suggested that the displacement of the oxygen from the ideal (¼,¼,¼) position was introduced during the film growth process.
2005. "Microstructure of Co-doped TiO₂ (110) Rutile by Ion Implantation." Journal of Applied Physics 97(7):99-104. Abstract Co-doped rutile TiO₂ was synthesized by injecting Co ions into single crystal rutile TiO₂ using high energy ion implantation. Microstructures of the implanted specimens were studied in detail using high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDS), electron diffraction, and HRTEM image simulations. The spatial distribution and conglomeration behavior of the implanted Co ions, as well as the point defect distributions induced by ion implantation, show strong dependences on implantation conditions. Uniform distribution of Co ions in the rutile TiO₂ lattice was obtained by implanting at 1075 K with a Co ion fluence of 1.25x10¹⁶ Co/cm². Implanting at 875 K leads to the formation of Co metal clusters. The precipitated Co metal clusters and surrounding TiO₂ matrix exhibit the orientation relationships Co<110>//TiO₂[001] and Co{111}//TiO₂(110). A structural model representing the interface between Co metal clusters and TiO₂ is developed based on HRTEM imaging and image simulations.
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. "Direct Imaging of Quantum Antidots in MgO Dispersed with Au Nanoclusters." Applied Physics Letters 87(15):153104 (3). Abstract Quantum antidots (a vacancy cluster) have been proposed to account for the non-linear optical behavior of magnesium oxide (MgO) dispersed with Au nanoclusters [1,2]. In this paper, we provide convincing evidence, based on direct observation using high-angle annular dark-filed imaging in aberration corrected scanning transmission electron microscope, that vacancies in excess of Au atoms are clustering together to form antidots at the immediate neighborhood of the Au clusters, leading to a spatially associated Au nanocluter and antidot. The antidots show a terraced layer structure and are typically facetted along the MgO{100} planes. Furthermore, we also directly observed that Au atom substitutes for Mg atom in the MgO lattice, which is consistently supported by image calculations.
2005. "Atomic Resolution Imaging of Au Nanocluster Dispersed in TiO₂, SrTiO₃, and MgO." Journal of the American Ceramic Society 88(11):3184-3191. Abstract Gold nanoclusters dispersed in single crystal TiO₂, MgO, and SrTiO₃ have been prepared by ion implantation at 300 K - 975 K and subsequent annealing at 1275 K for 10 hours. High resolution transmission electron microscopy and high-angle-annular-dark-field (HAADF) imaging in aberration corrected scanning transmission electron microscope (STEM) have been used to characterize the microstructure of the gold nanoclusters dispersed materials. STEM-HAADF imaging with atomic resolution has directly revealed for all three materials that Au atoms occupy cations lattice positions. Cavities of up to several tens nanometers were observed in the TiO₂ and SrTiO₃. The cavities and gold clusters appear to be spatially associated in SrTiO₃. The nanometer-sized cavities and the Au cluster are faceted along the same lattice plane of the matrix, indicating that the interfacial energy defined by the Au cluster and the matrix follows the same order of the surface energy for different lattice plane.
2005. "Changing Morphology of BaO/AI₂O₃ during NO₂ Uptake and Release." Journal of Physical Chemistry B 109(15):7339-7344. Abstract The changes in the morphology of Ba-oxide-based NOx storage/reduction catalysts were investigated using time resolved x-ray diffraction, transmission electron microscopy and energy dispersed spectroscopy. Large Ba(NO₃)₂ crystallites form on the alumina support when the catalyst is prepared by the incipient wetness method using an aqueous Ba(NO₃)₂ solution. Heating the sample to 873K in a He flow results in the decomposition of the Ba(NO₃)₂ phase and the formation of both a monolayer BaO film strongly interacting with the alumina support, and nano crystalline BaO particles. Upon NO₂ exposure of these BaO phases at room temperature, small (nano-sized) Ba(NO₃)₂ crystals and a monolayer of surface nitrate form. Heating this sample in NO₂ results in the coalescence of the nano crystalline Ba(NO₃)₂ particles into large crystals. The average crystal size in the re-formed Ba(NO₃)₂ layer is significantly smaller than that measured after the catalyst preparation. Evidence is also presented for the existence of a monolayer Ba(NO₃)₂ phase after thermal treatment in NO₂, in addition to these large crystals. These results clearly demonstrate the dynamic nature of the Ba-containing phases that are active in the NOx storage/reduction process. The proposed morphology cycle may contribute to the understanding of the changes observed in the performances of these catalysts during actual operating conditions.
2005. "Synthesis of supported carbon nanotubes in mineralized silica-wood composites." Carbon 43(5):1096-1098. Abstract Multiwall carbon nanotubes (MWNTs) form spontaneously upon graphitization of organic precursors bound to the internal surfaces of a high porosity mineralized sample of natural wood. Following HF etching, both mesostructured carbon and randomly distributed and intertwined MWNTs were seen throughout the wood cellular structure.
2005. "Synthesis of SiC ceramics by the carbothermal reduction of mineralized wood with silica." Advanced Materials 17(1):73-77. Abstract This paper describes the synthesis of SiC by the carbothermal reduction of mineralized wood with silica in acidic condition. The biomorphic cellular SiC ceramics were prepared by controlling the amount of silica and the size of SiC nanoparticles. Up to 20wt% of SiO2 was mineralized into wood cellular structures and the hierarchical structures such as cells, lumen, and pits were mainly retained after the thermal treatment at 1400oC.
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. "Oxygen Transport Studies in Nanocrystalline Ceria Films." Journal of Materials Research 20(5):1295-1299. Abstract Oxygen uptake and conductivity were measured by nuclear reaction analysis and ac impedance technique at the intermediate temperature range on sol-gel grown nanocrystalline ceria films with average grain-sizes 7 nm and 38 nm synthesized at 723 K and 1173 K, respectively. Higher oxygen uptake and lower ionic conductivity are observed in ceria films with ~7 nm grain-size. High permeation-assisted oxygen diffusion in nanocrystallites combined with oxygen trapping in the disordered region contributes in higher oxygen uptake. However the lower ionic conductivity in the film results from absence of long-range lattice ordering. Relationship between oxygen uptake and conductivity in ceria is discussed in details by considering grain-size dependent defect density, related surface area and enhanced oxygen mobility.
2005. "Ceria Incorporation into YSZ Columnar Nanostructures." Electrochemical and Solid-State Letters 8(10):A525-A527. Abstract We report the growth of porous yttria-stabilized zirconia (YSZ) columnar nanostructures by glancing angle deposition (GLAD) technique. CeO₂ sol solution was incorporated into YSZ creating high interface density columnar nanostructures. Initial experiments suggest higher conductivity in CeO₂/YSZ columnar nanostructures than polycrystalline CeO₂ and lower conductivity than single crystal YSZ in the intermediate temperature range of 600-825 K. GLAD/sol-gel process combination to create high density columnar nanostructures is discussed in the context of solid oxide fuel cells operating at intermediate temperatures.
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. "Growth of Cr-doped TiO₂ Films in the Rutile and Anatase Structures by Oxygen Plasma Assisted Molecular Beam Epitaxy ." Thin Solid Films 484(1-2):289-298. Abstract As part of a search for new spintronic materials, highly ordered films of CrxTi₁-xO₂ in both rutile and anatase structure and for several Cr concentrations ranging from x=0.02 to 0.16 were grown by oxygen-plasma assisted molecular beam epitaxy. X-ray photoelectron diffraction data of the Cr 2p level exhibit the same patterns and the same modulation amplitudes as those observed for Ti 2p, providing a strong indication that a large fraction of the Cr atoms occupy substitutional lattice sites in both structures. The Cr 2p core-level spectra as well as a Cr 3d related dopant signal above the valence band of TiO₂ are characteristic of Cr³⁺ ions. At room temperature, Cr-doped anatase films exhibit ferromagnetic order with a saturation magnetization of ~0.6 µB per Cr atom and strong in-lane anisotropy.
2005. "Characterization and Properties of Metallic Iron Nanoparticles: Spectroscopy, Electrochemistry, and Kinetics." Environmental Science and Technology 39(5):1221-1230. Abstract There are reports that nano-sized zero-valent iron (Fe⁰) exhibits greater reactivity than micro-sized particles of Fe⁰, which may impart advantages for groundwater remediation or other environmental applications. However, most of these reports are preliminary in that they leave a host of potentially significant (and often challenging) material or process variables either uncontrolled or unresolved. To better understand the reactivity of iron nanoparticles, we have used a variety of complementary techniques to characterize two widely studied nano Fe⁰ preparations: one synthesized by heat-reduction of goethite under H₂ (FeH₂) and the other by reductive precipitation with borohydride (FeBH). X-ray diffraction (XRD), transmission electron microscopy (STXM) showed particles of similar size (40-80 nm), but surface area measurements varied widely with method of measurement (4-60 m² g-¹). FeH₂ is a two-phase material consisting of ⍺-Fe⁰ and Fe₃O₄, doped with reduced sulfur, whereas FeBH is mostly metallic Fe with an oxide shell that is high in boron. Both materials exhibit corrosion potentials that are more negative than nano-sized Fe₂O₃, Fe₃O₄, micro-sized Fe⁰, or a solid Fe⁰ disk, consistent with their rapid reduction of oxygen, benzoquinone, and carbon tetrachloride. Benzoquinone-which presumably probes inner-sphere surface reactions-reacts more rapidly with FeBH than with FeH₂, whereas with carbon tetrachloride, FeBH and FeH₂ react at similar rates, presumably by outer-sphere electron transfer. Whether either material reacts more rapidly with the probes than micro-sized Fe⁰ is unclear due to uncertainties in the appropriate specific surface areas. The distribution of products from reduction of carbon tetrachloride is more favorable with FeH₂, which produces less chloroform than reaction with FeBH.
2005. "Misfit Dislocations at the Single Crystal Fe₂O₃/Al₂O₃ Interface ." Physical Review. B, Condensed Matter and Materials Physics 72(7):Article number 075403. doi:10.1103/PhysRevB.72.075403 Abstract The buried interface of epitaxially-grown ⍺-Fe₂O₃(0001)/ ⍺-Al₂O₃(0001) has been studied recently using ion scattering techniques and high resolution transmission electron microscopy (HRTEM) recently. The results reveal the existence of disordering at the interface attributed to misfit dislocations associated with lattice mismatch between the substrate and the film. Molecular dynamics (MD) calculations were carried out to understand the formation of misfit dislocations and the interface structural features. The calculations show that misfit dislocations form in the Al₂O₃ substrate and terminate at the interface, consistent with the experimental observations. Snapshots of the atomic positions generated by the MD calculations were used in Monte Carlo simulations of the ion channelling experiments. The hitting probabilities determined from these simulations are compared with the experimental surface and interface peaks obtained from the aligned RBS spectra. Combination of MD and the ion scattering simulations with RBS and HRTEM measurements show promising results in understanding the interface structures of this single crystal Fe₂O₃/Al₂O₃.
2005. "Negligible Magnetism in Excellent Structural Quality CrxTi₁-xO₂ Anatase: Contrast with High-Tc Ferromagnetism in Structurally Defective CrxTi₁-xO₂." Physical Review Letters 95:217203. doi:10.1103/PhysRevLett.95.217203 Abstract The mechanism of ferromagnetism in doped oxides is under active debate. We reexamine doped TiO₂ anatase, using epitaxial Cr:TiO₂ with excellent structural quality as a model system. In contrast to highly oriented but defective Cr:TiO₂ (~0.5 µB/Cr), these structurally superior single crystal films exhibit negligible ferromagnetism. We show for the first time that charge compensating oxygen vacancies alone, as predicted by F-center mediated exchange, are not sufficient to activate ferromagnetism. Instead, the onset of ferromagnetism correlates with the presence of structural defects.
2005. "Co-doped Anatase TiO₂ Heteroepitaxy on Si(001)." Journal of Applied Physics 97(7):073511-073511-10. doi:10.1063/1.1868854 Abstract Pure anatase TiO₂ and CoxTi₁-x O₂ (0.01 < x < 0.04) epitaxial thin films were deposited by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) on Si(001) for evaluation as a potential dilute magnetic semiconductor material suitable for Si-based spintronic devices. Epitaxial growth on Si(001) was facilitated by the deposition of ¼or ½ ML Sr metal on the clean Si(001) surface to form an oxidation resistant silicide layer, followed by deposition of a thin SrTiO₃ buffer layer. Using ½ ML Sr metal to form the silicide allowed the deposition of 10 ML SrTiO₃ without oxidation of the Si interface. Epitaxial anatase could be grown on this heterostructure, although use of the oxygen plasma during deposition resulted in significant SiO₂ formation. Pure anatase films consisted of epitaxial anatase surface particles on a continuous anatase film. For Co-doped films, Co segregation to surface particles of epitaxial anatase was observed by Auger electron spectroscopy and transmission electron microscopy (TEM); faceting of the particles was observed for low Co doping concentrations. Although no secondary phases containing Co were observed in Co-doped anatase films by x-ray diffraction or TEM, x-ray absorption near edge spectroscopy indicated Co was present in the films as a mixture of Co(0), Co(II), and Co(III). All samples were ferromagnetic at room temperature; for lower Co concentrations, the ferromagnetic remanence (9%) and coercive field (100 Oe) were similar to phase-pure Co:TiO₂ / LaAlO₃. However, the presence of Co(0) under strongly oxidizing growth conditions known to oxidize the Si interface implies that under no deposition conditions can Co metal be eliminated while simultaneously protecting the Si interface from oxidation.
2005. "Processing of Soot in an Urban Environment: Case Study from the Mexico City Metropolitan Area." Atmospheric Chemistry and Physics 5:3033-3043. Abstract Chemical composition, size, and mixing state of atmospheric particles are critical in determining their e ffects on the environment. There is growing evidence that soot aerosols play a particularly important role in both climate and human health, but still relatively little is known of their physical and chemical nature. In addition, the atmo- 5 spheric residence times and removal mechanisms for soot are neither well understood nor adequately represented in regional and global climate models. To investigate the effect of locality and residence time on properties of soot and mixing state in a polluted urban environment, particles of diameter 0.2–2.0 µm were collected in the Mexico City Metropolitan Area (MCMA) during the MCMA-2003 field campaign from various sites 10 within the city. Individual particle analysis by di fferent electron microscopy methods coupled with energy dispersed X-ray spectroscopy, and secondary ionization mass spectrometry show that freshly-emitted soot particles become rapidly processed in the MCMA. Whereas fresh particulate emissions from mixed-tra ffic are almost entirely carbonaceous, consisting of soot aggregates with liquid coatings suggestive of unburned 15 lubricating oil and water, ambient soot particles which have been processed for less than a few hours are heavily internally mixed, primarily with ammonium sulfate. Single particle analysis suggests that this mixing occurs through several mechanisms that require further investigation. In light of previously published results, the internally-mixed nature of processed soot particles is expected to a ffect heterogeneous chemistry on 20 the soot surface, including interaction with water during wet-removal.
2005. "Nitrogen Analysis Using Energetic Ion Beams." Surface and Interface Analysis 37(4):374-378. Abstract As a special case of nuclear reaction analysis (NRA), nuclear elastic scattering analysis (or non-Rutherford scattering analysis) is one of the important methods in ion-beam analysis, and is the preferred technique to analyze light elements in a heavy matrix. Compared to nuclear reaction, nuclear scattering usually has cross sections several orders of magnitude larger, which allows a quantitative analysis of light elements in a quicker and more convenient manner. Similar to NRA, this method complements the analysis of widely used Rutherford backscattering spectrometry. In this study, the scattering cross-sections for 14N(p,p)14N and 14N(alpha,alpha)14N at a laboratory angle of 150 degrees are measured over energy regions from 2.480 to 3.774 MeV using an amorphous film of Si3N4 on Si wafer. Examples for the analysis of lattice disorder on the N sublattice in Au2+-irradiated GaN single crystals will be demonstrated.
2005. "Cadmium Nanowire Formation Induced by Ion Irradiation." Advanced Materials 17(13):1602-1606. Abstract One-dimensional nanostructures, such as nanowires, of semiconductors and metals are of great technological interest due to their potential for many advanced technology applications. Utilization of these materials versus their bulk counterparts will not only allow for device miniaturisation, but also may improve device performance or create new functions. Here we report a novel method for the synthesis of crystalline Cd-nanowires without involving either templates or a “seeded” structure. Ion irradiation at low temperatures (≤ 295 K) has been used to induce material decomposition and phase segregation in a cadmium niobate pyrochlore (Cd2Nb2O7) wafer. During the formation and rupture of the gas-filled blisters in the material, soft metallic Cd is extruded/extracted as nanowires through pores in the exfoliated layer. The entire process may be readily controlled by changing the ion irradiation conditions (e.g., ion species, dose and energy) with minimal thermal constraints.
2005. "Optical, Physical and Chemical Properties of Tar Balls Observed During the Yosemite Aerosol Characterization Study." Journal of Geophysical Research. D. (Atmospheres) 110:Art. No. D21210. doi:10.1029/2004JD005728 Abstract The Yosemite Aerosol Characterization Study of summer 2002 (YACS) occurred during an active fire season in the western U. S., and provided an opportunity to investigate many unresolved issues related to the radiative effects of biomass burning aerosols. Single particle analysis was performed on field collected aerosol samples using an array of electron microscopy techniques. Amorphous carbon spheres, or “tar balls”, were present in samples collected during episodes of high particle light scattering coefficients that occurred during the peak of a smoke/haze event. The highest concentrations of light-absorbing carbon from a dual-wavelength aethalometer (λ = 370 and 880 nm) occurred during periods when the particles were predominantly tar balls, indicating they do absorb light in the UV and near-IR range of the solar spectrum. Closure experiments of mass concentrations and light scattering coefficients during periods dominated by tar balls did not require any distinct assumptions of organic carbon molecular weight correction factors, density, or refractive index compared to periods dominated by other types of organic carbon aerosols. Measurements of the hygroscopic behavior of tar balls using an environmental SEM indicate that tar balls do not exhibit deliquescence, but do uptake some water at high (~83 %) relative humidity. The ability of tar balls to efficiently scatter and absorb light, and to absorb water has important implications for their role in regional haze and climate fence.
2005. "Nano-Scaffold Mediates Hydrogen Release and Reactivity of Ammonia Borane." Angewandte Chemie International Edition 44(23):3578-3582. Abstract One of the imposing barriers to realizing the promise of an energy economy based on hydrogen is onboard hydrogen storage for fuel-cell-powered vehicles. New materials that enable the release of dense, plentiful and pure hydrogen at temperatures less than 85 ºC are necessary to move the world from an oil-based economy to a hydrogen economy. We report a novel approach in which we deposit a hydrogen-rich material into a nanoporous scaffold. The role of the scaffold is to impose a nano-phase structure on the hydrogen-rich material thus providing an additional handle on the kinetics and thermodynamics of hydrogen release. We demonstrate on the example of ammonia borane infused in the nanoporous silica that the kinetics of hydrogen release is improved while the purity of hydrogen is increased in comparison with the release from bulk ammonia borane. These findings suggest that hydrogen rich materials infused in nanoscaffolds offer the most promising approach to date for onboard hydrogen storage
2005. "Intrinsic Ferromagnetism in Insulating Cobalt Doped Anatase TiO₂ ." Physical Review Letters 94(15):Art. No. 157204. Abstract Using complementary experiments we show that the room temperature ferromagnetism observed in anatase Co:TiO₂ films is not carrier mediated, but coexists with the dielectric state. TEM and x-ray absorption spectroscopy reveal a solid solution of Co in anatase, where Co is not metallic but in the +2 state substituting for Ti. Measurements at 300 K yield a Ms of 1.1 µB/Co atom, while all films are highly insulating. The evidence of intrinsic ferromagnetism in the dielectric ground state of Co:TiO₂ leads to new considerations for the origin of ferromagnetism in transition metal doped oxides.
2005. "Cobalt-Doped Anatase TiO2: A Room Temperature Dilute Magnetic Dielectric Material." Journal of Applied Physics 97:10D320. Abstract Structural and magnetic properties of epitaxial CoxTi₁-xO₂ films with x~2%, grown by RF magnetron sputtering from composite oxide targets on lattice matched LaAIO₃(001) substrates have been investigated. The films were sputtered at a deposition rate of ~0.12 Å/s for a range of substrate temperatures from 300°C to 750°C, followed by UHV annealing for 1 hr at 400°C and aging in air for 3 months. XRD experiments determine the best quality of highly oriented anatase (991) phase in films deposited 500-750°C. Magnetic hysteresis loops at 5K and 300K and thermoremanence measurements from 5-365 K show ferromagnetism in all samples in the whole temperature range. Annealing and aging lead to an increase of spontaneous moment an order of magnitude of up to ~1.1 µB/ Co atom at 300 K. As=deposited, annealed, and aged films were found to be highly resistive changes both in surface morphology and distribution of spontaneous magnetization in the annealed films. Possible mechanisms of the ferromagnetic behavior of such dielectric transition metal-doped oxides will be discussed.
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.
2004. "Experimental Studies of Defects, Implants and their Processes in Ion-Irradiated Gallium Nitride Single Crystals." In Defects and Diffusion in Ceramics - an Annual Retrospective - VI, Defect and Diffusion Forum, vol. 226-228, ed. David J. Fisher, pp. 91-111. Trans Tech Publications, Uetikon-Zurich, Switzerland. Abstract This article reviews recent experimental results, obtained by the authors, on disorder accumulation, disorder recovery, and behavior of implanted species in ion-irradiated gallium nitride (GaN) single crystals. The disorder on both the Ga and N sublattices has been studied in situ using Rutherford backscattering spectrometry (RBS) and nuclear resonant scattering along the <0001> axis, while the damage states for as-irradiated and post-annealed specimens have been examined using transmission electron microscopy. The disorder accumulation has been investigated as a function of ion fluence, ion mass and irradiation temperature; disorder annealing has been studied under thermal and dynamic conditions. The behavior of gold implants in GaN during irradiation and thermal annealing also will be discussed.
2004. "Toward the Solution Synthesis of the Tetrahedral Au₂₀ Cluster." Journal of Physical Chemistry B 108(33):12259-12263. Abstract We report solution synthesis of the recently discovered tetrahedral Au₂₀ cluster coordinated with eight PPh₃ (Ph = Phenyl) ligands. The composition and molecular weight of the Au₂₀ ( PPh₃)₈ complex were confirmed by the isotopic pattern of its doubly charged cation using high resolution mass spectrometry. Collision-induced dissociation experiment showed that four PPh₃ can be easily dissociated from Au₂₀ ( PPh₃)₈²⁺, resulting in a highly stable Au₂₀ ( PPh₃)₄²⁺ molecule. This observation is consistent with the tetrahedral structure, in which the four apex sites are expected to be strongly bound to the PPh₃ ligands, and is confirmed by theoretical calculations, which predicted a highly stable Au₂₀ (PH₃)₄ complex with Au-PH₃ bond energies of ~1 eV. The current experimental and theoretical observations suggest that large quantity of ligand-stabilized tetrahedral Au₂₀ can be obtained, opening the door for exploring its anticipated novel chemical, optical, and catalytical properties.
2004. "Electronic Stopping Powers in Silicon Carbide." Physical Review. B, Condensed Matter and Materials Physics 69(20):205201, 1-9. Abstract The stopping powers in silicon carbide (SiC) of nine different ions, ranging from Be to Au, have been determined using a time-of-flight elastic recoil detection analysis (TOF ERDA) set-up. In transmission geometry, the energy loss of ions in a self-supporting SiC film is measured over a continuous range of recoil energies, from tens to hundreds keV/nucleon, using Time-of-Flight (TOF) spectrometry. By essentially calibrating the Si detector for each channel using the TOF spectrometer, the error resulting from nominal energy calibration is eliminated. An uncertainty of less than 4% is achieved in the stopping measurements. Stopping powers predicted by SRIM (The Stopping and Range of Ions in Matter) code are in reasonable agreement with much of the experimental data, and the SRIM 2003 predictions are in somewhat better agreement than SRIM 2000 for most ions. There are, however, still some discrepancies between SRIM predictions and the experimental data. For Ni, I and Au ions, the predicted values from SRIM 2003 are up to 40% less than the measured values. The stopping data from this study as well as previous work suggest that both the modified Bohr and Bloch formulas are suitable for scaling the stopping number of heavy ions in SiC within the classical interaction regime, and the Bloch formula is more appropriate to use at higher energies.
2004. "Effects of Implantation Temperature on Damage Accumulation in Al-Implanted 4H-SiC." Journal of Applied Physics 95(8):4012-4018. Abstract Damage accumulation in 4H-SiC under 1.1 MeV Al₂(₂⁺) irradiation is investigated as a function of dose at temperatures from 150 to 450 K. Based on Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) channeling spectra, the damage accumulation on both the Si and C sublattices have been determined, and a disorder accumulation model has been fit to the data. The model fits indicate that defect-stimulated amorphization is the primary amorphization mechanism in SiC over the temperature range investigated. The temperature dependence of the cross section for defect-stimulated amorphization and the critical dose for amorphization indicate that two different dynamic recovery processes are present, which are attributed to short-range recombination and long-range migration of point defects below and above room temperature, respectively. As the irradiation temperature approaches the critical temperature for amorphization, cluster formation has an increasing effect on disorder accumulation, and ion flux plays an important role on the nature and evolution of disorder. Dislocation loops, which are mostly formed under high ion flux, act as sinks for point defects, thereby reducing the disorder accumulation rate.
2004. "Anisotropic Thermal Properties of Single-Wall Carbon Nanotube Reinforced Nanoceramics." Philosophical Magazine Letters 84(2):419-423. Abstract Dense single-wall carbon nanotube (SWCNT) reinforced alumina nanocomposites have been fabricated by novel spark-plasma-sintering (SPS) technique. Anisotropic thermal properties have been found in the carbon nanotube composites. The introduction of ropes of SWCNT gives rise to a decrease of the transverse thermal diffusivity with increasing carbon nanotube content while it does not change the in-plane thermal diffusivity. This is scientifically interesting and technologically important for the development of materials for novel thermal barrier coatings.
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. "Ion-Beam Induced Defects and Nanoscale Amorphous Clusters in Silicon Carbide." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 216:25-35. Abstract Atomic-level simulations have been employed to study the defects and nanoscale disordering induced in 3C-SiC by C, Si, and Au ions with energies up to 50 keV. Energetic C and Si ions primarily produce interstitials, vacancies, antisite defects, and small defect clusters directly during the collision cascade. The overlap of Si cascades produces nanoscale defect clusters. In the case of energetic Au ions, nanoscale amorphous domains are produced directly within the Au cascade along with point defects and smaller clusters. In about 25% of the 50 keV Au cascades, one or more of the subcascades contain nanoscale clusters that exhibit a structure that is consistent with an amorphous state. Structural image simulations of the subcascade structures produced by energetic Si and Au recoils are consistent with experimental high-resolution transmission electron microscopy images. Simulations on close-pair production and recombination in SiC indicate that the activation energies for recombination of most close pairs range from 0.24 to 0.38 eV.
2004. "Precipitation of Au Nanoclusters in SrTiO₃ by Ion Implantation." Journal of Applied Physics 95(9):5060-5068. Abstract Gold nanoclusters dispersed in single crystal SrTiO₃(001) have been prepared by ion implantation at both 300 K and 975 K followed by annealing at 1275 K for 10 hours. Transmission electron microscopy (TEM), high-resolution TEM, high angle annular dark-filed (HAADF) imaging in an aberration corrected dedicated scanning TEM (STEM), and image simulations were used to study the morphology, size, and crystallographic orientation of the Au nanoclusters with respect to the SrTiO₃ matrix, as well as the interface structure between the Au nanoclusters and SrTiO₃. Implantation of gold at 300 K leads to amorphization of SrTiO₃ surface layer, which is corrugated to form bumps and valleys on the surface. Annealing at 1275 K for 10 hours leads to epitaxial re-crystallization of the amorphized layer within which Au clusters of several nanometers and a narrow size distribution were formed. Implantation at 975 K and subsequently annealing leads to Au cluster size of ~ 50 nm and a very wide size distribution. The precipitated Au clusters possess an epitaxial orientation with the SrTiO₃, such that Au[001]//SrTiO₃[001] and Au(100)//SrTiO₃(100). The critical size of the Au cluster for transition from strain-matched interface to dislocation relaxed interface has been found to be ~ 7 nm. Cavities formed by condensation of vacancies were faceted along {001} and {011} planes in the SrTiO₃ matrix. It is generally implied that implantation below a critical temperature may lead to a finer Au cluster size.
2004. "Microstructure of Precipitated Au Nanoclusters in TiO₂." Journal of Applied Physics 95(12):8185-8193. Abstract Gold nanoclusters dispersed in single crystal TiO2 (110) have been formed by 2 MeV Au2+ implantation to an ion fluence of 11017 cm-2 at 300 K and 975 K followed by annealing at 1275 K for 10 hours. The morphological features, size, crystallographic orientation of the Au nanoclusters with respect to the TiO2 matrix, and the interface structure between the Au nanoclusters and TiO2 have been investigated using conventional transmission electron microscopy (TEM), high-resolution TEM (HRTEM), electron diffractions, and high angle annular dark-filed (HAADF) imaging in an aberration corrected dedicated scanning TEM (STEM). STEM-HAADF image directly reveals that Au atoms are in the substitutional Ti atomic columns in the TiO2 lattice prior to nucleation of Au lattice. Atomic structural model of interface between Au and TiO2 was established based on HRTEM and image simulations. The precipitated Au clusters show typical (111) twins. Au clusters are facetted along Au{112}, Au{111}, and Au{220} planes. Two types of orientation relationship can be identified, Au<110>//TiO2[001] and Au{111}//TiO2(200), and Au<110>//TiO2[001] and Au{111}//TiO2(110). These orientation relationships as well as the {111} twining feature in Au clusters are similarly observed for Au clusters grown on stoichiometric TiO2(110) free surface, indicating that the presently established orientation corresponds to the lowest interfacial energy for Au contacted with TiO2. This is essential for understanding the catalytic properties of Au supported on TiO2.
2004. "Distortion of the Oxygen Sublattice in Pure Cubic-ZrO₂ ." Journal of Materials Research 19(5):1315-1319. Abstract Multi-layer films of pure ZrO₂ and CeO₂ were grown using molecular beam epitaxy on a yttria-stabilized zirconia (YSZ) substrate. Distinctive forbidden diffraction spots of (odd, odd, even) type were observed on the selected area electron diffraction patterns of the film. Dark-field imaging clearly revealed that these forbidden diffraction spots were solely due to the ZrO₂ layers. Comparison of the electron diffraction pattern with one simulated by dynamical calculations suggest that the pure ZrO₂ layers possess a cubic structure of space group with the oxygen sub-lattice being displaced diagonally, rather than along the c-axis as suggested for YSZ. Our results further suggest that the displacement of the oxygen from the ideal (¼,¼,¼) position might have been introduced during the film growth process.
2004. "Direct Observation of Substitutional Au Atoms in SrTiO3." Physical Review. B, Condensed Matter and Materials Physics 70(17):172201-172201-4. Abstract Ion implantation and subsequent high-temperature annealing is an effective way to prepare metal nanoclusters dispersed in a dielectric for useful optical and electrical properties. However, there is very little understanding of the nucleation and growth process of these nanoclusters, their correlations with the sites of the implanted ions, and the behavior of defects (such as vacancies) generated during the ion-implantation process. Using high-angle annular dark-field imaging in aberration-corrected scanning transmission electron microscopy, we have directly observed that, at a dilute concentration, Au atoms implanted in SrTiO3 are in a substitutional lattice position for both Sr and Ti. Congregation of Au atoms by diffusion to a critical concentration leads to the nucleation of a Au lattice within the SrTiO3. The Au nanocluster and SrTiO3 were found to maintain an orientation relationship of Au[001]//SrTiO3[001] and Au(100)//SrTiO3(100), which corroborated the results of a first-principles total-energy calculation. The interface between the Au cluster and SrTiO3 was bridged by an O/Ti plane. The Au-O bond length was found to be 2.2 Å, which is the same as Au-O bond length in AuO. Furthermore, the atomic planes adjacent to the interface in both Au and SrTiO3 were found to be slightly stretched. The high concentration of vacancies generated during the Au implantation aggregated to form cavities in the SrTiO3 lattice, and were faceted mostly along the SrTiO3 {100} and (110) planes. The Au and vacancy clusters were spatially associated, indicating a strong interaction. Thus, the formation of cavities in Au-implanted materials indicates that the vacancy-clustering process prevails over the Frenkel-pair recombination.
2004. "Ion-Beam Synthesis of Epitaxial Au Nanocrystals in MgO." Journal of Materials Research 19(5):1311-1314. Abstract The formation of Au nanoclusters in MgO using ion implantation and subsequent annealing has been investigated. Approximately 1200 and 1400 Au₂⁺ ions/nm₂ were implanted in MgO(100) substrates at 300 and 975 K, respectively. Subsequent annealing in air for 10 hours at 1275 K promoted the formation of Au nanostructures in MgO. The sample implanted at 300 K showed severe radiation damage. In addition, two-dimensional platelet-like structures with possible composition of Au and MgO were formed during implantation in the sample that was implanted at 300 K. In contrast, Au implantation at 975 K promoted the nucleation of Au nanostructures during implantation. Subsequent annealing of both samples show three-dimensional clusters in MgO. However, the 975 K implanted sample shows clean high quality single crystal Au clusters that have an epitaxial relationship to MgO(100).
2004. "Synthesis of Hierarchical Titanium Carbide from Titania-Coated Cellulose Paper." Advanced Materials 16(14):1212-1215. Abstract An aqueous-based, inexpensive, TiO2 precursor, Tyzor-LA, and cellulose paper have been used to prepare TiC via carbothermal reduction in an Ar flow. The hierarchical cellulose structures were also completely maintained through the agglomeration of 10-50nm TiC nanoparticles, and the lattice parameter of TiC increased with the increase of reaction temperature, while the oxygen content in the sample decreased (0.24wt% at 1500oC).
2004. "Distinguishibility of Oxygen Desorption From the Surface Region with Mobility Dominant Effects in Nanocrystalline Ceria Films." Journal of Applied Physics 96(10):5756-5760 . Abstract We present an investigation of oxygen (18O) uptake measurements in 1 m thick nanocrystalline ceria films grown on single crystal Al₂O₃ (0001) by nuclear reaction analysis (NRA). Oxygen uptake measurements were carried out in the temperature range of 200 0C-600 0C at a background 18O pressure of 4.0 x 10-6 Torr. Average grain-size in the as-grown films, synthesized by sol-gel process was ~ 3 nm confirmed by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) measurements. From the diffusion depth profiles, changes in intensity and slopes in surface and interface regions indicate dominating oxygen mobility effects. Oxygen desorption is clearly distinguishable in the film surface region as a result of shift in the oxygen peak intensity. It is argued that high defect density in nanocrystalline ceria which is associated with nano-grain surface combined with intermediate temperature reducing environment triggers multiple processes like diffusion, desorption and exchange interaction. The promising nature of NRA is realized as an effective tool to acquire the depth-dependent information regarding such complex reactions that exists in nanocrystalline environment.
2004. "The Effects of Template and Precursor Chemistry on Structure and Properties of Mesoporous TiO2 Thin Films." Langmuir 20(21):9095-9102. Abstract Mesoporous TiO2 thin films were synthesized by sol-gel processing using an aqueous-based, inexpensive and environmentally friendly precursor and cationic surfactants as templates under mild reaction conditions. The films were prepared by spin-coating on glass substrates followed by calcination to remove the surfactant. N2 sorption, x-ray diffraction and TEM were used to characterize the porosity, pore size and pore structure before and after calcination. Films were found to have wormlike pore structures after calcination, and surface areas on the order of 200 m2/g. These results show that the mesostructure and porosity of the thin films can be controlled by the surfactant template chemistry such as surfactant/Ti ratio, pH, and the rate of solvent evaporation.
2004. "Effects of Template and Precursor Chemistry on Structure and Properties of Mesoporous TiO2 Thin Films ." Langmuir 20(21):9095-9102. Abstract Mesoporous TiO2 thin films were synthesized by sol-gel processing using an aqueous-based, inexpensive and environmentally friendly precursor and cationic surfactants as templates. The films were prepared by spin-coating on glass substrates followed by calcination to remove the surfactant. N2 sorption, x-ray diffraction and TEM were used to characterize the porosity, pore size and pore structure before and after calcination. Films were found to have wormlike pore structures after calcination. These results show that the mesostructure and porosity of the thin films can be controlled by the surfactant template chemistry such as surfactant/Ti ratio, pH, and the speed of solvent removal.
2004. "Growth and Properties of molecular beam epitaxially grown ferromagnetic Fe-doped TiO2 rutile films on TiO2." Applied Physics Letters 84(18):3531-3533. Abstract We have grown epitaxial Fe-doped TiO₂ rutile films on rutile TiO₂(110) substrates, and have explored the resulting compositional, structural, morphological and magnetic properties. Clusters of mixed TiO₂ rutile and Fe₃O₄ form on the surface of a continuous rutile epitaxial film during growth. Room temperature ferromagnetism is observed, and is associated with the formation of secondary phase Fe₃O₄ rather than a true diluted magnetic oxide semiconductor.
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. "A Direct Route towards Assembly of Nanoparticle-Carbon Nanotube Composite Materials ." Langmuir 20(14):6019-6025. Abstract The exploration of nanoparticle-structured thin films as sensing materials desires maximum accessibility of analytes and effective mass transport within the nanostructure. This paper explores the viability of creating nanoparticle-carbon nanotube (CNTs) as composite interfacial materials to enhance such properties. We report findings of an investigation of the assembly of monolayer-protected gold nanoparticles on multi-walled CNTs. A simple and effective route has been demonstrated for assembling nanoparticles of 2-5 nm core sizes onto CNTs with controllable coverage and interparticle spatial properties. The composite nanomaterials can be dispersed in organic solvent and cast on interdigitated microelectrode surface. The skeleton-like nanocomposite materials have been examined for chemiresistor sensing of volatile organic compounds. The response profiles and sensitivities of the nanocomposites determined for the sorption of a series of vapors have been shown to exhibit different or enhanced sensing properties in comparison with similar but nanotube-free nanoparticle assemblies. The observation of these results can be attributed to a combination of three factors, the increased accessibility of analytes to the nanostructure, the enhanced mass transport characteristics, and the unique electronic properties of the nanocomposite materials. Implications of the findings to the design of nanostructured sensing materials are also discussed.
2004. "Growth and surface structure of vanadium oxide on anatase (001)." Surface Science 559(2-3):201-213. Abstract Oxygen plasma assisted molecular beam epitaxy (OPA-MBE) of vanadium oxide on (1×4)-reconstructed anatase (001) thin films was studied using reflection high energy electron diffraction (RHEED), low energy electron diffraction (LEED), x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), x-ray diffraction (XRD), and transmission electron microscopy (TEM). XPS and UPS results showed that the vanadium was predominantly in the 5+ oxidation state after deposition of a monolayer at 525 K. After 1 ML of vanadia was deposited, the anatase (1×4)/(4×1) LEED and RHEED patterns were replaced by (1×1) patterns indicating that the vanadia lifts the reconstruction and suggesting that the monolayer is pseudomorphic. At 525 K, the V⁵⁺ oxidation state predominated in thicker films, however, no discernible LEED or RHEED patterns were seen after a few monolayers were deposited indicating that V₂O₅ epitaxy cannot be continued beyond 1 ML. When the growth temperature was increased to 750 K, RHEED patterns indicated no change in the surface structure after more than 20 ML of vanadia were deposited. Under these conditions, XPS peak positions were consistent with VO₂. After growth at 775 K a c(2x2) LEED pattern attributed to half a monolayer of adsorbed oxygen on the VO₂ surface was observed. The surface characterization data all pointed towards pseudomorphic growth of VO₂ with a half monolayer of capping oxygen allowing the monolayer to achieve the V₂O₅ stoichiometry while maintaining the anatase structure. Bulk XRD data, however, were consistent with VO₂ (B), V₆O₁₃, and rutile VO₂ none of which expose surfaces with the periodicity observed with RHEED and LEED. The reasons for the differences between the surface and bulk characterization are discussed.
2004. "Control of Carbon Nanotube Morphology by Change of Applied Bias Field During Growth." Applied Physics Letters 85(22):5373-5375. Abstract Carbon nanotube morphology has been engineered via simple control of applied voltage during dc plasma chemical vapor deposition growth. Below a critical applied voltage, a nanotube configuration of vertically aligned tubes with a constant diameter is obtained. Above the critical voltage, a nanocone-type configuration is obtained. The strongly field-dependent transition in morphology is attributed primarily to the plasma etching and decrease in the size of nanotube-nucleating catalyst particles. A two-step control of applied voltage allows a creation of dual-structured nanotube morphology consisting of a broad base nanocone (~200 nm dia.) with a small diameter nanotube (~7 nm) vertically emanating from the apex of the nanocone, which may be useful for atomic force microscopy.
2004. "Carbon-Nanotube-Reinforced Polymer-Derived Ceramic Composites." Advanced Materials 16(22):2036-2040. Abstract Carbon nanotube reinforced ceramic composites were synthesized by using recently developed polymer-derived ceramics as matrices. Multi-wall carbon nanotubes, treated with a surfactant, were first dispersed in a liquid polymer precursor by sonication and mechanical stirring. The solution was then converted to fully dense ceramic composites with pressure-assist pyrolysis technique. Microstructural observation revealed that nanotubes were homogeneously dispersed throughout the ceramic matrix. Significant increases in mechanical and thermal properties were observed by adding only ~6vol% nanotubes. Strong nanotube pullout revealed by SEM observation suggested that the composites could possess high fracture toughness.
2003. "Synthesis, Characterization, and Manipulation of Helical SiO2 Nanosprings." Nano Letters 3(5):577-580. Abstract Amorphous helical SiO2 nanosprings (80 to 140 nm in diameter and up to 8 microns long) were synthesized with a chemical vapor deposition technique, characterized and manipulated by scanning (SEM) and transmission (TEM) electron microscopy and atomic force microscopy (AFM). The helical nanosprings were observed in the middle of a straight nanowire and were formed by a perturbation during the growth of the straight nanowire. Contraction and expansion of the helical nanosprings were observed under in situ electron beam heating during TEM, as well as bending induced by an AFM tip, suggesting that the helical nanosprings are highly flexible and may have potential applications in nanomechanical, nanoelectronmagnetic devices, and composite materials.
2003. "Effects of Implantation Temperature and Ion Flux on Damage Accumulation in Al-Implanted 4H-SiC." Journal of Applied Physics 93(4):1954-1960. Abstract The effects of implantation temperature and ion flux on damage accumulation on both the Si and C sublattices in 4H-SiC are investigated under 1.1 MeV Al₂₂⁺ irradiation at temperatures from 150 to 450 K. The rate of damage accumulation decreases dramatically and the damage profile sharpens due to significant dynamic recovery at temperatures close to the critical temperature for amorphization. At 450 K, the relative disorder and the density of planar defects increase rapidly with the increasing ion flux, exhibiting saturation at high ion fluxes. Planar defects are generated through the agglomeration of excess Si and C interstitials during irradiation and post-irradiation annealing at 450 K. Termination of (0001) planes is attributed to the accumulation of vacancies. A volume expansion of ~8% is observed for the peak damage region.
2003. "Supercritical Fluid Fabrication of Metal Nanowires and Nanorods Templated by Multi-walled Carbon Nanotubes." Advanced Materials 15(4):316-319. Abstract A rapid, direct and clean approach has been developed to synthesize metal (Pd, Ni and Cu) nanowires and nanorods using multi-walled carbon nanotubes (MWCNTs) as templates and supercritical CO2 as the reaction medium. Filling of metals into MWCNTs to form nanowires or nanorods was easily achieved by the simple hydrogen reduction of metal-b-diketone complexes in supercritical CO2.
2003. "Microstructure of Precipitated Au Nanoclusters in MgO." Journal of Applied Physics 93(10):6327-6333. Abstract Gold nanoclusters dispersed in single crystal MgO have been prepared by ion implantation at 975 K and subsequent annealing at 1275 K for 10 hours. The morphological features, size, and crystallographic orientation of the Au nanoclusters with respect to the MgO matrix, as well as the interface structure between the Au nanoclusters and MgO, have been investigated using transmission electron microscopy. During annealing, the Au clusters nucleate coherently in the MgO lattice, leading to an epitaxial orientation relationship of [010]MgO//[010]Au and (200)MgO//(200)Au that is maintained for all the Au clusters. Above a critical size of ~5 to 8 nm, a coherent-semicoherent interface transition is observed for the Au clusters in MgO. This critical cluster size is larger than the critical size, ~3 nm, based on energetic consideration. This discrepancy is discussed with respect to the point and extended defect structures at the interface between the Au clusters and the MgO matrix. The Au clusters larger than this critical size exhibit faceting on the {001} planes and internal dislocations. It is further suggested that the density of quantum antidot should depend on the size of the Au clusters.
2003. "Microstructrual Features of Al-Implanted 4H-SiC." Journal of Materials Research 18(4):772-779. Abstract The microstructural features of highly-damaged 4H-SiC implanted with Al₂₂⁺ ions at 450 K have been studied using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). Conventional TEM images reveal that the crystalline SiC domains are highly strained/distorted when the relative disorder on the Si sublattice ranges between about 0.4 and 0.8, as determined by Rutherford backscattering spectrometry in channeling geometry (RBS/C). As the relative disorder approaches to 1.0, the high strain contrast appears to be relieved and localized amorphized domains are observed. Plasmon-loss energy shows a red-shift following the implantation, and the magnitude of the red-shift increases with increasing relative disorder. Based on the red-shift, the estimated volume expansion is ~ 8% for highly-damaged crystalline SiC and ~ 16% for the amorphous state. Energy-loss near-edge-structure (ELNES) of both the C and Si K-edge reveals the existence of Si-Si and C-C bonding in the Al₂₂⁺implanted SiC.
2003. "Interface Structure of an Epitaxial Cubic Ceria Film on Cubic Zirconia." Journal of the American Ceramic Society 86(2):363-365. Abstract A cubic CeO₂ (001) film of thickness equal to ~58 nm was epitaxially grown on Y₂O₃-stablized cubic ZrO₂ by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface was characterized using high resolution transmission electron microscopy (HRTEM). The interface exhibited coherent regions separated by equally-spaced misfit dislocations. When imaged from the [100] direction, the dislocation spacing is 3.3 ? 0.5 nm, which is slightly shorter than the expected value of 4.9 nm calculated from the differences in lattice constants given in the literature, but is fairly consistent with the 3.9 nm lattice mismatch measured by electron diffraction. Thus, the results presented here indicate that the lattice mismatch between the film and the substrate is accommodated mainly by interface misfit dislocations above some critical thickness.
2003. "Interface Characteristics of Iso-Structural Thin Film and Substrate Pairs." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):1-9. Abstract Cubic-CeO₂ and -Fe₂O₃ thin films have been epitaxially grown on yttria-stabilized ZrO₂ and -Al₂O₃ substrates, respectively, by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface structural features between the films and the substrates were characterized by Rutherford backscattering spectrometry (RBS), high resolution transmission electron microscopy (HRTEM), and x-ray diffraction (XRD). RBS channeling spectra for both CeO₂/ZrO₂ and Fe₂O₃/Al₂O₃ show interface disorder-related scattering peaks. It is believed that the observed interface disorder-related scattering peaks on RBS spectra are attributed to the interface misfit dislocations. Cross sectional HRTEM reveals that interfaces of both systems are similarly characterized by coherent regions that are separated by misfit dislocations periodically distributed along the interface. The experimentally observed dislocation spacings are approximately consistent with those calculated from the lattice mismatch, implying that the lattice mismatch is accommodated mainly by interface misfit dislocations above the critical thickness.
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. "Ion-Beam-Irradiation Induced Defects in Gallium Nitride." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 206:1037-1041. Abstract Epitaxial single-crystal gallium nitride (GaN) films on sapphire were irradiated at low and room temperatures using O⁺ ions over a range of fluences. The accumulation of disorder on the Ga sublattice has been investigated based on He⁺ Rutherford backscattering analysis along the <0001>-axial channeling direction. The degree of disorder in the O⁺ irradiated GaN increases at low doses and saturates above 10 displacements per atom (dpa). The microstructures of two O⁺-irradiated specimens are characterized using high-resolution transmission electron microscopy. Similar planar defect structures at the saturated disorder level are observed for as-irradiated and thermally annealed GaN. The growth and annihilation of these defects during ion irradiation and thermal annealing are believed to contribute to the saturation and stability of defect concentrations.
2003. "Immobilization of Quantum Dots in the Photo-Cross-Linked Poly(ethylene glycol)-Based Hydrogel." Journal of Physical Chemistry B 107:10464-10469. Abstract An inorganic/organic composite hybrid nano-system has been successfully synthesized in which nanocrystalline quantum dots (QDs) were effectively immobilized within a photo-cross-linked poly(ethylene glycol) hydrogel. Organometallic synthesis of CdTe and CdSe QDs was accomplished with a trioctylphosphine oxide (TOPO) cap. Replacing the TOPO cap with mercaptoacetic acid groups further yielded modified water-soluble nanocrystals. The immobilization of these functionalized CdTe and CdSe QDs within PEG hydrogel network has been shown to be effective through utilization of physical trapping. The CdTe and CdSe QDs had a particle diameter of 4.5 and 2.5-6.0 nm, respectively. The most efficiently trapped QDs had a size of 4.5 nm or larger. Particle size determination was derived from spectroscopic (absorption and photoluminescence) and high-resolution transmission electron microscopic techniques. These QD-immobilized gel systems demonstrated photoluminescence characteristics unique to semiconductor QD nanocrystals. The authors have envisioned the utilization of the unique photophysical properties of this material as a convenient signal transducer for in vivo biosensing. The most promising application of the described QD/PEG-NC hybrid system is in the fields of in vivo fluorescence microscopy and as a monitoring system for drug delivery and wound healing.
2003. "Atomic-Level Simulations of Misfit Dislocation at the Interface of Fe2O3/Al2O3 System." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):63-71. Abstract When -Fe2O3 thin films are deposited on -Al2O3 (0001) substrates using oxygen plasma assisted molecular beam epitaxy (OPA-MBE), a periodic distribution of basal dislocations occurs due to lattice mismatch along the interfaces. High-resolution transmission electron microscopy (HRTEM) shows, when observed from zone axis, that these dislocations lie at the interface about 7.0 nm apart. Molecular dynamics simulations (MD) were performed in order to understand the formation of misfit dislocations and the interface structural features in Fe2O3/Al2O3 system. It is found that the misfit dislocations are mainly formed in Al2O3 substrates with Burger's vector of 1/3< >, and terminated at the interfaces, in consistent with experimental observations. These dislocations can dissociate into two partial dislocations with Burger's vectors of 1/3< > and 1/3< > by forming stacking faults on (0001) planes. The core structures of the misfit dislocations in semicoherent interfaces are analyzed in detail, and the misfit dislocations narrow cores in the plane of the interfaces.
2003. "Clusters and Magnetism in Epitaxial Co-doped TiO₂ Anatase." Applied Physics Letters 82(8):1257-1259. Abstract We show that under certain conditions, highly Co-enriched Ti0₂ anatase clusters nucleate on epitaxial Ti0₂ anatase grown on LaA₁O₃(001) by oxygen plasma assisted molecular beam epitaxy. In the most extreme cases, virtually all incident Co segregates to the clusters, yielding a nanoscale ferromagnitic phase that is not ferromagnetic in homogeneous films of the same Co concentration. The nucleation of this phase simultaneous with continuous epitaxial film growth must be carefully monitored in order to avoid drawing false conclusions about the film structure
2002. "Lithium-Assisted Self-Assembly of Aluminum Carbide Nanowires and Nanoribbons." Nano Letters 2(2):105-108. Abstract We report on the synthesis and self-assemlby of Al4C3 nanowires and nanoribbons using lithium as a catalyst. Large quantities of Al4C3 nanowires (diameters from 5 to 70 nm) and nanoribbons (5-70 nm thick and 20-5600 nm wide) tens of micrometers long were synthesized serendipitously in a solid-state reaction involving Al/C/Li at less than 780 degrees Celcius. High-resolution electron microscopy revealed that the nanowires all grew along the c-axis of hexagonal Al4C3, whereas the nanoribbons all grew within the basal plane. The facile synthesis of the Al4C3 nanowires and nanoribbons suggest similar nanostructures of other carbide and nitride materials may be made using the lithium-assisted self-assembly process.
2002. "Helical Crystalline SiC/SiO2 Core-Shell Nanowires." Nano Letters 2(9):941-944. Abstract Helical crystalline silicon carbide nanowires covered with a silicon oxide sheath (SiC/SiO2) have been synthesized by a chemical vapor deposition technique. The SiC core typically has diameters of 10-40 nm with a helical periodicity of 40-80 nm and is covered by a uniform layer of 30-60 nm thick amorphous SiO2. A screw-dislocation-driven growth process is proposed for the formation of this novel structure based on detailed structural characterizations. The helical nanostructures may find applications as building blocks in nanomechanical or nanoelectronic devices. The screw-dislocation-induced growth mechanism suggests that similar helical nanostructures of a wide range of materials may be synthesized.
2002. "The Characteristics of Interface Misfit Dislocations for Epitaxial alpha-Fe₂O₃ on alpha-Al₂O₃(0001)." Thin Solid Films 414:31-38. Abstract Alpha-Fe₂O₃(0001) films of thickness equal to ~7 nm and ~70 nm were epitaxially grown on alpha-Al₂O₃(0001) by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interfaces were characterized using high resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and x-ray diffraction (XRD). The interface exhibited coherent regions separated by equally-spaced misfit dislocations. When imaged from the [2110] direction, the dislocation spacing is 7.0 +- 1.1 nm for the 70 nm thick speciman, and 7.2 +- 0.1 nm for the 7 nm thick speciman. When imaged from the [0110] direction, the disslocation spacing is 4.5 +- 0.1 nm for the 7 nm thick speciman. The experimentally observed dislocation spacings are appproximately consistent with those calculated from the lattice mismatch between alpha-Al₂O₃ and alpha-Fe₂O₃, implying that the lattice mismatch is accomodated mainly be interface misfit dislocations above the critical thickness, which is less than 7 nm. This conclusion is also corroborated by the measured reidual strainof ~0.5% determined from x-ray diffraction for the 70 nm film . EELS analysis reveals that the Fe L₂,3-edge shows no measurable chemical shift relative to the L₂,3-edge of structural Fe₃⁺, indicating complete oxidation of Fe in the as-grown film.
2002. "HRTEM Characterization of Interface Between Iso-Structural Thin Solid Film and Substrate." Microscopy & Microanalysis 8(Suppl S02):1160-1161. doi:10.1017/S1431927602107975 Abstract Alpha-Fe₂O₃ and c-CeO₂ thin films have been epitaxially grown on alpha-Al₂O₃ and yttria- stabilized c-ZrO₂ substrates, respectively, by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface structural features between the films and the substrates were characterized by high resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), Rutherford backscattering spectrometry (RBS), and x-ray diffraction (XRD). For the two systems studied, the interfaces are similarly characterized by coherent regions that are separated by misfit dislocations periodically distributed along the interface. These results will be presented along with the results from molecular dynamics (MD) simulations of these interfaces.
2002. "Defect Clustering in GaN Irradiated with O⁺ Ions." Journal of Materials Research 17(11):2945-2952. Abstract Transmission electron microcopy (TEM) is used to study microstructures formed in GaN irradiated with 600 keV O⁺ ions at room temperature. Three types of defect clusters are identified in the irradiated GaN: (1) basal-plane stacking faults with dimensions ranging from 5 to 30 nm, (2) Pyramidal dislocation loops, and (3) local regions of highly-disordered material. High resolution TEM imaging clearly reveals that one type of the basal-plane stacking faults corresponds to insertion of one extra Ga-N basal-plane in the otherwise perfect GaN lattice. The interpretation of these results indicates that interstitials of both Ga and N preferentially condense on the basal plane to form a new layer of Ga-N under these irradiation conditions. The formation of these extended defects and their interactions with the point defects produced during irradiation contribute to a dramatic increase in the dynamic recovery of point defects in GaN at room temperature.
2002. "Disordering Behavior and Helium Diffusion in He⁺ Irradiated 6H-SiC." Journal of Materials Research 17(2):271-274. Abstract Single crystal 6H-SiC wafers were irradiated at 300 K with 50 keV He⁺ ions to fluences ranging from 7.5 to 250 He⁺/nm₂. Ion-channeling experiments with 2.0 MeV He⁺ Rutherford backscattering spectrometry (RBS) were performed to determine the depth profile of Si disorder. The measured profiles are consistent with SRIM-97 simulations at and below 45 He⁺/nm₂, but remarkably higher than the SRIM-97 prediction at both 100 and 150 He⁺/nm₂. Cross-sectional transmission electron microscopy (XTEM) study indicates a homogeneous-amorphization process in the material under the irradiation conditions. Results from elastic recoil detection analysis (ERDA) suggest that the implanted He atoms diffuse in a high-damage regime toward the surface.
2002. "Epitaxial Growth and Properties of MBE Grown Ferromagnetic Co-doped TiO₂ Anatase Films on SrTiO₃(001) and LaAlO₃(001)." Thin Solid Films 418(2):197-210. Abstract We have investigated the heteroepitaxial growth and materials properties of pure and Co-doped TiO₂ anatase on SrTiO₃(001) and LaAlO₃(001), grown by oxygen plasma assisted molecular beam epitaxy. This material is a promising new diluted magnetic semiconductor that shows large magnetization and a Curie temperature well above room temperature. We have found that epitaxial films with the highest crystalline quality and most uniform distribution of Co result when a rather slow growth rate (~0.01 nm/sec) is used over a substrate temperature range of 550 degrees Celcius to 600 degrees celcius. These conditions result in layer-by-layer growth of single-crystal films, and a very low density of extremely small nanocrystalline inclusions. In contrast, growth at a higher rate (~0.04 nm/sec) leads to extensive formation of secondary phase rutile nanocrystals to which Co diffuses and segregates. The rutile nanocrystals nucleate on the evolving anatase film surface in such a way that lattice strain between the two phases is minimized. Co appears to substitute for Ti in the lattice and exhibits a ⁺² formal oxidation state. Both pure and Co-doped films are typically n-type semiconductors despite the lack of intentional n-type doping, although a wide range of conductivities is observed.